JP4809541B2 - Semiconductor laser element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a self-aligned semiconductor laser device, and more particularly to a semiconductor laser device that can oscillate at high light output and has high long-term operational reliability.
[0002]
[Prior 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 oscillation laser beam in a resonator, and is usually manufactured using a GaAs compound semiconductor.
An example A of the layer structure of such a conventional semiconductor laser device is shown in FIG.
[0003]
In this device, a 0.5 μm thick buffer layer 2 made of n-GaAs, a 2.0 μm thick lower cladding layer 3 made of n-Al _0.3 Ga _0.7 As, and an i-Al _0.1 Ga _0.9 on a GaAs substrate 1. Lower optical confinement layer 50 made of As, active layer 5, i− 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 An upper optical confinement layer 6 made of Al _0.1 Ga _0.9 As and having a thickness of 50 nm is laminated, and an upper cladding layer 7 a made of p-Al _0.3 Ga _0.7 As and having a thickness of 500 nm is formed on the upper optical confinement layer 6. , n-Al _0.35 Ga _0.65 low refractive index layer 8 also serves as the current confinement layer having a thickness of 0.5μm made of As are stacked, an upper cladding layer which are made of p-Al _0.3 Ga _0.7 As having a thickness of 2.0μm It is embedded in b. A contact layer 9 made of p-GaAs and having a thickness of 0.5 μm is laminated on the upper clad layer 7b. 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.
[0004]
In the layer structure A of this element, 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 in the lateral (width) direction of light and current. A confinement structure is formed.
The width W of the channel 10, that is, the width of the portion where the electric field strength (light distribution) is the strongest in the channel is determined in relation to the transverse mode control of the oscillation laser light. Specifically, in the above-described semiconductor laser device, when the upper cladding layer 7a is made of p-Al _0.3 Ga _0.7 As and has a thickness of about 500 nm as described above, the high level of the laser light oscillated in the active layer 5 is increased. The width W of the channel 10 is designed to be about 2.5 μm in relation to the cut-off width necessary for operating the basic transverse mode by cutting off the next mode.
[0005]
This element 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. Then, an upper cladding layer 7a having a thickness of about 500 nm is formed, and then a layer 8 ′ to be a low refractive index layer is formed thereon to form the layer structure A ₀ shown in FIG.
[0006]
The active layer 5 is separated by a 10 nm thick barrier layer made of GaAs, each of which has a thickness of 7 nm, two quantum wells made of In _0.2 Ga0 _0.8 As, and 20 nm thick arranged on both sides of these quantum wells. The optical confinement layer is made of GaAs.
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 ' A layer structure A ₁ having a constriction layer (low refractive index layer) 8 is formed (FIG. 4). The side surface 10a of the channel 10 formed by the wet etching process is an inclined surface that spreads upward under the influence of etching anisotropy.
[0007]
Then placed in a crystal growth apparatus again a layer structure A _1, sequentially deposited upper cladding layer 7b and the contact layer 9 on a layer structure A _1, to form a layered structure A ₂ shown in FIG.
Then, after an upper electrode and a lower electrode are formed in this layer structure A ₂ , cleavage is performed so that the resonator length becomes 800 μm, and the layer structure A shown in FIG. 2 is formed, and one of the cleavage surfaces (front end surface) S _{1 is formed.} 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.
[0008]
The laser element manufactured with the above specifications has a threshold current of 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-described laser element in which the cut-off width in the low refractive index layer 8 is about 2.5 μm has the following problems.
[0009]
The first problem is that, in order to meet the recent demand for higher optical output of laser elements, if an attempt is made to further increase the output of the laser elements, a new problem occurs in addition to the problem of occurrence of kinks. Specifically, when operating at an optical output of about 500 mW, the light density at the front end face S ₁ increases and reaches several tens of MW / cm ² . For this reason, the front end face S ₁ is optically damaged, and so-called COMD (Catastrophic Optical Mirror Damage) occurs, resulting in a problem.
[0010]
The second problem is that the semiconductor material constituting the low refractive index layer 8 has a high Al composition ratio of Al _0.35 so that the low refractive index layer 8 has a lower refractive index than the upper cladding layers 7a and 7b. This is due to the formation of Ga _0.65 As. Specifically, since the surface (side surface) of the channel 10 formed in the low refractive index layer 8 is exposed after the layer structure A ₁ is formed and before the upper cladding layer 7b is laminated, the surface is exposed to the atmosphere. It is a problem that it may be oxidized by oxygen inside. When such surface oxidation of the channel 10 (low refractive index layer 8), particularly oxidation occurs on the side surface of the channel 10 close to the active layer 5, light output is achieved in a short time when the manufactured laser element is operated continuously. As a result, there is a problem that long-term reliability as an element is lowered.
