JPH08335750A - Semiconductor laser and its fabrication - Google Patents

Abstract

PURPOSE: To provide a semiconductor laser in which the width of stripe and the current constriction width are made narrow while sustaining the clad layer at a specified thickness. CONSTITUTION: The semiconductor laser is an AlGaAs based ridge waveguide type self-aligned semiconductor laser comprising a compound semiconductor laminate structure including an active layer 16, a lowermost optical guide layer 20 of Alx Ga1-x As, an upper clad layer 22 of Aly Ga1-y As, and a reverse mesa ridge type optical waveguide structure 50. The content (x) of Al in the optical guide layer is set in the range of 0.1<=x<=0.3 and the content (y) of Al in the clad layer is set higher than that of the optical guide layer so that the optical guide layer serves as an etching block layer at the time of etching the clad layer. The clad layer can be formed in reverse mesa type ridge shape by decreasing the content (y) of Al in the clad layer monotonously depending on the distance from the optical guide layer and using a hydrofluoric acid based etchant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AlGaAs-based ridge-guided self-aligned semiconductor laser and a method of manufacturing the same, and more particularly to an AlGa having a near-field near-field image.
The present invention relates to an As-based ridge waveguide type self-aligned semiconductor laser and a method for manufacturing the same.

[0002]

2. Description of the Related Art A semiconductor laser having a ridge type optical waveguide structure has been actively developed and put into practical use in order to improve the light confinement effect and the current confinement effect.
A conventional example of an AlGaAs based ridge waveguide type self-aligned semiconductor laser will be described below. Conventional Al G
a As series ridge waveguide type self-aligned semiconductor laser 7
0 (hereinafter simply referred to as a semiconductor laser 70) has a compound semiconductor laminated structure formed on a substrate, an optical waveguide structure formed in a mesa type ridge shape, and an optical waveguide structure, as shown in FIG. It has a buried structure for embedding and an electrode structure.

The compound semiconductor laminated structure has an n-type GaAs (1
00) The n-type lower clad layer 14, the active layer 16, and the p-type upper first clad layer 18, which are sequentially epitaxially grown on the substrate 12. The optical waveguide structure is formed as a mesa-type ridge portion 60 having a trapezoidal cross section, and the ridge portion 60 includes a p-type optical guide layer 20 and a p-type upper second layer thereon.
And a clad layer 22.

The buried structure is formed on both side surfaces of the mesa type ridge portion 60 and on the upper first cladding layer 18.
Type current blocking layer 28 and p formed on the current blocking layer 28
It is composed of the upper mold third clad layer 30. The electrode structure has an upper second cladding layer 2 forming the ridge portion 60.
P-type inversion layer 36 configured as a current injection path on
And the upper third cladding layer 3 in a region other than the p-type inversion layer 36.
0, a p-type contact layer 32, a p-type ohmic electrode 38 formed on the p-type inversion layer 36 and the contact layer 32, and an n-type ohmic electrode 40 formed on the lower surface of the substrate 12. Are formed from. In the conventional semiconductor laser 70 described above, the upper second cladding layer 22 above the optical guide layer 20 is selectively etched by a hydrofluoric acid-based wet etchant to form a ridge portion 60 having a trapezoidal cross section. There is. The self-alignment of the semiconductor laser 70 described above is performed by the ridge portion 60 including the light guide layer 20.
And the current blocking layer 28.

[0005]

The following points have been pointed out in the above-mentioned conventional semiconductor laser 70. First is the problem of far-field images. Semiconductor laser 7
The stripe width of 0 is substantially defined by the width of the bottom surface of the upper second cladding layer 22 formed on the optical guide layer in a trapezoidal cross section. On the other hand, the far-field image of a semiconductor laser is generally elliptical, and the divergence angle parallel to the bonding surface is smaller than the divergence angle in the vertical direction, and the extent of the divergence increases in inverse proportion to the stripe width. It is necessary to narrow the width of the bottom surface of the cladding layer in order to make the image closer to a circle. However, in the conventional semiconductor laser 70, since the ridge portion has a trapezoidal cross-sectional shape, if the width of the bottom surface of the clad layer is narrowed to a certain dimension or less, the cross-sectional shape becomes a triangle rather than a trapezoid, and the upper second clad layer There is a drawback that the thickness of 22 is reduced and the light confinement effect is reduced. Secondly, there is a problem of current constriction width. It is preferable to narrow the current confinement width because it is possible to suppress the lateral spatial hole burning that tends to occur during high-power operation of the semiconductor laser. However, in the case of the conventional mesa type ridge portion, in the case of both stripes in the [011] direction and the [01-1] direction,
The lower side of the ridge trapezoidal section becomes longer than the upper side,
There is a limit to shortening the length of the lower side while keeping the thickness of the cladding layer 22 at a predetermined thickness, that is, narrowing the current constriction width.

