Abstract:
The present invention discloses a method for fabricating a semiconductor device, comprising: providing a translucent portion; forming a covering layer comprised of one or more metals on the translucent portion by vapor deposition; providing kinetic energy to the covering layer for forming a periodic mask; forming a periodic structure on the translucent portion by using the periodic mask.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This Application claims the benefit of priority and is a Continuation application of the prior International Patent Application No. PCT/JP2005/015530, with an international filing date of Aug. 26, 2005, which designated the United States, and is related to the Japanese Patent Application No. 2004-251468, filed Aug. 31, 2004, the entire disclosures of all applications are expressly incorporated by reference in their entirety herein. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to a method for fabricating a semiconductor device and a semiconductor device. 
   (2) Description of Related Art 
   A semiconductor light-emitting element formed with a low-temperature deposition buffer layer (1986, H. Amano, N. Sawaki, I. Akasaki, and Y. Toyoda: Appl. Phys. Lett., 48 (1986) 353) has been proposed in the related art as this type of semiconductor light-emitting element. A semiconductor light-emitting element to which p-type conductivity control (1989, H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki: Jpn. J. Appl. Phys. 28 (1989) L2112) and n-type conductivity control (1991, H. Amano and I. Akasaki: Mat. Res. Soc. Ext, Abst., EA-21 (1991) 165) are applied has also been proposed. A semiconductor light-emitting element created by applying a highly efficient light emitting layer fabricating method (1991, N. Yoshimoto, T. Matsuoka, T. Sasaki, and A. Katsui, Appl. Phys. Lett., 59 (1991) 2251) has also been proposed. 
     FIG. 13  shows an exemplary constitution of a group III nitride semiconductor light-emitting element serving as an example of a semiconductor light-emitting element to which the techniques described above are applied. In the drawing, a group III nitride semiconductor light-emitting element  1  comprises a sapphire substrate  2 , and a low-temperature deposition buffer layer  3  is deposited on top of the sapphire substrate  2 . An n-GaN cladding layer  4 , a GaInN light-emitting layer  5 , a p-AlGaN barrier layer  6 , and a p-GaN contact layer  7  are deposited in succession on the low-temperature deposition buffer layer  3 . A p-electrode  8  is deposited on the uppermost p-GaN contact layer  7 , and an n-electrode  9  is deposited on the n-GaN layer, thereby forming the group III nitride semiconductor light-emitting element  1 . 
   In a group III nitride semiconductor light-emitting element, represented by the semiconductor light-emitting element constituted as described above, blue light, green light, and white light can be emitted at high intensity. In other types of semiconductor light-emitting element such as AlGaInP and AlGaAs, for example, a substantially identical layer structure can be produced using a substrate having an appropriate lattice constant, and thus a high light-emission efficiency can be realized. 
   Even in a semiconductor light-emitting element having high light-emission efficiency, if the efficiency with which light is extracted to the outside of the semiconductor light-emitting element is poor, the overall energy conversion efficiency of the semiconductor light-emitting element is also poor. Hence, improvement of the light extraction efficiency is important. One of the causes of poor light extraction efficiency is a semiconductor refractive index which is larger than the refractive index of air. When the refractive index of the semiconductor is larger than the refractive index of air, a large amount of the light emitted by the light-emitting later is reflected totally, thereby becoming sealed in the interior of the semiconductor light-emitting element. 
   To solve this problem, a method of molding a semiconductor light-emitting element using an epoxy resin or the like having a refractive index between the refractive index of the semiconductor light-emitting element and the refractive index of air is known (see Semiconductor Elements, Revision, Tetsuro Ishida and Azuma Shimizu, Corona, 1980, for example). A method of improving the light extraction efficiency by forming a large number of protrusions at a peak period of 500 nm or more on the surface layer of the semiconductor light-emitting element is also known (see Japanese Unexamined Patent Application Publication 2003-174191, for example). According to the former constitution, the extreme refractive index difference between the semiconductor light-emitting element and air can be reduced, enabling a reduction in total reflection and an improvement in the light extraction efficiency. In the latter constitution, the emitted light is reflected diffusely by the surface irregularities and can therefore be extracted, enabling an improvement in the light extraction efficiency. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention discloses a method for fabricating a semiconductor device, comprising: providing a translucent portion; forming a covering layer comprised of one or more metals on the translucent portion by vapor deposition; providing kinetic energy to the covering layer for forming a periodic mask; and forming a periodic structure on the translucent portion by using the periodic mask. 
   Another optional aspect of the present invention provides a method for fabricating a semiconductor device, wherein: the periodic mask is used as an etching mask. 
   One optional aspect of the present invention provides a method for fabricating a semiconductor device, wherein: the periodic mask is used as a crystal growth mask. 
   Another optional aspect of the present invention provides a method for fabricating a semiconductor device, wherein: the kinetic energy is provided for selective reduction in effective volume of the covering layer. 
   One optional aspect of the present invention provides a method for fabricating a semiconductor device, wherein: the covering layer is comprised of Au. 
   Another optional aspect of the present invention provides a method for fabricating a semiconductor device, further including: forming a highly reflective metallic layer on the periodic mask. 
   One optional aspect of the present invention provides a semiconductor device fabricated by a method, comprising: providing a translucent portion; forming a covering layer comprised of one or more metals on the translucent portion by vapor deposition; providing kinetic energy to the covering layer for forming a periodic mask; and forming a periodic structure on the translucent portion by using the periodic mask. 
   Another optional aspect of the present invention provides a semiconductor device, comprising: a translucent portion; and a periodic structure comprised of a plurality of juts distributed randomly on a surface of the translucent portion, with the periodic structure having space period lengths with a first standard deviation that is smaller than 20% of average length of the space periods. 
   One optional aspect of the present invention provides a semiconductor device, wherein: the average length of the space periods is shorter than twice of an average optical wavelength of a light through the translucent portion. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: the light through the translucent portion is emitted by a semiconductor layer included in the semiconductor device. 
   One optional aspect of the present invention provides a semiconductor device, wherein: an average height of the juts is greater than the average optical wavelength. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: a second standard deviation in heights of the juts is smaller than 20% of the average height of the juts. 
   One optional aspect of the present invention provides a semiconductor device, wherein: the translucent portion is a substrate. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: the translucent portion substrate is comprised of SiC. 
   One optional aspect of the present invention provides a semiconductor device, wherein: the periodic structure is formed on a surface on an opposite side of the substrate to a side on which the semiconductor layer is deposited. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: a group III nitride semiconductor layer is deposited between a substrate and the semiconductor layer, and the periodic structure is formed on an interface between the substrate and the group III nitride semiconductor layer. 
   One optional aspect of the present invention provides a semiconductor device, wherein: the translucent portion is a sealing portion that seals the semiconductor device. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: the sealing portion completely seals the semiconductor device. 
   One optional aspect of the present invention provides a semiconductor device, wherein: the sealing portion partially seals the semiconductor device. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: the juts are formed in a substantially pyramidal shape. 
   One optional aspect of the present invention provides a semiconductor device, wherein: a highly reflective metallic layer is formed on the periodic structure. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: the highly reflective metallic layer constitutes an electrode. 
   One optional aspect of the present invention provides a semiconductor device, comprising: a first semiconductor layer having a first side and a second side with the first semiconductor layer having a translucent property; a low temperature deposition buffer layer on the first side of the first semiconductor layer; a cladding layer on the low temperature deposition buffer layer; a light emitting layer on the cladding layer; a barrier layer on the light emitting layer; a contact layer on the barrier layer, with the light emitting layer, the barrier layer, and the contact layer selectively etched for exposing part of the cladding layer; a n-type electrode on the exposed part of the cladding layer; and a p-type electrode on the contact layer. 
   Another optional aspect of the present invention provides a semiconductor device, wherein: the second side of the first semiconductor layer is comprised of a periodic structure that is comprised of a plurality of juts. 
   One optional aspect of the present invention provides a semiconductor device, wherein: an average distribution space period of the juts is greater than a standard deviation of the distribution space period. 
   One optional aspect of the present invention provides a semiconductor device, wherein: an average heights of the juts is greater than a standard deviation of the average heights of the juts. 
   One optional aspect of the present invention provides a semiconductor device, wherein: the juts are formed by etching the second side of the first semiconductor layer using a periodic mask that is resistant to etching medium. 
   These and other features, aspects, and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred non-limiting exemplary embodiments, taken together with the drawings and the claims that follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     It is to be understood that the drawings are to be used for the purposes of exemplary illustration only and not as a definition of the limits of the invention. Throughout the disclosure, the word “exemplary” is used exclusively to mean “serving as an example, instance, or illustration.” Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
     Referring to the drawings in which like reference character(s) present corresponding parts throughout: 
       FIG. 1  is an exemplary schematic diagram of a semiconductor light-emitting element according to a first embodiment; 
       FIG. 2  is an exemplary perspective view of a periodic structure according to the first embodiment; 
       FIG. 3  is an exemplary histogram showing the light output of the semiconductor light-emitting element to which the present invention is applied; 
       FIG. 4  is an exemplary graph showing the relationship between optical transmittance and an average period; 
       FIG. 5  is an exemplary process diagram of a periodic structure according to the first embodiment; 
       FIG. 6  is an exemplary process diagram of the periodic structure according to the first embodiment; 
       FIG. 7  is an exemplary process diagram of the periodic structure according to the first embodiment; 
       FIG. 8  is an exemplary schematic diagram of a semiconductor light-emitting element according to a second embodiment; 
       FIG. 9  is an exemplary schematic diagram of a semiconductor light-emitting element according to a third embodiment; 
       FIG. 10  is an exemplary schematic diagram of a semiconductor light-emitting element according to a fourth embodiment; 
       FIG. 11  is an exemplary schematic diagram of a semiconductor light-emitting element according to a fifth embodiment; 
       FIG. 12  is an exemplary schematic diagram of a semiconductor light-emitting element according to a sixth embodiment; and 
       FIG. 13  is an exemplary schematic diagram of a semiconductor light-emitting element according to a conventional example. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed and or utilized. 
   (1) First Embodiment 
     FIG. 1  shows an exemplary outline of the structure of a group III nitride semiconductor light-emitting element as a semiconductor device according to a first embodiment of the present invention. In the drawing, a semiconductor light-emitting element  10  is constituted by a substrate  11  as a translucent portion, a low-temperature deposition buffer layer  12 , a cladding layer  13 , a light-emitting layer  14 , a barrier layer  15 , a contact layer  16 , a p-electrode  17 , and an n-electrode  18 , all of which are formed in a substantially plate-shaped form. In the drawing, the plate-form substrate  11  constituting the lowermost layer is consisted of SiC. The low-temperature deposition buffer layer  12  consisted of AlGaN (a group III nitride semiconductor), the cladding layer  13  consisted of n-GaN, the light-emitting layer  14  consisted of GaInN, the barrier layer  15  consisted of p-AlGaN, and the contact layer  16  consisted of p-GaN are deposited in succession onto the front side surface of the substrate  11 . The plate-form p-electrode  17  is deposited onto the contact layer  16  constituting the uppermost layer, and the n-electrode  18  is deposited on the cladding layer  13 . Periodic irregularities are formed on the back side of the substrate  11 . Note that the section extending from the cladding layer  13  consisted of n-GaN to the contact layer  16  consisted of p-GaN constitutes a light-emitting portion of the present invention. 
     FIG. 2  shows an exemplary back side (the opposite surface to the surface on which the light-emitting portion is deposited) of the substrate  11  seen diagonally. In the drawing, the back surface of the substrate  11  takes an indented form created by forming a large number of substantially conical juts  11   a ,  11   a ,  11   a , . . . thereon so as to protrude downward from the back side of the substrate  11 . Note that the juts  11   a ,  11   a ,  11   a , . . . are distributed periodically in a two-dimensional direction on the back surface of the substrate  11 , and are referred to collectively as a periodic structure A 1 . The average height of the juts  11   a ,  11   a ,  11   a , . . . is approximately 300 nm, and the standard deviation thereof is approximately 20 nm. Note that the heights of the juts  11   a ,  11   a ,  11   a , . . . are assumed to be the difference between the peak heights and base heights of the juts  11   a ,  11   a ,  11   a , . . . . The average distribution space period of the juts  11   a ,  11   a ,  11   a , . . . is approximately 200 nm, and the standard deviation of this distribution space period is approximately 15 nm. Note that the interval between the peaks of adjacent juts  11   a ,  11   a ,  11   a , . . . will be referred to as the distribution space period of the juts  11   a ,  11   a ,  11   a , . . . or the average period of the periodic structure A 1 . 
