Patent Publication Number: US-6904071-B1

Title: Semiconductor laser device and method of fabricating the same

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The present invention relates to a semiconductor laser device having a current blocking layer (a current confinement layer) for confining a region of a current flowing into an active layer and a method of fabricating the same. 
     BACKGROUND OF THE INVENTION 
     In semiconductor laser devices, structures for limiting a current in a striped shape have been widely used for the purpose of decreasing an operating current and limiting the position of a light emitting spot. One of the structures for limiting a current in a striped shape is a structure having a current blocking layer for cutting off a current in a region, other than an opening, provided in a striped shape. 
       FIG. 22  is a schematic sectional view showing an example of the construction of a conventional GaN based semiconductor laser device having a current blocking layer. 
     In a semiconductor laser device  101  shown in  FIG. 22 , an n-contact layer  103  composed of n-GaN, an n-cladding layer  104  composed of n-Al n Ga 1-a N, a multi quantum well active layer (hereinafter referred to as an MQW active layer)  105 , and a p-first cladding layer  106   a  composed of p-Al b Ga 1-b N are formed in this order on a sapphire substrate  102 . 
     The MQW active layer  105  has a multi quantum well layer constructed by alternately stacking a plurality of quantum well layers composed of In x Ga 1-x N and a plurality of quantum barrier layers composed of In y Ga 1-y N, where x&gt;y. 
     An n-current blocking layer  107  composed of n-Al c Ga 1-c N having a striped opening  108  is formed on the p-first cladding layer  106   a . A p-second cladding layer  106   b  composed of p-Al d Ga 1-d N and a p-contact layer  109  composed of p-GaN are formed in this order on the n-current blocking layer  107  and on the p-first cladding layer  106   a  inside the striped opening  108 . A dotted line drawn in the striped opening  108  indicates the boundary between the p-first cladding layer  106   a  and the p-second cladding layer  106   b . Here, 0≦a&lt;c, 0≦b&lt;c, and 0≦d&lt;c. 
     A partial region from the p-contact layer  109  to the n-contact layer  103  is etched away, so that a surface of the n-contact layer  103  is exposed. A p electrode  110  is formed on the p-contact layer  109 , and an n electrode  111  is formed on the exposed surface of the n-contact layer  103 . 
       FIG. 23  is a schematic sectional view showing another example of the construction of a conventional GaN based semiconductor laser device having a current blocking layer. 
     In a semiconductor laser device  201  shown in  FIG. 23 , an n-contact layer  203  composed of n-GaN, an n-cladding layer  204  composed of n-Al e Ga 1-e N, an MQW active layer  205 , and a p-first cladding layer  206   a  composed of p-Al f Ga 1-f N are formed in this order on a sapphire substrate  202 . 
     The MQW active layer  205  has a multi quantum well structure constructed by alternately stacking a plurality of quantum well layers composed of In a Ga 1-a N and a plurality of barrier layers composed of In t Ga 1-t N, where s&gt;t. 
     A p-second cladding layer  206   b  in a ridge shape composed of p-Al f Ga 1-f N is formed on the p-first cladding layer  206   a . An n-current blocking layer  207  composed of n-Al g Ga 1-g N having a striped opening  208  is formed on the p-first cladding layer  206   a  on both sides of the p-second cladding layer  206   b . A p-contact layer  209  composed of p-GaN is formed on the n-current blocking layer  207  and on the p-second cladding layer  206   b  inside the striped opening  208 . A dotted line drawn in the striped opening  208  indicates the boundary between the p-first cladding layer  206   a  and the p-second cladding layer  206   b . Here, 0≦e&lt;g and 0≦f&lt;g. 
     A partial region from the p-contact layer  209  to the n-contact layer  203  is etched away, so that a surface of the n-contact layer  203  is exposed. A p electrode  210  is formed on the p-contact layer  209 , and an n electrode  211  is formed on the exposed surface of the n-contact layer  203 . 
     In the semiconductor laser devices  101  and  201 , the Al composition ratios of the n-current blocking layers  107  and  207  are respectively higher than the Al composition ratios of the p-cladding layers  106   a  and  106   b  and the p-cladding layers  206   a  and  206   b . Accordingly, the refractive indexes of the n-current blocking layers  107  and  207  are respectively lower than the refractive indexes of the p-cladding layers  106   a  and  106   b  and the p-cladding layers  206   a  and  206   b . Consequently, effective refractive indexes in regions of the MQW active layers  105  and  205  under the striped openings  108  and  208  are respectively higher than effective refractive indexes in regions of the MQW active layers  105  and  205  under the n-current blocking layers  107  and  207 . Accordingly, light is concentrated on the regions under the striped openings  108  and  208 . A semiconductor laser device having a real refractive index guided structure is thus realized. 
     The semiconductor laser devices  101  and  201  shown in  FIGS. 22 and 23  can have a loss guided structure by respectively composing the n-current blocking layers  107  and  207  of InGaN having a smaller band-gap than those of the active layers. 
     In the conventional semiconductor laser device  101  shown in  FIG. 22 , the n-current blocking layer  107  has the striped opening  108  which is rectangular in cross section. The width W of the striped opening  108  is approximately constant irrespective of the depth thereof. 
     In the conventional semiconductor laser device  201  shown in  FIG. 23 , the n-current blocking layer  207  has the striped opening  208  which is trapezoidal in cross section. The width of the striped opening  208  gradually decreases as the depth thereof decreases, that is, the lower width W 2  is larger than the upper width W 1 . 
     In the semiconductor laser device  101  shown in  FIG. 22 , if the width W of the striped opening  108  is increased, an area occupied by the striped opening  108  in a plane shape of the semiconductor laser device  101  is increased. Even if the same operating voltage is applied to the semiconductor laser device  101 , a current flowing into the MQW active layer  105  from the p-contact layer  109  through the striped opening  108  is increased. If the same light output power is achieved, the operating voltage can be decreased. 
     If the width W of the striped opening  108  is increased, however, the width of a light emitting spot in a direction parallel to the MQW active layer  105  is increased. Accordingly, the aspect ratio of laser light emitted from the semiconductor laser device  101  (a vertical divergence/horizontal divergence of emitted laser light) is increased. 
     Conversely, if the width W of the striped opening  108  is decreased, the width of the light emitting spot in the direction parallel to the MQW active layer  105  is decreased. Accordingly, the aspect ratio of the emitted laser light is decreased. However, a current flowing into the MQW active layer  105  from the p-contact layer  109  through the striped opening  108  is decreased. Accordingly, the operating voltage must be increased in order to make light output power constant. 
     Similarly in the semiconductor laser device  201  shown in  FIG. 23 , when the widths W 1  and W 2  of the striped opening  208  are increased, an operating voltage for obtaining the same light output power can be decreased, while the aspect ratio of emitted laser light is increased. Conversely, if the widths W 1  and W 2  of the striped opening  208  are decreased, the aspect ratio of the emitted laser light can be decreased, while the operating voltage is increased. 
     On the other hand, the realization of a semiconductor laser device in which an operating voltage is low and low-noise characteristics is obtained depending on the use has been desired. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor laser device capable of reducing an operating voltage and improving the aspect ratio of emitted laser light and a method fabricating the same. 
     Another object of the present invention is to provide a semiconductor laser device capable of reducing an operating voltage and performing self-sustained pulsation and a method of fabricating the same. 
     A semiconductor laser device according to an aspect of the present invention comprises a first semiconductor layer including an active layer; a striped second semiconductor layer formed on the first semiconductor layer; and a current blocking layer formed on the first semiconductor layer on both sides of the second semiconductor layer, the second semiconductor layer including a cladding layer which comprises a lower layer having a first width at its lower end and an upper layer having a second width larger than the first width at its lower end and has a larger band-gap than that of the active layer. 
