JP2001230496A - Semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a self-alignment type semiconductor laser element with a high long term reliability where a COMD hardly takes place and that oscillates a high optical output. SOLUTION: In the self-alignment type semiconductor laser element where a low refractive index layer 8 acting also like a current constricting layer is formed in the vicinity of an active layer, the low refractive index layer 8 consists of chemical compound semiconductor layers 81, 82, ..., 8i each made of an AlδxΧGa1-xXAs (0<=x<= 1). The further the compound group semiconductor layer from the active layer 5, the lower is the refractive index. More specifically, the semiconductor laser element has the low refractive index layer 8 consisting of the AlGaAs layers where the Al composition ratio is the higher, the further the AlGaAs layer is away from the active layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-aligned semiconductor laser device, and more particularly, to a semiconductor laser device capable of oscillating with high light output and having high long-term operation reliability.

[0002]

2. Description of the Related Art A self-aligned semiconductor laser device is a laser device having a high optical output capable of simultaneously confining an injection current and an oscillating laser beam in a resonator.
It is manufactured using an s-based compound semiconductor. FIG. 2 shows an example A of the layer structure of such a conventional semiconductor laser device.

In this device, a GaAs substrate 1 is provided with n
A buffer layer 2, n of 0.5 μm thick made of GaAs
-Al _0.3 Ga _0.7 lower cladding layer having a thickness of 2.0μm made of As 3, i-Al _0.1 Ga thickness 50 of _0.9 As
nm lower light confinement layer 4, active layer 5 composed of a 7 nm thick quantum well layer made of In _0.2 Ga _0.8 As and a 10 nm thick barrier layer made of Al _0.1 Ga _0.9 As, i-A
An upper optical confinement layer 6 of l _0.1 Ga _0.9 As having a thickness of 50 nm is laminated, and a thickness 500 of p-Al _0.3 Ga _0.7 As is formed on the upper optical confinement layer 6.
nm upper cladding layer 7a and n-Al _0.35 Ga _0.65 As
A low-refractive-index layer 8 also serving as a current confinement layer having a thickness of 0.5 μm and having a thickness of 2.0 μm is formed.
It is buried in an upper cladding layer 7b made of _0.3 Ga _0.7 As. A p-type layer is formed on the upper cladding layer 7b.
A contact layer 9 made of GaAs and having a thickness of 0.5 μm is laminated. Note that an upper electrode (not shown) is formed on the contact layer 9, and a lower electrode (not shown) is formed on the back surface of the substrate 1.

In the layer structure A of the device, a channel 10 having a predetermined width reaching the upper cladding layer 7a is formed in the current confinement layer (low refractive index layer) 8 as a current injection path.
A confinement structure is formed in the lateral (width) direction of light and current. The width W of the channel 10, that is, the width of the portion where the electric field intensity (light distribution) is strongest in the channel is determined in relation to the transverse mode control of the oscillation laser light. In particular,
In the semiconductor laser device described above, the upper clad layer 7
If a is made of p-Al _0.3 Ga _0.7 As and the thickness is about 500 nm as described above, it is necessary to cut off the higher-order mode of the laser beam oscillated by the active layer 5 to operate in the basic transverse mode. The width W of the channel 10 is designed to be about 2.5 μm in relation to a suitable cutoff width.

This device is manufactured as follows. First, the buffer layer 2, the lower cladding layer 3, the lower optical confinement layer 4, the active layer 5, and the upper optical confinement layer 6 are sequentially formed on the substrate 1 by MOCVD or MBE.
Further after forming the upper cladding layer 7a having a thickness of about 500nm on it, to form a layered structure A ₀ shows the layer 8 'to be in the low refractive index layer thereon by depositing in FIG.

The active layer 5 is made of GaAs and has a thickness of 10 nm.
Each of which has a thickness of 7 nm.
_{It is formed of} two quantum wells made of _0.2 Ga _0.8 As, and a light confinement layer made of GaAs having a thickness of 20 nm disposed on both sides of the quantum wells. Then, take out the layered structure A ₀ from the crystal growth apparatus performs photolithography and wet etching process on the layer structure A _0, the current to form a channel 10 the channel width W is 2.5μm in the layer 8 ' constriction layer to the layer structure a ₁ having a (low refractive index layer) 8 (FIG. 4). The side surface 10a of the channel 10 formed by the above wet etching process
Is an inclined surface extending upward under the influence of the anisotropy of etching.

