Patent Publication Number: US-9419160-B2

Title: Nitride semiconductor structure

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 103124565, filed on Jul. 17, 2014. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The disclosure generally relates to a semiconductor structure, and, more particularly, to a nitride semiconductor structure. 
     2. Description of Related Art 
     In recent years, nitride photodiodes have been broadly used in various fields. In a nitride semiconductor structure, since the silicon substrate bears the merits of high thermal conductivity and low cost, a large dimension nitride semiconductor structure with a silicon substrate as a base is an essential element in a nitride photodiode. 
     Using a gallium nitride semiconductor layer as an exemplary illustration, the lattice difference between a gallium nitride semiconductor layer and a silicon substrate is about 17%, and the difference in thermal expansion coefficients between the two is about 54%. Aside from creating a rupture on the thin film due to the excessive thermal stress generated during the cooling period, the above differences may also generate internal strain in the gallium nitride semiconductor layer during the epitaxy process, resulting in a cracking of the thin film and a formation of defects. Accordingly, resolving the incompatibilities of lattices and thermal expansion coefficients between a nitride semiconductor layer and a silicon substrate to mitigate the rupture of wafers and to obviate the generation of defects is still an issue for the industry to endeavour. 
     SUMMARY OF THE INVENTION 
     The disclosure is directed to a nitride semiconductor substructure, in which the incompatibilities of lattices and thermal expansion coefficients between a nitride semiconductor layer and a silicon substrate are improved to reduce the rupture of wafers and obviate the generation of defects. 
     An exemplary embodiment of the disclosure provides a nitride semiconductor structure. The nitride semiconductor structure includes a substrate, a silicon carbide nucleation layer, a composite buffer layer and a nitride semiconductor layer. The silicon carbide nucleation layer is located on the substrate. The composite buffer layer is located on the silicon carbide nucleation layer. The nitride semiconductor layer is located on the composite buffer layer. Further, the nitride semiconductor structure is an aluminium nitride free (AlN free) semiconductor structure. 
     According to the above exemplary embodiment of the disclosure, the above composite buffer layer includes a first buffer layer and a second buffer layer, and the first buffer layer is in contact with the silicon carbide nucleation layer. 
     According to the above exemplary embodiment of the disclosure, the above first buffer layer includes an Al x GaN layer, wherein 0&lt;x&lt;1. 
     According to the above exemplary embodiment of the disclosure, the above second buffer layer includes a plurality of Al y Ga 1-y N layers and a plurality of Al z Ga 1-z N layers alternately arranged with each other, wherein 0&lt;y&lt;1, 0&lt;z&lt;1, and y is not equal to z. 
     According to the above exemplary embodiment of the disclosure, wherein x&gt;(y+z)/2. 
     According to the above exemplary embodiment of the disclosure, wherein the above second buffer layer includes an aluminium gallium nitride bulk layer. 
     According to the above exemplary embodiment of the disclosure, the above second buffer layer includes a graded aluminium gallium nitride layer with step graded aluminum content. 
     According to the above exemplary embodiment of the disclosure, the above second buffer layer includes a graded aluminium gallium nitride layer with continuously graded aluminum content. 
     According to the above exemplary embodiment of the disclosure, the above buffer layer further includes a third buffer layer positioned between the nitride semiconductor layer and the second buffer layer. 
     According to the above exemplary embodiment of the disclosure, the third buffer layer includes a silicon carbide layer. 
     According to the above exemplary embodiment of the disclosure, the third buffer layer includes a plurality of silicon carbide layers and a plurality gallium nitride layer alternately arranged with each other. 
     According to the above exemplary embodiment of the disclosure, the thickness of the third buffer layer is between 5 nanometers and 100 nanometers. 
     According to the above exemplary embodiment of the disclosure, the above silicon carbide nucleation layer has a cubic lattice. 
     According to the above exemplary embodiment of the disclosure, the thickness of the above silicon carbide nucleation layer is between 50 nanometers and 3000 nanometers. 
     According to the above exemplary embodiment of the disclosure, the thickness of the first buffer layer is between 0.1 micron and 3 microns. 
     According to the above exemplary embodiment of the disclosure, the thickness of the second buffer layer is between 0.1 micron and 3 microns. 
     According to the above exemplary embodiment of the disclosure, the total thickness of the first buffer layer and the second buffer layer is between 0.2 micron and 4 microns. 
