METHOD OF PRODUCING A SEMICONDUCTOR LASER

Abstract: A method of producing a semiconductor laser comprises a process of producing at least a support layer (8) and a mask layer (9); a process of producing an overhung portion (11) of mask layer (9) by etching these layers and side-etching the support layer (8) at above the semiconductor substrate (1) at the neighborhood of the resonator end surface; and a process of growing a lower cladding layer (4,4a), an active layer (5,5a), and an upper cladding layer (6,6a) successively on the substrate (1) by an MOCVD method; whereby a doublehetero junction structure in which the thicknesses of the respective layers at the neighborhood of the light emission end surface are less than those at the inside and central portion of the chip is obtained.

Description: FIELD OF THE INVENTION The present invention relates to a method of producing a semiconductorlaser, and more particularly to that method capable of obtaining a semiconductor laser having a narrow emission beam and a high light output property by conducting film production at good controllability and uniform quality on a large area wafer. BACKGROUND OF THE INVENTION Recently, semiconductor lasers are often used as a light source in light communication, an optical disk, and a laser printer. For these laser elements, properties such as narrow emission beam, high output operation, low threshold current, and lengthy lifetime are strongly desired. In order to improve these properties, a variety of kinds of lasers are proposed. Figure 4 shows an example of a substrate of such a semiconductor laser in its processed state and figure 5 shows a perspective view of a structure of a semiconductor laser which is obtained by conducting epitaxial growths of respective semiconductor layers successively on the substrate (EP-A-0155151). In figure 5, the center portion of the laser is cut and position shifted for easening the understanding. In these figures, four grooves 3 are produced at the four corners of a GaAs substrate 1 by etching, and thus two ridges 2 are provided at the both end surfaces of the GaAs substrate 1. The reference numerals 4 and 4a designate lower cladding layers comprising n type Al x Ga 1-x As which are produced at the central portion of the chip and on the ridges 2, respectively. The reference numerals 5 and 5a designate active layers comprising Al y Ga 1-y As which are produced at the central portion of the chip and on the ridges 2, respectively. The reference numerals 6 and 6a designate upper cladding layers comprising p type Al x Ga 1-x As which are produced at the central portion of the chip and on the ridges 2, respectively. The reference numeral 7 designates a p type GaAs contact layer. The production method of this semiconductor laser will be described. When Al x Ga 1-x As is epitaxially grown on the substrate 1 of such a construction, a growth usually is conducted at the central portion of the chip where there is no groove 3 without being affected by the groove 3, but at the neighborhood of the chip end surface, a phenomenon peculiar to the liquid phase epitaxy (hereinafter referred to as "LPE method"). That is, in the LPE method the growth rate at the neighborhood of the side of the ridge 2 becomes faster than usual and that onto the ridge 2 becomes slower than usual dependent on the isotropicity of the crystal surface. In the usual semiconductor laser, the thickness of the active layer is constant in the light emission direction, and when the thickness of the active layer is made thin uniformly in order to realize the narrow emission beam and high output operation, the gain is reduced thereby to increase the threshold current, and overheating occurs due to such as increase in the light power density and that in the light absorption at the neighborhood of the resonator end surface, thereby shortening the lifetime of the laser element. In the prior art device, however, the thickness of the active layer 5 at the inside and central portion of the chip which largely affects influences upon the lasing threshold current is made to be somewhat thick and only that at the neighborhood of the resonator end surface is made thin, whereby spreading of light into the upper and lower cladding layers 6a and 4a is increased and the light power density thereat is lowered. This and the accompanying reduction in the threshold current enable desired properties of lengthy lifetime, narrow emission beam, and high output operation. In the prior art semiconductor laser of such a construction, the layer thickness of the active layer at the neighborhood of the resonator end surface is made thinner than that at the inside and central portion of the chip by utilizing that the growth rate of the semiconductor layer in the LPE method is slower at the ridge portion than at the plain portion. In the LPE method, however, there is a limitation in the widening of the wafer area and this method is not desirable in view of the enhancement in the reproducibility, production yield, and quality. SUMMARY OF THE INVENTION The present invention is intended as an alternative and improved method of production. In accordance with the invention there is provided a method of producing a semiconductor laser comprising: providing a semiconductor substrate; growing a support layer upon a surface of said substrate; growing a mask layer upon said support layer; etching said mask layer to leave a remanent portion thereof at at least one position proximate to that position of a light emission end surface of the laser to be produced; etching said support layer thereby to leave a part of said remanent portion of the mask layer overhanging said surface of said substrate; and growing successively a lower cladding layer, an active layer and an upper cladding layer on said substrate by metal-organic chemical vapour deposition, whereby a double-heterojunction structure laser is produced wherein the thickness of each of said lower cladding layer, active layer, and upper cladding layer is in each respective case less at the light emission end thereof than at the inside and central position of the laser. While this method is being performed the circulation of gas is comparatively poor in that region that is beneath the overhanging mask layer. The amount of gas supplied to the adjacent re-entrant surface is thus reduced relative to the remainder surface and the film growth rate becomes slower. A doublehetero junction structure in which the thicknesses of the respective layers at the neighbourhood of the light emission end surface are thinner than those at the inside and central portion of the chip is thus obtained. This, as has been explained, results in the desired properties of lengthy lifetime, narrow beam emission, high output operation and a low threshold current. Since a metal-organic chemical vapour deposition technique is employed, the film production is readily controlled and can result in uniform film quality and a high production yield. Figure 1 is a perspective view showing a semiconductor laser as an embodiment of the present invention; Figure 2 is a diagram showing cross-sectional views at major production processes thereof; Figure 3 is a perspective view showing a semiconductor laser as another embodiment of the present invention; Figure 4 is a perspective view showing a substrate of a prior art semiconductor laser; and Figure 5 is a perspective view showing a prior art semiconductor laser in a state where the central portion is cut and position shifted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. Figure 1 shows a semiconductor laser as an embodiment of the present invention and figure 2 shows the production process thereof in the order of the processing steps. The same reference numerals designate the same or corresponding elements as those shown in figure 1. The reference numerals 8, 9, 10, and 11 designate layers constituting masks for producing the respective layers 4a to 6a. That is, the numeral 8 designates a support layer comprising n type Al x Ga 1-x As, the numeral 9 designates an n type GaAs mask layer, the numeral 10 designates a silicon dioxide film (SiO₂ film), and the numeral 11 designates an overhung portion comprising the mask layer 9 and the SiO₂ film 10, produced at the neighborhood of the light emission end surface. The production process of the respective layers will be described with reference to figure 2. Figure 2(a): 1) A support layer 8 comprising n type Al x Ga 1-x As is produced on the n type GaAs substrate 1 to a thickness of 2.5 to 3 µm and a mask layer 9 comprising n type GaAs is produced thereon to a thickness of about 0.5 µm both by an MOCVD method. (Metal organic chemical vapour deposition) Figure 2(b): 1) A SiO₂ film 10 is produced on the mask layer 9 by such as a thermal oxidation method to a thickness of about 1000 Å (10 Å = 1 nm). 2) A photoresist film (not shown) of about 5000 Å thickness is deposited thereon by a spinning method, a patterning for producing the semiconductor layers 4 to 6 and 4a to 6a is conducted to the SiO₂ film 10 by a photolithography technique, and an etching is conducted to the SiO₂ film 10. 3) The photoresist film is resolved to be removed. 4) The mask layer 9 is etched by ammonia hydrogen-peroxide water system etchant with the SiO₂ film 10 as mask. 5) The support layer 8 is etched by mixed solution of iodine and potassium iodide with the SiO₂ film 10 and the mask layer 9 as mask, and further side-etching is conducted to produce an overhung portion 11 comprising the SiO₂ film 10 and the mask layer 9 to a length of about 10 to 50 µm. Figure 2(c): 1) A cladding layer 4 comprising n type Al x Ga 1-x As of 1.0 to 1.5 µm thickness, an active layer 5 comprising Al y Ga 1-y As of 0.07 to 0.1 µm thickness, an upper cladding layer comprising p type Al x Ga 1-x As of about 1 µm thickness are successively grown on the n type GaAs substrate 1 thereby to produce a doublehetero junction. Herein, the circulating of material gas into below the mask layer producing the overhung portion comprising the SiO₂ film 19 and the mask layer 9 is bad and the growth rate of the film is slower, whereby thin growth layers 4a to 6a having about 80% thicknesses of the respective layers 4 to 6 are produced. Herein, the respective layers will not be epitaxially grown on the SiO₂ film 10. Figure 2(d) 1) The SiO₂ film 10 is etched to be removed by a dry etching using such as fluoro-carbon system gas. 2) The mask layer 9 is etched to be removed by an etching using ammonia hydrogen peroxide water series etchant. Figure 2(e) 1) A p type GaAs contact layer 7 of 1 to 1.5 µm thickness is produced on the upper cladding layers 6 and 6a by an MOCVD method. 2) A laser chip is produced by cleaving at the cleavage lines. In this embodiment, since an overhung portion comprising at least a support layer and a mask layer is produced at an upper portion of the semiconductor substrate at the neighborhood of the resonator end surface, and in this state respective semiconductor layers are successively grown on the substrate by an MOCVD method, the circulating of material gas into below the overhung portion becomes bad, and the amount of supplied gas thereto is reduced with relative to that to the other portion, and the growth rate of the film becomes slower. Thus, a doublehetero junction structure in which the thicknesses of the respective layers at the neighborhood of the resonator end surface are thinner than those at the inside and central portion of the chip is obtained. In the above-illustrated embodiment, a semiconductor laser having no mechanism for controlling the transverse mode is described for simplification, but the method of the present invention, of course, may be applied to a semiconductor laser having the same. Another embodiment of the present invention of such a structure is shown in figure 3. In figure 3, the reference numeral 12 designates an n type GaAs current blocking layer, the reference numeral 13 designates a stripe groove, and the reference numeral 14 designates a p type Al x Ga 1-x As embedded layer. The production processes of this embodiment are the same up to those of figures 2(a) to (d), and thereafter, an n type GaAs current blocking layer 12 is epitaxially grown, a stripe groove 13 is produced by etching the layer 12, and a p type Al x Ga 1-x As embedded layer 14 is epitaxially grown. The productions of the layers 12 and 14 are conducted by an MOCVD method. The processes thereafter are the same as those shown in figure 2(e). In the structure of this embodiment, the active layer 5 is provided close to the current blocking layer 12 so as to constitute a light waveguide mechanism which enables a transverse mode control. Also the thickness of the active layer is made less at the neighborhood of the resonator end surface than at the inside and central portion of the chip, resulting in the same effects as those of the first embodiment. As is evident from the foregoing description, according to the present invention, epitaxial growths of a lower cladding layer, an active layer, and an upper cladding layer which constitute a doublehetero junction are successively grown on the substrate by an MOCVD method in a state where an overhung portion of a mask layer is produced at above the semiconductor substrate at the neighborhood of the light emission end surface. Accordingly, the thicknesses of the respective layers at the neighborhood of the resonator end surface are made less than those at the inside and central portion of the chip, and a semiconductor laser having a narrow emission beam, a high output operation, a low threshold current, and a lengthy lifetime is obtained at a high production yield and uniform quality with good reproducibility.

Claims: 1. A method of producing a semiconductor laser comprising process steps conducted as follows: providing a semiconductor substrate (1); growing a support layer (8) upon a surface of said substrate; growing a mask layer (9, 10) upon said support layer; etching said mask layer to leave a remanent portion thereof at at least one position proximate to that position of a light emission end surface of the laser to be produced; etching said support layer thereby to leave a part (11) of said remanent portion of the mask layer overhanging said surface of said substrate; and growing successively a lower cladding layer (4), an active layer (5) and an upper cladding layer (6) on said substrate by metal-organic chemical vapour deposition, whereby a double-heterojunction structure laser is produced wherein the thickness of each of said lower cladding layer (4a), active layer (5a), and upper cladding layer (6a) is in each respective case less at the light emission end thereof than at the inside and central position of the laser. 2. A method of producing a semiconductor laser as claimed in claim 1, wherein said support layer (8) and said mask layer (9) are each grown by metal-organic chemical vapour deposition. 3. A method of producing a semiconductor laser as claimed in either claims 1 or 2, wherein said mask layer (9) comprises gallium arsenide (GaAs) and is etched by ammonium hydrogen-peroxide water system etchant. 4. A method of producing a semiconductor laser as claimed in any one of the preceding claims, wherein said support layer (8) comprises aluminium gallium arsenide (Al x Ga 1-x As) and is etched by mixed solution of iodine and potassium iodide.

Inventors: TAKAMI, AKIHIRO MITSUBISHI DENKI
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: H01S3
IPC codes: H01S5, H01L33, H01L21