   In this constitution, light can be emitted from the light-emitting layer  14  when a voltage is applied in a forward bias direction between the p-electrode  17  and n-electrode  18  of the semiconductor light-emitting element  10 . In the light-emitting layer  14 , light is emitted at a wavelength corresponding to the band gap thereof In the light-emitting portion of this embodiment, the average optical wavelength of the light is approximately 220 nm. Note that the optical wavelength is a value obtained by dividing the actual wavelength by the refractive index. Further, the wavelength of the light emitted by the light-emitting layer  14  is distributed within a wavelength bandwidth of several tens of nm, and the average value thereof is approximately 220 nm. The substrate  11 , low-temperature deposition buffer layer  12 , cladding layer  13 , barrier layer  15 , and contact layer  16  each possess a translucency, and hence the light emitted by the light-emitting layer  14  can be extracted from the back side of the substrate  11 . In other words, the back surface of the substrate  11  serves as a light extraction surface of the semiconductor light-emitting element  10 , and the light that is extracted from the extraction surface can be used for illumination and so on. 
   The light emitted from the light-emitting layer  14  penetrates the periodic structure Al formed on the back surface of the substrate  11 , and is discharged into the air on the exterior of the semiconductor light-emitting element  10 . The refractive index of the light is different in the air on the exterior of the semiconductor light-emitting element  10  and in the substrate  11  consisted of SiC, and hence the interface between the periodic structure A 1  and the air forms a reflective surface. Accordingly, light which enters the interface between the periodic structure A 1  and the air at an angle of incidence which exceeds a critical angle may be reflected on the interface and become sealed in the interior of the semiconductor light-emitting element  10 . However, in the present invention, the average period (approximately 200 nm) of the periodic structure A 1  is smaller than the optical wavelength (approximately 220 nm) of the emitted light, and hence the majority of the light that reaches the periodic structure A 1  feels a refractive index between that of the air and that of the substrate  11 . 
   The refractive index on the periodic structure A 1  may be considered to vary in accordance with the surface area ratio of the air which is distributed over a sliced surface obtained by slicing the periodic structure A 1  in a parallel direction to the back surface of the substrate  11 . In actuality, the air and the SiC of the substrate  11  are distributed non-uniformly over the sliced surface, but this non-uniform distribution exists in a shorter period than the average optical wavelength, and hence the majority of the light feels an intermediate refractive index that is dependent on the surface area ratio. On the sliced surface near the base of the periodic structure A 1 , the surface area ratio occupied by the air is small, and hence the refractive index of the substrate  11  contributes greatly at the height near the base of the periodic structure A 1 . Conversely, on the sliced surface near the peak of the periodic structure A 1 , the surface area ratio occupied by the air is large, and hence the refractive index of the air contributes greatly at the height of the peak of the periodic structure A 1 . In short, the periodic structure A 1  may be considered to have a refractive index (effective refractive index) which converges gradually toward the refractive index of the air from the refractive index of the substrate  11  as the light advances more deeply in the height direction of the periodic structure A 1 . 
   The transition of the refractive index corresponding to the height of the periodic structure A 1  depends on the shape of the juts  11   a ,  11   a ,  11   a , . . . . For example, when the juts  11   a ,  11   a ,  11   a , . . . incline linearly, as in this embodiment, the refractive index may be considered to vary in a continuous parabola. As a result, dramatic variation in the refractive index on the periodic structure A 1 , which constitutes the interface between the substrate  11  and the air, can be prevented, and light can be prevented from being reflected by the periodic structure A 1 . Note, however, that the shape of the juts  11   a ,  11   a ,  11   a , . . . is not limited to a conical shape, and the effects of the present invention can be exhibited with other shapes. In other words, any shape having a sectional area which varies gradually in accordance with the height may be employed, and accordingly the protrusions may be provided in the shape of triangular pyramids, quadrangular pyramids, hemispheres, or trapezoids, for example. 
   The height (approximately 400 nm) of the periodic structure A 1  in this embodiment is greater than the average optical wavelength of the light (approximately 220 nm) and the average period (approximately 200 nm), and hence the angle at which the incline of the juts  11   a ,  11   a ,  11   a , . . . intersects the substrate  11  can be set to a comparatively large angle (near 90 degrees). By forming the periodic structure A 1  to be high, dramatic variation in the refractive index can be prevented even in relation to light which enters the formation surface of the periodic structure A 1  at a shallow angle. Further, by forming the periodic structure A 1  to be high, the surface area ratio varies gently in accordance with the height of the periodic structure A 1 , and the gradient of linear variation in the refractive index can be reduced. In other words, dramatic variation in the refractive index can be suppressed, and a high reflection prevention ability can be realized. 
     FIG. 3  illustrates exemplary effects of the present invention in the form of a histogram. In the drawing, the abscissa shows a ratio between the light output of the present invention, formed as shown in  FIG. 1 , and the light output of a conventional semiconductor light-emitting element formed as shown in  FIG. 13 . The ordinate shows the number of samples corresponding to each light output ratio. Note that the light output was checked on 30 semiconductor light-emitting elements according to the present invention. It was found that with the samples to which the present invention was applied, a light output between 3.4 and 4.6 times (mode: 3.8 times) greater than the conventional semiconductor light-emitting element was obtained. It was also found that electric energy input into the semiconductor light-emitting element  10  could be extracted as optical energy with substantially no loss. 
   As noted above, the effects of the present invention are exhibited when the average period of the periodic structure A 1  is smaller than the average optical wavelength of the light, but by setting the standard deviation of the distribution space period of the juts  11   a ,  11   a ,  11   a , . . . within 20% (preferably within 10%) of the average period of the periodic structure A 1 , the effects of the present invention can be exhibited with certainty. Further, the standard deviation of the distribution space period of the juts  11   a ,  11   a ,  11   a , . . . is preferably as small as possible, but there is no need to form the juts  11   a ,  11   a ,  11   a , . . . regularly in a lattice shape or the like, for example. Note, however, that the juts  11   a ,  11   a ,  11   a , . . . are preferably distributed on the back surface of the substrate  11  in a two-dimensional direction in order to prevent anisotropy in effects of the present invention. The periodic structure A 1  may of course be formed in striped form, even though anisotropy occurs as a result. Further, variation in the height of the juts  11   a ,  11   a ,  11   a , . . . is preferably held within 20% (more preferably within 10%) of the average. 
     FIG. 4  shows an exemplary transmittance on the interface between the substrate  11  and the air in the form of a graph. In the diagram, the ordinate shows the optical transmittance and the abscissa shows the average period of the periodic structure A 1 . Note that the average period of the periodic structure A 1  shown on the abscissa is expressed as a multiple of the average optical wavelength (approximately 220 nm) of the emitted light. As is evident from the diagram, the transmittance improves in a region where the average period of the periodic structure A 1  is approximately 500 nm or less, i.e. 3 times the average optical wavelength (approximately 220 nm) or less. In a region where the average period of the periodic structure A 1  is double the average optical wavelength or less, a particularly high light extraction efficiency can be realized in the semiconductor light-emitting element  10 . By making the average period of the periodic structure A 1  equal to or less than the average optical wavelength, as in this embodiment, an optical transmittance of almost 100% can be realized. In other words, the average period of the periodic structure A 1  is preferably as small as possible. 
   The light emitted from the light-emitting layer  14  has an average optical wavelength of approximately 220 nm but a wavelength bandwidth of several tens of nm, and hence as the average period of the periodic structure A 1  decreases, the proportion of the emitted light that has a smaller optical wavelength than the period of the periodic structure A 1  increases. Accordingly, the optical transmittance can be raised gradually from the region in which the average period of the periodic structure A 1  is between 2 and 3 times greater than the average optical wavelength of the emitted light, and brought close to 100% in the region where the average period of the periodic structure A 1  is equal to or lower than the average optical wavelength of the emitted light. 
   Next, a fabricating method for the semiconductor light-emitting element  10  will be described. First, the substantially plate-form substrate  11  is prepared. Note that at this point in time, the periodic structure A 1  is not formed on the back side of the substrate  11 . The low-temperature deposition buffer layer  12  is formed at a predetermined thickness by growing AlGaN uniformly on the front side of the substrate  11  using a metal-organic chemical vapor deposition method. In a similar fashion, the cladding layer  13  is formed on the low-temperature deposition buffer layer  12  and the light-emitting layer  14  is formed on the cladding layer  13 . The barrier layer  15  is then formed on the light-emitting layer  14 , whereupon the contact layer  16  is formed by growing p-GaN on the barrier layer  15 . 
   After depositing the various layers in the manner described above, a covering layer  20  is formed on the back side of the substrate  11  by applying Au evenly thereto as a covering material through vapor deposition, as shown in  FIG. 5  (vapor deposition). Various vapor deposition methods may be applied to deposit the Au. For example, an EB vapor deposition apparatus which performs vapor deposition by heating the Au in a vacuum to cause the Au to transpire may be used. Further, the Au may be applied using a wet method, for example, as long as the Au can be distributed with a certain degree of uniformity over the back side of the substrate  11 . Note that in this embodiment, vapor deposition is performed such that the film thickness of the covering layer  20  is approximately 50 Å (50 m −10 ). 
   After forming the covering layer  20 , the semiconductor light-emitting element  10  is heated in an oven or the like (kinetic energy providing step for selective reduction in effective volume of the covering layer  30 ). At this time, the covering layer  20  formed on the back side of the substrate  11  is heated evenly to approximately 180° C., for example, over the entire surface of the covering layer  20 . In so doing, kinetic energy can be applied to each of the Au atoms constituting the covering layer  20 , and as a result, the Au atoms can be agglomerated on the back side surface of the substrate  11 . Then, by cooling the semiconductor light-emitting element  10 , a large number of Au particles  30 ,  30 ,  30 , . . . can be distributed over the back side surface of the substrate  11 , as shown in  FIG. 6 . The covering layer  20  is formed at an even film thickness and kinetic energy is applied evenly over the entire surface, as described above, and therefore the cohesive energy of the Au atoms may be considered uniform over the entire back side of the substrate  11 . Accordingly, the Au particles  30 ,  30 ,  30 , . . . can be distributed at even periods over the back side surface of the substrate  11 , as shown in  FIG. 6 . 
   Note that the distribution space period of the Au particles  30 ,  30 ,  30 , . . . may be controlled in accordance with the heating temperature, the film thickness of the covering layer  20 , and so on. In this embodiment, the covering layer  20  having a film thickness of approximately 50 Å (50 m −10 ) is heated to approximately 180° C., whereby the Au particles  30 ,  30 ,  30 , . . . can be distributed in an average period of approximately 200 nm. To increase the distribution space period of the Au particles  30 ,  30 ,  30 , . . . , the heating temperature may be raised or the film thickness of the covering layer  20  may be increased, for example. Conversely, to reduce the distribution space period of the Au particles  30 ,  30 ,  30 , . . . , the heating temperature may be lowered or the film thickness of the covering layer  20  may be decreased. Further, as long as kinetic energy can be applied to the covering layer  20  to the extent that the Au atoms can be agglomerated, the periodic Au particles  30 ,  30 ,  30 , . . . may be formed using a method other than heating. For example, kinetic energy may be applied to the covering layer through ion irradiation, electron irradiation, and so on. Note that the Au particles  30 ,  30 ,  30 , . . . form a periodic pattern having an average period which is equal to or lower than the average optical wavelength, and hence as a whole, the Au particles  30 ,  30 ,  30 , . . . constitute a periodic mask of the present invention (mask forming step). 
   After forming the Au particles  30 ,  30 ,  30 , . . . so as to be distributed periodically over the back side surface of the substrate  11  in the manner described above, the back side of the substrate  11  is etched using a reactive ion etching apparatus (etching step). In this embodiment, CF 4  gas is used as an etching medium. Needless to say, another etching gas may be used, or etching may be performed using an etching liquid. The etching resistance of Au to CF 4  gas is higher than the etching resistance of SiC to CF 4  gas, and hence the SiC may be etched selectively. The etching direction is perpendicular to the back surface of the substrate  11 , and etching may be performed only on the parts of the back side surface of the substrate  11  to which the Au particles  30 ,  30 ,  30 , . . . are not adhered. 
