CA1201191A - Semiconductor laser device - Google Patents

Abstract

SEMICONDUCTOR LASER DEVICE

Abstract of the Disclosure A semiconductor laser device comprises a semiconductor body of a first conductivity type having a major surface and a plurality of contiguous semiconductor layers disposed on this major surface. These contiguous semiconductor layers are comprised of:
i) a first semiconductor layer having a laser active region and having two opposite plane surfaces, ii) a second semiconductor layer disposed on the first of the two opposite plane surfaces, consisting of a material having a band gap broader than that of the first semiconductor layer, and consisting of a thin region having a thickness t1 not greater than 3r, where r is the distance in which the evanescent wave decays by 1/e, and a thick region having a thickness t2 larger than 3r, and iii) a third semiconductor layer disposed on the second surface of the two opposite plane surfaces and con-sisting of a material having a band gap broader than that of the first semiconductor layer.
The device operates stably in the lowest transverse mode and provides light-output versus current characteristics free of kinks or other anomalies.

Description

This invention relates to a double-hetero structure semiconductor diode laser device. More particularly, the invention relates to the structure of a semiconductor laser device having a stabilized lasing mode and high reliability.
Semiconductor laser devices are now indispensable elements as light sources in optical communication systems.
As is well known in the art, a double hetero ( DH ) structure is adopted for semiconductor devices that operate effectively at room temperature.
Before attempting to discuss either the prior art or the present invention the accompanying drawings will be listed~
Fig. l-a is a view showing a section o~ a conventional semiconductor laser device of the double hetero structure taken along a direction parallel to light propagation.
Fig. l-b is a view showing the section of the semi-con~uctor laser device of Fig. l-a taken along a direction perpendicular to light propagation.
Fig. 2 is a sectional view of a known buried hetero structure injection laser.
Fig. 3 is a sectional view showing the basic structure of one form of semiconductor laser device of the present inven-tion.
Fig. 4 is a view showing diagrammatically a wave-guide structure in the semiconductor device of Fig~ 3.
Figure 5 is a view showing guided wave profiles in the interior of the semiconductor device of Fig. 3.
Fi~. 6 illustrates typical light output versus current characteristics in a semiconductor laser de~ice o~
Embodiment 1 of the invention.
Fig. 7-a shows the far field intensity profile in the ., .~
i~

x-z face of the semiconductor laser device o Embodiment 1.
Fig. 7-b shows the far field intensity profile in the x z face of the conventional DH laser device shown in Figs. l-a and l-b.
Fig, 8 shows lasing characteristics observed when the thickness d of an active layer and the thickness tl of a thinner portion of a second semiconductor layer are changed in the semiconductor laser device of Embodiment 1.
Fig. 9 shows the relationship between a maximum thickness tM of a smaller thickness xegion of the second semiconductor layer and a threshold current density change Jth/Jtho to the refractive index of the third semiconductor layer in a semiconductor laser device of Embodiment 5 of the invention.
Fig. 10 shows a similar relationship in a semicon-ductor laser device of Embodiment 6~
Fig. 11 shows light output versus current charac-teristics of a semiconductor laser device of Embodiment 9.
Fig. 12 is a sectional view showing an embodiment of the present invention that includes a buffer layer.
Fig. 13 is a sectional view showing another embodiment of the present invention in which a channel is formed in the interior of a buffer layer~
Fig. 14 is a sectional view showing still another embodiment of the present invention wherein a channel piercing a buffer layer and reaching the substrate is formed.
Fig. 15 is a sectional view showing one embodiment of an InP-GaInAsP semiconductor series having the structure of the present invention.
Fig. 16 is a diagram illustrating a structure realizing a single mode lasing operation in a semiconductor device of the present invention.
Fig. 17 is a view illustrating the relationship between the threshold gain Gth and the thickness tl of a small thickness region of the second semiconductor layer when the thickness d of the active layer is 0.1 ~m, using the channel width W as a parameter.
Fig. 18 is a view showing the relationship between the change of the ratio of the threshold gain F'th of the first order transverse mode operation to the threshold gain Gth of the lowest transverse mode operation to tl and W~ when the thickness d of the active layer is 0.1 ~m.
Fig. 19 is a sectional view illustrating an example of an element realizing a single mode lasing operation in a semiconductor laser device of the present invention.
Fig. 20 shows the near field intensity profile of a semiconductor laser device constituting Embodiment 19.
Fig. 21 shows the results of evaluation of the stability of the lasing mode of a semiconductor device of the present invention in which d is 0.1 ~m, tl is in the range of from 0.1 to 0.8 ~m and W is in the range of from 1 to 10 ~m.
Fig. 22 is a sectional view showing an example of an element in which a thick portion and a thin portion are formed on the third semiconductor layer.
Fig. 23 is a sectional view illustrating a semi-conductor laser device in which the refractive index is changed in the transverse direction of the active region by curving the active layer.
Fig. 24 is a view illustrating the light waveguide structure of the semiconductor laser device shown in Fig. 23.
Fig. 25 is a sectional view showing an embodiment of semiconductor laser device in which optical confinement is attained in the transverse direction of the active region by curving the active layer.
Fig. 26 shows light output versus current character-istics o~ the semiconductor laser device of Embodiment 21.
Fig. 27-a shows ~he far field intensity profile in the x-z face of the semiconductor laser device of Embodiment 21.
Fig. 27-b shows the far field intensity profile of the semiconductor laser device shown in Figs. l-a and l-b.
Fig. 28 shows light outputs obtained when the semiconductor laser device shown in Fig. 25 and the semicon-ductor laser device shown in Figs. l-a and l-b are excited under pulse bias.
Fig. 29 is a diagram illustrating the concept of a semiconductor laser device in which the width of a stripe channel or protrusion is changed.
Fig. 30 shows the far field intensity profile in the x-z face of the semiconductor laser device shown in Fig.
29.
Fig. 31 is a diagram illustrating the concept of a semiconductor laser device in which a stripe channel or protrusion is curved.
Fig. 32 is a sectional view showing a modification of the semiconductor laser device shown in Fig. 25 in which the thickness of the active layer is changed.
Fig. 33a shows the near field intensity profile in the direction x in the light waveguide of the semiconductor laser device shown in Fig. 32.
Fig. 33b shows the near field intensity profile in the direction x in the light waveguide in the BH laser device shown in Fig. ~.
Fig. 34 is a sectional view showing an embodiment in which a plurality of semiconductor laser devices shown in Fig. 25 are connected to one another.
Fig. 35-a illustrates lasing spectra of semiconduc-tor laser devices of ~mbodiments 21 to 2~.
Fig. 35-b illustrates the lasing spectrum of the composite semiconductor laser device shown in Fig. 34.
Fig. 36 is a sectional view of an embodiment of the semiconductor laser device of the InP-GaInAsP series having a curved active layer.
A typical structure of the most advanced known GaAs-GaAQAs double-hetero structure semiconductor laser device is illustrated in Figs. l-a and l-b. Fig. la is a view showing the section of the element taken along a plane including the direction z parallel to light propagation and Fig. l-b is a view showing the section of the element of Fig. l-a taken along a direction perpendicular to light propagationO Such element is ordinarily prepared by growing on a n-GaAs substrate 1 a layer 2 of n-Gal xAQxAs (for examplel x is 0.3) corresponding to a second semiconductor, a layer 3 of p-GaAs corresponding to a first semiconductor, a layer 4 of p-Gal xAQxAs (for example, x is about 0.3) corresponding to a third semiconductor and a layer 5 of p-GaAs facilitating electric connection to a positive electrode 6, by successive liquid phase epitaxy, then forming électrodes and cleaving crystals to form a reflection face 8. Reference numeral 7 represents a negative electrode.
When this laser device is driven at 2~2 V and 100 mA (2 KA/cm2) ` at room temperature, a continuous light output power of ~8900 ~ and ~10 mW under CW conditions is obtained. The laser device having the above structure ca~ easily be prepared in a high production yield and it has a long life and a high reliability. However, a laser device of this type is defective in that the lasing mode becomes unstable. More specifically, in the above structure, the region of the first semiconductor layer 3 below the positive electrode 6 has a lasing action, but since the reEractive index is not intentionally changed in the direction x in this layer, the lasing mode in the direction x is determined by a slight refractive index profile and gain profile generated by application of an electric current. Such refractive index profile or gain profile is remarkably changed depending on changes in the excitation current or temperature and on configurations of the element such as the layer thickness. Accordingly, in general, the lasing mode shows very irregular changes and has no reproduci-bility. This instability of the transverse mode has bad influences on the linearity of the excitation current versus light output power. When modulation is conducted under pulse bias , unstable variations are caused in the light output power and the signal-to-noise ratio is degraded and the directivity of the output light is rendered unstable, and thexefore, it becomes difficult to introduce the light output power at a high efficiency stably to other optical systems such as light fibers. Thus, practical use of such laser device involves various problems.
Some attempts have heretofore been made to eliminate the foregoing disadvantages. For example, a so-called BH (buried hetero structure) laser device has been developed [T. Tsudaka, J. Appl. Phys., 45, 4897 (lJ75)]. The section of this laser device is illustrated in Fig. 2. Referring now to Fig. 2, a p-GaAs layer 3 which is a region having a lasing action is surrounded by a n-Gal xAQxAs layer 2, a p-Gal xAQxAs ~.~

y and a n Ga1_xAQxAs layer 9, each layer 2, 3 and 4 having a lower refractive index than that of the layer 9. In this arrangement, a definite change of the refractive index is present in the direction x. Accordingly, the transverse mode is stabilized, and characteristic difficulties involved in the element structure shown in Figs. l-a and l~b are eliminated.
In order to realize the element structure shown in Fig. 2, a n-Gal AQx~s layer 2, a p-GaAs layer 3 and a p-Gal xAQxAs layer 4 are grown on a n-GaAs substrate 1 by successive liquid phase epitaxy, and then the structure is processed into a mesa~
stripe form and a n-G2ll xAQxAs layer 9 is grown by liquid phase epitaxy. Accordingly, the production steps are compli-cated and the production yield is very low. In addition to these defects, there is another defect that during the produc-tion process, especially at the regrowth step, crystal defects are readily caused and have bad influences on factors concer-ning the practical utility, such às life and reliability.
A spot-like laser device capable of lasing in the single transverse mode is applicable as a light source for single mode fiber communication or a light source for light information processing devices such as a vid~o disk, and development of a laser device meeting requirements in these application fields is desired in the art. In these application fields, it generally is required that the width of spots of the light output power should be about 1 to about 8 ~m.
As the laser device in which single transverse mode operation is obtained in the junction face of the laser, there can be mentioned a buried hetero structure laser device as illustrated in FigO 2 and a transverse-junction stripe laser device (IEEE Journal of Quantum Electronics, Vol. QE-11, No. 7, July. 1975). The former device is defective in that the spot width is limited to about 1 ~Im or less and growth oE crys-tals must be conducted 2 times. The l~tte~ device is deEective in that zine must be diffused deeply even into the lasincJ active rec3ion after ~rowth of crystals arld a complete reliability cannot be attained.
I~ is a primary object of the presenk invention to provide ~ s-tructure of a semiconductor laser device that is free of the abovementioned defects of the double hetero structure, has a stabilized lasing mode and can easily be prepared.
Another object of the present invention is to provide a semiconductor laser device which has a low threshold current density enabling stabilization o~ the lasing mode.
Still another object of the present invention is to provide a structure o~a sellliconcluctol laSCI ~evicc havillg high reliability and a long life, and in which the lasing mode is stabilized.
A further object of the present invention is to provide a structure realizing a semiconductor laser in which a single fundamental operation is possible with a stabilized lasing mode.

