A SEMICONDUCTOR LASER

Abstract: A semiconductor laser for laser oscillation at long wavelength or short wavelength visible light comprises a substrate (4) for crystal growth having a striped channel (2), an active layer (7) for laser oscillation, and a cladding layer (6) containing Mg which is in contact with the side of said active layer (7) which faces the striped channel (2) in the substrate (4).

Description: This invention relates to the production of a semiconductor laser by a liquid phase epitaxial growth method. Semiconductor lasers for laser oscillation in the visible spectral range are known. For instance, there is disclosed in IEEE Journal of Quantum Electronics, Vol. QE-19, No.6, June 1983, pp 1009-1015, an AlGaAs double heterojunction laser having a V-channelled substrate inner stripe and an active layer sandwiched between a p-cladding layer and an n-cladding layer, the p-cladding layer being doped with magnesium. In the preparation of the inner stripe geometric structure of a semiconductor laser for laser oscillation at a long wavelength of 750 nm or more or of other semiconductor devices, a crystalline layer is grown on a V- or U-shaped channel substrate by an epitaxial growth technique, wherein the channel in the substrate must not be deformed during the epitaxial growth of the layer. When molecular beam epitaxy (MBE) or metal organic-vapour phase epitaxy (MO-VPE) is used as the epitaxial growth technique, the original shape of the channel is maintained. However, since the crystalline layer is formed in accordance with the shape of the channel in the substrate, the surface of the resulting growth layer cannot be a plane. On the other hand, as is shown in Figures 1 and 2 of the accompanying drawings, on the substrate 1 which is etched, in advance, to form the V-shaped channel 2, a crystalline layer 3 is grown using liquid phase epitaxy (LPE) as the epitaxial growth technique to completely fill the channel 2, creating the formation shown in Figure 2, resulting in a growth layer having a plane surface. However, each of the shoulder portions at both sides in the channel 2 undergoes meltback resulting in a rounded shape, so that the width W (shown in Figure 1) of the channel 2 expands to W' (shown in Figure 2). This is because the growth rate of the crystalline layer growing initially in the bottom of the channel 2 is so high that there is an insufficient amount of solute in the shoulder portions of the channel 2, so that meltback of the shoulder portions of the channel 2 occurs. The expansion of the width of the channel 2 from W to W' is undesirable Moreover, impurities from the substrate 1 are mixed into the growth layer 3 resulting in inferior characteristics of the device. According to the present invention, there is provided a method of producing a semiconductor laser having a heterostructure formed by III-V compound semiconductors, comprising the steps of: forming a striped channel in a substrate; growing a cladding layer on the substrate by liquid phase epitaxy using a crystal growth solution containing Mg in a concentration of at least 0.3 atomic percent; and forming an active layer for laser oscillation on said cladding layer, said active layer and said cladding layer constituting the heterostructure. Preferably the active layer and cladding layer are formed so as to be substantially planar. In one embodiment of the invention, the method further comprises forming a current blocking layer on said substrate prior to forming the striped channel which extends through the current blocking layer. Thus, the invention described herein makes possible the object of providing a novel and useful semiconductor laser for laser oscillation at a long wavelength of visible light, wherein the channel in the substrate is not deformed (i.e. the channel does not undergo meltback) during the epitaxial growth of the crystalline layer to fill the channel. 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: FIGURE 1 is a front sectional view of a substrate having a V-shaped channel with a width W; FIGURE 2 is a front sectional view showing an epitaxial growth layer, which was formed using a conventional liquid phase epitaxy method, on the substrate in Figure 1; FIGURE 3 is a front sectional view showing an epitaxial growth layer on the substrate according to this invention; and FIGURE 4 is a front sectional view of a semiconductor laser for laser oscillation at a long wavelength of visible light according to this invention. The semiconductor laser for laser oscillation at a long wavelength of visible light of this invention can be constructed based on the experimental results that deformation of the channel in the substrate is minimal and the original with W thereof as shown in Figure 3 is maintained when a crystalline layer is formed on the substrate by an epitaxial growth technique using a crystal growth solution containing Mg of 0.3 atomic percent concentration or more. Mg attains the reduction of the diffusion coefficient of the solute in the crystal growth solution, thereby decreasing the initial growth rate of the crystalline layer in the bottom of the channel. Thus, the vicinity of the shoulder of the channel is not short of solute, and accordingly the substrate does not undergo meltback, so that the width W of the channel is maintained and contamination by impurities from the substrate 1 to the growth layer 3 can be prevented, resulting in a device having excellent characteristics. Since Mg is a p-impurity, Mg alone is added to the crystal growth solution when the growth layer 3 is of a p-type (positive conducting type), while Mg is added together with n-impurities such as Te, etc., when the growth layer 3 is used as an n-type (negative conducting type). The following non-limitative Example further illustrates the invention. EXAMPLE A semiconductor device having high quality crystal layers such as a semiconductor laser for laser oscillation at a long wavelength of visible light is produced, for example, using a liquid phase epitaxial growth technique of a sliding type, wherein a graphite boat including a crystal growth solution therein is moved on a substrate holder to coat a substrate fixed to the holder with the crystal growth solution to form a growth layer. According to this technique, multiple-layered crystal can be grown on a substrate for crystal growth. Figure 4 shows a semiconductor laser for laser oscillation at a long wavelength of this invention using the above-mentioned technique, which was produced as follows: On the (100) face of a Zn-doped p-GaAs substrate (p=1 x 10¹⁹ cm⁻³)4 a Te-doped n-GaAs current blocking layer (n=3 x 10¹⁸ cm⁻³)5 was grown with a thickness of 0.6 µm, followed by etching to form a striped channel 2 having a width W in the (110) direction as an electroconductive region. This channel 2 penetrates the substrate 4. On the substrate 4 having the channel 2, a Mg-doped p-Ga 0.5 Al 0.5 As cladding layer (p=1 x 10¹⁸ to 2 x 10¹⁸ cm⁻³)6, a p-Ga 0.85 Al 0.15 As active layer 7, an n-Ga 0.5 Al 0.5 As cladding layer 8 and an N-GaAs cap layer 9 were successively grown to form a multiple-layered crystal for laser oscillation, resulting in a double heterostructure. An n-electrode and a p-electrode were connected to the cap layer 9 and the substrate 4, respectively. As a reference standard, a p-cladding layer containing Mg in a concentration of 0.1 atomic percent was formed in the same manner as the above-mentioned. The width of the channel in the resulting semiconductor laser was greater than the original, as shown in Figure 2. The reference standard laser oscillated at a wavelength of 780 nm and exhibited a far-field pattern which is at an angle ranging from 9.5 o to 10.5 o from the active layer. The width of the channel in the semiconductor laser of this invention obtained by the above-mentioned process was maintained at the original width W as shown in Figure 4. Each of the semiconductor laser units according to this invention exhibited uniform characteristics, especially uniform threshold current and a uniform lateral far-field pattern which is parallel to the junction, although a conventional semiconductor laser exhibited a non-uniform threshold current and a non-uniform lateral far-field pattern parallel to the junction. Moreover, in the semiconductor laser of this invention, contamination by impurities from the n-GaAs current blocking layer 5 to the p-cladding layer 6 is prevented resulting in improved reliability of the laser. As mentioned above, the p-cladding layer 6, which is formed on the current blocking layer 5 and the striped channel 2 by an epitaxial growth technique, fills the channel 2 during the growth thereof to thereby form a plane surface thereof. Therefore, the active layer 7 growing on the plane surface of the cladding layer 6 becomes flat and thin. Moreover, since the p-cladding layer 6 is grown using a Mg-doped GaAlAs solution (containing Mg of 0.3 atomic percent concentration or more) under the control of the initial growth rate by Mg in the bottom of the channel 2, the solute concentration in the GaAlAs solution is always maintained at a certain level during the epitaxial growth thereby preventing meltback in the shoulder portions of the channel 2 and contamination by components from the substrate 4 to the p-cladding layer 6. The Mg component which was doped to the p-cladding layer 6 is used as a p-impurity and the doping amount of Mg is selected to be 0.3 atomic percent concentration or more depending upon the desired carrier concentration. The crystal growth method in the Example is not limited to the GaAs-GaAlAs system, but it can be applied to liquid phase epitaxy for other semiconductor devices of an InP-InGaAsP system etc.

Claims: A method of producing a semiconductor laser having a heterostructure formed by III-V compound semiconductors, comprising the steps of: forming a striped channel in a substrate; growing a cladding layer on the substrate by liquid phase epitaxy using a crystal growth solution containing Mg in a concentration of at least 0.3 atomic percent; and forming an active layer for laser oscillation on said cladding layer, said active layer and said cladding layer constituting the heterostructure. A method according to Claim 1, wherein said active layer and said cladding layer are formed so as to be substantially planar. A method according to Claim 1 or 2 and further comprising forming a current blocking layer on said substrate prior to forming the striped channel which extends through the current blocking layer.

Inventors: HAYASHI, HIROSHI, MORIMOTO, TAIJI, YAMAMOTO, SABURO, YANO, SEIKI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1985
Main classification: H01S3
IPC codes: H01S5