   More specifically, etching may be performed using the periodic mask constituted by the large number of Au particles  30 ,  30 ,  30 , . . . as an etching resist. In so doing, the periodic structure A 1  may be formed as shown in  FIG. 8  (periodic structure forming step). Note that by increasing the etching speed, etching typically progresses perpendicular to the back surface of the substrate  11 , and as a result the angle of incline of the juts  11   a ,  11   a ,  11   a , . . . becomes almost perpendicular to the back surface of the substrate  11 . Conversely, by reducing the etching speed, side etching is performed, and hence the angle of incline of the juts  11   a ,  11   a ,  11   a , . . . becomes an acute angle in relation to the back surface of the substrate  11 . 
   By performing etching using the periodic mask in this manner, the shape of the periodic structure A 1  can be controlled to a desired shape. Furthermore, as long as etching is not performed excessively, the peaks of the juts  11   a ,  11   a ,  11   a , . . . can be aligned in height. As a result, variation in the height of the juts  11   a ,  11   a ,  11   a , . . . can be reduced. Note that the amount of side etching may be increased intentionally to remove the Au particle  30 , as shown on the third protruding portion  11   a  from the left in  FIG. 7 . Furthermore, the etching conditions may be set such that the Au particle  30  is removed by etching. Note that in this embodiment, the performance of the semiconductor light-emitting element  10  is not greatly diminished even when the Au particles  30 ,  30 ,  30 , . . . remain in the interior of the semiconductor light-emitting element  10 . However, the performance may be diminished depending on the material of the covering layer, and in such a case the periodic mask is preferably removed. 
   The material of the periodic mask is not limited to Au, and any material may be used as long as the etching resistance to the etching medium is greater than that of the substrate. Specifically, the etching selection ratio between the periodic mask and the substrate is preferably at least 0.1, and more preferably at least 1. Examples of periodic mask materials that are effective in relation to CF 4  gas include Ga, In, Al, Cu, Ag, Ni, Pt, Pd, SiN, SiO 2 , or an insulator. An appropriate periodic mask material is selected in accordance with the etching medium, and hence it goes without saying that other periodic mask materials may be applied. Note, however, that when atom or molecule agglomeration is employed in the periodic mask forming step, as in this embodiment, a gatherable covering material such as Au must be selected. 
   Furthermore, in this embodiment the periodic mask is formed using agglomeration of the covering layer, but a periodic mask may be formed using another method. For example, a periodic mask pattern may be formed using a stepper employing an excimer laser. Alternatively, a periodic mask pattern may be formed by subjecting a photosensitive mask material to electron beam exposure and so on or two-beam interference exposure. 
   After forming the periodic structure A 1  in the manner described above, the p-electrode  17  and n-electrode  18  are formed and the semiconductor light-emitting element  10  is packaged. Note that the cladding layer  13  may be exposed by selectively etching the uniformly deposited light-emitting layer  14 , barrier layer  15 , and contact layer  16  to form the n-electrode  18 , or the cladding layer  13  may be exposed by selectively growing the light-emitting layer  14 , barrier layer  15 , and contact layer  16  in advance to form the n-electrode  18 . Further, the periodic structure A 1  may be formed after forming the n-electrode  17  and n-electrode  18 . 
   Further, the various layers may be formed on the front side of the substrate  11  after forming the periodic structure A 1  on the back side of the substrate  11  in advance. Moreover, the substrate  11  may be consisted of a material other than SiC as long as it possesses a translucency. For example, a sapphire substrate, a GaN substrate, a Ga 2 O 3  substrate, a GaN substrate, and so on may be applied. Needless to say, the present invention is also applicable to another type of semiconductor light-emitting element such as AlGaInP or AlGaAs, for example. Note that the average optical wavelength of the emitted light varies according to the type of light-emitting layer, but as long as the periodic structure A 1  is formed in a period which is double the average optical wavelength or less (preferably no greater than the average optical wavelength), a high light extraction efficiency can still be realized. 
   (2) Second Embodiment 
     FIG. 8  shows an exemplary outline of the structure of a group III nitride semiconductor light-emitting element as a semiconductor device according to a second embodiment of the present invention. In the drawing, a semiconductor light-emitting element  110  is constituted by a substrate  111  as a translucent portion, a low-temperature deposition buffer layer  112 , a cladding layer  113 , a light-emitting layer  114 , a barrier layer  115 , a contact layer  116 , a p-electrode  117 , and an n-electrode  118 , all of which are formed in a substantially plate-shaped form. The plate-form substrate  111  constituting the lowermost layer is consisted of SiC. The low-temperature deposition buffer layer  112  consisted of AlGaN, the cladding layer  113  consisted of n-GaN, the light-emitting layer  114  consisted of GaInN, the barrier layer  115  consisted of p-AlGaN, and the contact layer  116  consisted of p-GaN are deposited in succession onto the front side surface of the substrate  111 . A periodical structure A 2  constituted by periodically arranged Au particles  130 ,  130 ,  130 , . . . is provided on the uppermost contact layer  116 , and a highly reflective metallic layer consisted of Cu and serving as the p-electrode  117  is deposited onto the contact layer  116  formed with the periodical structure A 2 . The back side of the substrate  111  is a flat surface, and the n-electrode  118  is deposited thereon. 
   With this constitution, light can be emitted from the light-emitting layer  114  by applying a voltage to the semiconductor light-emitting element  110  in a forward bias direction. In the light-emitting layer  114 , light is emitted in accordance with the band gap thereof, and the average optical wavelength of the light is approximately 220 nm. The substrate  111 , low-temperature deposition buffer layer  112 , cladding layer  113 , barrier layer  115 , and contact layer  116  each possess a translucency, and hence the light emitted by the light-emitting layer  114  can be extracted from the back side of the substrate  111 . In other words, the back side of the substrate  111  serves as a light extraction surface of the semiconductor light-emitting element  110 , and the light that is extracted from the extraction surface can be used for illumination and so on. 
   Meanwhile, the upper surface of the contact layer  116  is covered by the p-electrode  117 , which is consisted of highly reflective Cu, and by reflecting the emitted light, the light is prevented from leaking from the p-electrode  117  side. The reflected light can be extracted from the light extraction surface and used for illumination and so on. By forming the periodical structure A 2 , diffuse reflection can be promoted, and hence the reflectance on the interface between the contact layer  116  and the p-electrode  117  can be improved. As a result, the amount of light that is ultimately extracted from the light extraction surface of the semiconductor light-emitting element  110  can be increased, enabling an improvement in the light extraction efficiency to approximately 1.3 times the normal light extraction efficiency. 
   Next, a fabricating method for the semiconductor light-emitting element  110  will be described. First, the substantially plate-form substrate  111  is prepared. The low-temperature deposition buffer layer  112  is then formed at a predetermined thickness by growing AlGaN uniformly on the front side of the substrate  111  using a metal-organic chemical vapor deposition method. In a similar fashion, the cladding layer  113  is formed on the low-temperature deposition buffer layer  112  and the light-emitting layer  114  is formed on the cladding layer  113 . The barrier layer  115  is then formed on the light-emitting layer  114 , whereupon the contact layer  116  is formed by growing p-GaN on the barrier layer  115 . 
   After depositing the various layers in the manner described above, a similar covering layer to that of  FIG. 5  is formed on the surface of the contact layer  116  by applying Au evenly thereto as a covering material through vapor deposition. Various vapor deposition methods may be applied to deposit the Au. For example, an EB vapor deposition apparatus which performs vapor deposition by heating the Au in a vacuum to cause the Au to transpire may be used. Further, the Au may be applied using a wet method, for example, as long as the Au can be distributed with a certain degree of uniformity over the surface of the contact layer  116 . Note that in this embodiment, vapor deposition is performed such that the film thickness of the covering layer is approximately 50 Å (50 m −10 ). 
   After forming the covering layer, the semiconductor light-emitting element  110  is heated in an oven or the like. At this time, the covering layer formed on the surface of the contact layer  116  is heated to approximately 180° C., for example. In so doing, kinetic energy can be applied to each of the Au atoms constituting the covering layer, and as a result, the Au atoms can be agglomerated on the surface of the contact layer  116 . Then, by cooling the semiconductor light-emitting element  110 , a large number of Au particles  130 ,  130 ,  130 , . . . can be distributed over the surface of the contact layer  116 . As described above, the covering layer is formed at an even film thickness, and the cohesive energy of the Au atoms which agglomerate during heating may be considered uniform over the surface of the contact layer  116 . Accordingly, the Au particles  130 ,  130 ,  130 , . . . can be distributed in a uniform periodical form on the surface of the contact layer  116 , similarly to  FIG. 6 . 
   After forming the Au particles  130 ,  130 ,  130 , . . . so as to be distributed periodically over the surface of the contact layer  116  in the manner described above, Cu is applied to the contact layer  116  and the Au particles  130 ,  130 ,  130 , . . . through vapor deposition (highly reflective metallic layer forming step). An EB vapor deposition apparatus or the like may be used here to deposit the Cu, or Cu may be applied to the surface of the contact layer  116  using a method other than vapor deposition. In the initial stage of vapor deposition, the Au particles  130 ,  130 ,  130 , . . . form irregularities on the surface of the contact layer  116 , but as vapor deposition progresses, the gaps between the Au particles  130 ,  130 ,  130 , . . . are filled by the Cu such that eventually a flat surface is formed as the p-electrode  117 . In other words, a highly reflective metallic layer is formed as the p-electrode  117  so as to contact the interface with the periodical structure constituted by the Au particles  130 ,  130 ,  130 , . . . . 
   The substrate of this embodiment may be consisted of a material other than SiC as long as it possesses a translucency. For example, a sapphire substrate, a GaN substrate, a Ga 2 O 3  substrate, a GaN substrate, and so on may be applied. Needless to say, the present invention is also applicable to another type of semiconductor light-emitting element such as AlGaInP or AlGaAs, for example. Note that the average optical wavelength of the emitted light varies according to the type of light-emitting layer, but as long as the periodic structure A 2  is formed at a period which is no greater than the average optical wavelength, a high light extraction efficiency can still be realized. Furthermore, in this embodiment Cu is cited as an example of the material used to form the highly reflective metallic layer, but the highly reflective metallic layer may be consisted of Rh, Ag, Al, Ni, Pt, Cu, an alloy thereof, and so on. By using the highly reflective metallic layer as an electrode, a reduction in the number of fabricating steps can be realized. However, the highly reflective metallic layer and electrode may be formed separately. 
   (3) Third Embodiment 
     FIG. 9  shows an exemplary outline of the structure of a group III nitride semiconductor light-emitting element as a semiconductor device according to a third embodiment. In the drawing, a semiconductor light-emitting element  210  is constituted by a substrate  211 , a low-temperature deposition buffer layer  212 , a cladding layer  213 , a light-emitting layer  214 , a barrier layer  215 , a contact layer  216 , a p-electrode  217 , and an n-electrode  218 , all of which are formed in a substantially plate-shaped form. The plate-form substrate  211  constituting the lowermost layer is consisted of SiC. The low-temperature deposition buffer layer  212  consisted of AlGaN, the cladding layer  213  consisted of n-GaN, the light-emitting layer  214  consisted of GaInN, the barrier layer  215  consisted of p-AlGaN, and the contact layer  216  consisted of p-GaN are deposited in succession onto the front side surface of the substrate  211 . A periodical structure A 3  (with an average period of approximately 200 nm and an average height of 400 nm) is formed by a large number of juts protruding upward from the uppermost contact layer  216 . The p-electrode  217  consisted of Cu is deposited onto the periodical structure A 3 , and the n-electrode  218  is deposited onto the back side of the substrate  211 . 
   With this constitution, light can be emitted from the light-emitting layer  214  by applying a voltage to the semiconductor light-emitting element  210  in a forward bias direction. In the light-emitting layer  214 , light is emitted in accordance with the band gap thereof, and the average optical wavelength of the light is approximately 220 nm. The substrate  211 , low-temperature deposition buffer layer  212 , cladding layer  213 , barrier layer  215 , and contact layer  216  each possess a translucency, and hence the light emitted by the light-emitting layer  214  can be extracted from the back side of the substrate  211 . In other words, the back surface of the substrate  211  serves as a light extraction surface of the semiconductor light-emitting element  210 , and the light that is extracted from the extraction surface can be used for illumination and so on. 