Next, the layer structure A ₁ is placed in the crystal growth apparatus again, and the upper cladding layer 7 b and the contact layer 9 are sequentially formed on the layer structure A ₁ , and the layer structure A ₂ shown in FIG. Form. Then, after forming the upper and lower electrodes in the layer structure A _2, and the layer structure A cavity length is shown in Figure 2 is cleaved so that 800 [mu] m, one cleavage plane (front surface) S ₁ the reflectivity of 5% film is formed, a laser element is manufactured of interest by forming a reflection of 92% of the film to the other cleavage plane (rear surface) S _2.

The laser device manufactured according to the above specification is
The threshold current is 15 mA, and the maximum light output limited by the kink in the basic mode is about 350 mW. The oscillation wavelength is approximately 980 nm. By the way, the above-mentioned laser element having a cut-off width of about 2.5 μm in the low refractive index layer 8 has the following problem.

The first problem is that, in order to meet the recent demand for higher light output of the laser element, if the laser element is further increased in output, a new problem occurs in addition to the problem of kink generation. come. Specifically, operating at about optical output 500 mW, the optical density is higher at the front surface S _1, reaching up to several 10 MW / cm ^2. Therefore, the front end surface S ₁ is subjected to optical damage, the so-called COMD (Catastrop
hic Optical Mirror Damage) occurs.

The second problem is that, in order to make the low refractive index layer 8 lower in refractive index than the upper cladding layers 7a and 7b, the semiconductor material forming the low refractive index layer 8 must be made of A.
This is because they are formed of Al _0.35 Ga _0.65 As having a high 1 composition ratio. Specifically, after forming the layer structure A _1, before the upper cladding layer 7b are stacked, the surface of the channel 10 formed on the low refractive index layer 8 (side) is exposed, the air is its surface The problem is that it may be oxidized by the oxygen in it. Channel 1 like this
If the surface oxidation of the 0 (low-refractive-index layer 8), particularly the oxidation on the side surface of the channel 10 near the active layer 5, occurs, the light output decreases in a short time when the manufactured laser device is continuously operated. As a result, there arises a problem that the long-term reliability of the device decreases.

[0011]

The present invention solves the above-mentioned problem of the self-aligned semiconductor laser device having the layer structure shown in FIG. 5, and the COMD is reduced even when oscillating at a high optical output of 1000 mW or more. It is an object of the present invention to provide a self-aligned semiconductor laser device which is hardly generated and has high long-term reliability.

[0012]

According to the present invention, there is provided a self-aligned semiconductor laser device in which a low refractive index layer also serving as a current confinement layer is formed near an active layer. The low refractive index layer is made of Al _x Ga _1-x
A semiconductor laser device comprising a plurality of compound semiconductor layers made of As (0 ≦ x ≦ 1) and having a lower refractive index as the compound semiconductor layer is more distant from the active layer. Is done.

Specifically, there is provided a semiconductor laser device in which the compound semiconductor layer separated from the active layer has a higher Al composition ratio. Further, according to the present invention, in a self-aligned semiconductor laser device in which a low refractive index layer also serving as a current confinement layer is formed near an active layer,
The low-refractive-index layer is formed of Ga _y In _1-y As _z P _1-z (0 ≦ y ≦
0.5, 0 ≦ z ≦ 1), wherein the compound semiconductor layer separated from the active layer has a lower refractive index. Is provided.

[0014]

FIG. 1 shows an example B of a layer structure in a laser device of the present invention. The layer structure B is the same as the layer structure A shown in FIG. 2 except that the low refractive index layer 8 has a mode described later. The low refractive index layer 8 is made of GaAs such as AlGaAs, GaInAsP, or the like.
a plurality of layers 8 ₁ , 8 ₂ ,... 8 _i made of s-based compound semiconductor
It is composed of The layer located at a position farther from the active layer 5 has a lower refractive index.
That, in this low-refractive index layer 8, the refractive index of the layer ₈₁ which is deposited directly on the upper cladding layer 7a is the highest, is the refractive index toward the top becomes successively lower.

Such a low refractive index layer 8 is shown in FIG.
Layer structure A₀In forming the layer 8 ′, the Al composition ratio is different.
Al_xGa_1-xAs (0 ≦ x ≦ 1)
Formed as a layer having a composition change related to Al
can do. Specifically, p-Al_0.3Ga_0.7A
at least on the upper cladding layer 7a made of
The Al composition is the same as that of the upper cladding layer 7a, or
Layer 8 of n-AlGaAs having a smaller Al composition ratio₁
And the layer 8₁On top of this, the Al composition ratio is layer 8₁Greater than
Next layer 8 of AlGaAs_TwoAnd then further
Is layer 8 _TwoThe next layer is made of AlGaAs having a higher Al composition ratio.
The operation of forming a film is repeated, and the Al composition ratio increases as going upward.
Is a layer that rises stepwise.