In view of the above problems, the object of the present invention is to
It is an object of the present invention to provide an Al GaAs based ridge waveguide type self-aligned semiconductor laser in which the stripe width and the current constriction width are narrower than the conventional one while maintaining the thickness of the cladding layer at a predetermined thickness.

[0007]

The inventors of the present invention have a limit in reducing the length of the lower side while keeping the thickness of the upper second cladding layer at a predetermined thickness in the mesa type ridge shape.
It has been found that it is necessary to realize an inverted mesa type ridge-shaped optical waveguide structure in order to improve the above problems. Further, by studying a method for realizing an inverted mesa type ridge-shaped optical waveguide structure, the present inventor found that when the AlGaAs layer is wet-etched by using a hydrofluoric acid-based etchant,
The etching rate increases monotonically as the Al content of the layer increases, especially when the Al content is 0.3, especially 0.4.
The present invention has been completed, paying attention to the fact that the number suddenly increases when the value exceeds.

In order to achieve the above object, based on the above findings, the semiconductor laser according to the present invention is an AlGaAs system ridge waveguide type self-aligned semiconductor laser, and the active layer and the active layer are arranged from above and below. A compound semiconductor laminated structure having at least a pair of a lower clad layer and an upper clad layer sandwiched therebetween, an optical guide layer made of Al _x Ga _1-x As at the lowermost layer, and an upper layer made of Al _y Ga _1-y As thereon. And an optical waveguide structure formed on the compound semiconductor laminated structure in an inverted mesa type ridge shape, each having an Al cladding ratio X of 0.1 ≦ X ≦ 0.3. When the upper clad layer is etched, the optical guide layer acts as an etching stop layer when the Al content rate Y of the upper clad layer is controlled to be larger than the Al content rate X of the optical guide layer. And further Monotonically decreases in accordance with an Al content of Y in the clad layer to the distance from the light guide layer,
In addition, it is characterized in that the upper clad layer is wet-etched into an inverted mesa type ridge shape by using a hydrofluoric acid type etchant.

The Al content X of the light guide layer is 0.1 ≦ X ≦
The range was regulated to 0.3, but when the Al content was 0.3 or less, AlGaAs was used with a hydrofluoric acid-based etchant.
Since the etching rate when the layer is wet-etched is remarkably low, the optical guide layer can function as an etching stop layer for the upper clad layer in which the Al content Y is higher than the Al content X. Further, when X is 0.1 or less, the etching stop layer acts as an absorption layer for light having a wavelength of 800 nm or less, and therefore, when used in a semiconductor laser, particularly a semiconductor laser for compact discs, its optical damage level and consumption are reduced. This is because various characteristics such as electric power are deteriorated. The monotonous decrease of the Al content Y does not mean that the Al content Y decreases continuously, but does not necessarily mean that the Al content Y decreases only in a primary or secondary relationship.

According to the present invention, a method of manufacturing an AlGaAs based ridge waveguide type self-aligned semiconductor laser includes a first conductivity type substrate, a first conductivity type lower clad layer and an active layer, respectively. The first upper cladding layer of the second conductivity type different from the first conductivity type, and the Al content X is 0.1 ≦ X ≦ 0.
The light guide layer regulated to the range of 3 is sequentially epitaxially grown, and the Al content Y on the light guide layer is higher than the Al content X of the light guide layer and monotonically decreases according to the distance from the light guide layer. So as to epitaxially grow the upper second cladding layer of the second conductivity type, and then to epitaxially grow another GaAs layer of the second conductivity type, and a step of forming a stripe mask pattern on the other GaAs layer. And a step of transferring the pattern to a predetermined position in the depth direction of the upper second cladding layer using the stripe mask pattern as a mask, and a light guide by wet etching using a hydrofluoric acid-based etchant using the stripe mask pattern as a mask. The reverse mesa type ridge portion is formed by selectively removing only the upper second cladding layer while allowing the layer to function as an etching stop layer. Forming, it is characterized by comprising a step of transferring a pattern to the light guide layer by wet etching.

