A SEMICONDUCTOR LASER DEVICE

Abstract: A semiconductor laser device comprises a substrate having a main striped channel (21) for confining current therein and sub-striped channels (22a, 23a) formed in a parallel manner outside of said main striped channel (21), the width of said sub-striped channels being greater than that of said main striped channel, and an active layer (13) for laser oscillation, whereby the portions of said active layer corresponding to said sub-striped channels are formed into a concave, and the portion of said active layer positioned between the concaved por­ tions of said active layer results in a plane with a limited thickness.

Description: This invention relates to a semiconductor laser device. In order to achieve continuously stable laser oscillation in a single mode at a low threshold current level, semiconductor laser devices have been designed in a manner to define the active layer by cladding layers, and moreover to confine light and carriers with a high density within the laser oscillation area of the active layer by means of a current-confining striped structure and a waveguide structure. However, when the semiconductor laser devices operate at a high output power, the density of light in the active layer is increased, resulting in heat deterioration of the devices. To resolve this problem, TRS (a high-power, single-mode laser with twin-ridge-substrate structure) lasers have been proposed (Appl. Phys. Lett., vol. 42, No. 10, 15th May, 1983, p. 853) in which a thin active layer is formed to decrease the light density of the active layer thereby attaining high output power operation. The thin active layer is grown on the ridges of the base layer by liquid phase epitaxy. However, the growth of an active layer having a thickness of as thin as 0.1 µm or less using liquid phase epitaxy is extremely difficult. The output intensity of laser light depends upon the thickness of the active layer, and as the thickness of the active layer is decreased, the carrier density of the active layer increases resulting in a state for laser oscillation at a high output power at a low current level. However, even though a thin active layer is grown on the ridges of the base layer and accordingly a decrease in the light density thereof is achieved, the absorption of laser light at or around the facets does not decrease, causing damage and/or deterioration of the facets, which shortens the life time of these TRS lasers. On the other hand, window VSIS lasers have been proposed (Appl. Phys. Lett., vol. 42, No. 5, 1st May, 1983, p. 406) in which a thick and concaved active layer is used in the stimulated region inside of the facets and a thin and flat active layer is used in the window regions in the vicinity of the facets to thereby decrease the absorption of laser light at or near the facets, so that high output power operation can be attained. However, so long as the growth of the active layer is carried out using a common liquid phase epitaxial growth technique, the curvature of the active layer in the light stimulated region cannot be controlled within a fixed limit due to problems such as the saturation of the Ga solution and/or of the temperature distribution within the growth furnace. Thus, semiconductor laser devices having uniform characteristics cannot be mass-produced with reliability. Moreover, the application of such a concaved active layer to the devices results in an increase in part in the light density of the said concaved active layer with relation to the refractive index of the active layer, causing deterioration of the devices at high output power operation. In CA-A-1150388 there is disclosed a double heterostructure semiconductor laser having a substrate formed with three parallel striped channels, the central channel being narrower and shallower than the two flanking channels, and an active layer which has a planar portion of substantially uniform thickness over the central channel. According to the present invention, there is provided a semiconductor laser device comprising a substrate having a main striped channel for confining current therein and sub-striped channels formed in a parallel manner outside of said main striped channel, the width of said sub-striped channels being greater than that of said main striped channel, and an active layer for laser oscillation, the portions of said active layer which correspond to said sub-striped channels, being formed into a concave, characterised in that the portion of said active layer positioned between the concaved portions of the active layer has a thickness which is less than that of the concaved portions of the active layer for the purpose of controlling the thickness of the active layer of the semiconductor laser. The thickness of the portions of said active layer corresponding to said main striped channel in the vicinity of the facets is, in a preferred embodiment, thinner than that of the portion of said active layer corresponding to said main striped channel inside of the facets. In a more preferred embodiment, the sub-striped channels are formed in the direction of laser oscillation along the whole length of the main striped channel, the distance between the main striped channel and each of the sub-striped channels in the vicinity of the facets being smaller than that between the main striped channel and each of the sub-striped channels inside of the facets. Alternatively, the