JPH0821755B2 - Semiconductor laser manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a semiconductor laser, particularly an AlGaAs semiconductor laser.

[Outline of Invention]

The present invention provides a first epitaxial growth step in which a p-type first semiconductor layer is formed on a p-type semiconductor substrate, and then a base layer and a current confinement and light absorption layer are sequentially formed on the first semiconductor layer, and A step of forming a notch in the confinement and light absorption layer by a first etching step, leaving a part of the underlying layer;
And a step of exposing the first semiconductor layer by removing the underlying layer left in the cutout portion before performing the second epitaxial growth step. It is intended to manufacture a semiconductor laser which is in the transverse fundamental mode and has a small astigmatism and a low threshold current Ith.

[Conventional technology]

As a semiconductor laser as a light source for a compact disc (CD), a video disc (VD), etc., an AlGaAs compound semiconductor laser is widely used. In this type of semiconductor laser for CD or VD, a semiconductor laser that can obtain oscillation in the transverse fundamental mode in the longitudinal multimode is required to suppress noise due to oscillation mode hopping, and astigmatism is small and the threshold value is small. It is desirable that the current Ith is low.

The structure of the semiconductor laser can be roughly classified into a gain guide type and a refractive index guide type as its waveguide mechanism.
In the gain-guided laser, the oscillation mode is the longitudinal multimode and the transverse fundamental mode is easily obtained, but on the other hand, there are problems that the astigmatism is relatively large and the Ith is relatively large. On the other hand, although the refractive index guide type semiconductor laser has an advantage that astigmatism is small and Ith is small, it has a problem that it is difficult to obtain longitudinal multimode.

Usually, in a compound semiconductor, for example, an AlGaAs-based compound semiconductor, an AlGaAs-based semiconductor layer is epitaxially grown on a single crystal GaAs substrate to form a target semiconductor laser. However, the usage mode of this semiconductor laser and the circuit design are convenient. Above, the semiconductor substrate (so-called substrate)
It may be desired that the side be the anode side, ie the p-type substrate. As described above, when a semiconductor laser is formed on a p-type GaAs substrate, as a current confinement layer for concentrating current on the n-type cladding layer side in the oscillation region so as to form a gain guide type structure, a minority carrier electron When a semiconductor layer having a shielding effect is provided, since the diffusion distance of electrons is larger than that of holes, it is necessary to form the shielding layer with a sufficiently large thickness. In addition, when a double hetero structure having a current constriction on the p-type layer side is formed in the semiconductor layer by a molecular beam epitaxy method (MBE method), a metal organic chemical vapor deposition method (MOCVD method), or the like, there is a step. There are some difficulties in manufacturing this type of p-type gain-guided laser due to restrictions such as the fundamental thickness distribution being different from the liquid phase growth method. There is a problem that it is difficult to construct a semiconductor laser with a semiconductor substrate.

Incidentally, an AlGaAs semiconductor laser with a p-type substrate shown in FIG. 2 has been proposed (extended abstracts of the 17th conference on solid state device and materials (exte
nded Abstracts of the 17th Coference on Solid Stat
e Device and Materials) Tokyo 1985 pp 63-66).

As shown in the enlarged sectional view of FIG.
For example, a p-type GaAs buffer phase (2) having a thickness of, for example, 1 μm is formed on the GaAs semiconductor substrate, that is, the substrate (1).
And a p-type Al _0.43 Ga ₀₅₇ As buffer layer (3) and a current blocking layer (4) of a conductivity type different from this are sequentially grown, and a part of the center of the current blocking layer (4) is etched. After removing and including the removed portion again, p-type Al _0.43 Ga _0.57 As having a thickness of 1 μm is formed on the current blocking layer (4).
Clad layer (5) and an active layer (6) made of undoped Al _0.07 Ga _0.93 As with a thickness of 0.08 μm and a thickness of 1.3 mm
The n-type Al _0.43 Ga _0.57 As clad layer (7) and the n-type GaAs contact layer (8) are sequentially grown, and the contact (8) and the other back surface of the substrate (1) are formed. The counter electrodes (9) and (10) are arranged in the counter.

The semiconductor laser having this structure has a semiconductor structure of a p-type GaAs substrate (1), that is, a p-type substrate. In this case, the active layer (6) has a sufficient thickness, and each of the active layers ( Since the cladding layers (5) and (7) having a band gap considerably larger than that of 6) are arranged, the light is strongly confined in the active layer (6) and a substantially pure refractive index guide type is provided. Since it has a function, astigmatism and Ith can be reduced, but longitudinal multimode transmission cannot be obtained.

[Problems to be solved by the invention]

The present invention provides a method for manufacturing a semiconductor laser having a semiconductor laser structure using a p-type substrate (substrate) and capable of manufacturing a semiconductor laser having a small multi-longitudinal fundamental mode astigmatism and a small Ith. is there.

[Means for solving problems]

In the method for manufacturing a semiconductor laser according to the present invention, after forming a p-type first semiconductor layer (12) on a p-type semiconductor substrate (11), an underlayer (22) is formed on the first semiconductor layer (12). ) And a current confinement and light absorption layer (13) are sequentially formed, and a current confinement and light absorption layer (13) is removed by the first etching process leaving a part of the underlayer (22). The step of forming (13a) and the second epitaxial growth step are performed, and before the second epitaxial growth step is performed, the underlying layer (22) left in the cutout portion (13a) is removed to remove the first layer. It is characterized by including a step of exposing the semiconductor layer (12).

[Action]

In the manufacturing method described above, the active layer (15) formed by the second epitaxial growth step is bent along the recess (23) formed by the cutout portion (13a) of the current constriction and light absorption layer (13), and this active layer (15) is formed. Opposite bends (15A) of (15)
And (15B), a semiconductor laser with a p-type substrate in which a stripe-shaped optical oscillation region is formed is manufactured. This oscillation is basically caused by the concentration of current between the bends (15A) and (15B) at the center of the active layer (15) due to the constriction of the current path due to the current blocking by the power constriction and light absorption layer (13). Therefore, this is due to the gain-guiding function that forms the portion where the injected carrier density is large, that is, the portion where the gain is large, which causes oscillation in the longitudinal multimode and the transverse fundamental mode. At the same time, the existence of the bent portions (15A) and (15B) confine the light of the refractive index guide type in the lateral direction.
This refractive index type waveguide function also produces an effect of reducing astigmatism and Ith.

Further, in this manufacturing method, the base layer (22) is provided, and in the formation of the recess (23), the notch (13a) is formed so as to leave a part of the thickness of the base layer (22). By removing the underlying layer (22) left in the cutout portion (13a) before the second epitaxial growth step, the first portion in the recess (23) is removed.
It is possible to avoid deterioration of crystallinity at the interface with the second epitaxial growth layer due to the presence of a natural oxide film on the semiconductor layer (12).

〔Example〕

First, an example of a semiconductor laser obtained by the manufacturing method of the present invention will be described. In this example, as shown in the enlarged sectional view of FIG. 1, at least a p-type first clad layer (12), a current constriction and light absorption layer (13) are provided on a p-type semiconductor substrate (11). , P-type Al _x Ga _1-x As, x is 0.14 <x <0.
47 A guide layer (14) having a thickness d of 0 <d0.3 μm and Al _y Ga
An active layer (15) with y of 0.14 y 0.15 consisting of _1-y As,
The n-type second cladding layer (16) and the n-type second cladding layer (16) are sequentially epitaxially grown.

In particular, in the current confinement and light absorption layer (13), for example, a stripe-shaped cutout portion (13a) is formed at a portion corresponding to the central portion which becomes the oscillation region of the active layer (15) and is orthogonal to the paper surface in FIG. The guide layer (14), the active layer (15) and the second clad layer (16) are bent along the inner surface of the cutout portion (13a). Corresponding to the facing surfaces of the cutout portion (13a), that is, the stepped portion
A pair of bent portions (15A) and (15B) extending parallel to each other in a direction orthogonal to the plane of the drawing is formed in the active layer (15).

The band gap of the guide layer (14) is slightly larger than that of the active layer (15), but smaller than that of the first and second cladding layers (12) and (16). Selected so that both cladding layers (12) and (16) maintain this relationship.
_Z consisting of Al _z ga _1-z As was selected as 0.4z 0.47,
The layer has a band gap sufficiently larger than that of the active layer (15). The current confinement and light absorption layer (13) is made of, for example, GaAs having a band gap smaller than those of the active layer (15) and the guide layer (14) and a large refractive index, and its thickness Dab is selected to be, for example, 1 μm or more. To be done.

(17) is a contact layer provided on the second cladding layer, that is, an n-type cap layer, and (18) is an insulating layer such as SiO ₂ formed on the contact layer, which is formed in the center of the contact layer. One electrode (19) is ohmicly deposited through the electrode window (18a). Further, (20) indicates the other electrode provided on the back surface of the semiconductor substrate (11).

In the above configuration, the counter electrodes (19) and (2
The active layer (1
An optical oscillation region is formed between the bent portions (15A) and (15B) of 5). This oscillation is basically due to the constriction of the current path due to the current confinement due to the current confinement and light absorption layer (13).
Due to the concentration of current, and hence the portion where the injected carrier density is large, that is, the portion where the gain is large, is formed between the bent portions (15A) and (15B) at the center of the active layer (15) by the gain guiding function. Because of this, oscillation in multi-mode and transverse fundamental mode is performed. However, at the same time, the presence of the bent portions (15A) and (15B) confine the light of the refractive index guide type in the lateral direction. Further, in this case, a guide layer (14) having a smaller bandgap and weaker light and carrier confinement than that of the cladding layer (12) is present on both outer sides of the cladding layer (12). Although not functioning, on both sides of this, the light from the active layer (15) is absorbed by the current constriction and light absorption layer (13), so that the current confinement and light absorption layer (1
There is a difference in effective refractive index between the part facing 3) and the part not facing, and a weak index type waveguide function occurs. Therefore, astigmatism and Ith can be reduced.