   Meanwhile, the upper surface of the contact layer  216  is covered by the p-electrode  217  consisted of highly reflective Cu, and by reflecting the emitted light, the light can be prevented from leaking from the p-electrode  217  side. The reflected light can be extracted from the light extraction surface and used for illumination and so on. By forming the periodical structure A 3 , diffuse reflection can be promoted, and hence the reflectance on the interface between the contact layer  216  and the p-electrode  217  can be improved. As a result, the amount of light that is ultimately extracted from the light extraction surface of the semiconductor light-emitting element  210  can be increased, enabling an improvement in the light extraction efficiency. 
   Next, a fabricating method for the semiconductor light-emitting element  210  will be described. First, the substantially plate-form substrate  211  is prepared. The low-temperature deposition buffer layer  212  is then formed at a predetermined thickness by growing AlGaN uniformly on the front side of the substrate  211  using a metal-organic chemical vapor deposition method. In a similar fashion, the cladding layer  213  is formed on the low-temperature deposition buffer layer  212 , and the light-emitting layer  214  and barrier layer  215  are formed on the cladding layer  213 . The contact layer  216  is then formed by growing p-GaN on the barrier layer  115 . 
   After depositing the various layers in the manner described above, the periodic structure A 3  is formed on the surface of the contact layer  216 . A similar method to that of the first embodiment may be applied to form the periodic structure A 3 , and hence description thereof has been omitted here. Once the periodic structure A 3  has been formed, Cu is applied to the surface of the contact layer  216  through vapor deposition. In the initial stage of vapor deposition, the periodic structure A 3  forms irregularities on the surface of the contact layer  216 , but as vapor deposition progresses, the gaps in the periodic structure A 3  are filled by the Cu such that eventually a flat surface is formed as the p-electrode  217 . In the previous embodiment, the number of fabricating steps can be reduced by employing the Au particles as the periodic structure A 2 . In this embodiment, on the other hand, the shape of the periodic structure A 3  can be controlled by forming the periodic structure A 3  using the Au particles as a periodic mask. 
   (4) Fourth Embodiment 
     FIG. 10  shows an exemplary outline of the structure of a group III nitride semiconductor light-emitting element as a semiconductor device according to a fourth embodiment. In the drawing, a semiconductor light-emitting element  310  is constituted by a substrate  311 , a low-temperature deposition buffer layer  312 , a cladding layer  313 , a light-emitting layer  314 , a barrier layer  315 , a contact layer  316 , a p-electrode  317 , and an n-electrode  318 , all of which are formed in a substantially plate-shaped form. The plate-form substrate  311  constituting the lowermost layer is consisted of SiC. The low-temperature deposition buffer layer  312  consisted of AlGaN, the cladding layer  313  consisted of n-GaN, the light-emitting layer  314  consisted of GaInN, the barrier layer  315  consisted of p-AlGaN, and the contact layer  316  consisted of p-GaN are deposited in succession onto the front side surface of the substrate  311 . The p-electrode  317  is deposited onto the contact layer  316  constituting the uppermost layer, and the n-electrode  318  is deposited onto the back side of the substrate  311 . 
   An indented periodic structure A 4  is formed periodically on the front surface side of the substrate  311  as a translucent portion, and the low-temperature deposition buffer layer  312  and cladding layer  313  are formed in alignment with periodic structures  311   a ,  311   a ,  311   a , . . . . The front surface side of the cladding layer  313  is flat, and all of the layers above the cladding layer  313  are formed to be flat. 
   By forming the indented periodic structure A 4  periodically on the front surface side of the substrate  311  in this manner, reflectance on the interface between the substrate  311  and low-temperature deposition buffer layer  312  can be reduced. The refractive index of the substrate  311  is different to the refractive index of the low-temperature deposition buffer layer  312 , but by means of the periodic structure A 4 , dramatic variation in the refractive index can be suppressed. Further, by forming a layer having a thin film thickness such as the low-temperature deposition buffer layer  312 , the irregular form of the periodic structure A 4  is maintained, and hence the interface between the low-temperature deposition buffer layer  312  and the cladding layer  313  deposited thereon can also be formed in a periodically indented shape. Accordingly, reflectance on the-interface between the low-temperature deposition buffer layer  312  and the cladding layer  313  can also be reduced. 
   By forming the periodic structure on a plurality of interfaces in this manner, the light extraction efficiency can be further improved. Further, by forming a thin film layer (the low-temperature deposition buffer layer  312 ) on the periodic structure A 4  at a thickness which does not flatten the periodic structure A 4 , an irregular shape can be maintained on the surface of the thin film layer (low-temperature deposition buffer layer  312 ). Accordingly, by depositing an upper layer (the cladding layer  313 ) on the surface of the thin film layer (low-temperature deposition buffer layer  312 ) a periodic structure can be formed on the interface between the thin film layer (low-temperature deposition buffer layer  312 ) and the upper layer (cladding layer  313 ). In other words, steps for forming periodic structures individually on each interface need not be performed, and a semiconductor light-emitting element having a high light extraction efficiency can be manufactured at a low fabricating cost. 
   Next, a fabricating method for the semiconductor light-emitting element  310  will be described. First, the substantially plate-form substrate  311  is prepared. Next, the periodic structure A 4  is formed on the front side of the substrate  311 . A similar method to the method of forming the periodic structure A 1  on the back side of the substrate  11  in the first embodiment may be applied to form the periodic structure A 4 , and hence description thereof has been omitted here. After forming the periodic structure A 4 , the low-temperature deposition buffer layer  312  is formed in a shape corresponding to the periodic structure A 4  by growing AlGaN uniformly on the front side of the substrate  311  using a metal-organic chemical vapor deposition method. 
   The cladding layer  313  is then formed by growing n-GaN on the front side of the low-temperature deposition buffer layer  312  using a metal-organic chemical vapor deposition method. When the cladding layer  313  has been formed to a certain extent, the recessed portions of the periodic structure A 4  are buried by the n-GaN such that ultimately, a flat surface is formed. Once the flat surface of the cladding layer  313  has been formed, the light-emitting layer  314  is formed on the cladding layer  313 , and the barrier layer  315  is grown on the light-emitting layer  314 . The contact layer  316  is then formed by growing p-GaN on the barrier layer  315 . The p-electrode  317  is deposited onto the uppermost contact layer  316 , and the n-electrode  318  is deposited onto the back side of the substrate  311 . 
   (5) Fifth Embodiment 
     FIG. 11  shows an exemplary outline of the structure of a group III nitride semiconductor light-emitting element as a semiconductor device according to a fifth embodiment of the present invention. In the drawing, a semiconductor light-emitting element  410  is constituted by a substrate  411 , a low-temperature deposition buffer layer  412 , a cladding layer  413 , a light-emitting layer  414 , a barrier layer  415 , a contact layer  416 , a p-electrode  417 , and an n-electrode  418 , all of which are formed in a substantially plate-shaped form. The plate-form substrate  411  constituting the lowermost layer is consisted of SiC. The low-temperature deposition buffer layer  412  consisted of AlGaN, the cladding layer  413  consisted of n-GaN, the light-emitting layer  414  consisted of GaInN, the barrier layer  415  consisted of p-AlGaN, and the contact layer  416  consisted of p-GaN are deposited in succession onto the front side surface of the substrate  411 . The p-electrode  417  is deposited onto the contact layer  416 , and the n-electrode  418  is deposited onto the back side of the substrate  411 . Note that the p-electrode  417  is consisted of transparent mesh-form Ni/Au or the like, and is capable of transmitting light. The p-electrode  417  may employ a transparent electrode made of Ga 2 O 3 , ZnO, ITO, or the like, as long as it is capable of transmitting light to a certain extent. 
   The substrate  411 , low-temperature deposition buffer layer  412 , cladding layer  413 , light-emitting layer  414 , barrier layer  415 , contact layer  416 , and n-electrode  418  are deposited in flat plate form. An indented periodic structure A 5  is formed periodically on the surface of the contact layer  416 , and the p-electrode  417  is deposited onto the periodic structure A 5  so as to follow the indentations of the periodic structure A 5 . The surface of the p-electrode  417  is formed so as to maintain the irregularities of the periodic structure A 5 . 
   By forming the indented periodic structure A 5  periodically on the front side of the contact layer  416  in this manner, reflectance on the interface between the contact layer  416  and the p-electrode  417  can be reduced. The refractive index of the contact layer  416  is different to the refractive index of the p-electrode  417 , but by means of the periodic structure A 5 , dramatic variation in the refractive index can be suppressed. Further, by forming a layer having a thin film thickness such as the p-electrode  417 , the irregular form of the periodic structure A 5  is maintained, and hence the interface between the p-electrode  417  and the air can also be formed in a periodically indented shape. Accordingly, reflectance on the interface between the p-electrode  417  and the air can also be reduced. 
   Next, a fabricating method for the semiconductor light-emitting element  410  will be described. First, the substantially plate-form substrate  411  is prepared. Next, the low-temperature deposition buffer layer  412  is formed by growing AlGaN uniformly on the front side of the substrate  411  using a metal-organic chemical vapor deposition method. The cladding layer  413  is then formed by growing n-GaN on the front side of the low-temperature deposition buffer layer  412  using a metal-organic chemical vapor deposition method. The light-emitting layer  414  is then formed on the cladding layer  413 , and the barrier layer  415  is formed on the light-emitting layer  414 . The contact layer  416  is then formed by growing p-GaN on the barrier layer  415 . 
   The indented periodic structure A 5  is then formed periodically on the contact layer  416  as a translucent portion. A similar method to the method used to form the periodic structure A 1  on the back side of the substrate  11  in the first embodiment may be applied to form the periodic structure A 5 , and hence description thereof has been omitted here. After forming the periodic structure A 5 , the p-electrode  417  is deposited onto the periodic structure A 5  through coating or vapor deposition. Meanwhile, the n-electrode  418  is deposited onto the back side of the substrate  411 . 
   (6) Sixth Embodiment 
     FIG. 12  shows an exemplary outline of the structure of a group III nitride semiconductor light-emitting element according to a sixth embodiment. In the drawing, a hemispherical dome-shaped sealing portion  60  is formed, and the semiconductor light-emitting element  10  of the first embodiment is buried within the interior of the sealing portion  60  such that the light extraction surface is oriented upward on the paper surface. The sealing portion  60  is consisted of a synthetic resin such as transparent epoxy resin, and is capable of transmitting light emitted from the semiconductor light-emitting element  10  to the outside. The surface of the sealing portion  60  as a translucent portion is formed with a periodically indented periodic structure A 6 . A similar method to the method used to form the periodic structure A 1  on the back side of the substrate  11  in the first embodiment may be applied to form the periodic structure A 6 , and hence description thereof has been omitted here. 
   By forming the indented periodic structure A 6  periodically on the surface of the sealing portion  60  in this manner, reflectance on the interface between the sealing portion  60  and the outside air can be reduced. The refractive index of the air is different from that of the sealing portion  60 , but by means of the periodic structure A 6 , dramatic variation in the refractive index can be suppressed. Note that the semiconductor light-emitting element  10  may be sealed in the sealing portion  60  in various ways, and only the light extraction surface may be sealed in the sealing portion  60 . In this case also, the efficiency with which light is extracted to the outside of the sealing portion  60  can be improved by forming the periodic structure A 6  on the surface of the sealing portion  60 . 
   SUMMARY 
   According to the present invention described above, by forming the periodic structure A 1  on the light extraction surface of the semiconductor light-emitting element  10  in a period which is double the average optical wavelength of the light or less, the refractive index difference on the light extraction surface can be reduced. As a result, reflection on the light extraction surface can be prevented, enabling the realization of a high light extraction efficiency. Furthermore, a fine periodic mask can be formed by heating an Au thin film, and therefore the periodic structure A 1  can be formed easily and at low cost. 
   Further, a semiconductor light-emitting element may be formed by combining the various embodiments appropriately. For example, the semiconductor light-emitting elements of the first through fifth embodiments may be sealed inside the sealing portion  60  of the sixth embodiment. Further, a semiconductor light-emitting element may be formed by combining the constitution of the first embodiment with the constitution of the second or third embodiment, for example. According to this constitution, high reflectance can be realized on the opposite surface of the light-emitting portion to the light extraction surface while realizing high transmittance on the light extraction surface, and hence the light extraction efficiency can be improved synergistically. Although the invention has been described in considerable detail in language specific to structural features and or method acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as preferred forms of implementing the claimed invention. Therefore, while exemplary illustrative embodiments of the invention have been described, numerous variations and alternative embodiments will occur to those skilled in the art. For example, the material of the substrate can be changed. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention.