JP2000275418A - Optical semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical semiconductor element having high performance, high reliability, and high yield, by constituting a projecting part of a recessed/ projecting structure with a lamination structure comprising a GaAs layer, an intermediate layer, and a GaAs layer, and providing a diffraction grating having a semiconductor layer embedded in the recessed/projecting structure. SOLUTION: In a lamination structure comprising a downside GaAs layer 1, an intermediate layer 2, and an upside GaAs layer 3, a recess/projection having a recessed part with depth reaching the downside GaAs layer 1 is formed. A section of the intermediate layer 2 is exposed on a sidewall of the recessed part. Thus, migration of surface atoms occurring when a group III material in a crystal regrowth process is supplied from a gas phase is suppressed, and another semiconductor material can be embedded while keeping the recess/projection configuration. Deformation of the micro recess/projection configuration in a sub-micron order formed on the GaAs layer is prevented, the semiconductor can be embedded by regrowth while keeping the configuration, thus enabling the manufacture of the optical semiconductor element in high reliability and in high yield.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffraction grating formed by embedding irregularities formed on a certain layer, in particular, GaAs.
The present invention relates to an optical semiconductor device having a diffraction grating formed on a surface.

[0002]

2. Description of the Related Art As a structure for realizing dynamic single longitudinal mode oscillation in a semiconductor laser, distributed feedback (DFB: Distrib
A well-known semiconductor laser or a distributed Bragg reflector (DBR) type semiconductor laser is well known. Inside these structures, diffraction gratings each having a wavelength selection function are formed.

As a method for producing such a diffraction grating, Ga
Journal of Crystal Growth, Vol.7, using a DFB semiconductor laser as an example of an As / AlGaAs material semiconductor laser.
7, p637-642, 1986. "AlGaAs / GaAs Distributed Feedba
ck Laser Diodes Grown by MOCVD "T. Ohhata et al. In this example, the diffraction grating is formed at the interface between the first AlGaAs layer and the second AlGaAs layer.
It is manufactured by forming periodic irregularities on the surface of the AlGaAs layer by etching, growing a second AlGaAs layer having a different Al composition ratio thereon, and filling the irregularities with the second AlGaAs layer. According to the same document, when AlGaAs having an Al composition of 0.3 is recrystallized by MOCVD on irregularities formed on the GaAs surface, the GaAs / AlGaAs interface becomes flat and a diffraction grating cannot be formed. It is disclosed that the diffraction grating is formed by embedding AlGaAs with an Al composition of 0.5 on the surface while retaining the uneven shape.

According to the above-mentioned literature, it is estimated that the disappearance of the irregular shape on the GaAs surface is caused by accelerated migration on the surface by the group III atoms or decomposition products of the group III raw material adsorbed on the surface during the crystal regrowth process. Have been. In addition, the fact that the uneven shape is preserved on the AlGaAs surface can be explained by the difference in migration activity between Ga and Al existing on the underlying crystal surface. In order to prevent the disappearance of such surface irregularities, in the conventional diffraction gratings including the above-mentioned documents, AlGaAs is used as a base material for forming the irregularities for forming the diffraction grating.
Has been widely adopted.

[0005]

SUMMARY OF THE INVENTION However, AlGaAs
When the surface is taken out into the air and subjected to processing such as etching, a strong surface oxide film is quickly formed, and the crystallinity of the regrown crystal is impaired, or a high concentration of Impurities are accumulated. In the prior art described above, it is very difficult to perform good crystal regrowth because AlGaAs is exposed on the entire processed surface. Therefore, there is a problem that the characteristics of the optical semiconductor device having the diffraction grating constituted by such irregularities cannot be sufficiently brought out. In particular, in the case of a semiconductor laser, there is a possibility that it may cause an obstacle to an injection current or reduce the yield and reliability of the device.

An object of the present invention is to provide an optical semiconductor device having a diffraction grating formed by embedding another semiconductor layer while maintaining the fine irregularities on the sub-micron order formed on the GaAs surface. It is to provide reliability and high yield.

[0007]

According to the present invention, there is provided a diffraction grating having a convex / concave structure having a stacked structure composed of a GaAs layer / intermediate layer / GaAs layer, wherein the convex / concave structure is embedded with a semiconductor layer. An optical semiconductor element characterized by the above-mentioned.

In the present invention, the intermediate layer is desirably made of AlGaAs, and the Al composition of the AlGaAs intermediate layer is desirably 0.2 or less. With such a composition range, the influence of AlGaAs exposed on the side surface of the convex portion on the crystal regrowth is reduced, and a buried layer having good crystallinity can be obtained.

In the present invention, the intermediate layer is made of InGaAs.
It is preferable that the InGaAs intermediate layer
It is desirable that the In composition be 0.2 or less. By being in such a composition range, the influence of the strain caused by the difference in the lattice constant between GaAs and InGaAs can be reduced and a good crystalline laminated structure can be obtained.

In the present invention, the thickness of the intermediate layer is
It is desirable to be in the range of 2 to 40 nm. By being in this range, the influence of the intermediate layer exposed on the side surface of the convex portion on crystal regrowth is reduced, and a buried layer having good crystallinity can be obtained.

In the present invention, it is preferable that the semiconductor layer for embedding the uneven structure is made of AlGaAs. By doing so, a diffraction grating having a desired coupling efficiency can be easily obtained by accurately controlling the refractive index of the buried layer.

As a method of manufacturing a laminated structure and a buried layer constituting such a diffraction grating, MOCVD is used.
A vapor phase growth method such as a (organic metal chemical vapor deposition) method or an MBE method (molecular beam epitaxial method) is desirable.

Examples of the optical semiconductor device having such a diffraction grating include a semiconductor laser, an optical waveguide, an optical coupler, and an optical multiplexer / demultiplexer. Among them, a semiconductor laser is preferable.

[0014]

According to the experiments performed by the present inventors, the irregularities formed on the GaAs surface are affected by the thermal cycle up to the crystal growth temperature and
It was confirmed that it was stable with respect to the supply of the group V raw material, but disappeared quickly with the supply of the group III raw material. The present inventors hypothesized that the average migration length of Ga would be significantly reduced on the AlGaAs or InGaAs surface, and
Based on the hypothesis that if the width of GaAs or InGaAs exposed on the surface is larger than the average migration length of Ga, it is possible to effectively suppress the migration of Ga, we conducted an experiment on the thickness of AlGaAs or InGaAs and found that the thickness was 50 nm. It has been clarified that the buried layer can be formed with the following layer thickness while maintaining the uneven shape. This finding led to the present invention.

According to the present invention, for example, in a laminated structure composed of GaAs layer / intermediate layer / GaAs layer in which the intermediate layer is made of AlGaAs or InGaAs, the concave / convex portion having a depth reaching the lower GaAs layer is provided. Is formed, the cross section of the intermediate layer is exposed on the side wall of the concave portion. This suppresses migration of surface atoms that occurs when the group III raw material is supplied from the gas phase during the crystal regrowth process, and makes it possible to embed the layer with another semiconductor material while maintaining the uneven shape. Very little AlGaAs or InGa on the sidewalls of the machined recess
Although As is exposed, almost the entire surface is made of GaAs, so the formation of a surface oxide film is significantly suppressed compared to a diffraction grating fabricated on the conventional AlGaAs surface, and good quality regrown crystals and no impurity accumulation An interface is obtained. Therefore, the characteristics of the optical semiconductor device manufactured by such a method are stable, and the yield and reliability of the device can be improved.

[0016]

FIG. 1 shows a first embodiment of the present invention. Laminating the lower intermediate layer 2 made of Al _0.1 Ga _{0 .9} As on the GaAs layer 1 (thickness 10 nm) upper GaAs layer 3 (thickness 10 nm) serving as a base (Figure 1 (a)). A photoresist 4 is applied thereon (FIG. 1 (b)), and a resist pattern having a period of about 0.28 μm is formed by a normal two-beam interference exposure process (FIG. 1).
(c)). In this case, the length of the concave portion in the periodic direction is 0.
The range of 05 to 0.2 μm is appropriate. Next, the pattern is transferred to the laminated structure by a wet etching process.
At this time, the etching is performed until the lower GaAs layer 1 is etched to a depth of 10 nm, so that the bottom of the recess formed by the etching reaches the lower GaAs layer 1 (FIG. 1).
(d)).

Next, after removing the photoresist, the MOC
The semiconductor layer 5 made of Al _0.2 Ga _0.8 As is crystal-regrown by the VD method to bury the irregularities (FIG. 1E). In this manner, the uneven shape is preserved at the interface between the semiconductor layer 5 which is the Al _0.2 Ga _0.8 As regrown layer and the above-mentioned laminated structure, and a diffraction grating is formed by refractive index modulation.

In the second embodiment of the present invention, the material constituting the intermediate layer is changed from Al _0.1 Ga _0.9 As to In in the first embodiment.
_It is changed to _0.1 Ga _0.9 As, and the other components are the same as those in the first embodiment. As in the first embodiment, the concavo-convex shape is preserved at the interface between the Al _0.2 Ga _0.8 As regrown layer and the laminated structure, and a diffraction grating is formed by refractive index modulation or gain modulation. Note that the GaAs layer and the In _0.1 Ga _0.9 As intermediate layer can be etched at once with a known etchant.

In the above-described embodiment, the Al composition of the AlGaAs regrown layer is 0.2.
The Al composition of the regrown layer can adopt any value, and is not limited to the above embodiment. The period of the diffraction grating is determined from the effective refractive index of the guided mode and the desired Bragg wavelength. Further, the length of the concave portion in the periodic direction is not limited to the numerical range of the embodiment, and is appropriately determined by selecting the coupling coefficient of the diffraction grating.

Next, a D having such a diffraction grating inside
FIG. 2 shows a cross-sectional perspective view of the FB type semiconductor laser. This semiconductor laser comprises an n-AlGaAs cladding layer 7 / n-GaAs waveguide layer 8 / InGaAs active layer 9 / p-GaAs waveguide layer 10 / p-Al _0.1 Ga _0.9 As intermediate layer on an n-type GaAs substrate 6. (Thickness 10nm) 11 /
The structure is such that a p-GaAs cap layer 12 (10 nm thick) / p-AlGaAs cladding layer 13 / p-GaAs contact layer 14 are formed in this order, and a current confinement layer 15 made of n-GaAs is embedded. When the p-GaAs cap layer 12 is formed, irregularities reaching the p-GaAs waveguide layer 10 are formed on the surface,
The diffraction grating is formed by embedding the unevenness in the p-AlGaAs cladding layer 13.

[0021]

According to the present invention, it is possible to prevent the deformation of sub-micron irregularities formed on the GaAs surface and to bury them by crystal regrowth while maintaining the shape. Can be manufactured with high reliability and high yield.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of an optical semiconductor device according to the present invention;
1 is a cross-sectional perspective view of a type semiconductor laser.

[Explanation of symbols]

1... Lower GaAs layer, 2 .. Intermediate layer, 3.
· Upper GaAs layer, 4 · · · photoresist, 5 · · · semiconductor layer, 6 · · · GaAs substrate, 7 · · · n-AlGaAs cladding layer, 8 · · · n-GaAs waveguide layer, 9 · · · InGaAs active layer, 10 ..P-GaAs waveguide layer, 11 ..p-Al0.1
Ga0.9As intermediate layer, 12 ··· p-GaAs cap layer, 1
3 ··· p-AlGaAs cladding layer, 14 ··· p-GaAs
Contact layer, 15 ··· Current confinement layer

Claims (7)

[Claims] 1. An optical semiconductor device having a diffraction grating,
The convex part of the concavo-convex structure that constitutes the diffraction grating is GaAs
An optical semiconductor device having a stacked structure including an As layer, wherein the uneven structure is embedded in a semiconductor layer. 2. The optical semiconductor device according to claim 1, wherein said intermediate layer is made of AlGaAs. 3. An Al layer having an Al composition of 0.2.
The optical semiconductor device according to claim 2, wherein: 4. The optical semiconductor device according to claim 1, wherein said intermediate layer is made of InGaAs. 5. The InGaAs intermediate layer having an In composition of 0.2
The optical semiconductor device according to claim 4, wherein: 6. The optical semiconductor device according to claim 1, wherein said intermediate layer has a thickness in a range of 2 to 40 nm. 7. The semiconductor layer for embedding the uneven structure is AlGaAs.
The optical semiconductor device according to any one of claims 1 to 6, comprising: