Method of fabricating semiconductor laser device

Abstract: A method of fabricating a semiconductor laser device includes disposing a lower cladding layer (22), a superlattice active layer (23), an upper cladding layer (24), and a contact layer (25) in the named order on a substrate (21), disposing, on resonator end surfaces, films (26A,26B) containing a material which can cause disorder of the semiconductor superlattice structure at a high temperature, and causing current to flow between the substrate (21) and the contact layer (25) to cause laser oscillations. The laser oscillations cause laser light to be generated, which is absorbed at the resonator end surfaces (28A,28B). The resonator end surfaces (28A,28B) are locally heated due to absorption of laser light, whereby the disorder-causing material is diffused into the resonator end surfaces and the semiconductor superlattice structure in the vicinity of the resonant end surface is disordered to thereby form window regions (27A,27B).

Description: This invention relates to a so-called "window structure" semiconductor laser device operable to provide high output power, and more particularly to a method of making a semiconductor laser device including an active layer having a superlattice structure, and including a window structure having window regions formed in a self-aligning manner. BACKGROUND OF THE INVENTION Figure 1 shows a cross-section of a conventional semiconductor laser device having a so-called window structure, which can operate to provide high output power. This semiconductor laser device has an active layer of a superlattice structure. In Figure 1, disposed on an N-type GaAs substrate 1 is a stack of an N-type AlGaAs lower cladding layer 2, a GaAs/AlGaAs superlattice-structured active layer 3, a P-type AlGaAs upper cladding layer 4, and a P-type GaAs contact layer 5 successively formed in the named order with the N-type AlGaAs lower cladding layer being disposed on the substrate 1. Zn is diffused into the thus formed structure from opposite end portions of the surface of the contact layer 5 to thereby form Zn diffusion regions 6, 6. The Zn diffusion disorders opposite end portions of the superlattice structure of the active layer 3 to thereby form disordered regions 7, 7′. The disordered regions 7 and 7′ provide laser resonator end surfaces 8 and 8′. The semiconductor laser device of Figure 1 may be fabricated by a process such as one shown in Figures 2 and 3. As shown in Figure 2, the N-type AlGaAs lower cladding layer 2, the GaAs/AlGaAs superlattice active layer 3, the P-type AlGaAs upper cladding layer 4, and the P-type GaAs contact layer 5 are sequentially grown in the named order on the N-type GaAs substrate 1. Next, a film of, for example, Si 3 N 4 is deposited over the P-type GaAs contact layer 5, and photolithographic technigue is used to form a mask 9 with a desired pattern from the Si 3 N 4 film. Then, Zn is diffused into the structure through portions which are revealed through openings in the mask 9 to form disordered regions 60. Then, the mask 9 is removed, and the structure is cleaved in the thickness direction at the centre of each disordered region 60. Thus, semiconductor laser devices such as the one shown in Figure 1 result. Figures 4(a) and 4(b) illustrate another method of selectively disordering a GaAs/AlGaAs superlattice structure, which is shown in J. Appl. Phys. 64(7), 1 October 1988, pages 3439-3444. Figure 4(b) is an enlarged cross-sectional view of a portion of an area A shown in Figure 4(a). A semiconductor laser chip 10 having a supperlattice structure therein is placed on a computer-controlled X-Y table (not shown). The semiconductor laser chip 10 has an Si 3 N 4 coating 13 on its surface. An Ar laser beam 40 is directed via a mirror 30 and a condenser lens 20 to a predetermined location on the surface of the semiconductor laser chip 10 with the Si 3 N 4 coating 13 thereon. Ar laser beam energy is absorbed by that portion of the semiconductor laser chip 10 where the Ar laser beam impinges, so that that portion is locally heated to a high temperature, which causes Si in the Si 3 N 4 coating 13 to be diffused into a superlattice layer 11. This, in turn, promotes interdiffusion of GaAs and AlGaAs which constitute the superlattice layer 11, whereby a disordered region 12 is formed. By moving the X-Y table on which the semiconductor laser chip 10 is mounted, a disordered region can be formed in any desired pattern. By cleaving the semiconductor laser chip 10 in its thickness direction at the centre of the disordered region 12 shown in Figure 4(b), a semiconductor laser device similar to the one shown in Figure 1 is provided. The semiconductor laser device shown in Figure 1 is a typical example of semiconductor laser devices which may be fabricated by the techniques shown in Figures 2 and 3 and Figures 4(a) and 4(b). In Figure 1, the P-type GaAs contact layer 5 and the N-type GaAs substrate 1 are connected across an operating power supply to cause current to flow therebetween. This causes light to be generated in the active layer 3, and oscillations occur in a waveguide formed by the active layer 3 and the opposing two end surfaces 8 and 8', which operates as a resonator. In the regions 7 and 7' where the GaAs/AlGaAs superlattice structure of the active layer 3 is disordered due to the Zn diffusion, the bandgap is greater than that of GaAs, so that laser light generated in the active layer 3 is emitted without being absorbed. Accordingly, heat generated in the disordered regions 7 and 7' is significantly reduced, and, therefore, the laser device can be operated to provide high output power. In the conventional method shown in Figures 2 and 3, Zn is diffused from the surface of the P-type GaAs contact layer 5 in the thickness direction to reach the N-type AlGaAs lower cladding layer 2. According to this conventional method, because Zn has to be diffused relatively deep, thermal treatment for a long time is required for diffusion. Such a long time thermal treatment may disorder also superlattice structure portions into which no Zn has been diffused. Furthermore, the width a of the disordered regions 7, 7' (see Figures 1 and 3) cannot be made smaller than about one-half the depth of diffusion. A larger width of disordered regions adversely affect various laser characteristics. For example, it may increase laser beam astigmatism. In addition, in order to fabricate discrete laser devices from the semiconductor laser chip after the Zn diffusion, it is necessary to cleave the chip at the centre of the disordered regions 60, 60 (Figure 2), but if the cleavage at the centre of the disordered regions fails, a desired window structure cannot be obtained, and, therefore, this method provides a low yield. In the method shown in Figure 4, it is necessary to precisely position the region to be disordered relative to the Ar laser beam by means of the X-Y table. However, the positional adjustment is difficult, and sufficient precision is hardly attained. Furthermore, it is difficult to establish a precise distance between the condenser lens 20 and the semiconductor chip 10. An object of the present invention is to provide a method of fabricating a semiconductor laser device free of the above-stated drawbacks of the conventional methods. One approach to solving the problems aforesaid is described in Patent Abstracts of Japan, Vol.14, No. 414 (E-97), 7 September 1990 and Japanese Kokai JP-A-2159082. In the method of fabricating a semiconductor laser device described therein, the following steps are comprised: forming a lower cladding layer, an active layer having a semiconductor superlattice structure, an upper cladding layer, and a contact layer on a substrate in the named order with said lower cladding layer being adjacent to said substrate; and disposing, on respective resonator end surfaces, respective films containing a material which causes the semiconductor superlattice structure of said active layer to be disordered at a high temperature. The semiconductor laser device, as described therein, comprises a substrate of N-type GaAs, a lower cladding layer of N-type AlGaAs, an active layer formed of a GaAs/AlGaAs superlattice structure, an upper cladding layer of P-type AlGaAs, and a contact layer of P-type GaAs. The resonator end surfaces are exposed by dry etching using a developed and patterned photo-resist mask. Once the films are disposed on the resonator end surfaces, impurity is diffused into the etched resonator end surfaces by thermal diffusion using each film as a diffusion source. The present invention provides an alternative and is characterised by: causing current of predetermined magnitude to flow between said substrate and said contact layer to cause laser oscillations to occur so that laser light is generated, the thus generated laser light being absorbed at said resonator end surfaces to cause localised generation of heat which, in turn, causes said disorder-causing material to be diffused from said films into the superlattice structure of said active layer, in the vicinity of said resonator end surfaces, whereby the semiconductor superlattice structure of said active layer in the vicinity of said resonator end surfaces is disordered to form window regions. In the method of fabricating a semiconductor laser device of the present invention, a thin film containing a material which causes a semiconductor superlattice structure to be disordered at a high temperature is disposed on resonator end surfaces of the semiconductor laser device, and current of an appropriate magnitude is caused to flow in the device to cause laser oscillation. Light generated by the laser oscillation is absorbed by regions of the resonator end surfaces which are to be disordered, so that the regions are locally heated. The thus generated heat is used to disorder the structure at the resonator end surfaces. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a conventional window-structure type semiconductor laser device; Figures 2 and 3 are cross-sectional views at different manufacturing steps for use in explaining how to disorder a superlattice structure to manufacture the semiconductor laser device of Figure 1; Figure 4(a) illustrate another conventional method of disordering a superlattice structure; Figure 4(b) is an enlarged cross-sectional view of a portion of the device of Figure 4(a); Figure 5 is a cross-sectional view of a major portion of a semiconductor laser device having a window structure produced by the method according to the present invention; and Figure 6 is a cross-sectional view of the semiconductor laser device shown in Figure 5 in one manufacturing step of the method according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to Figure 5 which shows a semiconductor laser device manufactured in accordance with the present invention, an N-type AlGaAs lower cladding layer 22, an active layer 23 having a GaAs/AlGaAs superlattice structure, a P-type AlGaAs upper cladding layer 24, and a P-type GaAs contact layer 25 are formed in the named order on an N-type GaAs substrate 21, with the layer 22 being adjacent to the substrate 21. On opposite end surfaces of the semiconductor laser device, ZnO films 26A and 26B are disposed. Respective disordered regions 27A and 27B have been formed through the diffusion of Zn in the ZnO films 26A and 26B into the end portions of the superlattice structure active layer 23. The outer ends of the disordered regions 27A and 27B provide respective resonator end surfaces 28A and 28B of a laser oscillator section. The semiconductor laser device of Figure 5 is fabricated in the following manner. First, a wafer is formed by growing crystal to form on an N-type GaAs substrate, an N-type AlGaAs lower cladding layer, a GaAs/AlGaAs superlattice active layer, a P-type AlGaAs upper cladding layer, and a P-type GaAs contact layer, in the named order. The wafer is then cleaved in a conventional manner into a structure of predetermined size, which includes the N-type AlGaAs lower cladding layer 22, the GaAs/AlGaAs superlattice active layer 23, the P-type AlGaAs upper cladding layer 24, and the P-type GaAs contact layer 25 disposed in the named order on the N-type GaAs substrate 21. Then, by a conventional sputtering technique, using ZnO as a target, the ZnO films 26A and 26B are disposed on opposite end surfaces of the structure, as shown in Figure 6. Then, current of appropriate magnitude is caused to flow between the P-type GaAs contact layer 25 and the N-type GaAs substrate 21, which results in laser oscillations in the GaAs/AlGaAs superlattice structure active layer 23. In regions in the vicinity of the resonator end surfaces 28A and 28B, due to surface recombination, the current flowing in these regions does not contribute to generation of light, and, furthermore, these regions absorb laser light generated in the active layer 23, so that heat is generated in these regions. This causes these regions near the resonator end surfaces 28A and 28B to be locally heated to a high temperature, which, in turn, causes Zn to be diffused from the ZnO films 26A and 26B on the end surfaces 28A and 28B into the active layer 23. Due to the diffusion of Zn, the GaAs/AlGaAs superlattice structure in the end portions of the active layer 23 is locally disordered. The disordering of the GaAs/AlGaAs superlattice structure changes the regions into AlGaAs regions, which are transparent to and, therefore, do not absorb laser light generated in the active layer. Thus, the transparent regions provide a so-called "window" structure. The width of the regions which generate heat by absorbing light generated in the portions of the active layer 23 in the vicinity of the resonator end surfaces 28A and 28B during the process of disordering is only 1 µm at most. Accordingly, the width of the regions which are made transparent to laser light as a result of the disordering and provide window regions can be at most 1 µm or so. When current is caused to flow between the P-type GaAs contact layer 25 and the N-type GaAs substrate 21 of the structure shown in Figure 6, heat is generated at the resonator end surfaces 28A and 28B, but, before a temperature (e.g. about 1000 °C) at which crystals would melt is reached, Zn is diffused from ZnO, whereby the structure of Figure 6 changes into the structure shown in Figure 5. After that, laser light is no more absorbed and, therefore, no more heat is generated. Thus, the crystals do not melt. In the above-described embodiment, GaAs and AlGaAs are used as materials for the semiconductor laser device, but the present invention can be used for fabricating devices formed of InP/InGaAs, InP/InGaAsP, GaAs/AlGaInP, InP/AlGaInAs, or any other suitable materials. Further, in place films of ZnO, films of CdO, MgO, or BeO or films containing an element acting as a P-type dopant can be used. To sum up, according to the present invention, films are disposed on opposite end surfaces of a semiconductor laser resonator, which films contain a material that induces disorder of a semiconductor superlattice structure, and current is caused to flow in the resonator to cause laser oscillations to occur. The laser oscillations cause heat to be generated locally at the resonator end surfaces. The heat then causes the disorder-inducing material in the films to be diffused into the structure, whereby the semiconductor superlattice structure in the vicinity of the resonator end surfaces is disordered locally and in a self-aligning manner. According to this method, the semiconductor superlattice structure of the active layer in the portions other than those in the vicinity of the resonator end surfaces is not disordered at all. Accordingly, it is possible to fabricate a semiconductor laser device which can generate laser light at a precise wavelength as determined by the composition of the grown crystals. Furthermore, according to the present invention, the width of the window regions can be as small as 1 µm or so at the most, so that superior semiconductor laser devices with small astigmatism can be obtained.