sub-striped channels are formed in the direction of laser oscillation along the main striped channel only in the vicinity of the facets. The width of each of the sub-striped channels is, in a preferred embodiment, wider in the vicinity of the facets than that of each of the sub-striped channels inside of the facets. The depth of each of the sub-striped channels is, in a preferred embodiment, deeper in the vicinity of the facets than that of each of the sub-striped channels inside of the facets. The portion of the active layer in the vicinity of each of the facets is, in a preferred embodiment, a window region functioning as an optical waveguide. Thus, the invention described herein makes possible the objects of (1) providing a semiconductor laser device in which the growth of a thin active layer is attained with reproducibility resulting in a high output power semiconductor laser device; (2) providing a semiconductor laser device in which the active layer of the laser oscillation operation area is flat and the thickness of the flat portion of the active layer in the vicinity of the facets is thinner than that of the flat portion of the active layer inside of the facets, so that the absorption of laser light at or around the facets can be suppressed, thereby attaining continuous laser oscillation at a high output power at room temperatures; (3) providing a semiconductor laser device which can oscillate at a high output power and can be produced at an improved yield since the portion of the active layer corresponding to the main striped channel structure is necessarily formed into a flat shape; (4) providing a long life semiconductor laser device having window regions in which the absorption of laser light is minimized, and (5) providing a semiconductor laser device which can be mass-produced with reliability. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figures 1(A) to 1(E) are sectional side views showing a production process of a semiconductor laser device of this invention; Figure 2(A) is a perspective view showing the semiconductor laser device produced by the process shown in Figure 1; Figure 2(B) is a graph showing the relationship between the distance between the main striped channel and the sub-striped channel and the thickness of the active layer of the semiconductor laser device shown in Figure 2(A); Figure 3 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region; Figures 4(A) to 4(D) are perspective views showing a production process of the semiconductor laser device shown in Figure 3; Figure 5 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region; Figures 6(A) to 6(D) are perspective views showing a production process of the semiconductor laser device shown in Figure 5; Figure 7 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region: Figure 8 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region; Figures 9(A) to 9(D) are perspective views showing a production process of the semiconductor laser device shown in Figure 8; Figure 10 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region; Figure 11 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region; Figures 12(A) to 12(D) are perspective views showing a production process of the semiconductor laser device shown in Figure 11; and Figure 13 is a perspective view showing another semiconductor laser device of this invention which is cut at the interface between each of the window regions and the light stimulated region. Embodiment 1 Figures 1(A) to 1(E) show the production process of a semiconductor laser device of this invention. As shown in Figure 1(A), on the growth surface of a Zn-doped p-GaAs substrate (p=1 × 10¹⁹ cm⁻³) 10, striped mesa-channels 220a and 230a having a width of 8 µm and a depth of 1 µm are formed with distance of 40 µm therebetween by an etching technique. Then, on the GaAs substrate 10 including the striped mesa-channels 220a and 230a, as shown in Figure 1(B), a Te-doped n-GaAs current blocking layer (n=6 × 10¹⁸ cm⁻³) 11 is grown by liquid phase epitaxy in such a manner that the striped mesa-channels 220a and 230a are filled with the GaAs current blocking layer 11 having a flat surface, the thickness of which is 0.8 µm in the portion corresponding to the flat area of the substrate 10 other than the striped mesa-channels 220a and 230a. This current blocking layer 11 has a polarity different from the polarity of current to be injected. Then, a resist film 50 is formed on the current blocking layer 11, and etched off in part to form stripes 51, 52 and 53, as shown in Figure 1(C), followed by etching to form a main striped channel (the width thereof being 3 µm and the depth thereof being 1 µm) 21 into a V-shape in the portion corresponding to the stripe 51 and sub-striped channels (the width thereof being 8 µm and the depth thereof being 1 µm) 22a and 23a in the portions corresponding to the stripes 52 and 53, respectively, as shown in Figure 1(D). Since the sub-striped channels 22a and 23a are constituted by the current blocking layer 11 on the substrate 10, current does not flow therethrough. On the other hand, since the main striped channel 21 reaches the p-GaAs substrate 10, from which the current blocking layer 11 is removed, a current path is opened in this area. The current path is formed only in the main striped channel 21 which contributes to light emission, so that non-productive current which does not contribute to light emission can be reduced. Thereafter, the remaining resist film 50 is removed, and a p-GaAlAs cladding layer 12 forming a heterojunction with an active layer as mentioned below, a GaAlAs active layer 13 for laser oscillation, an n-GaAlAs cladding layer 14 forming a heterojunction with the above-mentioned active layer 13, and an n⁺-GaAs cap layer 15 achieving an ohmic contact with an electrode are successively grown by liquid phase epitaxy, resulting in a double-heterostructure multi-layered crystal for laser oscillation. Since the width of the sub-striped channels 22a and 23a is greater than that of the main striped channel 21, the portions of the p-cladding layer 12 corresponding to these sub-striped channels 22a and 23a is formed into a downward concave. The growth rate of the active layer 13 grown on the concaved portions of the p-cladding layer 12 is higher than that of the active layer 13 grown on the plane portion of the p-cladding layer 12, and thus As in the solution which comes into contact with the concaved portions of the p-cladding layer 12 is more quickly consumed for the growth of the active layer 13 than that in the solution in the surroundings of the concaved portions of the p-cladding layer 12, resulting in the diffusion of As from the solution in the surroundings of the concaved p-cladding layer 12 into that in the concaved p-cladding layer 12, causing a decrease in the concentration of As in the surroundings of the concaved p-cladding layer 12. Therefore, the growth of the active layer 13 in the surroundings of the concaved p-cladding layer 12 (i.e., in the plane portion of the p-cladding layer 12) is suppressed. That is, the concaved portions of the active layer 13 corresponding to the sub-striped channels 22a and 23a are formed with a great thickness, resulting in the suppression of the growth of the portion of the active layer 13 corresponding to the main striped channel 21 positioned between the sub-striped channels 22a and 23a. Because of the suppression of the growth of the active layer 13, the thickness of the active layer 13 becomes thin in the area corresponding to the main striped channel 21. In such a manner, the thickness of the active layer 13 can be easily controlled with reproducibility. The thickness of the p-cladding layer 12 is, of course, controlled to be thin enough such that light within the active layer 13 can be absorbed in part at the substrate 10. Figures 2(A) and 2(B) show the effect of the thinness of the active layer 13 on the lowering of the growth rate of the active layer 13 by changes in the distance W between the main striped channel 21 and each of the sub-striped channels 22a and 23a of the semiconductor laser device obtained by the above-mentioned process, wherein the width and the depth of the main striped channel 21 are 3 µm and 1 µm, respectively, and the width and the depth of each of the sub-striped channels 22a and 23a are 8 µm and 1 µm, respectively. The growth of the active layer was carried out at a supersaturation of 4°C for 2 seconds. When the active layer 13 is grown by liquid phase epitaxy, the active layer 13 becomes thinner as the distance W becomes small. When the distance W becomes greater than approximately 70 µm, the effect of the sub-striped channels 22a and 23a on the thinness of the active layer 13 cannot be observed. The p-sided electrode and the n-sided electrode are then formed on the above-mentioned double-heterostructure multi-layered crystal by a common method, and cleaved to form the facets, resulting in a semiconductor laser device unit, which can attain high output power operation two or three times as high as conventional semiconductor devices. Although the sub-striped channel was formed at each of both sides of the main striped channel in the above-mentioned example, it can be formed at one side of the main striped channel or a plurality of the sub-striped channels can be formed at each of both sides thereof. As the semiconductor material, a GaAlInP system and other materials can also be used. Embodiment 2 Figure 3 shows another semiconductor laser device of this invention which has an internal-cavity length of 25ρ µm and which comprises the window regions 1a and 1c having a length of 25 µm from the facets and the stimulated region 1b positioned between the window regions 1a and 1c. The structure of the semiconductor device is the same as that of the semiconductor device in Embodiment 1 except that the distance between the main striped channel 21 and each of the sub-striped channels 22a, 23a, 22c, and 23c in the window regions 1a and 1c is smaller than that between the main striped channel 21 and each of the sub-striped channels 22b and 23b in the stimulated region 1b. The upper surface of the p-cladding layer 12 is flat on the main striped channel 21, but it is downwardly concave on each of the sub-striped channels 22a, 23a, 22b, 23b, 22c, and 23c due to the shape of the sub-striped channels, and thus the active layer 13 grown on the concaved portions of the p-cladding layer 12 corresponding to the sub-striped channels 22a, 23a, 22c, and 23c are formed into a downward concave and the active layer 13 grown on the other portion of the p-cladding layer 12 is formed into a plane. The thickness of the portions of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c is thinner than that of the portion of the active layer 13 above the main striped channel 21 in the