Abstract:
There are provided a process for producing a semiconductor device and a semiconductor device which allow conductivity distribution to be formed without making refractive index distributed even in a material system of a semiconductor difficult to be subjected to ion implantation. The process for producing a semiconductor device includes the steps of forming a semiconductor layer containing a dopant; forming a concave and convex structure on the semiconductor layer by partially removing the semiconductor layer; and forming a conductivity distribution reflecting the concave and convex structure in the semiconductor layer by performing heat treatment on the semiconductor layer in which the concave and convex structure has been formed at a temperature at which a material forming the semiconductor layer causes mass transport and filling up a hole of a concave portion of the concave and convex structure with the material forming the semiconductor layer.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a process for producing a semiconductor device and a semiconductor device, and particularly to a process for producing a semiconductor device and a semiconductor device having a plurality of semiconductor regions having different conductivities in an in-plane direction. 
       BACKGROUND ART 
       [0002]    A group III nitride semiconductor is a promising material for a light emitting device in an ultraviolet to visible region and a next-generation advanced electronic device. 
         [0003]    In order to make semiconductor devices integrated, it is necessary to form a region with different conductivities in an in-plane direction. 
         [0004]    However, it has been known that it is very difficult to perform conductivity control on a very hard material system such as a group III nitride semiconductor by ion implantation after crystal growth. 
         [0005]    Conventionally, for example, Japanese Patent No. 3430206 discloses a process for producing a semiconductor device which carries out local conductivity control using mass transport without using ion implantation. 
         [0006]    Here, the above described mass transport will be briefly described. 
         [0007]    The mass transport is a phenomenon that some of the atoms forming a crystal move by heating and the atoms are rearranged so as to reduce surface free energy. 
         [0008]    It has been known that when a concave and convex structure is formed on a semiconductor surface and heated to a predetermined temperature or higher, mass transport occurs, which causes a phenomenon that the corners of the concave and convex structure are rounded or filled up. 
         [0009]    A theory and an experimental result of the mass transport are disclosed in Journal of Applied Physics Vol. 67, pp. 2434 (1990), for example. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: Japanese Patent No. 3430206 
       
     
       Non Patent Literature 
       [0000]    
       
         NPL 1: Journal of Applied Physics Vol. 67, pp. 2434 (1990) 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    According to Japanese Patent No. 3430206, as described above, since it is difficult to perform conductivity control on a material system such as a group III nitride semiconductor by ion implantation after crystal growth, a conductivity distribution reflecting a concave and convex structure formed after crystal growth is formed using mass transport without using ion implantation. 
         [0013]    However, as described later, the process for producing a semiconductor device disclosed in Japanese Patent No. 3430206 has a problem that it is difficult to form conductivity distribution without making refractive index distributed. 
         [0014]    More specifically, in order to control the region where mass transport occurs, Japanese Patent No. 3430206 uses the difference in mass transport characteristic due to the difference in Al composition. 
         [0015]    Here, the more Al is included, the more thermodynamically stable the semiconductor material becomes, and mass transport is difficult to occur. Therefore, it is so constructed that Al is not contained in a region where mass transport is desired to occur; and Al is contained in a region where mass transport is not desired to occur. 
         [0016]    However, this construction requires a combination of materials different in Al composition and thus the characteristics other than conductivity are also affected. 
         [0017]    For example, a layer containing Al has a smaller refractive index than a layer not containing Al. 
         [0018]    Therefore, it is difficult to form conductivity distribution without making refractive index distributed. 
         [0019]    Here, the description will be given on an example of a gain-coupled DFB laser (distributed feedback laser) which forms conductivity distribution without producing a refractive index distribution. 
         [0020]    The DFB laser is roughly divided into two types: an index-coupled DFB laser (structure whose refractive index is periodically modulated) and a gain-coupled DFB laser (structure whose gain is periodically modulated). 
         [0021]    The gain-coupled DFB laser is characterized in that a single mode oscillation is easier and a change in characteristic due to reflected light is smaller in comparison with the index-coupled DFB laser. 
         [0022]    However, in order to provide a pure gain-coupled DFB laser, gain or loss needs to be periodically modulated without modulating the refractive index. 
         [0023]    Modulation of the density of current injected into an active layer can induce modulation of gain, but the producing process of Japanese Patent No. 3430206 is difficult to produce a structure whose current density is modulated without modulating the refractive index. 
         [0024]    In view of the above problems, the present invention has been made, and an object of the present invention is to provide a process for producing a semiconductor device and a semiconductor device which allow conductivity distribution to be formed without making refractive index distributed even in a material system of a semiconductor difficult to be subjected to ion implantation. 
       Solution to Problem 
       [0025]    The present invention provides a process for producing a semiconductor device and a semiconductor device constituted as below. 
         [0026]    The process for producing a semiconductor device of the present invention comprises the steps of: forming a semiconductor layer containing a dopant; forming a concave and convex structure on the semiconductor layer by partially removing the semiconductor layer; and forming a conductivity distribution reflecting the concave and convex structure in the semiconductor layer by performing heat treatment on the semiconductor layer in which the concave and convex structure has been formed at a temperature at which a material forming the semiconductor layer causes mass transport and filling up a hole of a concave portion of the concave and convex structure with the material forming the semiconductor layer. 
         [0027]    Moreover, a semiconductor device of the present invention comprises a semiconductor device in which a region with different conductivity distributions is formed in an in-plane direction of the semiconductor layer, wherein the semiconductor layer is formed of a semiconductor layer containing a dopant, and a hole of a concave portion of a concave and convex structure formed in the semiconductor layer is filled up with a material forming the semiconductor layer by mass transport, and wherein a conductivity distribution reflecting the concave and convex structure is formed in the semiconductor layer. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0028]    The present invention can provide a process for producing a semiconductor device and a semiconductor device which allow conductivity distribution to be formed without making refractive index distributed even in a material system of a semiconductor difficult to be subjected to ion implantation. 
         [0029]    Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]      FIGS. 1A ,  1 B and  1 C illustrate a process for producing a semiconductor device according to an embodiment of the present invention. 
           [0031]      FIGS. 2A ,  2 B,  2 C,  2 D and  2 E each are a scanning electron microscope (SEM) photo of a sample in a process of forming a concave and convex structure on a semiconductor surface and performing heat treatment thereon, which is illustrated as a reference drawing for describing an experimental result. 
           [0032]      FIGS. 3A and 3B  each are an SEM photo of a sample prepared by performing heat treatment on the same structure illustrated in  FIG. 2A  as the reference drawing for 30 minutes at 900° C. for describing an experimental result. 
           [0033]      FIGS. 4A ,  4 B,  4 C,  4 D,  4 E and  4 F illustrate a producing process according to a first example of the present invention. 
           [0034]      FIG. 5  is a schematic view describing a structure example of a DFB laser using a semiconductor device according to a second example of the present invention. 
           [0035]      FIG. 6  is a schematic view describing a structure example of a DFB laser using a semiconductor device according to a third example of the present invention. 
           [0036]      FIG. 7  is a schematic view describing a structure example of a DFB laser using a semiconductor device according to a fourth example of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    Hereinafter, embodiments of the present invention will be described. 
         [0038]    The finding as described below enables conductivity distribution to be formed without making refractive index distributed in an in-plane direction of the semiconductor layer. 
         [0039]    More specifically, the present invention is based on a new finding that when a semiconductor material containing a dopant causes mass transport, the dopant concentration changes before and after movement of atoms. 
         [0040]    The present inventors conducted experiments and obtained experimental results suggesting that the dopant concentration of a region in which atoms were moved by mass transport was lower than before the atoms were moved. 
         [0041]    Hereinafter, by referring to  FIGS. 2A to 2E , the above mentioned experimental results will be described. 
         [0042]      FIG. 2A  is a photo before heat treatment. 
         [0043]    A two-dimensional photonic crystal with a hole pitch of 160 nm, a hole diameter (upper portion) of 90 nm, and a hole depth of 270 nm is formed by electron beam drawing and dry etching. 
         [0044]    In the figure, a region  200  below the dotted line is ud-GaN, and a region  210  above the dotted line is Mg-doped p-GaN. 
         [0045]    The Mg-doped region  210  appears brighter than the ud-GaN region  200 . The SEM photo contrast reflects the concave and convex profile, composition, crystallinity, magnetism, electric potential, or the like. In the SEM photo of  FIG. 2A , the reason why the p-GaN  210  appears brighter than the ud-GaN  200  is the difference in Mg composition and the resulting difference in electric potential therefrom. The higher the Mg concentration is, the brighter the SEM photo becomes. Therefore, the Mg concentration can be estimated from the contrast of the SEM photo. 
