A method of fabricating a semiconductor laser device having clad and contact layers respectively doped with Mg

Abstract: A semiconductor laser device having high performance, low operating voltage, and long service life, and a method for fabricating the same are provided. A semiconductor multilayer film (8) including an active layer (5) for use of laser beam oscillation is stacked on a substrate (2). Then a clad layer (9) composed of p-type AlGaAs doped with a p-type impurity Mg, and a contact layer (10) composed of p-type GaAs doped also with Mg are grown by an LPE growth process, and further a surface layer having a high-resistance portion present in the contact layer (10) and low in carrier concentration is removed. The active layer (5) for use of laser beam oscillation is arranged in a substantially center of an end surface from which the laser beam is emitted.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of fabricating a semiconductor laser device, for example to be used in minidisc players and the like. More specifically, the present invention relates to a method of fabricating a semiconductor laser device having high performance, low operating voltage, and long service life. 2. Description of the Prior Art A conventional semiconductor laser device 101 is constructed in the following way, as shown in Fig. 7. A semiconductor multilayer film 108 is stacked on a substrate 102 composed of, for example, n-type GaAs, the semiconductor multilayer film 108 being comprised of a buffer layer 103 composed of Se-doped n-type GaAs, a lower clad layer 104 composed of Se-doped n-type Al 0.5 Ga 0.5 As, an active layer 105 composed of Al 0.13 Ga 0.87 As, an upper clad layer 106 composed of Zn-doped p-type Al 0.5 Ga 0.5 As, and a current blocking layer 107 composed of Se-doped n-type AlGaAs. Further thereon stacked are a clad layer 109 composed of Zn-doped p-type Al 0.5 Ga 0.5 As and a contact layer 110 composed of Zn-doped p-type GaAs, one by one. On top and bottom surfaces of the semiconductor laser device, formed are a p (positive) electrode 111 and an n (negative) electrode 112 composed of, for example, Au-Ge and Au-Zn. The method for fabricating the above semiconductor laser device is shown in Figs. 8A, 8B, 8C, and 8D. First, as shown in Fig. 8A, a semiconductor multilayer film 108 is grown one layer after another by an MOCVD (Metal Organic Chemical Vapor Deposition) process on a wafer substrate 102a composed of, for example, n-type GaAs, the semiconductor multilayer film 108 comprising a buffer layer (thickness 2 µm) 103 composed of Se-doped n-type GaAs, a lower clad layer (thickness 1 µm) 104 composed of Se-doped n-type Al 0.5 Ga 0.5 As, an active layer (thickness 0.08 µm) 105 composed of Al 0.13 Ga 0.87 As, an upper clad layer (thickness 0.3 µm) 106 composed of Zn-doped p-type Al 0.5 Ga 0.5 As, and a current blocking layer (thickness 1 µm) 107 composed of Se-doped n-type AlGaAs. Thereafter, as shown in Fig. 8B, a striped groove (top width approx. 4 µm) 113 is formed by etching so as to be bored through the current blocking layer 107. Further thereon, as shown in Fig. 8C, a clad layer (thickness inside the striped groove 2 µm, thickness outside 1 µm) 109 composed of Zn-doped p-type Al 0.5 Ga 0.5 As and a contact layer (thickness 45 µm) 110 composed of Zn-doped p-type GaAs are grown one by one by a liquid phase epitaxial growth process (hereinafter, referred to as LPE growth process). Finally, a p (positive) electrode 111 and an n (negative) electrode 112 composed of, for example, Au-Ge and Au-Zn are formed and thereafter the wafer is divided into individual semiconductor laser devices. In the above method of fabricating a semiconductor laser device, the process using the LPE growth process is carried out by a growth slide board 115 made of carbon as shown in Figs. 9A through 9D. As shown in Fig. 9A, a substrate 102b on which the semiconductor multilayer film has been grown is set at a recess of a base 116, while an LPE-grown clad-layer growth use fused liquid 117 and a contact-layer growth use fused liquid 118 are set at growth fused liquid sumps. The liquid phase epitaxial growth (hereinafter, referred to as LPE growth) is carried out while the temperature is gradually lowered. As shown in Figs. 9B and 9C, a slider 120 is pulled by a slide bar 119 at a time point when a specified time or temperature is reached, so that the growth fused liquids 117, 118 are switched over. Finally, as shown in Fig. 9D, the slider 120 is further pulled so that the growth fused liquid 118 is removed from the substrate 102b. Thus, the growth is completed. The above conventional semiconductor light-emitting device has had the following problems: (1) A semiconductor laser device like the prior art example uses Zn as the p-type impurity for the LPE-grown clad layer and the contact layer. However, Zn is high in vapor pressure and therefore easy to evaporate such that even if it is charged in the growth fused liquid, its concentration in the fused liquid will not be stabilized, as a problem; (2) The aforementioned p-type impurity Zn is intensely diffused in the solid phase, being diffused into the lower semiconductor multilayer film during the growth at high temperature, affecting the carrier concentration. This makes a cause of deviations from the design structure of the semiconductor laser device and moreover leads to reduction in reproducibility, as another problem; (3) The contact layer of the semiconductor laser device such as shown in the prior art example is grown to a large thickness as much as 45 µm, requiring a long time to attain such a growth. However, the growth temperature decreases with time so that the p-type impurity Zn is captured into the growing layer in gradually decreasing amounts. Therefore, the resistance of the contact layer becomes higher as the growth progresses, so that the operating voltage of the semiconductor laser device becomes higher, as a further problem; (4) The slider is pulled at an end of the growth of the contact layer so that the contact layer growth use fused liquid is removed from the substrate on which the semiconductor multilayer film has been grown, whereby the growth is terminated. This method may cause the bottom portion of the slider to be damaged by an edge growth portion of the contact layer that forms an extremely thin, high grown portion around the substrate. This in turn may cause growth defects to occur in the next and following growths, as yet another problem; (5) Generally in semiconductor laser devices, the active layer for use of laser beam oscillation is advantageously arranged generally in the center of an end surface from which the laser beam is emitted, in terms of improvement in characteristics, relaxation of stress applied to the active layer, improvement in the operating life, and the like. In the semiconductor laser device and its fabricating method such as shown in the prior art example, however, an attempt to arrange the active layer generally in the center would require the contact layer to be grown to a thickness as much as 45 µm, so that the p-type impurity Zn would be diffused into the lower semiconductor multilayer film during the growth. This may cause the semiconductor laser device to be deteriorated in characteristics, as still another problem. "Extended Abstracts of the 16 th (1984) International Conference on Solid State Devices and Materials", Kobe, 1984, pp 153-156, discloses a method of fabricating a semiconductor laser device according to the preamble of claim 1. JP-A-01 152 789 discloses a semiconductor laser device comprising an AlGaAs upper cladding layer and a GaAs contact layer. The contact layer is doped with Mg. The present invention provides a method of fabricating a semiconductor laser device, the method comprising the steps of: forming a semiconductor multilayer structure including an Al y Ga 1-y As active layer for laser oscillation on a substrate; forming a cladding layer on the semiconductor multilayer structure, the cladding layer being composed of p-type Al x Ga 1-x As (where 0 ≤ y < x ≤ 1); forming a contact layer on the cladding layer, the contact layer being formed of p-type GaAs; the method being characterised in that the cladding layer and the contact layer are doped with Mg; in that the cladding layer and the contact layer are grown by a liquid phase epitaxial growth process; and in that the growth temperature during the step of forming the contact layer is reduced from an initial growth temperature to room temperature. In a preferred embodiment the method comprises the further step of removing a surface portion of the contact layer, the surface portion of the contact layer having a higher electrical resistivity and a lower carrier concentration than the other portion of the contact layer. In a preferred embodiment the surface portion of the contact layer has a thickness of substantially 5µm or more. In a preferred embodiment the active layer is formed such that it is arranged substantially in the centre of an end face of the laser device from which light is, in use, emitted. (1) The semiconductor laser device comprises a clad layer composed of p-type AlGaAs doped with Mg instead of the conventional p-type impurity Zn, and a contact layer composed of p-type GaAs doped also with Mg as the p-type impurity. Then, since Mg is lower in vapor pressure than Zn, the concentration of Mg in growth fused liquid is stabilized even if Mg is charged in the growth fused liquid.Also, since the p-type impurity Mg is less diffused in the solid phase than Zn, the effect on the lower part semiconductor multilayer film is suppressed during the growth at high temperature. (2) The active layer for use of laser beam oscillation is arranged substantially in the center of an end surface from which the laser beam is emitted. Then, even if the semiconductor laser device is applied to a minidisc player or the like, a light beam which is obliquely incident upon the minidisc through a grating and obliquely reflected from the minidisc is prevented from going back to the end surface from which the laser beam is emitted. Thus, occurrence of tracking errors can be prevented, because stray light does not occur.Also, since the semiconductor laser device itself can be made to have a large thickness, the wafer substrate can be prevented from cracking in the fabricating processes.Further, since the active layer for use of laser beam oscillation is formed at a position farthest from both electrodes, the stress applied to the active layer in the mounting of the semiconductor laser device to external can be reduced. (3) The method for fabricating the semiconductor laser device comprises a step of forming a clad layer composed of p-type AlGaAs doped with Mg as the p-type impurity, and a step of forming a contact layer composed of p-type GaAs doped with Mg as the p-type impurity. Also, the method uses an LPE growth process for forming the clad layer and the contact layer, and comprises a step of growing the contact layer from a growth start temperature to room temperature. If the clad layer and the contact layer are formed by the LPE growth process and the contact layer is grown from a growth start temperature to room temperature as described above, then it is unnecessary to pull a slider at an end of the growth, so that the slider can be prevented from any damage at its bottom portion that would make a cause of growth defects. (4) The method for fabricating the semiconductor laser device may comprise a step of removing a surface layer having a high-resistance portion present in the contact layer and lower in carrier concentration than that of the other portion of the contact layer. Also, the method may comprise a step of removing the surface layer of the contact layer to an extent of a 5 µm or more thickness.Since the surface layer having a high-resistance portion present in the contact layer and lower in carrier concentration than that of the other portion of the contact layer is removed or the surface layer of a 5 µm or more layer thickness is removed as described above, a portion of the grown layer having increased resistance can be removed efficiently. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Fig. 1 is a perspective view of a semiconductor laser device; Figs. 2A, 2B, 2C, 2D, and 2E are procedure diagrams showing a method for fabricating the semiconductor laser device of Fig. 1 according to the present invention; Figs. 3A, 3B, 3C, and 3D are process diagrams showing the steps of LPE growth of the clad layer and the contact layer in an embodiment of the present invention; Fig. 4 is a growth temperature profile chart of the clad layer and the contact layer in the LPE growth process used in an embodiment of the present invention; Fig. 5 is an I-V characteristic chart in comparison between the semiconductor laser device obtained by an embodiment of the present invention, and a semiconductor laser device of the prior art; Fig. 6 is a carrier concentration profile chart in the contact layer of the semiconductor laser device obtained by an embodiment of the present invention; Fig. 7 is a perspective view of a semiconductor laser device according to the prior art; Figs. 8A, 8B, 8C, and 8D are procedure diagrams showing a method for fabricating the semiconductor laser device according to the prior art; and Figs. 9A, 9B, 9C, and 9D are process diagrams showing the steps of LPE growth of the clad layer and the contact layer in the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now embodiments of the present invention are described with reference to the accompanying drawings. The Al mixed crystal ratio of an active layer and the Al mixed crystal ratio of a clad layer are set to 0.13 and 0.5, respectively, in the present embodiment. However, the Al mixed crystal ratios may be set to arbitrary ones only if they are within such a range that both are more than 0 and that the Al mixed crystal ratio of the clad layer is larger than the Al mixed crystal ratio of the active layer. Fig. 1 is a perspective view of a semiconductor laser device. A semiconductor laser device 1 is constructed as follows. A semiconductor multilayer film 8 is stacked on a substrate 2 composed of, for example, n-type GaAs, the semiconductor multilayer film 8 being comprised of a buffer layer 3 composed of Se-doped n-type GaAs, a lower clad layer 4 composed of Se-doped n-type Al 0.5 Ga 0.5 As, an active layer 5 composed of Al 0.13 Ga 0.87 As, an upper clad layer 6 composed of Zn-doped p-type Al 0.5 Ga 0.5 As, and a current blocking layer 7 composed of Se-doped n-type AlGaAs. Further thereon stacked are a clad layer 9 composed of Mg-doped p-type Al 0.5 Ga 0.5 As and a contact layer 10 composed of Mg-doped p-type GaAs, sequentially. On top and bottom surfaces of the semiconductor laser device, formed are a p electrode 11 and an n electrode 12 composed of, for example, Au-Ge and Au-Zn. Next, the method for fabricating the semiconductor laser device, which is an embodiment of the present invention, is described in detail. Figs. 2A, 2B, 2C, 2D and 2E are procedure diagrams showing the method for fabricating the semiconductor laser device according to the present embodiment. First, as shown in Fig. 2A, a semiconductor multilayer film 8 is grown by an MOCVD (Metal Organic Chemical Vapor Deposition) process on a wafer substrate 2a composed of, for example, n-type GaAs, the semiconductor multilayer film 8 being comprised of a buffer layer (thickness 2 µm) 3 composed of Se-doped n-type GaAs, a lower clad layer (thickness 1 µm) 4 composed of Se-doped n-type Al 0.5 Ga 0.5 As, an active layer (thickness 0.08 µm) 5 composed of Al 0.13 Ga 0.87 As, an upper clad layer (thickness 0.3 µm) 6 composed of Zn-doped p-type Al 0.5 Ga 0.5 As, and a current blocking layer (thickness 1 µm) 7 composed of Se-doped n-type AlGaAs. Next, as shown in Fig. 2B, a stripe-shaped groove (top width approx. 4 µm) 13 is formed by etching so as to be bored through the current blocking layer 7. Then, as shown in Fig. 2C, a clad layer (thickness inside the stripe-shaped groove 2 µm, thickness outside 1 µm) 9 composed of Mg-doped p-type Al 0.5 Ga 0.5 As and a contact layer (thickness 50 µm) 10 composed of Mg-doped p-type GaAs are sequentially grown by the LPE growth process. Further, as shown in Figs. 2C and 2D, a high-resistance layer 14 near the surface of the contact layer 10 is removed by, for example, etching or mechanical grinding. Finally, as shown in Fig. 2E, a p electrode 11 and an n electrode 12 composed of, for example, Au-Ge and Au-Zn and so on are formed and thereafter the wafer is divided into individual semiconductor laser devices. Figs. 3A through 3D are process diagrams showing the steps of the LPE growth of the clad layer and the contact layer, which are one of the points of the present invention. As shown in Fig. 3A, the LPE growth is carried out by using a growth slide board 15 made of carbon. A substrate 2b on which the semiconductor multilayer film has been grown is set at a recess of a base 16. An LPE-grown clad layer growth use fused liquid 17 and a contact layer growth use fused liquid 18 are set in growth fused liquid sumps. Next, as shown in Fig. 3B, a slider 20 is pulled by a slide bar 19, so that the LPE-grown clad layer growth use fused liquid 17 is put into contact with the substrate 2b. Then, while the temperature is gradually lowered, the LPE growth of the clad layer is performed. Further, as shown in Fig. 3C, at a time point when a specified time or temperature is reached, the slider 20 is pulled again by the slide bar 19, so that the contact layer growth use fused liquid 18 is put into contact with the substrate 2b. Then, while the temperature is gradually lowered, the LPE growth of the contact layer is carried out to room temperature. Thus, the growth is completed. After the growth of the contact layer is completed, the substrate 2b can be taken out directly from within the contact layer growth use fused liquid 18 as shown in Fig. 3D. Therefore, even if an edge growth portion is grown around the substrate 2b in a very thin and high form, the bottom portion of the slider 20 is prevented from damage. Fig. 4 is a growth temperature profile chart of the clad layer and the contact layer in the LPE growth process used in the present embodiment. As apparent from Fig. 4, in the present embodiment, the temperature is increased up to 800°C at a rate of 0.5°C/min, and thereafter the temperature is maintained at 800°C for 80 min with a view to well dissolving Al, GaAs, which is the supply source of As, p-type impurity Mg, and the like into Ga, which becomes a solvent in the LPE growth fused liquid, and also stabilizing the dissolution. Next, gradual cooling is done to 796°C, which is the growth start temperature for the clad layer, at a rate of 0.5°C/min, and thereafter the temperature is maintained at 796°C for 5 min to allow the temperature of the LPE growth fused liquid to be stabilized, where the LPE growth of the clad layer is started. Further, while the temperature is gradually lowered at a rate of 0.5°C/min, the LPE growth of the clad layer is performed. Subsequently, the contact layer is LPE grown while the temperature is firstly lowered gradually to the temperature of 700-750°C at a rate of 3.0°C/min and then rapidly lowered to the room temperature. It is needless to say that the LPE growth temperature may be set arbitrarily within a range of about 700 to 850°C, and the growth temperature profile may also be set arbitrarily depending on the growth conditions. Fig. 5 is an I-V characteristic (current-voltage characteristic) chart in comparison between the semiconductor laser device according to the present embodiment, and a semiconductor laser device according to the prior art. As compared with the I-V characteristic of the prior art semiconductor laser device using a contact layer composed of p-type GaAs doped with the conventional p-type impurity Zn, the I-V characteristic of the invention semiconductor laser device using a contact layer composed of p-type GaAs doped with the p-type impurity Mg of the present invention has been improved. Fig. 6 is a carrier concentration profile chart in the contact layer of the semiconductor laser device obtained in the present embodiment. As apparent from Fig. 6, the carrier concentration of the contact layer gradually decreases from the initial approximately 1 × 10 19 cm -3 in a direction toward the surface of the contact layer, such that the carrier concentration decreases to approximately 7 × 10 18 cm -3 at a depth of about 5 µm from the surface. Furthermore, the carrier concentration tends to begin abruptly dropping at a depth of about 2 µm from the surface. In the present embodiment, the carrier concentration that ensures an operating voltage equivalent to the prior art semiconductor laser device is approximately 7 × 10 18 cm -3 . Therefore, as can be understood from this, in order to remove the high-resistance layer in proximity to the surface of the contact layer in the present embodiment, it is sufficient to remove the contact layer surface to a thickness of about 5 µm. According to the present invention, (1) The semiconductor laser device is made up by using a clad layer composed of p-type AlGaAs doped with Mg instead of Zn as the p-type impurity, and a contact layer composed of p-type GaAs doped with Mg instead of Zn. Since Mg is lower in vapor pressure than Zn, its concentration in the fused liquid is stabilized even if Mg is charged in the growth fused liquid. Accordingly, the composition of the grown layer becomes easy to control.Also, since the p-type impurity Mg is less diffused in the solid phase than Zn, the effect on the lower part semiconductor multilayer film is suppressed during the growth at high temperature. Accordingly, it becomes possible to obtain desired design characteristics of the device. (2) The clad layer and the contact layer are formed by the LPE growth process and moreover the contact layer is grown from the growth start temperature to room temperature, eliminating the need of pulling the slider of the growth slide board at an end of growth. Accordingly, there is no possibility that the bottom portion of the slider may be damaged by an edge growth portion that is grown around the substrate to an extremely thin, high growth, during the growth of the contact layer. Accordingly, the slider becomes reusable repeatedly for the next and following growths. (3) Since the surface layer having a high-resistance portion present in the contact layer and low in carrier concentration may be removed or the surface layer may be removed to a 5 µm or more layer thickness, the portion of the grown layer having increased resistance can be removed efficiently. Accordingly, the operating voltage of the semiconductor laser device is not elevated but can be reduced, so that I-V characteristic improvement becomes feasible. (4) The active layer for use of laser beam oscillation may be arranged generally in the center of the end surface from which the laser beam is emitted. Accordingly, even if the semiconductor laser device of the present invention is applied to a minidisc player or the like, a light beam which is obliquely incident upon the minidisc through a grating and obliquely reflected from the minidisc is prevented from going back to the end surface from which the laser beam is emitted, so that occurrence of tracking errors can be prevented, because a stray light does not occur. Also, since the thickness of the semiconductor laser device itself can be made thick, the wafer substrate can be prevented from cracking in the fabricating processes. Further, since the active layer for use of laser beam oscillation is formed at a position farthest from both electrodes, the stress applied to the active layer in the mounting of the semiconductor laser device to external can be reduced. Accordingly, the operating life of the semiconductor laser device can be improved. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, which is defined by the following claims.

Claims: A method of fabricating a semiconductor laser device, the method comprising the steps of: forming a semiconductor multilayer structure (8) including an Al y Ga 1-y As active layer (5) for laser oscillation on a substrate (20); forming a cladding layer (9) on the semiconductor multilayer structure (8), the cladding layer being composed of p-type Al x Ga 1-x As (where 0 ≤ y < x ≤ 1); forming a contact layer (10) on the cladding layer (9), the contact layer being formed of p-type GaAs; the method being characterised in that the cladding layer (9) and the contact layer (10) are doped with Mg; in that the cladding layer (9) and the contact layer (10) are grown by a liquid phase epitaxial growth process; and in that the growth temperature during the step of forming the contact layer (10) is reduced from an initial growth temperature to room temperature. A method as claimed in claim 1 and comprising the further step of removing a surface portion (14) of the contact layer (10), the surface portion (14) of the contact layer having a higher electrical resistivity and a lower carrier concentration than the other portion of the contact layer (10). A method as claimed in claim 2 wherein the surface portion of the contact layer has a thickness of substantially 5µm or more. A method as claimed in claim 1, 2 or 3 wherein the active layer (5) is formed such that it is arranged substantially in the centre of an end face of the laser device from which light is, in use, emitted.

Inventors: HATA TOSHIO, ISHIZUMI TAKASHI, MIYAZAKI KEISUKE, MORIMOTO TAIJI, OHITSU YOSHINORI, SHIBATA ZENKICHI, HATA, TOSHIO, ISHIZUMI, TAKASHI, MIYAZAKI, KEISUKE, MORIMOTO, TAIJI, OHITSU, YOSHINORI, SHIBATA, ZENKICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1995
Main classification: H01S3
IPC codes: H01S5, H01L33