Abstract:
A method of growing a nitride single crystal layer, and a method of manufacturing a light emitting device using the method are disclosed. The method of growing a nitride single crystal layer comprises the steps of preparing a silicon substrate having an upper surface of a crystal plane ( 111 ), forming a buffer layer having the formula of Si x Ge 1-x , (where 0&lt;x≦1) on the upper surface of the silicon substrate, and forming a nitride single crystal on the buffer layer. Also, a nitride light emitting device using the method manufactured by the method, and a method of manufacturing the same are disclosed.

Description:
RELATED APPLICATION  
       [0001]     The present invention is based on, and claims priority from, Korean Application No. 2004-29477, filed on Apr. 28, 2004, the disclosure of which is incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method of growing a nitride single crystal, and, more particularly, to a method of growing a high quality nitride single crystal on a silicon substrate, a nitride semiconductor light emitting device using the same, and a method of manufacturing the nitride semiconductor light emitting device.  
         [0004]     2. Description of the Related Art  
         [0005]     A nitride semiconductor light emitting device is a high power optic device, which generates light having a short wavelength, such as blue or green light, and thereby enables full color to be realized, and is spotlighted in the field of related technologies. Generally, a nitride semiconductor light emitting device is made of a nitride single crystal having the formula Al x In y Ga (1-x-y) N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1).  
         [0006]     In order to manufacture such a nitride semiconductor light emitting device, it is necessary to provide a technology for growing a high quality nitride single crystal. However, there is a problem in that substrates currently used for growing the nitride single crystal are not appropriate due to differences in lattice parameters and thermal expansion coefficients between the substrate and the nitride single crystal.  
         [0007]     Generally, the nitride single crystal is grown on a dissimilar substrate, such as a sapphire (Al 2 O 3 ) substrate or a SiC substrate, by means of the MBE (Molecular Beam Epitaxy) process or a vapor phase growth process, such as the MOCVD (Metal Organic Chemical Vapor Deposition) process, the HVPE (Hydride Vapor Phase Epitaxy) process, etc.  
         [0008]     Since the dissimilar substrate, such as a sapphire (α-Al 2 O 3 ) substrate or a SiC substrate, is not only high priced, but also very restricted to a size of 2 or 3 inches, it is not appropriate for mass production.  
         [0009]     Accordingly, it is needed to use a Si substrate, which is most generally used as a substrate in the semiconductor industry, including the light emitting device industry. However, due to differences in lattice parameter and thermal expansion coefficient between the Si substrate and a GaN single crystal, there is a problem in that cracks can be created at an interface between the sapphire substrate and the GaN single crystal to such an extent that the GaN layer cannot be practically used. As for a method of relieving the differences, provision of a buffer layer on the Si substrate has been suggested, but this method is not regarded as an appropriate method for solving the problem as mentioned above.  FIGS. 1   a  and  1   b  show a GaN single crystal grown by use of a conventional AlN buffer layer and a buffer structure, which is combined with the AlN buffer layer and an AlGaN intermediate layer.  
         [0010]     First, as shown in  FIG. 1   a , a conventional AlN buffer layer  12  is formed on a (111) plane of a Si substrate  11 , and a GaN single crystal  15  having a thickness of 2 μm is grown on the AlN buffer layer  12 .  FIG. 2   a  is an optical micrograph showing a surface of the GaN single crystal  15  of  FIG. 1   a . As shown in  FIG. 2   a , it can be seen that a plurality of cracks are created on the surface of the GaN single crystal  15 . These cracks are created due to unresolved differences in lattice parameter and thermal expansion coefficient between the Si substrate and the GaN single crystal, thereby not only deteriorating performance of the device and life span thereof, but also making it impossible to use the GaN single crystal in practice.  
         [0011]     As an alternative method, as shown in  FIG. 1   b , with an AlN buffer layer  12  formed on a (111) plane of a Si substrate  11 , a Al x Ga 1-x N intermediate layer  13  having Al compositions (x) of 0.87 to 0.07 and a total thickness of 300 nm is formed on the AlN buffer layer  12 , and a GaN single crystal  15  having a thickness of 2 μm is grown thereon.  FIG. 2   b  is an optical micrograph showing a surface of the GaN single crystal  15  of  FIG. 1   b . As shown in  FIG. 2   b , it can be seen that, although the number of cracks created on the surface of the GaN single crystal  15  of  FIG. 1   b  is decreased in comparison to the GaN single crystal  15  of  FIG. 2   a , a number of cracks are still created on the surface of the crystal  15  of  FIG. 1   b . That is, it can be understood that the buffer structure suggested in  FIG. 1   b  cannot satisfy requirements for growing the high quality single crystal.  
         [0012]     Accordingly, in the field of the prior art, there is a need to provide a method of growing a high quality nitride single crystal layer, which does not create cracks, on an Si substrate, and a nitride semiconductor light emitting device using the same.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention has been made to solve the above problems, and it is an object of the present invention to provide a method of growing a nitride single crystal layer, using a buffer layer comprising Si and Ge in order to allow a high quality nitride single crystal layer to be grown on a silicon substrate.  
         [0014]     It is another object of the present invention to provide a nitride semiconductor light emitting device comprising a nitride single crystal layer grown on a silicon substrate, and a method of manufacturing the same.  
         [0015]     In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a method of growing a nitride single crystal layer, comprising the steps of: preparing a silicon substrate having an upper surface of a (111) crystal plane; forming a buffer layer having the formula of Si x Ge 1-x , (where 0&lt;x≦1) on the upper surface of the silicon substrate; and forming a nitride single crystal on the buffer layer.  
         [0016]     The method may further comprise forming an intermediate layer having the formula of Al y In z Ga (1-y-z) N, (where 0≦y≦1, 0≦z≦1, 0≦y+z≦1) on the buffer layer before forming the nitride single crystal.  
         [0017]     The buffer layer may have an Si composition (x) of about 0.1˜0.2, and more preferably of about 0.14.  
         [0018]     In order to efficiently reduce differences in lattice parameter and thermal expansion coefficient between the silicon substrate and the nitride single crystal, preferably, the buffer layer has an Si composition gradient (x) gradually decreasing from a portion, where the buffer layer contacts the silicon substrate, to an uppermost portion of the buffer layer. More preferably, the buffer layer has an Si composition gradient (x) gradually decreasing from 1 to 0.1 from the portion, where the buffer layer contacts the silicon substrate, to the uppermost portion of the buffer layer, respectively. Most preferably, the buffer layer has an Si composition gradient (x) gradually decreasing from 1 to 0.14 from the portion, where the buffer layer contacts the silicon substrate, to the uppermost portion of the buffer layer, respectively.  
         [0019]     The buffer layer may have a thickness of at least 20 nm in order to sufficiently secure a buffering function.  
         [0020]     In accordance with another aspect of the present invention, there is provided a nitride semiconductor light emitting device using the method of growing a nitride single crystal layer, the nitride semiconductor light emitting device comprising: a silicon substrate having an upper surface of a (111) crystal plane; a buffer layer having the formula of Si x Ge 1-x , (where 0&lt;x≦1) on the silicon substrate; a first conductive nitride semiconductor layer on the buffer layer; an active layer on the first conductive nitride semiconductor layer; and a second conductive nitride semiconductor layer on the first conductive nitride semiconductor layer.  
         [0021]     In accordance with yet another aspect of the present invention, there is provided a method of manufacturing a nitride semiconductor light emitting device by use of the method of growing a nitride single crystal layer, the method comprising the steps of: preparing a silicon substrate having an upper surface of a (111) crystal plane; forming a buffer layer having the formula of Si x Ge 1-x , (where 0&lt;x≦1) on the silicon substrate; forming a first conductive nitride semiconductor layer on the buffer layer; forming an active layer on the first conductive nitride semiconductor layer; and forming a second conductive nitride semiconductor layer on the first conductive nitride semiconductor layer.  
         [0022]     According to the present invention, the buffer layer employed for growing the nitride single crystal on the silicon substrate comprises a Si x Ge 1-x , layer, (where 0&lt;x≦1). Since Si and Ge are perfectly soluble in the Si x Ge 1-x  layer, there is an advantage in that the compositions of Si or Ge can be continuously varied from 0 to 1.  
         [0023]     Additionally, in case of the conventional AlN buffer layer, there are differences of 24.8% and 40.7% in thermal expansion coefficient between the GaN layer and the AlN layer and between the AlN layer and the Si substrate, respectively, causing a severe problem of cracks due to the differences in thermal expansion coefficient. However, according to the present invention, since the Si 0.14 Ge 0.86  buffer layer has a thermal expansion coefficient approximately the same as that of the GaN layer, the problems caused by the differences in thermal expansion coefficient can be effectively solved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The foregoing and other objects and features of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0025]      FIGS. 1   a  and  1   b  show structures of a nitride single crystal grown on a silicon substrate according to a conventional method;  
         [0026]      FIGS. 2   a  and  2   b  are optical micrographs showing surfaces of the nitride single crystals shown in  FIGS. 1   a  and  1   b;    
         [0027]      FIGS. 3   a  and  3   b  show structures of a nitride single crystal grown on a silicon substrate according to different embodiments of the present invention, respectively; and  
         [0028]      FIG. 4  is a section side elevation illustrating a nitride semiconductor light emitting device according to one embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Preferred embodiments will now be described in detail with reference to the accompanying drawings.  
         [0030]      FIGS. 3   a  and  3   b  show structures of a GaN single crystal grown by use of a SiGe buffer layer in accordance with the present invention.  
         [0031]     According to the embodiment of the present invention shown in  FIG. 3   a , a Si x Ge 1-x  layer (where 0&lt;x≦1) is provided as a buffer layer  34  on a silicon substrate  31 . At this time, the silicon layer  31  has an upper surface of a (111) crystal plane. A GaN single crystal  35  is grown on the Si x Ge 1-x  layer  34  by use of a well-known process of growing a nitride single crystal, such as the MOCVD process. According to the present invention, the Si x Ge 1-x  layer  34  preferably has an Si composition (x) of about 0.1˜0.2, and more preferably of about 0.14. When the Si x Ge 1-x  layer  34  comprises an Si composition (x) of about 0.14, since a difference in thermal expansion coefficient between the GaN layer and the Si x Ge 1-x  layer  34  is approximately 0, stress caused by the difference in thermal expansion coefficient therebetween can be remarkably reduced.  
         [0032]     The Si x Ge 1-x  layer  34  may be provided as a structure of a SiGe single layer or of Si/SiGe layers. Preferably, since Si and Ge are perfectly soluble with each other in the Si x Ge 1-x  layer  34 , and the Si composition therein can be controlled to be gradually decreased, the Si x Ge 1-x  layer  34  may have an Si composition gradient (x) gradually decreasing from a portion, where the Si x Ge 1-x  layer  34  contacts the silicon substrate  31 , to an uppermost portion of the Si x Ge 1-x  layer  34  (that is, to a portion where a GaN single crystal  35  will be formed). The Si composition preferably increase in the range of 1 to 0.1, and more preferably in the range of 1 to 0.14, from the portion where the Si x Ge 1-x  layer  34  contacts the silicon substrate  31  to the uppermost portion of the Si x Ge 1-x  layer  34 .  
         [0033]     Additionally, unlike the conventional AlN buffer layer, the Si x Ge 1-x  layer  34  may be grown to a thickness, which can sufficiently secure buffering effects between dissimilar materials. For instance, in the case of the conventional AlN buffer layer, it is difficult to grow it to a thickness of 1 μm or more, and thus, there is a problem in that a sufficient buffering region cannot be secured. However, since the Si x Ge 1-x  layer  34  can be grown to a thickness of several dozen nm, it is desirable that the Si x Ge 1-x  layer  34  be grown to a thickness of at least 20 nm in order to secure a sufficient buffering region.  
         [0034]     Alternatively, the present invention may be realized as the embodiment shown in  FIG. 3   b . As with  FIG. 3   a , according to the embodiment shown in  FIG. 3   b , after a Si x Ge 1-x , layer  34  (where 0&lt;x≦1) is formed on a silicon substrate  31 , which has an upper surface of a (111) crystal plane, an intermediate layer  33  having the formula of Al y In z Ga (1-y-z) N (where 0≦y≦1, 0≦z≦1, 0≦y+z≦1) may be formed on the Si x Ge 1-x  layer  34 . The Al y In z Ga (1-y-z) N intermediate layer  33  acts as a buffer layer, as with the AlGaN layer  13  illustrated in  FIG. 1   b . According to the embodiment of the present invention, with the stress due to differences in heat expansion coefficient between the layers removed by means of the Si x Ge 1-x  layer  34 , growth of a nitride single layer  35  can be imparted with enhanced quality by use of the Al y In z Ga (1-y-z) N intermediate layer  33 .  
         [0035]      FIG. 4  is a section side elevation illustrating a nitride semiconductor light emitting device according to another embodiment of the present invention.  
