JPH04111488A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To materialize a minute closed resonator semiconductor laser with low threshold by so constituting it as to shut in an active layer with a metallic reflecting film, which is made through a semiconductor layer of high resistance and doubles as an electrode, a Bragg reflecting film, which is positioned below the active layer and in which semiconductors are distributed, and a Bragg reflecting film, which is positioned above the active layer and in which dielectrics are distributed. CONSTITUTION:An active layer 4 is shut in by a metallic reflecting film 10, which is made through the semiconductor layer 8 of high resistance at the side and doubles as an electrode, a Bragg reflecting film 2, which is positioned below the active layer and in which semiconductors are distributed, and a Bragg reflecting film 7, which is positioned on the active layer and in which dielectrics are distributed. Those are arranged in a closed resonator, where only a very small number of scattering modes exist in the gain band. Therefore, it can produce the naturally emitted light in scattering mode, and the naturally emitted light coefficient can be enlarged extremely larger than that of the usual semiconductor laser. Moreover, the semiconductor layer 8 of high resistance not only breaks the current path not via the active layer 4 and lets leaked current flow, but also protects the side of the active layer 4 and stops the nonemitted light recoupling at interface and by these synergetic effects, a minute closed resonator where threshold is extremely low can be gotten.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a semiconductor laser, and particularly to a micro-closed-oscillator semiconductor laser with a low threshold value.

(Prior art) Lasers are light waves with a single frequency, and due to their straight propagation, good coherence, and narrow spectrum width, they are used as light sources in various fields such as optical information processing and optical z1 measurement. The use of is attracting attention.

In conventional semiconductor lasers, the resonator length is sufficiently large compared to the oscillation wavelength, and the sides of the resonator are open, so spontaneously emitted light is coupled with continuously distributed modes in free space and almost has a gain band. Shows broad luminescence over a wide area.

In this case, most of the spontaneously emitted light is emitted from the resonator in all directions, resulting in energy loss, which is one of the causes of the threshold value.

On the other hand, in a small closed resonator where the size of the resonator is a fraction of the wavelength to several times the emission wavelength, a situation in which there are only a very small number of discrete eigenmodes in the gain band is realized. be able to.

Under this situation, light is slowly emitted into a small number of discrete modes, whether it is spontaneous emission or stimulated emission, so even spontaneous emission light becomes monochromatic light with a frequency width that corresponds to the Q value of the closed resonator. . Furthermore, if the radiative recombination lifetime is sufficiently shorter than the non-radiative recombination lifetime, there will be no significant difference in luminous efficiency to the oscillation mode with respect to the injection energy between spontaneous emission and stimulated emission, resulting in a semiconductor laser with a very low threshold. be able to. These effects become larger as the number of modes decreases,
In particular, when there is one mode, the ratio of spontaneous emission light coupled to oscillation light (spontaneous emission light coefficient) is 1 in principle, so thresholdless operation is possible. Naturally, it can be used in the same way as a laser even in situations where spontaneous emission is dominant (hereinafter referred to as a microresonator laser).

Such a semiconductor laser can be easily analyzed by a simple analysis of its device characteristics (Tetsube Kobayashi et al., Proceedings of the Japan Society of Applied Physics 29z-B-88 (1)
982) Ura, Yokoyama and S, D, Bro
nson, J, Appl, Phys, BB, p4801
(1989)), however, no examples have actually been created, and no specific device structure has been proposed.

A vertical cavity surface emitting laser is currently considered to be the most structurally suitable for realizing this microresonator.

One such method is to form a resonator in a direction perpendicular to the substrate using a dielectric and semiconductor multilayer film.

For example, as shown in FIG. 8(a), an n-type GaAs substrate 6
1, Alo, + Gao, 9^S layer and ^10.
7 cao, 3 reflective film 62 consisting of a λ/4 multilayer film with As layer, n-type ^1゜<cao6^S cladding layer 63,
p-type GaAs active layer 64, p-type Alo4Gao, b^
S cladding layer 65, p type^1゜Gao, As through TU
It consists of a light emitting section formed by sequentially laminating a B facilitation layer 66, a dielectric 67 made of a multilayer film of an amorphous silicon layer and a silicon oxide layer, and an electrode feeding section formed around the light emitting section. There is. In addition, p type ^to, a Gao
The upper electrode 6 formed on the 6^S layer via the p-type GaAs contact layer is made of titanium-gold (TI-^U), and the lower electrode 70 is formed on the back surface of the n-type GaAs substrate 61. is gold germanium (Au-Ge)/nickel (
It is composed of Ni) / gold (^U).

Although the length of this resonator can be easily made to be about the same as the emission wavelength by controlling the growth film thickness during epitaxial growth, it is not a closed resonator because no light reflecting film is formed on the side surfaces. Therefore, the spectrum of spontaneously emitted light is a broad continuous spectrum, and it is also emitted in the lateral direction, which means that a clear threshold value arises in principle, just like a normal laser.

On the other hand, there is a JS type semiconductor laser that is modified to form a resonator in a direction perpendicular to the substrate.

For example, as shown in FIG. 8(b), an n-type GaAs substrate 6
1, Alo, r Gao, 9 As layer and Alo
, 7 Gao, 3 Reflective film 62 made of a λ/4 multilayer film with As layer, n-type ^1゜4Ga0.6^S cladding layer 63, p-type GaAs active layer 64, p-type AlO, 4Gao
, 6^S cladding layer 65, a dielectric 67 made of a multilayer film of an amorphous silicon layer and a silicon oxide layer, and a light emitting section formed by laminating in sequence, and a high concentration p + GaAs region 68 formed around this. , six upper electrodes made of Ti-Au formed so as to be in contact with this p + GaAs region 68 , and gold-germanium (^u-Ce ) /nickel (old) formed on the back surface of the n-type GaAs substrate 61 . /gold (Au) lower electrode 70.

In this structure, there is a region doped with a p-type impurity in a high concentration in the vibrator, so light absorption by free electrons becomes a problem.

In particular, as the resonator is made smaller, the volume ratio of the highly doped region to the total volume of the resonator increases. In this case, the resonator can be made smaller until the total thickness of the active layer and both the P and N Rad layers is about 0.3 μl, and the cross-sectional diameter or side length is about 1 to 1°5 μm. , most of the region inside the cavity is heavily doped, resulting in a significant increase in light absorption and a decrease in light emission efficiency in the active layer.

(Problems to be Solved by the Invention) As described above, there is a problem in that the structure of a conventional semiconductor laser cannot be directly applied to a micro-closed cavity laser.

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a micro-closed resonator laser body that is easy to manufacture and has high reliability.

[Structure of the invention]

(Means for Solving the Problem) Therefore, in the present invention, the side surface of the active layer is formed with a metal reflective film that also serves as an electrode and is formed via a high-resistance semiconductor layer having a bandgap larger than the emitted light energy. At the same time, the lower surface of the active layer is covered with a semiconductor distributed black reflective film, and the upper surface of the active layer is covered with a distributed black reflective film to form a closed resonant structure. The size of the eigenmode is such that it has one or a very small number of eigenmodes in a wavelength range of approximately the spectral width of .

Desirably, the length in the lateral direction is about a fraction of the wavelength to several wavelengths of the oscillation wavelength.

(Function) According to the above configuration, the active layer includes a metal reflective film that also serves as an electrode formed through a high-resistance semiconductor layer on the side surface, a semiconductor distribution black reflective film on the bottom surface of the active layer, and a dielectric layer on the top surface of the active layer. The body-distributed black reflective film is arranged inside a closed resonator that has only a very small number of discrete modes in the gain band. The emission coefficient can be made much larger than that of ordinary semiconductor lasers.

In addition, the high-resistance semiconductor layer not only blocks current paths that do not go through the active layer and eliminates leakage current, but also protects the sides of the active layer and minimizes non-radiative recombination at the interface. can. Due to these synergistic effects, a micro closed resonator with an extremely low threshold value can be obtained.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a diagram showing a vertical cavity surface emitting laser according to a first embodiment of the present invention.

This laser has n-Aft on an n-type GaAs substrate 1.
Ga+-xIAS layer and n Alx2Ga1-y2^S
Reflective film 2 consisting of a λ/4 multilayer film with n-type AIylG
a+-yl^S cladding layer 3, p-type GaAs active layer 4,
The laminate of the p-type Aly2Ga+-y2AS cladding layer 5 is formed so as to form a rectangular column with a side length of 1500 to 1.5 μm in cross section, and the side surface is Resistance A1. +ca t1^S In addition to covering the current blocking layer 8, the upper surface is covered with a dielectric distribution black reflective film 7 made of a multilayer film of an amorphous silicon layer and a silicon oxide layer, and furthermore, the current blocking layer 8 and the reflective film 7 are In between, there is a contact layer 9 made of p-^Ig2Ga+-t2As that contacts the cladding layer 5 from the side surface.
are provided, and a metal electrode/reflection film 10 made of ^g, ^g-^u, Ti-PL-Au, etc. is formed on the entire side surface of the prism formed by these.

In addition, on the back surface of the n-type GaAs substrate 1, ^u-Ge /
A Nl/^U lower electrode 11 is formed.

Further, in this structure, the cladding layer 3 may be omitted so that the active layer 4 and the reflective film 2 are in direct contact with each other.

The active layer 3 is formed to be narrower than the diameter of the cladding layer by selective meltback, and has a current confinement structure. A structure in which selective meltback is not performed and the widths of the active layer and the cladding layer are approximately the same is also possible.

Furthermore, a quantum well structure, a quantum wire, or a quantum well box structure may be used as the active layer. However, when using a quantum wire or a quantum well box structure, it is desirable to use a structure in which the sides of the thin wire or box are filled with a high-resistance semiconductor to eliminate leakage current, as shown in FIG.

In addition, the aluminum composition of the distributed black reflective film, cladding layer, and contact layer (xI + A2 + YY
2 + Z I + z2) is taken so that absorption of light emitted from the active layer does not become a problem.

Here, the thickness of each layer of the λ/4 multilayer film is λ. /4*l (
t −1,2). Further, the thicknesses of the active layer and both cladding layers are set so that the overall optical length thereof is A0.

Here λ. is the oscillation wavelength in vacuum, n I + n
2 are n-Alt+Ga+-ttAs layers and n-
It is the refractive index of the ^It2Ga+-m2As layer. The reflectance of a distributed black reflective film increases as the difference in refractive index of the laminated media increases, that is, as the difference between xl and x2 increases, and as the number of pairs increases, for example! + -0,1,
X2 =0.9~1. It is desirable that the number of pairs be about 20 to 30 pairs.

Ideally, the number of eigenmodes in a closed resonator should be one within the gain band of the active layer, but due to degeneracy or because the mode spacing is narrower than the gain band of the active layer, the number of eigenmodes is reduced to a few. The threshold value is also sufficiently low compared to that of a normal semiconductor laser. However, as the number of modes increases, the spontaneous emission coefficient decreases and the threshold value increases, so as a guide, it is desirable that the number of modes be within about 10.

Next, a method for manufacturing a laser according to an embodiment of the present invention will be described.

First, as shown in FIG. 2(a), an n-type GaAs substrate 1
on the surface of n Alt+Ga1-x1As layer and n
Reflective film 2 consisting of a λ/4 multilayer film with Alx2Ga+ and 2^S layers, n-type AIy+Ga+-y+^S cladding layer 3,
p-type GaAs active layer 4, p-type AIy2Ga+-y2AS
After the cladding layer 5 is sequentially deposited by epitaxial growth, a dielectric distribution black reflective film 7 consisting of a multilayer film of an amorphous silicon layer and a silicon oxide layer is formed, a resist R1 is applied on this upper layer, and this is coated with a butter by photolithography. ning.

Subsequently, as shown in FIG. 2(b), using this resist R1 as a mask, the substrate surface is etched by reactive ion beam etching to form columnar structures. Although the shape of this mask is square here, it may be circular, rectangular, circular, oval, or the like.

Further, as shown in FIG. 2(C), after removing the resist, the side walls are etched by wet etching.

Thereafter, as shown in FIG. 2(d), the side walls of the active layer are depressed by selective melt-back. This eliminates damage caused to the active layer during etching.

Next, as shown in FIG. 2(e), a high-resistance current blocking layer 8 is epitaxially grown halfway down the p-type cladding layer 5 while leaving the multilayer film 7 of amorphous silicon and silicon oxide, and then a contact layer is formed. form 9.

Next, as shown in FIG. 2(r), reactive ion beam etching is performed with this mask 12 left in place, and the etching is stopped with the high resistance current blocking layer 8 left on the substrate surface.