Next, an example of a method for manufacturing a semiconductor laser according to the present invention will be described in detail. In this case, first, a p-type GaAs semiconductor substrate (11) is prepared, a buffer layer (21) made of a p-type GaAs semiconductor layer having the same conductivity type as that of the p-type GaAs semiconductor substrate, and a p-type The above-mentioned Al _z Ga _1-z As is provided with a first cladding layer (12) having a thickness of, for example, 1 μm or more, and a p-type layer having a thickness of, for example, about 0.5 μm or less is formed on the first cladding layer (12). G
A current confinement and light absorption layer (13), that is, n having a thickness Dab of 1 μm, is formed on the underlying semiconductor layer (22) made of aAs.
Type MOCVD of high impurity concentration GaAs semiconductor layers
The first epitaxial work is performed by epitaxy by the MBE method or the MBE method.

Next, for example, a width of 2 μm to 10 μ is reached from the current confinement and light absorption layer (13) to a part of the thickness of the base semiconductor layer (22).
First, the groove (3a) is formed to form the notch (13a).
It is formed by chemical etching using a photolithography technique so as to extend in a direction orthogonal to the plane of the drawing. In this case, the depth of the groove may be selected so that a part of the thickness of the underlayer (22) is left.

After that, the second epitaxial growth work is performed. Prior to the second growth work, the removal part (13
Base semiconductor layer (22) left on the bottom surface through a)
Is removed by, for example, vapor phase etching in which AsCl ₃ or HCl is fed to form a recess (23) in the semiconductor layer (22) so as to connect to the recess (13a), and then the recess (23) is formed. The first clad layer (12) exposed through the window of the inner or cutout portion (13a) and the underlying conductor layer (22) therebelow
The guide layer (1
4), the active layer (15), the second cladding layer (16), and the cap layer (17) are successively subjected to a second epitaxial work of MOCVD method or MBE method. After that, an insulating layer (18) is formed on the cap layer (17), an electrode window (18a) is formed, and an electrode (19) is deposited on the back surface of the semiconductor substrate (11). To form.

In this way, the active layer (15) is bent along the recess (23) formed by the lacking portion (13a) of the current constriction and light absorption layer (13), and stripes are formed between the bent portions (15A) and (15B) facing each other. A semiconductor laser is formed by a p-type substrate in which a circular oscillation region is formed.

In this case, the recess (23) has a V-shaped or U-shaped cross section, preferably U-shaped.

Further, due to the existence of the step of forming the concave portion (23), the work divided into the first epitaxial work and the second epitaxial work is performed, but as described above, the underlying semiconductor layer (22) In the formation of the concave portion (23), a part of the thickness of the concave portion (23) is left to be removed by etching in the second epitaxial operation. It is possible to prevent the crystallinity of the interface with the guide layer (14) from being lowered due to the presence of a natural oxide film on the first cladding layer (12) in the recess (23).

 The base semiconductor layer (22) may be n-type.

〔The invention's effect〕

According to the present invention described above, a semiconductor laser structure having a p-type substrate is adopted, the crystallinity of the interface is good, the multi-mode longitudinal and transverse fundamental modes are oscillated with the gain guide type function, and the refractive index is increased. With the guide function, it is possible to manufacture a target semiconductor laser with a small astigmatism and a small Ith.
Therefore, it is possible to obtain a suitable semiconductor laser by using it as a light source for CD or VD.

[Brief description of drawings]

FIG. 1 is an enlarged schematic sectional view of an example of a semiconductor laser according to the present invention, and FIG. 2 is an enlarged schematic sectional view of a conventional semiconductor laser. (11) is a semiconductor substrate, (12) is the first cladding layer, (1
3) is the current constriction and light collection layer, (14) is the guide layer, and (15)
Is the active layer, (16) is the second cladding layer, and (19) and (2
0) is an electrode.

Claims (1)

[Claims] 1. A first epitaxial growth step of forming a p-type first semiconductor layer on a p-type semiconductor substrate and then sequentially forming a base layer and a current constriction / light absorption layer on the first semiconductor layer. And a second epitaxial growth step, in which the current confinement and light absorption layer is subjected to a first etching step to form a cutout portion while leaving a part of the underlayer, and the second epitaxial growth step is performed. A method of manufacturing a semiconductor laser, comprising a step of removing the underlying layer left in the cutout portion and exposing the first semiconductor layer.