CA1150809A - Semiconductor laser device - Google Patents

Abstract

Abstract of the Disclosure A semiconductor laser device has a stripe-shaped impurity-diffused region disposed at least in parts of semiconductor layers from a surface semiconductor layer of a semiconductor layer assembly to a second semiconductor layer lying in contact with a first semiconductor having an active region. The impurity-diffused region has the same conductivity type as that of the second semiconductor layer and extends at least from the surface semiconductor layer to a depth vicinal to the first semiconductor layer, the impurity region serving as a current path. The device is characterized in that a third semiconductor layer in which the diffusion rate of an impurity for use in the formation of the impurity-diffused region is lower than in the second semiconductor layer is disposed between the surface semiconductor layer and the second semiconductor layer.

Description

~5`S~ 9 Semiconductor Laser Device .

This invention relates to a semiconductor laser device.
In recent years, semiconductor laser devices have been proposed as light sources for optical communication, laser printers, instrumentation etc. In exciting such devices, a method of carrier injection has been employed because it has the advantages that a high light conversion efficiency can generally be attained and the optical output can be readily modulated by directly controlling the injection current.
When exciting a semiconductor laser device by this method of carrier injection, it is common practice to con-struct the device as a so-called stripe-geometry laser with a narrowed active region. This permits operation of the device with a low threshold current and facilitates control of the oscillation mode of the laser.
Concrete examples of a stripe-geometry laser are described in U.S. Patent 3,920,491, issued November 18, 1975 to Hiroo Yonezu and U.S. Patent Re. 29,395 issued September 13, 1977 to Hiroo Yonezu.
Such a semiconductor laser device includes a narrow elongated semiconductor region of the same conductivity type as that of another semiconductor region lying in the vicinity of the active region of the device. The elongated region extends in depth from the surface of the device to the vicinity of the active region. A surface semiconductor layer of the opposite conductivity type covers the entire surface of the device except for the elongated region.
Summary of the Invention An object of the present invention is to provide a semiconductor laser device employing a stripe-shaped impurity-diffused region for injecting current into an active region wherein a novel structure makes the per-centage of acceptable products of manufacture high, i.e. a low reject rate.
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~15~ 9 To this end the invention consists o~ a semiconductor laser device having a stripe-shaped impurity-diffused region disposed at least in parts of semiconductor layers from a surface semiconductor layer of a semiconductor layer assembly to a second semiconductor layer lying in contact with a first semiconductor layer having an active region, the impurity-diffused region having the same con-ductivity type as that of the second semiconductor layer and extending at least from the surface semiconductor layer to a depth vicinal to the first semiconductor layer, an electrode being disposed on the impurity-diffused region so that a current may flow from the electrode to the first semiconductor layer through the impurity-diffused region; characterized in that a third semiconductor layer in which a diffusion rate of an impurity for use in the formation of said impurity-diffused region is lower than in said second semiconductor layer is disposed between said surface semiconductor layer and said second semiconductor layer.
Since the diffusion rate of the impurity is low in the third semiconductor layer, control of the depth of the impurity-diffused region is facilitated.
In an example of a GaAs-GaAlAs-system, double-hetero-structure injection laser, GaAlAs is usually used for the second semiconductor layer, Zn or the like as the impurity for forming the impurity-diffused region, and GaAs for the surface semiconductor layer.
In order to realize a GaAs-GaAlAs-system semiconductor laser device that emits visible radiation, the mole fraction of AlAs (u) in Gal uAluAs needs to be made large. This aims to enhance the transfer efficiency of the visible radiation. However, when the mole fraction of AlAs in GaAlAs is large, the diffusion rate of ~n increases abruptly. Simultaneously, the depths of the impurity-diffused regions disperse very greatly and become difficult ~,~

to control in practice. Accordingly, troubles in which the impurity region extends over the greater part of the second semiconductor layer or even reaches the active region occur frequently.
Especially in the GaAlAs material in which the mole fraction of AlAs (u) is at least 0.45, not only the diffusion rate increases, but also the dispersion of the diffusion depths becomes conspicuous. When u is 0.5 or greater, the dispersion becomes very conspicuous, and such material is difficult to use in practice. Further, it becomes difficult to form a grown layer of good quality.
In a case where the GaAlAs material described above is used for the second semiconductor layer of the device, the third semiconductor layer is inserted between the second semiconductor layer and the surface semiconductor layer.
The impurity diffusion rate in this third semiconductor layer is then selected to be lower than that in the second semiconductor layer, whereby the diffusion depth of the ; impurity can be well controlled. In using GaAlAs for the third semiconductor layer, u should preferably be 0.1 to 0.35.
Figure 1 is a partial, vertical, cross-sectional view of a semiconductor laser device according to one embodiment of the invention;
Figure 2 is a perspective view of the device of Figure l;
Figure 3 is a partial, vertical, cross-sectional view of a semiconductor laser device according to one example of the prior art;
Figure 4 is a graph showing the relationship between the diffusion depth and the mole fraction of AlAs;
Figure 5 is a graph showing the threshold current distribution of semiconductor laser devices; and ." J
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t~8~9 Figures 6 and 7 are partial, vertical, cross-sectional views of semiconductor laser devices according to other embodiments of the invention.
Detailed Description of the Embodiments In Figs. 1 and 2, on an n-GaAs substrate 1 which has the (100) face as its upper surface, an n-GaO 4Alo 6As layer 2 is formed to a thickness of 1.5 ~m, an n-GaO 75Alo 25As layer 3 to a thickness of 0.1 ~m, a p-GaO 4Alo 6As layer 4 to a thickness of 1.5 ~m, a p-GaO 8Alo 2As layer 5 to a thickness of 1 ~m, and an n-GaAs layer 6 to a thickness of 1 ~m. Each semiconductor layer may be formed by the conventional liquid phase, epitaxial growth process.
The n-GaAs layer 3 corresponds to the first semiconduc-tor layer mentioned above, and has the active region. The p-GaO 4Alo 6As layer 4 corresponds to the second semi-conductor layer, the p-GaO 8Alo 2As layer 5 to the third semiconductor layer, and the n-GaAs layer 6 to the surface semiconductor layer.
In a double-heterostructure injection laser of the GaAs-GaAlAs-system, the first semiconductor layer is made of Gal xAlxAs (O < x < 0.5), and the cladding layers holding it therebetween are made of Gal yAlyAs (0.2 ~ y ~ 0.8), x and Y being so related that x ~ y. Regarding the thickness of the layers, the first semiconductor layer is set at 0.05 ~m - 0.3 ~m, and the cladding layers at 1.0 ~m - 3.0 ~m. The surface semiconductor layer is necessary (1) for preventing the semiconductor layers under manufacture from oxidizing, (2) for protecting the semiconductor layers - 30 when washing the semiconductor layer assembly, and (3) for reducing the contact resistance to an electrode disposed thereon. For these purposes, GaAs is the most preferred, and the thickness of the surface semiconductor layer is usually made 0.5 ~m - 1.5 ~m.
On the n-GaAs layer 6, an A1203 layer 8 is formed.

~i~t~8~)g In this layer 8, a window which is 3.0 ~m wide is provided by the well-known photolithographic process. Through the window, zn is selectively diffused to be 2 ~m deep, that is, to diffuse into the n-GaAs layer 6 and the p-GaO 8-Alo 2As layer 5. The regions indicated by symbols 7 and7' correspond to these Zn-diffused region.
Thereafter, stacked layers of Au and Cr are formed as a p-side electrode 9, and an Au-Ge-Ni alloy is deposited as an n-side electrode ~0. The crystal is cloven at the opposing (110) faces ( or faces equivalent thereto) to form an optical resonator and to construct the semicon-ductor laser device. The cavity length is 300~ m.
In an example of this embodiment, the laser could oscillate at a threshold current density of approximately

