Semiconductor light emitting device and method for fabricating the same

Abstract: In a II-VI group semiconductor laser, on an n type GaAs substrate, n type ZnSe layer, a multiquantum well layer of a ZnCdSe well layer and a ZnSe barrier layer, and a p type ZnSe layer are deposited in this order. A polycrystalline ZnO layer is provided on both sides of the p type ZnSe layer for constricting current. Multifilm reflecting mirrors, respectively constituted with a polycrystalline SiO 2 layer and a polycrystalline TiO 2 layer, for obtaining laser oscillation are provided on the p type ZnSe layer as well as on a surface of the n type ZnSe layer exposed by etching the GaAs substrate. Furthermore, a p type AuPd electrode and an n type AuGeNi electrode are respectively provided. Alternatively, on an n type GaAs substrate, an n type ZnSe epitaxial layer, an n type ZnMgSSe cladding layer, an n type ZnSSe optical waveguide layer, a ZnCdSe active layer, a p type ZnSSe optical waveguide layer, a p type ZnMgSSe cladding layer, a p type ZnTe contact layer and a polycrystalline ZnO burying layer are respectively formed. Furthermore, a p type AuPd electrode and an n type In electrode are respectively provided.

Description: 1. Field of the Invention The present invention relates to a semiconductor light emitting device as acknowledged in the opening clause of claim 1, such as a semiconductor laser and a light emitting diode, which is used in an optical disc apparatus, etc. as well as a method for fabricating such a semiconductor light emitting device as acknowledged in the opening clause of claim 3. 2. Description of the Related Art: Since a ZnSe series II-VI group compound semiconductor is a direct transition type semiconductor material which has a wide band gap, the development a semiconductor laser which emits blue laser light (referred to as a "blue semiconductor laser", hereinafter) employing this material is actively being conducted recently. Figure 5 is a cross-sectional view illustrating an example of a structure for a conventional blue semiconductor laser 2500 using a ZnSe series II-VI group semiconductor material. Specifically, the structure includes a Cl-doped n type ZnSe epitaxial layer 2502, a Cl-doped n type ZnMgSSe cladding layer 2503, a Cl-doped n type ZnSSe optical waveguide layer 2504, a ZnCdSe active layer 2505, an N-doped p type ZnSSe optical waveguide layer 2506, and an N-doped p type ZnMgSSe cladding layer 2507, which are deposited in this order on an Si-doped n type GaAs substrate 2501. The p type ZnMgSSe cladding layer 2507 is formed so that a portion thereof takes a stripe shape, and an N-doped p type ZnTe contact layer 2508 is formed on the stripe portion. On the other hand, on the portion of the p type ZnMgSSe cladding layer 2507 except the stripe portion is provided a polycrystalline SiO 2 burying layer 2509 so as to interpose the stripe portion therebetween. Furthermore, a p type AuPd electrode 2510 is provided on the p type ZnTe contact layer 2508 and the polycrystalline SiO 2 burying layer 2509. On the other hand, an n type In electrode 2511 is provided under the n type GaAs substrate 2501. A conventional blue semiconductor laser 2500 having a structure such as the one described above has the following problems to be solved. First, the polycrystalline SiO 2 burying layer 2509 used in the conventional blue semiconductor laser 2500 is formed by chemical vapor deposition or the like. However, the crystal growth temperature of a ZnSe series II-VI group compound semiconductor material is generally about 250°C, which is very low. Therefore, in order not to generate a crystal deterioration in the ZnSe series II-VI group compound semiconductor layer during the step of forming the polycrystalline SiO 2 burying layer 2509 which is conducted after the formation of a layer made of a ZnSe series II-VI group compound semiconductor material, the polycrystalline SiO 2 burying layer 2509 must be formed with a temperature lower than the growth temperature of the ZnSe series II-VI group compound semiconductor layer. However, SiO 2 formed with such low temperature becomes porous polycrystalline. As a result, the adhesion to a ZnSe layer or the like becomes extremely poor so that peeling or the like easily occurs. Therefore, it is questionable to use SiO 2 as an insulating burying layer. Moreover, even in a case where the polycrystalline SiO 2 burying layer 2509 is used as a mask during etching, side etch is likely to occur due to the poor adhesiveness or the porous property, and it is not a valuable mask material. Secondly, since the thermal conductivity of polycrystalline SiO 2 is considerably small, generated heat does not efficiently diffuse. As a result, the threshold value of the laser oscillation increases and the life of the light emitting device is shortened. Figure 6, on the other hand, is a cross-sectional view illustrating an example of a structure of a conventional vertical resonator type blue semiconductor laser 2600 using a ZnSe series II-VI group semiconductor material. Specifically, the structure includes a Cl-doped n type ZnSe epitaxial layer 2602, a ZnCdSe active layer 2603, an n-doped p type ZnSe epitaxial layer 2604, and a multi-film reflecting mirror 2605 comprising a set of polycrystalline SiO 2 layers and polycrystalline SiN layers, which are deposited in this order on an Si-doped n type GaAs substrate 2601. An aperture of a predetermined shape is provided in the GaAs substrate 2601. Another multi-layer reflecting mirror 2605, which also comprises a set of polycrystalline SiO 2 layers and polycrystalline SiN layers, is provided on the n type ZnSe epitaxial layer 2602 exposed through the aperture. Laser oscillation occurring in such structure by optical excitation is disclosed in, for example, P. D. Floyd, J. K. Furdyna, H. Luo, J. L. Merz, Y. Yamada, and T. Yokogawa, Proc. Int. Workshop on ZnSe-Based Blue-Green Laser Structures, Wurzburg, Germany, September 18-23, 1994, p.77, Phys. Stat. Sol. (b) 187 (1995) 355. The oscillated laser light is emitted in the direction indicated by an arrow in Figure 26, through the aperture of the GaAs substrate 2601 used as the "window". A conventional blue semiconductor laser 2600 having a structure such as the one described above has the following problems to be solved. First, in the conventional structure, the entire surface of the II-VI group semiconductor layer is covered with the multi-film reflecting mirrors 2605 formed of SiO 2 and SiN which are both insulators. For this reason, it is impossible that the current be injected through metal electrodes into the region of the active layer 2603 interposed by the multi-film reflecting mirrors 2605 and the path of the current be further constricted. As a result, a vertical resonator type blue semiconductor laser of a current driven type cannot be obtained. Secondly, there is no etching method for a II-VI group semiconductor layer which can maintain high etching speed with little damage and, at the same time, has excellent verticality. Consequently, it is impossible to form a structure which can effectively constrict the current flow. Thirdly, in the multi-film reflecting mirror 2605 constituted of the polycrystalline SiO 2 layer and the polycrystalline SiN layer, the difference between the refractive indices of SiO 2 and SiN is small and the absorption by the SiN is large for wavelengths in the blue region. Consequently, it is difficult to obtain high reflectivity. Fourthly, in order to form the multi-film reflective mirrors 2605 constituted of the polycrystalline SiO 2 layer and the polycrystalline SiN layer on the Cl-doped n type ZnSe epitaxial layer 2602, a part of the GaAs substrate 2601 on the rear surface must be removed to form a window from which laser output is to be taken out. At the same time, the surface of the Cl-doped n type ZnSe epitaxial layer 2602.must be made to be extremely smooth so as not to introduce a decrease in the reflectivity. However, an etchant which has high etching speed for GaAs but, at the same time, has low etching speed for ZnSe, (i.e . , an etchant with high selectivity for the both) is not available in conventional art. Thus, when a window is formed in the GaAs substrate 2601 by etching, the surface of the Cl-doped n type ZnSe epitaxial layer 2602 becomes rough. As a result, it introduces a decrease in the reflectivity. Fifthly, it is difficult to form an electrode by a low temperature process on the n type GaAs substrate 2601 after a window from which laser light is to be emitted is formed in the GaAs substrate 2601. A semiconductor light emitting device and a method for fabricating such a semiconductor light emitting device are know from the article of Shigeo HAYASHI et al "ZnSe-Based Laser Diodes on p-GaAs with Current Confinement by Nitrogen Ion Bombardment", International Conference on Solid State Devices and Materials, Chiba, Makuhari Messe (1993) August 29- Sept.1, Tokyo, JP, published in the Extend Abstracts of said International Conference, pp. 59-61. According to this disclosure a p type cladding layer, an active layer, a n type cladding layer, and a n type contact layer respectively made of a II-VI group compound semiconductor, are grown epitaxially in this order on a substrate; furthermore this semiconductor light emitting device comprises a burying layer into which nitrogen ions are implanted. SUMMARY OF THE INVENTION It is the object of the invention, to provide a II-VI group semiconductor light emitting device as well as a method for fabricating such a semiconductor light emitting device, which has a reduced threshold current density and a long life. This object is achieved by the features of the respective main claims. Suitable embodiments are defined by the features of the subclaims. Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor light emitting device in which a burying layer is formed from a material having a low refractivity suitable for providing light confinement as well as a high thermal conductivity, which is formed on epitaxial layers in the II-VI group compound semiconductor laser structure with high adhesiveness, as well as a method for fabricating the same. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a blue semiconductor laser in accordance with an embodiment of the present invention. Figure 2 is a graph showing current vs. optical output characteristics of the blue semiconductor laser shown in Figure 1. Figures 3A to 3D are cross-sectional views illustrating fabricating steps of the blue semiconductor laser shown in Figure 1. Figure 4 is a graph showing current vs. voltage characteristics of the burying layer (into which oxygen is ion-implanted) in the blue semiconductor laser shown in Figure 1. Figure 5 is a cross-sectional view illustrating a conventional blue semiconductor laser. Figure 6 is a cross-sectional view illustrating a conventional vertical resonator type blue semiconductor laser. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the figures. Figure 1 is a cross-sectional view illustrating the structure of a blue semiconductor laser 200 of the present embodiment using a ZnSe series II-VI group semiconductor. In the semiconductor laser 200, a Cl-doped n type ZnSe epitaxial layer 2052, a Cl-doped n type ZnMgSSe cladding layer 2053, a Cl-doped n type ZnSSe optical waveguide layer 2054, a ZnCdSe active layer 2055, an N-doped p type ZnSSe optical waveguide layer 2056, and an N-doped p type ZnMgSSe cladding layer 2057 are deposited in this order on an Si-doped n type GaAs substrate 2051. The p type ZnMgSSe cladding layer 2057 is formed so that a part thereof has a stripe shape, and an N-doped p type ZnTe contact layer 2058 is formed on the stripe portion. On the part of the p type ZnMgSSe cladding layer 2057 except the stripe portion, on the other hand, is formed a burying layer 2059 which is ion-implanted with oxygen so as to interpose the stripe portion therebetween. Furthermore, a p type AuPd electrode 2510 is provided on the p type ZnTe contact layer 2058 and the burying layer 2059. On the other hand, an n type In electrode 2511 is provided under the n type GaAs substrate 2051. As described above, in the semiconductor laser 200 of the present embodiment, a layer 2059 which is ion-implanted with oxygen is provided as the burying layer, which plays the role of constricting the currant and confining the light. Since such a layer has a good thermal radiation, the reduction of the threshold current density and the long life of the device can be realized. Moreover, a difference in refractivity in a preferred range with respect to the cladding layer 2057 is achieved. Thus, an effective light confinement is realized, and stable single transverse mode laser oscillation can be obtained. In the semiconductor laser 200 of the present structure, a continual oscillation at room temperature is realized at a wavelength of about 500 nm. Figure 2 is a graph illustrating current-optical output characteristics of the semiconductor laser 200, obtained when continuously oscillated at room temperature with the cladding layer 2057 having a 5 µm wide stripe. Next, an example of production steps of the semiconductor laser 200 in the present embodiment will be described with reference to Figures 3A through 3D. In the production steps of the present embodiment to be described below, molecular beam epitaxy is used as a growth method for ZnSe series II-VI group semiconductor materials. First, as shown in Figure 3A, an n type GaAs substrate 2051 is prepared. Then, as shown in Figure 3B, a Cl-doped n type ZnSe epitaxial layer 2052, a Cl-doped n type ZnMgSSe cladding layer 2053, a Cl-doped n type ZnSSe optical waveguide layer 2054, a ZnCdSe active layer 2055, an N-doped p type ZnSSe optical waveguide layer 2056, an N-doped p type ZnMgSSe cladding layer 2057, and an N-doped p type ZnTe contact layer 2058 are epitaxially grown in this order on the GaAs substrate 2051. Next, a pattern of a resist 2512 of stripe shape is formed by photolithography on the grown layers, specifically on the p type ZnTe contact layer 2058 which is the uppermost layer, and using the pattern as a mask, oxygen ions are implanted to the grown layers as shown in Figure 3C. Thereafter, the resist layer 2512 is removed using lift-off by acetone (see Figure 3D). And then, a p type AuPd electrode 2510 is formed on the entire upper surface of the wafer by vapor deposition as shown in Figure 3D. Furthermore, then, an n type In electrode 2511 is deposited on the rear surface side of the GaAs substrate 2051 also by vapor deposition. This completes the formation of the semiconductor laser 200. Conditions for the ion implantation are, for example, set for the acceleration voltage of 90 V and the dosage of 1 × 10 13 cm -3 . In this case, the range calculated from LSS theory is R p = 0.14 µm. As a result, oxygen ions 2513 are implanted into portions of the p type ZnMgSSe cladding layer 2057 and the p type ZnTe contact layer 2058 which are not covered by the resist 2512, thereby forming the implanted layer 2059. When oxygen is ion-implanted into a ZnSe series II-VI group semiconductor, it is preferable that the dosage be between 5 × 10 12 cm -2 and 1 × 10 14 cm -2 . The dosage in this range provides concentration of the implanted oxygen impurities of 1 × 10 14 cm -3 or more to a region of a thickness of 2R p (0.28 µm). Thus, the region obtains a high resistivity to be highly insulative. For a dosage exceeding this range, the influence of the irradiation damage deficiency on an active layer, characteristic to II-VI group materials, becomes large and it becomes difficult to obtain laser oscillation. For a dosage not reaching this range, it becomes difficult to obtain high resistivity and therefore a ZnSe series II-VI group semiconductor cannot be used as an insulating burying layer. Figure 4 illustrates current-voltage characteristics in the layer 2059 to which oxygen is ion-implanted in an attempt to show that the layer 2059 is insulative. From the figure, it can be seen that current does not flow at all when voltages in the range from -20 V to +20 V are applied to the layer 2059, thereby exhibiting effective insulation. Although a heat treatment at 275°C is conducted for ten minutes after the ion implantation in the present embodiment, this can possibly be omitted depending on the dosage.

Claims: A semiconductor light emitting device (200) comprising: a substrate (2051); a n type cladding layer (2053), an active layer (2055), a p type cladding layer (2057) and a p type contact layer (2058), respectively made of a II-VI group compound semiconductor, which are epitaxially grown in this order on the substrate (2051), wherein the p type contact layer (2058) is formed in a stripe-shape, and a burying layer (2059), into which oxygen ions are implanted, is formed on both portions of the p type cladding layer (2057), which are located on both sides of the stripe-shaped p type contact layer (2058). A semiconductor light emitting device (200) according to claim 1, wherein the dosage of the oxygen ions implantation is between 5 x 10 12 cm -2 and 1 x 10 14 cm -2 . A method of fabricating semiconductor light emitting device (200) comprising the steps of: epitaxially growing on a substrate (2051) a n type cladding layer (2053), an active layer (2055), a p type cladding layer (2057) and a p type contact layer (2058) in this order, respectively made of a II-VI group compound semiconductor; wherein the p type contact layer (2058) is etched into a striped shape; and oxygen ions are implanted into predetermined regions of the p type cladding layer (2057) which are located on both sides of the stripe-shaped p type contact layer (2058) to form a burying layer (2059). A method for fabricating a semiconductor light emitting device (200) according to claim 3, characterized in that the oxygen ions are implanted with a dosage between 5 x 10 12 cm -2 and 1 x 10 14 cm -2 .

Inventors: SASAI YOICHI, YOKOGAWA TOSHIYA, YOSHII SHIGEO, SASAI, YOICHI, YOKOGAWA, TOSHIYA, YOSHII, SHIGEO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H01S5
IPC codes: H01S5, H01L33