stimulated region 1b. The difference in the thickness of the active layer 13 therebetween arises for the following reason: The growth rate of the active layer 13 grown on the concaved portions of the p-cladding layer 12 corresponding to the sub-striped channels 22a, 23a, 22c, and 23c is higher than that of the active layer 13 grown on the plane portion of the p-cladding layer 12, resulting in the diffusion of As from the solution in the surroundings of the concaved p-cladding layer 12 into that in the concaved p-cladding layer 12, as described in Embodiment 1, causing a decrease in the concentration of As in the surroundings of the concaved p-cladding layer 12 (i.e., in the portion of the active layer 13 corresponding to the main striped channel 21). Thus, the growth of the active layer 13 in the surroundings of the concaved p-cladding layer 12 is suppressed. The suppression of the growth of the active layer 13 depends upon the distance between the main striped channel and the sub-striped channel, and accordingly the growth of the active layer 13 in the window regions 1a and 1c becomes slower than that in the stimulated region 1b, resulting in a difference in the thickness of the active layer 13 between each of the window regions 1a and 1c and the stimulated region 1b. In this example, the distance between the main striped channel 21 and each of the sub-striped channels 22a, 23a, 22c, and 23c in the window regions 1a and 1c was 10 µm, the distance between the main striped channel 21 and each of the sub-striped channels 22b and 23b in the stimulated region 1b was 30 µm, so that the thickness of the active layer 13 corresponding to the main striped channel 21 in the window regions 1a and 1c can be thinner than that of the active layer 13 corresponding to the main striped channel 21 in the stimulated region 1b. When current is applied to the above-mentioned semiconductor laser device, since the thickness of the active layer 13 in the window regions 1a and 1c is thinner than that of the active layer 13 in the stimulated region 1b, the injected electrons reach a conduction band at a higher level than that of the stimulated region 1b. Thus, this laser device functions as a window laser in which laser light oscillated in the stimulated regions 1b is not absorbed at the window regions 1a and 1c, thereby attaining high output power operation. Figures 4(A) to 4(D) show a production process of the semiconductor laser device shown in Figure 3. As shown in Figure 4(A), on a Zn-doped p-GaAs substrate (p=1 × 10¹⁹ cm⁻³) 10, striped channels 220a, 230a, 220b, 230b, 220c, and 230c with the same shape as and the same size as or a greater size than the sub-striped channels 22a, 23a, 22b, 23b, 22c, and 23c to be formed in the succeeding step are formed by an etching technique. Then, on the GaAs substrate 10, as shown in Figure 4(B), a Te-doped n-GaAs current blocking layer (n=6 × 10¹⁸ cm⁻³) 11 is grown by liquid phase epitaxy in such a manner that the striped channels 220a, 230a, 220b, 230b, 220c, and 230c are filled with the current blocking layer 11 having the flat surface, the thickness of which is 0.8 µm in the portion corresponding to the main striped channel 21 to be formed in the succeeding step. Then, as shown in Figure 4(C), the sub-striped channels 22a, 23a, 22b, 23b, 22c, and 23c having a width of 8 µm and a depth of 1.0 µm are formed on the current blocking layer 11 corresponding to the striped channels 220a, 230a, 220b, 230b, 220c and 230c, respectively, by an etching technique, and the main striped channel 21 having a width of 3 µm and a depth of 1.0 µm in the center portion between the sub-striped channels 22 (i.e., 22a, 22b and 22c) and 23 (i.e., 23a, 23b and 23c) by an etching technique in such a manner that it reaches the substrate 10, resulting in a current path therein. The sub-striped channels 22 do not constitute a current path since they do not contact the substrate 10 due to the current blocking layer 11. Thus, current flows through only the main striped channel 21, which serves to effectively confine the current therein. Thereafter, as shown in Figure 4(D), on the current blocking layer 11 including the main striped channel and the sub-striped channels, a p-GaAlAs cladding layer 12, a p-GaAlAs active layer 13, an n-GaAlAs cladding layer 14 and an n-GaAs cap layer 15 are successively grown by liquid phase epitaxy, resulting in a double-heterostructure multi-layered crystal for laser oscillation. In this embodiment, the thickness of the active layer 13 corresponding to the main striped channel 21 in the stimulated region 1b was about 0.10 µm, and that of the active layer 13 corresponding to the main striped channel 21 in each of the window regions 1a and 1c was about 0.04 µm, resulting in a window semiconductor laser device which created a high output power of 200 mW at the highest in continuous laser oscillation at room temperatures. In this embodiment, a p-GaAs semiconductor substrate was used, but is not limited thereto. An n-GaAs semiconductor substrate can be used. As the semiconductor material, an InGaAsP system, a GaAlAsSb system, etc., can, of course, be used. Embodiment 3 Figure 5 shows another semiconductor laser device of this invention, in which the internal-cavity length is approximately 250 µm, and the thickness of the active layer 13 for laser oscillation operation in the window regions 1a and 1c each of which is formed with a length of 25 µm from the facet is thinner than that of the active layer 13 for laser oscillation operation in the stimulated region 1b, so that the absorption of laser light at the window regions 1a and 1c can be suppressed. The portions of the active layer 13 corresponding to the sub-striped channels 22a, 23a, 22b, 23b, 22c and 23c positioned outside of the main striped channel 21 are formed into a concave, and the portion of the active layer 13 corresponding to the main striped channel 21 is formed into a plane. The growth rate of the plane portion of the active layer 13 depends upon the cross-sectional size and/or shape of the sub-striped channels 22a, 23a, 22c and 23c; that is, the thickness of the plane portion of the active layer 13 corresponding to the main striped channel 21 becomes thinner as the size of the cross section of the sub-striped channels 22a, 23a, 22c and 23c becomes large. In the semiconductor laser device of this embodiment, the width of the sub-striped channels 22a, 23a, 22c and 23c in the window regions 1a and 1c is greater than that of the sub-striped channels 22b and 23b in the stimulated region 1b, so that the thickness of the active layer 13 corresponding to the main striped channel 21 in the vicinity of the facets becomes thinner than that of the active layer 13 corresponding to the main striped channel inside of the facets. When current is applied to the device, since the thickness of the active layer 13 in the window regions 1a and 1c is thinner than that of the active layer 13 in the stimulated region 1b, the injected electrons reach a conduction band at a higher level than that of the stimulated region 1b. Thus, this laser device functions as a window laser in which laser light oscillated in the stimulated region 1b is not absorbed at the window regions 1a and 1c, thereby attaining high and stable output power operation. Figures 6(A) to 6(D) show a production process of the semiconductor laser device shown in Figure 5. As shown in Figure 6(A), on the portions of a p-GaAs substrate 10 corresponding to the sub-striped channels 22 and 23 to be formed in the succeeding step, a pair of channels 220 and 230 are formed by an etching technique. Then, as shown in Figure 6(B), on the substrate 10, an n-GaAs current blocking layer 11 is grown by liquid phase epitaxy in such a manner that the channels 220 and 230 are filled with the current blocking layer 11, the thickness of which is 0.8 µm at the portion corresponding to the main striped channel 21 to be formed in the succeeding step. Then, as shown in Figure 6(C), a pair of sub-striped channels 22 and 23 are formed corresponding to the pair of channels 220 and 230 by an etching technique in such a manner that the widths of the striped channels 22 and 23 in the window regions 1a and 1c and in the stimulated region 1b are 16 µm and 8 µm, respectively, and the depths thereof are 1 µm. Then, the main striped channel 21 having a width of 3 µm and a depth of 1 µm is formed in the center portion between the sub-striped channels 22 and 23. The main striped channel 21 reaches the p-GaAs substrate 10, resulting in a current path. Current required for laser oscillation flows only through the main striped channel 21. Thereafter, as shown in Figure 6(D), on the current blocking layer 11 including channels 21, 22 and 23, a p-GaAlAs cladding layer 12, a p-GaAlAs active layer 13, an n-GaAlAs cladding layer 14 and an n-GaAs cap layer 15 are successively grown by liquid phase epitaxy, resulting in a double-heterostructure multi-layered crystal for laser oscillation. In the growth process, the portions of the active layer 13 corresponding to the sub-striped channels 22 and 23 are grown with a downward curvature, in which the consumption of the amount of solution is greater than that in the plane portion of the active layer 13 corresponding to the main striped channel 21, so that the thickness of the plane portion of the active layer 13 positioned between the sub-striped channels 22 and 23 varies with the degree of the curvature of the concaved portions of the active layer 13. In order for the active layer 13 above the main striped channel 21 in the window regions 1a and 1c to be thinner than that of the active layer 13 above the main striped channel 21 in the stimulated region 1b, the width of the sub-striped channels 22a, 23a, 22c and 23c in the window regions 1a and 1c is wider than that of these sub-striped channels in the stimulated region 1b. In this example, the thickness of the active layer 13 above the main striped channel 21 in the stimulated region 1b is 0.08 µm, and that of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c is 0.03 µm, resulting in a window laser in which the absorption of laser light at the window regions is suppressed, thereby attaining a high output power of 200 mW at the highest in continuous laser oscillation at room temperatures. In the above-mentioned embodiment, a p-GaAs semiconductor substrate was used, but is not limited thereto. An n-GaAs semiconductor material can be used for the substrate. Other semiconductor materials such as those from an InGaAsP system, an AlGaAsSb system, etc., can also be used therefor. Embodiment 4 Figure 7 shows another semiconductor laser device of this invention, which is produced in the same manner as in Embodiment 3 except that a striped structure is formed in the cap layer 34. This device is produced as follows: The striped channels 21, 22a, 23a, 22b, 23b, 22c and 23c are formed on the p-substrate 10 in the same manner as in Embodiment 3, and then a p-cladding layer 31, a p-active layer 32, an n-cladding layer 33 and a p-cap layer (or a high ohmic contact) 34 are formed by liquid phase epitaxy, followed by diffusing n-impurities into the region of the p-cap layer 34 corresponding to the main striped channel to form an n-current path 35. The thickness of the active layer 32 above the main striped channel 21 in the window regions 1a and 1c is thinner than that of the active layer 32 above the main striped channel 21 in the stimulated region 1b. The main striped channel 21 of this example allows for the formation of an index guided waveguide in the active layer 32. Embodiment 5 Figure 8 shows another semiconductor laser device of this invention which has a VSIS (V-channeled substrate inner stripe) structure. Its internal-cavity length is 250 µm, and the window regions 1a and 1c, respectively are formed with a length of 25 µ m from the facets. The thickness of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c is thinner than that of the active layer 13 above the main striped channel 21 in the stimulated region 1b inside of the facets. The difference in the thickness of the active layer 13 between each of the window regions 1a and 1c and the stimulated region 1b can be achieved by the formation of the sub-striped channels 22a, 23a, 22c and 23c outside of the main striped channel 21 in the window regions 1a and 1c. For the same reason, as described in Embodiments 1 and 2, the growth rate of the active layer 13 can be controlled by these sub-striped channels. When current is applied to the above-mentioned semiconductor laser device, since the thickness of the active layer 13 above the main striped channel 21, which contributes to light emission, in the window regions 1a and 1c is thinner than that of the active layer 13 above the main striped channel 21 in the stimulated region 1b, the injected electrons reach a conduction band at a higher level than that of the stimulated region 1b. Thus, this laser device functions as a window laser in which laser light oscillated in the stimulated region 1b is not absorbed at the window regions 1a and 1c, thereby suppressing deterioration of the facets due to laser light and attaining high output power operation. Figures 9(A) to 9(D) show a production process of the semiconductor laser device shown in Figure 8. As shown in Figure 9(A), on the end portions of a Zn-doped p-GaAs substrate (p=1 × 10¹⁹ cm⁻³) 10 corresponding to the sub-striped channels 22a, 23a, 22c and 23c to be formed in the succeeding step, striped channels 220a, 230a, 220c and 230c which are slightly larger than the sub-striped channels 22a, 23a, 22c and 23c are formed by an etching technique. Then, as shown in Figure 9(B), on the substrate 10, a Te-doped n-GaAs current blocking layer (n=6 × 10¹⁸ cm⁻³) 11 is grown by liquid phase epitaxy in such a manner that the striped channels 220a, 230a, 220c and 230c are filled with the current blocking layer 11, the thickness of which is 0.8 µm at the portions corresponding to the main striped channel 21 to be formed in the succeeding step. Then, as shown in Figure 9(C), the sub-striped channels 22a, 23a, 22c and 23c having a width of 8 µm and a depth of 1 µm are formed corresponding to the striped channels 220a, 230a, 220c and 230c of the substrate 10 by an etching technique. The main striped channel 21 having a width of 3 µm and a depth of 1 µm is then formed from one end of the substrate 10 to the other end thereof in the center portion between the sub-striped channels 22 (i.e., 22a and 22c) and 23 (i.e., 23a and 23c) by an etching technique. The main striped channel 21 reaches the p-GaAs substrate 10, resulting in a striped current path therein, but current does not pass through sub-striped channels 22a, 23a, 22c and 23c due to the current blocking layer 11. Thereafter, as shown in Figure 9(D), on the current blocking layer 11 including the striped channels 21, 22 and 23, a p-GaAlAs cladding layer 12, a p-GaAlAs active layer 13, an n-GaAlAs cladding layer 14 and an n-GaAs cap layer 15 are successively grown by liquid phase epitaxy, resulting in a double-heterostructure multi-layered crystal for laser oscillation. In the growth process, the p-cladding layer 12 is, of course, formed into a concave corresponding to the sub-striped channels 22a, 23a, 22c and 23c and into a plane corresponding to the main striped channel 21, such that the active layer 13 can be formed with a downward curvature on the portions of the p-cladding layer 12 corresponding to the sub-striped channels 22a, 23a, 22c and 23c. Thus, for the afore-mentioned reasons, the thickness (e.g., 0.04 µm) of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c becomes thinner than that (e.g., 0.1 µm) of the active layer 13 above the main striped channel 21 in the stimulated region 1b. Since current is confined within the main striped channel 21, the region of the active layer 13 corresponding to the main striped channel 21 constitutes an index guided laser oscillation area. The semiconductor laser device obtained in this example created a high output power of 200 mW or more in continuous laser oscillation at room temperatures. In the above-mentioned example, the growth of a multi-layered crystal on the p-GaAs substrate was described, but is not limited thereto. The growth thereof on a semiconductor substrate having a different polarity can also be achieved. Other semiconductor materials such as those from a GaAsInP system, a GaAlAsSb system, etc., can be used. Embodiment 6 Figure 10 shows another semiconductor laser device of this invention, which is produced in the same manner as in Embodiment 5 except that a striped structure is formed in the cap layer 34. This device is produced as follows: The main striped channel 21 and the substriped channels 22a, 23a, 22c and 23c are formed on a p-GaAs substrate 30 in the same manner as in Example 5, and then on the substrate 30, a p-GaAlAs cladding layer 31, a p-GaAlAs active layer 32, an n-GaAlAs cladding layer 33 and a p-GaAs cap layer 34 are successively formed by liquid phase epitaxy, resulting in a multi-layered crystal structure for laser oscillation. Then, n-impurities are diffused into the region of the cap layer 34 corresponding to the main striped channel 21 to form a current path of a striped n-diffusion layer 35. The main striped channel 21 gives rise to the distribution of light intensities based on the exudation of light within the active layer 32, resulting in an index guided structure. Laser oscillation is achieved in the region of the active layer 32 positioned between the main striped channel 21 and n-diffusion layer 35. Embodiment 7 Figure 11 shows another semiconductor laser device of this invention having an internal-cavity length of 250 µm, which comprises the window regions 1a and 1c with a length of 25 µm from the facets and the stimulated region 1b positioned between the window regions 1a and 1c. The thickness of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c is thinner than that of the active layer 13 above the main striped channel 21 in the stimulated region 1b. The difference in the thickness of the active layer 13 between each of the window regions 1a and 1c and the stimulated region 1b results from the difference in the depth between the sub-striped channels 22a and 22b, 22b and 22c, 23a and 23b, and 23b and 23c. The depth of the sub-striped channels 22b and 23b in the stimulated region 1b is 1.5 µm, while that of the sub-striped channels 22a, 23a, 22c and 23c in the window regions 1a and 1c is approximately 2.5 µm. For the same reasons as described in Examples 1 and 2, As in the solution in the deeply concaved cladding layer is more quickly consumed for the growth of the active layer than that in the slightly concaved cladding layer, resulting in the suppression of the growth of the active layer in the surroundings of the deeply concaved cladding layer. Thus, the thickness of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c becomes thinner than that of the active layer 13 above the main striped channel 21 in the stimulated region 1b. When current is applied to the above-mentioned semiconductor laser device, since the thickness of the active layer 13 of the laser oscillation operation area in the window regions 1a and 1c is thinner than that of the active layer 13 of the laser oscillation operation area in the stimulated region 1b, the injected electrons reach a conduction band at a higher level than that of the stimulated region 1b. Thus, this laser device functions as a window laser in which laser light oscillated in stimulated region 1b is not absorbed at the window regions 1a and 1c, thereby attaining high output power operation. Figures 12(A) to 12(D) show a production process of the semiconductor laser device shown in Figure 11. As shown in Figure 12(A), on the portions of a p-GaAs substrate 10 corresponding to the sub-striped channels 22a, 23a, 22b, 23b, 22c and 23c to be formed in the succeeding step, striped channels 220 and 230 which are wider and deeper than the sub-striped channels 22a, 23a, 22c and 23c are formed into a rectangle with a width of 10 µm and a depth of 3 µm by an etching technique. Then, as shown in Figure 12(B), on the substrate 10, an n-GaAs current blocking layer 11 is grown by liquid phase epitaxy in such a manner that the striped channels 220 and 230 are filled with the current blocking layer 11, the thickness of which is 0.8 µm at the portion corresponding to the main striped channel 21 to be formed in the succeeding step. Then, as shown in Figure 12(C), using an etching technique, the sub-striped channels 22a, 23a, 22c and 23c having a width of 8 µm and a depth of 2.5 µm are formed in the window regions 1a and 1c and the sub-striped channels 22b and 23b having a width of 8 µm and a depth of 1.5 µm are formed in the stimulated region 1b. Then, the main striped channel 21 having a width of 3 µm and a depth of 1 µm is formed in the center portion between the sub-striped channels 22 (i.e., 22a, 22b and 22c) and 23 (i.e., 23a, 23b and 23c) by an etching technique. The main striped channel 21 reaches the GaAs substrate 10, resulting in a current path, while current does not pass through the sub-striped channels 22 and 23 due to the current blocking layer 11. Thus, current flows only through the main striped channel 21, so that occurrence of non-productive current which does not contribute to light emission can be suppressed. Thereafter, as shown in Figure 12(D), on the current blocking layer 11 including the channels 21, 22 and 23, a p-GaAlAs cladding layer 12, a p-GaAlAs active layer 13, an n-GaAlAs cladding layer 14 and an n-cap layer 15 are successively grown by liquid phase epitaxy, resulting in a double-heterostructure multi-layered crystal for laser oscillation. In the growth process, the p-cladding layer 12 is, of course, formed into a concave corresponding to the sub-striped channels 22a, 23a, 22b, 23b, 22c and 23c which are wider and deeper than the main striped channel 21 and into a plane corresponding to the main striped channel 21, such that the active layer 13 can be formed with a downward curvature on the concaved portions of the p-cladding layer 12 above the sub-striped channels 22a, 23a, 22b, 23b, 22c and 23c. Thus, for the afore-mentioned reasons, the thickness of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c becomes thinner than that of the active layer 13 above the main striped channel 21 in the stimulated region 1b, depending upon the degree of the curvature of the active layer 13 above the sub-striped channels 22a, 23a, 22b, 23b, 22c and 23c. In this embodiment, the thickness of the active layer 13 above the main striped channel 21 in the stimulated region 1b was 0.08 µm and that of the active layer 13 above the main striped channel 21 in the window regions 1a and 1c was 0.03 µm. The resulting device attained a high output power of 200 mW or more in continuous laser oscillation at room temperatures. In the above-mentioned embodiment, the growth of crystals on the p-GaAs semiconductor substrate was carried out, but is not limited thereto. A device structure having an opposite polarity can be applied to this invention. Other semiconductor materials such as those from an InGaAsP system, an AlGaAsSb system, etc., can also be used. Embodiment 8 Figure 13 shows another semiconductor laser device of this invention, which is produced in the same manner as in Embodiment 7 except that a striped structure is formed in the cap layer 34. This device is produced as follows: The main striped channel 21 and the sub-striped channels 22a, 23a, 22b, 23b, 22c and 23c are formed on an n-GaAs substrate 30 in the same manner as in Embodiment 7, and then on the substrate 30, an n-GaAlAs cladding layer 31, a non-doped GaAlAs active layer 32, a p-GaAlAs cladding layer 33 and an n-GaAs cap layer 34 are successively formed by liquid phase epitaxy, resulting in a multi-layered crystal structure for laser oscillation. Then, Zn is diffused into the region of the cap layer 34 corresponding to the main striped channel 21 to form a current path of a striped p-diffusion region 35 in which current is confined. Thus, laser oscillation is achieved in the region of the active layer 32 positioned between the p-diffusion region 35 and the main striped channel 21. The thickness of the active layer 32 of the laser oscillation area in the window regions 1a and 1c is thinner than that of the active layer 32 of the laser oscillation area in the stimulated region 1b, resulting in a window laser in which the absorption of laser light is suppressed.

Claims: A semiconductor laser device comprising a substrate (10) having a main striped channel (21) for confining current therein and sub-striped channels (22a, 23a) formed in a parallel manner outside of said main striped channel (21), the width of said sub-striped channels being greater than that of said main striped channel, and an active layer (13) for laser oscillation, the portions of said active layer (13) which correspond to said sub-striped channels (22a, 23a) being formed into a concave, characterised in that the portion of said active layer (13) positioned between the concaved portions of the active layer has a thickness which is less than that of the concaved portions of the active layer (13) for the purpose of controlling the thickness of the active layer of the semiconductor laser. A semiconductor laser device as claimed in Claim 1, wherein the thickness of the portions of the active layer (13) corresponding to the main striped channel (21) in the vicinity of the facets is thinner than that of the portion of said active layer corresponding to said main striped channel inside of the facets. A semiconductor laser device as claimed in Claim 2, wherein the sub-striped channels are formed in the direction of laser oscillation along the whole length of the main striped channel, the distance between the main striped channel and each of the sub-striped channels in the vicinity of the facets being smaller than that between the main striped channel and each of the sub-striped channels inside of the facets. A semiconductor laser device as claimed in Claim 2, wherein the sub-striped channels are formed in the direction of laser oscillation along the main striped channel only in the vicinity of the facets. A semiconductor laser device as claimed in Claim 2, 3 or 4, wherein the width of each of the sub-striped channels is wider in the vicinity of the facets than that of each of the sub-striped channels inside of the facets. A semiconductor laser device as claimed in Claim 2, 3, 4 or 5, wherein the depth of each of the sub-striped channels is deeper in the vicinity of the facets than that of each of the sub-striped channels inside of the facets. A semiconductor laser device as claimed in any one of Claims 2 to 6, wherein the portion of the active layer in the vicinity of each of the facets is a window region functioning as an optical waveguide.

Inventors: HAYASHI, HIROSHI, MORIMOTO, TAIJI, TANEYA, MOTOTAKA, YAMAMOTO, SABURO
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1986
Main classification: H01S3
IPC codes: H01S5