Semiconductor laser device and method of making same

Abstract: A semiconductor laser includes a groove (10) in a GaAs first current blocking layer (5), which extends to an AlxGa(1-x)As second clad layer (4) beneath the first blocking layer (5). The width of the groove (10) periodically changes between larger and smaller widths along the length of the resonator. Over the regions of the first current blocking layer (5) where the groove has the smaller width, a grating-shaped AlzGa(1-z)As second current blocking layer (6) is formed. An AlxGa(1-x)As third current blocking layer (7) is formed over the second current blocking layer (6) and the portions of the first current blocking layer (5) not covered by the second current blocking layer (6). The variation of the width of the groove (10) is provided by placing a wafer with the groove formed in the first current blocking layer (5) in a metal organic thermal decomposition crystal deposition apparatus and heating it, while causing hydrogen chloride to flow, or placing the wafer in a molecular beam epitaxial deposition apparatus and heating it while irradiating with an arsenic molecular beam, or by placing a liquid-phase epitaxial deposition apparatus and causing to pass over the wafer a melt which selectively melts the first current blocking layer (5) but not the second current blocking layer (6).

Description: The present invention relates to an AlGaAs semiconductor laser device and, more particularly, to a gain-coupled type semiconductor laser device and a method of making such a laser device. BACKGROUND OF THE INVENTION In Figure 1a, there is shown a conventional index-coupled type, distributed feedback (DFB) semiconductor laser device shown, for example, in the official gazette of Unexamined Japanese Patent Publication No. SHO 60-66484 laid open to public inspection on April 16, 1985. The laser device of Figure 1a comprises an N-type GaAs substrate 21, a first clad layer 22 of N-type Al 0.4 Ga 0.6 As formed on the substrate 21, an intrinsic (I) Al 0.1 Ga 0.9 As active layer 23 formed on the first clad layer 22, a P-type Al 0.25 Ga 0.75 As beam guide layer 24 formed on the active layer 23, and an N-type GaAs current blocking layer 25 formed on the beam guide layer 24. As will be described later with reference to Figure 1b, corrugations 31 and 32 each comprising periodically repeating ridges are formed by etching in the upper surface portion of the P-type Al 0.25 Ga 0.75 As beam guide layer 24 in a groove 30 in the N-type GaAs current blocking layer 25 and in the upper surface of the current blocking layer 25, respectively. A second clad layer 26 of P-type Al 0.4 Ga 0.6 As is formed over the groove 30 to fill it and also over the upper surface of the N-type GaAs current blocking layer 25. Over the second clad layer 26, formed is a P-type GaAs contact layer 27. An electrode layer 28 of, for example, Cr/Au overlies the P-type GaAs contact layer 27. An electrode layer 29 of, for example, Au-Ge/Au is formed to cover the bottom surface of the N-type GaAs substrate 21. This semiconductor laser device is fabricated in a manner stated below. For example, an MOCVD (metalorganic chemical vapor deposition) technique is employed to successively grow, on the N-type GaAs substrate 21, the N-type Al 0.4 Ga 0.6 As first clad layer 22, the intrinsic Al 0.1 Ga 0.9 As active layer 23, the P-type Al 0.25 Ga 0.75 A₅ beam guide layer 24 and the N-type GaAs current blocking layer 25. Next, referring to Figure 1b, a photoresist layer is formed over the current blocking layer 25, and is exposed to a pattern of laser light, using two-beam interference exposure technique, to thereby form a predetermined periodic striped pattern in the photoresist layer. Next, the current blocking layer 25 is etched by reactive ion etching technique, with the stripe-patterned photoresist layer used as a mask, whereby the corrugation 32 comprising periodically repeating ridges is formed in the current blocking layer 25. After that, a photoresist mask is formed over the current blocking layer 25, and the groove 30 of a predetermined width is formed to extend in the direction perpendicular to the direction of extension of the ridges. The conditions for this etching are determined such that the etching should advance vertically so that etching from the sides of the ridges can be prevented. Then, the periodic stripe pattern in the N-type GaAs current blocking layer 25 is retained during the etching and, ultimately, it is transferred to the surface of the the P-type Al 0.25 Ga 0.75 As beam guide layer 24. Thereafter, the photoresist is removed and the surface of the structure is cleaned. The MOCVD technique is again used to successively grow the P-type Al 0.4 Ga 0.6 As second clad layer 26 and the P-type GaAs contact layer 27. Then, the Cr/Au electrode layer 28 and the Au-Ge/Au electrode layer 29 are deposited over the contact layer 27 and the bottom surface of the substrate 21, respectively. Next, operation of the above-stated index-coupled type semiconductor laser device is described. The electrode layers 28 and 29 are connected respectively to the positive and negative terminals of a bias source (not shown). Then, current flows through the semiconductor laser so that carriers injected into the intrinsic Al 0.1 Ga 0.9 As active layer 23 are recombined, whereby light is emitted. As the injection current level is increased, stimulated emission begins, and, ultimately, laser oscillation results. Part of laser light is guided into the P-type Al 0.25 Ga 0.75 As beam guide layer 24. The period Λ of the ridges in the corrugation 31 in the surface of the beam guide layer 24 in the groove 30 is determined to be, Λ = m·λ₀/2N r where: m is a positive integer; N r is the index of refraction of the beam guide path; and λ₀ is the oscillation wavelength. Then, only light of the wavelength λ₀ is selected and, accordingly, single longitudinal mode oscillation results. During the manufacturing of the above-stated conventional AlGaAs index-coupled type DFB semiconductor laser device, the P-type Al 0.25 Ga 0.75 As beam guide layer 24 is exposed to air, during the photoresist removal and surface cleaning treatments. Since the beam guide layer 24 contains Al, the surface of the beam guide layer 24 is oxidized. The P-type Al 0.4 Ga 0.6 As second clad layer 26 is then formed through crystal growth over the corrugation 31 of the P-type Al 0.25 Ga 0.75 As beam guide layer 24 the surface of which has been oxidized. This results in a number of crystal defects in the vicinity of the hetero-interface between them, which degrades the crystallinity of the layers. Such crystal defects tend to increase during the operation of the semiconductor laser device, which shortens the life of the device. There is another type of DFB semiconductor laser device, namely, a gain-coupled type DFB semiconductor laser device. An example of the gain-coupled type DFB semicondutor laser device is shown in Figures 2a and 2b. This laser device comprises an N-type GaAs substrate 41, an N-type Al 0.40 Ga 0.80 As first clad layer 42 formed over the substrate 41, a P-type GaAs active layer 43 overlying the first clad layer 42, a P-type Al 0.25 Ga 0.75 As carrier confining layer 44 overlying the active layer 43, and a P-type GaAs beam guide layer 45 formed to overlie the carrier confining layer 44. As shown in Figure 2b, corrugation 51 comprising periodically repeating ridges repeating with a period Λ is formed in the surface of the P-type GaAs beam guide layer 45. Overlying the corrugated P-type GaAs beam guide layer 45, a P-type Al 0.40 Ga 0.80 As second clad layer 46 is formed, which in turn is overlain by a P-type GaAs contact layer 47. This device also includes electrode layers 48 and 49 similar to the ones used in the semiconductor laser device shown in Figure 1a. The gain-coupled type DFB semiconductor laser device shown in Figures 2a and 2b oscillates in the single longitudinal mode, when the electrode layers 48 and 49 are connected to positive and negative terminals, respectively, of a bias voltage source, which causes current to flow through the semiconductor laser device. By selecting the value in accordance with the aforementioned equation (1) for the period Λ of the corrugation 51, only light of the wavelength λ₀ is selected, and the device oscillates in the single longitudinal mode at the wavelength λ₀. When the gain-coupled type DFB semiconductor laser device shown in Figures 2a and 2b is fabricated, the corrugation 51 is formed in the P-type GaAs beam guide layer 45, and, therefore, the surface exposed to air during the manufacturing is the upper surface of the P-type GaAs beam guide layer 45 which does not contain Al. Since spontaneous oxidation of the surface of the layer 45 is little, the number of crystal defects at the re-grown hetero-interface as observed in the aforementioned index-coupled type DFB semiconductor laser device is less. Accordingly, this gain-coupled type DFB semiconductor laser device has no degradation of reliability or shortening of life caused by the degradation of the crystallinity. However, this device has another problems. That is, since the P-type GaAs beam guide layer 45 itself absorbs laser light, the internal loss within the laser resonator must be large so that the laser device tends to have problems of increase of the oscillation threshold current, decrease of the quantum efficiency. A semiconductor laser device comprising a n-type AlGaAs first clad layer formed on a n-type GaAs substrate, a AlGaAs active layer formed on said first clad layer, a p-type AlGaAs second clad layer on said active layer, and three current blocking layers, the first one being of GaAs formed on said second clad layer and having a groove extending in depth to said second clad layer, is known from the document "AlGaAs/GaAs Self-Aligned LD's Fabricated by the Process containing Vapor Phase Etching and Subsequent MOVPE Regrowth", Masaaki Nido et al., IEEE Journal of Quantum Electronics, June 1987. An object of the present invention is to provide an AlGaAs semiconductor laser device, in particularly, an AlGaAs gain-coupled type DFB semiconductor device which is free of the above-stated various problems and has low oscillation threshold, high quantum efficiency, high reliability and a long life. The present invention also provides a method of fabricating such a semiconductor laser device. A semiconductor laser device according to the present invention comprises a first-conductivity-type GaAs substrate, a first-conductivity-type Al x Ga (1-x) As first clad layer formed over the substrate, a first- or second-conductivity-type or intrinsic Al y Ga (1-y) As active layer formed over the first clad layer, a second-conductivity-type Al x Ga (1-x) As second clad layer formed over the active layer, and a first- or second-conductivity-type or intrinsic GaAs first current blocking layer formed over the second clad layer. The first current blocking layer has a groove formed therein to extend in the depth direction to the second clad layer. The width of the groove changes between broader and narrower widths with a predetermined period along the longitudinal direction of a resonator. The semiconductor laser device according to the present invention further includes a first- or second-conductivity-type or intrinsic, grating-shaped Al z Ga (1-z) As second current blocking layer comprising strips formed to extend in the transverse direction to the direction of the length of the resonator on those regions of the first current blocking layer where the groove formed in the first current blocking layer has the narrower width, a first-conductivity-type GaAs third current blocking layer formed over said second current blocking layer and the portions of said first current blocking layer which are free of said second current blocking layer, a second-conductivity-type Al x Ga (1-x) As third clad layer formed to fill the groove in said first current blocking layer and also to cover the surface of said third current blocking layer, and a second-conductivity-type GaAs contact layer formed over said third clad layer. A first embodiment of the method of fabricating a semiconductor laser device according to the present invention comprises the steps of: (1) epitaxially growing successively a first-conductivity-type Al x Ga (1-x) As first clad layer on a first-conductivity-type GaAs substrate, a first- or second-conductivity-type or intrinsic Al y Ga (1-y) As active layer on the first clad layer, a second-conductivity-type Al x Ga (1-x) As second clad layer on the active layer, a first- or second-conductivity-type or intrinsic GaAs first current blocking layer on the second clad layer, and a first- or second-conductivity-type or intrinsic Al z Ga (1-z) As layer, on the first current blocking layer, for providing a second current blocking layer; (2) forming a grating of a photoresist over the Al z Ga (1-z) As layer for providing the second current blocking layer; (3) selectively etching the Al z Ga (1-z) As layer with the photoresist grating used as an etching mask to thereby form a grating-shaped Al z Ga (1-z) As second current blocking layer; (4) epitaxially growing a first-conductivity-type GaAs third current blocking layer to fill the spaces between the strips of the grating-shaped second current blocking layer; (5) forming a groove in the third current blocking layer to extend in the direction transverse to the strips of the second current blocking layer, the groove extending in the depth direction to the first current blocking layer; (6) placing the wafer with the thus formed groove, in a device for use in growing crystal through metalorganic thermal decomposition, and heating the wafer, while causing hydrogen chloride to flow in the apparatus, to thereby etch to remove the first current blocking layer portion exposed at the bottom the groove, and also etch portions of the first current blocking layer with the strips of the grating-shaped second current blocking layer as an etching mask, whereby the width of the groove in the first current blocking layer changes between smaller and larger widths with a predetermined period along the length of the groove (the third current blocking layer being also similarly etched so as to change the width of the groove in the third current blocking layer in this step); and (7), after the step (6), in the same crystal growing apparatus, successively epitaxially growing by metalorganic thermal decomposition technique a second-conductivity-type Al x Ga (1-x) As third clad layer to fill the changing-width groove formed by the hydrogen chloride etching and to cover the surface of the third current blocking layer, and a second-conductivity-type GaAs contact layer. A second embodiment of the method of making the semiconductor laser device according to the present invention comprises the steps, following the steps (1) through (5) of the first embodiment, of (6′) placing the wafer with the groove formed therein in a molecular beam epitaxial deposition apparatus, irradiating the wafer with an arsenic molecular beam, while heating the wafer, to thereby etch to remove the first current blocking layer portion exposed at the bottom of the groove, and also etch portions of the first current blocking layer with the strips of the second current blocking layer used as an etching mask, whereby the width of the groove formed in the first current blocking layer changes between smaller and larger widths with a predetermined period (the third current blocking layer being also similarly etched so as to change the width of the groove in the third blocking layer in this step); and (7′), following the step (6′), in the same deposition apparatus, successively epitaxially growing by molecular beam epitaxy a second-conductivity-type Al x Ga (1-x) As third clad layer to fill the changing-width groove formed in the wafer and to cover the surface of the third current blocking layer, and a second-conductivity-type GaAs contact layer. According to a third embodiment of the method of making the semiconductor laser device of the present invention, following the steps (1) through (5) in the first embodiment, there are included the steps of (6″) placing the wafer with the groove formed therein in a liquid-phase epitaxial deposition apparatus, causing to pass over the wafer, a melt which can selectively melt the first current blocking layer but not the second current blocking layer, to thereby remove the first current blocking layer portion exposed at the bottom of the groove through melt-back process, and remove portions of the first current blocking layer with the strips of the second current blocking layer as a melt-back preventing layer, whereby the width of the groove formed in the first current blocking layer changes between smaller and larger widths with a predetermined period (portions of the third current blocking layer being also similarly melted back into the melt passing over the wafer, so as to change the width of the groove in the third current blocking layer); and (7″), after the step (6″), successively epitaxially growing, in the same liquid phase epitaxial deposition apparatus, a second-conductivity-type Al x Ga (1-x) As third clad layer to fill the changing-width groove in the first current blocking layer and to cover the surface of the third current blocking layer, and a second-conductivity-type GaAs contact layer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1a is a perspective view of a conventional index-coupled type semiconductor laser device, and Figure 1b is a perspective view for use in explaining part of the process for making it; Figure 2a is a perspective view of a conventional gain-coupled type semiconductor laser device, and Figure 2b is a perspective view for use in explaining part of the process for making it; Figure 3a is a perspective view of one embodiment of the gain-coupled type DFB semiconductor laser device according to the present invention; Figure 3b shows a cross-section of the semiconductor laser device shown in Figure 3a, at a location where a second current blocking layer is present; Figure 3c shows a cross-section of the semiconductor laser shown in Figure 3a, at a location where the second current blocking layer is not present; Figure 3d shows on what portions of a first current blocking layer the second current blocking layer is formed; and Figures 4a through 4g show various steps in the process of making the semiconductor laser device according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 3a is a perspective view showing the entire structure of a gain-coupled type DFB semiconductor laser device according to the present invention. The semiconductor laser device comprises, for example, an N-type GaAs substrate 1, an N-type Al₀·₅Ga₀·₅As first clad layer 2 formed on the substrate 1, a P-type Al₀·₁₅Ga₀·₈₅As active layer 3 on the first clad layer 1, a P-type Al₀·₅Ga₀·₅As second clad layer 4 on the active layer 3, and an N-type GaAs first current blocking layer 5 on the second clad layer 4. As shown in Figures 3a through 3d, on the regions of the first current blocking layer 5 where a groove 10 has a narrower width, that is, as shown in Figure 3b which shows the cross-section along the line X-X in Figure 3d, on the regions 11 in Figure 3d where the corresponding portions of the groove 10 has a width W₁, there is formed an N-type Al₀·₅Ga₀·₅As second current blocking layer 6 in the grating form having a predetermined period Λ (shown in Figure 3d). Over the second blocking layer 6 and also over the portion of the first current blocking layer 5 which is free of the second current blocking layer 6, an N-type GaAs third current blocking layer 7 is formed. P-type Al₀·₅Ga₀·₅As is deposited to fill the groove 10 extending in the first, second and third current blocking layers 5, 6 and 7 and also to cover the surface of the third current blocking layer 7, so that a third clad layer 8 is formed. Over this P-type Al₀·₅Ga₀·₅As third clad layer 8, a P-type GaAs contact layer 9 is formed. Electrode layers are provided on the P-type GaAs contact layer 9 and the N-type GaAs substrate 1, respectively. The electrode layer on the contact layer 9 is connected to the positive terminal of a bias source and the electrode layer on the substrate 1 is connected to the negative terminal. Then, current flows from the contact layer 9 toward the substrate 1 only through those regions of the device where the first, second and third current blocking layers 5, 6 and 7 are not present, that is, through the groove 10. Carriers injected into the P-type GaAs active layer by the current flow recombine to cause light to be emitted. As the level of the injected current is increased, stimulated emission begins and, then, the laser oscillation state is reached. Part of the laser light is guided into parts of the N-type Al₀·₅Ga₀·₅As first clad layer 2, the P-type Al₀·₅Ga₀·₅As second clad layer 4 and the P-type Al₀·₅Ga₀·₅AS third clad layer 8. As shown in Figure 3d, the width of the groove 10, which is formed through the first, second and third current blocking layers 5, 6 and 7 and which is filled with the material of the third clad layer 8, changes alternately between W₁ (smaller width) of Figure 3b and W₂ (larger width) of Figure 3c showing the cross-section along Y-Y in Figure 3d. The width changes with a period of Λ. By so arranging, only the wavelength λ₀ expressed by the equation (1) is selected and the single longitudinal mode of oscillations results. Next, referring to Figures 4a through 4g, the method of making the gain-coupled type DFB semiconductor laser device according to the present invention shown in Figures 3a through 3d is described. EXAMPLE 1 (1) As shown in Figure 4a, by epitaxial deposition technique, there are successively formed, the N-type Al₀·₅Ga₀·₅As first clad layer 2 on the N-type GaAs substrate 1, the P-type Al₀·₁₅Ga₀·₈₅As active layer 3 on the first clad layer 2, the P-type Al₀·₅Ga₀·₅As second clad layer 4 on the active layer 3, the N-type GaAs first current blocking layer 5 on the second clad layer 4, and a P-type Al₀·₅Ga₀·₅As layer 60 to be formed into the second current blocking layer. (2) Next, over the surface of the P-type Al₀·₅Ga₀·₅As layer 60 shown in Figure 4a, a grating of photoresist is formed by means of, for example, two-beam interference exposure technique. After that, the P-type Al₀·₅Ga₀·₅As layer 60 is selectively etched, using the photoresist grating as an etching mask, whereby the grating-shaped P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is formed. (3) Then, as shown in Figure 4c, the N-type GaAs third current blocking layer 7 is epitaxially grown to fill the grating-shaped second current blocking layer 6. (4) After that, a photoresist layer is formed on the third current blocking layer 7 shown in Figure 4c for etching the groove 10 in the third current blocking layer 7 and the second current blocking layer 6, which groove extends in the direction transverse to the direction of extension of the grating strips of the second current blocking layer 6 and also extends down to the first current blocking layer 5. (5) Next, the wafer having the groove 10 formed therein as shown in Figure 4d is placed in an apparatus for growing crystal by metalorganic thermal decomposition process. The wafer is heated, while hydrogen chloride is caused to flow over the wafer, so that the portion of the N-type GaAs first current blocking layer 5 exposed through the groove 10 is etched away and also portions of the first current blocking layer 5 are etched away with the strips of the grating-shaped second current blocking layer 6 acting as an etching mask, whereby the groove 10 in the first current blocking layer 5 changes in width between smaller and larger widths with the predetermined period Λ in the longitudinal direction of the groove 10. In this etching stage, portions of the third current blocking layer 7 are also similarly etched away to periodically change the width of the groove in the third current blocking layer 7, too. Thus, the structure as shown in Figure 4e is provided. Figure 4f shows the cross-section at portions of the structure shown in Figure 4e where the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is present. As shown in Figure 4f, the width of the groove at its bottom in these portions is W₁. Figure 4g shows the cross-section of the structure shown in Figure 4e at portions where the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is not present. The width of the groove 10 at its bottom in these portions is W₂ (W₂>W₁). (6) Following the step (5), the P-type Al₀·₅Ga₀·₅As third clad layer 8, which fills the groove 10 and covers the surface of the third current blocking layer 7, and the P-type GaAs contact layer 9 on the third clad layer 8 are epitaxially grown by the metalorganic thermal decomposition process in the same crystal growing apparatus, whereby the semiconductor laser device shown in Figure 3a results. EXAMPLE 2 The steps (1) through (4) of EXAMPLE 1 are followed. (5′) The wafer with the groove 10 formed therein as shown in Figure 4d is placed in an apparatus for growing crystal by molecular beam expitaxy. The wafer is heated, while being irradiated with an arsenic molecular beam, so that the portion of the N-type GaAs first current blocking layer 5 exposed at the bottom of the groove 10 is etched away and also portions of the first current blocking layer 5 are etched away with the strips of the grating-shaped second current blocking layer 6 acting as an etching mask, whereby the groove 10 in the first current blocking layer 5 changes in width, alternating between smaller and larger widths with the predetermined period Λ in the longitudinal direction of the groove 10. In this etching stage, portions of the third current blocking layer are also similarly etched away to periodically change the width of the groove in the third current blocking layer 7, too. Thus, the structure as shown in Figure 4e is provided. As stated previously with reference to EXAMPLE 1, Figure 4f shows the cross-section at portions of the structure shown in Figure 4e where the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is present. As shown in Figure 4f, the width of the groove at its bottom in these portions is W₁. Figure 4g shows the cross-section of the structure shown in Figure 4e at portions where the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is not present. The width of the groove 10 at its bottom in these portions is W₂ (W₂>W₁). (6′) Following the step (5′), the P-type Al₀·₅Ga₀·₅As third clad layer 8, which fills the groove 10 and covers the surface of the third current blocking layer 7, and the P-type GaAs contact layer 9 on the third clad layer 8 are epitaxially grown by the molecular beam epitaxy in the same crystal growing apparatus, whereby the semiconductor laser device shown in Figure 3a results. EXAMPLE 3 The steps (1)-(4) of EXAMPLE 1 are performed. (5″) The wafer having the groove 10 formed therein as shown in Figure 4d is placed in a liquid-phase epitaxial deposition apparatus. A melt which can melt the N-type GaAs first current blocking layer 5 and the N-type GaAs third current blocking layer 7 but cannot melt the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is caused to pass over the wafer, whereby the portion of the N-type GaAs first current blocking layer 5 exposed at the bottom of the groove 10 is removed through melt -back process and also portions of the N-type GaAs first current blocking layer 5 are removed with the strips of the grating-shaped Al₀·₅Ga₀·₅As second current blocking layer 6 acting as the melt-back preventing layer, whereby the groove 10 in the first current blocking layer 5 changes in width, alternating between a smaller width and a larger width with the period Λ in the longitudinal direction of the groove 10. In this stage, portions of the third current blocking layer 7 are also similarly removed through the melt-back process so as to periodically change the width of the groove in the third current blocking layer 7, too. Thus, the structure shown in Figure 4e results. As in the case of EXAMPLE 1, Figure 4f shows the cross-section of the structure of Figure 4e at portions where the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is present. In these portions, the width of the groove at the bottom is W₁. Figure 4g shows the cross-section of the structure of Figure 4e at portions where the P-type Al₀·₅Ga₀·₅As second current blocking layer 6 is not present. In these portions, the width of the groove at the bottom is W₂ (W₂>W₁). (6″) Following the step (5″), in the same crystal growing apparatus, the P-type Al₀·₅Ga₀·₅As third clad layer 8, which fills the groove 10 and also covers the surface of the third current blocking layer 7, and the P-type GaAs contact layer 9 overlying the third current blocking layer 7 are successively epitaxially grown. Thus, the semiconductor laser device shown in Figure 3a is completed. In the gain-coupled type DFB semiconductor laser device according to the present invention, the stripe-shaped groove is formed through the first through third current blocking layers 5, 6 and 7 to extend in the longitudinal direction of the resonator. Accordingly, there is substantially no portion which would act as laser-light absorbers. Furthermore, according to the present invention, since the groove 10 is formed to have a width which changes in the longitudinal direction between smaller and larger widths with a predetermined period Λ, the resulting AlGaAs gain-coupled type DFB semiconductor laser device can provide the single longitudinal mode of oscillations at the wavelength λ₀ with a low threshold current and with a high quantum efficiency. Furthermore, according to the method of making the semiconductor laser device of the present invention, during the step of removing portions of the first through third current blocking layers in order to form the varying-width groove and during the succeeding step of growing the third clad layer and the contact layer, it never occurs that the surfaces of these layers are exposed to air. Accordingly, there is no surface oxidation, and, therefore, the number of crystal defects at the re-grown heterointerface is greatly reduced relative to conventional devices, so that reliable, long-life semiconductor laser devices operable at low threshold current and at high quantum efficiency can be fabricated.

Claims: A semiconductor laser device comprising: a first-conductivity-type Al x Ga (1-x) As first clad layer (2) formed on a first-conductivity-type GaAs substrate (1); a first-conductivity-type, second-conductivity-type or intrinsic Al y Ga (1-y) As active layer (3) formed on said first clad layer (2); a second-conductivity-type Al x Ga (1-x) As second clad layer (4) on said active layer; a first-conductivity-type, second-conductivity-type or intrinsic GaAs first current blocking layer (5) formed on said second clad layer, said first current blocking layer (5) having a groove extending in depth to said second clad layer (4), said groove changing in width alternately between narrower and broader widths in the longitudinal direction of a resonator with a predetermined period; a first-conductivity-type, second-conductivity-type or intrinsic grating-shaped Al z Ga (1-z) As second current blocking layer (6) comprising strips extending in the direction transverse to said longitudinal direction of the resonator and overlying the portions of said first current blocking layer (5) where said groove in said first current blocking layer (5) has the narrower width; a first-conductivity-type GaAs third current blocking layer (7) overlying said second current blocking layer (6) and the portions of said first current blocking layer (5) which are free of said second current blocking layer (6); a second-conductivity-type Al x Ga (1-x) As third clad layer (8) filling said groove (10) and overlying said third current blocking layer (7); and a second-conductivity-type GaAs contact layer (9) formed over said third clad layer. A method of making a semiconductor laser device, comprising the steps of: successively epitaxially growing, in the named order, a first-conductivity-type Al x Ga (1-x) As first clad layer (2) on a first-conductivity-type GaAs substrate (1), a first-conductivity-type, second-conductivity-type or intrinsic Al y Ga (1-y) As active layer (3) on said first clad layer (2), a second-conductivity-type Al x Ga (1-x) As second clad layer (4) on said active layer (3), a first -conductivity-type, second-conductivity-type or intrinsic GaAs first current blocking layer (5) on said second clad layer (4), and a first-conductivity-type, second conductivity-type or intrinsic Al z Ga (1-z) As layer to be formed into a second current blocking layer(6); forming a grating of photoresist over said Al z Ga (1-z) As layer (6) to be formed into said second current blocking layer; selectively etching said Al zGa(1-z) As layer with said photoresist grating used as an etching mask to thereby form the grating-shaped Al z Ga (1-z) As second current blocking layer (6); epitaxially growing a first-conductivity-type GaAs third current blocking layer (7) such that said grating-shaped second current blocking layer is buried in said third current blocking layer; forming in said third current blocking layer a groove (10) extending in the direction transverse to the extension of the grating and also extending in depth to said first current blocking layer; placing the wafer with said groove formed therein in a metal-organic thermal decomposition crystal growing apparatus, and heating the wafer, while causing hydrogen chloride to flow in said apparatus, to thereby etch away the portion of said first current blocking layer (5) at the bottom of said groove (10) and also etch away portions of said first current blocking layer (5) with said grating-shaped second current blocking layer used as an etching mask so that the width of said groove formed in said first current blocking layer (5) changes alternately between smaller and larger widths with a predetermined period in the longitudinal direction of said groove; and after the preceding step, keeping said wafer in the same crystal growing apparatus and successively epitaxially forming through metalorganic thermal decomposition process a second-conductivity-type Al x Ga (1-x) As third clad layer (8) to fill said groove (10) in said wafer and also to cover the surface of said third current blocking layer (7), and a second-conductivity-type GaAs contact layer (9) on said third clad layer (8). A method of making a semiconductor laser device, comprising the steps of: successively epitaxially growing, in the named order, a first-conductivity-type Al x Ga (1-x) As first clad layer (2) on a first-conductivity-type GaAs substrate(1), a first-conductivity-type, second-conductivity-type or intrinsic Al y Ga (1-y) As active layer (3) on said first clad layer, a second-conductivity-type Al x Ga (1-x) As second clad layer (4) on said active layer (3), a first-conductivity-type, second-conductivity-type or intrinsic GaAs first current blocking layer (3) on said second clad layer (4), and a first-conductivity-type, second-conductivity-type or intrinsic Al z Ga (1-z) As layer to be formed into a second current blocking layer (6); forming a grating of photoresist over said Al z Ga (1-z) As layer to be formed into said second current blocking layer (6); selectively etching said Al z Ga (1-z) As layer with said photoresist grating used as an etching mask to thereby form the grating-shaped AlzGa(1-z)As second current blocking layer (6); epitaxially growing a first-conductivity-type GaAs third current blocking layer (7) such that said grating-shaped second current blocking layer (6) is buried in said third current blocking layer (7); forming in said third current blocking layer (7) a groove (10) extending in the direction transverse to the extension of the grating and also extending in depth to said first current blocking layer (6); placing the wafer with said groove (10) formed therein in a molecular beam epitaxy crystal deposition apparatus, and heating the wafer, while irradiating said wafer with an arsenic molecular beam, to thereby etch away the portion of said first current blocking layer (5) at the bottom of said groove and also etch away portions of said first current blocking layer (5) with said grating-shaped second current blocking layer used as an etching mask so that the width of said groove (10) formed in said first current blocking layer (5) changes alternately between smaller and larger widths with a predetermined period in the longitudinal direction of said groove (10); and after the preceding step, keeping said wafer in the same crystal deposition apparatus, and successively epitaxially growing through molecular beam epitaxy a second-conductivity-type Al x Ga (1-x) As third clad layer (8) to fill said groove (10) formed in said wafer and also to cover the surface of said third current blocking layer (7), and a second-conductivity-type GaAs contact layer on said third clad layer (9). A method of making a semiconductor laser device, comprising the steps of: successively epitaxially growing, in the named order, a first-conductivity-type Al x Ga(₁ -x )As first clad layer (2) on a first-conductivity-type, GaAs substrate (1), a first-conductivity-type, second-conductivity-type or intrinsic Al y Ga (1-y) As active layer (3) on said first clad layer (2), a second-conductivity-type Al x Ga (1-x) As second clad layer (4) on said active layer (3), a first-conductivity-type, second-conductivity-type or intrinsic GaAs first current blocking layer (5) on said second clad layer, and a first-conductivity-type, second-conductivity-type or intrinsic Al z Ga (1-z) As layer to be formed into a second current blocking layer (6) forming a grating of photoresist over said Al z Ga (1-z) As layer to be formed into said second current blocking layer (6); selectively etching said Al z Ga (1-z) As layer with said photoresist grating used as an etching mask to thereby form the grating-shaped A1 z Ga (1-z) As second current blocking layer (6); epitaxially growing a first-conductivity-type GaAs third current blocking layer (7) such that said grating-shaped second current blocking layer (6) is buried in said third current blocking layer (7); forming in said third current blocking layer (7) a groove (10) extending in the direction transverse to the extension of the grating and also extending in depth to said first current blocking layer (5); placing the wafer with said groove formed therein in a liquid-phase epitaxy crystal deposition apparatus, and causing to pass over said wafer a melt which can melt said first current blocking layer (5) but not said second current blocking layer (6), to thereby melt back to remove the portion of said first current blocking layer (5) at the bottom of said groove (10) and also to remove portions of said first current blocking layer (5) with said grating-shaped second current blocking layer used as an anti-melt-back layer so that the width of said groove formed in said first current blocking layer (5) changes alternately between smaller and larger widths with a predetermined period in the longitudinal direction of said groove; and after the preceding step, keeping said wager in the same crystal deposition apparatus, and successively epitaxially growing through liquid-phase epitaxy a second-conductivity-type Al x Ga( 1-x )As third clad layer (8) to fill said groove (10) formed in said wafer and also to cover the surface of said third curent blocking layer (7) and a second-conductivity-type GaAs contact layer on said third clad layer (9).

Inventors: KAGAWA HITOSHI C O MITSUBISHI, YAGI TETSUYA C O MITSUBISHI DE, KAGAWA, HITOSHI C/O MITSUBISHI DENKI K.K., YAGI, TETSUYA C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: H01S3
IPC codes: H01S5