     In the semiconductor laser device, the cladding layer comprises the lower layer having a first width at its lower end and the upper layer having a second width larger than the first width at its lower end. Accordingly, the resistance of the upper layer in the cladding layer is decreased. Consequently, an operating voltage is reduced. 
     The width at the lower end of the lower layer in the cladding layer is smaller than the width of the upper layer in the cladding layer. Accordingly, the width of a light emitting spot in a direction parallel to the active layer can be decreased. In this case, it is possible to decrease the aspect ratio of emitted laser light. 
     On the other hand, the width of a current injection region is defined by the width at the lower end of the lower layer in the cladding layer. When the width of the light emitting spot is defined by the width at the lower end of the upper layer in the cladding layer, therefore, the width of the current injection region is smaller than the width of the light emitting spot. In this case, both sides of the current injection region can function as a saturable light absorbing member, so that self-sustained pulsation occurs. As a result, low-noise characteristics can be obtained. 
     The cladding layer may have the function of confining light in the active layer. Consequently, the light is confined in a direction perpendicular to the active layer. 
     The semiconductor laser device may further comprise a third semiconductor layer formed on the cladding layer and having a carrier concentration which is not less than that of the cladding layer. The third semiconductor layer may be a contact layer. In this case, good ohmic contact can be obtained between the third semiconductor layer and an electrode. 
     The semiconductor laser device may further comprise a third semiconductor layer formed on the cladding layer and having a smaller band-gap than that of the cladding layer. The third semiconductor layer may be a contact layer. In this case, the carrier concentration of the third semiconductor layer can be increased. Accordingly, good ohmic contact can be obtained between the third semiconductor layer and an electrode. 
     The lower layer in the cladding layer may have the first width which is approximately constant from its lower end to its upper end, and the upper layer in the cladding layer may have a second width which is approximately constant from its lower end to its upper end. 
     In this case, a cladding layer having a reverse two-step shaped stripe structure comprising a lower layer having a side surface approximately perpendicular to an active layer and an upper layer having a side surface approximately perpendicular to the active layer is obtained. 
     The lower layer in the cladding layer may have the first width which is approximately constant from its lower end to its upper end, and the upper layer in the cladding layer may have a width which gradually decreases upward from the second width. 
     In this case, a cladding layer having a reverse two-step shaped stripe structure comprising a lower layer having a side surface approximately perpendicular to an active layer and an upper layer having a side surface inclined from the active layer is obtained. Particularly, the width at the upper end of the upper layer in the cladding layer can be increased by increasing the width at the lower end of the upper layer in the cladding layer. Consequently, the resistance of the upper layer in the cladding layer is decreased. 
     The first semiconductor layer may comprise a cladding layer of a first conductivity type, the active layer, and a first cladding layer of a second conductivity type in this order from its bottom, and the second semiconductor layer comprises a second cladding layer of a second conductivity type as the cladding layer. 
     When the refractive index of the current blocking layer is lower than the refractive indexes of the cladding layer of a first conductivity type and the first and second cladding layers of a second conductivity type, an effective refractive index in a region of the active layer under the second cladding layer is higher than an effective refractive index in a region of the active layer under the current blocking layer. Accordingly, light is concentrated on the region of the active layer under the second cladding layer. Consequently, a semiconductor laser device having a real refractive index guided structure in which the aspect ratio of emitted laser light is low is realized. 
     On the other hand, when the current blocking layer has a smaller band-gap than that of the active layer, light emitted in the active layer under the current blocking layer is absorbed by the current blocking layer. Accordingly, light is concentrated on the region of the active layer under the second cladding layer. Consequently, a semiconductor laser device having a loss guided structure in which the aspect ratio of emitted laser light is low is realized. 
     The first semiconductor layer may be a first nitride based semiconductor layer containing at least one of boron, thallium, gallium, aluminum, and indium, the second semiconductor layer may be a second nitride based semiconductor layer containing at least one of boron, thallium, gallium, aluminum, and indium, and the current blocking layer may be a third nitride based semiconductor layer containing at least one of boron, thallium, gallium, aluminum, and indium. In this case, a semiconductor laser device which emits light having a short wavelength is realized. 
     Particularly when the conductivity type of the second semiconductor layer is a p type, it is difficult to decrease the volume resistivity of the second semiconductor layer. In this case, the effect of reducing an operating voltage by increasing the width at the lower end of the upper layer in the cladding layer becomes significant. 
     A method of fabricating a semiconductor laser device according to another aspect of the present invention comprises the steps of forming a first semiconductor layer including an active layer; and forming a striped second semiconductor layer on the first semiconductor layer, and forming a current blocking layer on the first semiconductor layer on both sides of the second semiconductor layer, the step of forming the second semiconductor layer comprising the step of forming a cladding layer which comprises a lower layer having a first width at its lower end and an upper layer having a second width larger than the first width at its lower end and has a larger band-gap than that of the active layer. 
     In the semiconductor laser device fabricated by the fabricating method, the cladding layer comprises the lower layer having a first width at its lower end and the upper layer having a second width larger than the first width at its lower end. Accordingly, the resistance of the upper layer in the cladding layer is decreased. Consequently, an operating voltage is reduced. 
     The width at the lower end of the lower layer in the cladding layer is smaller than the width of the upper layer in the cladding layer. Accordingly, the width of a light emitting spot in a direction parallel to the active layer can be decreased. In this case, it is possible to decrease the aspect ratio of emitted laser light. 
     On the other hand, the width of a current injection region is defined by the width at the lower end of the lower layer in the cladding layer. When the width of the light emitting spot is defined by the width at the lower end of the upper layer in the cladding layer, therefore, the width of the current injection region is smaller than the width of the light emitting spot. In this case, both sides of the current injection region can function as a saturable light absorbing member, so that self-sustained pulsation occurs. As a result, low-noise characteristics can be obtained. 
     The step of forming the second semiconductor layer and the current blocking layer may comprise the step of forming a current blocking layer on the first semiconductor layer, the step of forming on the current blocking layer a first mask pattern having a first striped opening, the step of etching the current blocking layer inside the first striped opening of the first mask pattern by a first depth, to form a striped recess in the current blocking layer, the step of removing the first mask pattern, and then forming a second mask pattern having a second striped opening wider than the striped recess of the current blocking layer on the current blocking layer on both sides of the striped recess, the step of etching the current blocking layer inside the second striped opening of the second mask pattern to a second depth at which the first semiconductor layer is exposed, to form in the current blocking layer a striped opening which stepwise widens from a lower end to an upper end of the current blocking layer, and the step of removing the second mask pattern, and then forming the second semiconductor layer on the current blocking layer and on the first semiconductor layer inside the striped opening of the current blocking layer. 
     In this case, the current blocking layer is formed on the first semiconductor layer including the active layer, and the first mask pattern having the first striped opening is formed on the current blocking layer. The current blocking layer inside the first striped opening of the first mask pattern is etched by the first depth. At this time point, the first semiconductor layer has not been exposed yet, so that the striped recess is formed in the current blocking layer. 
     After the first mask pattern is removed, the second mask pattern having the second striped opening wider than the striped recess of the current blocking layer is formed on the current blocking layer on both sides of the striped recess. The current blocking layer inside the striped opening of the second mask pattern is etched to the second depth. Consequently, the first semiconductor layer is exposed in a region of the striped recess formed in the current blocking layer. Therefore, the striped opening which stepwise widens from its lower end to its upper end is formed in the current blocking layer. After the second mask pattern is removed, the second semiconductor layer is formed on the current blocking layer and on the first semiconductor layer inside the striped opening of the current blocking layer. 
     The cladding layer comprising the lower layer having the first width at its lower end and the upper layer having the second width larger than the first width at its lower end is thus formed. 
     The step of forming the second semiconductor layer and the current blocking layer comprises the step of forming a current blocking layer on the first semiconductor layer, the step of forming on the current blocking layer a first mask pattern having a first striped opening and composed of a first material, the step of forming a second mask pattern having a second striped opening narrower than the first striped opening of the first mask pattern and composed of a second material different from the first material on the current blocking layer inside the first striped opening and on the first mask pattern, the step of etching the current blocking layer inside the second striped opening of the second mask pattern by a first depth, to form a striped recess in the current blocking layer, the step of removing the second mask pattern, and then etching the current blocking layer inside the first striped opening of the first mask pattern to a second depth at which the first semiconductor layer is exposed, to form in the current blocking layer a striped opening which stepwise widens from a lower end to an upper end of the current blocking layer, and the step of removing the first mask pattern, and then forming the second semiconductor layer on the current blocking layer and on the first semiconductor layer inside the striped opening of the current blocking layer. 
     In this case, the current blocking layer is formed on the first semiconductor layer including the active layer, the first mask pattern having the first striped opening is formed on the current blocking layer, and the second mask pattern having the second striped opening narrower than the first striped opening of the first mask pattern is formed on the current blocking layer inside the first striped opening and on the first mask pattern. The current blocking layer inside the second striped opening of the second mask pattern is etched by only the first depth. At this time point, the first semiconductor layer has not been exposed yet, so that the striped recess is formed in the current blocking layer. 
     After the second mask pattern is removed, the current blocking layer inside the first striped opening of the first mask pattern is etched to the second depth. Consequently, the first semiconductor layer is exposed in a region of the striped recess formed in the current blocking layer. Therefore, the striped opening which stepwise widens from its lower end to its upper end is formed in the current blocking layer. After the first mask pattern is removed, the second semiconductor layer is formed on the current blocking layer and on the first semiconductor layer inside the striped opening of the current blocking layer. 
     The cladding layer comprising the lower layer having the first width at its lower end and the upper layer having the second width larger than the first width at its lower end is thus formed. 
     The step of forming the second semiconductor layer and the current blocking layer may comprise the step of forming a first current blocking layer on the first semiconductor layer, the step of forming on the first current blocking layer a first mask pattern having a first striped opening, the step of etching the first current blocking layer inside the first striped opening of the first mask pattern, to form a striped opening in the first current blocking layer, the step of removing the first mask pattern, and then forming a second semiconductor layer on the first current blocking layer and on the first semiconductor layer inside the striped opening of the first current blocking layer, the step of forming a striped second mask pattern in a region on the second semiconductor layer above the striped opening of the first current blocking layer, the step of etching the second semiconductor layer, except in a region of the second mask pattern to expose the first current blocking layer on both sides of the second mask pattern, to form in the second semiconductor layer a lower layer having the first width which is approximately constant from its lower end to its upper end and an upper layer having a width which gradually decreases upward from the second width, and the step of selectively forming a second current blocking layer on the first current blocking layer, except in a region on the second mask pattern. 
     In this case, the first current blocking layer is formed on the first semiconductor layer including the active layer, and the first mask pattern having the first striped opening is formed on the first current blocking layer. The first current blocking layer inside the first striped opening of the first mask pattern is etched, so that the striped opening is formed in the first current blocking layer. After the first mask pattern is removed, the second semiconductor layer is formed on the first current blocking layer and on the first semiconductor layer inside the striped opening of the first current blocking layer. 
     The striped second mask pattern is formed in a region on the second semiconductor layer above the striped opening of the first current blocking layer, and the second semiconductor layer, except in a region of the second mask pattern, is etched. Consequently, the first current blocking layer is exposed on both sides of the second mask pattern. In this case, the second mask pattern is formed in a region on the second semiconductor layer above the striped opening of the first current blocking layer. Accordingly, the second semiconductor layer remaining under the second mask pattern is overlapped with the striped opening of the first current blocking layer. The second semiconductor layer formed at this time comprises the lower layer having the first width which is approximately constant and the upper layer having the width which gradually decreases upward from the second width. The second current blocking layer is selectively formed on the first current blocking layer, except in a region on the second mask pattern. 
     The cladding layer comprising the lower layer having the first width at its lower end and the upper layer having the second width larger than the first width at its lower end is thus formed. 
     The step of forming the second semiconductor layer and the current blocking layer may comprise the step of forming on the first semiconductor layer a first mask pattern having a striped opening, the step of selectively growing a second semiconductor layer on the first semiconductor layer inside the striped opening and on the first mask pattern in the periphery of the striped opening, the step of removing the first mask pattern, and then forming a second mask pattern on an upper surface of the second semiconductor layer, and the step of selectively growing a current blocking layer on the first semiconductor layer on both sides of the second semiconductor layer, except on the second mask pattern. 
     In this case, the first mask pattern having the striped opening is formed on the first semiconductor layer, and the second semiconductor layer is selectively grown on the first semiconductor layer inside the striped opening and on the first mask pattern in the periphery of the striped opening. After the first mask pattern is removed, the second mask pattern is formed on the upper surface of the second semiconductor layer, and the current blocking layer is selectively grown on the first semiconductor layer on both sides of the second semiconductor layer, except on the second mask pattern. 
     The cladding layer comprising the lower layer having the first width at its lower end and the upper layer having the second width larger than the first width at its lower end is thus formed. 
     In this case, the cladding layer comprising the lower layer and the upper layer is formed by the selective growth, so that the crystallizability on a side surface of the cladding layer is improved. Consequently, the state of the interface of the cladding layer and the current blocking layer is improved, thereby reducing an invalid current flowing through the interface. As a result, device characteristics are improved. 
     The fabricating method may further comprise the step of forming on the cladding layer a third semiconductor layer having a smaller band-gap than that of the cladding layer. In this case, good ohmic contact can be obtained between the third semiconductor layer and an electrode. 
     The fabricating method may further comprise the step of forming on the cladding layer a third semiconductor layer having a carrier concentration which is not less than that of the cladding layer. In this case, the carrier concentration of the third semiconductor layer can be increased, so that good ohmic contact can be obtained between the third semiconductor layer and an electrode. 
     The step of forming the cladding layer may comprise the step of forming a lower layer having the first width which is approximately constant from its lower end to its upper end and an upper layer having the second width which is approximately constant from its lower end to its upper end. 
     In this case, a cladding layer having a reverse two-step shaped stripe structure comprising a lower end having a side surface approximately perpendicular to an active layer and an upper layer having a side surface approximately perpendicular to the active layer is obtained. 
     The step of forming the cladding layer may comprise the step of forming a lower layer having a first width which is approximately constant from its lower end to its upper end and an upper layer having a width which gradually decreases upward from the second width. 
     In this case, a cladding layer having a reverse two-step shaped stripe structure comprising a lower layer having a side surface approximately perpendicular to an active layer and an upper layer having a side surface inclined from the active layer is obtained. Particularly, the width at the upper end of the upper layer in the cladding layer can be increased by increasing the width at the lower end of the upper layer in the cladding layer. Consequently, the resistance of the upper layer in the cladding layer is decreased. 
     The step of forming the first semiconductor layer may comprise the step of forming a cladding layer of a first conductivity type, the active layer, and a first cladding layer of a second conductivity type in this order from its bottom, and the step of forming the second semiconductor layer may comprise the step of forming a second cladding layer of a second conductivity type as the cladding layer. 
     The first semiconductor layer may be a first nitride based semiconductor layer containing at least one of boron, thallium, gallium, aluminum, and indium, the second semiconductor layer may be a second nitride based semiconductor layer containing at least one of boron, thallium, gallium, aluminum, and indium, and the current blocking layer may be a third nitride based semiconductor layer containing at least one of boron, thallium, gallium, aluminum, and indium. In this case, a semiconductor laser device which emits light having a short wavelength is realized. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a first embodiment of the present invention; 
         FIG. 2  is a schematic perspective view showing the construction of the semiconductor laser device shown in  FIG. 1 ; 
         FIG. 3  is a schematic sectional view showing the steps of a first example of a method of fabricating the GaN based semiconductor laser device shown in FIG.  1 : 
         FIG. 4  is a schematic sectional view showing the steps of the first example of the method of fabricating the GaN based semiconductor laser device shown in  FIG. 1 ; 
         FIG. 5  is a schematic sectional view showing the steps of a second example of a method of fabricating the GaN based semiconductor laser device shown in  FIG. 1 ; 
         FIG. 6  is a schematic sectional view showing the steps of the second example of the method of fabricating the GaN based semiconductor laser device shown in  FIG. 1 ; 
         FIG. 7  is a diagram for explaining the effect of the GaN based semiconductor laser device in the first embodiment; 
         FIG. 8  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a second embodiment of the present invention; 
         FIG. 9  is a schematic sectional view showing the steps of an example of a method of fabricating the GaN based semiconductor laser device shown in  FIG. 8 ; 
         FIG. 10  is a schematic sectional view showing the steps of the example of the method of fabricating the GaN based semiconductor laser device shown in  FIG. 8 ; 
         FIG. 11  is a schematic sectional view showing the steps of the example of the method of fabricating the GaN based semiconductor laser device shown in  FIG. 8 ; 
         FIG. 12  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a third embodiment of the present invention; 
         FIG. 13  is a schematic sectional view showing the steps of an example of a method of fabricating the GaN based semiconductor laser device shown in  FIG. 12 ; 
         FIG. 14  is a schematic sectional view showing the steps of the method of fabricating the GaN based semiconductor laser device shown in  FIG. 12 ; 
         FIG. 15  is a diagram showing an effective refractive index distribution in an MQW active layer in the GaN based semiconductor laser device shown in  FIG. 12 ; 
         FIG. 16  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a fourth embodiment of the present invention; 
         FIG. 17  is a diagram showing a first example of band-gap energies in a cladding layer of a first conductivity type, an active layer, a cladding layer of a second conductivity type, and a contact layer of a second conductivity type in the semiconductor laser device according to the present invention; 
         FIG. 18  is a diagram showing a second example of band-gap energies in a cladding layer of a first conductivity type, an active layer, a cladding layer of a second conductivity type, and a contact layer of a second conductivity type in the semiconductor laser device according to the present invention; 
         FIG. 19  is a diagram showing a third example of band-gap energies in a cladding layer of a first conductivity type, an active layer, a cladding layer of a second conductivity type, and a contact layer of a second conductivity type in the semiconductor laser device according to the present invention; 
         FIG. 20  is a diagram showing a fourth example of band-gap energies in a cladding layer of a first conductivity type, an active layer, a cladding layer of a second conductivity type, and a contact layer of a second conductivity type in the semiconductor laser device according to the present invention; 
         FIG. 21  is a diagram showing a fifth example of band-gap energies in a cladding layer of a first conductivity type, an active layer, a cladding layer of a second conductivity type, and a contact layer of a second conductivity type in the semiconductor laser device according to the present invention; 
         FIG. 22  is a schematic sectional view showing the steps of a first example of the construction of a conventional GaN based semiconductor laser device; and 
         FIG. 23  is a schematic sectional view showing the steps of a second example of the construction of a conventional GaN based semiconductor laser device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a first embodiment of the present invention.  FIG. 2  is a schematic perspective view of the semiconductor laser device shown in FIG.  1 . Description is now made of a semiconductor laser device having the construction shown in FIG.  1  and having a real refractive index guided structure. 
     In a semiconductor laser device  1  shown in  FIG. 1 , an n-contact layer  3  composed of n-GaN having a thickness of 4.5 μm, an n-cladding layer  4  composed of n-Al a Ga 1-a N having a thickness of 1.0 μm, an MQW active layer  5 , and a p-first cladding layer  6   a  composed of p-Al b Ga 1-b N having a thickness of 0.1 μm are formed in this order on a sapphire substrate  2 . 
     The MQW active layer  5  has a multi quantum well structure constructed by alternately stacking three quantum well layers composed of In x Ga 1-x N having a thickness of 80 Å and four quantum barrier layers composed of In y Ga 1-y N having a thickness of 160 Å, where x&gt;y. In the present embodiment, x=0.13 and y=0.05. 
     An n-current blocking layer  7  composed of n-Al c Ga 1-c N having a striped opening  8  is formed on the p-first cladding layer  6   a . The striped opening  8  of the n-current blocking layer  7  has a step on both its inner side surfaces. That is, the upper width of the striped opening  8  of the n-current blocking layer  7  is gradually larger than the lower width thereof. A lower layer  7   a  in the n-current blocking layer  7  projects more inwardly, as compared with an upper layer  7   b  in the n-current blocking layer  7 . 
     The thickness t 1  of the whole of the n-current blocking layer  7  is 0.8 μm. The lower layer  7   a  in the n-current blocking layer  7  has a sufficient thickness t 2  to block a current, which is not less than 0.5 μm in the present embodiment. A p-second cladding layer  6   b  having a thickness of 0.9 μm composed of p-Al d Ga 1-d N and a p-contact layer  9  composed of p-GaN having a thickness of 0.05 μm are formed in this order on the n-current blocking layer  7  and on the p-first cladding layer  6   a  inside the striped opening  8 . The p-first cladding layer  6   a  and the p-second cladding layer  6   b  are composed of the same material. Here, 0≦a&lt;c, 0≦b&lt;c, and 0≦d&lt;c, and a=0.07, b=0.07, c=0.12, and d=0.07. 
     Si is used as an n-type dopant in each of the layers, and Mg is used as a p-type dopant in the layer. 
     A partial region from the p-contact layer  9  to the n-contact layer  3  is etched away, so that a surface of the n-contact layer  3  is exposed. A p electrode  10  is formed on the p-contact layer  9 , and an n electrode  11  is formed on the exposed surface of the n-contact layer  3 . 
     In the semiconductor laser device  1  according to the present embodiment, the Al composition ratio of the n-current blocking layer  7  is higher than the Al composition ratios of the p-first cladding layer  6   a  and the p-second cladding layer  6   b . Accordingly, the refractive index of the n-current blocking layer  7  is lower than the refractive indexes of the p-first cladding layer  6   a  and the p-second cladding layer  6   b . Consequently, an effective refractive index in a region of the MQW active layer  5  under the striped opening  8  is higher than an effective refractive index in a region of the MQW active layer  5  under the n-current blocking layer  7 . Accordingly, light is concentrated on the region under the striped opening  8  between lower layers  7   a  in the n-current blocking layer  7 . A semiconductor laser device  1  having a real refractive index guided structure in which an operating voltage is low and the width of a light emitting spot is small is thus realized. 
       FIGS. 3 and 4  are schematic sectional views showing the steps of a first example of a method of fabricating the GaN based semiconductor laser device  1  shown in FIG.  1 . 
     As shown in FIG.  3 ( a ), an n-contact layer  3 , an n-cladding layer  4 , an MQW active layer  5 , a p-first cladding layer  6   a , and an n-current blocking layer  7  are first continuously grown on a sapphire substrate  2  by MOCVD (Metal Organic Chemical Vapor Deposition) or the like. The thickness of the n-current blocking layer  7  is taken as t 1 . A first mask pattern  12  formed of an SiO 2  (silicon oxide) film, for example, having a first striped opening  13  is formed on the n-current blocking layer  7 . 
     As shown in FIG.  3 ( b ), the n-current blocking layer  7  inside the first striped opening  13  of the first mask pattern  12  is then removed by a first depth t 3  by dry etching such as RIE (Reactive Ion Etching) or RIBE (Reactive Ion Beam Etching). Consequently, a striped recess  14  is formed in the n-current blocking layer  7 . Thereafter, the first mask pattern  12  is removed. 
     As shown in FIG.  3 ( c ), a second mask pattern  15  is then formed in a region on the n-current blocking layer  7  on both sides of the striped recess  14 . In this case, the second mask pattern  15  has a second striped opening  16  which is wider than the striped recess  14 , and is formed spaced a predetermined distance apart from an edge of the striped recess  14 . 
     As shown in FIG.  3 ( d ), the n-current blocking layer  7  inside the second striped opening  16  of the second mask pattern  15  is then removed by a second depth t 4  by dry etching again. Consequently, the bottom of the striped recess  14  shown in FIG.  3 ( c ) reaches the p-first cladding layer  6   a , to expose the p-first cladding layer  6   a . Accordingly, a striped opening  8  is formed in the n-current blocking layer  7 . The second depth t 4  at which the n-current blocking layer  7  is removed at this time is smaller than the depth of the n-current blocking layer  7 . Accordingly, a step is formed in the striped opening  8 , so that a lower layer  7   a  in the n-current blocking layer  7  projects more inwardly, as compared with an upper layer  7   b  in the n-current blocking layer  7 . The thickness (t 2 =t 1 −t 4 ) of the lower layer  7   a  is determined by the second depth t 4  at which the n-current blocking layer  7  is etched away. The lower layer  7   a  has a sufficient thickness to block a current. Thereafter, the second mask pattern  15  is removed. 
     As shown in FIG.  4 ( e ), a p-second cladding layer  6   b  and a p-contact layer  9  are then formed in this order on the p-first cladding layer  6   a  inside the striped opening  8  and on the n-current blocking layer  7 . 
     Furthermore, as shown in FIG.  4 ( f ), a third mask pattern  18  is formed in a predetermined region on the p-contact layer  9 . 
     As shown in FIG.  4 ( g ), a region from the p-contact layer  9  to the n-contact layer  3 , except in a region of the third mask pattern  18 , is removed by dry etching, to expose a surface of the n-contact layer  3 . Thereafter, the third mask pattern  18  is removed. 
     Finally, as shown in FIG.  4 ( h ), a p electrode  10  is formed on the p-contact layer  9 , and an n electrode  11  is formed on the exposed surface of the n-contact layer  3 . 
       FIGS. 5 and 6  are schematic sectional views showing the steps of a second example of a method of fabricating the GaN based semiconductor laser device  1  shown in FIG.  1 . 
     As shown in FIG.  5 ( a ), an n-contact layer  3 , an n-cladding layer  4 , an MQW active layer  5 , a p-first cladding layer  6   a , and an n-current blocking layer  7  are first continuously grown on a sapphire substrate  2  by MOCVD or the like. A first mask pattern  20  having a first striped opening  22   a  composed of SiO 2  (silicon oxide) is formed on the n-current blocking layer  7 . Further, a second mask pattern  21  composed of Ni (nickel) having a second striped opening  22   b  is formed on the n-current blocking layer  7  inside the first striped opening  22   a  of the first mask pattern  20 . The second mask pattern  21  is formed such that the second striped opening  22   b  is positioned inside of an edge of the first striped opening  22   a  and spaced a predetermined distance apart therefrom so as to cover the first mask pattern  20 . The thickness of the current blocking layer  7  is taken as t 1 . 
     As shown in FIG.  5 ( b ), the n-current blocking layer  7  inside the second striped opening  22   b  of the second mask pattern  21  composed of Ni is then removed by a first depth t 3  by dry etching using CCl 4  (carbon tetrachloride). The first depth t 3  is smaller than the thickness t 1  of the n-current blocking layer  7 , so that a striped recess  23  is formed in the n-current blocking layer  7 . 
     Furthermore, as shown in FIG.  5 ( c ), in the first striped opening  22   a  of the first mask pattern  20  composed of SiO 2 , the n-current blocking layer  7  is removed by a second depth t 4  by dry etching using Cl 2  (chlorine), to expose the p-first cladding layer  6   a . Consequently, a striped opening  8  is formed in the n-current blocking layer  7 . In this case, the second mask pattern  21  composed of Ni and the current blocking layer  7  under the second mask pattern  21  are etched. However, the first mask pattern  20  composed of SiO 2  is not etched. Accordingly, a distance between upper layers  7   b  in the n-current blocking layer  7  is equal to the width of the first striped opening  22   a  of the first mask pattern  20 , and the second depth t 4  is smaller than the thickness t 1  of the n-current blocking layer  7 . Accordingly, a distance between lower layers  7   a  having a thickness t 2  in the n-current blocking layer  7  is decreased. Consequently, a striped opening  8  which stepwise widens from its lower end to its upper end is formed. Thereafter, the first mask pattern  20  is removed. 
     As shown in FIG.  5 ( d ), a p-second cladding layer  6   b  and a p-contact layer  9  are then grown in this order on the n-current blocking layer  7  and on the p-first cladding layer  6   a  inside the striped opening  8 . 
     Furthermore, as shown in FIG.  6 ( e ), a third mask pattern  26  is formed in a predetermined region on the p-contact layer  9 . 
     As shown in FIG.  6 ( f ), a region from the p-contact layer  9  to the n-contact layer  3 , except in a region of the third mask pattern  26 , is removed by dry etching, to expose a surface of the n-contact layer  3 . Thereafter, the third mask pattern  26  is removed. 
     Finally, as shown in FIG.  6 ( g ), a p electrode  10  is formed on the p-contact layer  9 , and an n electrode  11  is formed on the exposed surface of the n-contact layer  3 . 
     In the GaN based semiconductor laser device  1  according to the present embodiment, the width of the striped opening  8  of the current blocking layer  7  stepwise increases from W 2  to W 1 , as shown in FIG.  7 ( a ), for example, as the depth thereof decreases from its lower end to its upper end. 
     When the semiconductor laser device  1  according to the present embodiment shown in FIG.  7 ( a ) and the conventional semiconductor laser device  101 A shown in FIG.  7 ( b ) are compared with each other, the width W 1  at the upper end of the striped opening  8  in the semiconductor laser device  1  and the width W 1  of the striped opening  108  in the semiconductor laser device  101 A are the same. Accordingly, an operating voltage in the semiconductor laser device  1  and an operating voltage in the semiconductor laser device  101 A are approximately the same. On the other hand, the width W 2  at the lower end of the striped opening  8  in the semiconductor laser device  1  is smaller than the width W 1  of the striped opening  108  in the semiconductor laser device  101 A. Accordingly, the aspect ratio of laser light emitted by the semiconductor laser device  1  can be lower than the aspect ratio of laser light emitted by the semiconductor laser device  101 A. 
     When the semiconductor laser device  1  according to the present embodiment shown in FIG.  7 ( a ) and the conventional semiconductor laser device  101 B shown in FIG.  7 ( c ) are compared with each other, the width W 2  at the lower end of the striped opening  8  in the semiconductor laser device  1  and the width W 2  of the striped opening  108  in the semiconductor laser device  101 B are the same. Accordingly, the aspect ratio of laser light emitted by the semiconductor laser device  1  and the aspect ratio of laser light emitted by the semiconductor laser device  101 B are approximately the same. On the other hand, the width W 1  at the upper end of the striped opening  8  in the semiconductor laser device  1  is larger than the width W 2  of the striped opening  108  in the semiconductor laser device  101 B. Accordingly, an operating voltage in the semiconductor laser device  1  can be made lower than an operating voltage in the semiconductor laser device  101 B. 
       FIG. 8  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a second embodiment of the present invention. Description is now made of a semiconductor laser device having a real refractive index guided structure as the semiconductor laser device in the second embodiment having the construction shown in FIG.  8 . 
