JP3982940B2 - Method for manufacturing optical semiconductor element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diffraction grating produced by embedding irregularities formed on a certain layer, and more particularly to an optical semiconductor device having a diffraction grating produced on a GaAs surface.
[0002]
[Prior art]
As a structure for realizing dynamic single longitudinal mode oscillation in a semiconductor laser, a distributed feedback (DFB) type semiconductor laser or a distributed Bragg reflector (DBR) type semiconductor laser is well known. In each of these structures, a diffraction grating having a wavelength selection function is built.
[0003]
As a method for producing such a diffraction grating, Journal of Crystal Growth, Vol.77, p637-642, 1986. "AlGaAs / GaAs Distributed Feedback Laser Diodes" Grown by MOCVD "T.Ohata et al. In this example, the diffraction grating is formed at the interface between the first AlGaAs layer and the second AlGaAs layer. First, periodic irregularities are formed on the surface of the first AlGaAs layer by etching, and the Al composition is formed thereon. The second AlGaAs layer having a different ratio is grown, and the unevenness is embedded in the second AlGaAs layer. In this document, when AlGaAs with Al composition of 0.3 is regrown by MOCVD on the ruggedness formed on the GaAs surface, the GaAs / AlGaAs interface becomes flat and a diffraction grating cannot be formed, and AlGaAs with Al composition of 0.3 It is disclosed that a concavo-convex shape is preserved on the surface and a diffraction grating is formed by embedding with AlGaAs having an Al composition of 0.5.
[0004]
According to the above document, the disappearance of the irregular shape on the GaAs surface is presumed to be due to the accelerated migration of III-group atoms or group III source decomposition products adsorbed on the surface during the crystal regrowth process. . The fact that the uneven shape is preserved on the AlGaAs surface can be explained by the difference in the migration activity between Ga and Al present on the underlying crystal surface. In order to prevent the disappearance of such surface irregularities, AlGaAs is widely adopted as a base material for forming irregularities for constituting the diffraction grating in the conventionally proposed diffraction gratings including the above documents. .
[0005]
[Problems to be solved by the invention]
However, when the AlGaAs surface is taken out into the air and processed by etching or the like, a strong surface oxide film is quickly formed, so that the crystallinity of the regrowth crystal is impaired or the interface formed A high concentration of impurities is accumulated. In the prior art as described above, since AlGaAs is exposed on the entire processed surface, it is very difficult to perform good crystal regrowth. Therefore, there has been a problem that the characteristics of the optical semiconductor element having a diffraction grating constituted by such irregularities cannot be sufficiently extracted. In particular, in the case of a semiconductor laser, there is a possibility that it becomes a hindrance to the injection current and causes a reduction in device yield and reliability.
[0006]
An object of the present invention is to provide an optical semiconductor element having a diffraction grating formed by embedding with another semiconductor layer while maintaining a fine unevenness of submicron order formed on the GaAs surface, with high performance, high reliability, It is to provide at a high yield.
[0007]
[Means for Solving the Problems]
The optical semiconductor device according to the present invention has a diffraction grating in which the convex portion of the concavo-convex structure is composed of a laminated structure comprising a GaAs layer / InGaAs intermediate layer / GaAs layer, and the concavo-convex structure is embedded with a semiconductor layer. In the laminated structure composed of the GaAs layer / InGaAs intermediate layer / GaAs layer, and forming a concavo-convex portion having a depth reaching the lower GaAs layer, and performing crystal regrowth to form a concavo-convex shape. A method of manufacturing an optical semiconductor element, wherein the diffraction grating is formed by a method of manufacturing a diffraction grating having a step of manufacturing an embedded diffraction grating with another layer made of a semiconductor material.
[0009]
In the present invention, the InGaAs intermediate layer preferably has an In composition of 0.2 or less. By being in such a composition range, it is possible to reduce the influence of the strain caused by the difference in lattice constant between GaAs and InGaAs, and to obtain a good crystalline stacked structure.
[0010]
In the present invention, the thickness of the intermediate layer is preferably 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 the crystal regrowth can be reduced, and a buried layer having good crystallinity can be obtained.
[0011]
In the present invention, it is desirable that the semiconductor layer embedded in the concavo-convex structure is made of AlGaAs. By doing so, it is possible to easily obtain a diffraction grating having a desired coupling efficiency by accurately controlling the refractive index of the buried layer.
[0012]
Further, as a method for producing such a laminated structure and a buried layer constituting the diffraction grating, a vapor phase growth method such as MOCVD (metal organic chemical vapor deposition) method or MBE (molecular beam epitaxial method) is desirable.
[0013]
Examples of the optical semiconductor element provided with such a diffraction grating include a semiconductor laser, an optical waveguide, an optical coupler, and an optical multiplexer / demultiplexer. Among these, a semiconductor laser is preferable.
[0014]
[Action]