2 kA/cm at room temperature. The oscillation wavelength was 7,500 A , and the external quantum efficiency was approximately 40 %. Arrows in Figure 2 indicate the emerging directions of laser radiation.
The advantages of this construction will be described by referring to an example of a prior-art structure shown in Figure 3 which is a sectional view showing a typical known example of a GaAs-GaAlAs-system semiconductor laser.
On a GaAs substrate 1, an n-GaO 4Alo 6As layer 2 is formed to a thickness of 1.5 ~m, an n-GaO 75Alo 25As layer 3 to a thickness of 0.3 ~m, a p-GaO 4Alo 6As layer 4 to a thickness of 1.5 ~m, and an n-GaAs layer 6 to a thickness of 1 ~m. In parts of the n-GaAs layer 6 and the p-GaO 4Alo 6As layer 4, a Zn-diffused region at 7 and 7' is formed. The n-GaAs layer 3 corresponds to the first semiconductor layer described before, the p-Gao 4A10.6As layer 4 to the second semlcond~ctor layer, and the n-GaAs layer 6 to the surface semiconductor layer.
Numeral 8 designates an insulator layer, and numerals 9 and 10 respectively designate electrodes.
In a case where the diffusion depth of the impurity-8~9 diffused layer 7 is precisely controlled, as illustrated in Figure 3, no problem occurs. As stated before, however, when the mole fraction of AlAs in the p-GaO 4Alo 6As layer 4 becomes 0.45 or above, the diffusion rate of Zn increases and the diffusion depth thereof disperses greatly. In practice, accordingly, control becomes difficult.
Figure 4 illustrates the relationship of the diffusion depth to the mole fraction of AlAs at the time when Zn was diffused into a Gal xAlxAs crystal. Curves 11, 12 and 13 respectively indicate the characteristics obtained when Zn was diffused for 50 minutes at 700 C, 670 C and 640 C.
From this graph, it is understood that the diffusion depths disperse when the mole fraction of AlAs is 0.45 or greater. This tendency to dispersion was similarly noted in experiments that were conducted at diffusion tempera-tures of 600 - 800 C and for diffusion times of 1 - 300 minutes.
Shown in Figure 5 are the distributions of the thresh-old currents of manufactured samples of lasers having the structure of Figure 3, compared with a laser according to this invention. The curve 20 corresponds to a laser of this invention, while the curve 21 refers to the prior-art laser of Figure 3. The specifications of the respective lasers are as given above. It is shown that the disper-sion of the characteristics of the laser products becomesvery small with application of the present invention.
In the embodiment of Figs. 1 and 2, the third semiconductor layer 5 is made of p-type GaAlAs, and the surface semiconductor layer 6 is made of n-type GaAs.
As a result, when current is caused to flow through the laser in the forward direction, the interface between the p-GaAlAs layer and the n-GaAs layer is reverse-biased, so that leakage current is prevented. A greater band gap can be established at the junction between the GaAs and GaAlAs layers, than at a p-n junction in an identical material.

~1.~8~)9 The arrangement is thus useful for prevention of leakage current.
In addition, since the mole fraction of AlAs decreases in the order of the second semiconductor layer, the third semiconductor layer and the surface semiconductor layer, - crystal lattices are matched more easily.
In a case where the mole fraction of AlAs in the third semiconductor layer is less than 0.1, the advantage is also realized that the difference between the third semiconductor layer and the GaAs (crystal) as the surface semiconductor layer formed thereon becomes indistinct.
More specifically, although the diffusion depth needs to be measured from the surface of the GaAs surface layer formed on the GaAlAs layer, the boundary between the GaAlAs and GaAs layers cannot be distinguished in this case. Herein, the boundary between the GaAs and GaAlAs layers is able to be observed visually with a microscope when the polished portion of the crystal is etched with, for example, fluoric acid, hydrogen peroxide and water (at a mixing ratio of 1 : 1 : 5). The visual discrimination thus realized is very convenient and practical in the inspection of mass-produced articles.
The third semiconductor layer 5 is made at least 0.5 ~m thick. However, it is unnecessary to make the layer very thick. This is because a resistance which is connected in series with the active region of the semiconductor laser increases with the thickness of the layer.
Although a GaAs-GaAlAs system has been employed in the foregoing example, the invention is applicable to other material systems, for example, a Ga-Al-As-Sb system, a Ga-Al-As-P system, a Ga-As-P system and an In-Ga-As-P
system. The technical idea of this invention is also applicable to a semiconductor laser construction having conductivity types opposite those of the example.

5~8 Needless to say, this invention can be applied to various modified semiconductor lasers. Figure 6 is a sectional view showing another embodiment of the invention.
This embodiment differs from the embodiment of Figure 1 in that the substrate 1 is provided with a beltlike recess 15.
It is intended to improve mode control in the lateral direction by exploiting an optical characteristic change at the boundary of the recess 15. Mode control means is disclosed in, for example, Japanese published Patent Application No. 52-143787 (HITACHI).
In an example of the embodiment in Figure 6, a photo-resist film having a window 10 ~m wide was formed by the conventional photoresist process on an n-GaAs substrate 1 which had the (100) face as its upper surface. The surface ; 15 of the substrate was chemically etched through the window at 20 C by the use of, for example, phosphoric acid :
hydrogen peroxide : ethylene glycol = 1 : 1 : 3, whereby the groove 15 concave in the depth direction was formed.
The width of the groove was made about 10 ~Im (usually, 5 - 20 ~m), and the depth 1.5 ~m (usually 0.8 - 2.5 ~m).
Subsequently, the layers 2, 3, 4, 5 and 6 as in Figure 1 were grown on the resultant substrate by the continuous liquid phase growth method. Such method may conform with well-known parameters. However, the solution compositions and growth times that were used for forming the respective semiconductor layers are listed in Table 1 by way of example.

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.i~lti~8~9 Table 1 L ayer 2¦Layer 3¦Layer 4¦Layer 5¦Layer 6 ¦
Ga (gr) l 6 6 6 6 6 GaAs (mg) 400 400 400 400 400 SolutionAl (mg) 10 3 10 2 _ Composition Sn (mg) _ _ _ 200 Te (mg) 0.5 _ _ Ge (mg) _ _ _ 200 zn (mg) _ _ 30 _ 10Growth time 2 min.2 sec. 8 min.3 min.1 min.