         [0036]     Referring to  FIG. 4 , a nitride semiconductor light emitting device  40  according to the present invention comprises a buffer layer  44  having the formula of Si x Ge 1-x  (where 0&lt;x≦1) formed on a silicon substrate  41 . The nitride semiconductor light emitting device  40  further comprises a first conductive nitride semiconductor layer  45 , an active layer  46 , and a second conductive nitride semiconductor layer  47  sequentially formed on the buffer layer  44 . Additionally, the nitride semiconductor light emitting device  40  comprises an n-side electrode  49   a  on an upper surface of the first conductive nitride semiconductor layer  45 , where some portion of the second conductive nitride semiconductor layer  47  and active layer  46  is removed, a transparent electrode  48  on the second conductive nitride semiconductor layer  47  for enhancing contact resistance, and a p-side electrode  49   b  on the transparent electrode  48 .  
         [0037]     The first conductive nitride semiconductor layer  45  may comprise a first conductive GaN layer formed on the Si x Ge 1-x  buffer layer  44 , and a first conductive AlGaN layer on the first conductive GaN layer. The second conductive nitride semiconductor layer  47  may comprise a second conductive GaN layer formed on the active layer  46 , and a second conductive AlGaN layer on the second conductive GaN layer. The active layer  46  may be a GaN/InGaN active layer having a multi-well structure.  
         [0038]     The Si x Ge 1-x  layer  44  of the present invention preferably has an Si composition (x) of about 0.1˜0.2, and more preferably of about 0.14. When the Si x Ge 1-x  layer  34  has an Si composition (x) of about 0.14, since a difference in thermal expansion coefficient between the GaN layer and the Si x Ge 1-x  layer  34  is approximately 0, stress caused by the differences in thermal expansion coefficient therebetween can be remarkably reduced.  
         [0039]     Meanwhile, it should be noted that the present invention is not limited to the above embodiment. For instance, effects of the differences in thermal expansion coefficient between the layers is not restricted to a typical tension, even though the Si composition is reduced below 0.14, the present invention may be designed to intentionally generate a compression stress in order to complement a tension generated in a region between other layers.  
         [0040]     Preferably, since Si and Ge are perfectly soluble in the Si x Ge 1-x  layer  44 , and the Si composition therein can be controlled to be gradually decreased, the Si x Ge 1-x  layer  44  may have the Si composition gradient (x) gradually decreasing from a portion where the Si x Ge 1-x  layer  44  contacts the silicon substrate  41  to a portion where the Si x Ge 1-x  layer  44  contacts the first conductive nitride semiconductor layer  45 . The Si composition preferably increase in the range of 1 to 0.1, and more preferably in the range of 1 to 0.14, from the portion where the Si x Ge 1-x  layer  44  contacts the silicon substrate  41  to the uppermost portion of the Si x Ge 1-x  layer  44 . Since the Si x Ge 1-x  layer  34  can be grown to a thickness of several dozen nm, it is desirable that the Si x Ge 1-x  layer  44  be grown to a thickness of at least 20 nm in order to secure a sufficient buffering region.  
         [0041]     Furthermore, in the process of manufacturing the nitride semiconductor light emitting device, since the Si x Ge 1-x  buffer layer can be easily etched, it is advantageous in that the Si substrate can be lifted off, if necessary.  
         [0042]     Meanwhile, although the structure shown in  FIG. 4  has only the SiGe buffer layer, as with the embodiment shown in  FIG. 3   b , after the Si x Ge 1-x  layer  44  (where 0&lt;x≦1) is formed on the silicon substrate  41 , which has an upper surface of a (111) crystal plane, an intermediate layer having the formula of Al y In z Ga (1-y-z) N (where 0≦y≦1, 0≦z≦1, 0≦y+z≦1) may be formed on the Si x Ge 1-x  layer  44 .  
         [0043]     As apparent from the above description, according to the present invention, the method of growing a high quality nitride single crystal by use of the buffer layer comprising Si and Ge on the silicon substrate. The buffer layer of the present invention has a thermal expansion coefficient approximately similar to that of the GaN single crystal, sufficiently secures a growth thickness, and makes it possible to generate an intentional compression stress for compensating for a tension generated from other regions, enabling a high quality nitride single crystal to be grown on the silicon substrate.  
         [0044]     Accordingly, in manufacturing the nitride semiconductor light emitting device, the silicon substrate may be used as a substrate for growth of the nitride single crystal, instead of a sapphire substrate or a SiC substrate having a high price.  
         [0045]     It should be understood that the embodiments and the accompanying drawings as described above have been described for illustrative purposes and the present invention is limited by the following claims. Further, those skilled in the art will appreciate that various modifications, additions and substitutions are allowed without departing from the scope and spirit of the invention as set forth in the accompanying claims.