Claims: A method of fabricating a semiconductor laser device, comprising the steps of: forming a lower cladding layer (22), an active layer (23) having a semiconductor superlattice structure, an upper cladding layer (24), and a contact layer (25) on a substrate (21) in the named order with said lower cladding layer (22) being adjacent to said substrate (21); and disposing, on respective resonator end surfaces (28A,28B), respective films (26A,26B) containing a material which causes the semiconductor superlattice structure of said active layer (23) to be disordered at a high temperature; which method is characterised by: causing current of predetermined magnitude to flow between said substrate (21) and said contact layer (25) to cause laser oscillations to occur so that laser light is generated, the thus generated laser light being absorbed at said resonator end surfaces (28A,28B) to cause localised generation of heat which, in turn, causes said disorder-causing material to be diffused from said films (26A,26B) into the superlattice structure of said active layer (23), in the vicinity of said resonator end surfaces (28A,28B), whereby the semiconductor superlattice structure of said active layer (23) in the vicinity of said resonator end surfaces (28A,28B) is disordered to form window regions (27A,27B). A method of fabricating a semiconductor laser device according to claim 1 wherein said substrate (21) is of N-type GaAs, said lower cladding layer (22) is of N-type AlGaAs, said active layer (23) is formed of a GaAs/AlGaAs superlattice structure, said upper cladding layer (24) is of P-type AlGaAs, the contact layer (25) is of P-type GaAs, and the films (26A,26B) disposed on said resonator end surfaces (28A,28B) are formed of ZnO, CdO, MgO, BeO or any other suitable material which contains an element acting as a P-type dopant. A method of fabricating a semiconductor laser device, according to claim 1, wherein said active layer (23) is formed of an InP/InGaAs superlattice structure the materials of said substrate (21), said lower cladding layer (22), said upper cladding layer (24) and said contact layer (25) are selected from InP and InGaAs, respectively, as appropriate for laser construction, and said films (26A,26B) disposed on said resonator end surfaces (28A,28B) are of ZnO, CdO, MgO, BeO or any other suitable material which contains an element acting as a P-type dopant. A method of fabricating a semiconductor laser device, according to claim 1 wherein said active layer (23) is formed of an InP/InGaAsP superlattice structure, the materials of said substrate (21), said lower cladding layer (22), said upper cladding layer (24) and said contact layer (25) are selected from InP and InGaAsP, respectively, as appropriate for laser construction and said films (26A,26B) disposed on said resonator end surfaces (28A,28B) are of ZnO, CdO, MgO, BeO or any other suitable material which contains an element acting as a P-type dopant. A method of fabricating a semiconductor laser device, according to claim 1 wherein said substrate (21) is of N-type GaAs, said lower cladding layer (22) is of N-type AlGaInP, said active layer (23) is formed of a GaAs/AlGaInP superlattice structure, said upper cladding layer (24) is of P-type AlGaInP, said contact layer (25) is of P-type GaAs, and said films (26A,26B) disposed on said resonator end surfaces (28A,28B) are of ZnO, CdO, MgO, BeO or any other suitable material which contains an element acting as a P-type dopant. A method of fabricating a semiconductor laser device according to claim 1 wherein: said active layer (23) is formed of an InP/AlGaInAs superlattice structure; the materials of said substrate (21), lower cladding layer (22), upper cladding layer (24) and contact layer (25) are selected from InP and AlGaInAs, respectively, as appropriate for laser construction, and said films (26A,26B) disposed on said resonator end surfaces (28A,28B) are of ZnO, CdO, MgO, BeO or any other suitable material which contains an element acting as a P-type dopant. A method of fabricating a semiconductor laser device, according to claim 1, wherein the width of the window regions (27A,27B) formed at the resonator end surfaces (28A,28B) by disordering the semiconductor superlattice structure of said active layer (23) is 1 µm or less. A method of fabricating a semiconductor laser device according to any claim 1 to 7 wherein following said step of forming and preceding said step of disposing, a step of cleaving is performed to expose said resonator end surfaces (28A,28B).

Inventors: TAKAHASHI KAZUHISA, TAKAHASHI, KAZUHISA, Takahashi, Kazuhisa, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L33
IPC codes: H01S5, H01L33