In the present invention, the foreyoing objects can be attainecl by a se~icon~luctor laser device comprisillg a semi-conductor body o-f a Lirst conductivity type having a major surface and a plurality of contiguous semiconduct.or layers disposed on said major surface of said body, said plurality of contiguous semiconductor layers beillg comprised of: (i) a first semiconductor layer having a laser active region and having two opposite planc surfaces, (ii) a second semiconductor layer disposed on the .Eirst surface of said two opposite plane sur.Eaces, consisting of a material having a band gap broader than that of said first semiconductor layer, and consisting of a thin region having a thickness ~I.not greater thc~ll 3r, where B

;t;~ C i ll ~ l tllc (,`V~lllC'~`llt ~ ` (IC~IY; 1)~ l/c, .In~ .I thick rcgion having a thickllcss t2 lar~cr ~h~lll 3r, ~nd (i;i) .I tili~ emicon~lucto~ yer ~lispos~l on thc ~ccond surE;Ice oL said two oppositc plane sur~acc~ d con~isting Or ., ~ t~r~ c,ving ., balld g;lp bro~lcler th~lll th.lt o~ sai~l r i l`S t ~;~111 i COII~ C t OI` I .I~'(~I` .
The structure of a first type of semieonduetor laser aceording -to the present invention will first be deseribed.
Seeond and third semieonduetor layers, each consisting of a semiconduetor having a band gap broader than that of a first semiconductor and a refractive index smaller than that of the the Eirst semieonduetor, are bonded to both sides of a first layer eonsisting of the first semieonductor. In a-t least one of said second and third semiconductor layers, a thickness difference is made in at least a boundary between the lasing region and another region, and a semieonductor layer having a complex refraetive index different from said semiconductor layer is disposed along the thiekness-reduced region of said semiconduetor layer.
The semieonduetor layer that varies in thiekness (at least one of said second and third layers) is arranged so that the lasing region corresponds to a region of larger thiekness and the remaining reglon corresponds to a region of smaller thickness. By this disposition, good results are attained with res~ect to the optical gain. The reasons will be apparent from the deseription given hereinafter. Needless to say, even if the reverse arrangement is adopted, namely even if the thiekness is changed so that the lasing region corresponds to a region of smaller thiekness and the non-lasing region eorresponds to a region of larger thickness, an effect of confining lased light within the prescribed region ean similarly be attained.
According to one modification of semiconductor ~B

c~

laser device of the present invention, a fourth semiconductor layer is formed on the semiconductor substrate and a stripe channel is formed on the fourth semiconductor layer, irst to third semiconductor layers being disposed on said fourth semiconductor layer. When an electric conductivity type opposite to that of the substrate is given to the fourth s~miconductor layer, an electric current can be allowed to flow selectively in the portion of the channel. Further, ~hen the same conductivity type as that of the substrate is given to the fourth semiconductor layer and the speci~ic resistivity of the fourth semiconductor layer is higher than that of the semiconductors in the channel, spreading of the electric current can be reduced. Still further, this fourth semiconductor layer acts as a so-called buffer layer and defects are much reduced. Accordingly, a laser device having a long life can be obtained in a high production yield.
According to another modification of semiconductor laser device of the present invention, the width of the large thickness region of the second or third semiconductor layer is ad~usted in the range of from 2 ~m to 8 ~m and the loss of optical gain in the optical waveguide on the large thickness region is smaller by at least 40 cm 1 than the loss in the optical wa~eguide in the adjacent small thickness region, whereby a semiconductor laser device performing the lasing operation in the stabili~ed lowest transverse mode can be provided. The laser device of this modification fully meets the requirement of a semiconductor laser device capable of a single mode lasing operation.
According to still another modification of semiconductor laser device of the present invention, a stripe portion is formed on the second or third semiconductor layer ~?~Q~

on the side of the first semiconductor layer, and the first semiconductor layer is made protuberant along said portion, whereby the refractive index is changed with respect to the transverse direction of the laser active region of the first semiconductor layer.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
The basic principle of the present invention is now illustrated by reference to Fig. 3 which is a sectional view showing the main parts of a typical embodiment of the present invention.
Reference numeral 11 represents a substrate crystal having a stripe channel formed thereon, and layers described below are grown in succession on this substrate 11. A second semiconductor layer 12 has a portion projected toward the substrate 11. Reference numerals 13 and 14 represent a first semiconductor layer and a third semiconductor layer, respec-tively. The first semiconductor layer performing the lasing operation is sandwiched between the second and third semi-conductor layers, each consisting of a semiconductor having a band gap broader than that of the semiconductor of the first semiconductor layer and a~refractive index~ maller than that of the semiconductor of the first semiconductor layer. Thus, there is established a so-called double hetero structure in which the carriers and photons are confined at high density in the first semiconductor layer.
In this embodiment, the substrate crystal 11 constitutes a region absorbing waves evanescent from the thickness-reduced region of the second semiconductor layer 12, namely a region in which the imaginary portion of the complex refractive index is changed. This embodiment will now be described in detail by reference to the case where G~As and Ga~ As are used as semiconductor layers.
In general, the substrate consists of GaAs or Gal zAQzAs, the first semiconductor layer consists of Gal yAQyAs and each of the second and third semiconductor layers consists of Gal xAQxAs, in which a relation of O_z_y<x<l is ordinarily established. The thickness tl of the thickness-reduced portion of the second semiconductor layer 12 is so small that the tail of the evanescent wave along the first semiconductor layer reaches the substrate 11. The thickness t2 of the large thickness portion of the second semiconductor layer is sufficiently larger than the thickness tl .
This structure performs a function equivalent to the function of the waveguide in which either or both of the effective refractive index and the optical gain or loss are changed in the direction x of the waveguide layer. This waveguide structure is diagxammatically shown in Fig. 4.
As pointed out, the refractive index is changed in the direction y by the material per se constituting the semiconductor layers 12 and 14. In addition, also with respect to the direction x, regions 131 and 132 differing in ,their effective composite refractive index can be set. In regions 131 and 132 of the semiconductor layer 13, either or both of the effective refractive index and the optical loss or gain are made different from those of the semiconductor layer 13 by the penetrating effect of the evanescent wave into the substrate crystal 11. When the refractive index of the substrate crystal 11 is higher than that of the semi-conductor layer 12, the effective refractive index of each of the regions 131 and 132 is higher than that of the semiconductor layer 13. In the contrary case, the effective ~ 6~
~efractive index of each of the regions 131 and 132 is lower than that of the semi~conductor layer 13. The same ~olds yood with respect to the optical gain or loss.
In a practical device, suita~le electrodes are disposed on the sides o~ the substrate and the third semi-condustor layer 14. Reference numerals 15, 16, 17 a~d 18 rapresent portions constituting electrodes as illustrated in embodiments given hereinafter. In this s~ructure, guided light is distri~uted with the region 130 of the first semi-conductor layer 13 being the center, and is propagated in a direction Z perpendicular to the drawing surface. Character-istics of the semiconductor laser device of the present embodiment are strongly influenced by the thickness of the second semiconductor layer 12. As pointed out hereinbefore, the thickness of the thickness reduced portion of the semi~
conductor layer 12 is adjusted so that the tail of the evanescent wave reaches the substrate. Suppose that the depth in which the evanescent wave~from the semiconductor layer 12 in the direction y decays by l/e is defined as r, it can approximately by represented as follows:

r = ~ [ 1 ~ ( d 1 ~ + A' )2 wherein A - 2 2

2(nl3 - nl2 ~ ~ , A' = 2(nl3 ~ nl4 ~

where ~ represents a free space wave length, nl2 , n13 and n14 represent refractive indexes of the semiconductor layers 12, 13 and 14, respectivelyr and d denotes the ~hickness o the semiconductor ~13-i ~ J~b~
~yer 13.
Incidentally, the method for calculation of an approximate value of the penetrating depth is described in the thesis of E. A. J. Marchatili reported on Bell Syst.
Tech. J., 48, pp. 2071-2102 (1969).
When the above equation is -taken into consideration, it will be apparent that the evanescent wave does not sub-stantially penetrate over a depth exceeding 3r. In the present structure, it is apparent that, as the thickness tl is smaller, the transverse mode operation is more stabilized.
Referring to the a~ove equation, it may be said that, if the thickness tl of the thickness reduced portion of the semi-conductor layer 12 is set so that the condi-tion of tl < 3r is satisfied, good results will be obtained.
In the foregoing instance where GaAs and GaAQAs are used as semiconductor layers, d is adjusted in the range of about 0.05 to about 0.2 ~m in the actual operation. In this case, r is about 0.3 ~m. From a practical viewpoint, it is preferred that tl be about 0.3 to 0.7 ~m ( this holds good when the refractive inde~ of the second semiconductor layer is the same as that of the third semiconductor layer; the asymmetrical case where both the refractive indices are different will ~e described hereinafter ) and t2 be sufficiantly larger than the penetrating depth 3r, more specifically at least about 0. 9 ~m, for e~ample, about 1.5 ~m. In the case where a channel is formed on the substrate, from the practical viewpoint, it is preferred that the width W of the channel be in the range of about 1 to about 30 ~m. If the width W is ~roader than this range, it is difficult to form the top face of the protuberant portion of the second semiconductor layer as a plane face. A semiconductor laser de~ice having the ~?,~
~ ove-mentioned structure in which a channel is formed on a substrate crystal has the following advantages:
(1~ Since the structure can be formed hy one successive liquid phase epitaxy, produc-tion can be greatly facilitated.
(2) During the preparation process, mesa etching need not be performed for formation of semiconductor layers 12, 13 and 14.

(3) Since a planar stripe geometry is attained, the result-ing structure is excellent in the reliability and thermal diffusion characteristics.
In the foregoing arrangement, the second semi-conductor layer is formed to comprise a sufficiently thick protuberant portion corresponding to the lasing region and a thin portion corresponding to the other region.
The change of the effective loss coefficient of the waveguide in the direction x in the semiconductor laser device having the above structure will now be described.
In the portion of the second semiconductor layer 12 other than the protuberant portion, the tail of the evanescent wave reaches the crystal substrate 11 and a part of the wave is absorbed in this portion. On the other hand~
in the protuberant portion, such absorption does ndt take place. Accordingly, the effective loss coefficient of the waveguide in the other portion is sufficiently higher than that in the protuberant portion. Light waves are guided by this difference of the effective loss coefficient in the transverse direction parallel to the junction face, and the lasing operation is conducted stably in the transvexse mode with the region 130 of the first semiconductor layer 13 present above the protuberant portion of the second semiconductor layer 12 being the center. This feature is based on the waveguide effect due to the difference of ~he effec~ive loss coefficient. In general, this object can be attained by generating a difference of the/composite refractive index.
As will be apparent from the foregoing illustration, the above special effect is not due to the use of specific constituent materials. In other words, the present structure can be applied not only to lasers of materials of the Ga-A~-As and Ga-AQ-As-Sb series but also to lasers of materials of the Ga-AQ-As-P, Ga-As-P and In-Ga-As series, semiconductor compounds of elements of the groups III-V and II-V and other semiconductor materials.
As pointed out hereinbefore, the ob~ects of the present invention can be advantageously attained if the thickness tl of the semiconductor layer 12 is set so that the tail of the evanescent wave penetrates in the substrate.
In practice, the objects of the present invention can be attained more advantageously if the following laminate structure is adopted when the thickness tl is set.
The refractive indices of the second and third semiconductor layers sandwiching the first semiconductor layer therebetween are set in the following manner.
Namely, the refractive index of the semiconductor-layer having its thickness varied stepwise is made larger than the refractive index of the semiconductor layer disposed on the opposite side of the first semiconductor layer which is interposed therebetween. Referring now to Fig. 3, for example, the refractive index nl2 of the semiconductor layer 12 is made larger than the refractive index nl4 of the semi-conductor layer 14 (namely, the relation of nl2 > nl4 is established).
The guided wave profile attained in this case is illustrated in Fig. 5. The abscissa indicates the thic~ness of the crystal and the ordinate indicates the intensity of ~.~Q~
~ guided wave. Reference numex~ls 11, 12, 13 and 14 in Fig. 5 indicate regions of the semiconductor layers represented by the same reference numerals Curve 51 shows the guided wave profile observed when the refractive index of the semi-conduc~or layer 12 is the same as that of the semiconductor layer 14 (n 2= nl4 ) and curve 52 shows the guided wave profil~
observed when the relation of nl2>nl4 is established between the refractive indices of both the layers. In the case of nl2 = nl4, the guided wave intensity profile is symmetrical, with the fixst semiconductor layer 13 being the center. As pointed out hereinbefore, the thickness tl of the thin portion of the semiconductor layer 12 satis~ies the requirement of being < 3r. In contrast, in the case of nl2 ~ nl4, as is seen from the curve 52, the guided wave intensity profile is asymetrical, deviating toward the substrate crystal 11.
Accordingly, even if a value tll ( larger than to) were selected for the thickness of the thin portion of the semi-conductor layer 12, an effect equivalent to the efect in the case of nl2 = nl4 can be substantially obtained. Of course, also in this case, the requirement of tll _ 3r should be satisfied. ~y, virtue of such arrangement, the following advantages can be attained. I
A relatively large thickness may be selected for the second semiconductor layer (especially for the thin region o~ the layer 12 in Fig. 33 and hence, control of the thickness of the semiconductor layer 12 can be greatly facilitated.
~oreover, although interfacial defec~s are present on the inter-face between the substrate crystal 11 and the semiconductor layer 12, because it is a first crystal growth interface, since the thickness of the semiconductor layer 12 can be increased, the laser active layer (namely, the first semi-conductor layer~ is not influenced by such interfacial ~ ?,~3~
fects and the reliability of its characteristics can beenhanced. The degree oE increase of the thickness of the semi-conductor layer 12 is attainable by making the refractive index of the semiconductive layer 12 larger than that of the semi-conductor layer 14 can be approximately known from the following formulae.
Suppose that the refractive index of the first semiconductor layer 13 is n~3, the thickness of the layer 13 is d, the refractive index of the layer 12 is nl2, the ratio of the light energy present in the substrate crystal 11 to the total energy in the light distribution with respect to the direction y along the line a~a' in Fig. 3 is F, and the lasing light wave len~th is ~, the following relations are established k = ~ (2) and 2 2 2 2 G = k (nl3 nl2 ) then the ratio of tllto tl is expressed as follows.
tll = 2m2 . n m+l nF J 1 + d2G2 tl Qn 2 QnF ~ (m+l)~ +4m~dZG~-(m+l) (4) In the above formula (4), m is a parameter indi-cating the degree of asymmetry of the light distribution; and the refractive index nl4 of the semiconduct~r layer 14 and the refractive index nl2 of the semiconductor layer 12 have a relation represented by the following formula:

1?,(~

nl2 - nl4 =~ m2 ~21 {J (m~1~2 ~ 4d2G2m2 ~ (m~l)} (5) In the case of the symmetrical distribution (nl4 =
nl2), m is equal to 1, and in the case of the asymmetrical distribution, m is la.rger than 1.
From the above formulae (4) and (5), it will readily be understood t~at i~ the refractive index nl2 of the semiconductor layer 12 is made larger than the refractive index nl4 of the semiconductor layer 14 according to the present invention, m becomes large and tll/tl increases sharplyO /.~
The formula (4) i~dicates that the larger the value of m (the degree of asymmetry),the larger the value of tllitl. However, if the degree of asymetry is too high, the ratio of the lasing light present in the semiconductor layer 13 is reduced and therefore, the current dens.ity necessary for causing lasing operation (hereinafter referred to merely as "current.density") is increased. If the degree of asymmetry is m , the threshold current density Jth is expressed by the following formula:

Jth ¦ m(m~ l+d2~ - 13 (6) ~mll)2 + 4dZG~m~ - (m+l) wherein Jtho represents the threshold current density when m is equal to 1 (namely, the distribution is symmetrical and nl2 is equal to nl4).
As will be apparent from the formula t6), the degree of i.ncrease of Jth/Jtho attained by increasing m is much lower than the degree of increase of tll/tl. Therefore, according to the present structure, an effec~ive device can be designed, because a higher value of tll/tl can be obtained, while the increas~ of Jth is maintained at a low level.

The refractiYe indices of the above~mentioned semiconductor layers can be controlled by changing the compositions of the material~ constituting the respective layers. For examplel in the case of a material of the Gal_x AQx~s series, the refractive index can be lowered by increasing the composition ratio x of AQ. Furtherl in the case of a material of the AQyGal yAsxSbl x series, the refractive index can be lowered by increasing either or both of x and y.
Examples of the present invention will now be des-cribed in detail by reference to the fallowing Embodiments.
EMBODIM~NT 1 The section of a semiconductor laser device in Embodiment 1 of the present invention is as shown in Fig.
3. Reference numeral 13 represents a layer oE n-GaO 95 AQo 05As corresponding to a first semiconductor, and reference numeral 14 represents a layer of p-Gal xAQxAs ( x~ 0.3 ) corresponding to a third semiconductor. Reference numeral ll represents a n-GaAs substrate having a channel, reference numeral 12 represents a layer of n-Gal xAQxAs ( x ~ 0.3 ), reference numeral 15 represents a n-Ga~s layer, reference numeral 16 represents a positive electrode, reference numeral 17 represents a negative electrode, and reference numeral 1~ represents a Zn-diffused region. This semiconductor laser device is prepared in the following manner.
A photoresist film having a window having a width of 10 ~m is formed by customary photolithography on a n-GaAs substrate crystal having a face index (100) (Te is daped at a concentration of ~1 x 1013 cm 3). The surface of the sub-strate is chemically etched at 20C. through this windowwith a 1:1:3 mixture of phosphoric acid: hydrogen peroxide:
ethylene glycol to form a channel extending in the direction ~20-The width (W) of the channel is about 10 ~m and the depth~t4) is 105 ~m. Then, l~yers 12, ]3, 14 and 15 are formed on this substrate by successive liquid phase epitaxy. Compositions of solutions are used for formation of the respective semi-conductor layers and the growing times are as shown in Table 1.

Composition of Solution Layer 12Layer 13Layer 14 Layer 15 Ga (g) GaAs (mg)300 200 200 200 AQ (mg) 3 0.2 3 Sn (mg) 300 - ~ 100 Ge (mg~ - - 70 Growing Time (minutes) 2 1/30 8 The saturated solution for the layer 12 is cooled from 780C at a rate of about 0.4C/min, super-cooling is conducted for 3 minutes and the crystal is grown by successive liquid phase epitaxy. The thickness (tl) of the thin portion of the layer 12 is adjusted to 0.3 ~m. The thicknesses of the layers 13, 14 and 15 are O.l~m, 2 ~m and 1 ~m, respectively.
As an impurity additi~e, Sn is used ~or the n-type layer and Ge is used for the p-type layer. Then, Zn is diffused through a window of AQ2O3 formed by photolithography in the same manner as described above, to thereby form a p-type diffusion layer 18. Au and Cr and an Au-Ge~Ni alloy are vacuum-deposited to form positive electrode 16 and negative electrode 17.
Finally, the crystal is cleaved on a plane of a face index ~110) 50 that a confronting paralleI face is formed, whereby a reflector is formed. Thus, a laser deuice having a laser length of 300 ~m is prepared.

When epitaxial growth is effected on the top face o~ a substrate having a channel as in this Embodiment, if the top ~ace of the semiconductor layer 12 is made substantially plane, ~he ef~e~ts can be attained more advantageously.
More speci~ically, if the top face of the semiconductor layer 12 is made substantially plane, the thick and thin regions o the semiconductor layer 12 can be conveniently formed, and the active layer and the like can also be formed conveniently. This feature is attained if the saturated solution is cooled at a temperature reduction rate of up to 0.5C./min at the epitaxial growth step. From the practical viewpointt it is preferred that the temperature be lowered at a rate of about 0.01 to about 0.5C./min.
The above semiconductor laser device can perform a lasing operation at a threshold current density of ~KA/cm2 at room temperature~ The lasing wave leng~h is_ 8300 ~ and the external dif~erential quantum efficiency is about 40%.
Fig. 6 illustrates the relation between the exciting current and the light output power. Curve 61 shows results obtained with respect to the laser device of this Embodiment. Curve 62 shows results obtained with respect to the conventional structure shown in Figs. l-a and l-b. In the laser device of the present Embodiment, the transverse lasing mode is stable at an exciting current 2 times the threshold current~
which is practical for the operation of the laser devi~e, and non-linear characteristics of the output intensity and light output, such as curvatures called "kinks", and reduction of the signal-to-noise ratio at the pulse modulatibnl which are observed in the conventional structure, were not obsreved at all. Figs. 7-a and 7-b show the light ou~put profiles in the x~z plane o~ the above Embodiment and the above convention-al structure, respectively. Respective curves in Figs. 7-a ~nd 7-b show results obtained at various exciting currents.
More specifically, curves 71 and 74 show results a-t 1.3 Ith, curves 72 and 75 show results obtained at 1.2 Ith and curves 73 and 76 show results obtained at 1.07 Ith. Jth stands for the threshold current value ~milli-ampere unit).
From these results it is seen that, in the con-ventional structure the light output profile is irregularly changed, depending on the change of the exciting current, but when the str~cture of the present ~mbodiment is adopted, the output profile can be substantially stabilized. In connection with the lie of the element, there was observed no substantial difference between the structure of the present Embodiment and the conventional structure.
In the semiconductor device of this Embodiment, the lasing operation characteristics were examined while changing the thickness d of the ~irst semiconductor layer and the thickness tl of the thin portion of the second semi-conductor layer. Results are shown in Fig. 8. Symbols O,A, and X in Fig. 8 indicate the following characteristics:
O: A linear relation is established between the exciting current and the light output power at up to about 30 mW, and noise is very little~ Accord-ingly, it can be judged that the characteristics are excellent.
Q: A linear relation is established between the exciting current and the light output power at up to about 30 mW but generation of noise is relatively apparent ( the output of noise is a~out 20 to about 30~ of the light output~. It can be judged that the characteristics are good.
X: The relation between the exciting current and the i light output power is not linear and ~eneration of noise is conspicuaus. It can be judged that the characteristics- are bad.
In Fig. ~, curves 81, 82 and 83 illustrate the penetration depths r, 2r and 3r, respectively, as functions of the thickness of the first semiconductor layer.
From results shown in Fig. 8, it will clearly be understood that, when the penetration depth r and the thickness tl of the thin portion of the second semiconductor layer are arranged so that the requirement of tl _ 3r is satisfied, good results can be obtained, but it is preferred that the condition of tl < ~r be satisfied.
EMBODIMENTS 2 to 4 Tests were conducted in the same manner as des-cribed in Embodiment 1 while changing materials and dimensions of the respective layers 12, 13 and 14 as indicated in Table 2. Substantially the same effects were similarly attained.
Conditions other than those shown in Table 2 are the same as in Embadiment 1.

Embodi- Layer 12 Layer 13 Layer 14 Channel Thickness Thickness ment No. n~Gal_x n-Gal y p-Gal_z depth ~) of (~m) of AQxAsAQ As AQzAs (~m) thin por- layer 13 Y tion of Layer 12 2 ~=0.33y-0.05 z=0.36 1.5 0.45 0.1 3 x=0.33y=0.05 z=0.33 1.5 0.7 0.05

4 x=0.33y=0.05 z=0.40 1.5 0.2 0.25 In each of the semiconductor laser devides of Embodiments 2 to 4~ the threshold current density at room temperature is 2 KA/cm2, the lasing wave length is 8300 A
and the external differential quantum efficiency is about 40%.

~nly the fundamental transverse lasing mode is observed when the exciting current is up to 1.3 times the threshold current value. The relation between the exciting current above the threshold current value and the light output power is linear if the exciting current is in the above range.
Reduction oE the signal-to-noise ratio was not observed with pulse modulation.

Embodiments in which the refractive indices of the second and third semiconductor layers sandwiching the first semiconductor layer therebetween are asymmetrical will now be described.
The fundamental structure of each of these Embodiments is as shown in Fig. 3, and specific structures of respective layers are shown in Table 3.

!

Table 3 Embodi-Substrate Layer 12 Channel Channel Thickness ~m) Layer 13 Layer 1~! Layer 15 ment No.11 Depth Width of Thin Region ~-~m) (~m~ f Layer 12 n-GaAs n-GaO.67 1-5 10 (relation bet- undoped, Gal x ~aAs, AQ As ~een thickness GaO 95 A~ A 1 ~m 0-33 and character- A~ A x 5~ thlck istics is shown o.o5 s~ 2 ~m ln Tables ~l 0.1 ~m thicX
and 5) thick 6 n~GaAs n-GaO 7 1.5 10 ditto undoped, Srelation GaAs, A~ As GaO.95 between 1 ~m O. 3 A e As thickness ~hick O.05 ' and chara-0.1 ~m cteristics thick is shown in Tables and 5) 7 n-GaAs n-GaO 5 1.5 10 0.7 undoped, p-GaO 4 GaAs, Ae o . 5As - A~0 05A5' 0 6 thick O . 1 ~an thick 8 Te doped, Te doped, 1.5 10 1 undoped, Ge dope~ Ge doped, _Ga n-A~Q ~, GaAsO.88 P-A~o.5 GaAsO 8 s0.~8 GaO.6 SbO.12 GaO.5 SbO.12 SbO.12 A50.88 ASO.88 Sb0.12 Sbo.12 ~?,~
In each of the Embodiments 5 to 8, the layer 12 is Sn-doped and the carrier concentration is ~1018 cm 3, the layer 14 is Ge-doped and the carrier concentration is ~1018 cm 3 and the carrier concentration in the layer 13 is ~1016 cm 3. The region 18 is a p-type region formed by diffusing Zn in the semiconductor layers 14 and 15 according to a known method. The width of the region 18 in the direction x is 10 ~m, and the dif~usion depth in the direction y is about 1.5 ~m as measured from the boundary between 16 and 18.
Reference numerals 16 and 17 represent electrodes composed of, for example, Cr and Au and an Au-Ge-Ni alloy. These laser devices are prepared by a method substantiallY the same as the method described in Embodiment 1. In order to attain intended compositions in respective layers, amounts of the respective solutions charged and the crystal growing times are appropriately controlled.
In structures of Embddiments 5 and 6, the refractive index, namely the mole fraction of AQ, in Gal xAQxAs con-stituting the semiconductor layer 14, is changed to prepare various semiconductor laser devices. The maximum thickness tM of the thin region of the semiconductor layer 12 that allows the lasing operation in the stable transvers~ mode at a current density of up to 2 times the threshold current densi~y in Embodiment 5 or 6 and the threshold current density Jth at that time, are shown in Tables 4 and 5. The lasing wave length is 0.83 ~m.

~27-Table t, ( Embodlment 5 ) A.C Mole Refractive ~ m) Jth(KA/cm ) Remarks Fractlon, x Index of Layer 1/l 0-33 ~.39~ 0./l ~.0 + 1.0 sy~netrical structure o.~a ~.~G2 0.6 2.2 ~ 1.0 o.t,l ~.31,3 0-~ 2.3 ~ l.o o.~l~. 3.323 1.1 2.6 ~ l.o 0.~,6 3-31/' 1.5 ~.~ + l.o Table 5 ( Embodlrnent G ) Ae l~lole Refractive ~(,um) Jth(KA/cm2) hemarks Frac ti on, x Ind ~x o f Layer 14 o.30 3 .~ o.5 2 .0 ~ 1.0 ~;ymmetrical structure 0.37 3.370 o.~ 2.2 ~ 1.0 O.l,o 3.35~ 1.2 2.5 + l.o O.lll 3.3ll3 1.6 207 + 1.0 From the results shown in Tables 4 and 5, it will readily be understood that, if the refractive index of the semiconductor layer 12 is made larger than that of th~ semi-conductor layer 14, it is possi~le to increase the thickness of the semiconductor layer 12. This means that growth of crystals can be greatly facilitated. Further, good results can be obtained with respect to the reliability. More spec ifically, using a thermally accelerated aging test conducted at 70C., the operation can be performed continuously for several hundred hours ( 40,000 hours calculated at room temperature).
The above maximum time tM and the change Jth~Jtho :~0 (Jtho means the threshold current density attained when the refractive index of the second semiconductor layer is the ~ ?~
lme as that of the third semiconductor layer~ of the threshold current densitv in Embodiments 5 and 6 is shown in Figs. 9 and 10 (each of 91 and 101 represents Jth/Jtho and each of 92 and 102 shows tM)~ In each of ~igs. 9 and 10, these two factors are shown as functions of the refractive index of the semiconductor layer 14, namely the mole fraction of AQ.
In the case where the refractive indices of the second and third semiconductor layers are rendered asyrnmetrical as in the foregoing Embodiments, it is observed that the threshold current density is slightly increased, but the degree of the increase is within a range of the deviation of the threshold current density in each crystal, and no practical disadvantage is brought about by such increase of the threshold current density.
As will be apparent from the results of the fore-going Embodiments, wh~n the difference of the AQ mole fraction x between the semiconductor layers 12 and 14 is about 0.06, the maximum thickness tM is abo~t 1.5 times the ma~imum thick-ness attained when the AQ mole fraction ~ is the same in the layers 12 and 14, and, if the difference of x is about 0.085, the maximum thic~ness is about 2 times. In practical operation, the difference of x is appropriately determined ln view of the desired value of tM and increase of the threshold current density. From the above results, it is seen that, if the difference of x is 0.05 or more, effects are conspicuous.
In Embodiment 7, semiconductor materials to be laminated on the substrate crystal are changed. The lasing wave length is 0.83 ~m, and the threshold current density is 1.5 KA~cm2. In this Em~odiment, tM can be increased to Ibout 0.7 ~m, though tM is 0.35 ~m when the second and third semiconductor layers are composed of the same material, Ga0 5AQo 5As.
In Embodiment 8, a semiconductor de~ice is prepared by using materials differen-t from those used in Embodiment

5 as the materials o~ the substra~e crystal and the layers to be laminated thereon. Also in this Embodiment, tM can be increased to 1 ~m though when both the semiconductor layers 13 and 14 are composed of the same material, GaO 4AQo 6Aso 88Sbo 12 tM is 0.5 ~m. In this Embodiment, the lasing wave length is 1.1 ~m, and the threshold current density is 3KA/cm~.
In the foregoing Embodiments, the thickness d of the first semiconductor layer 13 is adjusted to 0.1 ~m.
As will readily be understood from the illustration given hereinbefore by reference to the formulae (4) and (5), the value d is not limited to a specific value.

Sem~conductor laser devices having a principle structure as shown in Fig. 3 are prepared, and details of the structures of these devices are shown in Table 6.
Devices of these Embodiments are preferred as large output power semiconductor laser devices.

Table 6 Embodi- SubstrateLayer 12 Thickness Layer 13Layer 14 Channel Chennel Layer 15 ment No. Cr,ystal 11 of Thin - Depth Width Portion of , ~m) t ~m) Layer 12 g n-GaA5~ 0 7 ~m n-GaAs, p-GaO 67 1.5 20 n-GaAs, Te doped 0.3 , 0.1 ~m 0.33 ' Sn doped, Sn do ped thi ck 2 ~m thi ck , thick ditto ditto 0.~ ~m 0.05 ~m ditto 1.5 20 ditto ~a thick 11 ditto ditto 0.5 ~m 0.15 ~m .ditto 1.5 20 ditto t~
thi ck ~ ?d~
The preparation method adop-ted for Embodiments 3, 10 and 11 is principall~ the same as the method described in Embodiment 1. In each case, the laser length is 300 ~m.
In ~eneral, in the structure shown in Fig. 3, the width of the region W pxovides the effective width of the laser. This width is determined from the lasing operation current and the light output power of the laser. In general, a width of 10 to 20 ~m is broadly adopted so as to obtain an output power of ~5 mA at an operation current of 100 to 200 mA.
As a result of our research work, it has been ~ound that, when the following arrangment is made in the structure of the semiconductor laser device of the present invention, it is possible to provide a semiconductor laser device having a large output power with a stable lasing mode. In the case of ~he structure shown in Fig. 3, for example, the wi~dth of the channel is adjusted to 10 ~m or more (preferably at least 12 ~m) ~nd the channel is designed so that ~he optical loss coefficient in the waveguide of the channel portion is smaller by at least 40 cm 1 than the optical loss coefficien~
in the waveguide of the adjacent region. Needless to say, in structures other than that shown in Fig. 3, if the above conditions are satisfied, the same effects can be attained equivalently.
The light output power characteristics of the device of Embodiment 9 are as shown in FIG. 11. The lasing wave length is 8800 A and the threshold current is 75 mA.
No kinks appear in the current versus light output power characteristics, and stable light output can be obtained.
Modulation is possible up to 800 Mbits/s. In the laser device of this Emhodiment, the difference o~ the effective absorption coef~ic~ent is 200 cm 1~ -32-~ lso in EmbodLments 1~ and 11, stable light ou-tputs can be obtained when the current density is up to 2 times the threshold current value.
In the semiconductor laser devices illustrated in the foregoing Embodiments, when the threshold current density is set, current expanded outside the channel and making no contribution to the lasing operation must be taken into consideration. Especially in the case of the structure shown in Fig. 3, in the channel portion, the thickness of the n~Gal xAQxAs layer is increased to 1-1.5 ~m. The specific resistivity of this portion is about O.lQ-cm, which is much higher than the specific resistivity of the substrate GaAs, i.e., about 0.003 Q-cm. Thus, the current tends to flow outside the channel.
Further, in the structure shown in Fig. 3, the thickness of the thin portion of the layer 12 is ordinarily 0.2 to 0.5 ~m. In this case, the reliability of the laser device depends greatly on the crystal quality of the substrate crystal, and in order to obtain a laser device having a long life, it is necessary to use a substrate having a low disclocation density and a high crystal quality.
According to a modification of the semiconductor laser device of the present invention, the foregoing defects can be effectively eliminated and channelled substrate polar laser devices having higher and more preferred characteristics are provided.
; According to this modification a four-th semiconductor layer is formed on the semiconductor substrate, a channel is formed on this fourth conductor layer and respective semiconductor layers are laminated on the channelled fourth sem~conductor layer. In this arrangement, if an electric conductivity type opposite to that of the s~bstrate is given to the fourth semiconductor layer, it is possible to cause an electric current to flow preferentially in the channel portion alone. If the electric conductivity type of the fourth semiconductor layer is made identical with that o~ the substrate and the specific resistivity of the fourth semiconductor layer is made higher than that of the semi-conductor in the channel, expansion oE the electric current can be reduced~ T~is fourth semiconductor performs the function of a so~called buffer layer and defects are sub-stantially reduced. Accordingly, long-life laser devices can be obtained in a high production yield.
The fore~oing modification of the semiconductor laser device of the present invention will now be described.

Fig. 12 illustrates the section of a semiconductor laser device of this Bmbodiment. A stripe channel 20 is formed on a p-GaAs layer. The structure shown in Fig. 12 is formed in the following manner.
The p-GaAs layer 19 is gxown by liquid phase epitaxy or gas phase epitaxy on a n-GaAs substrate (Te-doped, the electron density being ~1 x 1013 cm 3 ) having a crystal face of face index (100). The thickness is 1 to 1.5 ~m and the hole density is 1 x 1017 cm 3. As an impurity, Ge is used in the case of liquid phase epitaxy or Zn in the case of gas phase epitaxy. A photoresist window having a width of 2 to 20 ~m is formed by customary photolithography, and the semiconductor layer 19 is chemically etched through this window to the substrate to form a channel. Then, by successive liquid phase epitaxy on the so channelled semi-conductor layer 19, there are formed a layer 12 ofn-Gal_xAQxAs (x ~0.3, Sn-doped, electron density ~lx1017 cm ), a n-GaAs active layer 13 (undoped, electron density ~1x1016 cm 3), -3~-layer 14 of p-Gal_xA~x~s ~Ge-doped, the hole densit~
5 x 1017 cm )and a n-GaAs layer 15 (Sn-doped, the electron density ~l x 1017 cm 3~. The thicknesses of the layers 13, 14 and 1 5are about 0.1 ~m, about 2 ~m and about 1 ~m, respectively. The thickness of the thin portion of the layer 12 is about 0.~ ~Im. Zn is diffused selectively in the region 18 by using an AQ2O3 mask. Then, Au-Cr alloy and Au-Ge-Ni alloy are vacuum-deposited to form a positive electrode 16 and a negative electrode 17. Finally, the crystal is cleaved to obtain a laser element having a length of 300 ~m.
In the above lasser device, if the channel width is 10 ~m, the lasing operation is possible at a threshold current density of 1.2 KA/cm2 at room temperature. An n-p-n structure is formed in the portion of layers 12, 19 and 11 and no current ineffective for the lasin~ operation is allowed to flow. Accordingly, the apparent threshold current density can be reduced by about 40 %. Further, by introduction of the buffer layer 19, the median life under continuous operation is 80,000 hours at an ambient temperature of 30C., ~000 hours at 50C., or 800 hours at 70C. The output power of the laser at this operation is 3 mW per end face. In the case of a channelled laser of the conventional structure, the median life is 40,000 hours at 30C., 2000 hours at 50C. or 500 hours at 70C. Thus, by adoption of the structure of the present inventionr the reli~bility is enhanced about twice.
The threshold current density is not so increased, even if the channel wid-th W is narrowed, and when the channel width W is 5 ~m, the threshold current density is 1.4 KA~cm2.
Tn the case o~ the conventional struc~ure, if the channel width W is 5 ~m, the threshold current density is as high as 3.4 KA/cm .

EMBO~IMENT 13 A laser element is prepared in the same manner as in Embodiment 12 except that the layer 19 in Fig. 12 i~
composed of n-GaAs (undoped~. This layer 19 is grown by liquid phase epitaxy or gas phase epitaxy. The thickness is 1 to l.S ~Im~ and the electron density is 1015 cm 3. The specific resistivity is about 1 Q-cm .
In this structure, since the resistivity outside the channel is higher than the resistivity of the channel portion, the ineffective curren~ can be remarkably reduced and the threshold current is 1.3 KA/cm2 at room temperature.
The reliability is improved in this Embodiment as in Embodiment 12.

The section of a laser device of this Embodiment is shown in Fig. 13. Reference numeral 21 represents a n-GaAs layer (undoped, specific resistivity ~l Q-cm2). A channel 22 is formed on a part of the layer 21. Other structural elements are the same as in Embodiment 13~ In this Embodiment, the thickness of the layer 21 is adjusted to 1.1 to 3 ~m, and the depth of the channel is adjusted to 1 to 1.5 ~m so that the bottom of the channel is in the interior of th~ layer 21. Also in this structure, expansion o the electric current is reduced and the threshold current density is 1.3 KA/cm2 at room temperature. In this structure, if the thickness of the layer 12 below the channel bottom is large, the series resistance of the element is increased. Accordingly~ in the case of continuous operation, it is preerred that the thick-ness of the above portion be smaller than 1 ~m.

The section of a laser device of this embodiment is nown in Fig. 14. Reference numeral 11 represents a n-GaAs substrate (Te-doped, electron densit~ ~1018 cm 3) r reference numeral 23 represents a p~GaAs layer (~Ge-doped, hole density ~ 5 x 1017 cm 3 ~ and reference numeral 24 represents a channel extending to the interior of the substra-te. Other structural elements are the same as in Embodiment 12.
In this Embodiment, controlof chemical etching Eor formation of the channel is easier than in Embodiment 12. If the depth of the channel in the substrate is too large, the series resistance of the element is increased. Therefore, it is preferred that the above depth be smaller than 2 ~m.
In the laser device of this Embodiment, the threshold curxent density is 1.2 KA/cm2 at room temperature, and the reliability is improved as in Embodiment 12.

A laser device is formed in the same manner as in Embodiment 15 except that the layer 23 is composed of n-GaAs (undoped, electron density ~1 x 1015 cm 3 ). The threshold current density is 1.3 KA/cm2 at room temperature.

Laser elements of the GaAs-GaAQAs series are illustrated in the foregoing Embodiments 12 to 16. Needless to say, the present invention may be applied to ~ther semiconductors.
The sectibn of a laser element of Embodiment 17 is shown in Fig. 15. Reference numeral 31 represents a n-InP substrate ~Sn-doped, electron concentration ~2 x 1018cm 3), reference numeral 32 represents a layer of p-GaO 12In0 88 Aso 23Po 77 ( Zn-doped, hole density ~1 x 1017 cm 3), reference numeral 33 represents a n-InP layer Csn-doped~ electron density ~1 x lal7 cm 3~, reerence numer~l 34 represents a layer GaO.12InO.88AS0.23Po,77 ~undoped~, re~erence numeral 35 represents a layer of p-InP (Zn-doped, hole density ~1018 cm 3 ), and reference num~rals 36 and 37 represent ohmic electrodes. A channel 38 is formed on the layer 32.
The thickness of the layer 32 is 1.5 ~m, the thickness of the thin portion o the layer 33 is 0.~ ~m, the thickness of the layer 3~ is 0.1 ~Im, and the thickness of the layer 35 is 2 ~Im.
The above laser device is prepared by successive liquid phase epitaxy by using InP as the substrate. The ohmic electrodes are prepared by vacuum deposition of an Au-Zn alloy for a positive electrode and an Au-Sn alloy for a neg-ative electrode.
When the laser length is adjusted to 300 ~m, the threshold current density for the lasing operation is 2.3 ~A/cm ,~w~lich is lower by about 35% than the threshold current density of the conventional laser device of the substrate channelled structure. The median life is 40,000 hours at continuous operation at room temperature, which median life is significantly longer than the median life of the conventional structure.
In this Embodiment, p-GaInAsP is used asithe buffer layer and the channel bottom is in agreement with the surface of the substrate. It will be apparent that structures corresponding to those of Embodiments 12 to 16 may be adopted in this Emhodiment. It will also be apparent that ~ther compound semiconductors such as GaA~AsSb can be similarly employed in the present invention.
It is important that the band gap of the layer formed on the substrate is equal to or smaller than that of the active layer. If only this condition is satisfied, in Embodiment 17, a compound semiconductor consisting of other combination of Ga, In, As and P can be used for the layer 32.

In Embodiments 13 and 14, the layer l9 or 21 iscomposed of n-GaAs (Sn-doped, the electron density being 10l7 cm 3 ). In this structure, an electric current readily :Elows in portions other than the channel,and the threshold current density for the lasing operation is 2.3 K~/cm2.
Hdwever, by virtue of the action o the buffer layer, the reliability is improved as much as in Embodiment 12.
The modification of the semiconductor device of the present invention illustrated in the foregoing Embodiments 12 to 18 is summarized as follows:
l. A semiconductor laser device comprising a fourth semiconductor layer formed on the surface of a semi-conductor substrate, a stripe channel formed at least on the surface of said semiconductor layer, and a plurality of semi-conductor layers formed on said channel and the surface of said fourth semiconductor layer, one of said plurality of layers being a lasing active layer having a band gap equal to or lar~er than the band gap of the four.th semiconductor layer..
2. A semiconductor device as set forth in l above wherein the electric conductivity type of said fourth semi-çonductor layer is different from that of the semiconductor substrate and the bottom of the channel reaches the surface or interior of the substrate.
3. A semiconductor laser device as set forth in l above wherein the electric conductivity type of the fourth semiconductor layer is the same as that of the semi-conductor suhstrate, the bottom o~ the channel reaches the surface or the interior of the su~strate and the specific resistivity of the fourth semiconduc~or layer is higher than -39~

3~
at of the semiconductor present in the channel, 4. A semiconductor laser device as set forth in 1 above wherein the electric conductivity type of the fourth semiconductor layer is tne same as that o~ the semiconductor sutstrate, the bottom of the channel reaches the surface or interior oE the substrate and the specific resistivity of the Eourth semiconductor layer is equal to or higher than the specific resistivity of the semiconductor present in the channel.
As will be apparent from the foregoing illustration, if a buffer layer is formed in the semiconductor laser device of the present invention, the threshold current density can be reduced and the reliability can be improved. Accordingly, high practical effects can be attained.
Another modification of the semiconductor laser device of the present invention enables the abovementioned basic structure and modifications to be improved so that the lasing operation mode can be further stabilized and a semi-conductor laser device capable of much stabilized single fundamental mode operation can be provided.
This modification will now be described by re-ference to Fig. 16 illustrating an embodiment in which GaAs and GaAQAs are employed.
A channel 41 is formed on a n-GaAs ~ubstrate 11 ( n-type, electron density ~1013 cm 3 ). The depth of the channel 41 is, for example, 1 to 1.5 ~m. Reference numeral 12 ~epresents a n-Gal xA~xAs layer ( x~0.3, n-type, den~ity ~1017 cm 3 ) grown on the channelled substrate. Re~erence numeral 13 represents an undoped GaAs active layer ( n-type, density ~1016 cm 3, thickness ~0 1 ~m~. Re~erence numeral 14 represents a p-Gal xAQx~s layer ~hole density ~5 x 1017 cm 3~ p-type, thickness ~2~ m ~OReference numeral 25 represents p-GaAs layer (p type, density ~1013 cm 3, thickness ~1 ~m).
~eference numerals 16 and 17 represent ohmic electrodes. ~ight is distributed in the vertical direction in Fig. 16 wi~ the GaAs layer 13 being the center and is propagated in the direction rectangular to the drawing surface AS pointed out hereinbefo~e, light is confined in the transverse direction paralle~ to the junction face. The lasing operation takes place with the active region 40 on the channel being the center.
~ccordingly, the spot width of the lased light is substantially equal to the width W of the channel, and the spot width can easily be controlled by changing the width of the channel.
The effective absorption coefficient depends greatly on the degree of arrival of light at the substrate, namely the thickness d o~ the active layer 13 and the thickness tl of the n-Gal xAQxAs layer, as pointed ou~ h~reinbefore.
In view of the results o~ experiments made on the conventional double hetero structure, in order to reduce the threshold value for the lasing operation, d is ordinarily adjusted to 0~05 to 0.2 ~m. Fig. 17 illustrates the relation between the thickness tl of the thin portion of the n-Gal xAQx~s layer and the theoretical value of the threshold gain Gth necessary ~or initiation of the lasing operation when d is 0.1 ~m. The channel width W is adopted as the par~meter. In Fig. 17, curve 171 shows the results obtained when W is 2 ~m, curve 172 shows results obtained when T^J iS 3-~m, curve 173 shows results obtained when W is 4~m, curve 174 shows results obtained when l~ is 5 ~m, and curye 175 shows results obtained when W is 8 ~m.
In order to prevent the lasing operation from taking place in portions other than the channelj the threshold gain Gth must be suf~iciently smaller than the threshold gain Gth in the waveguide ot,her than the channel. In general, a relation of G~IJl 5 is su~stan~ially established between the ~,Q~
in G and the current densit~ J. ~ccordingly, i~ it isintended to obtain stable laser operation in the channel at a current density up to 1.5 times the threshold current density Jth, a condition of Gth _0.6 Gth must be satisfied. In view of the foregoing, 0.6Gth is indicated hy chain line A in Fig. 17. From Fig. 17, it will be apparent that when W is in the range o~ from 2 to 8 ~m, sufficiently stable lasing operation can be obtained in the channel portion if the condi-tion of tl~ 0.6 ~m is satisfied.
In Fig. 18, the ratio oE the threshold gain G'th in the first order transverse mode operation to the threshold gain Gth in the lowest transverse mode operation under conditions of d = 0.1 ~m and tl _0.6 ~m is plotted. The fact that the G'th/Gth ratio is high means that the single fundamental mode operation is possible even at a high current density.
In Fig. 18, curve 181 shows results obtained when W is 2 ~m, curve 182 shows results obtained when W is 3~m, curve 183 shows results obtained when W is 5 ~m, curve 184 shows results obtained when W is 7 ~m, curve 185 shows results obtained when W is 8 ~m, and curve 186 shows results obtained when W is 10 ~m. In the case of W~8 ~m, the relation of G'th/Gth _ 1.8 is e~tablished if tl_ 0.6 ~m. Thus, if this ratio is converted to the current density ratio it is seen that the lowest transverse mode operation is possible, if the current density is up to 1.5 times the threshold current density value.
If W is larger than 8 ~m, a higher order mode operation readily oc~urs.
From the theoretical results shown in Figs. 17 and 18, it is apparent that, in the case of d - 0.1 ~m and 2 ~m '0 < ~ ~8 ~m, stable single fundamental transverse mode operation takes place in the channel port~on if the condition of ~ 0.6 is satisfied. When d is other than 0.1 ~m, if theloss in the active region on the protuberance as a waveguide is smaller by at least 40 cm 1 than the loss in the w~veguide of the adjacent region, substantially same lasing characteristics as described above can be obtained. In short, the problem can be converted to a problem of the waveguide characteristics.
The waveguide characteristics are determined by the e~ective complex refractive index difference ~n which is written as follows:

~n - ~n ~ 2k ~

wherein ~n represents the effective refractive index difference,~ stands for the effective loss difference and ko stands for the wave number of lased light in vacuum.
As is well known, ~n is substantially in proportion to ~. Accor~ingly, in the structure of the pr~sent embodiment, the waveguide characteristics can be grasped by determining ~. The effective loss difference can be expressed as follows:

p2h2(p+r~e~2Pt2 ~= 2k Im t nt~ 2 ~ ] (7) ~o(l+ 2 ) (p +h ) (p-r) wherein Im ( ) stands for the imaginary part of the portion t ), ~0 stands for the light propagation constant in the lengthwise direction of :the laser in the channelled region, h stands for the propagation constant in the direction of the thickness of the layer 13 in the region outside the channel, p stands for the damping constant in the direction of the thickness in the layer 13 r and r stands for the damping constant in the direction of the thickness in the layer 11.

From the foregoing results, it is conclud~d tha-t the critical point and range o$ d = 0.1 ~m and t1 ~ 0.6 ~m in the structure of the present embodiment are well in conformity with ~ _ 40 cm 1. If d becomes large, the light distribution in the vertical direction in Fig. 16 is narrowed.
Accordingly, in order to obtain an equivalent effective loss di~Eerence, tl must be made smaller. More specifically, in order to attain N _ 40 cm 1, the condition of tl _ 0.5 ~m must be satisfied when d = 2.15 ~m, and the condition of tl _ 0.4 ~m must be satisfied when d = 0.2 ~m. If an equivalent effective loss difference is realized in the foregoing manner, similar lasing characteristics can be attained.
Tllis modification of the semiconductor laser device of the present invention will now be described in detail by reference to the following embodiments.

Fig. 19 shows the sectional structure of a laser device of this Embodiment. Reference numeral 15 represents a n-Ga~s layer and reference numeral 18 represents a zinc-diffused portion. Other structural elements are the same asin Fig. 16. Various samples are prepared by adjusting the width of a channel 41 to 5 ym in ~ig. 19 and changing the thickness d of the active layer and the thickness tl of the layer 3. A structure às shown in Fig. 19 is prepared in the following manner.
A photoresist window having a width of 5 ~m is formed on a n-GaAs substrate (Te doped ) having a face of face index (100) by customary photolithography, and the surface of the substrate is chemically etched through this window to form a channe~ having a depth of 1.5 ~m. Semi-conductor layers 12, 13 and 14 shown in Fig. 1~ are formed on the so channelled substrate according to customary successive lquid phase epitaxy. The thicknesses d and tl are controlledby controlling the growth times according to customary pro~
cedures, Then, an AQ2O3 film is formed and a window having a width of 5 ~m is ~ormed thereon accord;ng to photolithography.
Zinc is diffused throuth this window to form a p-type diffusion region 18. ~u-Cr alloy and Au-Ge alloy are vacuum-deposited to form oilmic electrodes 16 and 17, respectively. Finally, the crystal is cleaved to form a reflector, whereby a laser element having a laser length of 300 ~m is prepared.
In the above semiconductor laser element, when d = 0.1 ~m and tl = 0.5 ~m, the threshold current density is ~2.2 KA/cm at room temperature. Fig. 20 illustrates the near field intensity profile of the lasing operation in the direction x. The intensity profile is determined with respect to the direction paralled to the junction face on the cleft ~ace. The lasing operation is of the single transverse mode, and this operation mode is stable when the current value is up to two times the threshold value (up to 130 mA ). In Fig.
20, I/Ith is used as the parameter (curve 201: I/Ith = 1.1, curve 202: I/Ith = 1.5, curve 203: I/Ith =2 ).
Fig. 21 shows results of tests of the stability of the lasing operation mode made on elements in which d is fi~ed to 0.1 ~m, tl is changecl in the range of from 0.1 to 0.8 ~m, and W is chanqed in the range of 1 to 10 ~m. In Fig. 21, symbolo indicates that the single transverse mode operation ls possible when the current is up to 1.5 times the threshold value, symbol X indicates that a higher order mode operation takes place even if the current is lower than 1.5 times the threshold value, and symbolQ indicates that the single trans-verse mode operation is sllghtly unstable. From the results shown in Fig. 21, it is seen t~at the single transverse mode operation is always possi~le if conditions of 2 ~m ~ ~ < 8 ~m ~ ?~
nd tl _ 0.6 ~m are satisfied As illustrated hereinbefore, the value of 'c1 is determined by the optical loss in the waveguide.
EMBO~IMENT 20 In Fig. 19, the thic]cness d of the active layer is adjusted to 0.05 ~m or 0.15 ~m, and other structural elements alld preparation procedures are the same as described in Embodimentl9. Also in this ~mbodiment, when the channel width is at least 2 ~m but smaller than 8 ~m, if the loss in the semiconductor layer having a protuberance as the waveguide is smallerby at least 40 cm 1 than the loss in ~he waveguide in the adjacent region, stable single transverse mode operation can be obtained at a current up to 1.5 times the threshold value. For example, if d is 0.05 ~m, the above condition is satis~ied in case of tl ~ 0.8~m, and when d is 0.15 ~m, the above condition is satisfied in case of tl < 0.5 ~m.
In the foregoing Embodiments, GaAs and GaAQAs semiconductors are employed. However, it is apparent that in the present invention directed to a laser device includiny a waveguide, the materials of the semiconductors are not critical. Accordingly, other semiconductor materials, for example, three element type compound semiconductors such as GaInP, GaAsP and GaAQSb and four-element type compound semi~
conductors such as GaInAsP and GaAQAsSb, can be similarly used for the semiconductor laser device of the present in~ention.
Needless to say, the present modification of the semiconductor laser device is not limited to the structures shown specifically in the ~oregoing Embodiments 19 and 20, but this modiication can be ~pplied to any of the structures in which arrangement requirements of this modification are satisfied.

~?~

In semiconductor laser devices represented by a channelled substrate planar laser~ if the width o~ the lasing channel and the effective loss d(ifference are adjusted accordin~ to the present modiication of the present inventi~n, stable single transverse mode operation becomes possible.
Accordingly, the practical ef~ects of the present modification are extremely high.
~ s pointed out hereinbefore, the waveguide cha-racteristics are determined by the composite refractive index difference ~n between the channelled portion and the non-channelled portion9 which difference is expressed as follows:

~n - n +
2]~o In the foregoing Embodiments, the refractive index difference is realized in the transverse direction of the first semiconductor layer ~ active layer ) mainly by changing the effective loss corresponding to the imaginary part of the composite refractive index. However, the optical gain or loss difference can be realized in the transverse direction of the irst semiconductor layer ~ active layer ) ~y changing the effective refractive index corresponding to the real part of the composite refractive index while regarding the effect-ive loss ( imaginary part ) as being zero.
For example, in semiconductor laser devices shown ~n Figs. 12, 13 and 14, if Gal pAQpAs ( y < p ~ I, p~ x ) is used as the fourth semiconductor layer 19, 21 or 23, the composite refractive index difference can be brought about based on the efective refractive index difference, whereby the optical gain or loss can be chan~ed and light can be c~nfined in the transverse direction in the active region.
Gal pAQp~s includes two types~ nameIy the type of p~ x and the type of p~ x. In ~ach case, the same effects can be attained.
We succeeded in preparing semiconductor laser devices having good current versus light output power char-acteristics by using GaO 8AQo 2~s or GaO 5ARo 5As for the layer 19, 21 or 23.
In the foregiong Embodiments, elements in which a stripe protuberance is ~ormed on the second conductive layer 12 are illustrated. Semiconductor laser devices of the present invention can be prepared by forming a protuberance on the third semiconductor layer 14. More specifically, if the thicknesses t5 and t6 of the third semiconductor layer 14 are adjusted as in the foregoing Embodiments, as shown in Fig. 22, a difference of the optical gain or loss is brought about in the direction x of the lasing active layer 13, and the region below the protuberance 42 of the first semiconductor layer acts as a lasing active region and a semiconductor laser device having good light ou~put versus current characteristics can be provided.
In the semiconductor laser device shown in Fig. 22, n-GaAs is used as the substrate 11, n-GaO 7AQo 3As having ; a thickness of 2 ~m is used as the second semiconductor layer 12, undoped GaO 95AQo 05 having a thickness of 0.1 ~m is used as the first semiconductor layer 13, p-GaO 7AQo 3As ( t5 ~
0.4 ~m, t6 ~ 1.5 ~m3 is used as the third semiconductor layer 14, and n-GaAs is used as the semicond~ctor layer 15.
Reference numeral 18 represents a p-type ~n-dif~used layer, reference numeral 16 represents a positive electrode of Au-Cr, and reference numeral 1~ represents a negative electrode o~ an Au-~e-Ni alloy. In this structure, good resul~s are obtained.
It is possible to combine structures shown in Figs.
22 and 3. More specifically, it is possible to form a thick portion and a thin portion in each of the second semiconductor layer 12 and the third semiconductor layer 14.
Any of the materials having a composite refrac-tive index diEferent from that of the semiconductor layer 14 with respect to the guided lased wave may be used as the layer 15 instead of a semiconductor. For example, the light absorb-ing effect of metals such as Au can be utilized. Of course, materials other than semiconductors can be used as the sub-strate 11 in the structure shown in Fig. 3 and layers 19, 21 and 32 in structures shown in Figs. 12 to 15.
In semiconductor laser devices illustrated in the foregoing Embodiments, on the channel portion ( or the protubèrant portion ) formed on the substrate or the semi-conductor layer ~ormed on the su~strate, the ~econd semi-conductor layer having a plane surface covering said channel or pxotuberant portion is present, and also the active layer ( the first semiconductor layer ) formed on the second semi-conductor layer is a flat layer. Even if the active layer is protruded along the channel or protuberant portion, a semi~
conductor laser device equivalent to ~he waveguide structure shown in Fig. 4 can be obtained. This modifica~ion will now be described.
Referring now to Fig. 23, double hetero junction crystal layers are grown by the epitaxial method on a sub-strate 231 having a stripe channeI or stripe projection ~ projection in Fig. 23 ) extending in the direction z. A
layer 233 is composed of a first semiconductor crystal, and ~49-~?~
~ yers 232 and 234 are composed of second and third semi-conductor crystals, each having a band gap broader than that of the first semiconductor crystal and a refractive index smaller than that of the first semiconductor crystal. It is possible to omit provision of the layer 232 by using the second semiconductor for the substrate 231. ~nstead of a single crystal layer, a multilayer structure including a plurality of layers epitaxially grown, which is etched to form a channel or projection, may be used as the substrate 231. In other words, the crystal of the projection ( or the protuberant portion surrounding the 'channel ) may be different from the crystal of the substrate 231~ In short, the semi-conductor laser device according to the present modification of the present invention is characterized in that a projection or channel is formed on the substrate and the first semicon-ductor layer 233 is protruded along the projection or channel on the substrate.
According to the theoretical analysis of a di-electric rectangular waveguide, the section cf which is illustrated in Fig. 24, electromagnetic wave propagation characteristics along ~the region 241-are approximately determined by the refractive index, width a and thickness b of the region 241 and the refractive indices of regions 24~, 243, ~44 and 245, but they d~ not depend on characteristics of regions 246, 247, 248 and 249. This approximation is established except for the case where both a and b are simul-taneously smaller than the wave length ~g of the guided light in the waveguide. In ordinary semiconductor lasers, the relation of a>> ~g is established and good approximation is observed.
Referring to Fig. 23, the light propagated in the region 235, i.e,, a part of the layer 233 composed of the irst semiconductor, is equivalen-t to the light propagated in a rec.angular waveguide, the lower portion of which is defined by the second semiconductor and the left and right sides and upper portion of which are defined by the third semiconductor, and if the region 235 is excited, stabilized t~ansverse mode operation can be obtained.
In order to bring about the above-mentioned difference of the re~ractive index effectively in the direction x, the protuberant height T of ~he protuberant portion must be larger than the light penetration depth r in the direction y from the active region 235, and this penetration depth r is given by the formula (1) as pointed out hereinbefore.
In an ordinary laser element of the GaAs-GaAQ~s series, the value of r is about 0.3 ~m. Accordingly, if the condition of T > 0.3 ~m is satisfied~ a semiconductor laser element having a guide wave structure shown in Fig. 24 is provided.
In a laser element having a structure shown in Fig. 23, in order to change the re~ractive index in the horizontal direction in the region 235, the protuberant incline angle ~ must be larger than the critical angle ~.
In this case, a semiconductor laser element having a waveguide structure shown in Fiy. 24 is provided effectively. This critical angle ~ is expressed as follows:

-1 nl5 ~O= 9OO _ ~o_ goo _ ~in nl~

In the case of semiconductor lasers of the GaAs-GaAQAs series, if the condition of ~ _20 is satisfied, suf~icient effects can be expected. It also is important that the thickness c of the layer 232 below the lasing activè

region 235 must be sufficientl~ lar~er than the penetration ~pth r ( in order to prevent absorption of light b~ thesubstrate ). Sufficient results are obtained when the thickness c is larger than 1 ~m. Similarly, the thickness of the layer 15 must be larger than 1 ~m. If the height H of the projection ( or the channel ) is too larye, continuous junctions are not formed among the layers 13, 14 and 15, and if the height H is too small, no effective refractive index difference is brought about. Accordingly, it is preferred that the height H be 2 to 4 ~m.
When the respective layers are grown on the substrate by successive liquid phase epitaxy, if the --growing temperature is adjusted to 730to 780C. and the temperature lowering rate is adj~sted to 1 to 5C./min., the projection or channel on the substrate can be transferred substantially precisely on-to the first semiconductor layer, i.e., the lasing active layer, and the protuberant incline angle 9 in the laser active layer is about 20 or more.
It has been found that dimensions substantially the same as described above are suitable for laser elements of the GaAsSb series and of the GaInAsP-InP series.
As will be apparent from the foregoing illustration, the present modlfication is characterized in that in a first semiconductor layer where lasing operation is caused, a protuberant portion is formed between a region excited mainly by a current from an electrode disposed along the propagation direction of lased light and another region, and by virtue of this characteristic struc*ure, the following effects can be attained:
(1) From the optical viewpoint, there can be obtained a stable transverse mode operation equiYalent to the transverse mode operation attained when the entire periphery of the first s~miconductor is surrounded by the second or third semiconductor.
(2) In connection with the manufacture process, the semiconductor laser element can be prepared by one successive epitaxy and crystal growth can be controlled effectively. Thus, the pro~ess steps can be simplified and a product having a high reliability can be obtained without introduction of crystal defects.
This modification will now be described in detail by reference to the following Embodiments.

The section of a semiconductor laser device of this Embodiment is shown in Fig. 25. This laser device is prepared in the following manner.
A photoresist stripe having a width of 10 ~m is formed by photolithography on a n-GaAs substrate (Te-doped, concentration ~1013 cm 3 ) having a crystal face of face index (100), and the substrate is chemically etched along a depth of about 3 ~m by using the so formed stripe as a mask to thereby form a substrate 251 having a protuberant surface.
The direction of the stripe is at (011) and a 1:1:3 liquid 3 4 : H202 :C2E4 (OH)2 is used for the e~ching treatment. Layers 252, 253, 254 and 255 are formed on the substxate by customary successive liquid phase epitaxy.
Compositions of solutions used for formation of these layers are as follows:
1) First Layer 252 ( GaO 7AQo 3As ):
4 g of Ga, 3 mg of AQ, 200 mg of Sn and 200 mg of GaAs ~?,~
2) Second Layer 253 ( GaO 97AQo 03As ):
4 g Ga, 0.2 mg of AQ and 200 mg of GaAs 3) Third Layer 254 ( GaO 7AQo.3 4 g of Ga, 3 mg of A~, 70 mg of Ge and 200 mg o~ GaAs 4) Fourth Layer 255 (GaAs ):
4 g of Ga, 100 mg of Sn and 200 mg of GaAs An ordinary sliding boat is used for liquid phase epitaxy, and the solution is saturated at 760C. and cooled at a temperature-lowering rate of 3C./min. Growth is started at 740C. The growth times for the layers 252, 253, 254 and 255 are 3 minutes, 3 seconds, 3 minutes and 1 minute, respectively, and the thicknesses of these layers are 1.5 ~m, 0.2 ~m, ~ ~m and 1 ~m, respectively. The thickness t is about 1 ~m. As additive impurity, Sn is used for the n-type and Ge is used for the p-type. Then, an A~03 film is formed on the surface and a window having a width of 7 ~m is formed at a position corresponding to the protuberant portion, and Zn is selectively diffused up to the layer 244 through this window and the AQ203 film is then removed.
Positive and negative electrodes spaced by 300 ~m from each other are formed by vacuum deposition, and the crystal face -is cleaved (011), whereby a laser element is prepared. A
Cr-Au alloy is used for the positive electrode and an Au-Ge-Ni alloy is used for the negative electrode.
The lasing operation characteristics of the so prepared laser device and the conventional laser device shown in Figs. l-a and l-b as the compaxative element are shown in Figs. 26 to 28. Fig. 26 illustrates the relation between the exciting current and the light output power.
Curve 261 shows results of t~e laser device of the~present nvention shown in ~ig. 25, and curve 262 shows results of the conventional structure shown in Figs. l-a and l-b. In the laser device of the present invention, the threshold current density for the lasing operation is about 2 KA/cm2.
It is seen that the linearity of the relation between the current and the output power is highly improved in curve 261 over in curve 262.
Figs. 27-a and 27-b illustrate the far field in-tensity profile in the x-z face. Fig. 27-a shows results of the structure o~ the present invention shown in Fig. 25 and Fig. 27-b shows results of the conventional structure shown in Figs. l-a and l-b (curves 271 and 272: exciting current of 4 KA/cm , curves 273 and 274: exciting current of 3.3 KA/cm , curves 275 and 276: exciting current of 2.5 KA/cm ). It is seen that in Fig. 27-a, the intensity profile is irregularly changed depending on the change of the exoiting current, but in Fig. 27-a the intensity profile is much stabilized.
Fig. 28 illustrates the output power obtained when the element is excited by a pulsating current L having a length of 10 ns. Curve M shows the results of the structure shown in Fig. 25 and curve N shows the rffsults of the structure shown in Figs. l-a and l-b. In curve N, irregular changes of the output power are observed, but in curve M, such changes are not observed at all. The life of the laser device shown in Fig. 25 is more than 5000 hours at an output power of 5 mW
and is equivalent to the life of the laser device shown in Figs. l-a and l-b.
The foregoing results are those obtained with respect to the structure where the substrate has a protuberant ~ur~ace. Also in case of the structure ~here the substrate ~ 7'Q~
" . ~. o .as a caved surface, similar results were obtained.
In this Embodiment, since the width of the stripe is adjusted to about 10 ~m, a higher order mode lasing operation as shown in Fig. 27-a is caused. In order to obtain the lowest mode lasing operation, it is necessary that th~ stripe ~idth must be not larger than 2 ~m. In this case, the lasillg area is reduced and there is brought about a dis-advantage that a large output power can hardly be obtained.
In the present modification of the present invention, as will be apparent from Embodiments given hereinafter, it is possible to obtain a lower order mode lasing operation without reduction of the lasing area.

Fig. 29 illustrates the shape of a stripe formed on the surface of a substrate in Embodiment 22. Etching is carried out so that a caved stripe is Eormed. ~The width of the stripe is gradually changed from w = 20 ~m to e = 5 ~m.
On this substrate, respective layers are formed in the same manner as described in Embodiment 21. Both the ends are cleaved with a spacing Qof 300 ~m therebetween. The far field intensity profile in the face x-z in the so-prepared laser element is shown in Fig. 30. As is seen from Fig. 30, the laser operation obtained is of the lowest transverse mode and is very stable against the change of the exciting current ( curve301: 3.5 KA/cm2, curve 302: 3 KA/cm2, curve 303: 2.5 KA/cm2 ). Other characteristics are sub-stantially the same as those obtained in Embodiment 210 Incidentally, the stripe shape shown in Fig. 29 can be applied to not only the semiconductor laser structures of the present invention but also other various semicon-ductor laser structures.

., -~?~

The shape of a stripe on the surface of a substrate in Embodiment 23 is shown in Fig. 31. The stripe width w is adjusted to 20 ~m and the stripe is gradually curved. The length f is adjusted to 20 ~m. Other structural elements are the same as in Embodiment 22, and the laser device is prepared according to the same method as described in Embodiment 22. Characteristics of the laser device obtained in this Embodiment are substantially the same as those of the laser device obtained in Embodiment 22. A stable transverse mode lasing operation is obtained.
The stripe shape shown in Fig. 31 as well as that shown in Fig. 29 can be applied to not only the semiconductor laser structures of the present invention but also other various semiconductor laser structures.

The section of a laser element of this Embodiment is shown in Fig. 32. The width of a stripe on ~he surface of a substrate is adjusted to 5 ~m. When the growth speed is maintained at a relatively low level at successive liquid phase epitaxy, by the dependency of the growth speed on the ~ace direction, the thickness of a layer 253 is increased on both the sides as shown in Fig. 32~ When the growth temperature is 750C., the temperature-lowering rate is adjusted to 0.2C./min and the mesa height of the substrate i5 adjusted 5 ~m, the thickness of the layer ~s 0.2 ~m in the central portion and 1 ~m in both the side portions.
The near field intensity profile in the direction x in the waveguide o~ the above laser structure is shown in Fig. 33-~ ~ig. 33-b shows the near field intensity profile in the direction x in the waveguide of the buried hetero structure injection laser shown in Fig. 2 (Journal o~ AppliedPhysics, 45 , No. 11, pp. 4899~4906 ). If the thickness of the layer 253 is larger than 0.1 ~m, the optical gain is in inverse proportion to this thickness. Accordingly, the optical gain is larger in the central portion in the above structure. Therefore, even if the effective width of the wavequide reqion in the layer 253 is as broad as about 10 1lm, the lowest mode operation ( m ~ 0, m indicating the number of the order of the mode ) can be obtained sta~ly. If the width is larger than 10 ~m, a higher order ( m - 6 in Fig.
33 a ) mode operation is caused. In the structure shown in Fig. 32, however, the intensity of the output power is highest in the central portion, and a light output power can~be effectively introduced to other optical system such as optical fibers. In Figs. 33-a and 33-b, g corresponds to the width of the waveguide region. On the other hand, in the buried hetero structure injection laser, the thickness of the layer 3 is uniform and hence the optical gain is uniform throughout the layer 3. Accordingly, in order to obtain the lowest mode lasing operation, the width of the waveguide region must be smaller than 2 ~m. If the width is larger than 2 ~m, a higher order mode lasing operation is caused.
The case of m = 6 is shown in the drawing.
In the case where the light output powers of the laser of the present invention shown in Fig. 32 and of the buried hetero structure injection laser shown in FigO 2, which are obtained by a higher order mode lasing operation o~ m = 6, are introduced into the same multi-mode operation optical fibers through a cylindrical lens, the coupling efficiency is 45 % in case of the abov~ structure of the present invention and 18 % in case of the buried hetero structure ~njection laser.

EMBODIMENT ~5 A part of the section o~ a laser element of Embodiment 25 is shown in Fig. 34. Structural elements other than those shown in Fig. 34 are the same as in Embodiment 21. Excitation is effected on region5 h, i and j o~ a layer 253. In this structure, the waveguide consisting of the region h is weakly coupled with the wave guide consisting of the region j through the region i. Since the thickness of the region h is diEferen~ from that of the region j, the cor-responding propagation constants are different from eachother. Accordingly, the laser shown in Fig. 34 can be regarded as a composite laser device provided with a plurality of laser elements. In the case of the single element lasers shown in Embodiments 21 to 24, since there are present z axis modes having an approximate wave length, the lasing operation spectrum is as shown in Fig. 35-a and the singular-ity is not good ( multi-axis mode lasing operation ). On the other hand, in the laser structure of this Embodiment, it is possible to make only one lasing operation present in a gain spectrum, and as is seen from the lasing operation spectrum shown in Fig. 35-b, the singularity is highly improved~
In the foregiong Embodiment~s, substrates having a crystal face of face index (100) are used, but substrates having a crystal face of face index (110) or (111) can ~
similarly be used. Further, in the foregoin~ Embodiments, semiconductors of the GaAs-GaA~As series are used, but needless to say, semiconductors of other series can be used.
An ~mbodiment in which semiconductors of the InP-GaInAsP
series will now be descri~ed.