According to the experiments by the present inventors, the unevenness formed on the GaAs surface is stable with respect to the thermal cycle up to the crystal growth temperature and the supply of the group V material, but quickly disappears with the supply of the group III material. Confirmed to do. The inventors hypothesized that the average migration length of Ga is remarkably reduced on the AlGaAs or InGaAs surface, and if the width of the AlGaAs or InGaAs exposed on the surface is larger than the average migration length of Ga, the migration of Ga is performed. Based on the hypothesis that it can be effectively suppressed, experiments on the layer thickness of AlGaAs or InGaAs have revealed that a buried layer can be formed with a concavo-convex shape with a layer thickness of 50 nm or less. This finding led to the present invention.
[0015]
According to the present invention, the intermediate layer is made of InGaAs, and in the stacked structure composed of GaAs layer / InGaAs intermediate layer / GaAs layer, the concave and convex portions having the depth reaching the lower GaAs layer are formed. The cross section of the intermediate layer is exposed on the side wall of the recess. This suppresses the migration of surface atoms that occurs when the Group III raw material is supplied from the gas phase during the crystal regrowth process, and enables embedding with another layer made of a semiconductor material while maintaining the uneven shape. Although a very small amount of InGaAs is exposed on the side wall of the processed recess, almost the entire surface is made of GaAs, so that the surface oxide film formation is greatly suppressed compared to the conventional diffraction grating fabricated on the AlGaAs surface, which is good. A quality regrown crystal and an interface without impurity accumulation are 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]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention is shown in FIG. An intermediate layer 2 (thickness 10 nm) made of In _0.1 Ga _0.9 As and an upper GaAs layer 3 (thickness 10 nm) are stacked on the lower GaAs layer 1 serving as a base (FIG. 1A). A photoresist 4 is applied thereon (FIG. 1B), and a resist pattern having a period of about 0.28 μm is formed by a normal two-beam interference exposure process (FIG. 1C). In this case, the length in the period direction of the portion to be the recess is suitably in the range of 0.05 to 0.2 μm. 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. Therefore, the bottom of the recess formed by the etching reaches the lower GaAs layer 1 (FIG. 1 (d)). The GaAs layer and the In _0.1 Ga _0.9 As intermediate layer can be etched at once with a known etchant.
[0017]
Next, after removing the photoresist, the semiconductor layer 5 made of Al _0.2 Ga _0.8 As is regrown by MOCVD to fill the uneven shape (FIG. 1E). In this way, an uneven shape is preserved at the interface between the semiconductor layer 5 that is the Al _0.2 Ga _0.8 As regrowth layer and the laminated structure, and a diffraction grating by refractive index modulation is formed.
[0019]
In the above-described embodiment, the Al composition of the AlGaAs regrowth layer is set to 0.2. However, the Al composition of the AlGaAs regrowth layer can be any value in the practice of the present invention, and is not limited to the above embodiment. . The period of the diffraction grating is determined from the effective refractive index of the waveguide 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 can be appropriately determined by selecting the coupling coefficient of the diffraction grating.
[0020]
Next, a cross-sectional perspective view of a DFB type semiconductor laser having such a diffraction grating therein is shown. This semiconductor laser has an n-AlGaAs cladding layer 7 / n-GaAs waveguide layer 8 / InGaAs active layer 9 / p-GaAs waveguide layer 10 / p- In _0.1 Ga _0.9 As intermediate layer on an n-type GaAs substrate 6. (Thickness 10 nm) 11 / p-GaAs cap layer 12 (thickness 10 nm) / 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 buried. Structure. When the p-GaAs cap layer 12 is formed, irregularities reaching the p-GaAs waveguide layer 10 are formed on the surface, and the concave-convex shape is filled with the p-AlGaAs cladding layer 13 to form a diffraction grating.
[0021]
【The invention's effect】
An optical semiconductor device with a diffraction grating such as a dynamic single-mode semiconductor laser, which can be deformed by sub-micron-order fine irregularities formed on the GaAs surface and embedded 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 cross-sectional view showing an embodiment of the present invention.
FIG. 2 is a cross-sectional perspective view of a DFB type semiconductor laser which is an example of an optical semiconductor device according to the present invention.
[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- In _0.1 Ga _0.9 As intermediate layer,
12 .. p-GaAs cap layer, 13 .. p-AlGaAs cladding layer,
14 .. p-GaAs contact layer, 15 .. Current confinement layer

Claims (4)

An optical semiconductor element having a diffraction grating, wherein the convex part of the concave-convex structure constituting the diffraction grating is a laminated structure composed of a GaAs layer / InGaAs intermediate layer / GaAs layer, and the concave-convex structure is embedded by a semiconductor layer A method of manufacturing an optical semiconductor device, comprising: forming a concavo-convex portion having a recess reaching a lower GaAs layer in a laminated structure composed of the GaAs layer / InGaAs intermediate layer / GaAs layer; and crystal regrowth. A method for manufacturing an optical semiconductor element, characterized in that the diffraction grating is formed by a method for manufacturing a diffraction grating having a step of manufacturing an embedding diffraction grating with another layer made of a semiconductor material. 2. The method of manufacturing an optical semiconductor element according to claim 1, wherein the In composition of the intermediate layer made of InGaAs is 0.2 or less. 3. The method of manufacturing an optical semiconductor element according to claim 1, wherein the intermediate layer has a thickness in a range of 2 to 40 nm. 4. The method of manufacturing an optical semiconductor element according to claim 1, wherein another layer made of a semiconductor material for embedding the concavo-convex structure is made of AlGaAs.

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