The Al composition change in the low refractive index layer 8 may be a stepwise change in which the Al composition ratio is gradually increased as described above, but the Al composition ratio changes linearly. Alternatively, it may be changed in a parabolic manner. As described above, the change in the Al composition in the low refractive index layer 8 is caused by the equivalent refractive index (Σdini / Σ) of the entire low refractive index layer 8 with respect to the refractive indexes of the upper cladding layers 7a and 7b.
It is necessary to design so that が di is approximated; di is the thickness of each compound semiconductor layer, and ni is the refractive index of each compound semiconductor layer).

If the equivalent refractive index ratio between the portion of the channel 10 and the portion of the low refractive index layer 8 is designed to be smaller than that of the single low refractive index layer 8 shown in FIG. The basic lateral mode operation can be realized even if the width W of the bottom of the substrate is increased. Therefore, in the case of this laser device, the cutoff width can be made wider than that of the semiconductor laser device having the conventional structure shown in FIG.
Is less likely to occur. Conversely, high light output oscillation can be realized without COMD.

Since the Al composition ratio of the layer near the bottom of the channel 10 is smaller than that of the conventional structure, the vicinity of the bottom of the channel 10, that is, the layer structure A of the conventional structure shown in FIG. Oxidation near the active layer is suppressed. In other words, the oxidation of the bottom side surface where the electric field strength is highest is suppressed. Therefore, the long-term reliability of this laser device is higher than that of the conventional structure.

In order to prevent oxidation of the low refractive index layer 8 near the bottom of the channel 10, the lowermost layer of the low refractive index layer 8 may be formed of GaAs. In addition, if the equivalent refractive index of the low refractive index layer 8 is selected to be smaller than that of the upper cladding layers 7a and 7b, the low refractive index layer 8
Is replaced with Al _y GaAs instead of Ga _y In _1-y As _z P _1-z
(0 ≦ y ≦ 0.5, 0 ≦ z ≦ 1). In this case, since Ga _y In _1-y As _z P _1-z does not contain Al, a more remarkable effect can be obtained in suppressing oxidation.

In this case, the channel 1
If the design is such that the equivalent refractive index ratio of the portion of the low refractive index layer 8 other than the channel 10 is smaller than that of the single low refractive index layer 8 shown in FIG.
The basic lateral mode operation can be realized even if the width W of the bottom of 0 is widened. Therefore, also in the case of this laser device, the cutoff width can be made wider than that of the semiconductor laser device having the conventional structure shown in FIG. 5, so that the light density at the front end face during high light output oscillation becomes smaller, and COMD occurs. Is less likely to occur. Conversely, high light output oscillation can be realized without COMD.

[0021]

EXAMPLES width W of the channel 10 was 5 [mu] m, the low-refractive index layer 8, the thickness of 0.05μm. Before lowermost 8 ₁ of deposition Al _0.2 Ga _0.8 As (refractive index 3.53
Using 6), sequentially Al composition ratio, the layer ₈₂ is 0.25 (Thickness: 0.05 .mu.m, refractive index 3.514) Layer 8 ₃ 0.
3 (thickness: 0.1 [mu] m, refractive index 3.493) Layer 8 ₄ 0.35 (thickness: 0.15 [mu] m, refractive index 3.471) and will raise the linear, the uppermost 8 ₅ Thickness 0. 15 μm
Except that it is deposited from a 5-layer serving as the Al _0.4 Ga _0.6 As (refractive index 3.450), forms the same layer structure B with the layer structure A ₂ shown in FIG. 5 on the GaAs substrate did.

In this case, the equivalent refractive index of the low refractive index layer 8 is 3.48 when approximated by 近似 dini / Σdi (where di is the thickness of each layer and ni is the refractive index of each compound semiconductor layer). Then, after forming an electrode in this layer structure, the substrate was cleaved to a resonator length of 800 μm, and a film having a reflectivity of 5% and 92% was formed on the front end face S ₁ and the rear end face S ₂ to form a laser device.

