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 an end facet thereof. This method comprises the steps of: forming a multi-layered structure (13) including an active layer (12) for laser oscillation on a semiconductor substrate (11); etching the semiconductor substrate (11) and the multi-layered structure (13) to form a plurality of striped grooves parallel to each other in such a manner that a plurality of projections (21) are formed on the side face of the multi-layered structure (13) in the striped grooves; separating the projections (21) from the multi-layered structure (13) to form a pair of cleavage planes (23) each functioning as a resonator facet; forming a large-band-gap layer (16) on at least one of the cleavage planes (23) on the light-emitting side, the large-band-gap layer having a larger band gap than that of the active layer (12); forming a reflecting film (17,18) on the larger-band-gap layer; and finally cleaving the semiconductor substrate (11) and the multi-layered structure (13) to obtain a plurality of semiconductor laser devices.

Description: 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. 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 light-emitting facet, a semiconductor layer having a large band gap, i.e., a large-band-gap layer, may be formed on the facet. For example, it has been devised to form a semiconductor layer having a larger band gap than the energy of laser light emitted (see Japanese Patent Publication No. 55-27474 or Japanese Patent Application JP-A-1 033 987). The inventors have proposed the formation of a graded-band-gap layer on at least one of the cleavage planes of semiconductor crystals, which function as a pair of resonator facets (see Japanese Patent Application No. JP-A-2 239 680, which corresponds to European Patent Application No. EP-A-0 388 149). The graded-band-gap layer has a band gap which increases gradually with an increase in the distance from the cleavage plane. Thus, the carriers generated in the vicinity of the light-emitting facet are drawn strongly into the inside of semiconductor crystals due to the drift caused by the grading of the band gap, as well as the migration usually caused by the diffusion. This greatly reduces the probability that the carriers will be trapped in the surface state near the light-emitting facet. Furthermore, because the band gap of the graded-band-gap layer is larger than that of the laser oscillating region including the active layer, the absorption of light in the vicinity of the light-emitting facet is reduced. As a result, facet deterioration can be prevented, allowing the semiconductor laser device to attain stable oscillation at a high output power level. Figures 6a to 6c show a conventional process for producing a semiconductor laser device of the above-mentioned type having a large-band-gap layer (e.g., graded-band-gap layer). The following will describe the conventional process for producing a GaAs or GaAlAs semiconductor laser device of this type, which emits laser light from an end facet thereof. First, as shown in Figure 6a, on a substrate 111 made of, for example, GaAs, a multi-layered structure 113 including a GaAs or GaAlAs active layer 112 is grown by a known method such as liquid phase epitaxy or vapor phase epitaxy. Then, the wafer thus obtained is cleaved by a known cleavage method so as to obtain a prescribed cavity length, resulting in a plurality of laser bars 115 as shown in Figure 6b. At this time, cleavage planes 114, which function as a pair of resonator facets, can be obtained. After cleaving, as shown in Figure 6c, an SiO₂ film 116 is formed on the planes of the laser bar 115 other than the cleavage planes 114 by plasma chemical vapor deposition or the like. Then, on the cleavage planes 114, a GaAlAs large-band-gap layer (e.g., GaAlAs graded-band-gap layer) is formed by vapor deposition such as molecular beam epitaxy or metal organic chemical vapor deposition. The GaAlAs polycrystals which have been grown on the SiO₂ film 116 are removed by etching, followed by removal of the SiO₂ film 116. Next, metal electrodes are deposited on the upper face of the multi-layered structure 113 and the lower face of the substrate 111, respectively. Then, a reflecting film of low reflectivity is formed on the light-emitting facet, and a reflecting film of high reflectivity is formed on the other facet. Finally, the laser bar 115 is cleaved to form a plurality of semiconductor laser devices. In the above-described conventional process, however, the wafer composed of the substrate and the multi-layered structure is first cleaved to form cleavage planes, resulting in a plurality of laser bars. Thereafter, the large-band-gap (e.g., graded-band-gap layer) and the reflecting films are formed on each of the laser bars. Thus, the step of forming these layers and films must be performed for each of the laser bars, so that the production process becomes complicated. The complicated production process makes it difficult to attain a stable production of semiconductor laser devices with high quality. Furthermore, since semiconductor laser devices are obtained by the cleavage of each laser bar, only a small number of the semiconductor laser devices can be obtained by a single cleavage step, which reduces the productivity. Processes to form layers on the facets of semiconductor lasers prior to cleaving the wafer into single laser bars are described in Japanese Patent Applications No. JP-A-61 267 388 and JP-A-62 291 194. The formation of laser facets by breaking a protrusion off the semiconductor laser is known from Japanese Patent Application JP-A-61 077 385 and from the article "GRIN-SCH SQW laser/photodiode array by improved micro cleaved facet process" by H. Nobuhara et al. in Electronics Letters, Vol. 21, No. 17, pages 718-719. The method for the production of a semiconductor laser device according to the present invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises the steps of: forming on a semiconductor substrate a multi-layered structure including an active layer for laser oscillation; etching the semiconductor substrate and the multi-layered structure to form a plurality of striped grooves parallel to each other in such a manner that a plurality of projections are formed on the side faces of the multi-layered structure in the striped grooves, said projections formed on the side face of the multi-layered structure having a triangular cross-section in the plane of cleaving of the projection from the side; separating the projections from the multi-layered structure to form cleavage planes each functioning as a resonator facet; forming a large-band-gap layer on at least one of the cleavage planes on the light-emitting side, the large-band-gap layer having a larger band gap than that of the active layer; forming a reflecting film on the large-band-gap layer; and finally cleaving the semiconductor substrate and the multi-layered structure to obtain a plurality of semiconductor laser devices. In a preferred embodiment, the aforementioned large-band-gap layer is formed by a technique selected from the group consisting of metal organic chemical vapor deposition or molecular beam epitaxy. In a preferred embodiment, the aforementioned large-band-gap layer has a band gap increasing gradually with an increase in the distance from the cleavage plane. In a preferred embodiment, the aforementioned projections are formed by a technique selected from the group consisting of ion milling and selective etching. In a preferred embodiment, the aforementioned projections are separated by application of force in an atmosphere with substantially no oxygen. Thus, the invention described herein makes possible the objectives of (1) providing a method for the production of a semiconductor laser device having a large-band-gap layer (e.g., graded-band-gap layer) formed on at least the light-emitting facet, so that it can attain high reliability even when operated at a high output power level for a long period of time; and (2) providing a method for the production of such a semiconductor laser device with high productivity, in which the large-band-gap layer and the reflecting film are deposited on at least one of the cleavage planes formed without separating the wafer into laser bars, whereby the production steps can be simplified and the quality of the resulting semiconductor laser devices can be significantly improved. The present invention may be better understood and its numerous objectives and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a perspective view showing the front facet face of a semiconductor laser device produced by a method of the present invention. Figures 2a to 2e are sectional views showing the production of the semiconductor laser device of Figure 1. Figure 3 is a plan view showing the pattern of an SiO₂ film formed on the upper face of the multi-layered structure. Figure 4 is a perspective view showing the projections formed on the side face of the multi-layered structure by an ion-milling technique. Figure 5 is a schematic diagram showing the cross sections along lines X-X′ and Y-Y′ of Figure 4. Figures 6a to 6c are perspective views showing the conventional process for producing a semiconductor laser device. In the method for the production of a semiconductor laser device according to the present invention, a pair of cleavage planes are obtained by etching a wafer having a multi-layered structure on a semiconductor substrate to form a plurality of striped grooves parallel to each other in such a manner that a plurality of projections are formed on the side face of the multi-layered structure in the striped grooves, and then separating the projections from the multi-layered structure. The large-band-gap layer (e.g., graded-band-gap layer) and the reflecting film are deposited on at least one of the cleavage planes thus formed without separating the wafer into laser bars. This feature makes it possible to increase the productivity of a semiconductor laser device which can attain high reliability even when operated at a high output power level for a long period of time. The present invention will be further illustrated by reference to the following example, but this example is not intended to restrict the present invention. Figure 1 shows the front facet area of a semiconductor laser device which emits laser light from an end facet thereof. The semiconductor laser device was produced as follows. First, on a GaAs substrata 11, a multi-layered structure 13 including a GaAs or GaAlAs active layer 12 was grown by a known method such as liquid phase epitaxy or vapor phase epitaxy. Then, on the surface of the multi-layered structure 13, an SiO₂ film 19 was formed by plasma chemical vapor deposition. The entire surface of the SiO₂ film 19 was coated with a photoresist which was then formed into a prescribed resist pattern by photolithography. Using the resist pattern as a mask, the SiO₂ film 19 was etched into a pattern shown by the hatched areas in Figure 3. The reference numeral 20 in this figure designates a portion which will become a laser oscillation area of the semiconductor laser device to be produced. Next, using an ion-milling technique, the substrate 11 and the multi-layered structure 13 were etched from oblique directions, as shown by the thick arrows in Figure 4, to form a plurality of striped grooves in such a manner that a plurality of protections 21 were formed on the side face of the multi-layered structure 13 in the respective striped grooves. Figures 2a to 2e show the subsequent steps for the production of the semiconductor laser device. In these figures, the boundary lines for specifying the multi-layered structure 13 are omitted for simplicity. Figure 2a shows a cross section of the projections 21 along line Z-Z′ of Figure 4. Figure 5 shows cross sections of the projections 21 along lines X-X′ and Y-Y′ of Figure 4. Since the etching for the formation of the projections 21 was performed from oblique directions as described above, each of the projections 21 had a triangular cross section. As can be seen from Figure 2a, each of the projections 21 was connected to the substrate 11 and the multi-layered structure 13 only through a linking section 22, and the bottom of each of the projections 21 was separated from the substrate 11. The linking section 22 also had a triangular cross section which was smaller than that of each of the projections 21. The wafer including the substrate 11 and the multi-layered structure 13 with the projections 21 formed on the side face thereof was then placed in a vapor deposition apparatus such as one used for metal organic chemical vapor deposition or molecular beam epitaxy, followed by cleavage. The cleavage step was performed in a vacuum or in an atmosphere with substantially no oxygen, e.g., in an atmosphere of nitrogen or hydrogen. In this cleavage step, since the linking sections 22 were smaller than the projections 21 in cross section, the application of only a slight force was required for separating the projections 21 from the multi-layered structure 13 by cleavage at the linking sections 22. As a result, a striped groove having a rectangular cross section was formed in the substrate 11 and the multi-layered structure 13, as shown in Figure 2b. The cleavage planes 23 obtained in this step became resonator facets. For the formation of the projections 21, a selective etching technique (see, e.g., Appl. Phys. Lett., 40 , 289 (1982)) can also be used. Thereafter, using an apparatus such as one used for metal organic chemical vapor deposition or molecular beam epitaxy, a graded-band-gap layer 16 of Ga l-x Al x As was formed on the bottom and the side face of the striped groove, as shown in Figure 2c. A GaAs protective film 24 was formed thereon so as to prevent the graded-band-gap layer 16 from being oxidized. The graded-band-gap layer 16 had an Al mole fraction x increasing gradually with an increase in the distance from the surface of the cleavage plane 23, the smallest Al mole fraction being equal to that of the active layer 12. For example, in the production of a semiconductor laser device for emitting laser light with a wavelength of about 780 nm, the Al mole fraction x of the graded-band-gap layer 16 was set to gradually increase from 0.14 to 0.5. The Al mole fraction x of the graded-band-gap layer 16 is not limited to this range, so long as it increases gradually with an increase in the distance from the cleavage plane 23. The Al mole fraction x may increase linearly or parabolically. Furthermore, there may exist a step of the Al mole fraction at the cleavage plane 23 between the graded-band-gap layer 16 and the active layer 12. The thickness of the graded-band-gap layer 16 was set to be about 0.1 µm. In the case of metal organic chemical vapor deposition, the graded-band-gap layer 16 can be formed on both the cleavage planes 23 facing each other, as shown in Figure 2c. In the case of molecular beam epitaxy, the graded-band-gap layer 16 can also be formed on both the cleavage planes 23 by applying molecular beams to the wafer in directions inclined from the direction perpendicular to the upper face of the wafer, while rotating the wafer. The GaAs protective layer 24 was removed by heat etching or sputtering. Using an electron-beam deposition apparatus, a reflecting film 17 made of Al₂O₃ having low reflectivity and a reflecting film 18 made of Al₂O₃ and α-Si having high reflectivity were successively formed on the graded-band-gap layer 16 by deposition in directions inclined from the direction perpendicular to the wafer, as shown by thick arrows in Figure 2d. The reflecting films 17 and 18 can also be formed by sputtering. Since the graded-band-gap layer 16 and the reflecting films 17 and 18, formed on the SiO₂ film 19 as described above, were not made of single crystals but made of polycrystals, these layers and films were selectively removed by a known etching process. In this way, the reflecting film 17 having low reflectivity was formed on the light-emitting facet, and the reflecting film 18 having high reflectivity was formed on the other facet. Thereafter, the SiO₂ film 19 was removed from the wafer, followed by the formation of electrodes 14 and 15 on the upper face of the multi-layered structure 13 and on the bottom face of the substrate 11, respectively, using a known method. Finally, the wafer was cleaved as shown in figure 2e, resulting in a plurality of semiconductor laser devices. The resulting semiconductor laser devices were able to attain stable laser oscillation at a high output power level for a long period of time, indicating that these semiconductor laser devices had extremely high reliability. In the above example, the semiconductor laser devices having the graded-band-gap layers formed on the resonator facets were produced. The semiconductor laser devices which can be produced by the method of the present invention are not limited thereto, so long as a large-band-gap layer having a larger band gap than that of the active layer is formed on the resonator facet of the light-emitting side. For example, when a large-band-gap layer made of Ga 1-x Al x As having a constant Al mole fraction (e.g., Ga 0.5 Al 0.5 As) is formed on the light-emitting facet, the resulting semiconductor laser devices can also attain high reliability even when operated at a high output power level for a long period of time.

Claims: A method for the production of a semiconductor laser device which emits laser light from an end facet thereof, the method comprising: forming on a semiconductor substrate (11) a multi-layered structure (13) including an active layer (12) for laser oscillation; etching the multi-layered structure (13) and the semiconductor substrate to form a plurality of striped grooves parallel to each other in such a manner that a plurality of projections (21) are formed on the side faces of the multi-layered structure (13) in the striped grooves, said projections (21) formed on the side face of the multi-layered structure having a triangular cross-section in the plane of cleaving of the projection (21) from the side face; separating the projections (21) from the multi-layered structure (13) to form cleavage planes (23) each functioning as a resonator facet; forming a large-band-gap layer (16) on at least one of the cleavage planes (23) on the light-emitting side, the large-band-gap layer (16) having a larger band gap than that of the active layer (12); forming a reflecting film (17, 18) on the large-band-gap layer (16); and finally cleaving the semiconductor substrate (11) and the multi-layered structure (13) to obtain a plurality of semiconductor laser devices. A method according to Claim 1, characterised in that the large-band-gap layer (16) is formed by a technique selected from the group consisting of metal organic chemical vapor deposition or molecular beam epitaxy. A method according to Claim 1, characterised in that the large-band-gap layer (16) has a band gap increasing gradually with an increase in the distance from the cleavage plane (23). A method according to Claim 1, characterised in that the projections (21) are formed by a technique selected from the group consisting of ion milling and selective etching. A method according to Claim 1, characterised in that the projections (21) are separated by application of force in an atmosphere with substantially no oxygen.

Inventors: KAWANISHI HIDENORI, YAMAMOTO OSAMU, KAWANISHI, HIDENORI, YAMAMOTO, OSAMU
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L33
IPC codes: H01L33, H01S5