Patent Publication Number: US-8530255-B2

Title: Method of manufacturing semiconductor laser, semiconductor laser, optical pickup, optical disk device, method of manufacturing semiconductor device, semiconductor device, and method of growing nitride type group III-V compound semiconductor layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/038,329, filed Feb. 27, 2008, the entirety of which is incorporated herein by reference to the extent permitted by law. The present application claims priority to Japanese Patent Application Nos. 2007-282714 filed with the Japan Patent Office on Oct. 31, 2007 and 2007-050461 filed in the Japanese Patent Office on Feb. 28, 2007 the entireties of which also are incorporated by reference herein to the extent permitted by law. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of manufacturing a semiconductor laser, a semiconductor laser, an optical pickup, an optical disk device, a method of manufacturing a semiconductor device, and a method of growing a nitride type Group III-V compound semiconductor layer, and is preferable when applied, for example, to a ridge stripe type semiconductor laser having an end face window structure using a nitride type Group III-V compound semiconductor, and an optical pickup and an optical disk device which use the semiconductor laser as or in a light source. 
     In order to increase the maximum optical output of a semiconductor laser, it may inevitably be necessary to introduce an end face window structure in which an end face of a resonator is provided with a window transparent to the light coming from an active layer. 
     In a GaInP red light emitting semiconductor laser according to the related art, a method has been effective in which after the growth of a semiconductor layer forming a laser structure, Zn atoms are diffused into the semiconductor layer in the vicinity of a part to be a resonator end face so as to locally increase the band gap energy, thereby forming an end face window structure (refer to, for example, Japanese Patent Laid-open No. 2005-45009). 
     On the other hand, in recent years, semiconductor lasers based on a nitride type Group III-V compound semiconductor have been used as light sources in high-density optical disk devices and the like. Most of the nitride type Group III-V compound semiconductors are materials which are thermally and mechanically stabler than GaInP semiconductors. Therefore, in the semiconductor laser based on a nitride type Group III-V compound semiconductor, it is difficult to achieve formation of an end face window structure by diffusion of different kinds of atoms and wet etching, which have been effective in the case of the GaInP red light emitting semiconductor laser. 
     In view of this, with regard to the semiconductor lasers based on a nitride type Group III-V compound semiconductor, a variety of methods for forming an end face window structure have been proposed and put to experiment. Now, methods of forming an end face window structure which have been proposed will be described as follows. 
     It has been proposed to form an end face window structure through increasing the band gap energy in the vicinity of an end face of a resonator by utilizing an In elimination process caused by irradiation with laser light or exposure to a H 2  plasma after the formation of a laser bar by cleavage (refer to, for example, Japanese Patent Laid-open No. 2006-147814 and Japanese Patent Laid-open No. 2006-147815). However, for carrying out these methods, a high-vacuum chamber equipment may be needed, leading to a large-scale plant and equipment investment. Besides, processing the resonator end face after cleavage will generally leave a problem as to productivity. 
     Many proposals have been made regarding a method in which after a semiconductor layer for forming a laser structure is epitaxially grown on a substrate, a part of the semiconductor layer which is to be a resonator end face is dug by reactive ion etching (RIE), and a nitride type Group III-V compound semiconductor layer with a high band gap energy is again epitaxially grown in the dug area (refer to, for example, Japanese Patent Laid-open No. 2004-134555, Japanese Patent Laid-open No. 2003-60298, International Publication No. 03/036771 pamphlet, and Japanese Patent Laid-open No. 2002-204036). According to this method, however, a surface level would be formed at the surface dug by RIE, leading to the fear that light absorption and local heat generation may occur at the time of laser operation. 
     As another example, a method has been proposed in which a semiconductor layer for forming a laser structure is epitaxially grown on a substrate provided with a geometric step by RIE or insulating film deposition, whereby an end face window structure is formed (refer to, for example, Japanese Patent Laid-open No. 2005-191588, Japanese Patent Laid-open No. 2005-294394, Japanese Patent Laid-open No. 2003-198057, and Japanese Patent Laid-open No. 2000-196188). This method aims at a phenomenon in which a clad layer higher in band gap energy than an active layer functions as an end face window structure, in the traveling direction of laser light. 
     A typical example of this is shown in  FIG. 49 . As shown in  FIG. 49 , in this semiconductor laser, one principal surface of a substrate  101  is patterned by RIE to provide a recess  101   a , then an n-type semiconductor layer  102 , an active layer  103  and a p-type semiconductor layer  104  are sequentially grown over the recess  101   a , and thereafter a p-side electrode  105 , an isolation electrode  106  and a pad electrode  107  are formed over the p-type semiconductor layer  104 . In other words, steep geometric steps are generated in the n-type semiconductor layer  102 , the active layer  103  and the p-type semiconductor layer  104  due to the presence of the recess  101   a  in the substrate  101 , so that an optical waveguide loss would be generated in the vicinity of the steps. Besides, transparency acquired by gap widening in the active layer  103  in the vicinity of the resonator end face is not intended and, therefore, the semiconductor structure may fail to function as an effective end face structure. 
     SUMMARY OF THE INVENTION 
     As above-mentioned, the methods for forming the end face window structure in a semiconductor laser based on a nitride type Group III-V compound semiconductor in the past had many problems. 
     Thus, there is a need for a semiconductor laser using a nitride type Group III-V compound semiconductor, and a manufacturing method for the semiconductor laser, such that an end face window structure can be formed extremely easily, the optical waveguide loss can be suppressed, and light absorption and local heat generation at the time of laser operation due to the presence of a surface level can be restrained. 
     There is also a need for an optical pickup and an optical disk device which use the above-mentioned excellent semiconductor laser as or in a light source. 
     Furthermore, there is a need for a method of growing a nitride type Group III-V compound semiconductor layer by which it is possible to easily grow a nitride type Group III-V compound semiconductor containing at least In and Ga and having a part where band gap energy varies in at least one direction, and a semiconductor device and a manufacturing method therefor in which the growing method is utilized. 
     The present inventors made intensive and extensive studies for solving the above-mentioned problems. As a result of the studies, the present inventors have found out that in the case of growing a nitride type Group III-V compound semiconductor layer containing at least In and Ga, such as an InGaN layer, the band gap energy of a desired part of the nitride type Group III-V compound semiconductor layer can be controlled by selecting the width, spacing, shape, position and the like of portions of an insulating film mask, and they have come to make the present invention. The findings made by the present inventors themselves will be described as follows. 
     The following basic investigating experiments were made. 
     As shown in  FIGS. 1A and 1B , two SiO 2  film masks  2  having a stripe shape with a width w were formed on an n-type GaN substrate  1 , in parallel to each other with a spacing d therebetween. Here,  FIG. 1A  is a plan view, and  FIG. 1B  is a sectional view taken along line B-B of FIG.  1 A. Then, as shown in  FIGS. 2A and 2B , a GaN semiconductor layer  3  including an n-type AlGaN clad layer  3   a , an n-type GaN optical waveguide layer  3   b , an active layer  3   c  having an undoped Ga 1-x In x N (quantum well layer)/Ga 1-y In y N (barrier layer, x&gt;y) multiple quantum well structure, and an undoped InGaN optical waveguide layer  3   d , of layers forming a laser structure of a GaN semiconductor laser, was epitaxially grown over the n-type GaN substrate  1  provided with the SiO 2  film masks  2 . Here, the growth temperatures of the n-type AlGaN clad layer  3   a  and the n-type GaN optical waveguide layer  3   b  which are In-free layers were set in the range of 900 to 1100° C. for example; on the other hand, the growth temperatures of the active layer  3   c  having the Ga 1-x In x N/Ga 1-y In y N multiple quantum well structure and the undoped InGaN optical waveguide layer  3   d  which are In-containing layers were set in the range of 700 to 800° C., for example. In this case, the GaN semiconductor layer  3  is not substantially grown on the SiO 2  film masks  2 , and is grown only on the part, not covered with the SiO 2  film masks  2 , of the n-type GaN substrate  1 . 
     A specimen thus produced was irradiated with excitation light (hν), and the peak energy of the light emitted from the active layer  3   c  was evaluated by a microphotoluminescence method (see  FIG. 2B ). As a result, fundamental data on the dependency of the peak energy of emission from the active layer  3   c  on the width w and spacing d of the SiO 2  film masks  2  could be obtained. The measurement results are shown in  FIGS. 3 and 4 . 
     In the graph shown in  FIG. 3 , Δλ b  taken on the axis of ordinates is defined as follows. The wavelength corresponding to the peak energy of emission from the active layer  3   c  at a flat portion of the GaN semiconductor layer  3  formed at a position sufficiently far from the SiO 2  film mask  2  is represented by λ 1 . In this case, as one goes away from the SiO 2  film mask  2 , the wavelength corresponding to the emission peak energy is once shifted to the shorter wavelength side, and is again shifted to the longer wavelength side. The shortest wavelength corresponding to a maximum value of the emission peak energy is represented by λ min . In this instance, a definition of Δλ b =λ min −λ 1  is adopted. 
       FIG. 3  shows the variation in Δλ b  with variation in the spacing d, with the width w of the SiO 2  film masks  2  being kept constant. The width w was set at each of three levels of 5, 30, and 50 μm. As seen from  FIG. 3 , in general, Δλ b  tends to increase in the minus direction as the spacing d is larger and as the width w is larger. For example, where the width w was 5 μm and the spacing d was 10 μm, a Δλ b  value of about −9 nm was obtained. The Δλ b  value of about −9 nm corresponds to an increase of about 80 meV in band gap energy. This variation in the band gap energy is sufficient as a value of the end face window structure. 
     In the graph shown in  FIG. 4 , Δλ c  taken on the axis of ordinates is defined as follows. The wavelength corresponding to the peak energy of emission from a central part of the active layer  3   c  of a GaN semiconductor layer  3  grown in an area between the SiO 2  film masks  2  is represented by λ 2 . In this case, a definition of Δλ c =λ 2 −λ 1  is adopted. 
       FIG. 4  shows the variation in Δλ c  with variation in the spacing d, with the width w of the SiO 2  film mask  2  being kept constant. The width w was set to each of three levels of 5, 30 and 50 μm. As seen from  FIG. 4 , Δλ c  is shifted in the minus direction in the case where the width w is not less than 30 μm; where the width w is 5 μm, the Δλ c  tends to be shifted in the plus direction when the spacing d is not more than 5 μm but to be shifted in the minus direction when the spacing d is 10 to 50 μm. For example, a Δλ c  value of about +5 nm was obtained where the width w was 5 μm and the spacing d was 3 μm, and a Δλ c  value of about −5 nm was obtained where the width w was 5 μm and the spacing d was 20 μm. 
     It is seen from the data shown in  FIG. 3  that shift of the emission wavelength to the shorter wavelength side (increase in band gap energy of the active layer  3   c ) can be expected when only a single SiO 2  film mask  2  is used. Further, as seen from  FIG. 4 , surprisingly, it is possible to achieve Δλ c &gt;0, namely, to shift the emission wavelength to the longer wavelength side (decrease in band gap energy of the active layer  3   c ). From these it is understood that the band gap energy of the active layer  3   c  can be freely varied by arbitrary designing of the pattern of the SiO 2  film mask  2 . 
     The present inventors came to a conclusion that the reason why the band gap energy of the active layer  3   c  can be varied according to the portion of the GaN semiconductor layer  3  in the case where the GaN semiconductor layer  3  is epitaxially grown by use of the SiO 2  film mask  2  as above-mentioned lies in that the In diffusion length is very small as compared with the Ga diffusion length. Now, this reasoning will be described. 
     As shown in  FIGS. 2A and 2B , in the case where the active layer  3   c  of the GaN semiconductor layer  3  is grown over the part, not covered with the SiO 2  film mask  2 , of the n-type GaN substrate  1 , not only In and Ga are supplied to the part directly from the growth material sources but also In and Ga are supplied to the part through a diffusion process in which In and Ga supplied onto the SiO 2  film mask  2  are diffused over the SiO 2  film mask  2 . 
       FIGS. 5A ,  5 B and  5 C show variations in the concentrations of Ga and In diffused from an edge of the SiO 2  mask  2  formed on the n-type GaN substrate  1  toward the outside and variation in In content of the active layer  3   c , plotted against the distance measured from the edge along the direction orthogonal to the SiO 2  film mask  2 . As shown in  FIGS. 5A and 5B , where the In diffusion length is very small as compared with the Ga diffusion length, the In concentration becomes constant starting from a short distance ΔX 1 , whereas the Ga concentration becomes constant starting from a long distance ΔX 2 . Reflecting these, as shown in  FIG. 5C , the In content of the active layer  3   c  decreases to the distance ΔX 1 , to once take a minimum value, and then increases again, to become constant starting from the distance ΔX 2 . The distances ΔX 1  and ΔX 2  increase respectively with increases in the concentrations of Ga and In being diffused. 
       FIG. 6  shows the results of measurement of variations in ΔX 1  and ΔX 2  with the width w in the case where the spacing d between the SiO 2  film masks  2  was fixed to 5 μm. Besides,  FIG. 7  shows the results of measurement of variations in ΔX 1  and ΔX 2  with the spacing d in the case where the width w of the SiO 2  film masks  2  was fixed to 5 μm. It is seen from  FIGS. 6 and 7  that where the width w is 3 to 5 μm, at the growth temperature of the active layer  3   c , the maximum Ga diffusion length is about 20 μm, whereas the maximum In diffusion length is no more than about 3 μm, which is smaller than the maximum Ga diffusion length by a factor of about one order of magnitude. From this it is considered that in the case where the width w is 3 to 5 μm, even when the spacing d is enlarged to about 40 μm, it is possible to reduce the In content of the active layer  3   c  in a central area between the SiO 2  film masks  2  and to enlarge the band gap energy thereof. 
     While the case where the SiO 2  film mask  2  is used has been described above, the same control of the In content and band gap energy of the active layer  3   c  as above can be achieved even with the use of a mask formed of other insulating film such as a SiN film and an Al 2 O 3  film. In addition, the In content and band gap energy can be similarly controlled, not only for the active layer but also for any nitride type Group III-V compound semiconductor layer that contains In and Ga. 
     As a result of further investigations made by the present inventors based on the above-mentioned studies, the present invention has been completed. 
     According to a first embodiment of the present invention, there is provided a method of manufacturing a semiconductor laser. The method has an end face window structure, by growing over a substrate a nitride type Group III-V compound semiconductor layer including an active layer including a nitride type Group III-V compound semiconductor containing at least In and Ga. The method includes the steps of: forming a mask and growing the nitride type Group III-V compound semiconductor layer. The forming mask step includes an insulating film over the substrate, at least in the vicinity of the position of forming the end face window structure. The growing the nitride type Group III-V compound semiconductor layer step includes the active layer over a part, not covered with the mask, of the substrate. 
     According to a second embodiment of the present invention, there is provided a semiconductor laser having an end face window structure which has, over a substrate, a nitride type Group III-V compound semiconductor layer including an active layer including a nitride type Group III-V compound semiconductor containing at least In and Ga. A mask including an insulating film is formed over the substrate, at least in the vicinity of a part corresponding to the end face window structure. The nitride type Group III-V compound semiconductor layer including the active layer is formed over a part, not covered with the mask, of the substrate. 
     According to a third embodiment of the present invention, there is provided an optical pickup using a semiconductor laser as or in a light source. The semiconductor laser has an end face window structure having, on a substrate, a nitride type Group III-V compound semiconductor layer including an active layer including a nitride type Group III-V compound semiconductor containing at least In and Ga. A mask including an insulating film is formed over the substrate, at least in the vicinity of a part corresponding to the end face window structure. The nitride type Group III-V compound semiconductor layer including the active layer is formed over a part, not covered with the mask, of the substrate 
     According to a fourth embodiment of the present invention, there is provided an optical disk device using a semiconductor laser as or in a light source. The semiconductor laser has an end face window structure having, on a substrate, a nitride type Group III-V compound semiconductor layer including an active layer including a nitride type Group III-V compound semiconductor containing at least In and Ga. A mask including an insulating film is formed over the substrate, at least in the vicinity of a part corresponding to the end face window structure. The nitride type Group III-V compound semiconductor layer including the active layer is formed over a part, not covered with the mask, of the substrate. 
     In the first to fourth embodiments of the present invention, the width, spacing, shape, position and the like of the mask are appropriately determined according to the characteristics demanded of a semiconductor laser and the like factors, based on at least the above-mentioned findings made by the present inventors. The mask can be formed from any of various insulating films such as SiO 2  film, SiN film and Al 2 O 3  film. In an example of formation of the mask, a mask is formed over a substrate in the vicinity of the position of forming an end face window structure and on either one or both sides of the position of forming a laser stripe. The plan-view shape of the mask may be, but is not limited to, a trapezoid, a rectangle or the like. Alternatively, a configuration may be adopted in which a mask is formed over a substrate on one side of the position of forming a laser stripe along the position of forming the laser stripe, in such a manner that the spacing between the position of forming the laser stripe and the mask will be smaller, or larger, in the vicinity of the position of forming an end face window structure than in other areas. Or, a configuration may be adopted in which masks are formed over a substrate on both sides of the position of forming a laser stripe along the position of forming the laser stripe, in such a manner that the spacing between the masks on both sides of the position of forming the laser stripe will be larger in the vicinity of the position of forming an end face window structure than in other areas. In general, the width W 1  of the masks is selected to be smaller than the spacing W 2  between the masks, but this configuration is not limitative. In the case of forming the masks on both sides of the position of forming the laser stripe, generally, the relationships among the mask width W 3  and the mask spacing W 4  in the vicinity of the center of the resonator and the mask width W 5  and the mask spacing W 6  in the vicinity of the position of forming the end face window structure are so set as to satisfy W 3 &lt;W 4  and W 5 &lt;W 6 , but this design is not limitative. When a nitride type Group III-V compound semiconductor layer including an active layer is grown by use of these masks, the relationship between the In content x (or emission wavelength λ) of the active layer of the laser stripe in an area between the masks or in the vicinity of the masks and the In content y (emission wavelength λ′) of the active layer of the laser stripe in a mask-free area can be so set as to satisfy x&lt;y (λ&lt;λ′). In addition, the relationship between the thickness t 1  of the laser stripe in an area between the masks and the thickness t 2  of the laser stripe in a mask-free area can be so set as to satisfy t 2 &lt;t 1 . 
     The nitride type Group III-V compound semiconductor includes most generally Al x B y Ga 1-x-y-z In z As u N 1-u-v P v  (where 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦u≦1, 0≦v≦1, 0≦x+y+z&lt;1, 0≦u+v&lt;1), specifically Al x B y Ga 1-x-y-z In z N (where 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦x+y+z&lt;1), typically Al x Ga 1-x-z In z N (where 0≦x≦1, 0≦z≦1), and specific non-limitative examples thereof include GaN, InN, AlN, AlGaN, InGaN, and AlGaInN. The nitride type Group III-V compound semiconductor containing at least In and Ga includes most generally Al x B y Ga 1-x-y-z In z As u N 1-u-v P v  (where 0≦x≦1, 0≦y≦1, 0≦z≦1, 0≦u≦1, 0≦v≦1, 0≦x+y+z&lt;1, 0≦u+v&lt;1), typically Al x Ga 1-x-z In z N (where 0≦x≦1, 0≦z≦1), and specific non-limitative examples thereof include InGaN, and AlGaInN. The nitride type Group III-V compound semiconductor layer can typically be grown by various epitaxial growth methods such as metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy or halide vapor phase epitaxy (HVPE), and molecular beam epitaxy (MBE), which are not limitative. As the substrate, a conductive semiconductor substrate, particularly a nitride type Group III-V compound semiconductor substrate (most typically a GaN substrate) is preferably used. However, an insulating substrate such as a sapphire substrate may also be used; further, one of these substrates which has grown thereon at least one nitride type Group III-V compound semiconductor layer may also be used. 
     Preferably, after the nitride type Group III-V compound semiconductor layer including the active layer is grown over the part, not covered with the mask, of the substrate, a step is provided in which at least a part of a recess (groove) formed on the upper side of the mask through the growing of the nitride type Group III-V compound semiconductor layer, preferably a most part of the recess (groove), is filled with an insulating material, whereby the steps (differences in level) due to the presence of the recess is moderated. Most preferably, the recess is entirely filled up with the insulating material so as to eliminate the steps due to the recess and to obtain a flat surface. The moderation or elimination of the steps due to the recess ensures that, in the case of forming the insulating film (for example, the insulating film for current constriction which is formed in the areas inclusive of both sides of a ridge formed at an upper part of the nitride type Group III-V compound semiconductor layer so as to be a laser stripe) or an electrode or the like in a later step, the component to be thus formed can be formed favorably, without generating a step-induced interruption or the like. The insulating material may basically be any insulating material and is not particularly limited. Examples of the insulating material include application type insulating materials such as spin on glass (SOG), etc., organic materials such as polyimide, etc., oxides such as SiO 2 , Al 2 O 3 , etc., and nitrides such as SiN. The insulating material is preferably one that does not contain siloxane. Examples of such an application type insulating material as this include a phosphorus-doped silicate inorganic SOG. 
     The optical disk device includes those for reproduction (reading) only, those for recording (writing) only, and those applicable to both reproduction and recording. Besides, the reproduction and/or recording system is not particularly limited. The optical pickup is one that is suitable for use in such an optical disk device as this. 
     According to a fifth embodiment of the present invention, there is provided a method of manufacturing a semiconductor device by growing over a substrate a nitride type Group III-V compound semiconductor layer containing at least In and Ga, the semiconductor layer having a part where band gap energy varies in at least one direction along a surface of the substrate, the method including the steps of: forming a mask including an insulating film over the substrate in the vicinity of the part where band gap energy varies; and growing the nitride type Group III-V compound semiconductor layer over a part, not covered with the mask, of the substrate. 
     According to a sixth embodiment of the present invention, there is provided a semiconductor device having a nitride type Group III-V compound semiconductor layer containing at least In and Ga, the semiconductor layer having a part where band gap energy varies in at least one direction along a surface of the substrate. A mask including an insulating film is formed over the substrate in the vicinity of the part where band gap energy varies. The nitride type Group III-V compound semiconductor layer is formed over a part, not covered with the mask, of the substrate. 
     In the fifth and sixth embodiments of present invention, the semiconductor device includes not only semiconductor light emitting devices such as semiconductor lasers and light emitting diodes but also other various semiconductor devices such as FETs and electron transit devices, and the configuration of the nitride type Group III-V compound semiconductor layer is appropriately designed according to the relevant one of these devices. 
     The semiconductor laser may be a vertical cavity surface emitting laser (VCSEL). For example, in the case of manufacturing a surface emitting semiconductor laser of the structure in which an active layer including a nitride type Group III-V compound semiconductor containing at least In and Ga is provided between a first reflective layer and a second reflective layer, the active layer may be grown by a method in which a ask including an insulating film having a circular opening, for example, is preliminarily formed on a surface of a layer under the active layer and then the active layer is grown thereon, whereby a configuration can be obtained in which the In content and the refractive index in the portion in the vicinity of an edge of the circular opening are gradually reduced as one goes away from the edge and then the In content and the refractive index are gradually enhanced as one goes toward a central portion. Therefore, in this surface emitting semiconductor laser, a reduction in operating current can be promised, since light is easily concentrated into a central area of the circular opening in the mask including the insulating film at the time of operation. As each of the first reflective layer and the second reflective layer, a distributed Bragg reflector (DBR) is normally used. 
     Alternatively, in the case of picking up output light through, for example, the second reflective layer in the above-mentioned surface emitting semiconductor laser, the nitride type Group III-V compound semiconductor layer containing at least In and Ga may be used for a light outgoing part of the second reflective layer, whereby a lens part having a desired refractive index distribution can be formed in the nitride type Group III-V compound semiconductor layer. Specifically, the nitride type Group III-V compound semiconductor layer may be grown by a method in which a mask including an insulating film having a circular opening, for example, is preliminarily formed on a surface of a layer under the nitride type Group III-V compound semiconductor layer and the nitride type Group III-V compound semiconductor layer is grown thereon, whereby a configuration can be obtained in which the In content and the refractive index in a portion in the vicinity of an edge of the circular opening are gradually reduced as one goes away from the edge and then the In content and the refractive index are gradually enhanced as one goes toward a central portion, in the same manner as in the foregoing. As a result, a circular convex lens can be formed in the inside of the opening in the mask. When the In content of the nitride type Group III-V compound semiconductor layer constituting the convex lens is set to be lower than the In content of the active layer, the light emitted from the active layer can be prevented from being absorbed by the convex lens. 
     In the fifth and sixth embodiments of the present invention, as for the other items than the just-mentioned, the conditions as described above in relation to the first to fourth embodiments of the present invention are established unless they are against the desired properties. 
     According to a seventh embodiment of the present invention, there is provided a method of growing a nitride type Group III-V compound semiconductor laser containing at least In and Ga over a substrate, the semiconductor layer having a part where band gap energy varies in at least one direction along a surface of the substrate, the method including the steps of: forming a mask including an insulating film over the substrate in the vicinity of the part where band gap energy varies; and growing the nitride type Group III-V compound semiconductor layer over a part, not covered with the mask, of the substrate. 
     This method of growing a nitride type Group III-V compound semiconductor layer can be applied generally to the cases in which a part where band gap energy varies is formed in a nitride type Group III-V compound semiconductor layer containing at least In and Ga. For example, the method can be applied not only to production of such semiconductor devices as semiconductor lasers and light emitting diodes but also to production of optical component parts such as the above-mentioned convex lens and, further, to production of photonic crystals and the like. 
     In the seventh embodiment of the present invention, as for other items than the just-mentioned, the conditions as described above in relation to the first to sixth embodiments of the present invention are satisfied unless they are against the desired properties. 
     In the first to fourth embodiments of the present invention which are configured as above-described, when the mask including an insulating film is formed over the substrate at least in the vicinity of the position of forming the end face window structure and the active layer is grown over a part, not covered with the mask, of the substrate, it is ensured that the In content of the active layer in the part forming the end face window structure is lower than that in the other part, since the In diffusion length is extremely small as compared with the Ga diffusion length. In this case, formation of a recess in the substrate is not needed to form the end face window structure, and generation of a steep step in the nitride type Group III-V compound semiconductor layer including the active layer can be obviated by appropriately selecting the shape of the mask, so that the optical waveguide loss can be suppressed. In addition, since digging of the semiconductor layer in the portion for forming the end face window structure by RIE is not needed, a surface level is not formed, and it is possible to prevent light absorption or local heat generation from occurring at the time of laser operation. Furthermore, when at least a part of the recess formed on the upper side of the mask through the growth of the nitride type Group III-V compound semiconductor layer is filled up with the insulating material, the steps (differences in level) due to the recess can be moderated, so that in the case of forming an insulating film or an electrode or the like in a later step, the component to be formed can be favorably formed, without generating a step-induced interruption or the like. 
     In the fifth to seventh embodiments of the present invention configured as above, the mask including an insulating film is formed over the substrate in the vicinity of the part where band gap energy varies, and the nitride type Group III-V compound semiconductor layer is grown on the part, not covered with the mask, of the substrate, whereon the In content in the nitride type Group III-V compound semiconductor layer in the part in the vicinity of the mask is varied and the band gap energy is thereby varied, since the In diffusion length is extremely small as compared with the Ga diffusion length. Furthermore, when at least a part of the recess formed on the upper side of the mask through the growth of the nitride type Group III-V compound semiconductor layer is filled up with the insulating material, the steps (differences in level) due to the recess can be moderated, so that in the case of forming an insulating film or an electrode or the like in a later step, the component to be formed can be favorably formed, without generating a step-induced interruption or the like. 
     According to the present embodiments, it is possible to realize a semiconductor laser using a nitride type Group III-V compound semiconductor wherein an end face window structure can be formed extremely easily, the optical waveguide loss can be suppressed, and it is possible to prevent light absorption or local heat generation from occurring at the time of laser operation. With the excellent semiconductor laser used as or in a light source in an optical pickup, a high-performance optical disk device can be realized. 
     In addition, according to the present embodiments, it is possible to extremely easily grow a nitride type Group III-V compound semiconductor layer containing at least In and Ga and having a part where band gap energy varies in at least one direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are respectively a plan view and a sectional view which illustrate a specimen used in fundamental investigations made by the present inventors; 
         FIGS. 2A and 2B  are sectional views illustrating the specimen used in the fundamental investigations made by the present inventors; 
         FIG. 3  is a schematic diagram showing the variation in emission wavelength with variations in the width and spacing of SiO 2  film masks, in the specimen used in the fundamental investigations made by the present inventors; 
         FIG. 4  is another schematic diagram showing the variation in emission wavelength with variations in the width and spacing of the SiO 2  film masks, in the specimen used in the fundamental investigations made by the present inventors; 
         FIGS. 5A ,  5 B and  5 C are schematic diagrams showing distributions of Ga concentration, In concentration and In content when an InGaN layer is grown over the specimen used in the fundamental investigations made by the present inventors; 
         FIG. 6  is a schematic diagram showing the variations in ΔX 1  and ΔX 2  with variation in the width of the SiO 2  film masks, with the spacing between the SiO 2  film masks being kept constant, in the specimen used in the fundamental investigations made by the present inventors; 
         FIG. 7  is a schematic diagram showing the variations in ΔX 1  and ΔX 2  with variation in the spacing between the SiO 2  film masks, with the width of the SiO 2  film masks being kept constant, in the specimen used in the fundamental investigations made by the present inventors; 
         FIG. 8  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a first embodiment of the present invention; 
         FIGS. 9A and 9B  are sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 10A and 10B  are another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 11A ,  11 B and  11 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 12A ,  12 B and  12 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 13  is a perspective view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 14  is another plan view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 15  is still another sectional view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 16  is another perspective view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 17A ,  17 B and  17 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 18  is still another plan view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 19A and 19B  are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 20  is still another perspective view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 21  is still another plan view for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 22A ,  22 B and  22 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 23A ,  23 B and  23 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 24A ,  24 B and  24 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIGS. 25A ,  25 B and  25 C are still another sectional views for illustrating the method of manufacturing the GaN semiconductor laser according to the first embodiment of the present invention; 
         FIG. 26  is still another sectional view showing the GaN semiconductor laser manufactured according to the first embodiment of the present invention; 
         FIG. 27  is still another perspective view showing the GaN semiconductor laser manufactured according to the first embodiment of the present invention; 
         FIGS. 28A ,  28 B and  28 C are still another sectional views showing the GaN semiconductor laser manufactured according to the first embodiment of the present invention; 
         FIGS. 29A and 29B  are respectively a perspective view and a sectional view which illustrate a detailed structure of the GaN semiconductor laser manufactured according to the first embodiment of the present invention; 
         FIG. 30  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a second embodiment of the present invention; 
         FIG. 31  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a third embodiment of the present invention; 
         FIG. 32  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a fourth embodiment of the present invention; 
         FIG. 33  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a fifth embodiment of the present invention; 
         FIG. 34  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a sixth embodiment of the present invention; 
         FIG. 35  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a seventh embodiment of the present invention; 
         FIG. 36  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to an eighth embodiment of the present invention; 
         FIG. 37  is a plan view for illustrating the method of manufacturing a GaN semiconductor laser according to a ninth embodiment of the present invention; 
         FIG. 38  is a perspective view for illustrating the method of manufacturing a surface emitting GaN semiconductor laser according to an eleventh embodiment of the present invention; 
         FIGS. 39A and 39B  are respectively a sectional view of the surface emitting GaN semiconductor laser manufactured according to the eleventh embodiment of the present invention and a schematic diagram showing the distributions of In content and the total thickness of the grown layers; 
         FIG. 40  is a perspective view for illustrating the method of manufacturing a photonic crystal according to a twelfth embodiment of the present invention; 
         FIG. 41  is another perspective view for illustrating the method of manufacturing the photonic crystal according to the twelfth embodiment of the present invention; 
         FIG. 42  is a sectional view for illustrating the method of manufacturing a GaN semiconductor laser according to a thirteenth embodiment of the present invention; 
         FIG. 43  is a sectional view for illustrating the method of manufacturing a GaN semiconductor laser according to the thirteenth embodiment of the present invention; 
         FIG. 44  is a sectional view for illustrating the method of manufacturing a GaN semiconductor laser according to the thirteenth embodiment of the present invention; 
         FIG. 45  is a sectional view for illustrating the method of manufacturing a GaN semiconductor laser according to a fourteenth embodiment of the present invention; 
         FIG. 46  is a sectional view for illustrating the method of manufacturing a GaN semiconductor laser according to the fourteenth embodiment of the present invention; 
         FIG. 47  is a sectional view for illustrating the method of manufacturing a GaN semiconductor laser according to the fourteenth embodiment of the present invention. 
         FIG. 48  is a perspective view for illustrating the method of manufacturing a GaN semiconductor laser according to the fourteenth embodiment of the present invention; and 
         FIG. 49  is a sectional view of a GaN semiconductor laser manufactured by a method of forming an end face window structure according to the related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     Now, embodiments of the present invention will be described below referring to the drawings. Incidentally, in all the drawings relating to the embodiments, the same or corresponding parts are denoted by the same symbols. 
       FIGS. 8 to 29  illustrate the method of manufacturing a GaN semiconductor laser according to a first embodiment of the present invention. The GaN semiconductor laser has an end face window structure and a ridge stripe structure, wherein parts, in the vicinity of resonator end faces, of a p-side electrode are removed so that both end parts of the resonator are set as current non-injection region. 
     In the first embodiment, first, as shown in  FIG. 8  and  FIGS. 9A and 9B , a chip region  12  as a region to finally be one laser chip is defined on an n-type GaN substrate  11 . Then, in the chip region  12 , insulating film masks  16  having a trapezoidal plan-view shape are formed in the vicinity of resonator end face forming positions  13 ,  14 , where front-side and rear-side resonator end faces are to be finally formed by cleavage or the like, and on both sides of a ridge stripe forming position  15  where a ridge stripe is to be formed later, in line symmetry with respect to the ridge stripe forming position  15 . Here,  FIG. 8  is a plan view,  FIG. 9A  is a sectional view taken along line A-A of  FIG. 8 , and  FIG. 9B  is a sectional view taken along line B-B of  FIG. 8 . Of the pair of parallel edges of each insulating mask  16 , the longer edge is located at one edge of the ridge stripe forming position  15 . The insulating film mask  16  has a width w 1  at its part within a distance d 1  from the resonator end face forming position  13 ,  14 , and has a tapered shape in which the width is linearly reduced from w 1  to 0 along the resonator length direction at its part where the distance is in the range of d 1  to d 2 . One non-limitative example of the dimensions is d 1 =20 μm, d 2 =50 μm, and w 1 =5 μm. The insulating film masks  16  may each be composed of an insulating film such as SiO 2  film, SiN film, and Al 2 O 3  film. The insulating film masks  16  can be easily formed, for example, by forming an insulating film on the n-type GaN substrate  1  by vacuum evaporation, CVD or the like, and then patterning the insulating film by etching. The thickness of the insulating film masks  16  is, for example, about 300 nm, which is not limitative. The width of the ridge stripe forming position  15  is determined according to the characteristics demanded of the GaN semiconductor laser and the like factors, and a general but not limitative example of the width is about 1 to 20 μm (or about 1 to 12 μm). While the chip regions  12  in practice are present repeatedly in a matrix pattern on the n-type GaN substrate  11 , only one chip region  12  is shown in  FIG. 8 . In addition, while the insulating masks  16  in practice are formed over two or more adjacent chip regions  12  in the resonator length direction, only the insulating film masks  16  present in one chip region  12  are shown in  FIG. 8 . The shape and the size of the chip region  12  shown in  FIG. 8  are merely non-limitative examples. 
     Next, as shown in  FIGS. 10A and 10B , GaN semiconductor layers for forming a laser structure are epitaxially grown over the n-type GaN substrate  11  (provided thereon with the insulating film masks  16 ) by, for example, a metal organic chemical vapor deposition (MOCVD) process. Here,  FIG. 10A  is a sectional view taken along line A-A of  FIG. 8 , and  FIG. 10B  is a sectional view taken along line B-B of  FIG. 8 . As the GaN semiconductor layers for forming the laser structure, specifically, an n-type AlGaN clad layer  17 , an n-type GaN optical waveguide layer  18 , an active layer  19  of an undoped Ga 1-x In x N (quantum well layer)/Ga 1-y In y N (barrier layer, x&gt;y) multiple quantum well structure, an undoped InGaN optical waveguide layer  20 , an undoped AlGaN optical waveguide layer  21 , a p-type AlGaN electron barrier layer  22 , a p-type GaN/undoped AlGaN superlattice clad layer  23  and a p-type GaN contact layer  24  are sequentially grown epitaxially. Here, the growth temperatures of the n-type AlGaN clad layer  17 , the n-type GaN optical waveguide layer  18 , the undoped AlGaN optical waveguide layer  21 , the p-type AlGaN electron barrier layer  22 , the p-type GaN/undoped AlGaN superlattice clad layer  23  and the p-type GaN contact layer, which are In-free layers, are set in the range of 900 to 1100° C., for example. On the other hand, the growth temperatures of the active layer  19  of the Ga 1-x In x NGa 1-y In y N multiple quantum well structure and the undoped InGaN optical waveguide layer  20 , which are In-containing layers, are set in the range of 700 to 800° C., for example. These examples are non-limitative. Incidentally, in the following description, these layers forming the laser structure will be designated collectively as the GaN semiconductor layer  25 , as necessary. 
     The growing raw materials for the GaN semiconductor layers are as follows. Non-limitative examples of raw material for Ga include triethylgallium ((C 2 H 5 ) 3 Ga, TEG) and trimethylgallium ((CH 3 ) 3 Ga, TMG); non-limitative examples of raw material for Al include trimethylaluminum ((CH 3 ) 3 Al, TMA); non-limitative examples of raw material for In include triethylindium ((C 2 H 5 ) 3 In, TEI) and teimethylindium ((CH 3 ) 3 In, TMI); and non-limitative examples of raw material for N include ammonia (NH 3 ). As for dopants, non-limitative examples of n-type dopant include silane (SiH 4 ), and non-limitative examples of p-type dopant include bis(methylcyclopentadienyl)magnesium ((CH 3 C 5 H 4 ) 2 Mg), bis(ethylcyclopentadienyl)magnesium ((C 2 H 5 C 5 H 4 ) 2 Mg), and bis(cyclopentadienyl) magnesium ((C 2 H 5 ) 2 Mg). In addition, non-limitative examples of the carrier gas atmosphere used at the time of growing the GaN semiconductor layers include H 2  gas. A general but not limitative value of the flow rate ratio (V/III ratio) of the material for the Group V element to the material for the Group III element is in the range of 10 3  to 10 6  (for example, about 10 5 ). Besides, a non-limitative example of the pressure at the time of the growth is 760 Torr (normal pressure). 
     In this case, the n-type AlGaN clad layer  17 , the n-type GaN optical waveguide layer  18 , the active layer  19 , the undoped InGaN optical waveguide layer  20 , the undoped AlGaN optical waveguide layer  21 , the p-type AlGaN electron barrier layer  22 , the p-type GaN/undoped AlGaN superlattice clad layer  23  and the p-type GaN contact layer  24  are substantially not grown over the insulating masks  16 , but are grown only over the parts, not covered with the insulating masks  16 , of the n-type GaN substrate  11 . Such a growth can be easily realized by selecting the growing conditions by a known method. In this case, during the growth of the n-type AlGaN clad layer  17 , for the growth at the ridge stripe forming position  15  in the area between a pair of the insulating film masks  16 , not only the Al atoms and Ga atoms are supplied into this area directly from the growing raw materials, but also the Al atoms and Ga atoms supplied from the growing raw materials onto the insulating film masks  16  on both sides of this area are supplied into this area (to contribute to the growth) through diffusion over the insulating film masks  16 . Therefore, the thickness of the n-type AlGaN clad layer  17  in the area between the pair of the insulating film masks  16  is greater than in the other areas. Here, of each of the insulating film masks  16 , the part at a distance of d 1  to d 2  from the resonator end face forming position  13 ,  14  has a width which is linearly reduced from w 1  to 0, so that the quantities of the Al atoms and Ga atoms supplied to the ridge stripe forming position  15  in this area from over the insulating film masks  16  are gradually reduced along the resonator length direction. As a result of this, the thickness of the n-type AlGaN clad layer  17  in this area is gradually increased along the resonator length direction toward the resonator end face forming position  13 ,  14 . On the other hand, of each of the insulating film masks  16 , the part within a distance d 1  from the resonator end face forming position  13 ,  14  has a constant width w 1 , so that the quantities of the Al atoms and Ga atoms supplied from over the insulating masks  16  to the ridge stripe forming position in this area are constant along the resonator length direction. As a result of this, the thickness of the n-type AlGaN clad layer  17  in this area is constant. This applies also to the n-type GaN optical waveguide layer  18 . 
     On the other hand, during the growth of the active layer  19  containing In and Ga, for the growth at the ridge stripe forming position  15  in the area between a pair of the insulating film masks  16 , not only the In atoms and Ga atoms are supplied into this area directly from the growing raw materials, but also the In atoms and Ga atoms supplied from the growing raw materials onto the insulating film masks  16  on both sides of this area are supplied into this area (to contribute to the growth) through diffusion over the insulating film masks  16 . In this case, since the diffusion length of the In atoms at the growth temperature (e.g., 700 to 800° C.) of the active layer  19  is smaller than the diffusion length of the Ga atoms by a factor of about one order of magnitude, the quantity of the In atoms supplied from over the insulating film masks  16  to the ridge stripe forming position  15  in this area is smaller than that of the Ga atoms. As a result, the In content of the active layer  19  becomes uneven along the resonator length direction; specifically, the In content of the part corresponding to the area between the pair of the insulating film masks  16  becomes lower than that of other part. Therefore, the band gap energy in this part is higher than the band gap energy in other part, and, hence, this part will finally be the region of the end face window structure. This applies also to the growth of the undoped InGaN optical waveguide layer  20 . 
     The growth of each of the undoped AlGaN optical waveguide layer  21 , the p-type AlGaN electron barrier layer  22 , the p-type GaN/undoped AlGaN superlattice clad layer  23  and the p-type GaN contact layer  24  is similar to that of the n-type AlGaN clad layer  17  and the n-type GaN optical waveguide layer  18 . 
     Next, as shown in  FIG. 11A , an insulating film  26  is formed on the GaN semiconductor layer  25  (the uppermost layer of which is the p-type GaN contact layer  24 ) forming the laser structure, and then the insulating film  26  is coated with a resist  27 . A non-limitative example of the insulating film  26  is a SiO 2  film. Subsequently, the resist  27  is exposed by use of a photomask  28  provided with a mask pattern of a predetermined shape. 
     Next, as shown in  FIG. 11B , the resist  27  thus selectively exposed is subjected to development, thereby forming an opening  27   a . The plan-view shape of the opening  27   a  is a stripe shape corresponding to the shape of the ridge stripe to be formed later (the same shape as the shape of the ridge stripe forming position  15 ). In practice, a multiplicity of such openings  27   a  are formed in parallel at a predetermined pitch, but only one opening  27   a  is shown here. 
     Subsequently, as shown in  FIG. 11C , the insulating film  26  is etched by using the resist  27  as an etching mask, to form an opening  26   a . In the case of using a SiO 2  film as the insulating film  26 , for example, wet etching may be conducted by use of a hydrofluoric acid etchant, which is not limitative. 
     Next, as shown in  FIG. 12A , with the resist  27  left as it is, a Pd film  29  and a Pt film  30  are sequentially formed by, for example, vacuum evaporation from a direction orthogonal to the surface of the n-type GaN substrate  11 . Here, the thickness of the Pt film  30  is so set that the Pt film  30  will be substantially etched away to leave a very small thickness, for example, a thickness of 5 nm or below or a thickness of 3 nm or below, upon completion of dry etching by the RIE process conducted later for forming the ridge stripe. Specifically, for example, the thickness of the Pd film  29  is 150 nm, and the thickness of the Pt film is 30 nm, which values are not limitative. 
     Subsequently, the resist  27  is removed (lifted off) together with the Pd film  29  and the Pt film  30  formed thereon. Thus, as shown in  FIG. 12B , the Pd film  29  and the Pt film  30  in the shape of a stripe extending in one direction are formed. The width of the Pd film  29  and the Pt film  30  in the stripe shape is the same as the width of the ridge stripe forming position  15 , and is for example 1 to 20 μm, which is not limitative. 
     Next, as shown in  FIG. 12C , the insulating film  26  is etched away. In the case of using a SiO 2  film as the insulating film  26 , for example, wet etching is conducted by using a hydrofluoric acid etchant, which is not limitative.  FIG. 13  is a perspective view showing this condition, and  FIG. 14  is a plan view of this condition. 
     Subsequently, as shown in  FIG. 15 , while using the Pd film  29  and the Pt film  30  in the stripe shape as an etching mask, the GaN semiconductor layer  25  is dry etched to a predetermined depth by, for example, the RIE process using a chlorine-based etching gas, to form a ridge stripe  31 . In this case, as above-mentioned, the thickness of the Pt film  30  is so set that the Pt film  30  is substantially etched away to leave a very small thickness, for example, a thickness of 5 nm or below or a thickness of 3 nm or below, upon completion of the dry etching; therefore, the Pd film  29  is constantly covered with the Pt film  30  throughout the dry etching. Accordingly, there is no fear that roughening of the etched surface might be caused by deposition of Pd through sputtering of the surface of the Pd film  29  during the dry etching, and that such roughening might cause a trouble in carrying out the subsequent process or might exert bad influences on the reliability of the laser. The etching rates in the RIE process may be, for example, 0.01 μm/min for the Pt film  30 , and 0.13 μm/min for the GaN semiconductor layer  25 . The height of the ridge stripe  31  is, for example, 0.4 to 0.65 μm, which is not limitative.  FIG. 16  is a perspective view showing this condition. The ridge stripe  31  is formed, for example, to an intermediate depth of the p-type GaN/undoped AlGa superlattice clad layer  23  of the GaN semiconductor layer  25 . The Pd film  29  and the Pt film  30  thus finally left in the stripe shape constitute a p-side electrode  32 . 
     Next, as shown in  FIG. 17A , an insulating film  33  such as a SiO 2  film and an insulating film  34  such as an undoped Si film are sequentially formed over the whole surface, then a resist pattern (not shown) having an opening in the area corresponding to the ridge stripe  31  is formed thereon by lithography, and the portions, over the ridge stripe  31 , of the films  33  and  34  are selectively etched away by using the resist pattern as a mask. Thereafter, the resist pattern is removed. Subsequently, the whole surface inclusive of the area of the Pd film  29  and the Pt film  30  is coated with a resist  35 , and the resist  35  is exposed by using a photomask  36  provided with a mask pattern in a predetermined shape. 
     Next, as shown in  FIG. 17B , the thus selectively exposed resist  35  is subjected to development, to form an opening  35   a .  FIG. 18  is a plan view showing this condition.  FIG. 17B  corresponds to a sectional view taken along line B-B of  FIG. 18 . The plan-view shape of the opening  35   a  is a rectangle having a total width of  2   a  (a width of a in the resonator length direction on each side of the resonator end face forming position  13 ,  14 ) and a total length of  2   b  (a length of b on each side of the center line of the ridge stripe  31 ). 
     Subsequently, as shown in  FIG. 17C , with the resist  35  as an etching mask, wet etching is conducted using aqua regia to etch away the very thinly left Pt film  30  and the Pd film  29 . Here, the etching rate of the Pt film  30  by aqua regia is very low as compared with that of the Pd film  29 . However, since the Pt film  30  is extremely thin, the Pt film  30  can be etched away in a short time, and thereafter the Pd film  29  can be etched away at a sufficient etching rate. The rate of etching of the Pd film  29  by aqua regia is, for example, about 50 nm/min. In this manner, the Pd film  29  and the Pt film  30  inside the opening  35   a  in the resist  35  can be completely etched away. 
     Next, as shown in  FIGS. 19A and 19B , the resist  35  is removed.  FIG. 20  is a perspective view of the thus obtained condition, and  FIG. 21  is a plan view of this condition. Here,  FIG. 19A  is a sectional view taken along line A-A of  FIG. 21 , and  FIG. 19B  is a sectional view taken along line B-B of  FIG. 21 . 
     Subsequently, as shown in  FIG. 22A , an insulating film  37  is formed on the whole surface inclusive of the area of the Pd film  29  and the Pt film  30  by vacuum evaporation or the like. A non-limitative example of the insulating film  37  is a SiO 2  film. The thickness of the insulating film  37  may be, for example, 200 μm, which is not limitative. 
     Next, as shown in  FIG. 22B , the insulating film  37  is coated with a resist  38 , and the resist  38  is exposed by using a photomask  39  provided with a mask pattern in a predetermined shape. The thickness of the resist  38  may be, for example, 0.8 μm, which is non-limitative. 
     Subsequently, as shown in  FIG. 22C , the thus selectively exposed resist  38  is subjected to development, to form an opening  38   a  on the upper side of the ridge stripe  31 . 
     Next, as shown in  FIG. 23A , the resist  38  and the insulating film  37  are etched back by, for example, the RIE process to etch away the insulating film  38  on the upper side of the ridge stripe  31 , thereby exposing the Pt film  30 . 
     Subsequently, as shown in  FIG. 23B , the resist  38  is removed. 
     Next, as shown in  FIG. 23C , a resist  39  is applied to the insulating film  37  and the Pt film  30 , and the surface of the resist  39  is cured by treating it with chlorobenzene, to form a cured layer  40 . The total thickness of the resist  39  and the cured layer  40  may be, for example, 3.0 μm, which is not limitative. Subsequently, the resist  39  and the cured layer  40  are exposed by using a photomask  41  provided with a mask pattern in a shape corresponding to an isolation electrode. 
     Next, as shown in  FIG. 24A , the resist  39  and the cured layer  40  thus exposed are subjected to development, to form an opening  42  in a predetermined shape. In this instance, the cured layer  40  is in the eaves-like shape projecting toward the inside of the opening  42 . 
     Subsequently, as shown in  FIG. 24B , with the resist  39  and the cured layer  40  left as they are, for example, a Ti film, a Pt film and a Ni film are sequentially formed by, for example, vacuum evaporation from the direction orthogonal to the surface of the n-type GaN substrate  11 , to form a Ti/Pt/Ni film  43 . A non-limitative example of the configuration of the Ti/Pt/Ni film  43  is such that the lowermost Ti film has a thickness of 10 nm, the Pt film has a thickness of 100 nm, and the uppermost Ni film has a thickness of 100 nm. 
     Next, the resist  39  and the cured layer  40  are removed (lifted off) together with the Ti/Pt/Ni film  43  formed thereon. In this case, since the cured layer  40  has the eaves-like shape projecting toward the inside of the opening  42 , the lift-off operation can be easily carried out. In this manner, the isolation electrode  44  including the Ti/Pt/Ni film  43  is formed, as shown in  FIG. 24C . 
     Subsequently, as shown in  FIG. 25A , a resist  45  is applied to the whole surface so as to cover the isolation electrode  44 , and then the surface of the resist  45  is cured by treating it with chlorobenzene, to form a cured layer  46 . The total thickness of the resist  45  and the cured layer  46  may be, for example, 3.0 μm, which is non-limitative. Next, the resist  45  and the cured layer  46  are exposed using a photomask  47  provided with a mask pattern in a shape corresponding to a pad electrode. 
     Subsequently, as shown in  FIG. 25B , the resist  45  and the cured layer  46  thus exposed are subjected to development, to form an opening  48  in a predetermined shape. In this instance, the cured layer  46  is in an eaves-like shape projecting toward the inside of the opening  48 . 
     Next, as shown in  FIG. 25C , with the resist  45  and the cured layer  46  left as they are, for example, a Ti film, a Pt film and a Au film are sequentially formed by, for example, vacuum evaporation from the direction orthogonal to the surface of the n-type GaN substrate  11 , to form a Ti/Pt/Au film  49 . A non-limitative example of the configuration of the Ti/Pt/Au film  49  is such that the lowermost Ti film has a thickness of 10 nm, the Pt film has a thickness of 100 nm, and the uppermost Au film has a thickness of 300 nm. 
     Subsequently, the resist  45  and the cured layer  46  are removed (lifted off) together with the Ti/Pt/Au film  49  formed thereon. In this case, since the cured layer  46  has the eaves-like shape projecting toward the inside of the opening  48 , the lift-off operation can be easily carried out. In this manner, the pad electrode  50  including the Ti/Pt/Au film  49  is formed, as shown in  FIG. 26 . 
     Next, an n-side electrode  51  is formed on the back side of the n-type GaN substrate  11  in each chip region  12  by a lift-off method, for example. 
     Subsequently, the n-type GaN substrate  11  provided with the laser structures in the above-mentioned manner is subjected to cleavage along the resonator end face forming positions  13 ,  14  and the like operations, to form laser bars, thereby forming both resonator end faces. Next, the resonator end faces are subjected to end face coating, and then the laser bars are subjected to cleavage and the like operations, to divide them into chips. 
     In this manner, the objective GaN semiconductor laser is manufactured. 
     The GaN semiconductor laser thus obtained in a chip form is shown in  FIG. 27  and  FIGS. 28A ,  28 B and  28 C. Here,  FIG. 27  is a perspective view,  FIG. 28A  is a sectional view taken along line A-A of  FIG. 27 ,  FIG. 28B  is a sectional view taken along line B-B of  FIG. 27 , and  FIG. 28C  is a sectional view taken along line C-C of  FIG. 27 . In this GaN semiconductor laser, the p-side electrode  32  including the Pd film  29  and the Pt film  30  is not formed in the areas of width a along the resonator length direction from each of the resonator end faces, and these areas serve as current non-injection regions. 
       FIGS. 29A and 29B  illustrate a detailed structure of the GaN semiconductor laser. Here,  FIG. 29A  is a perspective view, and  FIG. 29B  is a sectional view taken along line B-B of  FIG. 29A . 
     According to the first embodiment of the present invention, the following merits can be obtained. By only preliminarily forming the insulating film masks  16  on the n-type GaN substrate  11  and growing thereon the GaN semiconductor layer  25  for forming the laser structure, the band gap energy of the active layer  19  in the areas in the areas near the resonator end face forming positions  13 ,  14  can be set greater than in the other area, so that the end face window structure can be formed very easily. In addition, the thickness of the GaN semiconductor layer  25  in the area between a pair of the insulating film masks  16  is gradually increased along the resonator length direction toward the resonator end face forming position  13 ,  14 , so that no steep step is generated. Therefore, the optical waveguide loss can be suppressed remarkably, as contrasted to the case where the semiconductor layer for forming the laser structure has a steep geometrical step in each area ranging from a recess  101   a  to the outer side thereof, as in a semiconductor laser according to the related art shown in  FIG. 49 . Besides, the semiconductor layer forming the laser structure need not be dug by RIE so as to form the end face window structure, so that formation of a surface level at the time of forming the end face window structure is obviated, and the problem of light absorption or local heat generation at the time of laser operation due to such a surface level can be obviated. 
     In addition, according to the first embodiment, it is possible to easily manufacture a GaN semiconductor laser having a structure in which a ridge stripe  31  is formed in a self-aligned manner in relation to the p-side electrode  32  including a Pd film  29  and a Pt film  30  formed in a stripe shape, wherein portions, in the vicinity of both resonator end faces, of the p-side electrode  32  are removed so that both end parts of the resonator serve as current non-injection regions. The GaN semiconductor laser, with both end parts of the resonator serving as current non-injection regions, can effectively prevent catastrophic optical damage (COD) to the resonator end faces, whereby enhanced lifetime and reliability are promised. 
     Now, a method of manufacturing a GaN semiconductor laser according to a second embodiment of the present invention will be described below. 
     In the second embodiment, first, as shown in FIG.  30 , a long insulating film mask  16  with a fixed width is formed on a n-type GaN substrate  11  on one side of the ridge stripe forming position  15  and along the whole length of the resonator in the resonator length direction. One edge of the insulating film mask  16  on the ridge stripe forming position  15  side is coinciding with one edge of the ridge stripe forming position  15 , in its central portion in the resonator length direction. However, in its portion within a distance d 1  from each of the resonator end face forming position  13 ,  14 , the one edge of the insulating film mask  16  is spaced from the ridge stripe forming position  15  by a spacing w 1 , and, in the portion at a distance d 1  to d 2  from each of the resonator end face forming positions  13 ,  14 , the spacing is gradually reduced from w 1  to 0. One non-limitative example of the dimensions is such that d 1  is 20 μm, d 2  is 50 μm, the width of the insulating film mask  16  is 5 μm, and w 1  is 5 to 10 μm. 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  for forming the laser structure is grown over the n-type GaN substrate  11  (provided with the insulating film mask  16 ) by the MOCVD method, for example. In this case, as for the active layer  19  containing In and Ga, the distance from the edge of the insulating film mask  16  to the ridge stripe forming position  15  in the portion within a distance d 2  from each of the resonator end face forming position  13 ,  14  is larger than in the central portion in the resonator length direction. Therefore, of the In atoms and Ga atoms supplied onto the insulating film mask  16 , the In atoms (the diffusion length of which is smaller than that of the Ga atoms by a factor of about one order of magnitude) are supplied to the ridge stripe forming position  15  in the portion within the distance d 2  in an amount smaller than that of the Ga atoms. As a result, the In content of the active layer  19  becomes uneven along the resonator length direction. Specifically, the In content is lower in the portions near the resonator end face forming positions  13 ,  14  than in the other portion, and the band gap energy in the portions near the end face forming positions  13 ,  14  is greater than the band gap energy in the other portion. Thus, the portions, where the band gap energy is greater, of the active layer  19  serve as the end face window structure. 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. 
     According to the second embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to a third embodiment of the present invention will be described below. 
     In the third embodiment, first, as shown in  FIG. 31 , the insulating film mask  16  used in the second embodiment is formed on the n-type GaN substrate  11  on each of both sides of the ridge stripe forming position  15 , in line symmetry. One non-limitative example of the dimensions is such that d 1  is 20 μm, d 2  is 50 μm, the width of the insulating film mask  16  is 5 μm, and the spacing w 1  between the insulating film mask  16  and the ridge stripe forming position  15  is 3 to 20 μm. 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  is grown on the n-type GaN substrate  11  (provided with the insulating film masks  16 ) by the MOCVD method, for example. In this case, as for the active layer  19  containing In and Ga, in the area between the pair of the insulating film masks  16  in the portions within a distance d 2  from the resonator end face forming positions  13 ,  14 , the spacing between the insulating film masks  16  is larger and the distance from the edge of the insulating film mask  16  to the ridge stripe forming position  15  is larger, as compared with those in a central area in the resonator length direction. Therefore, of the In atoms and Ga atoms supplied onto the insulating film masks  16  on both sides of this area between the pair of insulating film masks  16 , the In atoms (the diffusion length of which is smaller than that of the Ga atoms by a factor of about one order of magnitude) are supplied to the ridge stripe forming position  15  in the portion within the distance d 2  in an amount smaller than that of the Ga atoms. As a result, the In content of the active layer  19  becomes uneven along the resonator length direction. Specifically, the In content is lower in the portions near the resonator end face forming positions  13 ,  14  between the pair of insulating film masks  16  than in the other portion, and the band gap energy in the portions near the end face forming positions  13 ,  14  is greater than the band gap energy in the other portion. Thus, the portions, where the band gap energy is greater, of the active layer  19  serve as the end face window structure. 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. 
     According to the third embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to a fourth embodiment of the present invention will be described below. 
     In the fourth embodiment, first, as shown in  FIG. 32 , a long insulating film mask  16  with a fixed width is formed on the n-type GaN substrate  11  on one side of the ridge stripe forming position  15  and over the whole length of the resonator in the resonator length direction. The spacing between one edge of the insulating film mask  16  on the ridge stripe forming position  15  side and one edge of the ridge stripe forming position  15  is w 2  in the portion within a distance d 1  from each of the resonator end face forming positions  13 ,  14 , whereas in the portion at a distance of d 1  to d 2 , the spacing is linearly increased from w 2  to w 3 , to be w 3  in a central portion in the resonator length direction. Here, for example, w 2  is selected to be comparable to ΔX 1  in  FIG. 5C , and w 3  is selected to be comparable to or greater than ΔX 2  in  FIG. 5C . One non-limitative example of the dimensions is such that d 1  is 20 μm, d 2  is 50 μm, the width of the insulating film mask  16  is 5 μm, w 2  is 3 to 5 μm, and w 3  is 10 μm. 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  for forming the laser structure is grown on the n-type GaN substrate  11  (provided with the insulating film mask  16 ) by the MOCVD method, for example. In this case, as for the active layer  19  containing In and Ga, the distance from the edge of the insulating film mask  16  to the ridge stripe forming position  15  in the portion within the distance d 2  from each of the resonator end face forming position  13 ,  14  is selected to be comparable to ΔX 1  in  FIG. 5C , whereas in a central portion in the resonator length direction, the distance from the edge of the insulating film mask  16  to the ridge stripe forming position  15  is selected to be comparable to or greater than ΔX 2  in  FIG. 5C ; therefore, as seen from  FIG. 5C , the In content is lower in the portions near the resonator end face forming positions  13 ,  14  than in the other portion, and the band gap energy in the portions near the end face forming positions  13 ,  14  is greater than the band gap energy in the other portion. Thus, the portions, where the band gap energy is greater, of the active layer  19  serve as the end face window structure. 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. 
     According to the fourth embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to a fifth embodiment of the present invention will be described below. 
     In the fifth embodiment, first, as shown in  FIG. 33 , insulating film masks  16  rectangular in plan-view shape are formed on the n-type GaN substrate  11  in the vicinity of the resonator end face forming positions  13 ,  14  and on both sides of the ridge stripe forming position  15 , in line symmetry with respect to the ridge stripe forming position  15 . The edge of each insulating film mask  16  on the ridge stripe forming position  15  side is coinciding with the edge of the ridge stripe forming position  15 . Each of the insulating film masks  16  has a fixed width w 4  in the resonator length direction. One non-limitative example of the dimensions is such that d 1  is 20 to 50 μm, and the width of the insulating film mask  16  is 5 to 10 μm. 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  for forming the laser structure is grown on the n-type GaN substrate  11  (provided with the insulating film mask  16 ) by the MOCVD method, for example. In this case, as for the active layer  19  containing In and Ga, in the portion within a distance d 2  from each of the resonator end face forming positions  13 ,  14 , of the In atoms and Ga atoms supplied onto the insulating film masks  16  on both sides of this portion, the In atoms (the diffusion length of which is smaller than that of the Ga atoms by a factor of about one order of magnitude) are supplied to the ridge stripe forming position  15  in an amount smaller than that of the Ga atoms, the situation being different from the situation in the central portion in the resonator length direction. As a result, the In content of the active layer  19  becomes uneven along the resonator length direction. Specifically, the In content is lower in the portions near the resonator end face forming positions  13 ,  14  which portions are located between the pair of insulating film masks  16 , than in the other portion, and the band gap energy in the portions near the end face forming positions  13 ,  14  is greater than the band gap energy in the other portion. Thus, the portions, where the band gap energy is greater, of the active layer  19  serve as the end face window structure. 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. 
     According to the fifth embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to a sixth embodiment of the present invention will be described below. 
     In the sixth embodiment, first, as shown in  FIG. 34 , insulating film masks  16  trapezoidal in plan-view shape are formed on the n-type GaN substrate  11  in the vicinity of the resonator end face forming positions  13 ,  14  and on both sides of the ridge stripe forming position  15 , in line symmetry with respect to the ridge stripe forming position  15 . The edge of each insulating film mask  16  on the ridge stripe forming position  15  side is coinciding with the edge of the ridge stripe forming position  15 . The width of each of the insulating film masks  16  is linearly reduced from w 5  to w 6 , in the portion within a distance d 2  from each of the resonator end face forming positions  13 ,  14 . One non-limitative example of the dimensions is such that d 2  is 20 to 50 μm, w 5  is 10 to 20 μm, and w 6  is 5 μm. 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  for forming the laser structure is grown on the n-type GaN substrate  11  (provided with the insulating film mask  16 ) by the MOCVD method, for example. In this case, as for the active layer  19  containing In and Ga, in the portion within a distance d 2  from each of the resonator end face forming positions  13 ,  14 , of the In atoms and Ga atoms supplied onto the insulating film masks  16  on both sides of this portion, the In atoms (the diffusion length of which is smaller than that of the Ga atoms by a factor of about one order of magnitude) are supplied to the ridge stripe forming position  15  in an amount smaller than that of the Ga atoms, the situation being different from the situation in the central portion in the resonator length direction. As a result, the In content of the active layer  19  becomes uneven along the resonator length direction. Specifically, the In content is lower in the portions near the resonator end face forming positions  13 ,  14  which portions are located between the pair of insulating film masks  16 , than in the other portion, and the band gap energy in the portions near the end face forming positions  13 ,  14  is greater than the band gap energy in the other portion. Thus, the portions, where the band gap energy is greater, of the active layer  19  serve as the end face window structure. 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. 
     According to the sixth embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to a seventh embodiment of the present invention will be described below. 
     In the seventh embodiment, insulating film masks  16  are not formed directly on the n-type GaN substrate  11 , but, instead, for example, after the n-type AlGaN clad layer  17  is epitaxially grown on the whole surface of the n-type GaN substrate  11  in any of the first to sixth embodiments, the insulating film masks  16  are formed on the n-type AlGaN clad layer  17 . Thereafter, in the same manner as in the first embodiment, the n-type GaN optical waveguide layer  18 , the active layer  19 , the undoped InGaN optical waveguide layer  20 , the undoped AlGaN optical waveguide layer  21 , the p-type AlGaN electron barrier layer  22 , the p-type GaN/undoped AlGaN superlattice clad layer  23  and the p-type GaN contact layer  24  are sequentially grown epitaxially. One example of the condition upon the growth is shown in  FIG. 35 . FIG.  35  corresponds, for example, to a sectional view taken along line A-A of  FIG. 8 . 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. 
     According to the seventh embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to an eighth embodiment of the present invention will be described below. 
     In the eighth embodiment, as shown in  FIG. 36 , in defining chip regions  12  on the n-type GaN substrate  11 , a discarded region  52  to be finally discarded is provided between each adjacent pair of the chip regions  12  adjacent to each other in the resonator length direction. Both edges of the discarded region  52  coincide with resonator end face forming positions  13 ,  14 . An elongate insulating film mask  16  smaller in width than the discarded region  52  is provided in each discarded region  52 , in parallel to the resonator end face forming positions  52 . The width of the insulating film mask  16  is generally not less than 5 μm, for example, not less than 10 μm, but is not limited to such a value. 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  for forming the laser structure is grown on the n-type GaN substrate  11  (provided with the insulating film masks  16 ) by the MOCVD method, for example. In this case, as for the active layer  19  containing In and Ga, of the In atoms and Ga atoms supplied onto the insulating film masks  16 , the In atoms (of which the diffusion length is smaller than that of the Ga atoms by a factor of about one order of magnitude) are supplied to the ridge stripe forming position  15  in the portion at a predetermined distance from each of the resonator end face forming positions  13 ,  14  in an amount smaller than that of the Ga atoms. As a result, the In content of the active layer  19  becomes uneven along the resonator length direction. Specifically, the In content is lower in the portions near the resonator end face forming positions  13 ,  14  than in the other portion, and the band gap energy in the portions near the end face forming positions  13 ,  14  is greater than the band gap energy in the other portion. Thus, the portions, where the band gap energy is greater, of the active layer  19  serve as the end face window structure. 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. The discarded regions  52  are discarded upon the formation of the resonator end faces. 
     According to the eighth embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to a ninth embodiment of the present invention will be described below. 
     In the ninth embodiment, as shown in  FIG. 37 , in the same manner as in the eighth embodiment, a discarded region  52  is provided between each adjacent pair of chip regions  12  adjacent to each other in the resonator length direction, and an insulating film mask  16  is provided in each discarded region  52 . In addition, small-width strip-shaped insulating film masks  16  are provided in intermittently (in the form of broken lines) on the resonator end face forming positions  13 ,  14  and in a central area between each adjacent pair of ridge stripe forming regions  15 . 
     Next, in the same manner as in the first embodiment, the GaN semiconductor layer  25  for forming the laser structure is grown on the n-type GaN substrate  11  (provided with the insulating film masks  16 ) by the MOCVD method, for example. In this case, the In content of the portions near the resonator end face forming positions  13 ,  14  is lower than that in the other portion, and the band gap energy in the portions near the resonator end face forming positions  13 ,  14  is greater than that in the other portion. The portions, where the band gap energy is greater, of the active layer  19  constitute the end face window structures, in the same manner as in the eighth embodiment; it is to be noted, however, that the GaN semiconductor layer  25  for forming the laser structure is not grown on the insulating film masks  16  provided in the intermittent form on the resonator end face forming positions  13 ,  14 . 
     Thereafter, the subsequent steps are carried out in the same manner as in the first embodiment, to manufacture the objective GaN semiconductor laser. In this case, at the time of forming the resonator end faces, the cleavage along the resonator end face forming positions  13 ,  14  can be conducted easily and assuredly, since the GaN semiconductor layer  25  for forming the laser structure is not present on the insulating film masks  16  provided intermittently provided on the resonator end face forming positions  13 ,  14  and the mechanical strength is lower at the resonator end face forming positions  13 ,  14 . 
     According to the ninth embodiment, merits equivalent to those of the first embodiment can be obtained. 
     Now, a method of manufacturing a GaN semiconductor laser according to the tenth embodiment of the present invention will be described below. 
     The GaN semiconductor laser in this embodiment has a window structure and a ridge stripe structure, and is different from the first embodiment in that both end parts of the resonator are not made to be current non-injection regions. 
     According to the tenth embodiment, it is possible to obtain merits equivalent to those of the first embodiment, except for the merit obtained in the first embodiment owing to the configuration in which both end parts of the resonator are made to be the current non-injection regions. 
     Now, a vertical resonator surface emitting GaN semiconductor laser according to an eleventh embodiment of the present invention will be described below.  FIGS. 38 and 39A  illustrates the surface emitting GaN semiconductor laser. Here,  FIG. 38  is a perspective view, and  FIG. 39A  is a sectional view taken along line A-A of  FIG. 38 . 
     As shown in  FIGS. 38 and 39A , an insulating film mask  16  having a circular opening  16   a  is formed on an n-type GaN substrate  11 . The diameter of the opening  16  may be, for example, about 20 to 30 μm, which is not limitative. Next, a lower AlGaN clad layer  53 , an active layer  19 , an upper AlGaN clad layer  54 , a p-type DBR layer  55  and a p-type GaN contact layer  56  are sequentially grown epitaxially. An n-type DBR layer  57  is epitaxially grown on the back side of the n-type GaN substrate  11 . The active layer  19  has an undoped Ga 1-x In x N (quantum well layer)/Ga 1-y In y N (barrier layer, x&gt;y) multiple quantum well structure. The p-type DBR layer  55  includes a semiconductor multilayer film in which p-type Al z Ga 1-x N layers and p-type Al x Ga 1-x N layers (where z&gt;w, 0&lt;z, and w&lt;1) are alternately stacked; for example, these layers are stacked up to 25 cycles, to obtain a total thickness of about 3 μm. The n-type DBR layer  57  includes a semiconductor multilayer film in which n-type AlN layers and n-type GaN layers are alternately stacked; for example, these layers are stacked up to 35 cycles, to obtain a total thickness of about 4 μm. 
     In this case, since the insulating film mask  16  has the opening  16   a , the quantities of the Al atoms and Ga atoms supplied from over the insulating film mask  16  to the inside of the opening  16   a  are gradually increased along the diametrical direction of the opening  16   a . As a result of this, the thickness of the lower AlGaN clad layer  53  in this area is gradually increased as one goes along the diametrical direction of the opening  16   a  of the insulating film mask  16  toward the center of the opening  16   a . In a central area of the opening  16   a , the quantities of the Al atoms and Ga atoms supplied from over the insulating film mask  16  are constant in the diametrical direction of the opening  16   a . As a result of this, the thickness of the lower AlGaN clad layer  53  is constant in this area. At the time of growth of the active layer  19 , in addition to the In atoms and Ga atoms supplied into the inside of the opening  16   a  of the insulating film mask  16  directly from the growing raw materials, the In atoms and Ga atoms supplied from the growing raw materials onto the insulating film mask  16  are also supplied into this area (to contribute to the growth) through diffusion. In this case, since the diffusion length of the In atoms at the growth temperature (e.g., 700 to 800° c.) of the active layer  19  is smaller than that of the Ga atoms by a factor of about one order of magnitude, the In content of the active layer  19  is reduced in the portion near the edge of the opening  16   a . Thence, the In content of the active layer  19  is again increased as one goes toward the central portion. As a result, the In content of the active layer  19  becomes uneven along the diametrical direction of the opening  16   a . Specifically, the In content in the portion near the opening  16   a  is lower than that in the other portion, so that the band gap energy in this is greater than the band gap energy in the other portion, and this portion forms a low-refractive-index region. On the other hand, since the In content in a central area of the opening  16   a  is high, the band gap energy in this area becomes smaller, and this area forms a high-refractive index region. 
     Next, a circular p-side electrode  32 , for example, is provided on the p-type GaN contact layer  56  in the central area of the opening  16   a  in the insulating film mask  16 . Subsequently, a ring-shaped n-side electrode  51  is provided on the n-type DBR layer  57  on the back side of the n-type GaN substrate  11 . 
       FIG. 39B  shows distributions of the In content (refractive index) and the total thickness of the grown layers in the section shown in  FIG. 39A . 
     According to the eleventh embodiment, a difference in refractive index can be produced between a central part of the GaN semiconductor layer for forming the laser structure and the outside thereof by a single run of epitaxial growth, whereby light can be confined in the central portion of the resonator. Therefore, it is possible to easily realize a surface emitting GaN semiconductor laser which has a low threshold current density and needs less operating current. 
     Now, a method of manufacturing a photonic crystal according to a twelfth embodiment of the present invention will be described below. 
     In the twelfth embodiment, as shown in  FIG. 40 , an insulating film mask  16  having a plurality of circular openings  16   a  in a two-dimensional array is formed on an n-type GaN substrate  11 . In this case, the openings  16   a  in each x-direction array have the same diameter, but the openings  16   a  in each y-direction array are increased stepwise in diameter (for example, increased stepwise in the range of 5 to 100 μm). As shown in  FIG. 41 , an InGaN layer  58  is grown on the n-type GaN substrate  11  provided thereon with the insulating film mask  16 . As a result, the InGaN layer  58  is grown in a cylindrical shape on the n-type GaN substrate  11  in the inside of each of the openings  16   a . In this case, the In content of the InGaN layer  58  is higher as the diameter of the InGaN layer  58  is larger. Therefore, the refractive index is changed stepwise along the y-direction. 
     According to the twelfth embodiment, a photonic crystal including a two-dimensional array of InGaN layers  58  of which the refractive index is changed stepwise along one direction can be manufactured by a single run of epitaxial growth. 
     Incidentally, the shape of the openings  16   a  in the insulating film mask  16  is not limited to the circle, and may be an ellipse, for example. In addition, the InGaN layer  58  may be grown into a conical shape, for example. 
     Now, a method of manufacturing a GaN semiconductor laser according to a thirteenth embodiment of the present invention will be described below. 
     In the thirteenth embodiment, as shown in  FIG. 42 , the steps are carried out in the same manner as in the first embodiment, whereby a GaN semiconductor layer  25  for forming a laser structure such as an active layer  19  is grown over the parts, not covered with the insulating film mask  16 , of the n-type GaN substrate  11 , and then a recess  59  formed on the upper side of the insulating film mask  16  through the growth of the GaN semiconductor layer  25  is filled up with an insulating material  60  to obtain a flattened (planarized) surface. Specifically, for example, a phosphorus-doped silicate inorganic SOG as the insulating material  60  is applied by spin coating to fill up the recess  59  with the insulating material  60 , followed by a heat treatment to remove the solvent, thereby solidifying the insulating material  60 . Alternatively, an organic material, such as polyimide, or SiO 2  or the like as the insulating material  60  is applied to the whole surface area by sputtering or vacuum evaporation or the like to fill up the recess  59  with the insulating material  60 , and thereafter the insulating material  60  is etched back until the GaN semiconductor layer  25  is exposed. 
     Next, an insulating film (not shown) such as, for example, a SiO 2  film is formed on the surface flattened (planarized) in the above-mentioned manner, and then the insulating film is patterned into a predetermined shape by etching. Subsequently, as shown in  FIG. 43 , with the insulating film as an etching mask, the GaN semiconductor layer  25  is dry etched to a predetermined depth by, for example, an RIE method using a chlorine-based etching gas, to form grooves  61 ,  62 , with a ridge stripe  31  formed between the grooves  61 ,  62 . Next, while the insulating film used as the etching mask is left as it is, an insulating film  33  such as a SiO 2  film and an insulating film  34  such as an undoped Si film are sequentially formed over the whole surface area, then a resist pattern (not shown) having an opening in the area corresponding to the ridge stripe  31  is formed by lithography, and, with the resist pattern as a mask, the insulating films  33 ,  34  present on the upper side of the ridge stripe  31  are selectively removed by etching. Thereafter, the resist pattern is removed. By these steps, the insulating films  33 ,  34  being thick as a whole are formed in the areas outside the grooves  61 ,  62 . Here, the insulating film  33  in the areas outside the grooves  61 ,  62  includes the insulating film which has been used as the etching mask. 
     Subsequently, as shown in  FIG. 44 , a p-side electrode  32  is formed on the ridge stripe  31 , and, further, a pad electrode  50  is formed so as to cover the p-side electrode  32 . 
     Thereafter, the required steps are carried out, to manufacture the objective GaN semiconductor laser. 
     According to the thirteenth embodiment, in addition to merits equivalent to those of the first embodiment, the following merit can also be obtained. Since the recess  59  formed on the upper side of the insulating film mask  16  through the growth of the GaN semiconductor layer  25  over the parts, not covered with the insulating film mask  16 , of the n-type GaN substrate  11  is filled up with the insulating material  60  so as to obtain a flattened surface and to eliminate the surface steps due to the recess  59 , the formation of the insulating films  33 ,  34  and the pad electrode  50  in the later steps can be favorably carried out, without generating a step-induced interruption or the like. 
     Now, a method of manufacturing a GaN semiconductor laser according to a fourteenth embodiment of the present invention will be described below. 
     In the fourteenth embodiment, as shown in  FIG. 45 , the steps are carried out in the same manner as in the fourth embodiment, whereby a GaN semiconductor layer  25  for forming a laser structure such as an active layer  19  is grown over the parts, not covered with an insulating film mask  16 , of an n-type GaN substrate  11 , and thereafter a recess  59  formed on the upper side of the insulating film mask  16  through the growth of the GaN semiconductor layer  25  is filled up with an insulating material  60  to obtain a flattened surface. Specifically, for example, a phosphorus-doped silicate inorganic SOG as the insulating material  60  is applied by spin coating to fill up the recess  59  with the insulating material  60 , followed by a heat treatment to remove the solvent and to solidify the insulating material  60 . Alternatively, an organic material, such as polyimide, or SiO 2  or the like as the insulating material  60  is applied to the whole surface area by spattering, deposition or the like method so as to fill up the recess  59  with the insulating material  60 , and then the insulating material  60  is etched back until the GaN semiconductor layer  25  is exposed. 
     Next, an insulating film (not shown) such as, for example, a SiO 2  film is formed on the surface flattened (planarized) in the above-mentioned manner, and then the insulating film is patterned into a predetermined shape by etching. Subsequently, as shown in  FIG. 46 , with the insulating film as an etching mask, the GaN semiconductor layer  25  is dry etched to a predetermined depth by, for example, an RIE method using a chlorine-based etching gas, to form grooves  61 ,  62 , with a ridge stripe  31  formed between the grooves  61 ,  62 . Next, while the insulating film used as the etching mask is left as it is, an insulating film  33  such as a SiO 2  film and an insulating film  34  such as an undoped Si film are sequentially formed over the whole surface area, then a resist pattern (not shown) having an opening in the area corresponding to the ridge stripe  31  is formed by lithography, and, with the resist pattern as a mask, the insulating films  33 ,  34  present on the upper side of the ridge stripe  31  are selectively removed by etching. Thereafter, the resist pattern is removed. By these steps, the insulating films  33 ,  34  being thick as a whole are formed in the areas outside the grooves  61 ,  62 . Here, the insulating film  33  in the areas outside the grooves  61 ,  62  includes the insulating film which has been used as the etching mask. 
     Subsequently, as shown in  FIG. 47 , a p-side electrode  32  is formed on the ridge stripe  31 , and, further, a pad electrode  50  is formed so as to cover the p-side electrode  32 . A perspective view showing this condition is schematically shown in  FIG. 48 . 
     Thereafter, the required steps are carried out, to manufacture the objective GaN semiconductor laser. 
     According to the fourteenth embodiment, in addition to merits equivalent to those of the fourth embodiment, the following merit can also be obtained. Since the recess  59  formed on the upper side of the insulating film mask  16  through the growth of the GaN semiconductor layer  25  over the parts, not covered with the insulating film mask  16 , of the n-type GaN substrate  11  is filled up with the insulating material  60  so as to obtain a flattened surface and to eliminate the surface steps due to the recess  59 , the formation of the insulating films  33 ,  34  and the pad electrode  50  in the later steps can be favorably carried out, without generating a step-induced interruption or the like. 
     While the embodiments of the present invention have been specifically described above, the invention is not limited to the above-described embodiments, and various modifications are possible based on the technical thought of the invention. 
     For example, the numerical values, structures, substrates, processes and the like mentioned in the embodiments above are merely examples, and numeral values, structures, substrates, processes and the like which are different from those mentioned above may also be used, if necessary. 
     Specifically, for example, while an edge of the insulating film mask  16  is located at an edge of the ridge stripe forming position  15  in the first, second, third, fifth and sixth embodiments, the insulating film mask  16  may be so formed that the edge of the insulating film mask  16  is located at a position spaced from the edge of the ridge stripe forming position  15 . 
     Besides, two or more of the above-described first to tenth embodiments may be combined, as necessary. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.