Semiconductor laser device and method for manufacturing the same semiconductor laser device

A current blocking structure of a semiconductor laser includes a p-type InP buried layer, an n-type InP current blocking layer, and a p-type InP current blocking layer laminated along the mesa side surface of a ridge. In the structure, an upper end part of the n-type InP current blocking layer is covered with the p-type InP buried layer and the p-type InP current blocking layer. The n-type InP current blocking layer is prevented from contacting n-type and p-type InP cladding layers. Creation of an ineffective current path from one of the n-type InP cladding layers through the n-type InP current blocking layer to a p-type InP cladding layer of the semiconductor laser is prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device and method for manufacturing the same, and more specifically to a buried semiconductor laser device and method for manufacturing the same.

2. Background Art

With the expansion of optical fiber communications network in recent years, demands for semiconductor laser devices that enable high-speed operation at high temperatures have been increasing. As a product to realize such demands, a semiconductor laser device using an AlGaInAs-based material for an active layer is attracting attention.

Since Al-containing semiconductor materials are easily oxidized, oxides of Al are easily formed in such semiconductor materials. If electric current is injected into the active layer of such a semiconductor laser, the defects formed by the oxides of Al create non radiative recombination centers to deteriorate the device. Therefore, a semiconductor laser wherein an AlGaInAs-based material is applied in a multiple quantum well (MQW) active layer has been fabricated using a structure and method for preventing the oxidation of AlGaInAs.

In Non-Patent Document “Japanese Journal of Applied Physics, Vol. 43, No. 10A, pp. L1247-L1249”, a method for manufacturing a semiconductor laser that prevents the oxidation of AlGaInAs is disclosed. The manufacturing method will be described below.

First, an n-type InP clad layer, a lower light confinement layer, an MQW active layer, an upper light confinement layer, and a p-type InP clad layer are sequentially laminated on an n-type InP substrate in the order from the bottom. These layers are formed by a metal organic vapor phase epitaxy (MOVPE) method or the like. The lower light confinement layer, the MQW active layer, and the upper light confinement layer are formed using an AlGaInAs-based material.

Next, a mask pattern is formed on the p-type InP clad layer, and is used as a mask to etch the p-type InP clad layer, the upper light confinement layer, the MQW active layer, the lower light confinement layer, and the n-type InP clad layer. As a result, forward tapered mesa side surfaces are formed on both sides of the mask pattern. Then, a buried layer is formed along the mesa side surfaces. Next, the mask pattern is removed to form a p-type InP clad layer on the entire surface.

In the above-described manufacturing method, the etching step to form the mesa side surfaces is carried out using HCl gas in a reactor of metal organic chemical vapor deposition (MOCVD) equipment. Furthermore, in the same reactor, a buried layer is formed. Specifically, in the same reactor of the same equipment, the formation of mesa side surfaces and the formation of a buried layer can be sequentially performed.

By thus forming the layers, the oxidation of AlGaInAs exposed on the mesa side surfaces can be prevented.

SUMMARY OF THE INVENTION

The structure of a semiconductor laser wherein a buried layer is formed by inversing n-type and p-type conductivity types and laminating p-type, n-type, and p-type InP layers on the mesa side surface in the order from the bottom in the above-described manufacturing method is shown inFIG. 11.

The buried layer is formed by laminating a p-type InP buried layer9, an n-type InP current blocking layer10, and a p-type InP current blocking layer11along the mesa side surface8aand the mesa bottom surface8bin the order from the bottom.

A (111) B surface is exposed on the mesa side surface8a, and has a predetermined angle to the major surface of the p-type InP substrate1. Therefore, the end part of the n-type InP current blocking layer10is exposed on the surface of the buried layer, and contacts the n-type InP clad layer12. As a result, an ineffective current path23is created from the n-type InP clad layer12through the n-type InP current blocking layer10to the p-type InP clad layer2. Thereby, the current injection efficiency to the MQW active layer4when applying current is lowered.

In order to solve the above-described problems, it is an object of the present invention to provide a semiconductor laser device and a method for manufacturing the same using an AlGaInAs-based material wherein the deterioration of the device due to the oxidation of an active layer on a mesa side surface is prevented, and the lowering of current injection efficiency by an ineffective current path that does not pass through a active layer is suppressed.

The above object is achieved by a semiconductor laser device comprising a substrate of a first conductivity type, a ridge part formed on said substrate, whose side surface has a sequentially tapered shape having a predetermined angle to the major surface of said substrate, wherein a semiconductor layer of a first conductivity type, an active layer that generates laser beams, and a second semiconductor layer of a second conductivity type are sequentially laminated in the order from the bottom, a current blocking structure wherein a third semiconductor layer of a first conductivity type, a fourth semiconductor layer of a second conductivity type, and a fifth semiconductor layer of a first conductivity type are sequentially laminated in the order from the bottom so as to bury both sides of said ridge part, wherein the end part of said fourth semiconductor layer is covered with said third semiconductor layer and said fifth semiconductor layer, and a sixth semiconductor layer of a second conductivity type that covers the upper surface of said ridge part and the upper surface of said current blocking structure.

The above object is achieved by a method for manufacturing a semiconductor laser device comprising a first step for processing a laminated film formed by laminating a first semiconductor layer of a first conductivity type, an active layer that generates laser beams, and a second semiconductor layer of a second conductivity type on a substrate of a first conductivity type in the order from the bottom, to form a ridge part whose side surface has a sequentially tapered shape having a predetermined angle to the major surface of said substrate, wherein said active layer is exposed on said side surface,a second step for forming a third semiconductor layer of a first conductivity type on both sides of said ridge part so as to cover the exposed part of said active layer, a step for forming a fourth semiconductor layer of a second conductivity type on both sides of said ridge part so as to cover said third semiconductor layer, a step for removing the upper end part of said fourth semiconductor layer to expose the end part of said third semiconductor layer, a step for forming a fifth semiconductor layer of a first conductivity type on both sides of said ridge part so as to cover the end part of said third semiconductor layer and said fourth semiconductor layer, and a step for forming a sixth semiconductor layer of a second conductivity type so as to cover said second semiconductor layer and said fifth semiconductor layer.

According to the present invention, in a semiconductor laser device using an AlGaInAs-based material and a method for manufacturing such a semiconductor laser device, the deterioration of the device due to the oxidation of an active layer on a mesa side surface can be prevented. The lowering of current injection efficiency by an ineffective current path that does not pass through an active layer can also be suppressed.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below referring to the drawings. In the drawings, the same or equivalent parts will be denoted by the same symbols, and the description thereof will be simplified or omitted.

EMBODIMENT

The structure of a semiconductor laser device according to this embodiment is shown inFIG. 1. The semiconductor laser device is formed on a p-type InP substrate1. A p-type InP clad layer2is formed thereon. On the p-type InP clad layer2, a lower light confinement layer3, a multiple quantum well (hereafter abbreviated as “MQW”) active layer4, and an upper light confinement layer5are laminated in the order from the bottom (hereafter, the structure formed by laminating these layers is referred to as a “laminated structure6”). By injecting electrons and holes into the MQW active layer4, laser beams can be generated. An n-type InP clad layer7is formed on the laminated structure6.

A ridge part A is formed of the p-type InP clad layer2, the laminated structure6, and the n-type InP clad layer7. A (111) B surface is exposed on the side surface of the ridge part A. The ridge part A has a sequentially tapered mesa shape wherein the side thereof has a predetermined angle (about 54.7°) to the major surface of the p-type InP substrate1. The p-type InP clad layer2extends horizontally from the lower edge part of the side surface of the ridge part A on both outsides. (Hereafter, the side surface of the ridge part A is referred to as “mesa side surface8a”; and the surface extending horizontally from the lower edge part of the mesa side surface8aon both sides is referred to as “mesa bottom surface8b”.)

A p-type InP buried layer9, an n-type InP current blocking layer10, and a p-type InP current blocking layer11along the mesa side surface8aand the mesa bottom surface8bare stacked in order, so as to bury both sides of the ridge part A (hereafter, the structure formed by laminating these layers is referred to as a “current blocking structure B”). The end part of the n-type InP current blocking layer10is covered with the p-type InP buried layer9and the p-type InP current blocking layer11.

An n-type InP clad layer12is formed so as to cover the upper surface of the ridge part A and the upper surface of the current blocking structure B. An n-type InGaAs contact layer13is formed thereon; and an n-type electrode14is formed on the n-type InGaAs contact layer13.

An isolation trench15is formed so as to isolate the laminated films from the p-type InP clad layer2to the n-type InGaAs contact layer13at the location outside the ridge part A. An insulation film16is formed so as to cover the inner surface of the isolation trench15, the end part of the upper surface of the n-type InGaAs contact layer13, and the end part of the n-type electrode14. A p-type back electrode17is formed on the back surface of the p-type InP substrate1.

When applying current to the above-described semiconductor laser device, holes are injected into the MQW active layer4from the p-type InP clad layer2side; and electrons are injected into the MQW active layer4from the n-type InP clad layer7side. By combining these holes and electrons, laser beams can be generated in the MQW active layer4.

As described above, the end part of the n-type InP current blocking layer10is covered with the p-type InP buried layer9and the p-type InP current blocking layer11. Therefore, the n-type InP current blocking layer10has a structure that contacts neither the n-type InP clad layer7nor the n-type InP clad layer12.

Thereby, no ineffective current path is created from the n-type InP clad layer12through the n-type InP current blocking layer10to the p-type InP clad layer2. Therefore, the creation of the ineffective current path that does not pass through the MQW active layer4can be prevented, and the lowering of current injection efficiency into the MQW active layer4can be suppressed.

Next, a method for manufacturing a semiconductor laser device according to this embodiment will be described referring to cross-sectional views shown inFIGS. 2 to 10.

The semiconductor laser device is formed on the major surface of a p-type InP substrate (wafer). First, asFIG. 2shows, a p-type InP clad layer2is formed on a p-type InP substrate1. Next, a laminated structure6wherein a lower light confinement layer3, an MQW active layer4, and an upper light confinement layer5are sequentially laminated is formed on the p-type InP clad layer2. The laminated structure is composed of an AlGaInAs-based material. The MQW active layer4is used to generate laser beams. Next, an n-type InP clad layer7, an n-type InGaAs cap layer18and an n-type InP cap layer19are formed on the laminated structure6.

From the step for forming the p-type InP clad layer2to the step for forming the n-type InP cap layer19, the layers are formed using a metal organic vapor phase epitaxy (hereafter abbreviated as “MOVPE”) method or the like.

Then, asFIG. 3shows, a silicon oxide film20is formed on the n-type InGaAs cap layer18. Next, a resist pattern (not shown) is formed on the silicon oxide film20using photo lithography. Then, the resist pattern is used as a mask to selectively etch the silicon oxide film20. As a result, a mask pattern20ais formed on the n-type InGaAs cap layer18.

Next, using the mask pattern20aas a mask, the n-type InGaAs cap layer18, the n-type InP clad layer7, the laminated structure6, and the p-type InP clad layer2are selectively etched. This etching is performed in a reactor of the MOVPE equipment using an HCl-added gas. As a result, a mesa side surface8ahaving a sequentially tapered shape is formed from the lower end parts of both sides of the mask pattern20aobliquely downward asFIG. 5shows. The mesa side surface8ais composed of a (111) B surface, and the MQW active layer4is exposed on this surface. At the same time, a mesa bottom surface8bextending from the lower end part of the mesa side surface8ain the horizontal directions is formed. As a result, a ridge part A composed of the upper surface of the n-type InP clad layer7and the mesa side surface8ais formed. The ridge part A has a sequentially tapered shape having a predetermined angle (about 54.7°) to the major surface of the p-type InP substrate1.

Next, asFIG. 6shows, a p-type InP buried layer9is formed on both sides of the ridge part A along the mesa side surface8aand the mesa bottom surface8b. At this time, the part exposed on the mesa side surface8aof the MQW active layer4is covered with the p-type InP buried layer9. This step is continuously carried out after the step for forming the mesa side surface8aand the mesa bottom surface8bwithout taking the p-type InP substrate1(wafer) out of the MOVPE equipment.

By thus forming the layers, the oxidation of the side of the laminated structure6exposed on the mesa side surface8acan be prevented. In other words, the oxidation of AlGaInAs exposed on the mesa side surface8acan be prevented.

Furthermore, in the MOVPE equipment, n-type InP current blocking layers10are formed on both sides of the ridge part A so as to cover the p-type InP buried layer9. As described above, a (111) B surface is exposed on the mesa side surface8a, a (111) B surface has a predetermined angle to the major surface of the p-type InP substrate1. Therefore, the end parts of the n-type InP current blocking layers10are grown to the vicinity of the upper end parts21(the parts surrounded by dotted lines) of the mesa side surface8a.

Next, a part of the upper end parts21of the n-type InP current blocking layers10is selectively etched off using dry-etching equipment. As a result, asFIG. 7shows, the end part of the p-type InP buried layer9is exposed on the upper end parts21of the mesa side surface8a.

Next, p-type InP current blocking layers11are formed on both sides of the ridge part A so as to cover the end part of the p-type InP buried layer9, and the n-type InP current blocking layers10. These layers are formed using MOVPE equipment. As a result, asFIG. 8shows, a structure wherein the end part of the n-type InP current blocking layers10is covered with the p-type InP buried layer9and the p-type InP current blocking layers11can be obtained.

Next, although not shown in the drawings, the mask pattern20aand the n-type InGaAs cap layer18are removed.

Next, asFIG. 9shows, an n-type InP clad layer12is formed on the entire surface so as to cover the n-type InP clad layer7and the p-type InP current blocking layers11. Furthermore, an n-type InGaAs contact layer13and an n-type InP cap layer22are sequentially laminated thereon. These layers are formed using MOVPE equipment. Next, although not shown in the drawings, the n-type InP cap layer22is removed.

Next, asFIG. 10shows, photo lithography, etching and the like are performed to form an isolation trench15so as to isolate the laminated films from the p-type InP clad layer2to the n-type InGaAs contact layer13at the location outside the ridge part A. Next, an n-type electrode14is formed on the n-type InGaAs contact layer13. Then, an insulation film16is formed so as to cover the inner surface of the isolation trench15, the upper end part of the n-type InGaAs contact layer13, and the end part of the n-type electrode14. Furthermore, the back surface of the p-type InP substrate1is polished to form a p-type back electrode17on the back surface of the p-type InP substrate1.

By the above-described method for manufacturing a semiconductor laser device, the semiconductor laser device shown inFIG. 1can be formed. According to this manufacturing method, the oxidation of AlGaInAs exposed on the mesa side surface8acan be prevented. Thereby, the deterioration of a semiconductor laser device caused by oxides can be suppressed. In this embodiment, a semiconductor laser device having a material containing aluminum (Al), which is easily oxidized in an MQW active layer4, and a manufacturing method thereof have been described. However, an equivalent effect can be obtained even from other materials as long as an oxide can be formed from such materials.

In addition, a semiconductor laser device can also be formed wherein the creation of an ineffective current path that does not pass through the MQW active layer4can be prevented.

In the above-described embodiment, an example wherein a (111) B surface is exposed on the mesa side surface8a, and has a predetermined angle (about 54.7°) to the mesa bottom surface8bhas been described. When the angle of the mesa side surface8ato the mesa bottom surface8bis smaller than the above-described predetermined angle, the n-type InP current blocking layers10grow along the mesa side surface8a. When the angle is larger than the above-described predetermined angle, the n-type InP current blocking layers10grow while forming a (111) B surface along the mesa side surface8aand a (001) surface. Even when the angle of the mesa side surface8ato the mesa bottom surface8bis 90°, the n-type InP current blocking layers10also grow while forming a (111) B surface. Therefore, in any of the above-described cases, there is possibility of causing the connection of the n-type InP clad layer12and the n-type InP current blocking layers10at the top of the mesa side surface8a, which is an object of the present invention.

Therefore, in the present invention, the angle of the mesa side surface8ato the mesa bottom surface8bis not limited to the above-described predetermined angle (about 54.7°). In other words, the same effect is obtained if the mesa side surface8ahas a sequentially tapered shape or a vertical shape before forming the current blocking structure B.

In the semiconductor laser device and the manufacturing method thereof described in this embodiment, an example using a p-type semiconductor substrate has been shown. However, the equivalent effect can be obtained even when an n-type semiconductor substrate is used and all the polarities of n- and p-types are inversed.