         [0046]    The sample was heated at a temperature of 1025° C. to cause mass transport and a part of the hole was filled up. 
         [0047]      FIGS. 2B and 2C  illustrate the results.  FIG. 2C  corresponds to a region enclosed by the dotted line in  FIG. 2B . In addition, the dotted trapezoid in  FIG. 2C  indicates a hole shape before heat treatment. 
         [0048]    By referring to  FIG. 2B , the contrast reflecting the hole shape before filling up is confirmed on the sample surface. In addition, the inside of the trapezoid in  FIG. 2C  (i.e., a portion filled up by mass transport) appears darker than the surrounding region. This result suggests that the Mg concentration of the material moved by mass transport is reduced in comparison with the Mg concentration before movement. 
         [0049]    As a comparative example, the experimental results using ud-GaN in the same manner are illustrated in  FIGS. 2D and 2E . Here, the contrast observed in  FIGS. 2B and 2C  was not observed. 
         [0050]    The difference between the p-GaN and the ud-GaN used in this experiment is only whether Mg is doped or not. 
         [0051]    Therefore, it can be concluded that the contrast reflecting the concave and convex structure before heat treatment, observed after mass transport caused by heat treatment, represents the difference in doped Mg concentration. 
         [0052]    A low dopant concentration means a low carrier concentration in the semiconductor, leading to an increase in electrical resistance. 
         [0053]    Therefore, conductivity distribution can be controlled by intentionally changing the dopant concentration. 
         [0054]    Moreover, the dopant concentration is smaller than the concentration of the semiconductor material of the base material by several orders of magnitude, and thus a certain change in dopant concentration involves a very small change in refractive index. 
         [0055]    Therefore, the present invention can provide a structure whose conductivity is modulated with a uniform refractive index. 
         [0056]    The present invention is based on the above findings and provides a process for producing a semiconductor device and a semiconductor device utilizing such findings. 
         [0057]    Next, by referring to  FIGS. 1A to 1C , the description will focus on the process for producing a semiconductor device according to the present embodiment. First, as illustrated in  FIG. 1A , a semiconductor layer  110  containing a dopant is formed on a substrate  100 . 
         [0058]    Subsequently, as illustrated in  FIG. 1B , a part of the semiconductor layer  110  is removed by lithography and etching to form a concave and convex structure having a concave portion  120  and a convex portion  130  on the semiconductor layer. 
         [0059]    Subsequently, heat treatment is performed at a temperature at which the semiconductor material causes mass transport to fill up the hole of the depressed portion of the concave and convex structure. 
         [0060]    The shape after filling up is formed so as to reduce surface free energy, and thus does not always reflect the concave and convex structure, but may be mostly a flat shape as illustrated by a solid line in  FIG. 1C . 
         [0061]    However, as described above, the dopant concentration after filling up reflects the concave and convex structure. 
         [0062]    Thus, a conductivity distribution reflecting the concave and convex structure can be formed in the semiconductor layer. More specifically, the dopant concentration is low in a region  140  corresponding to the concave portion before filling up and the dopant concentration is higher in a region  150  corresponding to the convex portion before filling up in comparison with the region  140 . 
         [0063]    The reason why the dopant concentration becomes low after mass transport is mostly unknown, but is supposed such that when a crystal is decomposed by heat treatment and atoms are moved, a phenomenon occurs that the dopant is diffused in the atmospheric gas. As a result, the dopant concentration is not changed in a region in which atoms are not moved, and the dopant concentration is reduced in a region to which atoms are moved. 
         [0064]    When mass transport is caused by heat treatment, a part or all of a hole is filled up. How much of the hole is filled up depends on the aspect ratio and the side wall verticality of the concave and convex structure before heat treatment, the heat treatment temperature, the heat treatment time, and the like. 
         [0065]    More specifically, the smaller the aspect ratio, the smaller the side wall inclination to the substrate surface (i.e., worse verticality), the higher the heat treatment temperature, and the longer the heat treatment time, the easier the hole is filled up completely without voids. Of them, it is the aspect ratio that has the greatest effect. If the aspect ratio is defined as the ratio between the depth and the width of a concave portion of a striped concave and convex structure, the aspect ratio required to fill up the concave portion completely without voids is preferably equal to or less than 1. 
         [0066]    If the above conditions are appropriately set, a structure only whose side wall portion is filled up can be formed. 
         [0067]      FIGS. 3A and 3B  each illustrate an SEM photo of a sample prepared by performing heat treatment at a temperature of 900° C. for 30 minutes on the same structure as the structure illustrated in  FIG. 2A .  FIG. 3B  corresponds to a region enclosed by the dotted line in  FIG. 3A . In addition, the dotted trapezoid in  FIG. 3B  indicates a hole shape before heat treatment. 
         [0068]    Around the hole, the inside region of the trapezoid (i.e., a portion filled up by mass transport) appears darker than the outside of the trapezoid. 
         [0069]    More specifically, the inside region has a low Mg concentration and a high resistance in comparison with the outside of the trapezoid. 
         [0070]    An advantage of forming this structure is that current flowing near the hole surface can be reduced and thereby a loss such as non-radiative recombination on the surface can be suppressed. 
         [0071]    The semiconductor material used as the base material according to present invention is not limited to a particular material, but may be any material which can cause mass transport by heat treatment. 
         [0072]    Well known examples of the semiconductor materials causing mass transport include a group III-V semiconductor such as GaN, InP, and GaAs. 
         [0073]    A generally available material can be used as the dopant. Examples of the n-type dopant include Si, Ge, Se, S, Sn, Te, and C. Examples of the p-type dopant include Mg, Be, Zn, Cd, Ca, Sr, and Ba. 
         [0074]    Only one of the above dopants may be contained, or a plurality of dopants may be contained at the same time. 
         [0075]    Moreover, either one or both of the p-type dopant and the n-type dopant may be contained. 
         [0076]    If a plurality of dopants are contained, the relative concentration may be changed before and after mass transport. 
         [0077]    The cause for this is that the interatomic bond cleavability in a crystal and the mobility on the crystal surface are different depending on the dopant type. 
         [0078]    Thus, the carrier concentration after mass transport and the difference between the p-type and the n-type can be precisely controlled by appropriately designing each dopant concentration before mass transport. 
         [0079]    When a group III-V semiconductor is used, heat treatment for causing mass transport is generally performed in a group V material gas atmosphere. 
         [0080]    For example, when a MOVPE apparatus is used to perform heat treatment on GaN, the heat treatment is performed in a mixed atmosphere of nitrogen and hydrogen used as the carrier gas and ammonia which is a group V material gas. The concave and convex structure can be formed by a general lithography and etching process. For example, photolithography or electron beam lithography can be used as the lithography. 
         [0081]    Wet etching or dry etching can be used as the etching. A chemical solution or etching gas suitable for each semiconductor material to be processed is used. 
       EXAMPLES 
       [0082]    Hereinafter, examples of the present invention will be described. 
       Example 1 
       [0083]    By referring to  FIGS. 4A to 4F , a process for producing a semiconductor device according to the present example will be described. 
         [0084]    In  FIGS. 4A to 4F , are illustrated a sapphire substrate  400 , a ud-GaN buffer layer  401 , an n-GaN layer  402 , a photoresist  403 , a concave portion  420 , and a convex portion  430 . 
         [0085]    First, as illustrated in  FIG. 4A , a multi-layer wafer of group III nitride semiconductors is formed by a MOVPE apparatus. 
         [0086]    Then, the ud-GaN buffer layer  401  is formed on the sapphire substrate  400 , on which the Si-doped n-GaN layer  402  and the Mg-doped p-GaN layer  410  are grown. 
         [0087]    Here, the Mg concentration of the p-GaN layer  410  was designed to be about 4×10 19  cm −1 . After growth, the wafer was taken out of a growth furnace and was annealed for five minutes at a temperature of 900° C. in a nitrogen atmosphere to activate Mg. 
         [0088]    Subsequently, a photoresist  403  was applied to the wafer surface ( FIG. 4B ); a striped pattern was formed by photolithography ( FIG. 4C ); and a pattern was transferred to a p-GaN surface by ICP-RIE using chlorine gas ( FIG. 4D ). 
         [0089]    The striped concave portion  420  formed on the p-GaN surface was 10 μm wide and 1 μm deep. The remaining photoresist is removed by sulfuric acid-hydrogen peroxide mixture rinsing ( FIG. 4E ); then ultrasonic cleaning using an organic solvent and an alkaline solution was performed; and then the wafer was introduced into the MOVPE apparatus again to perform heat treatment. 
         [0090]    Heat treatment was performed for five minutes at a temperature of 1000° C. in an nitrogen and ammonia atmosphere to cause mass transport and fill up the striped pattern ( FIG. 4F ). 
         [0091]    As described above, the smaller the aspect ratio of the concave and convex structure (the ratio between the depth and the width of a groove), the easier the hole of the concave portion of the concave and convex structure is filled up completely without voids. According to the present example, the aspect ratio is relatively small such as 0.1, and thus the pattern is filled up without voids. 
         [0092]    As illustrated in  FIG. 4F , the shape after filling up is approximately flat, but the concentration of Mg which is a dopant reflects the striped pattern, whereby a structure whose conductivity is modulated in the striped pattern is provided. 
         [0093]    The Hall effect was measured to measure the electrical resistance of each region. The electrical resistivity of the region  450  corresponding to the convex portion before filling up was 1.2 Ωcm, but the electrical resistance of the region  440  corresponding to the concave portion was too large to be measured by our measuring apparatus. 
       Example 2 
       [0094]    By referring to  FIG. 5 , a structure example of a DFB laser using a semiconductor device according to the present example will be described. 
         [0095]    In particular, the present example focuses on a gain-coupled DFB laser made of a semiconductor device produced by the same producing process as the producing process described in example 1. The gain-coupled DFB laser according to the present example is designed to be operated near a wavelength of 405 nm. 
         [0096]    On an n-type GaN substrate  500 , there were laminated an n-Al 0.07 Ga 0.93 N cladding layer  504 , an n-GaN layer  505 , an active layer  506 , a p-GaN layer  507 , a p-Al 0.07 Ga 0.93 N cladding layer  508 , and a p + -GaN contact layer  509 . 
         [0097]    In addition, a p-electrode  511  and an n-electrode  512  were provided on the top and the bottom thereof respectively. 
         [0098]    In the n-GaN layer  505 , there were formed a high resistance region  540  and a low resistance region  550  by the same producing process as the producing process described in example 1. 
         [0099]    The high resistance region  540  and the low resistance region  550  form a periodic structure using the half length of a laser oscillation wavelength as one period. 
         [0100]    When a voltage is applied to the upper and lower electrodes, current flows. At this time, the current flows so as to avoid the high resistance region  540  and thereby a periodic current density distribution is formed. 
         [0101]    When the current injected in the active layer  506  is periodically modulated, a periodic modulation of gain occurs, and thus the DFB laser of the present example operates as the gain-coupled DFB laser. 
         [0102]    If the distance between the high resistance region  540  and the active layer  506  is too far, the current spreads and the current density in the active layer  506  becomes uniform. Thus, the DFB laser does not operate as the gain-coupled DFB laser. 
         [0103]    In order to prevent this, the distance between the high resistance region  540  and the active layer  506  needs to be set close enough not to spread the current. According to the present example, the distance was set to 10 nm. 
         [0104]    According to the present example, the high resistance region  540  was filled up without voids, and thus refractive index modulations are caused to a negligible extent, if any, could be substantially ignored. 
         [0105]    Thus, a pure gain-coupled DFB laser only whose gain was modulated was achieved. 
         [0106]    Note that the high resistance region and the low resistance region according to the present example form a periodic structure using the half length of a laser oscillation wavelength as one period, but even a periodic structure using an integer multiple of the half length of the laser oscillation wavelength as one period can operate as the gain-coupled DFB laser. 
       Example 3 
       [0107]    By referring to  FIG. 6 , a structure example of a DFB laser using a semiconductor device according to the present example will be described. 
         [0108]    In particular, the present example focuses on a gain-coupled DFB laser made of a semiconductor device produced by the same producing process as the producing process described in example 1 but different in structure from the example 2. 
         [0109]    The gain-coupled DFB laser according to the present example is designed to be operated near a wavelength of 405 nm. 
         [0110]    The present example is different from the example 2 in the following two points: 
         [0000]    (1) A high resistance region  640  is formed not in the n-GaN layer  505  but in the p-GaN layer  507 .
 
(2) The high resistance region  640  is 100 nm far from the active layer  506  and the current density of the active layer  506  is uniform.
 
         [0111]    The reason and advantages of the above mentioned differences will be described below. 
         [0112]    In example 2, a periodic structure of gain by the current density distribution is formed, but in the present example, the distance between the high resistance region and the active layer is large and thus the current density of the active layer is approximately uniform. Thus, instead of the current density, the degree of light absorption is so configured as to have a periodic structure. 
         [0113]    More specifically, the net gain obtained by subtracting loss from gain is periodically modulated. Thereby, the gain-coupled DFB laser is enabled. 
         [0114]    In general, a semiconductor containing a dopant shows a higher light absorption than an undoped semiconductor. The light absorption coefficient of ud-GaN is about 10 cm −1  at a wavelength of 400 nm, but the absorption coefficient of Si-doped n-GaN is about 30 cm −1  and the light absorption coefficient of Mg-doped p-GaN is further increased to about 100 cm −1 . 
         [0115]    Therefore, as illustrated in  FIG. 6 , formation of a periodic structure of dopant concentration allows a periodic structure of light absorption to be formed. In order to increase the modulation, use of p-GaN is more advantageous than the use of n-GaN. 
         [0116]    The present example has an advantage over the example 2 in that the distance between the high resistance region  640  and the active layer  506  can be relatively large. 
         [0117]    In the structure of example 2, if the concave and convex structure is not sufficiently flat after mass transport, the active layer grown thereon may be damaged. In order to avoid this problem, when the concave and convex structure is fabricated after the active layer is grown, it is necessary to dig in near the active layer by dry etching. As a result, the active layer may be damaged. 
         [0118]    In contrast to this, according to the structure of the present example, a uniform current density of the active layer does not cause any problem and thus the distance between the high resistance region and the active layer can be relatively large. Thus, the present example can produce a structure less damaging the above mentioned active layer. 
         [0119]    As described above, the distance between the high resistance region and the active layer is desirably far enough not to damage the active layer. 
         [0120]    However, if the distance is too far, light guided near the active layer cannot sense a periodic structure of dopant concentration made of the high resistance region and the low resistance region and thus cannot be optically coupled. 
         [0121]    Thus, the gain-coupled DFB laser is disabled. In the present example, as an appropriate distance, the distance between the high resistance region and the active layer is set to 100 nm. 
       Example 4 
       [0122]    By referring to  FIG. 7 , a structure example of a DFB laser using a semiconductor device according to the present example will be described. 
         [0123]    In particular, the present example focuses on an index-coupled DFB laser made of a semiconductor device produced by the same producing process as the producing process described in example 1. 
         [0124]    The index-coupled DFB laser according to the present example was designed to be operated near a wavelength of 405 nm. 
         [0125]    The index-coupled DFB laser structure of the present example is substantially the same structure as the structure of example 3, but is differently configured as follows. 
         [0126]    More specifically, the present example is different from example 3 in that the hole of a concave portion of the concave and convex structure is covered with a material forming the semiconductor layer so as to surround a void remaining therein, and thus a void  760  remains in a high resistance region  740 . 
         [0127]    In order to leave a void, the aspect ratio of the concave and convex structure formed before heat treatment needs to be increased. The present example formed a striped concave portion with an aspect ratio of 3 and then performed heat treatment. 
         [0128]    As a result, a structure was formed such that the high resistance region  740  surrounded the void  760 . 
         [0129]    Since voids are periodically arranged, the DFB laser of the present example operates as an index-coupled DFB laser. 
         [0130]    The difference from a general index-coupled DFB laser is that each void is surrounded by a high resistance region. 
         [0131]    In such a structure, a region covered with a material forming the semiconductor layer so as to surround the void has a lower dopant concentration than a region further surrounding the region. 
         [0132]    For this reason, current flowing near an interface between the void and the semiconductor can be reduced and as a result, non-radiative recombination on the interface can be suppressed. 
         [0133]    Thus, the index-coupled DFB laser operating with less loss and low threshold can be realized. 
         [0134]    Hereinbefore, each example of the present invention has been described, but the semiconductor device and the producing process of the present invention are not limited to these examples. 
         [0135]    The semiconductor materials and the shape of the concave and convex structure can be appropriately modified within the scope of the present invention. 
         [0136]    The above examples use a laser oscillation wavelength of 405 nm, but any oscillation wavelength may be used by selection of an appropriate material and structure. 
         [0137]    The above examples focus on the DFB laser, but the present invention can be applied not only the laser but also any device operating as an optical wavelength filter or an optical amplifier with the same structure. 
         [0138]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
         [0139]    This application claims the benefit of Japanese Patent Application No. 2009-178412, filed Jul. 30, 2009, which is hereby incorporated by reference herein in its entirety.