CA1065460A - Buried-heterostructure diode injection laser - Google Patents
Buried-heterostructure diode injection laserInfo
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- CA1065460A CA1065460A CA252,590A CA252590A CA1065460A CA 1065460 A CA1065460 A CA 1065460A CA 252590 A CA252590 A CA 252590A CA 1065460 A CA1065460 A CA 1065460A
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- 238000002347 injection Methods 0.000 title abstract 3
- 239000007924 injection Substances 0.000 title abstract 3
- 239000000758 substrate Substances 0.000 claims abstract description 85
- 239000000463 material Substances 0.000 claims abstract description 83
- 230000006798 recombination Effects 0.000 claims abstract description 8
- 238000005215 recombination Methods 0.000 claims abstract description 8
- 239000010410 layer Substances 0.000 claims description 261
- 239000004065 semiconductor Substances 0.000 claims description 31
- 239000002344 surface layer Substances 0.000 claims description 25
- 238000000034 method Methods 0.000 claims description 20
- 238000004519 manufacturing process Methods 0.000 claims description 15
- 229910001218 Gallium arsenide Inorganic materials 0.000 claims description 14
- 230000015572 biosynthetic process Effects 0.000 claims description 14
- 238000009792 diffusion process Methods 0.000 claims description 13
- 229920002120 photoresistant polymer Polymers 0.000 claims description 8
- 210000001364 upper extremity Anatomy 0.000 claims description 8
- 230000005855 radiation Effects 0.000 claims description 5
- 239000002253 acid Substances 0.000 claims 2
- 230000003287 optical effect Effects 0.000 abstract description 7
- 239000011149 active material Substances 0.000 description 16
- 238000009740 moulding (composite fabrication) Methods 0.000 description 13
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 7
- 230000006911 nucleation Effects 0.000 description 7
- 238000010899 nucleation Methods 0.000 description 7
- 239000011701 zinc Substances 0.000 description 7
- 229910052725 zinc Inorganic materials 0.000 description 7
- 108091006146 Channels Proteins 0.000 description 5
- 239000007791 liquid phase Substances 0.000 description 5
- 210000003141 lower extremity Anatomy 0.000 description 5
- 230000000994 depressogenic effect Effects 0.000 description 4
- 238000005530 etching Methods 0.000 description 4
- LYCAIKOWRPUZTN-UHFFFAOYSA-N Ethylene glycol Chemical compound OCCO LYCAIKOWRPUZTN-UHFFFAOYSA-N 0.000 description 3
- 239000000969 carrier Substances 0.000 description 3
- 210000003414 extremity Anatomy 0.000 description 3
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 2
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 2
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 2
- 229910052782 aluminium Inorganic materials 0.000 description 2
- 239000003153 chemical reaction reagent Substances 0.000 description 2
- 238000011109 contamination Methods 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 230000000873 masking effect Effects 0.000 description 2
- 239000000155 melt Substances 0.000 description 2
- 229920006395 saturated elastomer Polymers 0.000 description 2
- 108010075750 P-Type Calcium Channels Proteins 0.000 description 1
- 229910000147 aluminium phosphate Inorganic materials 0.000 description 1
- 239000003708 ampul Substances 0.000 description 1
- 230000004888 barrier function Effects 0.000 description 1
- 230000000903 blocking effect Effects 0.000 description 1
- 238000003486 chemical etching Methods 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000000407 epitaxy Methods 0.000 description 1
- 238000007654 immersion Methods 0.000 description 1
- 238000002513 implantation Methods 0.000 description 1
- 238000010348 incorporation Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 238000001451 molecular beam epitaxy Methods 0.000 description 1
- 238000005086 pumping Methods 0.000 description 1
- 238000000992 sputter etching Methods 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/24—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a grooved structure, e.g. V-grooved, crescent active layer in groove, VSIS laser
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/223—Buried stripe structure
- H01S5/2232—Buried stripe structure with inner confining structure between the active layer and the lower electrode
- H01S5/2234—Buried stripe structure with inner confining structure between the active layer and the lower electrode having a structured substrate surface
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S5/00—Semiconductor lasers
- H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
- H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
- H01S5/223—Buried stripe structure
- H01S5/2237—Buried stripe structure with a non-planar active layer
Landscapes
- Physics & Mathematics (AREA)
- Geometry (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Semiconductor Lasers (AREA)
- Liquid Deposition Of Substances Of Which Semiconductor Devices Are Composed (AREA)
- Led Devices (AREA)
Abstract
BURIED-HETEROSTRUCTURE
DIODE INJECTION LASER
ABSTRACT OF THE INVENTION
A buried-heterostructure (BH) diode injection laser capable of operating at low room temperature thresholds and in the lowest order TE, TM or TEM modes. The laser has an elongated groove in a substrate with the groove extending through a pump current confining layer with a central portion of the active layer substantially completely within the groove and substantially completely surrounded by light guiding and carrier confining layers of material having a lower index of refraction than the index of refraction of the material of the active layer. The light guiding and carrier confining layer in contact with the substrate has a central depression within the elongated groove and the central portion of the active layer is bowl-shaped and within the depression such that light waves produced by carrier recombination within the central portion of the active layer when the laser is forward biased are guided in the central portion of the active layer such that the laser produces a light beam having reduced width v. height dimensional variations such that the light beam can be used with symmetrical optical elements such as round lenses.
DIODE INJECTION LASER
ABSTRACT OF THE INVENTION
A buried-heterostructure (BH) diode injection laser capable of operating at low room temperature thresholds and in the lowest order TE, TM or TEM modes. The laser has an elongated groove in a substrate with the groove extending through a pump current confining layer with a central portion of the active layer substantially completely within the groove and substantially completely surrounded by light guiding and carrier confining layers of material having a lower index of refraction than the index of refraction of the material of the active layer. The light guiding and carrier confining layer in contact with the substrate has a central depression within the elongated groove and the central portion of the active layer is bowl-shaped and within the depression such that light waves produced by carrier recombination within the central portion of the active layer when the laser is forward biased are guided in the central portion of the active layer such that the laser produces a light beam having reduced width v. height dimensional variations such that the light beam can be used with symmetrical optical elements such as round lenses.
Description
lO~ iO
~3AC~GROUND OF THE INVENTION
In order to lower the threshold current density of a double heterojunction diode laser below the critical limit for CW operation, it is necessary to have the thickness of the active layer on the order of 0.5 mic-ons or less. The usual cleaved and sawed double heterojunction lasers have cross-sectional areas which are typically on the order of 0.2 microns x 20 microns. To make the laser beam produced by heterojunction diode lasers compatible with optical systems utilizing round lenses, or other symmetric optical elements, it is desirable to reduce the large dimensional unbalance between active area width and thickness from the order of several hundred to one to as close to a one to one ratio as possible, with a five to one ratio being satisfactory.
Recently, a double heterojunction diode laser was disclosed in which the width of the filamentary area of the active layer was substantially reduced. The disclosed diode laser is called a buried-heterostructure (BH) injecticn laser since the filamentary active laser region is completely surrounded by ~ region of lower index of refraction material, that is, surrounded by GaAlAs when the active region is GaAs. The typical fabrication process for the disclosed BH
laser is composed of four main steps: (i) a liquid phase epitaxial growth step to produce on a GaAs substrate a first GaAlAs layer, an active layer of GaAs, and a second GaAlAs layer, (ii) a mesa etching step which removes part of the two GaALAs layers and part of the GaAs layer to define the active filamentary area, (iii) a second liquid phase epitaxial growth step to provide a GaALAs layer around the mesa to thereby completely surround the active filamentary area with 1~54~0 material having a lower index of refraction than that of the active filamentary area, that is, burying or surrounding the active filamentary area with GaAlAs when the active region material is GaAs, and finally (iiii) a selective diffusion of a p-type dopent (zinc) to provide a p-type channel from the non-substrate end of the device to the GaAlAs layer adjacent the active filamentary area. The last step requires that an apertured masking layer be formed on the non-substrate end of the device with the aperture in precise alignment with the top of the mesa.
The described process has the readily apparent disadvantage of requiring two separate epitaxial growth steps.
Another disadvantage is that several layers of varying com-position and thickness must be etched, or otherwise removed, and these variances make the etching or removal difficult to control. A further disadvantage is that subsequent to the etching step and prior to thè second epitaxial growth, the exposed surfaces of the mesa can get oxidized because of the problem of aluminum contamination with such contamination creating defects in the active filamentary area. Also, the second epitaxial growth can cause melt back of the regions formed by the first epitaxial growth with the likelihood of further defects in the active region.
As noted, the zinc diffusion of the described process to form a non-rectifying channel to the GaAlAs layer on the non-substrate end of the active filamentary area, must be through a masking aperture which is precisely aligned with the mesa top. Such alignment is difficult to maintain because the top of the mesa is hidden by the second epitaxial growth layers and because tolerances of better than a micron must ~0~54~0 be maintained. In further reference to a zinc diffusion, the diffused region must extend through a relatively thick (5 micron) GaAlAs layer and terminate in a relatively thin (1 micron) GaAl~s layer. If the diffusion is not deep enough a rectifying barrier will be created that will prevent pump-current flow and if the diffusion is too deep the active region ~.5 microns thick) could be penetrated. The zinc diffusion is hard to control due to the different thicknesses ~etween layers, as discussed, and also due to the varying thickness of each layer. Thus, the zinc diffusion must be controlled extremely accurately. Another difficulty with the mesa producing process is that the mesa is a very long, thin plateau which is easily disturbed, that is, chipped or broken off during the subsequent epitaxial growth and zinc diffusion steps.
OBJECTS OF THE INVENTION
It is an object of an aspect of the invention to provide an improved laser.
It is an object of an aspect of the invention to provide an improved buried-heterojunction diode laser.
- It is an object of an aspect of the invention to provide an improved buried heterojunction diode laser capable of operating at low room temperature thresholdsand in the lowest order TE, TM or TEM modes.
It is an object of an aspect of the invention to provide an improved laser having an output beam compatible with symmetrical optical elements.
It is an object of an aspect of the invention to provide an improved method of making a buried-heterojunction diode laser.
It is an object of an aspect of the present 10~i54f~i0 invention to provide an improved method of making a double heterojunction diode laser that requires few process steps.
It is an object of an aspect of the present invention to provide an improved method of making a buried heterojunction diode laser that requires only one epitaxial growth.
It is an object of an aspect of the present invention to provide an improved method of making a buried-heterojunction diode laser wherein diffusion is easily controlled.
SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there is provided a heterojunction diode laser comprising: a sub~
strate body having a surface with an elongated groove formed in said surface, a first layer of a first material deposited on said surface, said first layer having a surface configurat-ion remote from said surface of said substrate with a portion which is concave toward said substrate within a substantial portion of said groove, a second layer of a second material deposited on said first layer, said second material having an index of refraction greater than said first material, said second layer having substantially planar portions on both sides of said groove and a substantially thicker cross-sectional portion substantially within said groove and in contact with said concave portion of said surface of said first layer remote from said substrate, a third layer of a third material deposited on said second layer, said third material having a lower index of refraction than said second material, said second layer having a conductivity type different from either said first layer or said third layer such that a first rectifying junction exists at the interface between said second layer and either said first layer or said third layer, and means located on both sides of said groove - s _ , B q~
10~541~0 formed in said surface of said substrate for restricting the flow of pump current to a path through said groove and through said thicker cross-sectional portion of said second layer when said rectifying junction is forward biased, whereby radiation recombination will occur in said thicker cross-sectional portion of said second layer.
In accordance with another aspect of this invention there is provided a heterojunction diode laser for producing a substantially symmetrically output light beam comprising:
a substrate body of a semiconductor material having a surface with an elongated groove formed in said surface, a first layer of a semiconductor material in integral contact with said surface of said substrate, said first layer having a sub-stantially planar portion on each side of said groove and a portion within said groove with a surface of said portion within said groove inwardly arched toward said substrate body, a second layer of a semiconductor material in integral contact with said first layer, said second layer having a central portion substantially within said groove and substantially planar portions on each side of said central portion, said central portion having a surface conforming to the shape of said inwardly arched surface of said first layer and a sub-stantially flat surface adjacent said inwardly arched surface, :
a third layer of semiconductor material in integral contact with said second layer, said material of said second layer having an index of refraction greater than the index of re-fraction of both the material of said first layer and the material of said third layer to thereby provide a hetero-geneous laser, at least two of said first, second and third layers being of a different conductivity type such that a rectifying junction exists between two of said first, second ~ - 5a -10~;S4~0 and third layers, and means located on both sides of said groove formed in said surface of said substrate for restricting the flow of pump current to a path through said central portion of said second layer when said rectifying junction is forward biased, whereby radiation recombination will occur in said.
central portion of said second layer.
In accordance with another aspect of this invention there is provided a method of making a heterojunction diode laser comprising the steps of: forming in a semiconductor material substrate of one conductivity type a surface layer of second conductivity type, removing both a portion of said : ~ surface layer and an additional part of said substrate material adjacent thereto to provide an elongated groove in said sub-strate, said groove dividing said surface layer into distinct . parts, forming on said grooved surface of said substrate a . first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second . 20 layer of a semiconductor material of higher index of refraction `~ than that of said material of said first layer and of either conductivity type, with formation of said second layer being concluded when the exposed surface of said second layer directly over the concave surface of said first layer is sub-stantially flat, and forming on said second layer a third layer of a semiconductor material of lower index of refraction ; than that of said second layer and of a second conductivity ~ .
` type.
In accordance with another aspect of this invention there is provided the method of making a heterojunction diode laser comprising the steps of: forming in a semiconductor ,~ ~ - 5b -iO~S4~0 material substrate of one conductivity type a non-electrically conducting surface layer, removing both a portion of said surface layer and an additional part of said substrate adjacent thereto to provide an eiongated groove in said substrate, said groove dividing said surface layer into distinct parts, form-ing on said grooved surface of said substrate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second layer of a semiconductor material of higher index of refraction than . that of said material of said first layer and of either con-ductivity type, with formation of said second layer being con-cluded when the portion of said second layer within said groove is substantially bowl-shaped, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said second layer and of a second :~ .
conductivity type.
: In accordance with another aspect of this invention there is provided a method of making a heterojunction diode laser comprising the steps of: forming in a semiconductor `
material substrate of one conductivity type surface layer of the opposite conductivity type, removing both a portion of said surface layer and an additional part of said substrate ~ :
material adjacent thereto to provide an elongated groove in ~ :
said substrate, said groove dividing said surface layer into : :
: distinct parts, forming on said grooved surface of said sub-strate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of said exposed surface of said : first layer in said groove is curved toward said substrate, - 5c -)~
;54~0 said first layer forming secondary rectifying junctions with each of said distinct parts of said surface layer, forming on said first layer a second layer of a material of higher index of refraction than that of said material of said first layer, with formation of said second layer being concluded when the portion of said second layer within said groove has a thicker cross-sectional extent in the central region of said groove and a very shallow cross-sectional extent above the upper extremities of said groove, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said material of said second layer, at least one of said second and third layers being of the opposite conductivity type than that of said first layer such that a primary rectifying junction exists between these layers.
In an embodiment of this invention, a buried-heterojunction (BH) diode laser is provided which is capable of operating at room temperature thresholds and at the lowest order TE, TM, or TEN mode. The BH diode laser is characterized by an active region which is completely surrounded (buried) by material of lower index of refraction and higher band gap.
The filamentary area of the active region is substantially bowl-shaped, that is, thicker in the middle than at the ends and is substantially completely within an elongated channel of the laser substrate. This filamentary area con-figuration and placement is effective to favour emitted light in the central portion of the active region which permits the ; width of the light beam produced to be small (1-2~) and thus, provides a laser having a fairly symmetrical light output beam with lowest order transverse modes in both directions and threshold currents as low as 10 milliamps.
The BH diode laser is made by a process in which ~ - 5d -10654~0 the filamentary area of the active layer is formed substantially within a groove etched in the diode substrate. First, a p-type surface layer is provided in the n-type substrate to pro-vide, subsequently, a current blocking junction. Next, an elongated groove is etched in the substrate to a depth _ 5e - ' ,~ .
, 10~;54~i0 deeper than the surface layer thickness. Following formation of the groove, a layer of a light guiding and carrier confin-ing material, a layer of active region material and a second layer of light guiding and carrier confining material are grown successively on the grooved surface of the substrate by conventional liquid phase epitaxial growth or molecular beam epitaxy, with the active region being doped such that it orms a rectifying junction with one of the light guiding layers~ Due to the shape of the groove, nucleation sites for the first growth layer are more prevalent near the bottom extremities of the groove than at other portions of the groove and thus the first growth layer has a depressed central area which is filled in by the growth of the active material layer to provide an active region having a bowl-shaped filamentary area.
