JPH05267776A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To protect the end face of a semiconductor laser device against damage even when the device outputs laser rays of high power by a method wherein a surface level is restrained from being formed due to an oxide at the light emission end face. CONSTITUTION:A layer 51 of material which contains InP or InGaP is formed on the light emitting end face of a semiconductor laser device. As the lattice constant of the layer 51 is larger than the of an AlGaAs active layer 13, it is sharply enlarged in band gap a given compressive stress. Even surface level is a little generated on the surface of the layer 51, carriers trapped by the surface level concerned are injected again into the conduction band or the valence band of the AlGaAs active layer 13 through a tunneling effect and used for emitting light, so that the surface of the layer 51 is restrained from rising in temperature. That is, the layer 51 functions as a kind of a window layer. Therefore, the end face of an AlGaAs semiconductor laser device is hardly damaged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AlGaAs type edge emitting type high power semiconductor laser device.

[0002]

2. Description of the Related Art As a light source for an optical disk device
GaAs semiconductor laser devices have been widely used. When the semiconductor laser device is used as a light source for a writable write-once optical disc device, it is required to have high reliability even in a high light output state of 50 mW or more. When used as a light source for excitation of a YAG (Yttrium Aluminum Garnet) laser device or a light source for a high-definition printer, a high output of 100 mW or more is required.

As a practical method for increasing the output of a semiconductor laser device, a low reflectance protective film made of, for example, Al ₂ O ₃ having a reflectance of 10% is provided on the light emitting side. The reflectance on the opposite end surface is 90%,
For example, there is a method of increasing the external quantum efficiency by forming a high reflectance protective film composed of a multilayer film of an Al ₂ O ₃ layer and a Si layer. A high output characteristic of 100 mW or more has been confirmed in a 780 nm band semiconductor laser device in which the transverse mode is controlled by this method.

[0004]

However, in the semiconductor laser device in which the low reflectance protective film and the high reflectance protective film are simply formed on the end face, there is a problem in reliability of high output characteristics for a long period of time. The main cause is deterioration of the end surface. This is because the light emitting end face has a high light density and thus locally generates heat. This heat generation occurs when electrons or holes as carriers are captured by the surface level existing on the light emitting end face, and energy is released through a non-light emitting process. In particular, when laser oscillation is performed at a high output, the amount of heat generated increases and the temperature of the light emitting end face rises, eventually reaching the melting point of the crystal, and the end face destruction (CO
D) occurs.

A method of forming a film containing S (sulfur) on the light emitting end face in order to prevent the end face deterioration due to the heat generation of the light emitting end face has been proposed (for example, Extend.
edAbstracts of the 21st Conference on Solid State
Devices and Materials, Tokyo, (1989) p337-340). By this method, the maximum optical output of the semiconductor laser device is improved to 200 mW or more, which is twice the normal maximum output. This is because the S atomic layer or the S molecular layer terminates dangling bonds existing on the light emitting end surface of the semiconductor laser and inactivates the light emitting end surface. In other words, it is possible to significantly reduce the surface state density, which has been an obstacle to improving the light emission characteristics of the semiconductor laser.

However, even with the proposed method of forming the above S-containing film on the light emitting end face, there remains a problem in the long-term reliability of the semiconductor laser device. This is because the bond between the dangling bond on the light emitting end face and S is weak, and as a result of driving in a high output state for a long period of time, O (oxygen), which causes a surface state, is replaced by S on the light emitting end face. This is because an oxide film is formed in association with the bond.

By the way, although the target is GaAs, in order to improve the surface condition of the GaAs,
There is known a method of forming an InP layer having a larger lattice constant than aAs by several atomic layers (Extended Abstracts of the 23r
d Conference on Solid State Devices and Materials,
Yokohama (1991) pp683-685). This is GaAs
The oxide film that causes the formation of surface states on the surface of
A kind of window layer is formed on the GaAs surface. This InP layer serves as the window layer because the InP layer is the G of the laser element.
This is because the band gap of the InP layer remarkably increases due to the effect of strain caused by receiving compressive stress from aAs.

However, the end faces of the optical element, especially Al
The end face of the GaAs semiconductor laser device has not been attempted to be improved by forming an ultrathin film on the surface as described above. This is because Al was a very active substance, and its oxide film was stable, and it was thought that a good AlGaAs surface having a small number of surface states could not be formed by the above method.

An object of the present invention is to provide an AlGaAs semiconductor laser device capable of suppressing the surface level caused by an oxide at the light emitting end face and hardly causing end face breakdown even in a high output state.

[0010]

In the semiconductor laser device of the present invention, one layer of a laser element formed by laminating a plurality of layers is A
In a semiconductor laser device including an lGaAs active layer,
A layer made of a material containing InP or InGaP is formed on the light emitting end face of the laser element, and the above object is achieved by this.

A window layer made of AlGaAs having a higher Al mixed crystal ratio than the AlGaAs active layer may be formed between the light emitting end face and a layer formed on the end face.

The layer formed on the light emitting end face is formed of InGa.
It may be formed of a material containing P, and the composition ratio of In in InGaP may be greater than 49% with respect to the total of Group III elements In and Ga.

The thickness of the layer formed on the light emitting end surface is
It may be thinner than the critical layer thickness at which dislocations occur in the layer.

[0014]

In the semiconductor laser device of the present invention, a layer made of a material containing InP or InGaP is formed on the light emitting end face thereof. Although a surface level is slightly formed on the surface of the layer formed on the light emitting end face, the lattice constant of the layer formed on the light emitting end face is larger than that of the AlGaAs active layer. Alternatively, the band gap of the InGaP intermediate layer becomes extremely large. Therefore, the carriers trapped in the surface level are re-injected into the conduction band or valence band of the AlGaAs active layer by the tunneling (resonance) effect and used for light emission, and thus do not lead to heat generation on the surface. That is, the layer formed on the light emitting end face acts as a kind of window layer.

It is more effective to form a window layer made of AlGaAs having a higher Al mixed crystal ratio than the AlGaAs active layer between the light emitting end face and the layer formed on the light emitting end face. ..

In order to make the lattice constant of the layer formed on the light emitting end face larger than that of the AlGaAs active layer, there is no problem if the layer formed on the light emitting end face is made of a material containing InP. When it is formed of a material containing In, the composition ratio of In in InGaP may be greater than 49% with respect to the total of Group III elements In and Ga.

The layer formed on the light emitting end face is A
Although it is a lattice-mismatched system with respect to the 1GaAs system, if it is formed thinner than the critical layer thickness at which dislocations occur, the crystallinity can be prevented from being affected.

[0018]

EXAMPLES Examples of the present invention will be described below.

AlGaA as a base material to which the present invention is applied
The s-semiconductor laser device may be of any known end-face emission type that has been conventionally used, but here, VSIS (V-channeled substrate) is used.
inner stripe) laser (eg Applied Physics Le
tters vol.40 (1982) 372) as an example.

<< First Embodiment >> FIG. 1 is a perspective view showing the vicinity of an end face of a semiconductor laser device according to a first embodiment. The VSIS laser as a base material to which the present invention is applied is p-Ga.
N-GaAs current confinement layer 21, p-Al on As substrate 11
_0.45 Ga _0.55 As clad layer 12, p-Al _{0. 15} Ga
_{A 0.85} As active layer 13, an n-Al _0.45 Ga _0.55 As clad layer 14, and an n-GaAs cap layer 15 are laminated in this order, and electrodes 31 and 32 are further formed on the upper and lower sides thereof. Of these, the n-GaAs current confinement layer 21 has a V-shaped groove formed to form a current path and an optical waveguide. This laser device is for the 780 nm band, and
It is produced by a single LPE (Liquid Phase Epitaxy) growth.

A layer 51 of InP having a thickness of 15 angstroms is formed on the light emitting end face of the laser device.
A low reflection protective film 61 made of Al ₂ O ₃ is formed on it. Hereinafter, the layer 51 is referred to as an intermediate layer. A high reflection protection film is formed on the end face opposite to the light emitting side. Middle layer 5
1 is MOCVD (Metal Organic Chemic) using TMI (trimethylindium) and PH ₃ (phosphine) after cleaving the light emitting end face of the semiconductor laser device.
al Vapor Deposition) method. In addition, the protective film 61
Was prepared by the electron beam evaporation method.

FIG. 2 shows the current vs. light output characteristics of the semiconductor laser device by curve A, and for comparison, the characteristics of the semiconductor laser device having the same structure as the conventional one in which a protective film is directly formed on the end face after cleavage. Is shown by curve B. As can be seen from this figure, in the conventional example, the laser element is deteriorated due to the end face destruction at the optical output of about 120 mW, whereas in the present example, the maximum optical output of about 250 mW is obtained.
Moreover, no end face destruction occurred at that time.

A test was conducted in order to elucidate the reason why a high optical output was obtained in this example as described above. Al _0.15 Ga _0.85
A sample obtained by simply growing an As single layer on a GaAs substrate,
Another sample having an InP layer of 15 angstrom formed on the surface of the same sample was prepared and evaluated by the photoluminescence method. As a result, the latter was 50 times stronger than the former. From this, it is understood that the surface level caused by the oxide film was reduced in this example.

Al _0.15 Ga _0.85 A with InP layer formed
The surface improvement effect in the s active layer can be explained as follows using the energy state diagram near the end face shown in FIG.
FIGS. 3A and 3B are state diagrams with and without the InP layer formed. When the InP layer is not formed, Al _0.15 Ga _0.85 As contains Al, so many levels are formed on the surface, and as shown in FIG. 3B, the level overflows from the AlGaAs active layer and is trapped. The generated carriers are non-radiatively recombined to generate heat. on the other hand,
When the InP layer is formed, as shown in FIG.
The surface of the _0.15 Ga _0.85 As active layer serves as an interface with the InP layer, and Al _0.15 Ga _0.85 As and InP are firmly bonded to each other, so that the surface level hardly occurs. At this time, In
When the P layer is subjected to compressive stress, the band gap energy is significantly increased, and the state shown in the figure is obtained. In this case, a slight surface level remains on the surface of the InP layer.
Since the P layer does not contain Al, the density of the surface level is extremely small. Even if carriers are trapped in this level, the InP layer is thin and the carriers are tunneled (resonance) in the Al _0.15 Ga _0.85 As semiconductor layer. ) It is re-injected by the effect and is used for the light emission process and does not contribute to heat generation. Therefore, a kind of window effect occurs and the output characteristics are improved.

As a result of the same evaluation of the semiconductor laser device using GaAs as the active layer, the improvement of the photoluminescence intensity due to the formation of the InP layer is about 10 times, and the semiconductor laser of the present invention using AlGaAs as the active layer. It was found that the improvement of the surface condition was greater in the case of the device. Therefore, it is presumed that the large effect of improving the surface state of the AlGaAs semiconductor layer as described above is caused by the presence of Al, which is contrary to what has been conventionally considered.

As described above, the effect of reducing the surface state of the AlGaAs active layer is to terminate and inactivate the dangling bonds on the surface with the S atomic film or the molecular film, which is the conventional method as described above. But it can be achieved. In fact, a light output of about 220 mW has been observed even in a 780 nm band VSIS laser whose surface is coated with an S film (Extended Abstracts of t
he 21st Conference on Solid State Devices and Mate
rials, Tokyo, (1989) p337-340). However, In
When the P layer is used, the coupling with AlGaAs is S and Al.
Since it is much stronger than the bond with GaAs, the surface condition can be kept good for a long time. To prove this, 780 nm band VS with InP layer formed on the surface
The IS laser and the same laser on which the S film is formed are heated at 50 ° C for 5
After driving for 500 hours under the condition of 0 mW, the maximum light output was measured. In the former semiconductor laser device, an optical output of up to 250 mW was observed as in the case before long-time driving, but in the latter semiconductor laser device, only 130 mW was observed, which is about the level of an ordinary semiconductor laser device. That is, it was proved that the present invention in which the InP layer was formed was far more durable.

<Second Embodiment> The second embodiment of the present invention is as follows.
The intermediate layer 51 made of InP in FIG. 1 showing the first embodiment is an intermediate layer made of In _0.9 Ga _0.1 P having a thickness of 20 Å. In the case of this intermediate layer also AlG
The lattice constant is larger than that of the aAs system, and Al of the laser element is
When a compressive stress is applied from the semiconductor layer made of GaAs, the band gap energy increases. Also in the semiconductor laser device having this structure, a maximum optical output of 260 mW was obtained for the same reason as in the first embodiment, and it was confirmed that the effect was double that of the usual case.

<< Third Embodiment >> FIG. 4 shows a third embodiment of the present invention. In the semiconductor laser device of this embodiment, after growing an AlGaAs type VSIS laser as a base, an end face is formed by the cleavage method or etching, and M is formed on the light emitting end face.
OCVD method in a thickness 0.2μm of Al _0.5 Ga _{0 .5} As window layer 52, an intermediate layer 51 made of a thickness of 10 Å InP successively grown further by sputtering to form a protective film 61 is formed thereon It was made.

This semiconductor laser device and Al _0.5 G on the end face
When a semiconductor laser device in which only the a _0.5 As window layer 52 and the protective film 61 are formed and the intermediate layer 51 is not formed is compared, the latter maximum optical output is 340 mW,
In the former case, a maximum optical output of 500 mW was obtained, and the output characteristics were also greatly improved.

The reason for the above improvement can be explained as follows using the energy phase diagram as before. Figure 5
(A) shows the energy state in the vicinity of the surface of the semiconductor laser device of this embodiment, and (b) shows the semiconductor laser device in which the intermediate layer is omitted. In the case of a semiconductor laser device in which the intermediate layer is omitted, as shown in FIG. 5B, the Al _0.5 Ga _0.5 As window layer 52 prevents light absorption near the end face and carrier diffusion, but flows on the surface. small leakage current 101 Al _{0. 5} G
The light is trapped by the surface level formed on the surface of the a _0.5 As window layer 52, and the heat is slightly generated through the non-light emitting process. On the other hand, in the case of the present embodiment, the presence of the intermediate layer 51 eliminates the surface level of the Al _0.5 Ga _0.5 As window layer 52, and instead a slight surface level is formed on the surface of the intermediate layer 51. Only. The carriers due to the minute leak current trapped in this level are tunneling (resonance).
As a result, it is re-injected into the AlGaAs active layer of the laser element and used for light emission, so that it does not generate heat. Therefore, the effect of increasing the maximum light output as described above can be obtained.

The semiconductor laser device of this embodiment has excellent characteristics in terms of long-term reliability. That is, 60 semiconductor laser devices have an Al _0.5 Ga _0.5 As window layer without an intermediate layer. 1000 ° C under the driving condition of 70mW
After a lapse of time, the failure rate of 1% was shown, whereas the failure rate of the semiconductor laser device of this example was 0.1% under the same conditions.
Met.

In the above first, second and third embodiments,
The intermediate layer formed on the light emitting end face of the AlGaAs laser device was made of InP or In _0.9 Ga _0.1 P, but the material of the intermediate layer has a lattice constant larger than that of the AlGaAs active layer, that is, is subjected to compressive stress. Any material having a bandgap energy larger than that of the AlGaAs active layer may be used. Specifically, when the material is In _x Ga _{1 -x} P, x>
It may be 0.49. The intermediate layer is InP or InG
The main component may be aP, and some other elements may be included. The thickness of the intermediate layer was as thin as 10 to 20 angstroms in the above example, but the thickness may be within a range in which dislocation does not occur in the crystal of the intermediate layer due to strain and a surface level is not formed. .. Also, the applicable laser element is VSI
The structure is not limited to the S laser and may have any structure.

[0033]

As is apparent from the above description, since the semiconductor laser device of the present invention has the intermediate layer made of the material containing InP or InGaP on the light emitting end face thereof, it does not lead to the generation of heat. The emission end face is less likely to be destroyed, and the maximum light output can be improved. Further, when the window layer is provided, the effect can be further enhanced. In the case of the InGaP intermediate layer, the lattice constant of the intermediate layer can be made larger than that of the semiconductor layer made of AlGaAs if the In composition ratio is larger than 49% with respect to the total of In and Ga. Although the intermediate layer is a lattice-mismatched system with respect to the AlGaAs system, when it is formed thinner than the critical layer thickness at which dislocations occur, the crystallinity can be prevented from being adversely affected.

[Brief description of drawings]

FIG. 1 is a perspective view showing the vicinity of an end face of a semiconductor laser device according to a first embodiment.

FIG. 2 is a diagram showing the relationship between the current and the light output in the semiconductor laser device of the first embodiment together with the relationship of the conventional example.

FIG. 3A is an energy state diagram near the emission end surface of the semiconductor laser device of the first embodiment, and FIG. 3B is an energy state diagram near the emission end surface of the conventional semiconductor laser device.

FIG. 4 is a perspective view showing the vicinity of an end face of a semiconductor laser device according to a third embodiment.

5A is an energy state diagram in the vicinity of an emission end face of a semiconductor laser device of a third embodiment, and FIG. 5B is an energy state diagram in the vicinity of an emission end face of a conventional semiconductor laser device.

[Explanation of symbols]

 11 p-GaAs substrate 12 p-AlGaAs clad layer 13 p-AlGaAs active layer 14 n-AlGaAs clad layer 15 n-GaAs cap layer 21 n-GaAs current constriction layer 31, 32 electrode 51 InP or InGaP layer 52 AlGaAs window Layer 61 Protective film 101 Minute leak current flowing on the surface

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadashi Takeoka 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture Sharp Corporation (72) Masanori Watanabe 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture Co., Ltd. (72) Inventor Osamu Yamamoto 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture Sharp Corporation (72) Hiroshi Nakatsu 22-22, Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture Osaka Prefecture

Claims (4)

[Claims] 1. A semiconductor laser device in which one layer of a laser element formed by laminating a plurality of layers is an AlGaAs active layer, wherein a layer made of a material containing InP or InGaP is formed on a light emitting end face of the laser element. Semiconductor laser device. 2. A window layer made of AlGaAs having a higher Al mixed crystal ratio than the AlGaAs active layer is formed between the light emitting end face and a layer formed on the end face.
The described semiconductor laser device. 3. The layer formed on the light emitting end face is InG
3. The semiconductor laser device according to claim 1, wherein the semiconductor laser device is formed of a material containing aP, and the composition ratio of In in InGaP is greater than 49% with respect to the total of group III elements In and Ga. 4. The thickness of the layer formed on the light emitting end face is thinner than a critical layer thickness at which dislocation occurs in the layer.
2. The semiconductor laser device according to 2 or 3.