~?~

The section of a semiconductor laser device of Embodiment 26 is shown in Fig. 36. Reference numeral 361 represents a p-InP substrate ( Zn-doped, concentration ~1 ~ 1013 cm 3 ) having a face (lll)B. A layer 362 of n-GaO lInO 9ASo 2Po 8 having a thickness of 0-5 ~m and a l layer 363 of n-InP having a thickness of 2 ~m are grown on the substrate 361 by liquid phase epitaxy. Sn is used as an n-tupe impurity. Negative and positive electrodes 364 and 365 are formed by vacuum deposition of an Au-Ge-Ni alloy and a Cr-Au alloy, respectively. Other preparation procedures are the same as described in Embodiment 21.
In the so-prepared laser device, a laser operation can be obtained at a current density of 2.5 KA/cm2 at room temperature. The wave length is 1.1 ~m. The stability of the transverse mode lasing operation is substantially the same as in Embodiment 21.
In the foregoing Embodiments 21 to 26, so-called Fabry-Pérot type semiconductor lasers are illustrated. The modification of the present invention illustrated in these Embodiments can be applied not only to distribution feedback type lasers having a built-in diffraction lattice but also to dye lasers and solid lasers. Furthermore, the structure of this modification of the present inven~ion can be used as a waveguide or optical element when an optical integrated circuit is formed.
As is apparent from the foregoing illustration, the laser device of the present modification characterized by a curved lasing active region formed on a substrate having a stripe channel or protrusion has various characteristic 3~ advantages such as stable transverse mode lasing operation.
Further, it can be manu~actured very simply with high reprod-ucibility and it is excellent in life and reliability. Thus, ~ne laser device of the present modification of the present invention has very high practiaal effects.
I
.

Claims (30)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A semiconductor laser device comprising a semiconductor body of a first conductivity type having a major surface and a plurality of contiguous semiconductor layers disposed on said major surface of said body, said plurality of contiguous semiconductor layers being comprised of:
(i) a first semiconductor layer having a laser active region and having two opposite plane surfaces, (ii) a second semiconductor layer disposed on the first surface of said two opposite plane surfaces, consisting of a material having a band gap broader than that of said first semiconductor layer, and consisting of a thin region having a thickness t1 not greater than 3r, where r is the distance in which the evanescent wave decays by l/e, and a thick region having a thickness t2 larger than 3r, and (iii) a third semiconductor layer disposed on the second surface of said two opposite plane surfaces and consisting of a material having a band gap broader than that of said first semiconductor layer, (iv) wherein the first surface faces the main surface of said body and the second surface faces away from the main surface of said body, and said semiconductor body consists of a material having a complex refractive index different from that of said second semiconductor layer, and said second semiconductor layer is adjacent to said body and covers said major surface of said body.

2. A semiconductor laser device according to claim 1 wherein said third semiconductor layer has a thickness t3 larger than 3r.

3. A semiconductor laser device according to claim 1 wherein said semiconductor body has a projection on said major surface, and said thin region is disposed on said projection and said thick region is disposed on the remaining part of said major surface excluding the portion of said projection.

4. A semiconductor laser device according to claim 1 wherein said second semiconductor layer has said thickness t1 equal to or smaller than 2r.

5. A semiconductor laser device according to claim 1 wherein said second semiconductor layer has said thickness t1 equal to or smaller than r.

6. A semiconductor laser device according to claim 1 wherein said second semiconductor layer has said first conductivity type, and said third semiconductor layer has a second conductivity type opposite to said first conductivity type.

7. A semiconductor laser device according to claim 1 wherein the refractive index of said second semiconductor layer is the same as that of said third semiconductor layer.

8. A semiconductor laser device according to claim 1 wherein the refractive index of said second semiconductor layer is larger than that of said third semiconductor layer.

9. A semiconductor laser device according to claim 8 wherein said thickness t1 is larger than the thickness t3 of said third semiconductor layer.

10. A semiconductor laser device according to claim 1 which further comprises first and second electrodes coupled to the top layer of said plurality of contiguous semiconductor layers and to said body, respectively.

11. A semiconductor laser device according to claim 1, wherein said plurality of contiguous semiconductor layers are further comprised of a fourth semiconductor layer disposed between said second semiconductor layer and said body and consisting of a material having a complex refractive index different from that of said second semi-conductor layer.

12. A semiconductor laser device according to claim 11 wherein said fourth semiconductor layer has a channel on the surface opposite to said second semiconductor layer.

13. A semiconductor laser device according to claim 12 wherein the channel bed is on said major surface of said body.

14. A semiconductor laser device according to claim 12 wherein the channel bed is in said fourth semiconductor layer,

15. A semiconductor laser device according to claim 12 wherein said channel reaches the interior of said body through said fourth semiconductor layer.

16. A semiconductor laser device according to claim 13, claim 14 or claim 15 wherein said fourth semiconductor layer has said first conductivity type and a higher resistivity than that of said second semiconductor layer.

17. A semiconductor laser device according to claim 13 or claim 15 wherein said fourth semiconductor layer has said second conductivity type.

18. A semiconductor laser device according to claim 11 wherein said fourth semiconductor layer has a band gap as small as that of said first semiconductor layer or less.

19. A semiconductor laser device according to claim 1 wherein said body consists of Gal-zA?zAs (0?z<l), said first semiconductor layer consists of Gal-yA?yAs(z?y<l), said second semiconductor layer consists of Gal-xA?xAs (y<x?l), and said third semiconductor layer consists of Gal-xA?x'As(y<x'<l).

20. A semiconductor laser device according to claim 19 wherein said thickness t1 is from 0.3 µm to 0.7 µm both inclusive, said thickness t2 is as large as 0.9 µm or more, the thickness t2 of said first semiconductor layer is from 0.05 µm to 0.2µm both inclusive, and x is equal to x'.

21. A semiconductor laser device according to claim 18 wherein said fourth semiconductor layer consists of Gal-pA?pAs (0?p<l), said body consists of Gal-zA?zAs (0?z<l), said first semiconductor layer consists of Gal-yA?y As (p?y<l), said second semiconductor layer consists of Gal-xA?x As (y<x?l), and said third semiconductor layer consists of Gal-x'A?x'As (y<x'?l).

22. A semiconductor laser device according to claim 18 wherein said body consists of Gal-zA?zAs (0?z<l), said first semiconductor layer consists of Gal-yA?yAs (0?y<l), said second semiconductor layer consists of Gal-xA?xAs (y<x?l), said third semiconductor layer consists of Gal-x'A?x'As (y<x'?l), and said fourth semiconductor layer consists of Gal-qA?qAs (y<q?l, q = x).

23. A semiconductor laser device according to claim 1, wherein the first surface of said two opposite plane surfaces faces away from said main surface of said body and the second surface of said two opposite plane surfaces faces towards said main surface of said body, and said plurality of semiconductor layers are further comprised of a fifth semiconductor layer disposed on said second semi-conductor layer and consisting of a material having a complex refractive index different from that of said second semiconductor layer.

24. A semiconductor laser device according to claim 23 wherein said second semiconductor layer has a projection on the far surface of said second semiconductor layer from said first semiconductor layer.

25. A semiconductor laser device according to claim 1, wherein said thick layer has a width being in the range of from 2 µm inclusive to 8 µm exclusive.

26. A semiconductor laser device comprising a semiconductor body of a first conductivity type having a major surface and a plurality of contiguous semiconductor layers disposed on said major surface of said body, said plurality of contiguous semiconductor layers including:
(i) a first semiconductor layer having a laser active region and having two opposite plane surfaces, (ii) a second semiconductor layer disposed on the first surface of said two opposite plane surfaces, consist-ing of a material having a band gap broader than that of said first semiconductor layer, and consisting of a thin region having a thickness not greater than that which will permit a significant amount of the wave energy therein to emerge therefrom during operation of the laser, and a thick region having a thickness larger than that of the said thin region, and (iii) a third semiconductor layer disposed on the second surface of said two opposite plane surfaces and consisting of a material having a band gap broader than that of said first semiconductor layer, (iv) wherein the first surface faces the main surface of said body and the second surface faces away from the main surface of said body, and said semiconductor body consists of a material having a complex refractive index different from that of said second semiconductor layer, and said second semiconductor layer is adjacent to said body and covers said major surface of said body, and (v) wherein said third semiconductor layer has a thickness larger than that of said thin region.

27. A semiconductor laser device according to claim 26 wherein said body consists of GaAs, said first semi-conductor layer consists of Gal-yAlyAs (0<y<0.2), said second semiconductor layer consists of Gal-xAlxAs (0.2<x<0.8), and said third semiconductor layer consists of Gal-x'Alx'As (0.2<x'<0.8).

28. A semiconductor laser device according to claim 27 wherein said thickness of said thin region is from 0.3 µm to 0.6 µm both inclusive, said thickness of said thick region is from 0.9 µm to 1.5 µm, the thickness of said first semiconductor layer is 0.2 µm and x is equal to x'.

29. A solid state diode laser with mode control comprising:
a semiconductor body including a substrate and a plurality of layers, at least one of said layers being an active region layer, said layers being doped to provide a rectifying junction adjacent said active region layer, carriers injected across said rectifying junction upon sufficient forward biasing of said rectifying junction undergoing radiative recombination in said active region layer to generate stimulated coherent radiation including ordered transverse modes, means for restricting the path of pump current to only a portion of said active region layer, a layer of non-uniform thickness adjacent said active region layer and separating said active region layer from semiconductor material having a bandgap lower than the bandgap of the material of said layer of non-uniform thick-ness, the portion of said layer of non-uniform thickness adjacent said pumped portion of said active region layer being thicker than other portions of said layer of non-uniform thickness whereby the energy distribution of the lowest order mode of said ordered transverse modes is primarily within said thicker portion of said layer of non-uniform thickness and the energy distributions of the higher order modes of said ordered transverse modes extend significantly into said material of lower bandgap, said energy distributions of said higher order transverse modes being attenuated significantly by said semiconductor material of lower bandgap and said lowest order transverse mode not being attenuated significantly by the material of said layer of non-uniform thickness whereby lowest order mode operation is achieved, wherein said thicker portion of said layer of non-uniform thickness is at least twice as thick as said other portions of said layer of non-uniform thickness.

30. A solid state diode laser with mode control comprising:
a semiconductor body including a substrate and a plurality of layers, at least one of said layers being an active region layer, said layers being doped to provide a rectifying junction adjacent said active region layer;
resonator means for providing an optical resonant cavity including said active region layer; electrode means on opposite sides of said semiconductor body, carriers injected across said rectifying junction upon sufficient forward biasing of said rectifying junction by a voltage differential applied to said electrode means undergoing radiative recombination in said active region layer to generate stimulated coherent radiation including ordered transverse modes; means for restricting the path of pump current to only a portion of said active region layer; a layer of non-uniform thickness adjacent said active region layer and separating said active region layer from semi-conductor material having a bandgap lower than the bandgap of the material of said layer of non-uniform thickness, the portion of said layer of non-uniform thickness adjacent said pumped portion of said active region layer being at least twice as thick as other portions of said layer of non-uniform thickness adjacent thereto whereby the energy distribution of the lowest order mode of said ordered transverse modes is primarily within said thicker portion of said layer of non-uniform thickness and the energy distributions of the higher order modes of said ordered transverse modes extend significantly into said material of lower bandgap, said energy distributions of said higher order transverse modes being attentuated significantly by said semiconductor material of lower bandgap and said lowest order transverse mode not being attentuated significantly by the material of said layer of non-uniform thickness whereby lowest order mode operation is achieved.