The threshold current of this laser device was 20 mA, and the maximum light output limited by kink in the fundamental transverse mode was 500 mW. Further, when the light output was further increased, COMD occurred at 1200 mW. On the other hand, the conventional laser device fabricated from the layer structure A ₂ shown in FIG. 5 (a width in the channel bottom 2.5 [mu] m, the low refractive index layer is a single layer) COMD occurs in 500mW For. Therefore, the performance of the laser device of the present invention is greatly improved as compared with the conventional laser device.

When the laser was continuously operated under the conditions of a temperature of 60 ° C. and an optical output of 250 mW, and the reduction rate of the optical output after the lapse of 1000 hours was measured, in the case of the laser device of the present invention, it was 0.1 to 0.5. %Met. On the other hand, in the case of the conventional laser element, it was 1 to 5%. As is apparent from this, the laser device of the present invention has higher long-term reliability than the conventional laser device.

Also, in the layer structure B of FIG. 1 using a GaAs substrate, a semiconductor laser device having the same specifications as in the above embodiment is manufactured except that the low refractive index layer 8 has the following layer structure. did. That is, the lowermost layer 8 ₁ in the low refractive index layer 8 is formed of GaAs (refractive index 3.54), and the energy gap is sequentially increased to make the uppermost layer Ga _0.89 In _0.01.
It was deposited by As _{0. 78} P _0.22 (refractive index 3.45).

In the case of this semiconductor laser device, the same performance as that of the semiconductor laser device in which the low refractive index layer was formed of AlGaAs was exhibited. Further, in the case of this semiconductor laser device, since the low-refractive-index layer 8 is entirely formed of an Al-free layer containing no Al, the device does not deteriorate due to oxidation of the material. Note that the present invention is not limited to the above-described embodiment, and any combination may be used as long as the equivalent refractive index of the low refractive index layer 8 is appropriately selected as described above.

Further, an etching stop layer may be provided between the upper cladding layer 7a and the low refractive index layer 8 for controlling the etching of the low refractive index layer 8.

[0028]

As is apparent from the above description, the self-aligned laser device of the present invention oscillates at a high light output,
OMD is unlikely to occur and has high long-term reliability. This is an effect brought about by forming the low refractive index layer from a semiconductor material having a gradient composition such that the refractive index decreases as the distance from the active layer increases.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a layer structure B of a laser device of the present invention.

FIG. 2 is a perspective view showing a layer structure A of a conventional self-aligned laser device.

FIG. 3 shows a conventional layer structure A when manufacturing the layer structure A of FIG. 2;
₀ is a sectional view showing a.

4 is a sectional view showing a conventional layer structure A _1.

5 is a cross-sectional view illustrating a conventional layered structure A _2.

[Explanation of symbols]

Reference Signs List 1 GaAs substrate 2 Buffer layer (n-GaAs) 3 Lower cladding layer (n-Al _0.3 Ga _0.7 A
s) 4 Lower optical confinement layer (i-Al _0.1 Ga _0.9
As) 5 active layer (In _0.2 Ga _0.8 As / Al _0.1
Ga _0.9 As) 6 upper optical confinement layer (i-Al _0.1 Ga _0.9
As) 7a, 7b Upper cladding layer (p-Al _0.3 Ga _0.7 A
s) 8 low refractive index layer _{(n-AlGaAs) 8 1,} 8 2, ...... 8 i Al composition ratios are different n-AlGaAs
aAs layer (layer having different refractive index) 9 contact layer (p-GaAs) 10 channel 10a side surface of channel 10

Claims (3)

[Claims] 1. A self-aligned semiconductor laser device in which a low refractive index layer also serving as a current confinement layer is formed near an active layer, wherein the low refractive index layer has a composition formula: Al _x Ga _{1 -x} As (0 ≦ x
≦ 1) A semiconductor laser device comprising a plurality of layers of a compound semiconductor represented by the formula (1) and having a lower refractive index as the compound semiconductor layer is more distant from the active layer. 2. The semiconductor laser device according to claim 1, wherein an Al composition ratio is higher in the compound semiconductor layer that is more distant from the active layer. 3. A self-aligned semiconductor laser device in which a low refractive index layer also serving as a current confinement layer is formed near an active layer, wherein the low refractive index layer has a composition formula: Ga _y In _1-y As _z P
_1-z (0 ≦ y ≦ 0.5, 0 ≦ z ≦ 1) The compound semiconductor layer composed of a plurality of layers and separated from the active layer has a lower refractive index. A semiconductor laser device.