[0011]
[Problems to be solved by the invention]
The present invention solves the above-mentioned problem of the self-aligned semiconductor laser device having the layer structure shown in FIG. 5, and does not easily generate COMD even when oscillating at a high optical output of 1000 mW or more, and has long-term reliability. An object is to provide a high self-aligned semiconductor laser device.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, in a self-aligned semiconductor laser device in which a low refractive index layer that also serves as a current confinement layer is formed in the vicinity of an active layer,
The low refractive index layer is composed of a plurality of compound semiconductor layers made of Al _x Ga _1-x As (0 ≦ x ≦ 1), and the compound semiconductor layer separated from the active layer has a lower refractive index. and which, among the compound semiconductor layer for forming the low refractive index layer, a semiconductor laser device lowermost layer formed on said active layer and said GaAs Tona Rukoto is provided.
[0013]
Specifically, there is provided a semiconductor laser device having a higher Al composition ratio as the compound semiconductor layer is separated from the active layer.
In the present invention, in the self-aligned semiconductor laser element in which the low refractive index layer that also serves as the current confinement layer is formed in the vicinity of the active layer,
The low refractive index layer is composed of a plurality of compound semiconductor layers made of Ga _1-y In _y As _z P _1-z (0 ≦ y ≦ 0.5, 0 ≦ z ≦ 1), and from the active layer as the compound semiconductor layer which is remote has become the low refractive index, of the compound semiconductor layer for forming the low refractive index layer, the lowermost layer formed on said active layer and said GaAs Tona Rukoto A semiconductor laser device is provided.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An example B of the layer structure in the laser device of the present invention is shown in FIG. This layer structure B is the same as that of the layer structure A shown in FIG. 2 except that the low refractive index layer 8 is in the form described later.
The low refractive index layer 8 is composed of a plurality of layers 8 ₁ , 8 ₂ ,... 8 _i made of a GaAs compound semiconductor such as AlGaAs or GaInAsP. And the layer located in the location which is separated from the active layer 5 is a layer with a low 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.
[0015]
Such a low refractive index layer 8 is formed by sequentially forming Al _x Ga _1-x As (0 ≦ x ≦ 1) having different Al composition ratios when forming the layer 8 ′ in the layer structure A ₀ shown in FIG. By doing so, it can be formed as a layer having a composition change with respect to Al.
Specifically, a layer of n-AlGaAs having an Al composition ratio that is at least the same as or smaller than that of the upper cladding layer 7a is formed on the upper cladding layer 7a made of p-Al _0.3 Ga _0.7 As. 8 ₁ forming a, on top of that layer 8 ₁ forming a next layer ₈₂ with an Al composition ratio of the layer ₈₁ is greater than AlGaAs, further on a high Al composition ratio than layer ₈₂ on its By repeating the operation of depositing the next layer with AlGaAs, the Al composition ratio is increased stepwise as it goes upward.
[0016]
The Al composition change in the low refractive index layer 8 may be a step-like change in which the Al composition ratio increases sequentially as described above, but the Al composition ratio varies linearly. Alternatively, it may be changed into a parabolic shape.
The composition change of Al as described above in the low refractive index layer 8 is approximated by the equivalent refractive index (Σdini / Σdi) 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 the thickness of the compound semiconductor layer, ni, is the refractive index of each compound semiconductor layer).
[0017]
If the equivalent refractive index ratio between the channel 10 portion and the low refractive index layer 8 portion is designed to be smaller than that of the single low refractive index layer 8 shown in FIG. Even if the width W is increased, the basic transverse mode operation can be realized.
Therefore, in the case of this laser element, the cut-off width can be made wider than that of the semiconductor laser element having the conventional structure shown in FIG. 5, so that the light density at the front end face at the time of high light output oscillation is reduced, and the occurrence of COMD is reduced. It is hard to happen. In other words, high light output oscillation can be realized without causing COMD.
[0018]
Further, 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 vicinity of the active layer is compared with the case of the layer structure A having the conventional structure shown in FIG. Is inhibited from oxidation. In other words, the oxidation of the side surface of the bottom where the electric field strength is strongest is suppressed. Therefore, the long-term reliability of this laser element is higher than that of the conventional structure.
[0019]
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 Ga _1-y In _y As _z instead of AlGaAs. P _1-z (0 ≦ y ≦ 0.5, 0 ≦ z ≦ 1) can also be used. In this case, since Ga _{1 -y} In _y As _z P _{1 -z} does not contain Al, a more remarkable effect can be obtained in suppressing oxidation.
[0020]
Here, the reason why the y value is defined as 0 ≦ y ≦ 0.5 is that when the y value is larger than 0.5, lattice matching with the GaAs substrate is not achieved and good crystallinity cannot be obtained. is there.
Even in this case, the equivalent refractive index ratio between the channel 10 portion and 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. With this design, the basic transverse mode operation can be realized even if the width W of the bottom of the channel 10 is increased. Therefore, in this laser element as well, the cutoff width can be made wider than that of the conventional semiconductor laser element shown in FIG. 5, so that the light density at the front end face at the time of high optical output oscillation is reduced, and the occurrence of COMD Is difficult to occur. In other words, high light output oscillation can be realized without causing COMD.
[0021]
【Example】
Width W of the channel 10 was 5 [mu] m, the low-refractive index layer 8, using a thickness 0.05μm Al _0.2 Ga _0.8 As (refractive index 3.536). Before lowermost 8 ₁ of the film forming, successively Al The composition ratio of the layer 8 ₂ is 0.25 (thickness: 0.05 μm, refractive index 3.514), the layer 8 ₃ is 0.3 (thickness: 0.1 μm, refractive index 3.493), and the layer 8 ₄ is 0.35 (thickness: 0.15 [mu] m, refractive index 3.471) and will raise linearly, from 5 layers as the top layer 8 ₅ in thickness 0.15μm Al _0.4 Ga _0.6 as (refractive index 3.450) Except for the film formation, the same layer structure B as the layer structure A ₂ shown in FIG. 5 was formed on the GaAs substrate.
[0022]
In this case, the equivalent refractive index of the low refractive index layer 8 is 3.48 when approximated by Σdini / 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, it was cleaved to a resonator length of 800 μm, and films with reflectivities of 5% and 92% were formed on the front end face S ₁ and the rear end face S ₂ to form a laser element.
[0023]
The threshold current of this laser element was 20 mA, and the maximum light output limited by the kink in the fundamental transverse mode was 500 mW.
Further, when the light output was further increased, CMD was generated at 1200 mW. On the other hand, in the case of the conventional laser element manufactured from the layer structure A ₂ shown in FIG. 5 (the width at the bottom of the channel is 2.5 μm and the low refractive index layer is a single layer), COMD is generated at 500 mW. Therefore, the performance of the laser element of the present invention is greatly improved as compared with the conventional laser element.
[0024]
Further, when continuously operating under the conditions of a temperature of 60 ° C. and an optical output of 250 mW and measuring the decrease rate of the optical output after 1000 hours, it was 0.1 to 0.5% in the case of the laser element of the present invention. It was. On the other hand, in the case of the conventional laser element, it was 1 to 5%. As is clear from this, the laser element of the present invention has higher long-term reliability than the conventional laser element.
[0025]
Further, in the layer structure B of FIG. 1 using a GaAs substrate, a semiconductor laser device having the same specifications as in the above example was manufactured, except that the low refractive index layer 8 had the following layer structure.
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 form the uppermost layer as Ga _0.89 In _0.11 As _0.78 P _0.22 (refractive index 3.45). ).
[0026]
In the case of this semiconductor laser element, the same performance as that of the semiconductor laser element in which the low refractive index layer is formed of AlGaAs was exhibited.
Further, in the case of this semiconductor laser element, since the low refractive index layer 8 is entirely composed of an Al-free layer that does not contain Al, the element does not deteriorate due to oxidation of the material.
The present invention is not limited to the above-described embodiments, 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.
[0027]
Further, an etching stop layer may be provided between the upper cladding layer 7 a and the low refractive index layer 8 for controlling the etching of the low refractive index layer 8.
[0028]
【The invention's effect】
As is apparent from the above description, the self-aligned laser element of the present invention oscillates at a high light output, does not easily generate CMD, and has high long-term reliability. This is an effect brought about by forming the low refractive index layer with 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 cross-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 element.
FIG. 3 is a cross-sectional view showing a conventional layer structure A ₀ when manufacturing the layer structure A of FIG. 2;
FIG. 4 is a cross-sectional view showing a conventional layer structure A ₁ .
FIG. 5 is a cross-sectional view showing a conventional layer structure A ₂ .
[Explanation of symbols]
1 GaAs substrate 2 Buffer layer (n-GaAs)
3 Lower clad layer (n-Al _0.3 Ga _0.7 As)
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 As)
8 Low refractive index layer (n-AlGaAs)
8 ₁ , 8 ₂ ,... 8 _i n-AlGaAs layers with different Al composition ratios (layers with different refractive indexes)
9 Contact layer (p-GaAs)
10 Channel 10a Side surface of channel 10

Claims (3)

In a self-aligned semiconductor laser device in which a low refractive index layer that also serves as a current confinement layer is formed in the vicinity of the active layer,
The low refractive index layer is composed of a plurality of layers of compound semiconductors of the same conductivity type represented by the composition formula: Al _x Ga _1-x As (0 ≦ x ≦ 1), and is separated from the active layer. The compound semiconductor layer has a lower refractive index, and the plurality of compound semiconductor layers forming the lower refractive index are at least 2 different from the lowest GaAs layer in the range of 0 <x ≦ 1 and x value. A semiconductor laser device comprising two compound semiconductor layers .   The semiconductor laser device according to claim 1, wherein the Al composition ratio is higher as the compound semiconductor layer is separated from the active layer. In a self-aligned semiconductor laser device in which a low refractive index layer that also serves as a current confinement layer is formed in the vicinity of the active layer,
The low refractive index layer includes a plurality of layers of compound semiconductors of the same conductivity type represented by a composition formula: Ga _{1 -y} In _y As _z P ₁ -z (0 ≦ y ≦ 0.5, 0 ≦ z ≦ 1) And the compound semiconductor layer which is separated from the active layer has a lower refractive index, and the plurality of compound semiconductor layers forming the low refractive index layer include a lowermost GaAs layer and 0 < A semiconductor laser element comprising: at least two compound semiconductor layers satisfying y ≦ 0.5 and having different y values .

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