A method for the production of a semiconductor laser device

Abstract: There is provided a method for the production of a semiconductor laser device which emits laser light from a facet (307). The method includes the steps of: growing a multi-layered structure containing an active layer (304) for laser oscillation on a semiconductor substrate (301) to form a wafer; etching the multi-layered structure to form a striped groove (320) perpendicular to the direction of an optical waveguide, resulting in a pair of resonator facets (307); bringing the facets (307) into contact with a sulfur-containing solution (308); subjecting the facets (307) to heat treatment; growing a semiconductor layer (309) on the surface of the facets (307), which layer (309) has a band gap greater than that of the active layer (304); and cleaving the wafer along the striped groove (320) to obtain a semiconductor laser device.

Description: 1. Field of the Invention: This invention relates to a method for the production of a semiconductor laser device which emits laser light from an end facet thereof, and more particularly, it relates to an improved method for the production of such a semiconductor laser device which can attain high reliability even when operated at a high output power level for a long period of time. 2. Description of the Prior Art: A semiconductor laser device which emits laser light from an end facet thereof is a typical example of the semiconductor devices produced by use of the cleavage of semiconductor crystals. A semiconductor laser device of this type has a Fabry-Perot resonator having a pair of semiconductor facets and functioning on the basis of the difference in refractive index between the semiconductor crystals and the air outside the device. In recent years, semiconductor laser devices such as described above have widely come into practical use as light sources for optical disc driving units and the like. When semiconductor laser devices are used as the light sources for write-once optical disc driving units or rewritable optical disc driving units, they are required to have high reliability even at a high output power level of about 40 to 50 mW. Furthermore, for the purpose of attaining higher operational speed of an entire system including an optical disc driving unit, there is a demand for semiconductor laser devices which can attain laser oscillation at a still higher output power level. When semiconductor laser devices are used as the light sources for high-resolution laser printers or for optical pumping of solid state laser devices such as a YAG laser, they are required to attain laser oscillation at an output power level of 100 mW or more. The high output power operation of such a semiconductor laser device, however, causes the deterioration of its end facet from which laser light is emitted. The deterioration in the light-emitting facet increases the current required for driving the semiconductor laser device, and eventually it becomes impossible for the laser device to attain laser oscillation. Therefore, with respect to semiconductor laser devices, it is difficult to attain high reliability at a high output power level. The principal cause for the deterioration of the light-emitting facet is now described. First, heat is generated locally at the light-emitting facet due to the high optical density at this facet and also due to non-radiative recombination caused by the surface state. As the temperature in the area near the facet increases, the band gap in that area becomes smaller, which in turn increases the absorption of light. The increase in the light absorption generates carriers, which are then trapped in the surface state, and non-radiative recombination of the carriers occurs. This further generates heat in the area near the light-emitting facet. This process is repeated until the temperature in the area near the facet reaches the melting point of the semiconductor, resulting in facet breakdown. For the prevention of such deterioration in the resonator facet, there has been proposed a semiconductor laser device having a semiconductor layer with a band gap larger than that of an active layer (i.e., a large-bond-gap layer) formed on the facet. In the production of such a semiconductor laser device, resonator facets are first formed on a single wafer, and then large-band-gap layers such as mentioned above are formed on the facets. For example, in a conventional method for producing this type of semiconductor laser device, resonator facets are formed on a wafer by dry etching, after which semiconductor layers having a band gap larger than that of an active layer are formed on the resonator facets. This process will be described in detail below. Figures 2a to 2c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of the conventional method for producing a semiconductor layer device with a large-band-gap layer. First, as shown in Figure 2a, on an n-GaAs substrate 101 , an n-Al 0.33 Ga 0.67 As first cladding layer 102 , an n-Al 0.08 Ga 0.92 As active layer 103 , a p-Al 0.33 Ga 0.67 As second cladding layer 104 , and a GaAs contact layer 105 are successively grown by epitaxy, resulting in a buried BCM wafer. Next, on the entire surface of the GaAs contact layer 105 , a layered structure composed of two photoresists and a Ti film interposed therebetween is deposited, and then formed into stripes perpendicular to the direction of an optical waveguide, resulting in a multi-layer resist 106 with a prescribed striped pattern. Using the multi-layer resist 106 as a mask, the wafer is subjected to reactive ion beam etching. As a result, striped channels 120 perpendicular to the optical waveguide are formed in the wafer. The side walls of the striped channels 120 become resonator facets 107 , as shown in Figure 2b. The above-described process of forming resonator facets by the use of dry etching is reported by Uchida et al., in "Preprints for the 32nd Annual Meeting of the Society of Applied Physics, Spring 1985 (1 P-ZB-8)". After the formation of the resonator facets, the wafer is taken out into the ambient air, followed by the removal of the multi-layer resist 106 . Then, an AlGaAs layer 108 which is a semiconductor crystal layer with a band gap larger than that of the n-Al 0.08 Ga 0.92 As active layer 103 is formed on each resonator facet 107 by vapor deposition such as metal organic chemical vapor deposition or molecular beam epitaxy, as shown in Figure 2c. Then, a p-sided electrode 109 and an n-sided electrode 110 are formed on the upper face of the GaAs contact layer 105 and on the bottom face of the n-GaAs substrate 101 , respectively, as shown in Figure 2c. Finally, the wafer is cleaved along each striped channel 120 , and divided into a plurality of semiconductor laser devices. In this manner, a semiconductor laser device with large-band-gap layers on the resonator facets thereof is produced. Another conventional method for producing this type of semiconductor laser device has been proposed which employs chemical etching to form resonator facets on a wafer. In this method, chemical etching is first performed to form resonator facets on a wafer, and then semiconductor layers with a band gap larger than that of an active layer are formed on the facets. This conventional process will be described in detail below. Figures 3a to 3c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of the conventional process for producing a semiconductor laser device with a large-band-gap layer. First, as shown in Figure 3a, on a p-GaAs substrate 201 , an n-GaAs current confining layer 202 , a p-Al x Ga 1-x As first cladding layer 203 , a p-Al 0.08 Ga 0.92 As active layer 204 , an n-Al x Ga 1-x As second cladding layer 205 , an n-GaAs contact layer 206 and an n-Al z Ga 1-z As layer 207 are successively grown by epitaxy, resulting in an inner striped BTRS wafer. The Al mole fraction z of the Al z Ga 1-z As layer 207 is obtained by adding 0.1 to the Al mole fraction x of the Al x Ga 1-x As first and second cladding layers 203 and 205 . Then, on the entire surface of the Al z Ga 1-z As layer 207 , a layered structure composed of two photoresists and a Ti film interposed therebetween is deposited, and then formed into stripes perpendicular to the direction of an optical waveguide, resulting in a multi-layer resist 208 with a prescribed striped pattern. Using the multi-layer resist 208 as a mask, the wafer is subjected to chemical etching by the use of a solution containing H₂SO₄, H₂O₂ and H₂O in a volume ratio of 1:8:1. As a result, striped channels 220 perpendicular to the optical waveguide are formed in the wafer. The side walls of the striped channels 220 become resonator facets 209 , as shown in Figure 3b. The above-described process of forming resonator facets on a wafer by chemical etching is reported by Wada et al., in "Preprints for the 45th Annual Meeting of the Society of Applied Physics, Autumn 1984 (13P-R-5)". After the formation of the resonator facets, in the same manner as in the conventional process described with reference to Figures 2a to 2c, the wafer is taken out into the air, followed by the removal of the multi-layer resist 208 . Then, an AlGaAs layer 210 which is a semiconductor crystal layer with a band gap larger than that of the p-Al 0.08 Ga 0.92 As active layer 204 is formed on each resonator facet 209 , as shown in Figure 3c. Thereafter, an n-sided electrode 212 and a p-sided electrode 211 are formed on the upper face of the Al z Ga 1-z As layer 207 and on the bottom face of the p-GaAs substrate 201 , respectively, also as shown in Figure 3c. Finally, the wafer is cleaved along each striped channel and divided into a plurality of semiconductor laser devices. In this manner, a semiconductor laser device with large-band-gap layers on the resonator facets thereof is produced. In the above-described conventional processes, the wafer on which resonator facets have been formed is taken out into the ambient air before a large-band-gap layer is formed on each facet. Thus, the resonator facets are oxidized while being exposed to the air. Large-band-gap layers are formed on the thus oxidized resonator facets. Because of the oxidized portions of the resonator facets, the semiconductor laser device produced by the conventional processes has a surface state at the interface between each resonator facet and the large-band-gap layer formed thereon, thereby causing deterioration in the resonator facet. JP-A-1 039 789 and Patents Abstracts of Japan Vol. 13, No. 232 (E-765) disclose a method for the production of a semiconductor laser device which, in use, emits laser light from a facet, the method comprising the steps of: growing a multi-layered structure containing an active layer for laser oscillation on a semiconductor substrate to form a wafer; etching the multi-layered structure to form a striped groove perpendicular to the direction of an optical waveguide, resulting in a pair of resonator facets; and growing a semiconductor layer on the surface of the facets (307), the semiconductor layer having a band gap greater than that of the active layer. In this prior method, the facets are cleaned using hydrogen radicals before the large band-gap layer is grown on the facets. Extended Abstracts of the 21st Conference on Solid State Devices and Materials, August 1989, Tokyo, pages 337 to 340 discloses a method for the production of a semiconductor laser device in which a sulphur layer is grown on the facets. A large band-gap layer is then grown on the sulphur layer, which is not removed. Extended Abstracts of the 20th Conference on Solid State Devices and Materials, August 1988, Tokyo, pages 263 to 266 discloses a method of growing a sulphur layer on a GaAs surface. Summary of the Invention According to the present invention there is provided a method for the production of a semiconductor laser device as claimed in claim 1. In a preferred embodiment, the aforementioned sulfur-containing solution is selected from the group consisting of undiluted (NH₄)₂S solutions, aqueous (NH₄)₂S solutions, undiluted (NH₄)₂S x solutions, aqueous (NH₄)₂S x solutions, undiluted Na₂S solutions and aqueous Na₂S solutions. In a preferred embodiment, the aforementioned heat treatment is conducted at a temperature of 500°C to 600°C. In a preferred embodiment, the aforementioned semiconductor layer formed on the surface of the facets has a band gap increasing gradually with an increase in the distance from the facets. Thus, the invention described herein makes possible the objective of providing a method for the production of a semiconductor laser device which has excellent resonator facets for emitting laser light so as to attain high reliability even when operated at a high output power level for a long period of time. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figures 1a and 1c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of producing a semiconductor laser device according to this invention; and Figure 1b is a perspective view of the wafer which is being subjected to a surface treatment according to this invention. Figures 2a to 2c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of a conventional process for producing a semiconductor laser device. Figures 3a to 3c are sectional views of a wafer taken along the direction of an optical waveguide, in the steps of another conventional process for producing a semiconductor laser device. DESCRIPTION OF THE PREFERRED EMBODIMENTS The method for the production of a semiconductor laser device according to this invention includes a surface treatment in which resonator facets formed by etching on a wafer are brought into contact with a solution containing sulfur, e.g., a solution of ammonium sulfide. Through this surface treatment, portions of the resonator facets which have been oxidized by being exposed to the ambient air are removed so that the resonator facets can be made clean. Also in this treatment, sulfur in the solution adheres to the clean resonator facets to protect them from another oxidization. The wafer thus treated is placed in an apparatus such as one used for metal organic chemical vapor deposition or molecular beam epitaxy, after which the wafer is heated to such a temperature that the sulfur is removed through sublimation from the resonator facets. Thereafter, on the exposed clean resonator facets, semiconductor layers with a band gap larger than that of an active layer are grown. In this way, in the method of this invention, a large-band-gap layer can be grown on a clean resonator facet having no oxidized portion. This eliminates the possibility that a surface state will arise at the interface between each resonator facet and the large-band-gap layer thereon in the resultant semiconductor laser device. Thus, the semiconductor laser device produced by the method of this invention has excellent resonator facets, thereby attaining high reliability even when operated at a high output power level for a long period of time. This invention will be further illustrated by reference to the following example. Examples Figures 1a to 1c show the production of a semiconductor laser device according to a method of this invention. First, on a p-GaAs substrate 301 , an n-GaAs current confining layer 302 , a p-Al 0.45 Ga 0.55 As first cladding layer 303 , a p-Al 0.15 Ga 0.85 As active layer 304 , an n-Al 0.45 Ga 0.55 As second cladding layer 305 and a GaAs contact layer 306 are successively grown by epitaxy, resulting in an inner striped VSIS wafer. Then, in the same manner as in the conventional processes described above, a resist with a prescribed striped pattern (not shown) is formed on the GaAs contact layer 306 , after which the wafer is subjected to etching by the use of the resist as a mask. As a result of the etching, striped channels 320 perpendicular to the direction of an optical waveguide are formed in the wafer. The side walls of the striped channels 320 become resonator facets 307 , as shown in Figure 1a. The wafer is then taken out into the ambient air, followed by the removal of the resist. While the wafer is exposed to the air, the resonator facets are likely to be oxidized. According to this invention, however, the oxidized portions of the resonator facets can be removed through a surface treatment as will be described below. As shown in Figure 1b, the wafer is immersed in a 10% (NH₄)₂S aqueous solution 308 at a room temperature for 3 minutes. The wafer is then cleaned with pure water, and dried using N₂ gas. As a result, the portions of the resonator facets which have been oxidized by being exposed to the air are removed, so that the resonator facets can be made clean. Also in this treatment, sulfur adheres to the clean resonator facets so as to protect them from another oxidization. The wafer thus treated is placed in an apparatus used for metal organic chemical vapor deposition, and then heated to a temperature of 500 to 600°C in an atmosphere of AsH₃ gas, thereby removing the sulfur through sublimation from the resonator facets. Then, the wafer is heated to a temperature of 800°C, so that a non-doped Al 0.5 Ga 0.5 As layer 309 having high resistance is grown on the clean resonator facets, as shown in Figure 1c. The Al 0.5 Ga 0.5 As layer 309 thus formed is a large-band-gap layer with a band gap larger than that of the p-Al 0.15 Ga 0.85 As active layer 304 . During the above-described deposition, the Al 0.5 Ga 0.5 As layer is also grown on the upper face of the GaAs contact layer 306 . This portion of the Al 0.5 Ga 0.5 As layer formed on the GaAs contact layer 306 is removed by selective etching. Then, an n-sided electrode 311 and a p-sided electrode 310 are formed on the upper face of the GaAs contact layer 306 and on the back face of the p-GaAs substrate 301 , respectively. The wafer is then cleaved along each striped channel 320 , and divided into a plurality of chips. On the Al 0.5 Ga 0.5 As layer 309 on one resonator facet 307 of each chip, an Al₂O₃ reflecting film with a 10% reflectance is formed, so that this resonator facet 307 of the chip becomes a front light-emitting facet. On the Al 0.5 Ga 0.5 As layer 309 on the other resonator facet 307 of each chip, a layered reflecting film made of an Al₂O₃ film and an Si film and having a 75% reflectance is formed, so that this resonator facet 307 of the chip becomes a rear light-emitting facet. In this way, a semiconductor laser device with large-band-gap layers (Al 0.5 Ga 0.5 As layer 309 ) on its resonator facets ( 307 ) is produced. The semiconductor laser device thus produced exhibited excellent reliability even when operated at a high output power level of 150 mW. Also, when this semiconductor laser device was operated for a long period of time, the resonator facets thereof did not deteriorate. In the above-described example, metal organic chemical vapor deposition is used to form a large-band-gap layer on each resonator facet. Alternatively, large-band-gap layers can be formed by molecular beam epitaxy. In the latter case, a wafer placed in an apparatus used for molecular beam epitaxy is first heated to a temperature of 500°C to 600°C to remove sulfur from each resonator facet, and then heated to a temperature of 700°C in a vacuum so as to grow a large-band-gap layer on each resonator facet. Furthermore, the large-band-gap layer to be formed on each resonator facet may be a graded-band-gap layer which has a band gap increasing gradually with an increase in the distance from the resonator facet. In a semiconductor laser device having such graded-band-gap layers on the resonator facets thereof, carriers generated in the vicinity of the resonator facets are drawn strongly into the inside of the semiconductor crystals due to the drift caused by the grading of the band gap, thereby reducing leakage current from the active layer. This attains further reliability in the operation of the semiconductor laser device. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention as defined by the amended claims.

Claims: A method for the production of a semiconductor laser device which, in use, emits laser light from a facet, the method comprising the steps of: growing a multi-layered structure containing an active layer (304) for laser oscillation on a semiconductor substrate (301) to form a wafer; etching the multi-layered structure to form a striped groove (320) perpendicular to the direction of an optical waveguide, resulting in a pair of resonator facets (307); forming a sulphur layer on the facets (307) by bringing the facets (307) into contact with a sulphur-containing solution (308); placing the wafer in an apparatus for epitaxial growth; removing the sulphur layer from the facets (307) in the apparatus for epitaxial growth by subjecting the facets to heat treatment; growing a semiconductor layer (309) on the surface of the facets (307), the semiconductor layer (309) having a band gap greater than that of the active layer (304); and cleaving the wafer along the striped groove (320) to obtain a semiconductor laser device. A method according to claim 1, wherein the sulphur-containing solution (308) is selected from the group consisting of undiluted (NH₄)₂S solutions, aqueous (NH₄)₂S solutions, undiluted (NH₄)₂S x solutions, aqueous (NH₄)₂S x solutions, undiluted Na₂S solutions and aqueous Na₂S solutions. A method according to claim 1 or 2, wherein the heat treatment is conducted at a temperature of 500°C to 600°C. A method according to claim 1, 2 or 3, wherein the semiconductor layer (309) formed on the surface of the facets (307) has a band gap increasing gradually with an increase in the distance from the facets.

Inventors: KONDO MASAKI, MATSUMOTO MITSUHIRO, SASAKI KAZUAKI, KONDO, MASAKI, MATSUMOTO, MITSUHIRO, SASAKI, KAZUAKI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L33
IPC codes: H01S5, H01L21