[0011]

[Operation] Al Ga A using hydrofluoric acid type etchant
The wet etching rate of the s layer monotonically increases as the Al content of the layer increases.
Therefore, the Al content rate X of the optical guide layer made of Al Ga As layer is smaller than that of the upper cladding layer, and 0.1 ≦ X ≦
By limiting the range to 0.3, the optical guide layer can function as an etching stop layer when the upper clad layer is etched. Further, the Al content Y of Al _y Ga _1-y As constituting the clad layer above the light guide layer is higher than the Al content of the light guide layer and monotonically decreases as the distance from the light guide layer increases, and By wet etching using a system etchant,
The etching cross-sectional shape of the clad layer is an inverted mesa type ridge portion in which the upper side is longer than the lower side. With the above configuration,
The stripe width can be reduced without reducing the thickness of the upper clad layer.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the accompanying drawings. In the following description, the numbers exemplified for the thickness and composition of each layer are one example for specifically explaining the present invention, and it goes without saying that other numbers can be taken. It does not depart from the gist.

Embodiment of Semiconductor Laser FIG. 1 is a sectional view showing a layer structure of an embodiment of a semiconductor laser according to the present invention. As shown in FIG. 1, the semiconductor laser 10 of the present embodiment has a compound semiconductor laminated structure formed on a substrate, an optical waveguide structure formed in an inverted mesa type ridge shape, and an embedded structure for embedding the optical waveguide structure. , And an electrode structure. The compound semiconductor laminated structure is
A lower clad layer 14 of n-type Al _u Ga _1-u As (u = 0.5) with a thickness of 2 μm and an intrinsic Al _v G epitaxially grown on an n-type GaAs (100) substrate 12.
An active layer 16 made of a _1-v As (v = 0.13) and having a thickness of 0.06 μm, and an upper portion made of p-type Al _w Ga _1-w As (w = 0.5) and having a thickness of 0.3 μm It is composed of the first clad layer 18.

The optical waveguide structure is formed as a so-called inverted mesa type ridge portion 50 having an inverted trapezoidal cross section, and the ridge portion 50 is made of p-type Al _x Ga _1-x As (x = 0.3). 0.09 μm thick optical guide layer 20 and p-type Al _y Ga _1-y
A second upper 2 μm thick layer made of As (Y is described later).
And a clad layer 22. The Al content Y of the upper second cladding layer 22 is 0.5 at a point in contact with the active layer 16 as shown in FIG. 2I, for example, and gradually increases monotonically according to the distance L from the active layer 16. Decreased p-type G
At the point of contact with the As layer 24, ie, L = 0.3 μm
At that point, it will be 0.45. That is, in this embodiment, Y
Decreases linearly with the distance L, and Y = − (5/3
0) The relationship is L + 0.5. The Al content Y of the upper second cladding layer 22 has only to gradually and monotonically decrease according to the distance L from the active layer 16, and as described above, Y and L are primary. You don't have to be in a relationship. Further, FIG. 2 (I) shows the Al content of each compound semiconductor layer on the horizontal axis corresponding to FIG. 2 (a).

The buried structure is formed on both side surfaces of the inverted mesa type ridge portion 50 and on the upper first cladding layer 18.
The thickness of the mold is Al _w Ga _1-w As (w = 0.5).
The current blocking layer 28 having a thickness of 5 μm and the upper third cladding layer 30 having a thickness of 1.5 μm and made of p-type Al _w Ga _1-w As (w = 0.5) formed on the current blocking layer 28. It is configured. The electrode structure is the p-type inversion layer 3 formed as a current injection path on the upper second cladding layer 22 of the ridge portion 50.
6 and a p-type GaAs layer formed on the third upper cladding layer 30 in a region other than the p-type inversion layer 36.
5 μm contact layer 32 and p-type ohmic electrode 38 formed on p-type inversion layer 36 and contact layer 32
And the n-type ohmic electrode 4 formed on the lower surface of the substrate 12.
It is formed from 0 and. In this embodiment, the semiconductor laser 10 has a length between both end faces of 250 μm.

With the above structure, in this embodiment, the stripe width can be made narrower than in the conventional case, and the thickness of the upper second cladding layer 22 can be maintained at a predetermined thickness. This,
A far-field image close to a circle can be realized, and lateral spatial hole burning can be suppressed. Further, in the semiconductor laser 10 of the present embodiment, the lateral refractive index step width can be narrowed at the same time, so that the lateral mode is stable during high-power operation.

Manufacturing Method of Example FIG. 2A to FIG. 2C, FIG. 3D to FIG.
(G) and (h) are cross-sectional views showing the layer structure of the substrate at each manufacturing stage of the semiconductor laser 10 of the example. Figure 2 (a)
, The n-type lower clad layer 1 is formed on the n-type substrate 12 by a vapor phase growth method, for example, a metal organic chemical vapor deposition method.
4, active layer 16, p-type upper first cladding layer 18, p-type optical guide layer 20, p-type upper second cladding layer 22, p-type Ga
The As layer 24 is sequentially epitaxially grown. At this time, the p-type Al _y G forming the upper second cladding layer 22 is formed.
The Al content Y of the a _1-y As compound semiconductor depends on the active layer 16
According to the distance L from, Y =-(5/30) L + 0.5
The control program is operated so that the flow rate of the raw materials such as trimethylaluminum (TMAl) and trimethylgallium (TMG) is controlled by the mass flow controller connected to the computer. Next, a stripe mask pattern 26 made of a photoresist having a width of 4 μm and extending in the [01-1] direction is formed on the above-mentioned wafer surface, that is, the GaAs layer 24 by a normal photolithography technique (FIG. 2A). ).

Next, using the mask pattern 26 as a mask, the GaAs layer 24 is removed by wet etching using, for example, a mixed solution of dilute hydrochloric acid and hydrogen peroxide solution, and the upper second cladding layer 22 is formed in the depth direction thereof. The pattern is transferred to a position near the middle (FIG. 2 (b)).

Next, by using the mask pattern 26, wet etching using a hydrofluoric acid-based etchant is performed to obtain G.
a As layer 24 is used as a layer for the upper second cladding layer 22.
Only the portion is selectively shaved to form the ridge portion 50 (see FIG. 2).
(C)). In wet etching using a hydrofluoric acid-based etchant, the Al content in the upper second cladding layer 22 decreases from the bottom to the top, and the etching rate decreases in proportion to it. The shape is an inverted mesa type. At this time, the light guide layer 20 having a low Al content of 0.3 or less functions as an etching stop layer in the wet etching of the upper second cladding layer 22 using a hydrofluoric acid-based etchant. Etching
The interference fringes are observed by naked eyes or the like immediately before reaching the light guide layer 20, and when the etching actually reaches the light guide layer 20, the interference fringes disappear. Therefore, by observing the generation and disappearance of the interference fringes, it is confirmed that the etching progresses to remove the upper second cladding layer 22 in the region other than the ridge portion 50 and reach the optical guide layer 20 which is the etching stop layer. Reliable recognition and etching control with good reproducibility.

Subsequently, the light guide layer 2 is wet-etched using a mixed solution of ammonia water and hydrogen peroxide water.
The pattern is transferred to 0, the eave-shaped overhang of the GaAs layer 24 is removed, and the mask pattern 26 is removed (FIG. 3D). Next, a second epitaxial growth is performed by using a vapor phase growth method, for example, a metal organic chemical vapor deposition method, to form a second stacked structure on the entire surface of the substrate. That is, the n-type current blocking layer 28, the p-type upper third cladding layer 30, and the p-type contact layer 32 are sequentially formed on the entire surface of the substrate (FIG. 3E). Next, a photoresist film is applied with a film thickness smaller than the level difference of the irregularities of about 2 μm generated by the triangular protrusions 52 formed on the substrate surface. For example,
Such a photoresist film can be formed by spin coating MP1400-17 at 4000 rpm for 20 seconds. Then, the photoresist film is removed by ashing, for example, oxygen plasma ashing to expose the contact layer 32 on the top of the ridge. At this stage, the cross section of the photoresist film 34 is shown in FIG.
As shown in (f).