The saturated solution had its temperature lowered at a rate of about 0.4 C/min. from 780 C and was overcooled for 3 minutes. Thereafter, the solutions were successively brought into contact with the substrate. Thus, the layer 2 had its thicker part made 2 ~m thick and had its thinner part made 0.3 ~m thick. The thicknesses of the layers 3, 4, 5 and 6 were 0.1 ~m, 2 ~m, 2 ~m and 1 ~m respectively.
As dopant impurities, Sn was used for the n-type layers, and Ge for the p-type layers. Subsequently, through a window in the A12O3, formed by the same photoresist process as in the previous case, Zn was diffused at 700C
for 10 minutes, to form the p-type diffused regions at 7 and 7' which were 1.0 - 3.0 ~m deep. Thereafter, Au and Cr, and an Au-Ge-Ni alloy were respectively deposited as the positive electrode 9 and the negative electrode 10.
Lastly, the crystal was cloven at the (110) faces so as to obtain opposite parallel surfaces. A reflector was then formed to construct the laser device. The laser length was 300 ~m.

Such a semiconductor laser could osc;llate at a thresh-old current density of approximately 2 kA/cm2 at room temperature. The oscillation wavelength was approximately 7,500 A, and the external quantum efficiency was approxi-mately 40 %.
This invention is also applicable to a so-called buried heterostructure, injection laser whose active region is buried in a different kind of semiconductor region. Figure 7 is a sectional view showing such an embodiment. The first semiconductor layer 3 is held between burying side layers 16 and 16'. This structure is described in, for example, U.S. Patent No. 4,121,177 issued October 17, 1978 to Toshihisa Tsukada. Also in this case, the objective can be satisfactorily accomplished by diffusing Zn into the third semiconductor layer 5 and the surface semiconductor layer 6 and thus forming the impurity-diffused region at 7 and 7'. In the figure, parts assigned the same numerals as in Figure 1 are the same.
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Claims (8)

Claims:

1. A semiconductor laser device having a stripe-shaped impurity-diffused region disposed at least in parts of semiconductor layers from a surface semiconductor layer of a semiconductor layer assembly to a second semiconduetor layer lying in contact with a first semiconductor layer having an active region, the impurity-diffused region having the same conductivity type as that of the second semiconductor layer and extending at least from the surface semiconductor layer to a depth vicinal to the first semi-conductor layer, an electrode being disposed on the impurity-diffused region so that a current may flow from the electrode to the first semiconductor layer through the impurity-diffused region; characterized in that a third semiconduetor layer in which a diffusion rate of an impurity for use in the formation of said impurity-diffused region is lower than in said second semiconductor layer is disposed between said surface semiconductor layer and said second semiconductor layer.

2. A semiconductor laser device according to claim 1, wherein said first semiconduetor layer is sandwiched between said second layer and a fourth semiconductor layer, and said second semiconductor layer has an opposite conductivity type to that of said fourth semiconductor layer.

3. A semiconductor laser device according to claim 1 or 2, wherein said second semiconductor layer and said third semiconductor layer have the same conductivity type.

4. A semiconductor laser device according to claim 2, wherein said first semiconductor layer is a Ga1xAlxAs layer, said second semiconductor layer is a Ga1-yAlyAs layer (where x < y), said fourth semiconductor layer is a Ga1-y,Aly,As layer (where x < y') and said second semiconductor layer and said third semiconductor layer have the same conductivity type.

5. A semiconductor laser device according to claim 1 wherein said second semiconductor layer and said third semiconductor layer have the same conductivity type, and said surface semiconductor layer has the conductivity type opposite thereto.

6. A semiconductor laser device according to claim 2, wherein said first semiconductor layer is a Ga1-xAlxAs layer, said second semiconductor layer is a p-type Ga1-yAlyAs layer (where x < y), said third semiconductor layer is a p-type Ga1-zAlzAs layer, said fourth semiconductor layer is a n-type Ga1-y,Aly,As layer (where x < y') said surface semiconductor layer is a GaAs layer, the impurity is an impurity turning GaAlAs into p-type, and y > z.

7. A semiconductor laser device according to claim 1 or 6, wherein said impurity is Zn.

8. A semiconductor laser device according to claim 5, wherein said second semiconductor layer is a p-type Ga1-yAlyAs (0.45 < y < 0.8) layer, said third semi-conductor layer is a p-type Ga1-zAlzAs (0.1 < z < 0.35) layer, and said surface semiconductor layer is an n-type GaAs layer.