Furthermore, as shown in Figure 2 (g), ^g,
After depositing Ag-Au and Ti-PL-^U, a resist (not shown) is applied, the surface is flattened, and then etched back to expose the metal on the top surface of the cylindrical body. Using a resist as a mask, the metal on the upper surface of this cylindrical body is removed by etching to form a metal electrode 10.

Finally, as shown in FIG. 2(h), the back surface of the substrate is polished, and a u-Ge/old/Au lower old type Au lower electrode 11 is placed on the back surface of the n-type GaAs substrate 1, as shown in FIG. A laser similar to the above is completed.

Embodiment 2 Next, as a second embodiment of the present invention, as shown in FIG. 3, a dielectric distribution black reflective film 7 may be formed on the entire upper surface of the cylindrical body.

The rest is completely the same as the semiconductor laser of Example 1 shown in FIG.

Next, a method for manufacturing this semiconductor laser will be explained.

First, as shown in FIG. 4(a), an n-type GaAs substrate 1
On the surface, ^IQ, l Gao, 9^S layers and AI.

, 7 Gao, 3^S reflective film 2 consisting of a λ/4 multilayer film, n-type^1゜4 Gao, 6As cladding layer 3
, p-type GaAs active layer 4, p-type Alo', 4 Gao
, 6 As cladding layers 5 are sequentially deposited by epitaxial growth, and then a silicon oxide film 12 is formed and patterned by photolithography.

Subsequently, as shown in FIG. 4(b), using this silicon oxide film 12 as a mask, the substrate surface is etched by reactive ion beam etching to form a columnar structure.

The shape of this mask is square, rectangular, circular, oval, etc.

Furthermore, as shown in FIG. 4(e), the side walls are etched by wet etching.

Thereafter, as shown in FIG. 4(d), the side wall of the active layer is depressed by selective melt-back. This eliminates damage caused to the active layer during etching.

Next, as shown in FIG. 4(e), a high-resistance current blocking layer 8 is epitaxially grown to the middle of the p-type rad layer 5 while leaving the silicon oxide mask 12, and then a contact layer 9 is formed. .

Next, as shown in FIG.
Stop etching at least half way before the bottom surface reaches the substrate surface.

Furthermore, as shown in Figure 4(g), Ag
, Ag-^u, TI-r'L-Au were deposited,
After applying a resist (not shown) and flattening the surface, etch back is performed to expose the metal on the top surface of the cylindrical body, and then, using the resist as a mask, remove the metal on the top surface of the cylindrical body by etching. At the same time, this silicon oxide mask 12 is removed and a metal electrode 10 is formed.

Then, on this upper layer, as shown in FIG. 4(h), a dielectric distribution black reflective film 7 made of a multilayer film of an amorphous silicon layer and a silicon oxide layer is formed.

Finally, the back side of the substrate is polished and the nl4J, GaAs
A ^-Ge/old/^U lower electrode 11 is deposited on the back surface of the substrate 1, and a laser as shown in FIG. 3 is completed.

This semiconductor laser also has an extremely low threshold and high oscillation efficiency, similar to the one shown in Example.

Example 3 Next, a third embodiment of the present invention will be described.

In this example, as shown in FIG. 5, p-type A1m1Ga+-+ As/ p
Type^1t2Ga+-t□^S or non-doped^1m+
A Ga+-1As/^1t2Ga+-x2^S semiconductor distribution black reflective film 17 is used, and the etching of the n-type semiconductor distribution black reflective film 2 on the substrate surface is stopped midway, leaving it on the substrate surface, and a silicon oxide film 22 is formed on the flat part. However, unnecessary electrodes on the surface of the substrate were removed.

In this structure, p-type A1. , Ga , ^s/p type A
I, 2Ga1-82^S or non-doped A1. ,
By using the Ga,, As/^1x2Ga, -12^S distribution black reflective film 17, the substantial contact area with the p-side electrode increases and the series resistance of the element decreases.

In addition, since the etching of the n-type black reflective film 2 on the substrate surface is stopped midway through and is left on the substrate surface, when selectively melting back the active layer, the active layer can be sufficiently melted without excessively melting the substrate. Can be backed.

Furthermore, by removing unnecessary electrodes on the substrate and using an insulating film 7 having a lower dielectric constant than that of a semiconductor, the capacitance of the element can be reduced, making it possible to further increase the speed.

Example 4 Next, a fourth embodiment of the present invention will be described.

In this example, as shown in FIG. 6, an example of application to a semiconductor laser that emits light in a direction parallel to a substrate is shown.

In addition to the structure of Example 3, p-type ^1. , Ga.

^s/p type^1,2Ga+-m□^S or non-doped A1. A metal film 18 is further formed on the upper layer of the lGa8^S/^It2Ga+-w□^S semiconductor distribution black reflective film 17, so as to cover it in two directions.

In this case, the semiconductor distributed black reflective film] 7 and the metal film 18
Since the light is reflected by both, sufficient reflectance can be obtained even with a multilayer film made up of about 5 pairs.

In the above embodiment, an n-type GaAs substrate was used as the substrate, but it goes without saying that a p-type substrate may also be used. In that case, the conductivity type of each layer may be reversed.

Furthermore, the composition ratio, impurity concentration, and thickness of each semiconductor layer are not limited to the examples and can be changed as necessary, and can also be applied to semiconductor lasers using materials other than AIGaAsP-based materials. It goes without saying that it is possible. For example, it is also possible to use 1nGaAIP type materials. However, lno of the cladding layer, (Ga+-y
Al-) o, q P and active layer lno, (ca
ty Aly ) 0. P is feasible for all x>y. Further, the active layer may be composed of a quantum wire or a quantum wire structure as shown in FIG.

In addition, the present invention can be implemented with various modifications without departing from the spirit thereof.

〔effect〕

As described above, according to the present invention, a metal reflective film that also serves as an electrode is formed via a high-resistance semiconductor layer that has a forbidden band width larger than the optical energy emitted to the side surface of the active layer, and Since the semiconductor distributed black reflective film on the lower surface of the layer and the distributed black reflective film on the upper surface of the active layer are configured to confine the active layer, it is possible to obtain a micro closed resonant semiconductor laser with a low threshold value.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a semiconductor laser according to a first embodiment of the present invention, FIGS. 2(a) to 2(h) are diagrams showing the manufacturing process of the same conductor laser, and FIG. FIGS. 4(a) to 4(1) are diagrams showing the semiconductor laser of the second embodiment of the invention, and FIGS. 5 and 6 are diagrams showing the manufacturing process of the same conductor laser.
The figures show semiconductor lasers according to third and fourth embodiments of the present invention, FIG. 7 shows a quantum well box and an active layer with a thin wire structure, and FIGS. 8(a) and 8(l). ) is a diagram showing a conventional surface emitting semiconductor laser. ]... n-type GaAs substrate, 2... semiconductor distributed black reflective film, 3-n-^mGa1-v) AS cladding layer,
4--p-type GaAs active layer, 5-=p-type^1 y
2Ga + -y 2AS cladding layer, 7... Dielectric distribution black reflective film, 8... High resistance^(t+Ga
+I As current blocking layer, 9-p A1.2Ga+
-2As contact layer, 10...metal electrode/reflection film,
11... Lower electrode, 12... Silicon oxide film, 17
...Semiconductor distributed black reflective film, 22...Silicon oxide film, R1...Resist, 61...N-type GaAs
Substrate, 62... Reflective film, 63-n type AI0.4 Ga
o6As cladding layer, 64-D type GaAs active layer, 65
-9 type AI0.4 Gao6^S cladding layer, 66...
・Dielectric material, 68... High concentration p + GaAs region, 69
. . . upper electrode, 70 . . . lower electrode. Figure 1 Figure Output light Figure 5 Figure

Claims (1)

[Scope of Claims] An active layer, first and second cladding layers sandwiching the active layer, and a high resistance layer covering side surfaces of the active layer and having a forbidden band width larger than optical energy emitted from the active layer. It has a closed resonator structure composed of a metal reflective film that also serves as an electrode formed through a semiconductor layer, a semiconductor distributed black reflective film that covers the bottom surface of the active layer, and a distributed black reflective film that covers the top surface of the active layer. A semiconductor laser characterized in that the size of the closed resonator is such that it has one or a few eigenmodes in a wavelength region that is approximately the spectral width of spontaneous emission light in the free space of the light emitting medium.