Next, by wet etching using a mixed solution of phosphoric acid and hydrogen peroxide solution, the p-type contact layer 32 and the upper third cladding layer 30 forming the protrusion 52 are removed and planarized (see FIG. 3 (g)). As a result, the unevenness on the surface is suppressed to 0.5 μm or less. Further, the photoresist film 34 is removed (FIG. 3 (h)). Next, the inversion layer 36 is formed by utilizing the fact that the rate of zinc diffusion is higher in Al GaAs than in GaAs. That is, zinc is diffused into the portion of the n-type Al GaAs current blocking layer 28 exposed from the p-type GaAs contact layer 32 by flattening the surface, and the conductivity type is inverted to form the p-type inversion layer 36. Formed (FIG. 3 (i)). This ensures the current injection path. Next, after depositing the p-type ohmic electrode 38 on the surface of the substrate, lapping the back surface of the substrate until the thickness of the substrate reaches a predetermined thickness, for example, 100 μm, and then covering the n-type ohmic electrode 40. Form. After that, it is cleaved to a predetermined length, for example, 250 μm, whereby the semiconductor laser 10 according to the present invention shown in FIG. 1 can be obtained.

In the method of this embodiment, the fact that the etching rate of the Al Ga As layer increases in proportion to the Al content in wet etching using a hydrofluoric acid-based etchant is utilized, and The second upper cladding layer 22 having a gradually decreasing Al content can be etched to form an inverted mesa type ridge. Also,
The upper second clad layer 22 can be etched with good reproducibility by causing the light guide layer 20 to function as an etching stop layer.

[0023]

EFFECT OF THE INVENTION Using a hydrofluoric acid type etchant, Al
The wet etching rate of the GaAs layer monotonically increases as the Al content of the layer increases, and in particular, increases sharply when the Al content exceeds 0.4.
In the Al GaAs based ridge waveguide type self-aligned semiconductor laser (hereinafter referred to as a semiconductor laser) according to the present invention,
Utilizing this, by monotonically decreasing the Al content Y of the Al _y Ga _1-y As clad layer which is located above the optical guide layer and constitutes the ridge portion, according to the distance from the optical guide layer, An inverted mesa ridge-shaped optical waveguide structure is realized in which the cross-sectional shape of the upper clad layer is longer on the upper side than on the lower side. As a result, firstly, even when the stripe width, that is, the current constriction width is narrowed to 2 μm or less, the thickness of the clad layer does not decrease. An image can be realized and lateral spatial hole burning can be suppressed. Secondly, in the semiconductor laser according to the present invention, the lateral refractive index step width can be narrowed at the same time, so that the lateral mode is stable during high-power operation. Thirdly, compared to the case of etching a conventional clad layer having a uniform Al composition, in a semiconductor laser having the same step width of refractive index, the current constriction width can be widened and the series resistance can be reduced, so that the operating voltage is lowered. It is possible to increase the energy conversion efficiency. Fourthly, in the conventional semiconductor laser, partial crystal growth inhibition was likely to occur between the upper surface and the side surface of the ridge, but in the semiconductor laser according to the present invention, the upper clad of the ridge portion is formed. Since the Al content in the layer portion can be made lower than before, the surface shape after the second epitaxial crystal growth becomes better than before.

In the method of manufacturing a semiconductor laser according to the present invention, the Al content of the upper clad layer constituting the ridge portion is gradually decreased from the bottom to the top of the layer, and a hydrofluoric acid type etchant is used. By wet etching,
It becomes easy to form the upper clad layer in an inverted mesa ridge shape. Further, by making the light guide layer function as an etching stop layer, the upper clad layer can be etched with good reproducibility. Therefore, the semiconductor laser manufacturing method according to the present invention can easily and reproducibly manufacture the above-described semiconductor laser according to the present invention.