     In a semiconductor laser device  51  shown in  FIG. 8 , an n-contact layer  53  composed of n-GaN having a thickness of 4.5 μm, an n-cladding layer  54  composed of n-Al a Ga 1-a N having a thickness of 1.0 μm, an MQW active layer  55 , and a p-first cladding layer  56   a  composed of p-Al b Ga 1-b N having a thickness of 0.1 μm are formed in this order on a sapphire substrate  52 . 
     The MQW active layer  55  has a multi quantum well structure constructed by alternately stacking three quantum well layers composed of In x Ga 1-x N having a thickness of 80 Å and four barrier layers composed of In y Ga 1-y N having a thickness of 160 Å, where x&gt;y. In the present embodiment, x=0.13 and y=0.05. 
     An n-current blocking layer  57  composed of n-Al c Ga 1-c N having a striped opening  58  is formed on the p-first cladding layer  56   a.    
     A p-second cladding layer  56   b  composed of p-Al c Ga 1-c N having a thickness of 0.8 μm is formed on the p-first cladding layer  56   a  inside the striped opening  58 . A p-contact layer  59  composed of p-GaN having a thickness of 0.05 μm is formed on the p-second cladding layer  56   b  and on the n-current blocking layer  57 . Here, 0≦a&lt;c and 0≦b&lt;c. In the present embodiment, a=0.07, b=0.07, and c=0.12. 
     The p-second cladding layer  56   b  comprises a lower layer  56   b   1  formed on the p-first cladding layer  56   a  and an upper layer  56   b   2  formed on the lower layer  56   b   1 . The lower layer  56   b   1  is rectangular in cross section, has a height t 2  which is not less than 0.5 μm, for example, and has an approximately constant width W 5  of 3.5 μm. On the other hand, the upper layer  56   b   2  is trapezoidal in cross section, and has a height (t 1 −t 2 ) of 0.3 μm, for example, where the length of its upper bottom is 3.5 μm, and its lower bottom is longer than the upper bottom. That is, the upper layer  56   b   2  has a width which gradually decreases upward from a width W 4  (a second width) which is not less than the width W 5  (a first width) of the lower layer  56   b   1 . 
     Si is used as an n-type dopant in each of the layers, and Mg is used as a p-type dopant in the layer. 
     A partial region from the p-contact layer  59  to the n-contact layer  53  is etched away, so that a surface of the n-contact layer  53  is exposed. A p electrode  60  is formed on the p-contact layer  59 , and an n electrode  61  is formed on the exposed surface of the n-contact layer  53 . 
     In the semiconductor laser device  51  according to the present embodiment, the Al composition ratio of the n-current blocking layer  57  is higher than the Al composition ratios of the p-first cladding layer  56   a  and the p-second cladding layer  56   b . Accordingly, the refractive index of the n-current blocking layer  57  is lower than the refractive indexes of the p-first cladding layer  56   a  and the p-second cladding layer  56   b . Consequently, an effective refractive index in a region, of the MQW active layer  55 , having the width W 5 , under the striped opening  58  is higher than an effective refractive index in a region of the MQW active layer  55  under the n-current blocking layer  57 . Accordingly, light is concentrated on the region having the width W 5  under the striped opening  58 . A semiconductor laser device  51  having a real refractive index guided structure in which an operating voltage is low and the width of a light emitting spot is small is thus realized. 
       FIGS. 9 ,  10  and  11  are schematic sectional views showing the steps of an example of a method of fabricating the GaN based semiconductor laser device  51  shown in FIG.  8 . 
     As shown in FIG.  9 ( a ), an n-contact layer  53 , an n-cladding layer  54 , an MQW active layer  55 , a p-first cladding layer  56   a , and an n-current blocking layer  57  are first continuously grown on a sapphire substrate  52  by MOCVD or the like. A first mask pattern  62  having a first striped opening  63  is formed on the n-current blocking layer  57 . In order to perform vertical etching in the subsequent steps. SiO 2  (silicon oxide), for example, which is not relatively difficult to etch is used for the first mask pattern  62 . 
     As shown in FIG.  9 ( b ), the n-current blocking layer  57  inside the first striped opening  63  of the first mask pattern  62  is then removed by dry etching such as RIBE (Reactive Ion Beam Etching) using Cl 2  (chlorine) or FIB (Focussed Ion Beam). Consequently, a striped opening  64  having an approximately vertical wall surface is then formed in the n-current blocking layer  57 . Thereafter, the first mask pattern  62  is removed. 
     As shown in FIG.  9 ( c ), a p-second cladding layer  56   b  is then grown on the p-first cladding layer  56   a  inside the striped opening  64  of the n-current blocking layer  57  and on the n-current blocking layer  57 . 
     As shown in FIG.  9 ( d ), a striped second mask pattern  66  is then formed of Ni (nickel) in a region on the p-second cladding layer  56   b  above the striped opening  64  of the n-current blocking layer  57 . 
     As shown in FIG.  10 ( e ), the p-second cladding layer  56   b , except in a region of the striped second mask pattern  66 , is then removed, to expose the n-current blocking layer  57 . At this time, when RIE using CCl 4  (carbon tetrachloride), for example, is performed, a peripheral part of the second mask pattern  66  composed of Ni is gradually etched, to narrow. Accordingly, a facet of the p-second cladding layer  56   b  is obliquely etched. 
     Furthermore, as shown in FIG.  10 ( f ), an n-current blocking layer  57  is selectively grown on the n-current blocking layer  57  and on a side surface of the p-second cladding layer  56   b  utilizing the striped second mask pattern  66  using the previous dry etching. The thickness of the n-current blocking layer  57  is made equal to the thickness of the p-second cladding layer  56   b . A dotted line drawn in the n-current blocking layer  57  indicates the boundary between a lower n-first current blocking layer and an upper n-second current blocking layer. 
     As shown in FIG.  10 ( g ), the striped second mask pattern  66  is then removed, and a p-contact layer  59  is then grown on the n-current blocking layer  57  and on the p-second cladding layer  56   b.    
     As shown in FIG.  10 ( h ), a third mask pattern  70  is formed in a predetermined region on the p-contact layer  59 . 
     As shown in FIG.  11 ( i ), a region from the p-contact layer  59  to the n-contact layer  53 , except in a region of the third mask pattern  70 , is then removed by dry etching, to expose a surface of the n-contact layer  53 . Thereafter, the third mask pattern  70  is removed. 
     Finally, as shown in FIG.  11 ( h ), a p electrode  60  is formed on the p-contact layer  59 , and an n electrode  61  is formed on the exposed surface of the n-contact layer  53 . 
     In the GaN based semiconductor laser device  51  according to the present embodiment, the p-second cladding layer  56   b  (a second semiconductor layer) comprises a lower layer  56   b   1  having an approximately constant width W 5  (a first width) in the thickness direction and an upper layer  56   b   2  having a width which gradually decreases upward from a width W 4  (a second width) which is not less than the width W 5 . 
     Therefore, the width W 5  of the lower layer  56   b   1  in the p-second cladding layer  56   b  is smaller than a width W 6  obtained at a lower end of the lower layer  56   b   1  upon gradually increasing downward from the width W 4 . Consequently, the width of a light emitting spot in a direction parallel to the active layer  55  is smaller, as compared with that in the conventional semiconductor laser device  201 . Accordingly, the aspect ratio of emitted laser light can be reduced. 
     On the other hand, the width W 3  of the upper layer  56   b   2  is increased irrespective of the width W 5  of the lower layer  56   b   1 , so that the widths W 3  and W 4  of the upper layer  56   b   2  can be increased. Accordingly, the resistance of the upper layer  56   b   2  can be decreased to decrease an operating voltage in the semiconductor laser device  51 . 
     Although in the first and second embodiments, the insulating sapphire substrates  2  and  52  are used, the sapphire substrates  2  and  52  may be respectively replaced with conductive substrates such as GaN substrates or SiC substrates. In the case, a mask pattern forming process and a dry etching process are not carried out after the p-contact layers  9  and  59  are grown, to form the p electrodes  10  and  60  on the p-contact layers  9  and  59  and form the n electrodes  11  and  61  on the conductive substrates, for example, the GaN substrates. 
     Furthermore, the n-current blocking layers  7  and  57  may be respectively formed of materials having smaller band-gaps than those of the active layers  5  and  55 . In this case, light emitted in the regions of the active layers  5  and  55  under the n-current blocking layers  7  and  57  is absorbed by the n-current blocking layers  7  and  57 . Accordingly, the light is concentrated on the active layers  5  and  55  respectively having the widths W 2  and W 5  under the striped openings  8  and  58  of the p-first cladding layers  6   a  and  56   a . Consequently, a semiconductor laser device having a loss guided structure is realized. 
     Also in this case, the insulating sapphire substrates  2  and  52  may be respectively replaced with conductive substrates such as GaN substrates or SiC substrates, to form the n electrodes  11  and  61  on the conductive substrates, for example, the GaN substrates, as in the semiconductor laser device having a real refractive index guided structure. 
     Although in the first and second embodiments, description was made of a case where the first cladding layers  6   a  and  56   a  and the second cladding layers  6   b  and  56   b  have the same composition, they may have different compositions. 
       FIG. 12  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a third embodiment of the present invention. Description is now made of a semiconductor laser device having the construction shown in FIG.  12  and having a real refractive index guided structure. 
     In a semiconductor laser device  71  shown in  FIG. 12 , an n-contact layer  73  composed of n-GaN having a thickness of 4.5 μm, an n-cladding layer  74  composed of n-Al a Ga 1-a N having a thickness of 1.0 μm, an MQW active layer  75 , and a p-first cladding layer  76   a  composed of p-Al b Ga 1-b N having a thickness of 0.15 μm are formed in this order on a sapphire substrate  72 . 
     The MQW active layer  75  has a multi quantum well structure constructed by alternately stacking three quantum well layers composed of In x Ga 1-x N having a thickness of 80 Å and four quantum barrier layers composed of In y Ga 1-y N having a thickness of 160 Å, where x&gt;y. In the present embodiment, x=0.13 and y=0.05. 
     An n-current blocking layer  77  having a thickness of 0.3 μm composed of n-Al c Ga 1-c N having a striped opening is formed on the p-first cladding layer  76   a . The striped opening of the n-current blocking layer  77  has a step on both its inner side surfaces. That is, the upper width of the striped opening of the n-current blocking layer  77  is stepwise larger than the lower width thereof. 
     The thickness t 1  of the whole of the n-current blocking layer  77  is 0.3 μm. A lower layer  77   a  in the n-current blocking layer  77  has a sufficient thickness t 2  to block a current, which is 0.1 μm in the present embodiment. 
     A p-second cladding layer  76   b  having a thickness of 0.3 μm composed of p-Al d Ga 1-d N is formed on the p-first cladding layer  76   a  inside the striped opening of the n-current blocking layer  77 . The width W 2  of a lower layer in the p-second cladding layer  76   b  is 2 μm, and the width W 1  of an upper layer in the p-second cladding layer  76   b  is 2.5 μm. A p-contact layer  79  having a thickness of 0.1 μm composed of p-GaN is formed on the p-second cladding layer  76   b  and on the current blocking layer  77 . 
     The p-first cladding layer  76   a  and the p-second cladding layer  76   b  are composed of the same material. Here, 0≦a&lt;c, 0≦b&lt;c, and 0≦d&lt;c, and a=0.07, b=0.07, c=0.12, and d=0.07. 
     Si is used as an n-type dopant in each of the layers, and Mg is used as a p-type dopant in the layer. The carrier concentrations of the p-first cladding layer  76   a  and the p-second cladding layer  76   b  are 1×10 17 ˜3×10 17 /cm 3 , and the carrier concentration of the p-contact layer  79  is 4×10 17 ˜8×10 17 /cm 3 . 
     A partial region from the p-contact layer  79  to the n-contact layer  73  is etched away, so that a surface of the n-contact layer  73  is exposed. A p electrode  80  is formed on the p-contact layer  79 , and an n electrode  81  is formed on the exposed surface of the n-contact layer  73 . 
     In the semiconductor laser device  71  according to the present embodiment, the Al composition ratio of the n-current blocking layer  77  is higher than the Al composition ratios of the p-first cladding layer  76   a  and the p-second cladding layer  76   b . Accordingly, the refractive index of the n-current blocking layer  77  is lower than the respective refractive indexes of the p-first cladding layer  76   a  and the p-second cladding layer  76   b . Consequently, an effective refractive index in a region of the MQW active layer  75  under the p-second cladding layer  76   b  is higher than an effective refractive index in a region of the MQW active layer  75  under the n-current blocking layer  77 . Accordingly, light is concentrated on the region of the MQW active layer  75  under the p-second cladding layer  76   b . A semiconductor laser device  71  having a real refractive index guided structure is thus realized. 
       FIGS. 13 and 14  are schematic sectional views showing the steps of an example of a method of fabricating the GaN based semiconductor laser device  71  shown in FIG.  12 . 
     As shown in FIG.  13 ( a ), an n-contact layer  73 , an n-cladding layer  74 , an MQW active layer  75 , and a p-first cladding layer  76   a  are first continuously grown on a sapphire substrate  72  by MOCVD or the like in a crystal growing device. The sapphire substrate  72  on which the above-mentioned layers  73 ,  74 ,  75  and  76   a  have been formed is taken out of the crystal growing device, to form on the p-first cladding layer  76   a  a first mask pattern  82  composed of SiO 2  (silicon oxide), for example, having a striped opening  83  along a &lt;1{overscore (1)}00&gt; direction of the p-first cladding layer  76   a.    
     As shown in FIG.  13 ( b ), the sapphire substrate  72  is then returned to the crystal growing device, to grow a p-second cladding layer  76   b . In this case, the substrate temperature is maintained at approximately 1000° C., thereby selectively growing the p-second cladding layer  76   b  only on the p-first cladding layer  76   a  inside the striped opening  83  and on the first mask pattern  82  in the periphery of the striped opening  83 . As a result, the p-second cladding layer  76   b  has a structure having a step, that is, a reverse two-step shaped stripe structure in which the width of an upper layer (a layer far from the MQW active layer  75 ) is larger than the width of a lower layer (a layer close to the MQW active layer  75 ). In this case, a side surface of the upper layer in the p-second cladding layer  76   b  is approximately perpendicular to the MQW active layer  75 . 
     The sapphire substrate  72  is taken out of the crystal growing device again, to chemically remove the first mask pattern  82  by hydrofluoric acid, for example. Thereafter, as shown in FIG.  13 ( c ), a second mask pattern  84  composed of SiO 2 , for example, is formed on an upper surface in the p-second cladding layer  76   b . The sapphire substrate  72  is then returned to the crystal growing device, to grow an n-current blocking layer  77 . In this case, the substrate temperature is maintained at approximately 1000° C. thereby selectively growing the n-current blocking layer  77  only on the p-first cladding layer  76   a  exposed, except in a region on the second mask pattern  84  formed on the n-second cladding layer  76   b.    
     Thereafter, the sapphire substrate  72  is taken out of the crystal growing device, to remove the second mask pattern  84 . The sapphire substrate  72  is then returned to the crystal growing device, to grow a p-contact layer  79  on the p-second cladding layer  76   b  and on the n-current blocking layer  77 , as shown in FIG.  14 ( d ). Further, the sapphire substrate  72  is taken out of the crystal growing device, to form a striped third mask pattern  85  composed of SiO 2 , for example, on the p-contact layer  79 . The third mask pattern  85  is arranged so as to cover a region above the p-second cladding layer  76   b.    
     As shown in FIG.  14 ( e ), a region from the p-contact layer  79  to the n-contact layer  73 , except in a region of the third mask pattern  85 , is removed by dry etching, to expose a surface of the n-contact layer  73 . Thereafter, the third mask pattern  85  is removed. 
     Finally, as shown in FIG.  14 ( f ), a p electrode  80  is formed on the p-contact layer  79 , and an n electrode  81  is formed on the exposed surface of the n-contact layer  73 . 
     According to the fabricating method shown in  FIGS. 13 and 14 , the striped p-second cladding layer  76   b  is formed by selective growth, so that the crystallizability on the side surface of the p-second cladding layer  76   b  is improved. Consequently, the state of the interface of the p-second cladding layer  76   b  and the n-current blocking layer  77  is improved, thereby making it possible to reduce an invalid current flowing through the interface. As a result, device characteristics are improved. 
     In the GaN based semiconductor laser device  71  according to the present embodiment, the width W 1  at an upper end of the n-current blocking layer  77  is larger than the width W 2  at a lower end thereof. That is, the width W 1  at an upper end of the p-second cladding layer  76   b  is larger than the width W 2  at a lower end thereof. Consequently, the resistance of the upper layer in the p-second cladding layer  76   b  is decreased. Accordingly, an operating voltage in the semiconductor laser device  71  is reduced. 
       FIG. 15  is a diagram showing an effective refractive index distribution in the MQW active layer  75  in the semiconductor laser device  71  shown in FIG.  12 . 
     As shown in  FIG. 15 , the width W 1  of the upper layer in the p-second cladding layer  76   b  is larger than the width W 2  of the lower layer in the p-second cladding layer  76   b . Accordingly, an effective refractive index is the highest in a region having the width W 2  at the center of the MQW active layer  75 , takes an intermediate value in a region having the width W 1  larger than the width W 2  of the MQW active layer  75 , and is the lowest on both sides of the region having the width W 1  of the MQW active layer  75 . The effective refractive index distribution in the MQW active layer  75  thus has a stepped shape. 
     In this case, the width of a light emitting spot is W 1 , and the width of a current injection region is W 2 . Consequently, both sides of the current injection region function as a saturable light absorbing member, so that self-sustained pulsation occurs. As a result, a semiconductor laser device  71  having low-noise characteristics is realized. 
       FIG. 16  is a schematic sectional view showing the construction of a GaN based semiconductor laser device in a fourth embodiment of the present invention. 
     In a semiconductor laser device  71 A shown in  FIG. 16 , a p-second cladding layer  76   b  comprises a lower layer having an approximately constant width W 5  in the thickness direction and an upper layer having a width which gradually decreases upward from a width W 4  larger than the width W 5 . 
     The width W 5  of the lower layer in the p-second cladding layer  76   b  is 2 μm, and the width W 4  at a lower end of the upper layer in the p-second cladding layer  76   b  is 2.5 μm. Further, the width W 3  at an upper end of the upper layer in the p-second cladding layer  76   b  is 2.3 μm. The construction of the other portions of the semiconductor laser device  71 A shown in  FIG. 16  is the same as the construction of the semiconductor laser device  71  shown in FIG.  12 . 
     In the GaN based semiconductor laser device  71 A according to the present embodiment, the width W 3  at the upper end of the upper layer in the p-second cladding layer  76   b  can be also increased by increasing the width W 4  at the lower end of the upper layer in the p-second cladding layer  76   b . Consequently, the resistance of the upper layer in the p-second cladding layer  76   b  is decreased. Accordingly, an operating voltage in the semiconductor laser device  71 A is reduced. 
     The width W 4  at the lower end of the upper layer in the p-second cladding layer  76   b  is larger than the width W 5  at the lower end of the lower layer in the p-second cladding layer  76   b . Accordingly, an effective refractive index is the highest in a region having the width W 5  at the center of an MQW active layer  75 , takes an intermediate value in a region having the width W 4  larger than the width W 5  of the MQW active layer  75 , and is the lowest on both sides of the region having the width W 4  of the MQW active layer  75 . 
     In this case, the width of a light emitting spot is W 4 , and the width of a current injection region is W 5 . Consequently, both sides of the current injection region function as a saturable light absorbing member, so that self-sustained pulsation occurs. As a result, a semiconductor laser device  71 A having low-noise characteristics is realized. 
     In fabricating the semiconductor laser device  71 A according to the present embodiment, the striped opening  83  of the first mask pattern  82  is formed along a &lt;11{overscore (2)}0&gt; direction of the p-second cladding layer  76   a  at the step shown in FIG.  13 ( a ), whereby both side surfaces of the p-second cladding layer  76   b  can be inclined by approximately 62° to the MQW active layer  75  at the step shown in FIG.  13 ( b ). The other steps of fabricating the semiconductor laser device  71 A shown in  FIG. 16  are the same as those shown in  FIGS. 13 and 14 . 
       FIGS. 17 ,  18 ,  19 ,  20 , and  21  are diagrams showing examples of band-gap energies in a cladding layer of a first conductivity type, an active layer, a cladding layer of a second conductivity type, and a contact layer of a second conductivity type in the semiconductor laser device according to the present invention. 
     In the example shown in  FIG. 17 , the band-gap of the cladding layer of a second conductivity type composed of AlGaN is larger than the band-gap of the active layer and the band-gap of the contact layer of a second conductivity type composed of GaN, and the band-gap of the contact layer of a second conductivity type is larger than the band-gap of the active layer. 
     In the example shown in  FIG. 18 , the band-gap of the cladding layer of a second conductivity type composed of AlGaN is larger than the band-gap of the active layer, and the band-gap of the contact layer of a second conductivity type composed of AlGaN is larger than the band-gap of the cladding layer of a second conductivity type. 
     In the example shown in  FIG. 19 , the band-gap of the cladding layer of a second conductivity type composed of AlGaN is larger than the band-gap of the active layer, and the band-gap of the contact layer of a second conductivity type composed of AlGaN is equal to the band-gap of the cladding layer of a second conductivity type. 
     In the example shown in  FIG. 20 , the band-gap of the cladding layer of a second conductivity type composed of AlGaN is larger than the band-gap of the active layer and the band-gap of the contact layer of a second conductivity type composed of InGaN, and the band-gap of the contact layer of a second conductivity type is larger than the band-gap of the active layer. 
     In the example shown in  FIG. 21 , the band-gap of the cladding layer of a second conductivity type composed of AlGaN is larger than the band-gap of the active layer and the band-gap of the contact layer of a second conductivity type composed of InGaN, and the band-gap of the contact layer of a second conductivity type is smaller than the band-gap of the active layer. 
     In the examples shown in  FIGS. 17  to  21 , the carrier concentration of the contact layer of a second conductivity type is set to not less than the carrier concentration of the cladding layer of a second conductivity type. 
     Particularly in the example shown in  FIG. 17 , the contact layer of a second conductivity type is formed of GaN, thereby making it possible to increase the carrier concentration thereof. Consequently, good ohmic contact can be obtained between the contact layer of a second conductivity type and an electrode. As a result, an operating voltage in the semiconductor laser device is reduced, thereby making it possible to prevent the semiconductor laser device from generating heat. As a result, it is possible to reduce a threshold current and improve reliability. 
     Furthermore, the band-gap of the contact layer of a second conductivity type is larger than the band-gap of the active layer. Accordingly, the light emission efficiency is prevented from being decreased by the absorption of light in the contact layer of a second conductivity type. 
     The material of each of the layers in the semiconductor laser device according to the present invention is not limited to the above-mentioned materials in the embodiments. It is possible to use various types of nitride based semiconductors containing at least one of B, Tl, Ga, Al, and In. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.