Method of fabricating semiconductor laser diode

1. Field of the Invention

The invention relates to method of fabricating a semiconductor laser diode using selective growth.

2. Description of the Related Art

A conventional method of fabricating a semiconductor laser diode using selective growth is disclosed in Japanese patent application laid-open No. 6-260727.

According to the conventional method, strip-shaped masks for selective growth are formed on a semiconductor substrate. Then, using metal organic chemical vapor deposition (MOCVD), a lower clad layer, a multiple quantum well (MQW) active layer and an upper clad layer are formed between the selective growth masks in the stated order. With this method, not using etching, a ridge waveguide structure is fabricated.

However, according to the conventional method of fabricating a semiconductor laser diode, in a step for removing the selective growth mask, side walls of the ridge structure are exposed to the air. Although a top surface of the MQW active layer is covered with the upper clad layer, side portions of the active layer are exposed. Therefore, surface state is generated in the MQW active layer so that property of the semiconductor laser diode becomes worse.

It is therefore an object of the invention to provide a method of fabricating a semiconductor laser diode, counters against the above-described problem.

According to the present invention, such a method includes forming a mask layer on a semiconductor substrate and forming a ridge waveguide structure including an active layer on the semiconductor substrate by selective growth method using the mask layer. The mask layer is formed to be along a direction with an angle to [011] direction of the semiconductor substrate

Side portions of the active layer are mostly covered with the upper grown layer so that portions of the active layer exposing to the air are relatively small. Therefore, comparing to the conventional method, it is possible to reduce the surface state of the active layer.

The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:

FIGS. 1 ( a ) to 1 ( c ) are sectional views illustrating a manufacturing process of the embodiment according to the invention.

FIG. 2 is a plan view of a selective growth mask for manufacturing a semiconductor laser diode according to the invention.

The following will describe embodiments of the present invention with reference to the drawings. In the drawings, the size, the shape, and the mutual relationship in arrangement of each component are just roughly shown for the understanding of the present invention, so that the present invention is not limited to the exemplified embodiments.

With reference to FIGS. 1 ( a )- 1 ( c ) and FIG. 2, the embodiment of the invention is described. FIGS. 1 ( a ) to 1 ( c ) are sectional views illustrating a manufacturing process of the embodiment according to the invention. FIG. 2 is a plan view of a selective growth mask used for the invention.

First, as shown in FIG. 1 ( a ), a pair of strip-shaped selective growth masks 12 are formed on an n-InP substrate 10 which has a (100) surface. An SiO 2 film is patterned by conventional photolithography so as to form the selective growth masks 12 . For example, the thickness of the selective growth masks 12 is 0.2 μm and the masks 12 are formed with 2 μm interval.

As shown in FIG. 2, the selective growth masks 12 are formed to be along a direction with an angle θ to [011] direction. Therefore, a laser resonator is formed along the direction with the angle θ to [011] direction. The angle θ is, for example, 5 degrees.

Consequently, in a gap between the selective growth masks 12 , an InGaAsP optical confinement layer 14 (600 Å), an MQW active layer 16 , an InGaAsP optical confinement layer 18 (600 Å) and a p-InP layer 20 (500 Å) are formed by MOCVD in the stated order. The MQW active layer includes, for example, five layers of InGaAsP well and a layer of InGaAsP barrier. As a result, a ridge waveguide structure including an active layer is formed.

As described above, using the selective growth mask formed along the direction with the angle θ to [011] direction, each layer grows to cover the under layer as shown in FIG. 1 ( a ).

Consequently, the selective growth masks 12 are removed. In this step, the ridge waveguide structure including the MQW active layer 16 is exposed to the air. Because side portions of the MQW active layer 16 are mostly covered with the upper optical confinement layer 18 , exposed portions of the MQW active layer are relatively small. Therefore, comparing to the conventional method, it is possible to reduce the surface state of the MQW active layer 16 .

Then, as shown in FIG. 1 ( b ), on the whole surface, p-InP clad layer 22 (500 Å), p-InGaAsP etching stopper layer 24 (200 Å), p-InP clad layer 26 (3 μm), p-InGaAs contact layer 28 (0.2 μm) are formed by MOCVD in the stated order.

Consequently, an etching mask 30 is formed on the contact layer 28 . An SiO 2 film is patterned by conventional photolithography so as to form the etching masks 30 . For example, the thickness of the etching masks 12 is 0.2 μm.

Consequently, using the etching mask 30 as a mask, p-InGaAs contact layer 28 and the p-InP clad layer 26 are etched. This etching is, for example, wet etching using etchant including chlorine. According to the etching, a reverse mesa including the contact layer 28 and the p-InP clad layer 26 are formed on the ridge waveguide structure including the MQW active layer 18 .

Then, as shown in FIG. 1 ( c ), after removing the etching mask 30 , polyimide layers 32 are formed on both side of the reverse mesa. Consequently, a P-type electrode 34 consisting of Au and Zn is formed on the contact layer 28 . Also, an N-type electrode 36 consisting of Au, Ge and Ni is formed on the back surface of the substrate 10 . According to the above-described steps, a semiconductor laser diode is manufactured.

According to the invention, the selective growth masks are formed to be along the direction with the angle θ to [011] direction. Using this selective growth masks, each layer grows to cover the under layer one after the other. Therefore, side portions of the active layer are mostly covered with the upper grown layer so that portions of the active layer exposing to the air are relatively small. As a result, comparing to the conventional method, it is possible to reduce the surface state of the active layer.