     According to the above exemplary embodiments of the disclosure, the material of the above substrate includes silicon, aluminium oxide, or glass. 
     According to the above exemplary embodiments of the disclosure, the above substrate is a patterned substrate. 
     According to the above exemplary embodiments of the disclosure, the thickness of the above nitride semiconductor layer is between about 1 micron and 8 microns. 
     According to the nitride semiconductor structure of the disclosure, through the disposition of the silicon carbide nucleation layer and the composite buffer layer, the incompatibilities of the lattices and the thermal expansion coefficients between gallium nitride and silicon are improved to overcome the problem of excessive defects present between the two. Moreover, the conventional nitride semiconductor structure typically includes aluminium nitride, and the epitaxy process time in forming aluminium nitride by MOCVD is long, resulting in a low throughput of the equipment. According to the disclosure, a nitride semiconductor structure, which is aluminium nitride free, is provided; hence, the epitaxy process time is reduced and the frequency of preventive maintenance of the equipment is also reduced. 
     Other objectives, features and advantages of the present invention will be further understood from the further technological features disclosed by the embodiments of the present invention wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of modes best suited to carry out the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic cross-sectional view of a nitride semiconductor structure of an exemplary embodiment of the disclosure. 
         FIG. 2  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. 
         FIG. 3  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. 
         FIG. 4  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. 
         FIG. 5  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. 
         FIG. 6  is an X-ray diffraction pattern of a nitride semiconductor structure of another exemplary embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  is a schematic cross-sectional view of a nitride semiconductor structure of an exemplary embodiment of the disclosure.  FIG. 2  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. 
     Referring to  FIG. 1 , a nitride semiconductor structure  100   a  of the disclosure includes a substrate  102 , a silicon carbide nucleation layer  104 , a composite buffer layer  120   a  and a nitride semiconductor layer  112 . The material of the substrate  102  includes, but is not limited to, silicon, aluminium oxide (Al 2 O 3 ) or glass. In one exemplary embodiment, the substrate  102  can be a silicon substrate, wherein its crystal orientation is (111). The substrate  102  may be a patterned substrate, for example, a patterned silicon substrate. The patterns of the silicon substrate may be regular or irregular micro-patterns or nano-patterns. In one exemplary embodiment, the substrate  102 , after being patterned, includes a plurality of recesses (not shown) rendering the epitaxial layer (such as silicon carbide nucleation layer  104 ) to grow laterally on the substrate  102  and the generation of dislocation is reduced. 
     The silicon carbide nucleation layer  104  is located on the substrate  102 . In this exemplary embodiment, the silicon carbide nucleation layer  104  may be in contact with the substrate  102 ; however, it should be understood that the above embodiment is presented by way of example and not by way of limitation. The silicon carbide nucleation layer  104  is formed by, for example, a chemical vapour deposition (CVD) method, wherein the reaction temperature is about 1200° C. to 1300° C. Further, the silicon carbide nucleation layer  104  may be formed on a flat silicon substrate or a patterned silicon substrate. The thickness of the silicon carbide nucleation layer  104  ranges from about 50 nanometers to about 3000 nanometers. The silicon carbide nucleation layer  104  has, for example, a cubic lattice, wherein the crystal orientation is (111), for example. 
     The composite buffer layer  120   a  is located on the silicon carbide nucleation layer  104 . In this exemplary embodiment, the composite buffer layer  120   a  is in physical contact with the silicon carbide nucleation layer  104 ; however, it should be understood that the above embodiment is presented by way of example and not by way of limitation. In another exemplary embodiment (not shown), the composite buffer layer  120   a  and the silicon carbide nucleation layer  104  may include an intermediate layer there-between. In one exemplary embodiment, the composite buffer layer  120   a  at least includes a first buffer layer  106  and a second buffer layer  108 . The first buffer layer  106  is in contact with the silicon carbide nucleation layer  104 , for example. The first buffer layer  106  includes an Al x GaN layer, for example, wherein 0&lt;x&lt;1. The first buffer layer  106  is formed by a metal organic chemical vapour deposition (MOCVD) method, wherein the reaction temperature is, for example, about 1000° C. to about 1100° C. The first buffer layer  106  is about 0.1 micron to about 3 microns thick, for example. 
     The second buffer layer  108  is positioned on the first buffer layer  106 . In this exemplary embodiment, the second buffer layer  108  may be a bulk layer, for example an aluminium gallium nitride (Al y GaN) bulk layer, wherein 0&lt;y&lt;1. In one embodiment, the aluminium content of the Al y GaN bulk layer is a predetermined value. Further, the aluminium content of the Al x GaN layer in the first buffer layer  106  is greater than the aluminium content of the Al y GaN bulk layer in the second buffer layer  108 . The second buffer layer  108  can be formed by a MOCVD method, wherein the reaction temperature is, for example, about 1000° C. to about 1100° C. The thickness of the second buffer layer  108  is about 0.1 micron to about 3 microns. In one exemplary embodiment, the thickness of the second buffer layer  108  is greater than that of the first buffer layer  106 . The total thickness of the first buffer layer  106  and the second buffer layer  108  is between about 0.2 micron and about 4 microns. In another embodiment, the total thickness of the first buffer layer  106  and the second buffer layer  108  is between about 2 microns and about 4 microns. 
     In another exemplary embodiment, the composite buffer layer  120   a  may optically include a third buffer layer  110 , as shown in  FIG. 1 . The third buffer layer  110  is located on the second buffer layer  108 , for example. The third buffer layer  110  may serve as a stress relaxation layer for relaxing the stress between the second buffer layer  108  and the subsequently formed nitride semiconductor layer  112 . In one exemplary embodiment, the third buffer layer  110  may be a single layer of a low temperature buffer layer, for example, a silicon carbide layer as shown in  FIG. 1 . The third buffer layer  110  can be formed by a MOCVD method, wherein the reaction temperature is between 800° C. and 900° C. The thickness of the third buffer layer  110  is between about 5 nanometers and about 100 nanometers. 
     Still referring to  FIG. 1 , the nitride semiconductor layer  112  is located on the composite buffer layer  120   a . In one exemplary embodiment, the nitride semiconductor layer  112  is located on the third buffer layer  110 , for example. Alternatively speaking, the third buffer layer  110  is located between the nitride semiconductor layer  112  and the second buffer layer  108 . The nitride semiconductor layer  112  is formed with a gallium nitride material. The nitride semiconductor layer  112  can be formed by a MOCVD method, wherein the reaction temperature is between about 1000° C. and about 1100° C. In one exemplary embodiment, the thickness of the nitride semiconductor layer  112  is between about 1 micron and about 8 microns. In another exemplary embodiment, the thickness of the nitride semiconductor layer  112  is between about 2 microns and about 8 microns. In yet another exemplary embodiment, the thickness of the nitride semiconductor layer  112  is between about 4 microns and 8 about microns. 
     In the exemplary embodiment as illustrated in  FIG. 1 , the third buffer layer  110  is exemplified as a single layer. However, it should be understood that the above embodiments are presented by way of example and not by way of limitation. In another exemplary embodiment, the third buffer layer may be a multi-layer structure, for example, the third buffer layer  114  as illustrated in  FIG. 2 . The third buffer layer  114  may include a superlattice structure containing a plurality of silicon carbide layers  114   a  and a plurality of gallium nitride layers  114   b  alternately arranged with each other. The third buffer layer  114  can be formed by a MOCVD method, wherein the reaction temperature is between about 800° C. and about 900° C. The third buffer layer  114  is about 5 nanometers to about 50 nanometers thick. It is noted that only some of the layers of the superlattice structure is exemplified in  FIG. 2 , and the number of the silicon carbide layers  114   a  and the number of the gallium nitride layers  114   b  are not limited thereto. 
     It is noted that in the nitride semiconductor structures  100   a  and  100   b  of the disclosure, a silicon carbide nucleation layer  104  is used to replace the conventional aluminium nitride layer, in which the long epitaxy process of aluminium nitride, leading to a low throughput of the equipment can be obviated. Further, in the composite buffer layers  120   a  and  120   b  of the disclosure, the lower first buffer layer  106  (which is the Al x GaN layer) has a higher aluminium content, which can compensate the surface defects of the silicon carbide nucleation layer  104  to improve the quality of the epitaxial layer. Further, in the composite buffer layer  120   a  of the disclosure, the upper second buffer layer  108  (which is the Al y GaN layer) has a lower aluminium content, which can provide a compress strain to reduce the generation of tensile stress resulted from the difference in the thermal expansion coefficients between gallium nitride and silicon, thereby preventing the rupture of wafers. Alternatively speaking, the nitride semiconductor structures  100   a  and  100   b  of the disclosure are aluminium nitride free semiconductor structures, in which the epitaxy process time is reduced and the frequency of preventive maintenance (PM) of the equipment is also reduced. 
     Further, in the nitride semiconductor structures  100   a  and  100   b  as shown in  FIGS. 1 and 2 , the second buffer layer  108  is a bulk layer; however, it should be understood that the above embodiments are presented by way of example and not by way of limitation. In other exemplary embodiments, the second buffer layer may be a graded layer or a superlattice structure, as shown in  FIGS. 3 and 4 . 
       FIG. 3  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. The difference between the nitride semiconductor structure  200  in  FIG. 3  and the nitride semiconductor structure  100   a  in  FIG. 1  lies in that the compositions of the second buffer layers are different. 
     Referring to  FIG. 3 , the nitride semiconductor structure  200  of the disclosure includes a substrate  102 , a silicon nitride nucleation layer  104 , a composite buffer layer  220  and a nitride semiconductor layer  112 . The composite buffer layer  220  includes a first buffer layer  106 , a second buffer layer  208  and an optional third buffer layer  110 . The materials and the forming methods of substrate  102 , the silicon carbide nucleation layer  104 , the first buffer layer  106 , the third buffer layer  110  and the nitride semiconductor layer  112  are similar to those discussed above and will not be further reiterated herein. Further, in the structure as shown in  FIG. 3 , the third buffer layer  110  is exemplified as a single-layer structure; however, it should be noted that this embodiment is only presented by way of example and not by way of limitation. In other exemplary embodiments, the third buffer layer  110  may be a multi-layer structure or a superlattice structure. 
     The second buffer layer  208  is located on the first buffer layer  106 . The second buffer layer  208  includes a plurality of graded layers, for example, the content of the aluminium gradually changes in the aluminium gallium nitride (Al y GaN) layer, wherein 0&lt;y&lt;1. In one exemplary embodiment, the second buffer layer  208  includes a graded aluminium gallium nitride layer with step graded aluminium content. As shown in  FIG. 3 , the second buffer layer  208  may include (but is not limited to) a sub-buffer layer  208   a , a sub-buffer layer  208   b  and a sub-buffer layer  208   c . The aluminium contents of sub-buffer layers  208   a ,  208   b  and  208   c  decrease in a stepwise manner. For example, the aluminium content of the sub-buffer layer  208   a  is greater than the aluminium content of the sub-buffer layer  208   b , and the aluminium content of the sub-buffer layer  208   b  is greater than that of the sub-buffer layer  208   c;  however, it should be understood that the above embodiment is presented by way of example and not by way of limitation. In another exemplary embodiment, the aluminium contents of sub-buffer layers  208   a ,  208   b  and  208   c  increase in a stepwise manner. In yet another exemplary embodiment, the second buffer layer  208  includes a graded aluminium gallium nitride layer with continuously graded aluminium content; for example, the aluminium content reduces along the thickness of the graded aluminium gallium nitride layer; however, it should be understood that the above embodiment is presented by way of example and not by way of limitation. In another exemplary embodiment, the aluminium content increases along the thickness of the graded aluminium gallium nitride layer. In an exemplary embodiment, the aluminium content of the first buffer layer  106  (that is the Al x GaN layer) is greater than the maximum aluminium content of the second buffer layer  208  (that is the graded Al y GaN layer). In another exemplary embodiment, the aluminium content of the first buffer layer  106  (that is the Al x GaN layer) is greater than the average aluminium content of the second buffer layer  108  (that is the graded Al y GaN layer). 
     In the composite buffer layer  220  as illustrated in  FIG. 3 , the lower first buffer layer  106  (which is the Al x GaN layer) has a higher aluminium content, which can compensate the surface defects of the silicon carbide nucleation layer  104  to improve the quality of the epitaxial layer. Moreover, in the composite buffer layer  220 , the upper second buffer layer  108  includes a graded aluminium gallium layer in which the aluminium content gradually changes to reduce the stress generated between gallium nitride and silicon due to the difference in the thermal expansion coefficients. Further, the nitride semiconductor structure  200  of the disclosure is an aluminium nitride free semiconductor structure; therefore, the epitaxy process time is reduced and the frequency of preventive maintenance of the equipment is also reduced. 
       FIG. 4  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. The difference between the nitride semiconductor structure  300  in  FIG. 4  and the nitride semiconductor structure  100   a  in  FIG. 1  lies in that the compositions of the second buffer layers are different. 
     Referring to  FIG. 4 , the nitride semiconductor structure  300  of the disclosure includes a substrate  102 , a silicon nitride nucleation layer  104 , a composite buffer layer  320  and a nitride semiconductor layer  112 . The composite buffer layer  320  includes a first buffer layer  106 , a second buffer layer  308  and an optional third buffer layer  110 . The materials and the forming methods of substrate  102 , the silicon carbide nucleation layer  104 , the first buffer layer  106 , the third buffer layer  110  and the nitride semiconductor layer  112  are similar to those discussed above and will not be further reiterated herein. Further, in the structure as shown in  FIG. 4 , the third buffer layer  110  is exemplified as a single-layer structure; however, it should be noted that it is only presented by way of example and not by way of limitation. In other exemplary embodiments, the third buffer layer may be a multi-layer or a superlattice structure. 
     The second buffer layer  308  is located on the first buffer layer  106 . The second buffer layer  308  includes a superlattice structure containing a plurality of Al y Ga 1-y N layers  308   a  and a plurality of Al z Ga 1-z N layers  308   b  alternately arranged with each other, wherein 0&lt;y&lt;1, 0&lt;z&lt;1 and y is not equal to z. It is noted that only some of the layers of the superlattice structure are exemplified in  FIG. 4 , and the number of the Al y Ga 1-y N layers  308   a  and the number of the Al z Ga 1-z N layers  308   b  are not limited thereto. Further, the average aluminium content of the second buffer layer  308  is less the aluminium content of the first buffer layer  106 . In other words, the relationship of the aluminium contents of the Al x GaN layer, the Al y Ga 1-y N layer  308   a  and the Al z Ga 1-z N layer  308   b  is represented by the mathematical expression x&gt;(y+z)/2. In one exemplary embodiment, the aluminium content of the first buffer layer  106  (which is Al x GaN layer) is greater than the maximum of the aluminium contents of the Al y Ga 1-y N layer  308   a  and the Al z Ga 1-z N layer  308   b.    
     In the composite buffer layer  320  in  FIG. 4 , the lower first buffer layer  106  (which is the Al x GaN layer) contains a higher aluminium content, which can compensate the surface defects of the silicon carbide nucleation layer  104  to improve the quality of the epitaxial layer. Further, in the composite buffer layer  320 , the upper second buffer layer  308  has a superlattice structure (which contains Al y Ga 1-y N layers/Al z Ga 1-z N layers) and the average aluminium content of the second buffer layer  308  is less than the aluminium content of the first buffer layer  106  so that the stress generated due to the difference of the thermal expansion coefficients between gallium nitride and silicon is reduced. Further, the nitride semiconductor structure  300  of the disclosure is an aluminium nitride free semiconductor structure, in which the epitaxy process time is reduced and the frequency of preventive maintenance of the equipment is also reduced. 
     In the nitride semiconductor structures  100   a ,  100   b ,  200  and  300  as illustrated in  FIGS. 1 to 4 , the first buffer layer  106  is a single layer and an aluminium nitride free structure; it should be understood that the above embodiments are presented by way of example and not by way of limitation. In another exemplary embodiment, the first buffer layer may be a multi-layer structure and optionally contains aluminium nitride, as shown in  FIG. 5 . 
       FIG. 5  is a schematic cross-sectional view of a nitride semiconductor structure of another exemplary embodiment of the disclosure. The difference between the nitride semiconductor structure  400  in  FIG. 5  and the nitride semiconductor structure  100   a  in  FIG. 1  lies in that the structures of the first buffer layers are different. 
     Referring to  FIG. 5 , the nitride semiconductor structure  400  of the disclosure includes a substrate  102 , a silicon carbide nucleation layer  104 , a composite buffer layer  420  and a nitride semiconductor layer  112 . The composite buffer layer  420  includes a first buffer layer  206 , a second buffer layer  108  and an optional third buffer layer  110 . The materials and the forming methods of the substrate  102 , the silicon carbide nucleation layer  104  and the nitride semiconductor layer  112  are similar to those discussed above and will not be further reiterated herein. Further, in the structure as shown in  FIG. 5 , the second buffer layer  108  and the optional third buffer layer  110  are respectively exemplified as a single-layer structure; however, it should be noted that the embodiment is only presented by way of example and not by way of limitation. Alternatively speaking, the second buffer layer  108  may include a plurality of graded layers as the illustrated nitride semiconductor structure  200  or may include a supperlattice structure as the illustrated nitride semiconductor structure  300 , and the optional third buffer layer  110  may be a superlattice structure as in the illustrated nitride semiconductor structure  100   b.    
     Still referring to  FIG. 5 , the first buffer layer  206  includes an Al x GaN layer and an AlN layer, wherein 0&lt;x&lt;1. In one exemplary embodiment, the first buffer layer  206  may include (but is not limited to) an Al x GaN layer  20   ba , an AlN layer  206   b  and an Al x GaN layer  206   c . The AlN layer  206   b  is, for example, located between the Al x GaN layers  206   a  and  206   c;  however, it should be noted that the disclosure is not limited as such. The compositions of the Al x GaN layers  206   a  and  206   c  may be the same or different. In one exemplary embodiment, the aluminium content of the first buffer layer  206  is greater than that of the second buffer layer  108 . The method used in forming the Al x GaN layer  206   a , the AlN layer  206   b  and the Al x GaN layer  206   c  includes a MOCVD method, wherein the reaction temperature is about 1000° C. to about 1100° C. The thicknesses of each of the Al x GaN layers  206   a  and  206   c  is about 0.5 microns to about 2 microns, for example. The thickness of the AlN layer  206   b  is about 0.01 micron to about 0.05 micron, for example. 
     In the exemplary embodiment of  FIG. 5 , in addition to the higher aluminium content of the first buffer layer  206  for compensating the surface defects of the silicon carbide nucleation layer  104 , the disposition of the AlN layer  206   b  in between the Al x GaN layers  206   a  and  206   c  further enhances the overall uniformity of the nitride semiconductor structure  400 ; hence, the stress compensation characteristics of the structure is enhanced to thereby improve the incompatibilities of the lattices and the thermal expansion coefficients between gallium nitride and silicon. 
       FIG. 6  is an X-ray diffraction pattern of a nitride semiconductor structure of an exemplary embodiment of the disclosure. 
     The nitride semiconductor structure of an exemplary embodiment of the disclosure includes a silicon substrate, a silicon carbide nucleation layer, a composite buffer layer and a nitride semiconductor layer. The thickness of the silicon carbide nucleation layer is about 300 nanometers. The composite buffer layer is a graded aluminium gallium nitride layer with continuously graded aluminium content and has a thickness of about 2 microns. The thickness of the nitride semiconductor layer is about 2 microns. 
     Referring to  FIG. 6 , at the position where the diffraction angle (2θ) is 14.24 degrees, the Si(111) plane is observed. At the position where the diffraction angle is 17.32 degrees, the GaN (0002) plane is observed. At the position where the diffraction angle is 17.83 degrees, the SiC (111) plane is observed. At the position where the diffraction angle is 16.5 degrees, the peak of Si a C 1-a  is observed, and this peak demonstrates that a transition layer is present between the silicon substrate and the nucleation layer. According to the above results, with the silicon carbide nucleation layer and the composite buffer layer of the disclosure, a gallium nitride semiconductor layer can be successfully grown on a silicon substrate. 
     Based on the above, in the nitride semiconductor structure of the disclosure, through the disposition of the silicon carbide nucleation layer and the composite buffer layer, the incompatibilities of the lattices and the thermal coefficients between gallium nitride and silicon are improved to resolve the problem of excessive defects present there-between. Moreover, the composite buffer layer of the disclosure may be a multi-layer structure, a graded layer, a superlattice structure or a combination thereof, and the average aluminium content of the upper layer(s) is less than the average aluminium content of the lower layer(s) to reduce the stress generated due the difference in the thermal expansion coefficients between gallium nitride and silicon. Further, the conventional nitride semiconductor structure typically includes aluminium nitride; the epitaxy process time in forming aluminium nitride by MOCVD is long, resulting in a low throughput of the equipment. According to the disclosure, a nitride semiconductor structure, which is aluminium nitride free, is provided, in which the epitaxy process time is reduced and the frequency of preventive maintenance of the equipment is also reduced. 
     The foregoing description of the exemplary embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to best explain the principles of the invention and its best mode practical disclosure, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.