Abstract:
A semiconductor laser device contains a pair of electrodes; a conductive substrate connected to one of the pair of electrodes; a lower cladding layer formed on the conductive substrate; a lower optical waveguide layer formed on the lower cladding layer; a quantum well active layer formed on the lower optical waveguide layer; an upper optical waveguide layer formed on the quantum well active layer; an upper cladding layer formed on the upper optical waveguide layer; and a contact layer formed on the upper cladding layer. The other of the pair of electrodes is formed on the contact layer, and the conductive substrate is made of InGa material. The lower cladding layer is made of one of InGaN and InGaAlN material and has a composition which causes a strain not less than −0.01 and not greater than 0.01 between the lower cladding layer and the conductive substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device having a layered structure which is lattice-matched with a substrate. The semiconductor laser devices referred to in this specification may include semiconductor laser diodes, semiconductor optical amplifying devices, and any other semiconductor laser devices. 
     2. Description of the Related Art 
     Currently, short-wavelength semiconductor laser devices using GaN materials and II-VI group materials are being studied. For example, Japanese Journal of Applied Physics, Vol. 37 (1998) pp.L309-L312 reports a semiconductor laser device which is constructed by forming, on a GaN substrate, an n-type GaN buffer layer, an n-type InGaN crack preventing layer, an n-AlGaN/GaN modulation-doped superlattice cladding layer, an n-type GaN optical waveguide layer, an n-InGaN/InGaN multiple quantum well active layer, a p-type AlGaN carrier block layer, a p-type GaN optical waveguide layer, an p-AlGaN/GaN modulation-doped superlattice cladding layer, and a p-type GaN contact layer, where the GaN substrate is formed by selective growth using a mask of an SiO 2  film on a GaN layer formed on a sapphire substrate. Oscillation of the 410 nm band is realized by the above semiconductor laser device. 
     However, there is a great degree of lattice mismatching (i.e., a great strain) between the GaN substrate and the active layer in the above construction of layers. Therefore, it is impossible to increase the indium content in the active layer, and thus conventional semiconductor laser devices which operate at wavelengths equal to or longer than 450 nm are not reliable. 
     The strain between the grown layer and the substrate is defined as (a−a s )/a s , where a s  denotes a lattice constant of the substrate, and a denotes a lattice constant of the grown layer. Generally, the “lattice matching” is defined by the condition that the strain is not less than −0.01 and not greater than 0.01. 
     As described above, in order to obtain reliable semiconductor laser devices which oscillate at wavelengths equal to or longer than 450 nm, it is necessary to increase the indium content in the active layer. However, conventionally, it is difficult to realize such semiconductor laser devices due to the increase in the strain because the great strain between the grown layers and the substrate is liable to cause cracking or dislocation. Therefore, reliability of the conventional semiconductor laser devices is decreased in high power operations. 
     In addition, in semiconductor laser devices, it is desirable to thicken cladding layers to reduce overflow currents and optical losses. However, conventionally, it is also impossible to realize thick cladding layers due to the above great strain, which is liable to cause cracking. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a short-wavelength semiconductor laser device which is reliable in high power operations. 
     The object of the present invention is accomplished by the present invention, which provides a semiconductor laser device which contains a conductive substrate connected to one of a pair of electrodes, a lower cladding layer, a lower optical waveguide layer, a single or multiple quantum well active layer, an upper optical waveguide layer, an upper cladding layer, a contact layer, and the other of the pair of electrodes, which are stacked in this order, wherein the conductive substrate is made of InGa material, and the lower cladding layer has a composition which causes a strain which is not less than −0.01 and not greater than 0.01 between the lower cladding layer and the conductive substrate, and is made of one of InGaN and InGaAlN material. 
     The above conductive substrate may be formed on a first substrate which is formed on a second substrate by selective growth, where the first substrate may be made of InGa material. 
     Since the InGaN substrate is used, instead of the conventional GaN substrate, the range of compositions realizing lattice matching is extended. Therefore, it becomes possible to use InGaAlN material for the cladding layer so that the cladding layer lattice-matches the InGaN substrate. Hence, generation of cracking or dislocation can be prevented, and the indium content in the active layer can be increased. Thus, high-power long-wavelength oscillation up to 550 nm can be realized. 
     In addition, since generation of cracking is prevented by the lattice-matching between the cladding layers and the substrate, it is possible to realize cladding layers having thickness equal to or more than one micrometer, which is sufficient to reduce the amounts of overflow currents and optical losses in the optical waveguide. Thus, reliability is increased. 
     In the above semiconductor laser device according to the present invention, the contact layer may be made of InGaN material. In this case, the contact resistance can be reduced, and therefore temperature increase in high power operations can be reduced. Therefore, reliability of the semiconductor laser device in high power operations is increased. 
     Since, according to the present invention, all or almost all of the layers above the InGaN substrate of the semiconductor laser device include indium, the number of changes in growth temperature during the formation of the layers can be reduced. Therefore, it is possible to reduce growth interruption time, which is necessary for raising or lowering the growth temperature, and it is also possible to reduce probability of defect generation during the growth interruptions. 
     In the above semiconductor laser device according to the present invention, the cladding layers may have a superlattice structure, and compositions of the cladding layers may be such that the strain in the cladding layer is not less than −0.01 and not greater than 0.01. 
     In addition, the above cladding layers may have a modulation-doped superlattice structure in which impurity is doped into barrier layers in the superlattice structure, and compositions of the cladding layers may be such that the strain in the cladding layer is not less than −0.01 and not greater than 0.01 . Further, the cladding layers may have a superlattice structure in which impurity is doped into both of the well layers and the barrier layers in the superlattice structure. 
     Further, preferably, the first substrate may be one of sapphire, SiC, ZnO, LiGaO 2 , LiAlO 2 , ZnSe, GaAs, GaP, Ge, and Si. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the short-wavelength semiconductor laser device as the first embodiment of the present invention at a first intermediate production stage. 
     FIG. 2 is a cross-sectional view of the short-wavelength semiconductor laser device as the first embodiment of the present invention at a second intermediate production stage. 
     FIG. 3 is a cross-sectional view of a complete construction of the short-wavelength semiconductor laser device as the first embodiment of the present invention. 
     FIG. 4 is a plan view of the short-wavelength semiconductor laser device as the second embodiment of the present invention. 
     FIG. 5 is a cross-sectional view of the short-wavelength semiconductor laser device as the second embodiment of the present invention at an intermediate production stage. 
     FIG. 6 is a cross-sectional view of a complete construction of the short-wavelength semiconductor laser device as the second embodiment of the present invention. 
     FIG. 7 is a plan view of the short-wavelength optical amplifying device as the third embodiment of the present invention. 
     FIG. 8 is a cross-sectional view of the short-wavelength optical amplifying device as the third embodiment of the present invention at a first intermediate production stage. 
     FIG. 9 is a IX—IX cross-sectional view of a complete construction of the short-wavelength optical amplifying device as the third embodiment of the present invention. 
     FIG. 10 is a plan view of an optical amplifying device as the fourth embodiment of the present invention. 
     FIG. 11 is a cross-sectional view of the optical amplifying device as the fourth embodiment of the present invention taken along line XI—XI in FIG.  10 . 
     FIG. 12 is a cross-sectional view of the optical amplifying device as the fourth embodiment of the present invention taken along line XII—XII in FIG.  10 . 
     FIG. 13 is a diagram illustrating an exemplary construction using the optical amplifying device, where the optical amplifying device is connected with a master light source realized by a single-mode semiconductor laser device which has a feedback function using a diffraction grating. 
     FIG. 14 is a diagram illustrating an exemplary construction using the optical amplifying device, where the optical amplifying device is connected with a master light source realized by a single-mode semiconductor laser device which is controlled by using a wavelength filter. 
     FIG. 15 is a diagram illustrating an exemplary construction using the optical amplifying device, where the optical amplifying device is connected with a master light source realized by a solid-state laser device which uses a second harmonic generating element using semiconductor laser excitation of infrared light. 
     FIG. 16 is a plan view of the short-wavelength light emitting device as the fifth embodiment of the present invention. 
     FIG. 17 is a cross-sectional view of the short-wavelength light emitting device as the fifth embodiment of the present invention taken along line XVII—XVII in FIG.  16 . 
     FIG. 18 is a cross-sectional view of the semiconductor laser device as the sixth embodiment of the present invention at a first intermediate production stage. 
     FIG. 19 is a cross-sectional view of the semiconductor laser device as the sixth embodiment of the present invention at a second intermediate stage at a second intermediate production stage. 
     FIG. 20 is a cross-sectional view of a complete construction of the semiconductor laser device as the sixth embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention are explained in detail below with reference to drawings. 
     First Embodiment 
     FIGS. 1 to  3  are cross-sectional views of a short-wavelength semiconductor laser device as the first embodiment of the present invention, where FIGS. 1 and 2 are cross-sectional views at first and second intermediate production stages, respectively, and FIG. 3 is a cross-sectional view of a complete construction of the short-wavelength semiconductor laser device as the first embodiment of the present invention. 
     As illustrated in FIG. 1, a GaN buffer layer  2  having a thickness of about 20 nm is formed at a temperature of 500° C. on a (0001) C face of a sapphire substrate  1  by using organometallic vapor phase epitaxy. Then, a GaN layer  3  having a thickness of about 2 micrometers is formed at a temperature of 1,050° C. on the GaN buffer layer  2 . Materials used in the growth of the layers are trimethyl gallium (TMG), trimethyl indium (TMI), trimethyl aluminum (TMA) and ammonia. In addition, silane gas is used as an n-type dopant gas, and cycropentadienyl magnesium (CP2Mg) is used as a p-type dopant gas. 
     On the above GaN layer  3 , an SiO2 layer  4  having a thickness of 3 micrometers is formed. Next, the SiO2 layer  4  in stripe areas, which are spaced out and oriented in the [1{overscore (1)}00] direction, are removed by using conventional lithography so that remaining stripe areas spaced at intervals of about 10 micrometers form a line-and-space pattern. Thus, a base  4 ′ for selective growth is formed. Then, an In x1 Ga 1−x1 N layer  5  having a thickness of about 20 micrometers is formed at a temperature of 750° C. by selective growth. In the operation of forming the In x1 Ga 1−x1 N layer  5 , finally, the surface of the In x1 Ga 1−x1 N layer  5  is planarized by growth in the lateral directions. Further, a silicon-doped In x1 Ga 1−x1 N layer  6  of a thickness of about 5 micrometers is formed at the same temperature. 
     Subsequently, a Si-doped In x2 (Al z2 Ga 1−z2 ) 1−x2 N (2.5 nm)/In x3 (Al z3 Ga 1−z3 ) 1−x3 N (2.5 nm) superlattice cladding layer  7  having 240 layers, a silicon-doped In x3 (Al z3 Ga 1−z3 ) 1−x3 N optical waveguide layer  8 , an In x3 Ga 1−x3 N/In x4 Ga 1−x4 N (2.5 nm) multiple quantum well active layer  9 , a magnesium-doped In x3 (Al z3 Ga 1−z3 ) 1−x3 N (2.5 nm) optical waveguide layer  10 , a Mg-doped In x2 (Al z2 Ga 1−2 ) 1−x2 N/In z3 (Al z3 Ga 1−z3)   1−x3 N (2.5 nm) superlattice cladding layer  11 , and a magnesium-doped In x1 Ga 1−x1 N contact layer  12  are formed in this order. In order to activate the p-type magnesium impurity, heat treatment may be performed in a nitrogen atmosphere after the above growth. Alternatively, the growing operation may be performed in a nitrogen-rich atmosphere. The compositions in the above layers are determined within the ranges of: 0.05&lt;x1≦0.1, 0&lt;x2&lt;x1&lt;x3&lt;x4≦0.4, 0≦z2≦1, and 0≦z3≦1. 
     In addition, a SiO 2  layer  13  is formed on the above contact layer  12 . Thereafter, the SiO 2  layer  13 , except for a stripe area thereof having a width of 4 micrometers, is removed by conventional lithography, and then selective etching is performed by using a reactive ion etching (RIE) apparatus until a part of the thickness of the superlattice cladding layer  11  is etched. Thus, ridges  14  are formed as illustrated in FIG.  2 . The remaining thickness of the superlattice cladding layer  11  is such that the basic transverse mode oscillation is realized. Then, the remaining portion of the SiO 2  layer  13  is removed. Thereafter, a SiO 2  film  15  is formed, then the SiO 2  film  15 , except for a stripe area thereof having a width of 5 micrometers, is removed by conventional lithography. Next, etching is performed by using reactive ion etching (RIE) until the Si-doped In x1 Ga 1−x1 N layer  6  is exposed, and then the SiO 2  film  15  is removed. Further, an insulation layer  16 , an n electrode  17  made of Ti/Au, and a p electrode  18  made of Ni/Au are formed by conventional lithography as illustrated in FIG. 3, where the p electrode  18  is formed on the surface of the p contact layer  12 , and has a stripe shape. Thereafter, end surfaces of the resonant cavity are formed by polishing the substrate and cleaving the layered materials, and high-reflection coating and low-reflection coating are laid on the end surfaces of the resonant cavity, respectively. Then, the construction of FIG. 3 is formed into a chip. 
     The wavelength band of oscillation in the short-wavelength semiconductor laser device formed as described above can be controlled in the range of 400&lt;λ&lt;550 (nm) by varying the indium content in the In x4 Ga 1−x4 N active layers. 
     Although impurity is doped only in the barrier layer in the supperlattice cladding layer in the above-described embodiment, it may be doped into both the well layer and the barrier layer of the superlattice structure. 
     It is possible to change the types of conductivity (i.e., alternate the n-types and the p-types) in the above construction of the first embodiment. 
     Second Embodiment 
     Next, the second embodiment of the present invention will be explained. FIG. 4 is a plan view of a short-wavelength semiconductor laser device as the second embodiment of the present invention, FIG. 5 is a cross-sectional view of an intermediate construction during the formation of the short-wavelength semiconductor laser device of FIG. 4, and FIG. 6 is a cross-sectional view of a completed construction of the short-wavelength semiconductor laser device of FIG.  4 . 
     In the second embodiment, first, the same layers as the layers  1  to  13  in the construction of FIG. 1 are formed in the same manner as explained for the first embodiment. Then, the SiO 2  film  13 , except for a stripe area thereof having a width of 20 to 200 micrometers, is removed by conventional lithography. Next, etching is performed by using reactive ion etching (RIE) until the Si-doped In x1 Ga 1−x1 N layer  6  is exposed as illustrated in FIG. 5, and then the remaining SiO 2  film  13  is removed. Further, an insulation layer  16 - 1 , an n electrode  17 - 1  made of Ti/Au, and a p electrode  18 - 1  made of Ni/Au are formed by conventional lithography as illustrated in FIG. 6, where the p electrode  18 - 1  is formed on the surface of the p contact layer  12 , and has a stripe shape. Thereafter, end surfaces of the resonant cavity are formed by polishing the substrate and cleaving the layered materials, and high-reflection coating  19  and low-reflection coating  20  are laid on the end surfaces of the resonant cavity, respectively. Then, the construction of FIG. 6 is formed into a chip. Thus, a high-power short-wavelength semiconductor laser device of the second embodiment is realized. The wavelength band of oscillation in the short-wavelength semiconductor laser device formed as described above can also be controlled in the range of 400&lt;λ&lt;550 (nm) by varying the indium content in the In x4 Ga 1−x4 N active layers. 
     It is also possible to change types of conductivity (i.e., alternate the n-types and the p-types) in the above construction of the second embodiment. 
     Third Embodiment 
     The third embodiment of the present invention will be explained below. 
     FIG. 7 is a plan view of a short-wavelength optical amplifying device as the third embodiment of the present invention, FIG. 8 is a cross-sectional view of the intermediate construction during the formation of the short-wavelength optical amplifying device of FIG. 7, and FIG. 9 is a cross-sectional view of a complete construction of the short-wavelength optical amplifying device of FIG. 7 taken along the line IX—IX. 
     In the third embodiment, first, the same layers as the layers  1  to  13  in the construction of FIG. 1 are formed in the same manner as explained for the first embodiment. Then, the SiO 2  film  13 , except for an area thereof having a tapered-shape as illustrated in FIG. 7, is removed by conventional lithography, where the tapered-shape widens from the input port size of 3 micrometers with a total tapered angle of about 8 degrees. Next, etching is performed using reactive ion etching (RIE) until the Si-doped In x1 Ga 1−x1 N layer  6  is exposed as illustrated in FIG. 8, and then the remaining SiO 2  film  13  is removed. Further, an insulation layer  16 - 2 , an n electrode  17 - 2  made of Ti/Au, and a p electrode  18 - 2  made of Ni/Au are formed by conventional lithography, as illustrated in FIG. 9, where the p electrode  18 - 2  is formed on the surface of the p contact layer  12 , and has a stripe shape. Thereafter, end surfaces of a resonant cavity are formed by polishing the substrate and cleaving the layered materials, and nonreflection coating  20 ′ is laid on the end surfaces of the resonant cavity. Then, the construction of FIG. 9 is formed into a chip. Thus, the high-power short-wavelength optical amplifying device as the third embodiment is realized. The amplifiable wavelength band of the optical amplifying device formed as described above can also be controlled in the range of 400&lt;λ&lt;550 (nm) by varying the indium content in the In x4 Ga 1−x4 N active layers. 
     It is also possible to change types of conductivity (i.e., alternate the n-types and the p-types) in the above construction of the third embodiment. 
     Fourth Embodiment 
     The fourth embodiment of the present invention will be explained below. 
     FIG. 10 is a plan view of an optical amplifying device as the fourth embodiment of the present invention, FIG. 11 is a cross-sectional view of the optical amplifying device as the fourth embodiment of the present invention at the XI—XI cross-section indicated in FIG. 10, and FIG. 12 is a cross-sectional view of the optical amplifying device as the fourth embodiment of the present invention at the XII—XII cross-section indicated in FIG.  10 . The optical amplifying device as the fourth embodiment of the present invention has ridges on a light-incident side thereof, where the ridges are provided for realizing an index-guided waveguide in the optical amplifying device. 
     In the fourth embodiment, the layers  1  to  13  are formed in the same manner as the third embodiment, and an insulation layer  16 - 3 , an n electrode  17 - 3  made of Ti/Au, and a p electrode  18 - 3  made of Ni/Au are also formed in the same manner as the insulation layer  16 - 2 , the n electrode  17 - 2 , and the p electrode  18 - 2  in the third embodiment, respectively. However, in the fourth embodiment, as illustrated in FIG. 11, ridges are formed by photolithography on only the left side (light-incident side) of the cross-section C-C′ indicated in FIG.  10 . The amplifiable wavelength band of the optical amplifying device formed as described above can also be controlled in the range of 400&lt;λ&lt;550 (nm) by varying the indium content in the In x4 Ga 1−x4 N active layers. 
     Each of the optical amplifying devices in the third and fourth embodiments is used with a master light source connected to the input port of the optical amplifying device. Three examples of the master light source are illustrated in FIGS. 13 to  15 . The master light source illustrated in FIG. 13 is a single-mode semiconductor laser device which has a feedback function using a diffraction grating. The master light source illustrated in FIG. 14 is a single-mode semiconductor laser device which is controlled by using a wavelength filter. The master light source illustrated in FIG. 15 is a solid-state laser device which uses a second harmonic generating element using semiconductor laser excitation of infrared light. 
     Fifth Embodiment 
     The fifth embodiment of the present invention will be explained below. 
     FIG. 16 is a plan view of a short-wavelength light emitting device as the fifth embodiment of the present invention, and FIG. 17 is a cross-sectional view of the short-wavelength light emitting device at the XVII—XVII cross-section indicated in FIG.  16 . 
     In the fifth embodiment, the layers  1  to  13  are formed in the same manner as the second embodiment. Then, the Sio 2  film  13 , except for an area thereof having a round shape, is removed by conventional lithography. Next, etching is performed using reactive ion etching (RIE) until the Si-doped In x1 Ga 1−x1 N layer  6  is exposed, and then the remaining SiO 2  film  13  is removed. Further, an insulation layer  16 - 4 , an n electrode  17 - 4  made of Ti/Au, and a p electrode  18 - 4  made of Ni/Au are formed by conventional lithography, as illustrated in FIGS. 16 and 17, where the p electrode  18 - 4  is formed on the surface of the p contact layer  12 . Thereafter, the substrate is polished and the layered materials are cleaved. Then, the construction illustrated in FIGS. 16 and 17 is formed into a chip. Thus, the high-power short-wavelength light emitting device of the fifth embodiment is realized. The wavelength of emitted light in the short-wavelength light emitting device formed as described above can also be controlled in the range of 400&lt;λ&lt;550 (nm) by varying the indium content in the In x4 Ga 1−x4 N active layers. 
     It is also possible to change types of conductivity (i.e., the n-types and the p-types with each other) in the above construction of the fifth embodiment. 
     Sixth Embodiment 
     The sixth embodiment of the present invention will be explained below. 
     FIG. 18 is a cross-sectional view of the semiconductor laser device as the sixth embodiment of the present invention at a first intermediate production stage. FIG. 19 is a cross-sectional view of the semiconductor laser device as the sixth embodiment of the present invention at a second intermediate production stage. FIG. 20 is a cross-sectional view of a complete construction of the semiconductor laser device as the sixth embodiment of the present invention. 
     As illustrated in FIG. 18, a GaN buffer layer  102  having a thickness of about 20 nm is formed at a temperature of 500° C. on a (0001) C face of a sapphire substrate  101  by using the organometallic vapor phase epitaxy technique. Then, a GaN layer  103  having a thickness of about 2 micrometers is formed at a temperature of 1,050° C. on the GaN buffer layer  102 . Materials used in the growth of the layers are trimethyl gallium (TMG), trimethyl indium (TMI), trimethyl aluminum (TMA)and ammonia. In addition, silane gas is used as an n-type dopant gas, and cycropentadienyl magnesium (CP2Mg) is used as a p-type dopant gas. 
     On the above GaN layer  103 , a SiO2 layer  104  having a thickness of 3 micrometers is formed. Next, the SiO2 layer  104  in stripe areas, which are spaced out and oriented in the [1{overscore (1)}00] direction, is removed by using conventional lithography, so that remaining stripe areas spaced at intervals of about 10 micrometers form a line-and-space pattern. Thus, a base  104 ′ for selective growth is formed. Then, an In x1 Ga 1−x1 N layer  105  (0.05&lt;x1≦0.1) of a thickness of about 20 micrometers is formed at a temperature of 750° C. by selective growth. In the operation of forming the In x1 Ga 1−x1 N layer  105 , finally, the surface of the In x1 Ga 1−x1 N layer  105  is planarized by growth in the lateral directions. Further, a silicon-doped In x1 Ga 1−x1 N layer  106  of a thickness of about 100 to 200 micrometers is formed. Then, the sapphire substrate  101 , the GaN buffer layer  102 , the GaN layer  103 , the SiO2 layer  104 , and the In x1 Ga 1−x1 N layer  105  are removed by polishing to use the Si-doped In x1 Ga 1−x1 N layer  106  as a substrate. 
     Subsequently, a silicon-doped In x1 Ga 1−x1 N layer  107 , an Si-doped In x1 (Al z2 Ga 1−z2 N (2.5 nm)/In x3 (Al z3 Ga 1−x3 N (2.5 nm) superlattice cladding layer  108  having 240 layers, a silicon-doped In x3 (Al z3 Ga 1−z3 ) 1−x3 N optical waveguide layer  109 , an In x3 Ga 1−x3 N (5 nm)/In x4 Ga 1−x4 N (2.5 nm) multiple quantum well active layer  110 , a magnesium-doped In x3 (Al z3 Ga 1−z3 ) 1−x3 N optical waveguide layer  111 , a Mg-doped In x2 (Al z2 Ga 1−x2 ) 1−x2 N (2.5 nm)/In x3 (Al z3 Ga 1−z3 ) 1−x3 N (2.5 nm) superlattice cladding layer  112 , and a magnesium-doped In x1 Ga 1−x1 N contact layer  113  are formed in this order. In order to activate the p-type magnesium impurity, heat treatment may be performed in a nitrogen atmosphere after the above growth. Alternatively, the growing operation may be performed in a nitrogen-rich atmosphere. The compositions in the above layers are determined within the ranges of: 0.05&lt;x1≦0.1, 0&lt;x2&lt;x1&lt;x3&lt;x4≦0.4, 0≦z2≦1, and 0≦z3≦1. 
     In addition, a SiO 2  layer  114  is formed on the above contact layer  12 . Thereafter, the SiO 2  layer  114 , except for a stripe area thereof having a width of 4 micrometers, is removed by conventional lithography, and then selective etching is performed using a reactive ion etching (RIE) apparatus until a part of the thickness of the superlattice cladding layer  112  is etched. The remaining thickness of the superlattice cladding layer  112  is such that basic transverse mode oscillation is realized. Next, the remaining portion of the SiO 2  layer  114  is removed. Thereafter, an SiO 2  film  115  is formed, then the SiO 2  film  115  in the top area is removed by conventional lithography. A p electrode  116  having a shape of a stripe and being made of Ni/Au is then formed on the surface of the p contact layer  113 . Thereafter, the rearside of the Si-doped In x1 Ga 1−x1 N layer  106  is polished, and an n electrode  117  made of Ti/Au is formed on the polished surface as illustrated in FIG.  20 . Then, end surfaces of the resonant cavity are formed by polishing the substrate and cleaving the layered materials, and high-reflection coating and low-reflection coating are laid on the end surfaces of the resonant cavity, respectively. The construction of FIG. 20 is then formed into a chip to realize the semiconductor laser device as the sixth embodiment of the present invention. 
     The wavelength band of oscillation in the semiconductor laser device formed as described above can be controlled in the range of 400&lt;λ&lt;550 (nm) by varying the indium content in the In x4 Ga 1−x4 N active layers. 
     Although impurity is doped only in the barrier layer in the supperlattice cladding layer in the above-described embodiment, it may be doped into both the well layer and the barrier layer of the superlattice structure. 
     It is also possible to change types of conductivity (i.e., the n-types and the p-types with each other) in the above construction of the sixth embodiment. 
     In addition, the above Si-doped In x1 Ga 1−x1 N layer  106  may be formed by using the hydride vapor phase epitaxy. 
     Others 
     Further, the n electrodes  17 - 1  and  17 - 2  in the semiconductor devices of the second and third embodiments may also be formed on the rearside of the Si-doped In x1 Ga 1−x1 N layer  6  in the same manner as the sixth embodiment. 
     The substrates in the first to sixth embodiments may be made of SiC, ZnO, LiGaO 2 , LiAlO 2 , ZnSe, GaAs, GaP, Ge, and Si, instead of sapphire. 
     When GaAs, GaP, Ge, or Si substrate is used in the sixth embodiment, this substrate may be removed by selective chemical etching. 
     In addition, all of the contents of Japanese Patent Application, No.11(1999)-30050 are incorporated into this specification by reference.