[Brief description of drawings]

FIG. 1 is a sectional view showing a layer structure of an embodiment of a semiconductor laser according to the present invention.

FIGS. 2A to 2C are cross-sectional views showing the layer structure of the substrate at each manufacturing stage of the embodiment of the semiconductor laser according to the present invention. In FIG. 2 (I), the abscissa indicates the Al content in each compound semiconductor layer, corresponding to FIG. 2 (a).

3 (d) to 3 (f) are cross-sectional views showing the layer structure of the substrate in each manufacturing step of the embodiment of the semiconductor laser according to the present invention, following FIG. 2 (c).

4 (g) and 4 (i) are cross-sectional views showing the layer structure of the substrate in each manufacturing step of the embodiment of the semiconductor laser according to the present invention, following FIG. 3 (f).

FIG. 5 is a cross-sectional view showing a layer structure of a conventional semiconductor laser.

[Explanation of symbols]

10 Example of Al GaAs-based ridge waveguide type self-aligned semiconductor laser according to the present invention 12 n-type GaAs substrate 14 n-type lower clad layer 16 active layer 18 p-type upper first clad layer 20 p-type optical guide layer 22 p-type upper second clad layer 24 p-type GaAs layer 26 mask pattern 28 n-type current blocking layer 30 p-type upper third clad layer 32 p-type contact layer 34 photoresist film 36 p-type inversion layer 38 p-type ohmic electrode 40 n-type Ohmic Electrode 50 Reverse Mesa Ridge Part 52 Convex Part 60 Mesa Ridge Part 70 Conventional Al Ga As Ridge Waveguide Self-Aligned Semiconductor Laser

Claims (2)

[Claims] 1. An AlGaAs based ridge waveguide type self-aligned semiconductor laser, comprising: a compound semiconductor laminated structure having at least an active layer and a pair of a lower cladding layer and an upper cladding layer sandwiching the active layer from above and below. , An optical guide layer made of Al _x Ga _1-x As at the bottom layer, and an upper clad layer made of Al _y Ga _1-y As thereon, respectively, and an inverted mesa type ridge is formed on the compound semiconductor laminated structure. And a light guide structure formed in a shape, and controlling the Al content X of the optical guide layer within the range of 0.1 ≦ X ≦ 0.3, and controlling the Al content Y of the upper clad layer to the optical guide layer. The Al content rate X of the above is set so that the optical guide layer acts as an etching stop layer when the upper clad layer is etched, and the Al content rate Y of the upper clad layer is further adjusted from the optical guide layer. Monotonically decreases with distance And A, wherein the fluoride using etchant acid was to be wet etched upper cladding layer inverted mesa ridge
l GaAs based ridge waveguide type self-aligned semiconductor laser. 2. A method of manufacturing an Al GaAs based ridge waveguide type self-aligned semiconductor laser, comprising: a first conductivity type lower clad layer, an active layer, and a first conductivity type lower clad layer, respectively. An upper first clad layer of a second conductivity type different from the first conductivity type, and the Al content X is 0.1 ≦
An optical guide layer regulated in the range of X ≦ 0.3 is sequentially epitaxially grown, and the Al content Y on the optical guide layer is larger than the Al content X of the optical guide layer and the distance from the optical guide layer is changed. Epitaxially growing an upper second cladding layer of the second conductivity type so as to monotonically decrease, and then epitaxially growing another GaAs layer of the second conductivity type, and a stripe mask pattern on the other GaAs layer. And a step of transferring the pattern to a predetermined position in the depth direction of the upper second cladding layer using the stripe mask pattern as a mask, and a wet process using a hydrofluoric acid-based etchant with the stripe mask pattern as a mask. While the light guide layer functions as an etching stop layer by etching, the upper second
An Al GaAs based ridge waveguide type characterized by including a step of selectively shaving only the cladding layer to form an inverted mesa type ridge portion, and a step of transferring a pattern to the optical guide layer by wet etching. Manufacturing method of self-aligned semiconductor laser.