Abstract:
According to this GaN-based HFET, resistivity ρ of a semi-insulating film forming a gate insulating film is 3.9×10 9 Ωcm. The value of this resistivity ρ is a value derived when the current density is 6.25×10 −4  (A/cm 2 ). By inclusion of the gate insulating film by a semi-insulating film having a resistivity ρ=3.9×10 9 Ωcm, a withstand voltage of 1000 V can be obtained. Meanwhile, the withstand voltage abruptly drops as the resistivity of the gate insulating film exceeds 1 ×10 11 Ωcm, and the gate leak current increases when the resistivity of the gate insulating film drops below 1 ×10 7 Ωcm.

Description:
TECHNICAL FIELD 
     The present invention relates to a nitride semiconductor. The present invention particularly relates to the structure of a channel layer for improving the life of a nitride semiconductor device. 
     BACKGROUND ART 
     Structures based on heterojunctions of AlGaN and GaN are generally used as electronic devices containing a nitride semiconductor. 
     A particular structure includes a buffer layer which is placed on a sapphire or Si substrate and which is made of a nitride semiconductor, a channel layer which is placed on the buffer layer and which is generally made of GaN, a barrier layer which is placed on the GaN channel layer and which is made of AlGaN, a source electrode, a drain electrode, and a gate electrode placed between the source and drain electrodes. The source and drain electrodes form an ohmic contact with a two-dimensional electron gas region formed at the interface between the AlGaN barrier layer and the GaN channel layer. 
     In the case where a nitride semiconductor is formed on a sapphire or SiC substrate, there is not a very serious problem. In the case of using a Si substrate having a thermal expansion coefficient less than that of a nitride semiconductor, the Si substrate warps to form a downwardly convex curve after the growth of a nitride semiconductor layer and cracks are formed in crystals by stress. Therefore, the Si substrate is not suitable to fabricate an electronic device. 
     A technique for reducing the difference in thermal expansion coefficient between a Si substrate and a nitride semiconductor is a “semiconductor electronic device” disclosed in Japanese Unexamined Patent Application Publication No. 2005-85852 (Patent Literature 1). In the semiconductor electronic device, a buffer layer, a GaN electron travel layer (500 nm), an AlGaN electron supply layer (20 nm), and a GaN contact layer are stacked on a GaN intervening layer formed on a silicon substrate. The buffer layer is composed of one or more first layers made of GaN and one or more second layers made of AlGaN, the first and second layers being alternately stacked in that order. Since the buffer layer, which is composed of the first and second layers different in material, is interposed, the direction of dislocation defects propagating from a lower side is bent and therefore the propagation of the dislocation defects in a growth direction is suppressed. 
     However, the semiconductor electronic device disclosed in Patent Literature 1 has a problem below. 
     A mechanism to form a two-dimensional electron gas in a nitride semiconductor is shown in  FIG. 6 . As shown in  FIG. 6 , an AlGaN layer (the AlGaN electron supply layer described in Patent Literature 1) having a thickness insufficient to cause stress relaxation and a small lattice constant is placed on a GaN layer (the GaN electron travel layer described in Patent Literature 1) which is stress-relieved and which has substantially a bulk lattice constant. In this case, piezoelectric polarization P pe  is induced by the difference in spontaneous polarization P sp  between the GaN layer and the AlGaN layer and the fact that the AlGaN layer on the GaN layer is strained in-plane by stress +σ. As a result, the two-dimensional electron gas (2DEG) is formed at the interface therebetween. 
     In the case where a buffer layer (the GaN (Al composition=0)/AlGaN (1≧Al composition&gt;0) buffer layer described in Patent Literature 1) formed by alternately growing AlGaN layers having different Al compositions as shown in  FIG. 7  is considered as an average block, the buffer layer can be considered to be equivalent to a stress-relieved AlGaN layer by the same principle. Thus, a GaN layer (the GaN electron travel layer described in Patent Literature 1) formed on the stress-relieved AlGaN layer has a lattice constant larger than that of the AlGaN layer and therefore is strained by stress −σ in contrast to the case shown in  FIG. 6  and a two-dimensional hole gas (2DHG) is formed at the interface therebetween. 
     There is a problem in that a two-dimensional hole gas formed in the electronic device as described above causes a leakage current to reduce device properties. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2005-85852 
     Non Patent Literature 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is an object of the present invention to provide a nitride semiconductor capable of suppressing the formation of a two-dimensional hole gas in the case of using a superlattice buffer layer prepared by alternately and repeatedly stacking AlGaN layers having different compositions. 
     Solution to Problem 
     In order to solve the above problem, a nitride semiconductor according to the present invention includes a substrate, an initial growth layer formed on the substrate, a buffer layer formed on the initial growth layer, a superlattice buffer layer formed on the buffer layer, a channel layer which is formed on the superlattice buffer layer and which is composed of a plurality of layers, a barrier layer formed on the channel layer. The superlattice buffer layer is formed by alternately stacking high-Al content layers which have the composition Al x Ga 1-x N (0.5≦x≦1.0) and a thickness a and low-Al content layers which have the composition Al y Ga 1-y N (0≦y≦0.3) and a thickness b. The channel layer is joined to the superlattice buffer layer and is formed by stacking at least an Al z Ga 1-z N layer and a GaN layer in that order from the superlattice buffer layer side, and the Al composition of the Al z Ga 1-z N layer is the same as the average Al composition of the superlattice buffer layer. 
     In the nitride semiconductor according to an embodiment, the Al composition z of the Al z Ga 1-z N layer of the channel layer is given by the following equation:
 
 z =( a×x+b×y )/( a+b ).
 
     In the nitride semiconductor according to an embodiment, the barrier layer includes an Al w Ga 1-w N layer and the Al composition w of the Al w Ga 1-w N layer is greater than the Al composition z of the Al z Ga 1-z N layer of the channel layer. 
     In the nitride semiconductor according to an embodiment, the thickness a of the high-Al content layers in the superlattice buffer layer ranges from 1 nm to 5 nm and, the thickness b of the low-Al content layers ranges from 22 nm to 30 nm. 
     Advantageous Effects of Invention 
     As is clear from the above, in a nitride semiconductor according to the present invention, a channel layer joined to an AlGaN superlattice buffer layer is formed by stacking an Al z Ga 1-z N layer and a GaN layer in that order from the AlGaN superlattice buffer layer side and the Al composition of the Al z Ga 1-z N layer is the same as the average Al composition of the AlGaN superlattice buffer layer. Thus, the Al z Ga 1-z N layer can be considered to have substantially the same lattice constant as that of the AlGaN superlattice buffer layer, which is equivalent to a stress-relieved AlGaN layer. Therefore, it can be suppressed that strain is induced at the interface between the AlGaN superlattice buffer layer and the Al z Ga 1-z N layer by stress −σ and therefore a two-dimensional hole gas is formed. 
     Thus, a leakage current can be reduced in such a manner that a two-dimensional hole gas formed between the superlattice buffer layer and the GaN layer is compensated for by the Al z Ga 1-z N layer, which is formed on the superlattice buffer layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of a nitride semiconductor epitaxial wafer as a nitride semiconductor according to the present invention. 
         FIG. 2A  is a graph showing C-V measurement results of a HEMT formed using the nitride semiconductor epitaxial wafer shown in  FIG. 1 . 
         FIG. 2B  is a graph showing C-V measurement results of a HEMT formed using the nitride semiconductor epitaxial wafer shown in  FIG. 1 . 
         FIG. 3  is an illustration of a C-V measurement method. 
         FIG. 4  is a sectional view of a nitride semiconductor epitaxial wafer different from the one shown in  FIG. 1 . 
         FIG. 5A  is a graph showing C-V measurement results of a HEMT formed using the nitride semiconductor epitaxial wafer shown in  FIG. 4 . 
         FIG. 5B  is a graph showing C-V measurement results of a HEMT formed using the nitride semiconductor epitaxial wafer shown in  FIG. 4 . 
         FIG. 6  is an illustration of a mechanism to form a two-dimensional electron gas. 
         FIG. 7  is an illustration of a mechanism to form a two-dimensional hole gas. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention are described below in detail with reference to the attached drawings. 
     First Embodiment 
       FIG. 1  is a sectional view of a nitride semiconductor epitaxial wafer as the nitride semiconductor according to this embodiment. As shown in  FIG. 1 , the following layers are formed on a Si substrate  1  in this order: an AlN initial growth layer  2  which is made of AlN and which has a thickness of 100 nm and an Al 0.2 Ga 0.8 N buffer layer  3  having a thickness of 20 nm. Subsequently, a superlattice buffer layer  4  is formed thereon. The superlattice buffer layer  4  has 100 repeated periods of AlN layers having a thickness of 4 nm and Al 0.1 Ga 0.9 N layers having a thickness of 23 nm, the AlN layers and the Al 0.1 Ga 0.9 N layers being alternately and repeatedly stacked. 
     Subsequently, a channel layer  5  composed of a plurality of layers is formed on the superlattice buffer layer  4 . The channel layer  5  includes an Al z Ga 1-z N layer  6  and GaN channel region  7  stacked in that order. The GaN channel region  7  is the above GaN layer. 
     Supposing that the superlattice buffer layer  4  is formed by alternately stacking high-Al content layers which have the composition Al x Ga 1-x N (0.5≦x≦1.0) and a thickness “a (nm)” and low-Al content layers which have the composition Al y Ga 1-y N (0≦y≦0.3) and a thickness “b (nm)”, the Al composition z of the Al z Ga 1-z N layer  6  of the channel layer  5  is given by the following equation:
 
 z =( a×x+b×y )/( a+b )  (1).
 
     Thus, in this embodiment, the Al composition z of the Al z Ga 1-z N layer  6  is as follows: z=(4×1+23×0.1)/(4+23)=0.23. The Al 0.23 Ga 0.77 N layer  6  is grown to a thickness of 1 μm. 
     Thereafter, the GaN channel region  7  is grown on the Al 0.23 Ga 0.77 N layer  6  so as to have a thickness of 20 nm, so that the channel layer  5  is formed. 
     The thickness “a” of Al x Ga 1-x N layers (0.5≦x≦1.0) that are the high-Al content layers in the superlattice buffer layer  4  preferably ranges from 1 nm to 5 nm and the thickness “b” of Al y Ga 1-y N layers (0≦y≦0.3) that are the low-Al content layers preferably ranges from 22 nm to 30 nm. This is due to a reason below. 
     That is, in the case where the superlattice buffer layer  4  is formed by repeatedly stacking the Al x Ga 1-x N layers that are the high-Al content layers and the Al y Ga 1-y N layers that are the low-Al content layers, the difference between the thickness “a” of the high-Al content layers and the thickness “b” of the low-Al content layers needs to be at least 17 nm or more in order to effectively reduce the warpage of the obtained nitride semiconductor epitaxial wafer. Furthermore, the thickness “a” of the high-Al content layers, which are likely to be warped, needs to be less than the thickness “b” of the low-Al content layers, which are unlikely to be warped. In this case, when the thickness “a” of the high-Al content layers is less than 1 nm, the warpage cannot be effectively reduced because the superlattice buffer layer  4  is close to a configuration equivalent to the case where the high-Al content layers are a single-layer film. When the thickness “b” of the low-Al content layers is greater than 30 nm, the warpage cannot be effectively reduced because the superlattice buffer layer  4  is close to a configuration equivalent to the case where the low-Al content layers are a single-layer film. Therefore, it is effective that the thickness “a” of the high-Al content layers and the thickness “b” of the low-Al content layers are set in the above range. 
     Thereafter, an AlGaN barrier layer  8  is grown on the GaN channel region  7  of the channel layer  5  so as to have a thickness of 15 nm and Al 0.4 Ga 0.6 N. The Al composition “w” of the Al w Ga 1-w N barrier layer  8  is preferably greater than the Al composition “z” of the Al z Ga 1-z N layer  6  of the channel layer  5 . This is due to a reason below. 
     That is, in the case where a HEMT (high-electron mobility transistor) is formed using the nitride semiconductor epitaxial wafer obtained in this embodiment, a two-dimensional electron gas needs to be formed at the interface between the Al w Ga 1-w N barrier layer  8  and the GaN channel region  7  in such a manner that strain is induced at the interface therebetween by stress +σ as shown in  FIG. 6 . In this case, strain is induced at the interface between the GaN channel region  7  and the Al z Ga 1-z N layer  6  by stress −σ as shown in  FIG. 7  because the GaN channel region  7  is placed on the Al z Ga 1-z N layer  6 . Therefore, a strain larger than that induced at the interface between the GaN channel region  7  and the Al z Ga 1-z N layer  6  needs to be induced at the interface between the Al w Ga 1-w N barrier layer  8  and the GaN channel region  7 . Thus, the Al composition “w” of the Al w Ga 1-w N barrier layer  8  is needs to be greater than the Al composition “z” of the Al z Ga 1-z N layer  6  of the channel layer  5 . 
     In order to improve mobility, an AlN intermediate layer (not shown) made of AlN may be grown between the GaN channel region  7  and the AlGaN barrier layer  8  in some cases. A GaN capping layer (not shown) made of GaN may be grown on the AlGaN barrier layer  8 . 
     As described above, the channel layer  5  is formed by stacking the Al z Ga 1-z N layer  6 , which has the Al composition given by Equation (1), and the GaN channel region  7  in that order on the AlGaN superlattice buffer layer  4 , which is formed by alternately stacking the high-Al content layers which have a thickness “a (nm)” and the composition Al x Ga 1-x N (0.5≦x≦1.0) and the low-Al content layers which have a thickness “b (nm)” and the composition Al y Ga 1-y N (0≦y≦0.3), in other words, the AlGaN layer  6 , which has the same Al composition as the average. Al composition of the superlattice, is formed on the AlGaN superlattice buffer layer  4 , so that the nitride semiconductor epitaxial wafer is obtained. 
     In this embodiment, the channel layer  5  used is a combination of the AlGaN layer  6  and the GaN channel region  7 . The channel layer  5  is not limited to this combination. 
       FIG. 2A  and  FIG. 2B  show C-V measurement (capacity measurement) results of the HEMT, which is formed using the obtained nitride semiconductor epitaxial wafer.  FIG. 2A  corresponds to this embodiment, in which the channel layer  5  used is the combination of the AlGaN layer  6  and the GaN channel region  7 .  FIG. 2B  corresponds to a comparative example in which a channel layer used is a GaN layer only. The horizontal axis “au” in each figure represents the relative distance in a substrate direction based on a surface of the wafer. 
     In the C-V measurement, a bias voltage is applied between a gate electrode G of a HEMT  10  and a stage  11  using an LCR meter  12  as shown in  FIG. 3 . 
     As is clear from  FIG. 2B , in the case where the channel layer used is GaN only, a carrier concentration peak  13  is observed between the GaN layer and a superlattice layer. This suggests the presence of a two-dimensional hole gas. However, in  FIG. 2A , which corresponds to the case where the AlGaN layer  6 , which has the same Al composition as the average Al composition of the superlattice, is formed on the AlGaN superlattice buffer layer  4 , no carrier peak due to a two-dimensional hole gas is observed. This suggests that the formation of the two-dimensional hole gas is suppressed. 
     In accordance with the nitride semiconductor epitaxial wafer according to this embodiment, the AlGaN layer  6 , which has the same Al composition as the average Al composition of the superlattice, is formed between the AlGaN superlattice buffer layer  4  and the GaN channel region  7  as described above. 
     In this case, the AlGaN superlattice buffer layer  4 , which is formed by alternately growing AlGaN layers having different Al compositions, can be considered to be equivalent to a single stress-relieved AlGaN layer. The AlGaN layer  6 , which is formed on the single stress-relieved AlGaN layer, has the same Al composition as the average Al composition of the superlattice buffer layer  4 , which is equivalent to the single stress-relieved AlGaN layer, and therefore can be considered to have substantially the same lattice constant as that of the superlattice buffer layer  4 . Thus, it can be suppressed that strain is induced at the interface between the superlattice buffer layer  4  and the AlGaN layer  6  by stress −σ and therefore a two-dimensional hole gas is formed. 
     Thus, a leakage current can be reduced in such a manner that a two-dimensional hole gas formed between the superlattice buffer layer  4  and the GaN layer is compensated for by the AlGaN layer  6 , which is formed on the superlattice buffer layer  4 . 
     Second Embodiment 
       FIG. 4  is a sectional view of a nitride semiconductor epitaxial wafer as the nitride semiconductor according to this embodiment. As shown in  FIG. 4 , the following layers are formed on a Si substrate  21  in this order: an AlN initial growth layer  22  which is made of AlN and which has a thickness of 100 nm and an Al 0.2 Ga 0.8 N buffer layer  23  having a thickness of 20 nm. Subsequently, a superlattice buffer layer  24  is formed thereon. The superlattice buffer layer  24  has 100 repeated periods of AlN layers having a thickness of 3 nm and Al 0.1 Ga 0.9 N layers having a thickness of 25 nm, the AlN layers and the Al 0.1 Ga 0.9 N layers being alternately and repeatedly stacked. 
     Subsequently, a channel layer  25  composed of a plurality of layers is formed on the superlattice buffer layer  24 . The channel layer  25  includes an Al z Ga 1-z N layer  26 , Al composition-graded AlGaN layer  27 , and GaN channel region  28  stacked in that order. The GaN channel region  28  is the above GaN layer. 
     Supposing that the superlattice buffer layer  24  is formed by alternately stacking high-Al content layers which have the composition Al x Ga 1-x N (0.5≦x≦1.0) and a thickness “a (nm)” and low-Al content layers which have the composition Al y Ga 1-y N (0≦y≦0.3) and a thickness “b (nm)”, the Al composition z of the Al z Ga 1-z N layer  26  of the channel layer  25  is given by Equation (1). 
     Thus, in this embodiment, the Al composition z of the Al z Ga 1-z N layer  26  is as follows: z=(3×1+25×0.1)/(3+25)=0.20. The Al 0.2 Ga 0.8 N layer  26  is grown to a thickness of 1 μm. 
     Thereafter, the Al composition-graded AlGaN layer  27  is grown on the Al 0.2 Ga 0.8 N layer  26  so as to have a thickness of 100 nm. In the Al composition-graded AlGaN layer  27 , the Al composition is continuously graded from 0.2 to 0 from the Si substrate  21  side. Furthermore, the GaN channel region  28  is grown to a thickness of 20 nm, so that the channel layer  25  is formed. 
     In this embodiment, the thickness “a” of Al x Ga 1-x N layers (0.5≦x≦1.0) that are the high-Al content layers in the superlattice buffer layer  24  ranges from 1 nm to 5 nm and the thickness “b” of Al y Ga 1-y N layers (0≦y≦0.3) that are the low-Al content layers ranges from 22 nm to 30 nm. Thus, the warpage of the obtained nitride semiconductor epitaxial wafer can be effectively reduced in such a manner that the difference between the thickness “a” of the high-Al content layers and the thickness “b” of the low-Al content layers is adjusted to at least 17 nm or more. 
     Thereafter, an AlGaN barrier layer  29  having Al 0.4 Ga 0.6 N is grown on the GaN channel region  28  of the channel layer  25  so as to have a thickness of 15 nm. 
     In order to improve mobility, an AlN intermediate layer (not shown) made of AlN may be grown between the GaN channel region  28  and the AlGaN barrier layer  29  in some cases. A GaN capping layer (not shown) made of GaN may be grown on the AlGaN barrier layer  29 . 
     As described above, the channel layer  25  is formed by stacking the AlGaN layer  26 , which has the Al composition given by Equation (1), the GaN Al composition-graded AlGaN layer  27 , and the GaN channel region  28  in that order on the superlattice buffer layer  24 , which is formed by alternately stacking the high-Al content layers which have Al x Ga 1-x N (0.5≦x≦1.0) and a thickness “a (nm)” and the low-Al content layers which have Al y Ga 1-y N (0≦y≦0.3) and a thickness “b (nm)”, in other words, the AlGaN layer  26 , which has the same Al composition as the average Al composition of the superlattice, is formed on the superlattice buffer layer  24 , so that the nitride semiconductor epitaxial wafer is obtained. 
     In this embodiment, the channel layer  25  used is a combination of the AlGaN layer  26 , the Al composition-graded AlGaN layer  27 , and the GaN channel region  28 . The channel layer  25  is not limited to this combination. 
       FIG. 5A  and  FIG. 5B  show the C-V measurement results of the HEMT which is formed using the obtained nitride semiconductor epitaxial wafer.  FIG. 5A  corresponds to this embodiment, in which the channel layer  25  used is the combination of the GaN channel region  28 , the Al composition-graded AlGaN layer  27 , and the AlGaN layer  26 .  FIG. 5B  corresponds to a comparative example in which a channel layer used is a GaN layer only. 
     The C-V measurement method is the same as that described in the first embodiment ( FIG. 3 ). 
     As is clear from  FIG. 5B , in the case where the channel layer used is GaN only, a carrier concentration peak  30  is observed between the GaN layer and a superlattice layer. This suggests the presence of a two-dimensional hole gas. However, in  FIG. 5A , which corresponds to the case where the AlGaN layer  26  which has the same Al composition as the average Al composition of the superlattice is formed on the superlattice buffer layer  24 , no carrier peak due to a two-dimensional hole gas is observed. This suggests that the formation of the two-dimensional hole gas is suppressed. 
     In the nitride semiconductor epitaxial wafer according to this embodiment, the Al z Ga 1-z N layer  26 , which has the same Al composition as the average Al composition of the superlattice, is formed on the AlGaN superlattice buffer layer  24  as described above. Thus, as is the case with the first embodiment, a leakage current can be reduced in such a manner that a two-dimensional hole gas formed between the superlattice buffer layer  24  and the GaN layer is compensated for by the AlGaN layer  26 , which is formed on the superlattice buffer layer  24 . 
     Furthermore, in this embodiment, the channel layer  25  is obtained by forming the Al composition-graded AlGaN layer  27  between the AlGaN layer  26  and the GaN channel region  28 . In order to form a two-dimensional electron gas, the channel layer  25  needs an AlGaN layer (the AlGaN barrier layer  29 ) having a thickness insufficient to cause stress relaxation and a small lattice constant and a GaN layer (the GaN channel region  28 ) which forms a heterojunction and which has a bulk lattice constant as shown in  FIG. 6 . In this case, the GaN channel region  28  is stacked on the AlGaN layer  26 , so that a structure in which the two-dimensional hole gas is formed is obtained as shown in  FIG. 7 . 
     Therefore, the strain induced in the GaN channel region  28  is relieved in such a manner that the Al composition-graded AlGaN layer  27  is formed between the Al z Ga 1-z N layer  26  and the GaN channel region  28  such that the Al composition of the Al composition-graded AlGaN layer  27  is continuously graded from z to 0 from the Si substrate  21  side, whereby the two-dimensional hole gas is inhibited from being formed at an interface. 
     Thus, this embodiment can further reduce a leakage current as compared to the first embodiment. 
     As described above, in this embodiment, the strain induced in the GaN channel region  28  is relieved in such a manner that the Al composition-graded AlGaN layer  27  is formed between the Al z Ga 1-z N layer  26  and the GaN channel region  28 . Thus, a strain larger than that induced at the interface between the GaN channel region  28  and the Al composition-graded AlGaN layer  27  can be induced at the interface between the Al w Ga 1-w N barrier layer  29  and the GaN channel region  28  and therefore the two-dimensional electron gas can be formed. 
     As is the case with the first embodiment, the Al composition “w” of the Al w Ga 1-w N barrier layer  29  is preferably greater than the Al composition “z” of the Al z Ga 1-z N layer  26  of the channel layer  25  in order to form the two-dimensional electron gas. 
     In this embodiment, a nitride semiconductor chip can be obtained as another example of the nitride semiconductor by dicing the nitride semiconductor epitaxial wafer. 
     As described above, a nitride semiconductor according to the present invention includes a substrate  1  or  21 , an initial growth layer  2  or  22  formed on the substrate  1  or  21 , a buffer layer  3  or  23  formed on the initial growth layer  2  or  22 , a superlattice buffer layer  4  or  24  formed on the buffer layer  3  or  23 , a channel layer  5  or  25  which is formed on the superlattice buffer layer  4  or  24  and which is composed of a plurality of layers, and a barrier layer  8  or  29  formed on the channel layer  5  or  25 . The superlattice buffer layer  4  or  24  is formed by alternately stacking high-Al content layers which have the composition Al x Ga 1-x N (0.5≦x≦1.0) and a thickness a and low-Al content layers which have the composition Al y Ga 1-y N (0≦y≦0.3) and a thickness b. The channel layer  5  or  25  is joined to the superlattice buffer layer  4  or  24  and is formed by stacking at least an Al z Ga 1-z N layer  6  or  26  and a GaN layer  7  or  28  in that order from the superlattice buffer layer  4  or  24  side. The Al composition of the Al z Ga 1-z  layer  6  or  26  is the same as the average Al composition of the superlattice buffer layer  4  or  24 . 
     The superlattice buffer layer  4  or  24  can be considered to be equivalent to a stress-relieved AlGaN layer. Thus, when the channel layer  5  or  25 , which is joined to the superlattice buffer layer  4  or  24 , is formed of a GaN layer only, a GaN channel layer formed on the stress-relieved AlGaN layer is strained on a tension side by stress −σ because the GaN channel layer has a lattice constant larger than that of the AlGaN layer; hence, a two-dimensional hole gas (2DHG) is formed at the interface therebetween. 
     According to the above configuration, the channel layer  5  or  25 , which is joined to the superlattice buffer layer  4  or  24 , is formed by stacking the Al z Ga 1-z  layer  6  or  26  and the GaN layer  7  or  28  in that order from the superlattice buffer layer  4  or  24  side and the Al composition of the Al z Ga 1-z  layer  6  or  26  is the same as the average Al composition of the superlattice buffer layer  4  or  24 . Thus, the Al z Ga 1-z  layer  6  or  26  can be considered to have substantially the same lattice constant as that of the superlattice buffer layer  4  or  24 , which is equivalent to the stress-relieved AlGaN layer. Therefore, it can be suppressed that strain is induced at the interface between the superlattice buffer layer  4  or  24  and the Al z Ga 1-z  layer  6  or  26  by stress −σ and therefore a two-dimensional hole gas is formed. 
     Thus, a leakage current can be reduced in such a manner that a two-dimensional hole gas formed between the superlattice buffer layer  4  or  24  and the GaN layer  7  or  28  is compensated for by the Al z Ga 1-z  layer  6  or  26 , which is formed on the superlattice buffer layer  4  or  24 . 
     In a nitride semiconductor according to an embodiment, the Al composition z of the Al z Ga 1-z  layer  6  or  26  of the channel layer  5  or  25  is given by the following equation:
 
 z =( a×x+b×y )/( a+b ).
 
     According to this embodiment, the Al composition z of the Al z Ga 1-z  layer  6  or  26  of the channel layer  5  or  25  is given by the equation “z=(a×x+b×y)/(a+b)”. Thus, the Al composition of the Al z Ga 1-z  layer  6  or  26  can be equalized to the average Al composition of the superlattice buffer layer  4  or  24 . 
     In a nitride semiconductor according to an embodiment, the barrier layer  8  includes an Al w Ga 1-w N layer  8  and the Al composition w of the Al w Ga 1-w N layer  8  is greater than the Al composition z of the Al z Ga 1-z  layer  6  of the channel layer  5 . 
     In the case where a HEMT is formed using an obtained nitride semiconductor, a two-dimensional electron gas needs to be formed at the interface between the Al w Ga 1-w N layer  8  of the barrier layer  8  and the GaN layer  7  of the channel layer  5  in such a manner that strain is induced at the interface therebetween by stress +σ. In this case, the channel layer  5  is formed by stacking the GaN layer  7  on the Al z Ga 1-z  layer  6  and therefore strain is induced at the interface between the GaN layer  7  and the Al z Ga 1-z  layer  6  by stress −σ. Therefore, a strain larger than that induced at the interface between the GaN region  7  and the Al z Ga 1-z  layer  6  needs to be induced at the interface between the Al w Ga 1-w N layer  8  of the barrier layer  8  and the GaN layer  7  of the channel layer  5 . 
     According to this embodiment, the Al composition w of the Al w Ga 1-w N layer  8  is set to a value greater than the Al composition z of the Al z Ga 1-z  layer  6  of the channel layer  5 . Thus, a strain larger than that induced at the interface between the GaN region  7  and the Al z Ga 1-z  layer  6  can be induced at the interface between the Al w Ga 1-w N layer  8  of the barrier layer  8  and the GaN layer  7  of the channel layer  5  and therefore a two-dimensional electron gas can be formed at the interface therebetween. 
     In a nitride semiconductor according to an embodiment, the thickness a of the high-Al content layers in the superlattice buffer layer  4  or  24  ranges from 1 nm to 5 nm and the thickness b of the low-Al content layers ranges from 22 nm to 30 nm. 
     According to this embodiment, the warpage of the obtained nitride semiconductor can be effectively reduced in such a manner that the difference between the thickness “a” of the high-Al content layers in the superlattice buffer layer  4  or  24  and the thickness “b” of the low-Al content layers is adjusted to at least 17 nm or more and the thickness “a” of the high-Al content layers, which are likely to be warped, is adjusted below the thickness “b” of the low-Al content layers, which are unlikely to be warped. 
     REFERENCE SIGNS LIST 
       1 ,  21  Si substrate 
       2 ,  22  AlN initial growth layer 
       3 ,  23  AlGaN buffer layer 
       4 ,  24  Superlattice buffer layer 
       5 ,  25  Channel layer 
       6 ,  26  Al z Ga 1-z N layer 
       7 ,  28  GaN channel region 
       8 ,  29  AlGaN barrier layer 
       10  HEMT 
       11  Stage 
       12  LCR meter 
       13 ,  30  Carrier concentration peak 
       27  Al composition-graded AlGaN layer