DESCRIPTIO~ OF THE DR~WI~GS
Figure 1 is an end view of a BH laser is accordance with the invention.
Figure 2 is a symbolic representation of light wave energy distribution in the laser of Figure 1.
Figure 3A-3E shows various process steps in the production of the laser of Figure 1.
DESCRIPTIO~ OF THE PREFERRED EMBODIME~T
Referring now to Figure 1, there is shown an end view of a BH diode laser 2 in accordance with the invention.
Laser 2 includes a substrate 4, a diffused layer 6, a first light wave guiding and carrier confining layer 8, an active material layer having central portion 10 and end portions 10', a second light wave guiding and carrier confining layer 12, and a contact-facilitating layer 14. The central portion 10~4~() of the layer 8 and the central portion 10 of the active material layer are within a groove 16 formed in the substrate 4 and extending through the diffused layer 6. Groove 16 is defined by lower extremities 1 and upper extremities 2.
Layer 8 and active material layer 10'-10-10' are of different conductivity type to provide a rectifying junction 20 therebetween. Contacts 16 and 18 are provided in contact with substrate 4 and layer 14, respectively, to provide means for forward biasing rectifying junction 20 at the interface of layer 8 and active material layer 10'-10-10'. Layers 4 and 8 are of different conductivity type than layer 6 such that second and third rectifying junctions 22 and 23 exist at the interface between layers 4 and 6 and 6 and 8, respectively.
When junction 20 is forward biased, junction 22 is also forward biased and junction 23 is back biased. More specifically, substrate 4 can be n-type GaAs, layer 6 can be p-type GaAs, light wave guiding and carrier confining layer 8 can be n-type GaAlAs, active layer 10'-10-10' can be p-type GaAs, light wave guiding and carrier confining layer 12 can be p-type GaAlAs, and contact-facilitating layer 14 can be p-type GaAs. Layer 10'-10-10' can be n-type GaAs in which case a rectifying junction 20' would exist between the layer of active material 10'-10-10' and layer 12, and layer 10'-10-10' can be undoped to provide a rectifying junction somewhere intermediate layers 8 and 12.
As discussed in detail hereinafter, the bowl-shape of the central portion 10 of the active material layer is controlled, in part, by the shape of the first light wave guiding and carrier confining layer 8 which has a central trough or elongated depression 8' which results from the 10t;54f~0 groove 16 and the tendancy for nucleating atoms to attach themselves more readily at places that require less energy for bonding, which, in fact, are those places which have the highest density of neighboring atoms. From Figure 1, it can be seen that the groove angles at lower extremities 1 are about 125 degrees whereas the groove angles at upper extremi-ties 2 are about 235 degrees. Thus, there is a higher density of neighboring atoms at lower extremities 1 than at upper extremities and hence nucleation and incorporation of growth material into the substrate lattice can occur more easily at lower extremities 1 than at upper extremities 2. Other nucleation control factors will be discussed when the method of making the diode 2 is described.
As noted, the central portion 10 of the active material layer has a bowl-shaped cross-section, being deeper in the central region and very shallow adjacent the upper extremities 2. Since the light beam rays generated as a result of the recombination of carriers when junction 20 is forward biased are guided by material having a high index of reraction than adjacen t layers 8 and 12, the filamentary area of the laser, defined symbolically as existing between lines lOa and 10b, is confined to the central portion 10 of the active material layer. Referring to Figure 2, the emitted light energy distribution pattern is illustrated symbolically by light energy distribution patterns 25 and 25' for the center and ends, respectively, of the bowl-shaped, central portion 10 of the active material layer. A
majority of the laser light emitted is concentrated in the middle of central portion 10, that is, within the filamentary area defined by lines 10a and 10bo Since the filamentary lO~S4~iO
area has a maximum depth on the order of 1 micron and a width of about 1-2 microns, the laser output beam produced by laser 2 has an approximately round shape which makes it compatible with external round lenses thereby eliminating the need for complicated lens arrangement which are required with lasers having a mesa structure, as previously described.
The laser 2 of Figure 1 is made by a process which requires only a single epitaxial growth step, which can be a liquid phase epitaxial growth or a molecular beam epitaxial growth. The process is initiated by placing in a diffusion ampoule the substrate 4 which as previously noted can be n-type GaAs and a p-type dopent, such as zinc, and diffusing the dopent into the substrate 4 to form p-type region 6, as shown in Figure 3A. Substrate 4 preferably has a dopent level of 1-5 x 10 /cm and layer 6 preferably has a doping level slightly greater than the doping level of substrate 4. Next a layer of a conventional photoresist, such as the ultraviolet sensitive photoresist Shipley AZ 1350 is deposited over the layer 6 followed by exposure of the resist, as shown in Figure 3B, wherein the dotted portions of resist layer 30 has been exposed making those portions insensitive to a reagent, such æ Shipley developer when the photoresist is Shipley AZ
1350. The unexposed portion of the photoresist, which can be 1-2 microns in width or still smaller, is then removed such as by immersion of the substrate wafer of Figure 3B in a bath of a suitable photoresist developer reagent to provide the structure of Figure 3C. A groove or channel 16 is then formed in the substrate 4 in the area not protected by resist 30 to provide the substrate wafer configuration of Figure 3D. The depth of groove 16 is not critical but it is necessary that _g_ 10~54~;0 the groove 16 extend through the layer 6 in the substrate 4.
For example, layer 6 can be 0.6 microns thick in which case groove 16 would be about 1.5 microns deep. The depth of layer 6 is not critical and the depth of groove 16 is also not critical, for example, layer 6 can have a thickness range from 0.1 to 2 microns or more and groove 16 can be .2 to 2.5 microns or more deep, provided that groove 16 extends through layer 6 in substrate 4. Groove 16 can be formed by conventional chemical etching, ion milling or a combination of these techniques or other known techniques for removing the substrate material. When the substrate material is as previously specifically specified, that is, GaAs, an etch bath composed of 20 parts ethylene glycol, 5 parts phosphoric acid, and 1 part hydrogen peroxide is satisfactory, with the etch bath maintained at room temperature and with the etch bath being stirred during the etching process.
Since p-type material (the material of layer 6) etches faster than n-type material (the material of substrate 4), the etched groove 16 takes on the sloping sidewall con-figuration of Figure 3D, with the sloping walls having a (111) A or GaAs plane atomic surface and the top surface of substrate 4 having a (100) crystalographic orientation. The upper width of the groove 16 is controlled by the opening in the resist 30, and due to undercutting of the resist 30 by the etch bath, the upper width of groove 16 can be slightly wider than the opening of resist 30. The width of the bottom of the groove 16 will depend greatly upon groove depth but is generally on the order of the width of the opening in the resist 30. It is noted again that the angle ~ formed between each of the sidewalls and the bottom of the groove 16, that is, at lower extremities 1 is less than 180 degrees whereas the 10~5~0 angle ~ formed between the sidewalls and the top of the layer 6, that is, at the upper extremities 2 is greater than 180 .
These angles are important for nucleation site purposes.
Following formation of groove 16, the remaining photoresist 30 is removed from the substrate wafer configuration of 3D and the layers 10'-10-10', 12 and 14 are grown consecutively by means of conventional liquid phase epitaxial growth to produce the device of Figure 3E. Other method of layer growth could also be used such as mulecular beam epitaxy. The layer 8 has a depressed region in channel 16 due, in part, to the prevalence of nucleation sites at extremities 1 which cause a greater growth rate at those extremities. Layer 8 could have a thickness tl adjacent to layer 6 of about 1 micron, although it could have a thickness range of 0.5 to several microns. It is important, however, that the growth of layer 8 be terminated before the depressed central area is smoothed over. The portions 10' of the active material layers are substantially planar with a width t2 of about 0.2 microns (although a range of t2 from 0.1 to 1 micron or more is acceptable) and the central portion 10 of the active material layer is bowl-shaped and within the groove 16. When thickness t2 is 0.2 microns, the thickness t3 of the bowl-shaped central portion 10 would be about 0.4-0.8 microns. Once again, the configuration of the bowl-shaped central portion 10 of the active material layer is controlled by nucleation sites on layer 8 with such sites being greater in the depressed central area of layer 8.
Actually, the reasons layers 8 and 10'-10-10' grow as they do is dependent upon other factors besides nucleation sites.
Some of these factors, which are well known to those versed in the semiconductor fabrication art, are depth of groove 10~4~iO
16, width of groove 16, depth of diffusion of layer 6, crystallographic orientation of substrate 4, growth times, growth temperatures, cooling rates, degree of clean wiping melts, and whether the melts are saturated or super saturated.
the Growth of the active material layer 10'-10-10' can be continued until the entire top surface is substantially smooth, however, this additional growth will substantially increase the width of the active material that will provide guiding of the light beam produced by carrier recombination, that is, increase the width of the filamentary area, and thus the width of the output laser beam will be substantially greater than the height of the output laser beam thereby making symmetric optical elements uncompatible.
The GaAlAs layer 12 is typically 1.5 microns thick although the thickness range may be from 1 to 3 microns or more. Layer 14 is typically 1-4 microns thick and substrate 4 is typically 100 microns thick. The concentration of aluminum in layers 8 and 12 is typically 0.4, although a range from 0.15 to 0.7 is acceptable. The doping levels of layers 10, 12 and 14 are typically 1016-1017/cm3, 1017-1018/cm3, and 1017-1019/cm3, respectively. The process is csmpleted by applying electrodes 16 and 18 in a conventional manner. The method is also directly applicable to the fabrication of optical waveguides, modulators, directional couplers and other integrated optical components.
In operation, when the diode 2 is forward biased, that is, by applying a voltage to electrode 18 of approxi-mately 1.4 volts greater than the potential applied to electrode 16, the junction 20 (or the junction 20' when the active material is n-type) is forward biased and electrons are injected from the centra~ port~on of layer 8 into the bowl-shaped central por~ion 10 of the active layer and are confined there by the surrounding heterojunction layers 8 and 12. With sufficient pump current, population inversion is achieved and gain is obtained with light produced by radiative recombination of the carriers in layer 10. This light is guided by layers 8 and 12 due to the lower index of refraction of these layers relative to that of layer 10.
Pump current flow is restricted to a path through channel 16 due to the back bias on junction 23 when the junction 20 is forward biased. This pump current path confine-ment could be achieved by other than diffusion techniques. For example, substrate 4 can be provided with an intrinsic layer instead of layer 6, or protron implantation could be used to create insulating regions in place of layer 6. Also, by selective growth, layer 6 could be grown instead of diffused.
The laser disclosed is capable of operating at low room temperature thresholds (approximately 10 milliamps) and operating in the lowest order TE, TM or TEM modes. What makes this operation possible is that the central portion 10 of the active layer is completely or almost completely surrounded (buried) in the groove 16 by layers 8 and 10 of lower index of refraction materiai. The pump current is substantially restricted to a flow path through the central portion 10 of the active layer within the groove 16 because regions 6 provide p-n junctions 23 which are back biased when diode laser is forward biased for pumping mainly the "buried" portion of the active region of the diode laser.
The major advantage of this structure is that most of the carriers for radiation recombination are injected into the "buried", bowl-shaped region 10 of the active layer and substantially all of the light waves produced can be confined to the bowl-shaped region 10 of the active layer, and more particularly to the filamentary area of the active layer because the effective index of refraction of the active layer decreases as the active layer thickness decreases.
Thus, the light waves produced tend to be focused in the center of the "buried" active layer because that is where the active layer is thickest and thus has the highest index of refraction. Accordingly, by controlling active region geometry, it is possible to provide CW room temperature lasers with lowest order transverse modes in both directions and threshold currents as low as 10 milliamps.
~3AC~GROUND OF THE INVENTION
In order to lower the threshold current density of a double heterojunction diode laser below the critical limit for CW operation, it is necessary to have the thickness of the active layer on the order of 0.5 mic-ons or less. The usual cleaved and sawed double heterojunction lasers have cross-sectional areas which are typically on the order of 0.2 microns x 20 microns. To make the laser beam produced by heterojunction diode lasers compatible with optical systems utilizing round lenses, or other symmetric optical elements, it is desirable to reduce the large dimensional unbalance between active area width and thickness from the order of several hundred to one to as close to a one to one ratio as possible, with a five to one ratio being satisfactory.
Recently, a double heterojunction diode laser was disclosed in which the width of the filamentary area of the active layer was substantially reduced. The disclosed diode laser is called a buried-heterostructure (BH) injecticn laser since the filamentary active laser region is completely surrounded by ~ region of lower index of refraction material, that is, surrounded by GaAlAs when the active region is GaAs. The typical fabrication process for the disclosed BH
laser is composed of four main steps: (i) a liquid phase epitaxial growth step to produce on a GaAs substrate a first GaAlAs layer, an active layer of GaAs, and a second GaAlAs layer, (ii) a mesa etching step which removes part of the two GaALAs layers and part of the GaAs layer to define the active filamentary area, (iii) a second liquid phase epitaxial growth step to provide a GaALAs layer around the mesa to thereby completely surround the active filamentary area with 1~54~0 material having a lower index of refraction than that of the active filamentary area, that is, burying or surrounding the active filamentary area with GaAlAs when the active region material is GaAs, and finally (iiii) a selective diffusion of a p-type dopent (zinc) to provide a p-type channel from the non-substrate end of the device to the GaAlAs layer adjacent the active filamentary area. The last step requires that an apertured masking layer be formed on the non-substrate end of the device with the aperture in precise alignment with the top of the mesa.
The described process has the readily apparent disadvantage of requiring two separate epitaxial growth steps.
Another disadvantage is that several layers of varying com-position and thickness must be etched, or otherwise removed, and these variances make the etching or removal difficult to control. A further disadvantage is that subsequent to the etching step and prior to thè second epitaxial growth, the exposed surfaces of the mesa can get oxidized because of the problem of aluminum contamination with such contamination creating defects in the active filamentary area. Also, the second epitaxial growth can cause melt back of the regions formed by the first epitaxial growth with the likelihood of further defects in the active region.
As noted, the zinc diffusion of the described process to form a non-rectifying channel to the GaAlAs layer on the non-substrate end of the active filamentary area, must be through a masking aperture which is precisely aligned with the mesa top. Such alignment is difficult to maintain because the top of the mesa is hidden by the second epitaxial growth layers and because tolerances of better than a micron must ~0~54~0 be maintained. In further reference to a zinc diffusion, the diffused region must extend through a relatively thick (5 micron) GaAlAs layer and terminate in a relatively thin (1 micron) GaAl~s layer. If the diffusion is not deep enough a rectifying barrier will be created that will prevent pump-current flow and if the diffusion is too deep the active region ~.5 microns thick) could be penetrated. The zinc diffusion is hard to control due to the different thicknesses ~etween layers, as discussed, and also due to the varying thickness of each layer. Thus, the zinc diffusion must be controlled extremely accurately. Another difficulty with the mesa producing process is that the mesa is a very long, thin plateau which is easily disturbed, that is, chipped or broken off during the subsequent epitaxial growth and zinc diffusion steps.
OBJECTS OF THE INVENTION
It is an object of an aspect of the invention to provide an improved laser.
It is an object of an aspect of the invention to provide an improved buried-heterojunction diode laser.
- It is an object of an aspect of the invention to provide an improved buried heterojunction diode laser capable of operating at low room temperature thresholdsand in the lowest order TE, TM or TEM modes.
It is an object of an aspect of the invention to provide an improved laser having an output beam compatible with symmetrical optical elements.
It is an object of an aspect of the invention to provide an improved method of making a buried-heterojunction diode laser.
It is an object of an aspect of the present 10~i54f~i0 invention to provide an improved method of making a double heterojunction diode laser that requires few process steps.
It is an object of an aspect of the present invention to provide an improved method of making a buried heterojunction diode laser that requires only one epitaxial growth.
It is an object of an aspect of the present invention to provide an improved method of making a buried-heterojunction diode laser wherein diffusion is easily controlled.
SUMMARY OF THE INVENTION
In accordance with one aspect of this invention there is provided a heterojunction diode laser comprising: a sub~
strate body having a surface with an elongated groove formed in said surface, a first layer of a first material deposited on said surface, said first layer having a surface configurat-ion remote from said surface of said substrate with a portion which is concave toward said substrate within a substantial portion of said groove, a second layer of a second material deposited on said first layer, said second material having an index of refraction greater than said first material, said second layer having substantially planar portions on both sides of said groove and a substantially thicker cross-sectional portion substantially within said groove and in contact with said concave portion of said surface of said first layer remote from said substrate, a third layer of a third material deposited on said second layer, said third material having a lower index of refraction than said second material, said second layer having a conductivity type different from either said first layer or said third layer such that a first rectifying junction exists at the interface between said second layer and either said first layer or said third layer, and means located on both sides of said groove - s _ , B q~
10~541~0 formed in said surface of said substrate for restricting the flow of pump current to a path through said groove and through said thicker cross-sectional portion of said second layer when said rectifying junction is forward biased, whereby radiation recombination will occur in said thicker cross-sectional portion of said second layer.
In accordance with another aspect of this invention there is provided a heterojunction diode laser for producing a substantially symmetrically output light beam comprising:
a substrate body of a semiconductor material having a surface with an elongated groove formed in said surface, a first layer of a semiconductor material in integral contact with said surface of said substrate, said first layer having a sub-stantially planar portion on each side of said groove and a portion within said groove with a surface of said portion within said groove inwardly arched toward said substrate body, a second layer of a semiconductor material in integral contact with said first layer, said second layer having a central portion substantially within said groove and substantially planar portions on each side of said central portion, said central portion having a surface conforming to the shape of said inwardly arched surface of said first layer and a sub-stantially flat surface adjacent said inwardly arched surface, :
a third layer of semiconductor material in integral contact with said second layer, said material of said second layer having an index of refraction greater than the index of re-fraction of both the material of said first layer and the material of said third layer to thereby provide a hetero-geneous laser, at least two of said first, second and third layers being of a different conductivity type such that a rectifying junction exists between two of said first, second ~ - 5a -10~;S4~0 and third layers, and means located on both sides of said groove formed in said surface of said substrate for restricting the flow of pump current to a path through said central portion of said second layer when said rectifying junction is forward biased, whereby radiation recombination will occur in said.
central portion of said second layer.
In accordance with another aspect of this invention there is provided a method of making a heterojunction diode laser comprising the steps of: forming in a semiconductor material substrate of one conductivity type a surface layer of second conductivity type, removing both a portion of said : ~ surface layer and an additional part of said substrate material adjacent thereto to provide an elongated groove in said sub-strate, said groove dividing said surface layer into distinct . parts, forming on said grooved surface of said substrate a . first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second . 20 layer of a semiconductor material of higher index of refraction `~ than that of said material of said first layer and of either conductivity type, with formation of said second layer being concluded when the exposed surface of said second layer directly over the concave surface of said first layer is sub-stantially flat, and forming on said second layer a third layer of a semiconductor material of lower index of refraction ; than that of said second layer and of a second conductivity ~ .
` type.
In accordance with another aspect of this invention there is provided the method of making a heterojunction diode laser comprising the steps of: forming in a semiconductor ,~ ~ - 5b -iO~S4~0 material substrate of one conductivity type a non-electrically conducting surface layer, removing both a portion of said surface layer and an additional part of said substrate adjacent thereto to provide an eiongated groove in said substrate, said groove dividing said surface layer into distinct parts, form-ing on said grooved surface of said substrate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second layer of a semiconductor material of higher index of refraction than . that of said material of said first layer and of either con-ductivity type, with formation of said second layer being con-cluded when the portion of said second layer within said groove is substantially bowl-shaped, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said second layer and of a second :~ .
conductivity type.
: In accordance with another aspect of this invention there is provided a method of making a heterojunction diode laser comprising the steps of: forming in a semiconductor `
material substrate of one conductivity type surface layer of the opposite conductivity type, removing both a portion of said surface layer and an additional part of said substrate ~ :
material adjacent thereto to provide an elongated groove in ~ :
said substrate, said groove dividing said surface layer into : :
: distinct parts, forming on said grooved surface of said sub-strate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of said exposed surface of said : first layer in said groove is curved toward said substrate, - 5c -)~
;54~0 said first layer forming secondary rectifying junctions with each of said distinct parts of said surface layer, forming on said first layer a second layer of a material of higher index of refraction than that of said material of said first layer, with formation of said second layer being concluded when the portion of said second layer within said groove has a thicker cross-sectional extent in the central region of said groove and a very shallow cross-sectional extent above the upper extremities of said groove, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said material of said second layer, at least one of said second and third layers being of the opposite conductivity type than that of said first layer such that a primary rectifying junction exists between these layers.
In an embodiment of this invention, a buried-heterojunction (BH) diode laser is provided which is capable of operating at room temperature thresholds and at the lowest order TE, TM, or TEN mode. The BH diode laser is characterized by an active region which is completely surrounded (buried) by material of lower index of refraction and higher band gap.
The filamentary area of the active region is substantially bowl-shaped, that is, thicker in the middle than at the ends and is substantially completely within an elongated channel of the laser substrate. This filamentary area con-figuration and placement is effective to favour emitted light in the central portion of the active region which permits the ; width of the light beam produced to be small (1-2~) and thus, provides a laser having a fairly symmetrical light output beam with lowest order transverse modes in both directions and threshold currents as low as 10 milliamps.
The BH diode laser is made by a process in which ~ - 5d -10654~0 the filamentary area of the active layer is formed substantially within a groove etched in the diode substrate. First, a p-type surface layer is provided in the n-type substrate to pro-vide, subsequently, a current blocking junction. Next, an elongated groove is etched in the substrate to a depth _ 5e - ' ,~ .
, 10~;54~i0 deeper than the surface layer thickness. Following formation of the groove, a layer of a light guiding and carrier confin-ing material, a layer of active region material and a second layer of light guiding and carrier confining material are grown successively on the grooved surface of the substrate by conventional liquid phase epitaxial growth or molecular beam epitaxy, with the active region being doped such that it orms a rectifying junction with one of the light guiding layers~ Due to the shape of the groove, nucleation sites for the first growth layer are more prevalent near the bottom extremities of the groove than at other portions of the groove and thus the first growth layer has a depressed central area which is filled in by the growth of the active material layer to provide an active region having a bowl-shaped filamentary area.
DESCRIPTIO~ OF THE DR~WI~GS
Figure 1 is an end view of a BH laser is accordance with the invention.
Figure 2 is a symbolic representation of light wave energy distribution in the laser of Figure 1.
Figure 3A-3E shows various process steps in the production of the laser of Figure 1.
DESCRIPTIO~ OF THE PREFERRED EMBODIME~T
Referring now to Figure 1, there is shown an end view of a BH diode laser 2 in accordance with the invention.
Laser 2 includes a substrate 4, a diffused layer 6, a first light wave guiding and carrier confining layer 8, an active material layer having central portion 10 and end portions 10', a second light wave guiding and carrier confining layer 12, and a contact-facilitating layer 14. The central portion 10~4~() of the layer 8 and the central portion 10 of the active material layer are within a groove 16 formed in the substrate 4 and extending through the diffused layer 6. Groove 16 is defined by lower extremities 1 and upper extremities 2.
Layer 8 and active material layer 10'-10-10' are of different conductivity type to provide a rectifying junction 20 therebetween. Contacts 16 and 18 are provided in contact with substrate 4 and layer 14, respectively, to provide means for forward biasing rectifying junction 20 at the interface of layer 8 and active material layer 10'-10-10'. Layers 4 and 8 are of different conductivity type than layer 6 such that second and third rectifying junctions 22 and 23 exist at the interface between layers 4 and 6 and 6 and 8, respectively.
When junction 20 is forward biased, junction 22 is also forward biased and junction 23 is back biased. More specifically, substrate 4 can be n-type GaAs, layer 6 can be p-type GaAs, light wave guiding and carrier confining layer 8 can be n-type GaAlAs, active layer 10'-10-10' can be p-type GaAs, light wave guiding and carrier confining layer 12 can be p-type GaAlAs, and contact-facilitating layer 14 can be p-type GaAs. Layer 10'-10-10' can be n-type GaAs in which case a rectifying junction 20' would exist between the layer of active material 10'-10-10' and layer 12, and layer 10'-10-10' can be undoped to provide a rectifying junction somewhere intermediate layers 8 and 12.
As discussed in detail hereinafter, the bowl-shape of the central portion 10 of the active material layer is controlled, in part, by the shape of the first light wave guiding and carrier confining layer 8 which has a central trough or elongated depression 8' which results from the 10t;54f~0 groove 16 and the tendancy for nucleating atoms to attach themselves more readily at places that require less energy for bonding, which, in fact, are those places which have the highest density of neighboring atoms. From Figure 1, it can be seen that the groove angles at lower extremities 1 are about 125 degrees whereas the groove angles at upper extremi-ties 2 are about 235 degrees. Thus, there is a higher density of neighboring atoms at lower extremities 1 than at upper extremities and hence nucleation and incorporation of growth material into the substrate lattice can occur more easily at lower extremities 1 than at upper extremities 2. Other nucleation control factors will be discussed when the method of making the diode 2 is described.
As noted, the central portion 10 of the active material layer has a bowl-shaped cross-section, being deeper in the central region and very shallow adjacent the upper extremities 2. Since the light beam rays generated as a result of the recombination of carriers when junction 20 is forward biased are guided by material having a high index of reraction than adjacen t layers 8 and 12, the filamentary area of the laser, defined symbolically as existing between lines lOa and 10b, is confined to the central portion 10 of the active material layer. Referring to Figure 2, the emitted light energy distribution pattern is illustrated symbolically by light energy distribution patterns 25 and 25' for the center and ends, respectively, of the bowl-shaped, central portion 10 of the active material layer. A
majority of the laser light emitted is concentrated in the middle of central portion 10, that is, within the filamentary area defined by lines 10a and 10bo Since the filamentary lO~S4~iO
area has a maximum depth on the order of 1 micron and a width of about 1-2 microns, the laser output beam produced by laser 2 has an approximately round shape which makes it compatible with external round lenses thereby eliminating the need for complicated lens arrangement which are required with lasers having a mesa structure, as previously described.
The laser 2 of Figure 1 is made by a process which requires only a single epitaxial growth step, which can be a liquid phase epitaxial growth or a molecular beam epitaxial growth. The process is initiated by placing in a diffusion ampoule the substrate 4 which as previously noted can be n-type GaAs and a p-type dopent, such as zinc, and diffusing the dopent into the substrate 4 to form p-type region 6, as shown in Figure 3A. Substrate 4 preferably has a dopent level of 1-5 x 10 /cm and layer 6 preferably has a doping level slightly greater than the doping level of substrate 4. Next a layer of a conventional photoresist, such as the ultraviolet sensitive photoresist Shipley AZ 1350 is deposited over the layer 6 followed by exposure of the resist, as shown in Figure 3B, wherein the dotted portions of resist layer 30 has been exposed making those portions insensitive to a reagent, such æ Shipley developer when the photoresist is Shipley AZ
1350. The unexposed portion of the photoresist, which can be 1-2 microns in width or still smaller, is then removed such as by immersion of the substrate wafer of Figure 3B in a bath of a suitable photoresist developer reagent to provide the structure of Figure 3C. A groove or channel 16 is then formed in the substrate 4 in the area not protected by resist 30 to provide the substrate wafer configuration of Figure 3D. The depth of groove 16 is not critical but it is necessary that _g_ 10~54~;0 the groove 16 extend through the layer 6 in the substrate 4.
For example, layer 6 can be 0.6 microns thick in which case groove 16 would be about 1.5 microns deep. The depth of layer 6 is not critical and the depth of groove 16 is also not critical, for example, layer 6 can have a thickness range from 0.1 to 2 microns or more and groove 16 can be .2 to 2.5 microns or more deep, provided that groove 16 extends through layer 6 in substrate 4. Groove 16 can be formed by conventional chemical etching, ion milling or a combination of these techniques or other known techniques for removing the substrate material. When the substrate material is as previously specifically specified, that is, GaAs, an etch bath composed of 20 parts ethylene glycol, 5 parts phosphoric acid, and 1 part hydrogen peroxide is satisfactory, with the etch bath maintained at room temperature and with the etch bath being stirred during the etching process.
Since p-type material (the material of layer 6) etches faster than n-type material (the material of substrate 4), the etched groove 16 takes on the sloping sidewall con-figuration of Figure 3D, with the sloping walls having a (111) A or GaAs plane atomic surface and the top surface of substrate 4 having a (100) crystalographic orientation. The upper width of the groove 16 is controlled by the opening in the resist 30, and due to undercutting of the resist 30 by the etch bath, the upper width of groove 16 can be slightly wider than the opening of resist 30. The width of the bottom of the groove 16 will depend greatly upon groove depth but is generally on the order of the width of the opening in the resist 30. It is noted again that the angle ~ formed between each of the sidewalls and the bottom of the groove 16, that is, at lower extremities 1 is less than 180 degrees whereas the 10~5~0 angle ~ formed between the sidewalls and the top of the layer 6, that is, at the upper extremities 2 is greater than 180 .
These angles are important for nucleation site purposes.
Following formation of groove 16, the remaining photoresist 30 is removed from the substrate wafer configuration of 3D and the layers 10'-10-10', 12 and 14 are grown consecutively by means of conventional liquid phase epitaxial growth to produce the device of Figure 3E. Other method of layer growth could also be used such as mulecular beam epitaxy. The layer 8 has a depressed region in channel 16 due, in part, to the prevalence of nucleation sites at extremities 1 which cause a greater growth rate at those extremities. Layer 8 could have a thickness tl adjacent to layer 6 of about 1 micron, although it could have a thickness range of 0.5 to several microns. It is important, however, that the growth of layer 8 be terminated before the depressed central area is smoothed over. The portions 10' of the active material layers are substantially planar with a width t2 of about 0.2 microns (although a range of t2 from 0.1 to 1 micron or more is acceptable) and the central portion 10 of the active material layer is bowl-shaped and within the groove 16. When thickness t2 is 0.2 microns, the thickness t3 of the bowl-shaped central portion 10 would be about 0.4-0.8 microns. Once again, the configuration of the bowl-shaped central portion 10 of the active material layer is controlled by nucleation sites on layer 8 with such sites being greater in the depressed central area of layer 8.
Actually, the reasons layers 8 and 10'-10-10' grow as they do is dependent upon other factors besides nucleation sites.
Some of these factors, which are well known to those versed in the semiconductor fabrication art, are depth of groove 10~4~iO
16, width of groove 16, depth of diffusion of layer 6, crystallographic orientation of substrate 4, growth times, growth temperatures, cooling rates, degree of clean wiping melts, and whether the melts are saturated or super saturated.
the Growth of the active material layer 10'-10-10' can be continued until the entire top surface is substantially smooth, however, this additional growth will substantially increase the width of the active material that will provide guiding of the light beam produced by carrier recombination, that is, increase the width of the filamentary area, and thus the width of the output laser beam will be substantially greater than the height of the output laser beam thereby making symmetric optical elements uncompatible.
The GaAlAs layer 12 is typically 1.5 microns thick although the thickness range may be from 1 to 3 microns or more. Layer 14 is typically 1-4 microns thick and substrate 4 is typically 100 microns thick. The concentration of aluminum in layers 8 and 12 is typically 0.4, although a range from 0.15 to 0.7 is acceptable. The doping levels of layers 10, 12 and 14 are typically 1016-1017/cm3, 1017-1018/cm3, and 1017-1019/cm3, respectively. The process is csmpleted by applying electrodes 16 and 18 in a conventional manner. The method is also directly applicable to the fabrication of optical waveguides, modulators, directional couplers and other integrated optical components.
In operation, when the diode 2 is forward biased, that is, by applying a voltage to electrode 18 of approxi-mately 1.4 volts greater than the potential applied to electrode 16, the junction 20 (or the junction 20' when the active material is n-type) is forward biased and electrons are injected from the centra~ port~on of layer 8 into the bowl-shaped central por~ion 10 of the active layer and are confined there by the surrounding heterojunction layers 8 and 12. With sufficient pump current, population inversion is achieved and gain is obtained with light produced by radiative recombination of the carriers in layer 10. This light is guided by layers 8 and 12 due to the lower index of refraction of these layers relative to that of layer 10.
Pump current flow is restricted to a path through channel 16 due to the back bias on junction 23 when the junction 20 is forward biased. This pump current path confine-ment could be achieved by other than diffusion techniques. For example, substrate 4 can be provided with an intrinsic layer instead of layer 6, or protron implantation could be used to create insulating regions in place of layer 6. Also, by selective growth, layer 6 could be grown instead of diffused.
The laser disclosed is capable of operating at low room temperature thresholds (approximately 10 milliamps) and operating in the lowest order TE, TM or TEM modes. What makes this operation possible is that the central portion 10 of the active layer is completely or almost completely surrounded (buried) in the groove 16 by layers 8 and 10 of lower index of refraction materiai. The pump current is substantially restricted to a flow path through the central portion 10 of the active layer within the groove 16 because regions 6 provide p-n junctions 23 which are back biased when diode laser is forward biased for pumping mainly the "buried" portion of the active region of the diode laser.
The major advantage of this structure is that most of the carriers for radiation recombination are injected into the "buried", bowl-shaped region 10 of the active layer and substantially all of the light waves produced can be confined to the bowl-shaped region 10 of the active layer, and more particularly to the filamentary area of the active layer because the effective index of refraction of the active layer decreases as the active layer thickness decreases.
Thus, the light waves produced tend to be focused in the center of the "buried" active layer because that is where the active layer is thickest and thus has the highest index of refraction. Accordingly, by controlling active region geometry, it is possible to provide CW room temperature lasers with lowest order transverse modes in both directions and threshold currents as low as 10 milliamps.
Claims (19)
1. A heterojunction diode laser comprising:
a substrate body having a surface with an elongated groove formed in said surface, a first layer of a first material deposited on said surface, said first layer having a surface configuration remote from said surface of said substrate with a portion which is concave toward said substrate within a substantial portion of said groove, a second layer of a second material deposited on said first layer, said second material having an index of refraction greater than said first material, said second layer having substantially planar portions on both sides of said groove and a substantially thicker cross-sectional portion sub-stantially within said groove and in contact with said concave portion of said surface of said first layer remote from said substrate, a third layer of a third material deposited on said second layer, said third material having a lower index of refraction than said second material, said second layer having a conductivity type different from either said first layer or said third layer such that a first rectifying junction exists at the interface between said second layer and either said first layer or said third layer, and means located on both sides of said groove formed in said surface of said substrate for restricting the flow of pump current to a path through said groove and through said thicker cross-sectional portion of said second layer when said rectifying junction is forward biased, whereby radiation re-combination will occur in said thicker cross-sectional portion of said second layer.
a substrate body having a surface with an elongated groove formed in said surface, a first layer of a first material deposited on said surface, said first layer having a surface configuration remote from said surface of said substrate with a portion which is concave toward said substrate within a substantial portion of said groove, a second layer of a second material deposited on said first layer, said second material having an index of refraction greater than said first material, said second layer having substantially planar portions on both sides of said groove and a substantially thicker cross-sectional portion sub-stantially within said groove and in contact with said concave portion of said surface of said first layer remote from said substrate, a third layer of a third material deposited on said second layer, said third material having a lower index of refraction than said second material, said second layer having a conductivity type different from either said first layer or said third layer such that a first rectifying junction exists at the interface between said second layer and either said first layer or said third layer, and means located on both sides of said groove formed in said surface of said substrate for restricting the flow of pump current to a path through said groove and through said thicker cross-sectional portion of said second layer when said rectifying junction is forward biased, whereby radiation re-combination will occur in said thicker cross-sectional portion of said second layer.
2. The laser of Claim 1 wherein said first and third layers are of the same material.
3. The laser of Claim 2 wherein said second layer is GaAs, and said first and third layers are GaAlAs.
4. The laser of Claim 2 wherein said first layer in n-type and said second and third layers are p-type such that said rectifying junction exists between said first and second layers.
5. The laser of Claim 1 wherein said means for restrict-ing the path of pump current flow are additional rectifying junctions disposed on each side of said groove and formed at the interface between said substrate and said first layer, said additional junctions being back biased when said first rectifying junction is forward biased.
6. The laser of claim 5 wherein said additional rectifying junctions are formed by a diffused layer within said substrate at said surface of said substrate, said diffused layer having a conductivity type different than the conductivity type of said first layer.
7. The laser of Claim 1 wherein said means for restricting the path of pump current flow are separate non-conducting regions located on each side of said groove in said surface of said substrate.
8. The laser of claim 1 wherein said groove in said surface of said substrate has sloping sidewalls.
9. The laser of claim 1 wherein the bottom of said groove makes an angle of less than 180° with the sloping sidewalls of said groove.
10. The laser of claim 1 wherein said second layer has substantially planar portions on each side of said bowl-shaped portion with said planar portions being coupled to said bowl-shaped portion with additional portions of said second layer which are thinner than said planar portions of said second layer.
11. The laser of claim 1 wherein the width of said bowl-shaped portion of said second layer is on the same order of magnitude as the height of said bowl-shaped portion.
12. A heterojunction diode laser for producing a substantially symmetrically output light beam comprising:
a substrate body of a semiconductor material having a surface with an elongated groove formed in said surface, a first layer of a semiconductor material in integral contact with said surface of said substrate, said first layer having a substantially planar portion on each side of said groove and a portion within said groove with a surface of said portion within said groove inwardly arched toward said substrate body, a second layer of a semiconductor material in integral contact with said first layer, said second layer having a central portion substantially within said groove and substantially planar portions on each side of said central portion, said central portion having a surface conforming to the shape of said inwardly arched surface of said first layer and a substantially flat surface adjacent said inwardly arched surface, a third layer of semiconductor material in integral contact with said second layer, said material of said second layer having an index of refraction greater than the index of refraction of both the material of said first layer and the material of said third layer to thereby provide a heterogeneous laser, at least two of said first, second and third layers being of a different conductivity type such that a rectifying junction exists between two of said first, second and third layers, and means located on both sides of said groove formed in said surface of said substrate for restricting the flow of pump current to a path through said central portion of said second layer when said rectifying junction is forward biased, whereby radiation recombination will occur in said central portion of said second layer.
a substrate body of a semiconductor material having a surface with an elongated groove formed in said surface, a first layer of a semiconductor material in integral contact with said surface of said substrate, said first layer having a substantially planar portion on each side of said groove and a portion within said groove with a surface of said portion within said groove inwardly arched toward said substrate body, a second layer of a semiconductor material in integral contact with said first layer, said second layer having a central portion substantially within said groove and substantially planar portions on each side of said central portion, said central portion having a surface conforming to the shape of said inwardly arched surface of said first layer and a substantially flat surface adjacent said inwardly arched surface, a third layer of semiconductor material in integral contact with said second layer, said material of said second layer having an index of refraction greater than the index of refraction of both the material of said first layer and the material of said third layer to thereby provide a heterogeneous laser, at least two of said first, second and third layers being of a different conductivity type such that a rectifying junction exists between two of said first, second and third layers, and means located on both sides of said groove formed in said surface of said substrate for restricting the flow of pump current to a path through said central portion of said second layer when said rectifying junction is forward biased, whereby radiation recombination will occur in said central portion of said second layer.
13. A method of making a heterojunction diode laser com-prising the steps of:
forming in a semiconductor material substrate of one conductivity type a surface layer of second conductivity type, removing both a portion of said surface layer and an additional part of said substrate material adjacent thereto to provide an elongated groove in said substrate, said groove dividing said surface layer into distinct parts, forming on said grooved surface of said substrate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second layer of a semiconductor material of higher index of refraction than that of said material of said first layer and of either con-ductivity type, with formation of said second layer being concluded when the exposed surface of said second layer directly over the concave surface of said first layer is sub-stantially flat, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said second layer and of a second conductivity type.
forming in a semiconductor material substrate of one conductivity type a surface layer of second conductivity type, removing both a portion of said surface layer and an additional part of said substrate material adjacent thereto to provide an elongated groove in said substrate, said groove dividing said surface layer into distinct parts, forming on said grooved surface of said substrate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second layer of a semiconductor material of higher index of refraction than that of said material of said first layer and of either con-ductivity type, with formation of said second layer being concluded when the exposed surface of said second layer directly over the concave surface of said first layer is sub-stantially flat, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said second layer and of a second conductivity type.
14. The method of Claim 13 wherein said surface layer is formed by diffusion of material of said opposite conductivity type, and said first, second and third layers are formed by epitaxial growth.
15. The method of Claim 13 wherein said groove is formed by providing a photoresist over selected portions of said surface layer of said substrate and then immersing said sub-strate in an acid bath.
16. The method of making a heterojunction diode laser comprising the steps of:
forming in a semiconductor material substrate of one conductivity type a non-electrically conducting surface layer, removing both a portion of said surface layer and an additional part of said substrate adjacent thereto to provide an elongated groove in said substrate, said groove dividing said surface layer into distinct parts, forming on said grooved surface of said substrate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second layer of a semiconductor material of higher index of refraction than that of said material of said first layer and of either con-ductivity type, with formation of said second layer being con-cluded when the portion of said second layer within said groove is substantially bowl-shaped, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said second layer and of a second conductivity type.
forming in a semiconductor material substrate of one conductivity type a non-electrically conducting surface layer, removing both a portion of said surface layer and an additional part of said substrate adjacent thereto to provide an elongated groove in said substrate, said groove dividing said surface layer into distinct parts, forming on said grooved surface of said substrate a first layer of semiconductor material of said one conductivity type, with formation of said first layer being concluded while the portion of the exposed surface of said first layer in said groove is concave, forming on said first layer a second layer of a semiconductor material of higher index of refraction than that of said material of said first layer and of either con-ductivity type, with formation of said second layer being con-cluded when the portion of said second layer within said groove is substantially bowl-shaped, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said second layer and of a second conductivity type.
17. The method of Claim 16 wherein said first, second and third layers are formed by epitaxial growth.
18. The method of Claim 16 wherein said groove is formed by providing a photoresist over selected portions of said surface layer of said substrate and then immersing said substrate in an acid bath.
19. A method of making a heterojunction diode laser comprising the steps of:
forming in a semiconductor material substrate of one conductivity type surface layer of the opposite conduct-ivity type, removing both a portion of said surface layer and an additional part of said substrate material adjacent thereto to provide an elongated groove in said substrate, said groove dividing said surface layer into distinct parts, forming on said grooved surface of said substrate a first layer of semiconductor material of said one conduct-ivity type, with formation of said first layer being con-cluded while the portion of said exposed surface of said first layer in said groove is curved toward said substrate, said first layer forming secondary rectifying junctions with each of said distinct parts of said surface layer, forming on said first layer a second layer of a material of higher index of refraction than that of said material of said first layer, with formation of said second layer being concluded when the portion of said second layer within said groove has a thicker cross-sectional extent in the central region of said groove and a very shallow cross-sectional extent above the upper extremities of said groove, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said material of said second layer, at least one of said second and third layers being of the opposite conductivity type than that of said first layer such that a primary rectifying junction exists between these layers.
forming in a semiconductor material substrate of one conductivity type surface layer of the opposite conduct-ivity type, removing both a portion of said surface layer and an additional part of said substrate material adjacent thereto to provide an elongated groove in said substrate, said groove dividing said surface layer into distinct parts, forming on said grooved surface of said substrate a first layer of semiconductor material of said one conduct-ivity type, with formation of said first layer being con-cluded while the portion of said exposed surface of said first layer in said groove is curved toward said substrate, said first layer forming secondary rectifying junctions with each of said distinct parts of said surface layer, forming on said first layer a second layer of a material of higher index of refraction than that of said material of said first layer, with formation of said second layer being concluded when the portion of said second layer within said groove has a thicker cross-sectional extent in the central region of said groove and a very shallow cross-sectional extent above the upper extremities of said groove, and forming on said second layer a third layer of a semiconductor material of lower index of refraction than that of said material of said second layer, at least one of said second and third layers being of the opposite conductivity type than that of said first layer such that a primary rectifying junction exists between these layers.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/589,277 US4033796A (en) | 1975-06-23 | 1975-06-23 | Method of making buried-heterostructure diode injection laser |
| US05/589,120 US3978428A (en) | 1975-06-23 | 1975-06-23 | Buried-heterostructure diode injection laser |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1065460A true CA1065460A (en) | 1979-10-30 |
Family
ID=27080453
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA252,590A Expired CA1065460A (en) | 1975-06-23 | 1976-05-14 | Buried-heterostructure diode injection laser |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS523392A (en) |
| AU (1) | AU501061B2 (en) |
| CA (1) | CA1065460A (en) |
| DE (1) | DE2626775C2 (en) |
| FR (1) | FR2315785A1 (en) |
| GB (1) | GB1546729A (en) |
| NL (1) | NL7606798A (en) |
Families Citing this family (19)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5234686A (en) * | 1975-09-10 | 1977-03-16 | Sumitomo Electric Ind Ltd | Double hetero junction type semiconductor laser element and its manufa cturing process |
| GB1530323A (en) * | 1975-12-22 | 1978-10-25 | Standard Telephones Cables Ltd | Semiconductor waveguide structures |
| US4326176A (en) * | 1976-04-16 | 1982-04-20 | Hitachi, Ltd. | Semiconductor laser device |
| DE2757470A1 (en) * | 1977-12-22 | 1979-07-05 | Siemens Ag | METHOD OF MANUFACTURING A SEMICONDUCTOR ARRANGEMENT |
| CA1127282A (en) * | 1978-05-22 | 1982-07-06 | Takashi Sugino | Semiconductor laser and method of making the same |
| GB2046983B (en) * | 1979-01-18 | 1983-03-16 | Nippon Electric Co | Semiconductor lasers |
| JPS55158691A (en) * | 1979-05-30 | 1980-12-10 | Sumitomo Electric Ind Ltd | Semiconductor light emitting device manufacture thereof |
| JPS56161688A (en) * | 1980-05-16 | 1981-12-12 | Matsushita Electric Ind Co Ltd | Semiconductor laser |
| JPS5910039Y2 (en) * | 1980-03-18 | 1984-03-29 | 大日本印刷株式会社 | Product display box |
| JPS5723292A (en) * | 1980-07-16 | 1982-02-06 | Sony Corp | Semiconductor laser device and manufacture thereof |
| JPS5763885A (en) * | 1980-10-06 | 1982-04-17 | Mitsubishi Electric Corp | Semiconductor laser device |
| JPS5791574A (en) * | 1980-11-28 | 1982-06-07 | Nec Corp | Light emitting diode |
| JPS5792880A (en) * | 1980-12-02 | 1982-06-09 | Toshiba Corp | Light emitting diode |
| JPS57162484A (en) * | 1981-03-31 | 1982-10-06 | Fujitsu Ltd | Semiconductor luminous device |
| JPS58225683A (en) * | 1982-06-22 | 1983-12-27 | Mitsubishi Electric Corp | Semiconductor laser |
| GB2123604B (en) * | 1982-06-29 | 1985-12-18 | Standard Telephones Cables Ltd | Injection laser manufacture |
| JPS599990A (en) * | 1982-07-07 | 1984-01-19 | Mitsubishi Electric Corp | Manufacturing method of semiconductor laser |
| JPS6042890A (en) * | 1983-08-18 | 1985-03-07 | Mitsubishi Electric Corp | Surface-emitting semiconductor laser and its manufacturing method |
| JP2553580B2 (en) * | 1987-08-19 | 1996-11-13 | 三菱電機株式会社 | Semiconductor laser device |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2859178A (en) * | 1955-12-19 | 1958-11-04 | Exxon Research Engineering Co | Method of lubricating bearings |
| GB1273284A (en) * | 1970-10-13 | 1972-05-03 | Standard Telephones Cables Ltd | Improvements in or relating to injection lasers |
| IT963303B (en) * | 1971-07-29 | 1974-01-10 | Licentia Gmbh | SEMICONDUCTOR LASER |
| JPS51114887A (en) * | 1975-04-01 | 1976-10-08 | Nippon Telegr & Teleph Corp <Ntt> | Semiconductor device |
-
1976
- 1976-05-14 CA CA252,590A patent/CA1065460A/en not_active Expired
- 1976-05-26 FR FR7615955A patent/FR2315785A1/en active Granted
- 1976-06-15 DE DE2626775A patent/DE2626775C2/en not_active Expired
- 1976-06-16 JP JP7091576A patent/JPS523392A/en active Granted
- 1976-06-18 GB GB25356/76A patent/GB1546729A/en not_active Expired
- 1976-06-22 NL NL7606798A patent/NL7606798A/en not_active Application Discontinuation
- 1976-06-23 AU AU15210/76A patent/AU501061B2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE2626775A1 (en) | 1976-12-30 |
| FR2315785A1 (en) | 1977-01-21 |
| DE2626775C2 (en) | 1983-04-21 |
| JPS5653237B2 (en) | 1981-12-17 |
| JPS523392A (en) | 1977-01-11 |
| NL7606798A (en) | 1976-12-27 |
| GB1546729A (en) | 1979-05-31 |
| FR2315785B1 (en) | 1983-03-25 |
| AU501061B2 (en) | 1979-06-07 |
| AU1521076A (en) | 1978-01-05 |
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