Patent Publication Number: US-7903710-B2

Title: Nitride semiconductor light-emitting device

Description:
PRIORITY STATEMENT 
     This application is a divisional application of U.S. application Ser. No. 10/554,991 (filed Nov. 1, 2005), which is a national phase of PCT Application No. PCT/JP2004/007681 (filed May 27, 2004). PCT Application No. PCT/JP2004/007681 claims priority to Japanese Patent Application No. 2003-153621 (filed May 30, 2003). The entire contents of U.S. application Ser. No. 10/554,991, PCT Application No. PCT/JP2004/007681 and Japanese Patent Application No. 2003-153621 are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a nitride semiconductor light-emitting device such as a nitride semiconductor laser device. 
     BACKGROUND ART 
     There have been fabricated prototypes of semiconductor laser devices that oscillate in a region ranging from ultraviolet to visible light by the use of a nitride semiconductor material as exemplified by GaN, AlN, InN, and composite crystals thereof. For such purposes, GaN substrates are typically used, and therefore they have been intensively researched by a host of research-and-development institutions. At the moment, however, no semiconductor laser devices offer satisfactorily long useful lives, and accordingly what is most expected in them is longer useful lives. It is known that the useful life of a semiconductor laser device strongly depends on the density of defects (in the present specification, defects refer to, for example, vacancies, interstitial atoms, and dislocations in a crystal) that are present in a GaN substrate from the beginning. The problem here is that substrates with low defect density, however effective they may be believed to be in achieving longer useful lives, are difficult to obtain, and therefore researches have been eagerly done to achieve as much reduction in defect density as possible. 
     For example, Non-Patent Reference 1 reports fabricating a GaN substrate by the following procedure. First, on a sapphire substrate, a 2.0 μm thick primer GaN layer is grown by MOCVD (metalorganic chemical vapor deposition). Then, on top of this, a 0.1 μm thick SiO 2  mask pattern having regular stripe-shaped openings is formed. Then, further on top, a 20 μm thick GaN layer is formed again by MOCVD. Now, a wafer is obtained. This technology is called ELOG (epitaxially lateral overgrown), which exploits lateral growth to reduce defects. 
     Further on top, a 200 μm thick GaN layer is formed by HVPE (hydride vapor phase epitaxy), and then the sapphire substrate serving as a primer layer is removed. In this way, a 150 μm thick GaN substrate is produced. Next, the surface of the obtained GaN substrate is ground to be flat. The thus obtained substrate includes, within a substrate surface, a defect-concentrated region and a low-defect region, and, in general, it is classified into a defect-concentrated region including many defects in a part of SiO 2  and a low-defect region being all the remaining part of SiO 2 . 
     The problems here is, however, that the characteristics of a semiconductor laser device fabricated by growing a nitride semiconductor layer, by a growing process such as MOCVD, on a substrate including a defect-concentrated region and low-defect region vary, resulting in a remarkably low yield rate. 
     As a result of an intensive research on why the characteristics of a semiconductor laser device fabricated by growing a nitride semiconductor layer, by a growing process such as MOCVD, on a substrate including a defect-concentrated region and low-defect region vary, resulting in a remarkably low yield rate, the applicant of the present invention has found out that this is because poor flatness of the film surface results in poor surface morphology. Specifically, when a nitride semiconductor layer (particularly, an InGaN layer used as an active layer) is grown on an irregular surface of the film, the thickness and composition of the layer vary depending on the surface irregularities of the film, and thus greatly deviate from the set values. Furthermore, the applicant has found out that the poor surface morphology greatly depends on the shape of the defect-concentrated region in the nitride semiconductor layer. That is, the applicant has found out that the growth direction and mode of a thin film strongly depends on the shape of the defect-concentrated region, and therefore the irregularly-shaped defect-concentrated region degrades the flatness of the film surface, leading to poor surface morphology. Growing a thin film such as an active layer on such an irregular surface causes the device characteristics to vary. 
     These results are obtained in experiments conducted in the following manner. First, a case where a nitride semiconductor layer is grown on a substrate including a defect-concentrated region and a low-defect region will be described.  FIG. 16(   a ) is a sectional view of a conventional semiconductor laser device, and  FIG. 16(   b ) is a top view of  FIG. 16A . Reference numeral  10  represents a substrate including a defect-concentrated region and a low-defect region, reference numeral  11  represents a defect-concentrated region, reference numeral  12  represents a low-defect region, reference numeral  13  represents a nitride semiconductor layer, and reference numeral  13   a  represents a surface of the nitride semiconductor layer. 
     If a nitride semiconductor layer is grown directly on the substrate  10  (i.e., without performing any preliminary treatment for the substrate, etc.), the growth rate of the defect-concentrated region is greatly different from that of the low-defect region, because the defect-concentrated region has lower crystallinity than the low-defect region and may have a growth surface that does not appear in the low-defect region. As a result, the defect-concentrated region grows at a lower growth rate than the low-defect region, and thus growth hardly occurs in the defect-concentrated region. 
       FIG. 17(   a ) is a top view showing how a nitride semiconductor layer having defect-concentrated regions in the shape of lines grows, and  FIG. 17(   b ) is a top view showing how a nitride semiconductor layer having defect-concentrated regions in the shape of dots grows. In either case, since growth hardly occurs in the defect-concentrated regions, growth is started at the defect-concentrated region x and proceeds in the direction indicated by arrow A, and growth is started at the defect-concentrated region y and proceeds in the direction indicated by arrow B. When growth occurs in two different directions in this way, the layer thickness in a growth meet portion becomes different from that elsewhere, leading to poor surface flatness. 
       FIG. 18  shows measurements of the roughness as measured in the direction [11-20] perpendicular to the line-shaped defect-concentrated region and in the direction [1-100] parallel thereto. The measurements were made by using the “DEKTAK3ST” model manufactured by A SUBSIDIARY OF VEECO INSTRUMENTS INC. The measurement was conducted under the following conditions: measurement length: 600 μm; measurement time: 3 s; probe pressure: 30 mg; and horizontal resolution: 1 μm/sample. The level difference between the highest and lowest parts, within the 600 μm wide region in which the measurement was taken, was found to be 200 nm. Here, the large grooves in the defect-concentrated regions are not considered. 
     In addition, the growth meet portion was found to be a non-luminous region. Thus, it can be said that the difference in thickness between the layers within a wafer surface causes the device characteristics to vary. 
     DISCLOSURE OF THE INVENTION 
     In view of the conventionally encountered problems mentioned above, it is an object of the present invention to provide a nitride semiconductor light-emitting device that offers uniform characteristics within a wafer surface and improves the yield rate. 
     To achieve the above object, according to one aspect of the present invention, in a nitride semiconductor light-emitting device, a substrate or a nitride semiconductor layer has a defect-concentrated region and a low-defect region corresponding to a region other than the defect-concentrated region, and, in a portion thereof including the defect-concentrated region, an engraved region engraved so as to be located lower than the low-defect region. 
     In this way, by engraving the defect-concentrated region, the growth direction is made uniform and the surface flatness is improved, offering uniform characteristics within a wafer surface. This makes it possible to improve the yield rate. 
     In this nitride semiconductor light-emitting device, the defect-concentrated region has the shape of a line or a dot, and the engraved region has the shape of a line. Preferably, the engraving depth of the engraved region is 0.5 μm or more but 50 μm or less. Advisably, it is preferable that the distance from an edge of the engraved region to an edge of the defect-concentrated region is 5 μm or more. Moreover, it is preferable that the nitride semiconductor layer has a ridge portion serving as a laser light waveguide region, and the ridge portion is so formed as to be located 5 μm or more away from an edge of the engraved region. 
     Moreover, according to another aspect of the present invention, in a nitride semiconductor light-emitting device, a substrate or a nitride semiconductor layer has a defect-concentrated region and a low-defect region corresponding to a region other than the defect-concentrated region, the defect-concentrated region or the low-defect region has a depression, and there is provided an engraved region obtained by engraving a portion including the depression. 
     In this way, when there is a depression, a portion including the depression is engraved. This makes it possible to achieve uniform growth and improve the surface flatness. 
     In this nitride semiconductor light-emitting device, it is preferable that the depression measures 0.5 μm or more in depth and 1 μm or more in width. Advisably, it is preferable that the engraving depth of the engraved region is 0.5 μm or more but 50 μm or less. Moreover, it is preferable that the distance from an edge of the engraved region to an edge of the defect-concentrated region is 5 μm or more. Furthermore, it is preferable that the nitride semiconductor layer has a ridge portion serving as a laser light waveguide region, and the ridge portion is so formed as to be located 5 μm or more away from an edge of the engraved region. 
     Moreover, according to still another aspect of the present invention, in a nitride semiconductor light-emitting device, a substrate or a nitride semiconductor layer has a defect-concentrated region and a low-defect region corresponding to a region other than the defect-concentrated region, the nitride semiconductor layer has a ridge portion serving as a laser light waveguide region, and there is provided, between the ridge portion and the defect-concentrated region, an engraved region engraved so as to be located lower than the low-defect region. 
     In this way, the engraved region does not necessarily have to be provided in the defect-concentrated region, but may be provided between the ridge portion and the defect-concentrated region to achieve improved surface flatness. 
     In this nitride semiconductor light-emitting device, the engraved region has the shape of a line. Preferably, the engraving depth of the engraved region is 0.5 μm or more but 50 μm or less. Advisably, it is preferable that the distance from an edge of the engraved region to an edge of the defect-concentrated region is 5 μm or more. Moreover, it is preferable that the ridge portion is so formed as to be located 5 μm or more away from an edge of the engraved region. Furthermore, it is preferable that the width of the engraved region is 3 μm or more but 150 μm or less. 
     As described above, according to the present invention, when a substrate or a nitride semiconductor layer has a defect-concentrated region and a low-defect region corresponding to a region other than the defect-concentrated region, the nitride semiconductor layer or the substrate is provided, in a predetermined portion thereof, with an engraved region engraved so as to be located lower than the low-defect region. In this way, the growth direction is made uniform and the surface flatness is improved, offering uniform characteristics within a wafer surface. This makes it possible to improve the yield rate. 
     Moreover, by providing an engraved region, it is possible to release the strains present within the nitride semiconductor layer and thus suppress development of cracks. 
     It is to be noted that, in the present specification, a negative index indicating a crystal plane or direction is represented by its absolute value headed with a negative symbol “−” instead of overscoring the figure as conventionally practiced in crystallography, because it is impossible to do so herein. 
     Used as a substrate in the present invention may be a GaN substrate in a freestanding state by removing a primer therefrom, as used in the conventional example described earlier, or a GaN substrate just as it is without removing a sapphire primer layer. That is, the following description deals with examples that use a substrate having a defect-concentrated region and a low-defect region on a surface thereof on which a thin film of a nitride semiconductor laser has not yet been grown by MOCVD. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1(   a ) is a sectional view of a nitride semiconductor laser device of a first embodiment, and  FIG. 1(   b ) is a top view of  FIG. 1(   a ); 
         FIG. 2  is a sectional view showing the layer structure of a nitride semiconductor layer; 
         FIG. 3(   a ) is an enlarged top view showing an example of a defect-concentrated region,  FIG. 3(   b ) is an enlarged top view showing an example of a defect-concentrated region, and  FIG. 3(   c ) is an enlarged top view showing an example of a defect-concentrated region; 
         FIG. 4(   a ) is a top view of a substrate of the first embodiment, and  FIG. 4(   b ) is a sectional view of  FIG. 4(   a ); 
         FIG. 5(   a ) is a diagram showing the surface flatness as measured in the direction [11-20], and  FIG. 5(   b ) is a diagram showing the surface flatness as measured in the direction [1-100]; 
         FIG. 6(   a ) is a top view showing how a nitride semiconductor layer having defect-concentrated regions in the shape of lines grows, and  FIG. 6(   b ) is a top view showing how a nitride semiconductor layer having defect-concentrated regions in the shape of dots grows; 
         FIG. 7  is a diagram showing the relationship between the depth X of an engraved region and the yield rate; 
         FIG. 8(   a ) is a top view of a substrate having defect-concentrated regions in the shape of lines, and  FIG. 8(   b ) is a top view of a substrate having defect-concentrated regions in the shape of dots; 
         FIG. 9  is a diagram showing the relationship between the distance Y and the yield rate; 
         FIG. 10  is a top view of a substrate of a second embodiment having defect-concentrated regions in the shape of dots; 
         FIG. 11(   a ) is a top view of a substrate of a third embodiment, and  FIG. 11(   b ) is a sectional view of  FIG. 11(   a ); 
         FIG. 12(   a ) is a top view of a substrate of a fourth embodiment, and  FIG. 12(   b ) is a sectional view of  FIG. 12(   a ); 
         FIG. 13(   a ) is a top view showing how a nitride semiconductor layer having no engraved region grows, and  FIG. 13(   b ) is a top view showing how a nitride semiconductor layer having an engraved region grows; 
         FIG. 14(   a ) is a top view of a substrate of a fifth embodiment, and  FIG. 14(   b ) is a sectional view of  FIG. 14(   a ); 
         FIG. 15(   a ) is a sectional view of a nitride semiconductor laser device of the fifth embodiment, and  FIG. 15(   b ) is a top view of  FIG. 15(   a ); 
         FIG. 16(   a ) is a sectional view of a conventional semiconductor laser device, and  FIG. 16(   b ) is a top view of  FIG. 16(   a ); 
         FIG. 17(   a ) is a top view showing how a conventional nitride semiconductor layer having defect-concentrated regions in the shape of lines grows, and  FIG. 17(   b ) is a top view showing how a conventional nitride semiconductor layer having defect-concentrated regions in the shape of dots grows; and 
         FIG. 18(   a ) is a diagram showing the surface flatness of a conventional nitride semiconductor laser device as measured in the direction [11-20], and  FIG. 18(   b ) is a diagram showing the surface flatness of a conventional nitride semiconductor laser device as measured in the direction [1-100]. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1(   a ) is a sectional view of a nitride semiconductor laser device, and  FIG. 1(   b ) is a top view of  FIG. 1(   a ). An n-type GaN substrate  10  includes, as a part thereof, a defect-concentrated region  11 , all the remaining part thereof being a low-defect region  12 . 
     Note that, in the present specification, a defect-concentrated region denotes a region where, as a result of subjecting a substrate or a nitride semiconductor layer fabricated on a substrate to etching by dipping it in a mixed acid liquid, namely a mixture of sulfuric acid and phosphoric acid, heated to 250° C., many etch pits are observed, attesting to concentration of defects (or dislocations, for example) therein. On the other hand, a low-defect region denotes a region with EPDs (etch pit densities) of the order of 10 4  to 10 5 /cm 2 . The defect-concentrated region has three or more orders of magnitude greater EPDs. The measurement of the EPD can be made possible by the use of gas-phase etching such as RIE (reactive ion etching). Alternatively, suspension of growth in a MOCVD furnace followed by exposure to a high temperature (about 1,000° C.) also makes the measurement of the EPD possible. The measurement itself can be achieved by the use of an AFM (atomic force microscope), CL (cathode luminescence), microscopic PL (photo luminescence), or the like. 
     On the substrate  10 , a nitride semiconductor layer  13  (an epitaxially grown layer) is formed. In the substrate  10 , an engraved region  14  is so formed as to include the defect-concentrated region  11 . The engraved region  14  is engraved by RIE. Moreover, on the top of the nitride semiconductor layer  13 , a ridge portion  15  that serves as a laser light waveguide structure and a SiO 2  layer  16  for current constriction are formed, and on top of this, a p-type electrode  17  is formed. Furthermore, on the bottom face, of the substrate  10 , an n-type electrode  18  is formed. 
     Note that, in the present specification, the distance from the center of the ridge portion  15  to an edge of the engraved region  14  is represented by d. In  FIG. 1(   a ), it is assumed that d=40 μm. 
       FIG. 2  is a sectional view showing the layer structure of the nitride semiconductor layer  13 . The nitride semiconductor layer  13  has the following layers formed one on top of another in the order mentioned on the surface of an n-type GaN layer  20  (with a film thickness of 3.5 μm): an n-type Al 0.062 Ga 0.938 N first clad layer  21  (with a film thickness of 2.3 μm), an n-type Al 0.1 Ga 0.9 N second clad layer  22  (with a film thickness of 0.2 μm), an n-type Al 0.062 Ga 0.938 N third clad layer  23  (with a film thickness of 0.1 μm), an n-type GaN guide layer  24  (with a film thickness of 0.1 μm), an InGaN/GaN-3MQW active layer  25  (with an InGaN/GaN film thickness of 4 nm/8 nm), a p-type Al 0.3 Ga 0.7 N vaporization prevention layer  26  (with a film thickness of 20 nm), a p-type GaN guide layer  27  (with a film thickness of 0.05 μm), a p-type Al 0.062 Ga 0.938 N clad layer  28  (with a film thickness of 0.5 μm), and a p-type GaN contact layer  29  (with a film thickness of 0.1 μm). 
     As shown in  FIG. 1(   b ), the line-shaped defect-concentrated region  11  extends in the direction [1-100]. The defects, which are linear as seen from above, may have different shapes depending on their defect density and type. Examples of the shape of the defect-concentrated region are shown in  FIGS. 3(   a ) to  3 ( c ). There are, for example, defect-concentrated regions in the shape of lines ( FIG. 3(   a )), defect-concentrated regions in the shape of holes, ( FIG. 3(   b )), and closely-spaced defect-concentrated regions in the shape of fine holes ( FIG. 3(   c )). The size of the holes and linear cores here is of the order of about 1 nm to several tens of μm. This embodiment deals with a case shown in  FIG. 3(   a ). Note that the same advantages are obtained in cases shown in  FIGS. 3(   b ) and  3 ( c ). 
     Next, a fabricating procedure will be described. As in the conventional example described earlier, the GaN substrate  10  having defect-concentrated regions in the shape of lines is fabricated through the following procedure. On a sapphire substrate, a 2.5 μm thick primer GaN layer is grown by MOCVD. Then, on top of this, a SiO 2  mask pattern having regular stripe-shaped openings is formed (with a period of 20 μm), and then a 15 μm thick GaN layer is formed again by MOCVD to produce a wafer. The film does not grow on SiO 2 , and thus starts to grow inside the openings. As soon as the film becomes thicker than the SiO 2 , the film then starts to grow horizontally away from the openings. At the center of every SiO 2  segment, different portions of the film growing from opposite sides meet, producing, where they meet, a defect-concentrated region  11  with high defect density. Since the SiO 2  is formed in the shape of lines, defect-concentrated regions are also formed in the shape of lines. Here, the width of the defect-concentrated region  11  is about 40 μm, and the defect-concentrated regions  11  are formed at about 400 μm intervals. 
     Here, the substrate is produced by ELOG. It should be understood, however, that other fabricating methods may be used. Specifically, the only requirement is to use a substrate including a defect-concentrated region and a low-defect region and grow a nitride semiconductor layer on the substrate. The substrate may be a substrate of sapphire, or a substrate of another material, for example, a substrate of SiC, GaN, GaAs, Si, spinel, or ZnO. 
     Next, all over the surface of the substrate  10 , SiO 2  or the like is vapor-deposited by electron beam deposition so as to have a thickness of 400 nm. Then, by common photolithography, stripe-shaped windows are formed with photoresist in the direction [1-100] so as to have a width of 60 μm each and include a defect-concentrated region each. Then, by ICP or RIE, the SiO 2  and the GaN substrate  10  are etched. The GaN substrate  10  is etched to a depth of 4 μm. Thereafter, the SiO 2  is removed with an etchant such as HF. This is the end of the treatment of the substrate to be performed before a nitride semiconductor layer  13  is grown thereon. 
       FIG. 4  shows the thus obtained substrate  10 .  FIG. 4(   a ) is a top view of the substrate  10 , and  FIG. 4(   b ) is a sectional view of  FIG. 4(   a ). Reference numeral  14   a  represents the regions etched by RIE so as to include the defect-concentrated region  11 . Symbol X represents the etching depth. In the present specification, the etching may be achieved by the use of gas-phase etching, or by the use of a liquid etchant. 
     Then, the nitride semiconductor layer  13  is laid on the top of the substrate  10 , followed by the formation of the ridge portion  15 , the SiO 2  layer  16 , the p-electrode  17 , and the n-electrode  18 . 
     When the defect-concentrated region  11  in the substrate  10  is engraved by RIE, and the nitride semiconductor layer  13  is then laid on the top of the substrate  10 , the surface flatness of the engraved region  14  is greatly degraded to the same level of roughness as that of the conventional nitride semiconductor laser device shown in  FIG. 16  (see  FIG. 18 ). However, as shown in  FIG. 5(   b ), the level difference between the highest and lowest parts, within the 600 μm wide region, except the engraved region  14 , in which the measurement was taken, was found to be 20 nm or less. Here, the drops corresponding to the groove portions shown in  FIG. 5(   a ) are not considered. 
     The reasons are explained by using  FIG. 6 .  FIG. 6(   a ) is a top view showing how the nitride semiconductor layer  13  having the defect-concentrated regions  11  in the shape of lines grows, and  FIG. 6(   b ) is a top view showing how the nitride semiconductor layer  13  having the defect-concentrated regions  11  in the shape of dots grows. Unlike the case in  FIG. 17 , where the growth direction varies depending on the shape of the defect-concentrated region  11 , the formation of the engraved region  14  makes it possible to achieve approximately the same growth direction as indicated by arrows C and D shown in  FIG. 6 , preventing the growth meet portion from being produced due to the difference in the growth direction. This prevents the thickness of the individual layers from being varied within the surface, making uniform the layer thickness thereof. 
     Moreover, as shown in  FIGS. 6(   a ) and  6 ( b ), the engraved region  14  makes it possible to achieve the same growth direction within the surface regardless of the shape of the defect-concentrated region  11 , and is thus effective in improving the surface flatness. 
     By forming the ridge portion  15  on the thus obtained extremely flat region, it is possible to suppress the in-surface distribution of the device characteristics and thus improve the yield rate dramatically. The useful lives of the thus obtained semiconductor laser devices were tested with the devices driven under APC at 60° C. and at an output of 30 mW. Here, the useful life is defined as the length of time required for I op  (a current value when the optical output is kept at 30 mW) to become 1.5 times the initial level thereof. In the test, the devices emitted at wavelengths of 405±5 nm. From each wafer, 50 semiconductor laser devices were randomly picked out, and the number of devices of which the useful lives exceeded 3,000 hours was counted as the yield rate. 
     Here, the yield rate was more than 80%. Note that, when the nitride semiconductor layer  13  was grown directly on the substrate  10  shown in the conventional example described earlier, the yield rate was 30% or less. Accordingly, it can be said that better surface flatness of the nitride semiconductor layer  13  (except the engraved region  14 ) makes uniform the layer thickness and the composition of the individual layers within the wafer surface, leading to better yield rate. 
     Now, the depth X of the engraved region  14  shown in  FIG. 4  will be explained.  FIG. 7  shows the relationship between the engraving depth X and the yield rate. Although  FIG. 7  shows an example in which the deepest engraving depth X is 5 μm, the yield rate was found to be more than 80% even when the depth was more than 5 μm. If the engraving depth X is less than 0.5 μm, the engraved region is filled quickly when the primer n-type GaN grows. Thus, the poor surface flatness of the engraved region  14  spreads out of it to degrade the surface flatness of the region outside the engraved region  14 . Moreover, it has been found that, if X=50 μm or more, when, in general, in the device separation process, the substrate is polished and ground, cracks or the like develop, resulting in a low yield rate. Hence, it is preferable that the engraving depth X be 0.5 μm or more but 50 μm or less. 
     Now, the position of the engraved region  14  will be explained.  FIG. 8(   a ) is a top view of the substrate  10  having the defect-concentrated regions in the shape of lines, and  FIG. 8(   b ) is a top view of the substrate  10  having the defect-concentrated regions in the shape of dots. As shown in  FIGS. 8(   a ) and  8 ( b ), the distance from an edge of the defect-concentrated region  11  to an edge of the engraved region  14  is represented by Y. Here, although the distance Y on one side of the defect-concentrated region  11  in the width direction differs from the distance Y on the other side of the defect-concentrated region  11 , a shorter one is defined as the distance Y. 
       FIG. 9  shows the relationship between the distance Y and the yield rate. If the distance Y is less than 5 μm, the engraved region  14  cannot accommodate all the low-crystallinity portions of the defect-concentrated region  11 , letting them be outside the engraved region  14 , resulting in a low yield rate. Hence, it is preferable that the distance Y be 5 μm or more. 
     Now, the position of the ridge portion  15  will be explained. The position of the ridge portion  15  is defined by the distance d shown in  FIG. 1 . If the distance d is less than 5 μm, there appear edge growths (i.e., the growth rate of the edge portions of the unengraved region increases, making the layer thicker), resulting in the variation in the layer thickness. This is undesirable. Hence, no problem arises when the distance d is 5 μm or more. 
     In the nitride semiconductor laser device shown in the conventional example described earlier, the number of cracks observed per 1 cm 2  area on the nitride semiconductor laser device  13  is five to seven. The reason is believed to be strains produced by the differences in lattice constant or in the thermal expansion coefficient between the AlGaN clad layer and the GaN layer included in the nitride semiconductor layer  13 . Such cracks present in a chip greatly affect the characteristics of a nitride semiconductor device, resulting in a low yield rate. 
     By contrast, in the nitride semiconductor laser device of this embodiment, the number of cracks observed per 1 cm 2  area is zero. Thus, with this embodiment, it is possible to greatly reduce the number of cracks in the nitride semiconductor layer  13 . The reason is believed to be that the strains present within the nitride semiconductor layer  13  are released by the presence of the engraved region  14 . 
     Second Embodiment 
     This embodiment deals with a case where the defect-concentrated region  11  has the shape of a dot. This embodiment has the same process and configuration, etc. as those of the first embodiment except in that, here, the defect-concentrated region  11  in the substrate  10  has the shape of a dot. 
     When no engraved region  14  was formed as in the conventional example described earlier, the nitride semiconductor layer  13  grew concentrically away from the defect-concentrated region  11 , and the flatness in a growth meet portion was greatly degraded. We then measured the surface roughness, and observed that the level difference between the highest and lowest parts on the surface was as great as 200 nm. 
       FIG. 10  is a top view of the substrate  10  having the defect-concentrated region  11  in the shape of a dot. By forming the engraved region  14  in the shape of a line so as to include the dot-shaped defect-concentrated regions  11 , it is possible to improve the surface flatness. 
     We measured the surface flatness of the wafer produced in the manner described in the first embodiment, and observed that the level difference between the highest and lowest parts on the surface was 20 nm or less. Moreover, the obtained yield rate was approximately the same as that of the first embodiment. Furthermore, it is preferable that the depth X of the engraved region, the distance Y, and the distance d be made equal to those in the first embodiment. 
     Third Embodiment 
     In this embodiment, a substrate having a depression is used. This depression may be formed elsewhere than in the defect-concentrated region  11 . This embodiment has the same process and configuration, etc. as those of the first embodiment except for the substrate to be used. 
     The depression can take different shapes. Examples of the shape of the depression are shown in  FIG. 11 .  FIG. 11(   a ) is a top view of the substrate, and  FIG. 11(   b ) is a sectional view of  FIG. 11(   a ). Here, assuming that these depressions  30   a  to  30   c  have the width Z and the depth V. Experiments have proved that, if the width Z is 1 μm or more and the depth V is 0.5 μm or more, the growth of the nitride semiconductor layer  13  occurs in different directions according to the shape of the depression. On the other hand, although the depression smaller and shallower than that described above is filled quickly and thus does not affect the growth direction, it degrades the surface flatness. 
     The region having such a depression is engraved, as in the first embodiment, by the use of gas-phase etching such as RIE. Then, a wafer is produced in the same manner as in the first embodiment. We then measured the surface flatness, and observed that the level difference between the highest and lowest parts on the surface was 20 nm or less. On the other hand, when the nitride semiconductor layer  13  is grown without forming the engraved region  14  as in the conventional example described earlier, the level difference between the highest and lowest parts on the surface was greatly degraded to 200 nm or more. 
     Moreover, the obtained yield rate was approximately the same as that of the first embodiment. Furthermore, it is preferable that the depth X of the engraved region, the distance Y, and the distance d be made equal to those in the first embodiment. 
     Fourth Embodiment 
     In this embodiment, a semiconductor laser device having an engraved region other than the engraved region  14  including the defect-concentrated region  11  will be explained. This embodiment has the same process and configuration, etc. as those of the first embodiment except for the position of the engraved region on the substrate  10 . 
       FIG. 12(   a ) is a top view of the substrate of the fourth embodiment, and  FIG. 12(   b ) is a sectional view of  FIG. 12(   a ). There exist, as the engraved region, the engraved region  14  including the defect-concentrated region  11  and an engraved region  14   a  formed in the low-defect region  12 . 
     The engraved region  14   a  is provided for the purpose of preventing, when an abnormal growth portion such as a region where defects or growth surfaces are different than elsewhere is included in the low-defect region, such a portion from affecting a widespread area.  FIG. 13(   a ) is a top view showing how the nitride semiconductor layer  13  having no engraved region  14   a  grows, and  FIG. 6(   b ) is a top view showing how the nitride semiconductor layer  13  having the engraved region  14   a  grows. As shown in  FIG. 16(   a ), when there exists an irregular defect or the like in the low-defect region  12 , abnormal growth occurs there, because there is no engraved region, and then spreads over the low-defect region  12 . However, it has been found that, by forming the engraved region  14   a  also in the low-defect region  12 , it is possible to prevent the abnormal growth from spreading out of it as shown in  FIG. 16(   b ). Specifically, the engraved region  14   a  prevents the abnormal growth occurred in a low-defect region  12   a  of  FIG. 16(   b ) from spreading out of it, allowing the low-defect region  12   b  to maintain better surface flatness. 
     It has been found that the width of the engraved region  14   a  should be 3 μm or more to prevent the abnormal growth occurred in the low-defect region  12   a  from spreading out of it into the low-defect region  12   b  and keep the level difference of the surface flatness to be 20 nm or less. If the width was 3 μm or less, the engraved region  14   a  was filled, making it impossible to prevent abnormal growth. If the width is 200 μm or more, however, the area of the low-defect region  12  is reduced. This makes narrower the region on which the p-electrode  17  or the like is to be formed, resulting in low process yield. This is undesirable. 
     For the same reason as stated in the first embodiment, it is preferable that the engraving depth X of the engraved region  14   a  be 0.5 μm or more but 50 μm or less. 
     Note that a plurality of engraved regions  14   a  may be provided between the engraved regions  14 , and the same advantage can be achieved by forming them anywhere within the low-defect region. 
     Fifth Embodiment 
     This embodiment deals with a case where, instead of engraving the defect-concentrated region  11  by the use of etching such as RIE, engraved regions are formed on both sides of the defect-concentrated region  11  to improve the surface flatness of the nitride semiconductor layer  13 , achieving greatly improved in-surface yield rate of the characteristics of a semiconductor laser device. This embodiment has the same process and configuration, etc. as those of the first embodiment except for the position of the engraved region on the substrate  10 . 
       FIG. 14(   a ) is a top view of the substrate of the fifth embodiment, and  FIG. 14(   b ) is a sectional view of  FIG. 14(   a ). Here, engraved regions  14   b  are provided on both sides of the defect-concentrated region  11 . For example, the width of the engraved region  14   b  can be set to 20 μm and the depth thereof at 3 μm. Reference numeral  175  represents a low-defect region between the engraved regions  14   b , and is referred to as a ridge portion formation region. The ridge portion formation region is a region where a ridge portion that serves as a light waveguide region formed on the top of the nitride semiconductor layer  13  grown on the substrate  10  for the purpose of producing a nitride semiconductor laser device. 
     On the substrate  10  of  FIG. 14 , the nitride semiconductor layer  13  is epitaxially grown. Then, a nitride semiconductor laser device is fabricated on the thus obtained wafer.  FIG. 15(   a ) is a sectional view of the nitride semiconductor laser device of the fifth embodiment, and  FIG. 15(   b ) is a top view of  FIG. 15(   a ). Also here, just as in  FIG. 1(   a ), the distance from the center of the ridge portion  15  to an edge of the engraved region  14   b  is represented by d, and d=100 μm. 
     When, as in this embodiment, the engraved regions  14   b  are provided on both sides of the defect-concentrated region  11  in the substrate  10 , and the nitride semiconductor layer  13  is grown on the substrate  10 , the surface flatness of a region that includes the defect-concentrated region  11  and is sandwiched between the engraved regions  14   b  is greatly degraded. 
     However, even after the nitride semiconductor layer  13  was epitaxially grown, the level difference between the highest and lowest parts, within the 600 μm wide region in which the surface flatness of the ridge portion formation region  12   c  shown in  FIG. 14  was measured, was found to be 20 nm or less. The reason is believed to be that, with action similar to the engraved region  14   a  of  FIG. 13 , an abnormal growth region can be prevented from spreading. Thus, it has been found that it is possible to improve the surface flatness by forming the engraved region  14   b  in the low-defect region  12  so as to include the defect-concentrated region  11  without having to engrave the defect-concentrated region  11 . With this engraved region, it is possible to suppress the in-surface distribution of the device characteristics and thus improve the yield rate dramatically. 
     Moreover, the obtained yield rate was approximately the same as that of the first embodiment. Furthermore, it is preferable that the depth X of the engraved region, the distance Y, and the distance d be made equal to those in the first embodiment. 
     Moreover, to keep the level difference of the surface flatness to be 20 nm or less, it is preferable that the width of the engraved region  14   b  be 3 μm or more but 150 μm or less. If the width is 3 μm or less, the engraved region  14   b  is filled, making the abnormal growth in the defect-concentrated region  11  spread into the low-defect region  12 . If the width is 150 μm or more, however, the area of the low-defect region  12  is reduced. This makes narrower the region on which the p-electrode  17  or the like is to be formed, resulting in low process yield. This is undesirable. 
     Furthermore, in the nitride semiconductor laser device of this embodiment, the number of cracks observed per 1 cm 2  area is zero. Thus, with this embodiment, it is possible to greatly reduce the number of cracks in the nitride semiconductor layer  13  for the same reason as stated in the first embodiment. 
     INDUSTRIAL APPLICABILITY 
     A nitride semiconductor light-emitting device according to the present invention can be used effectively, especially in a nitride semiconductor laser device.