Abstract:
A semiconductor substrate includes: an AlN layer provided on a silicon substrate; an AlGaN layer that is provided on the AlN layer and has an Al composition ratio of 0.3 to 0.6; and a GaN layer provided on the AlGaN layer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to semiconductor substrates and semiconductor devices using the same. More particularly, the present invention relates to a semiconductor substrate having a gallium nitride (GaN) layer formed on a silicon (Si) substrate, and a semiconductor device such as a semiconductor substrate. 
         [0003]    2. Description of the Related Art 
         [0004]    Semiconductor devices using GaN are used as power devices operating at high frequencies and outputting high power, light-emitting diodes and laser diodes emitting lights of short wavelengths. Among these semiconductor devices, particularly, there has been considerable activity in the development of semiconductor devices capable of amplifying microwaves, quasi-millimeter waves or millimeter waves, such as high electron mobility transistors (HEMT), and light-emitting devices such as laser diodes (LD), light-emitting diodes and vertical cavity surface emitting laser diodes (VCSEL). 
         [0005]    Generally, a sapphire substrate or silicon carbide (SiC) is used as a substrate for growing a GaN layer. Since the sapphire substrate and SiC substrate are expensive, an alternative technique for growing the GaN layer on a Si substrate has been developed. Silicon and gallium react easily. Thus, an aluminum nitride (AlN) layer is provided as a barrier layer between the Si substrate and the GaN layer. However, the GaN layer having good crystallinity cannot be obtained when the GaN layer is grown directly on the AlN layer. Japanese Patent Application Publication No. 2004-524250 discloses a technique taking the above into consideration in which an AlGaN layer is provided between the AlN layer on the Si substrate and the GaN layer and the Al composition continuously changes. Japanese Patent Application Publication No. 2000-277441 discloses a technique in which an AlGaN layer having a stepwise changing composition is provided between the AlN layer on the silicon substrate and the GaN layer. 
         [0006]    However, the techniques disclosed in the above publications do not realize the GaN layer that is grown on the Si substrate and has good crystallinity. For example, a measurement result shows that the X-ray diffraction rocking curve full width at half-maximums (XRC-FWHM) of the (002) or (102) plane of a GaN layer formed on a Si substrate using the conventional technique ranges from 600 seconds to 800 seconds. Further, the substrate having the GaN layer grown on the Si substrate may have a crack on the surface or a warp due to the difference in thermal expansion coefficient between the GaN layer and the Si substrate. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention has been made in view of the above-mentioned circumstances, and provides a semiconductor substrate having a GaN layer having improved crystallinity and reduced warp and a semiconductor device using such as a semiconductor substrate. 
         [0008]    According to an aspect of the present invention, there is provided a semiconductor substrate including: an AlN layer provided on a silicon substrate; an AlGaN layer that is provided on the AlN layer and has an Al composition ratio of 0.3 to 0.6; and a GaN layer provided on the AlGaN layer. 
         [0009]    According to another aspect of the present invention, there is provided a semiconductor device including: an AlN layer provided on a silicon substrate; an AlGaN layer that is provided on the AlN layer and has an Al composition ratio of 0.3 to 0.6; a GaN layer provided on the AlGaN layer; and an operating layer provided on the GaN layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1A and 1B  show a method for manufacturing a semiconductor device in accordance with a first embodiment; 
           [0011]      FIG. 2  is a graph of an X-ray diffraction rocking curve full width at half-maximums (XRC-FWHM) as a function of the Al composition ratio of an AlGaN layer; 
           [0012]      FIG. 3  is a graph of the quantity of warp of a semiconductor substrate as a function of the Al composition ratio of an AlGaN layer; and 
           [0013]      FIG. 4  is a cross-sectional view of a HEMT in accordance with a second embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    A description will now be given of embodiments of the present invention with reference to the accompanying drawings. 
       First Embodiment 
       [0015]    A first embodiment will now be described with reference to  FIGS. 1A and 1B . Referring to  FIG. 1A , a surface oxide film on a Si substrate  10  is removed by hydrofluoric acid (HF). Next, the Si substrate  10  is placed in a reactor of metal organic chemical vapor deposition (MOCVD) apparatus. Then, the temperature of the Si substrate  10  is raised to 1050° C. by using hydrogen (H 2 ) gas as carrier gas. This thermally cleans the surface of the Si substrate  10 . Then, trimethyl aluminum (TMA) and ammono (NH 3 ) are supplied to grow an AlN (aluminum nitride) layer  12  having a thickness of about 250 nm on the Si substrate  10 . The AlN layer  12  is an almost lattice-relaxed layer because the Si substrate  10  is heated to 1050° C. Then, the temperature of the Si substrate  10  is raised to 1100° C., and TMA, trimethyl gallium (TMG) and NH 3  are supplied to grow an AlGaN (aluminum gallium nitride) layer  14  having a thickness of about 250 nm on the AlN layer  12 . The Al composition ratio of the AlGaN layer  14  will be described later. Further, the Si substrate  10  is heated to 1150° C., and TMG and NH 3  are supplied to grow a GaN (gallium nitride) layer  16  having a thickness of about 1 μm on the AlGaN layer  14 . 
         [0016]    Referring to  FIG. 1B , the temperature of the Si substrate  10  is decreased to 1050° C., and TMA, TMG and NH 3  are supplied to grow an AlGaN electron supply layer  18  on the GaN layer  16  in which the AlGaN electron supply layer  18  has a thickness of about 30 nm and an Al composition ratio of 0.25. The semiconductor substrate of the first embodiment formed using MOCVD is completed through the above-mentioned steps. 
         [0017]    The inventors investigated the crystallinity of the GaN layer  16  when the AlGaN layer  14  has an Al composition ratio of 0.40 in the state shown in  FIG. 1A  by using the X-ray diffraction method. The XRC-FWHM of the (002) plane of the GaN layer  16  was 450 seconds and was excellent. In the case of the (102) plane of the GaN layer  16  shows 570 seconds and a very good value. Cracks or pits on the surface of the GaN layer  16  were not observed. Further, the inventors confirmed that the semiconductor substrate has only a small warp. 
         [0018]      FIG. 2  shows the XRC-FWHM of the (102) plane of the GaN layer  16  for different Al compositions of the AlGaN layer  14 . The XRC-FWHM abruptly increases when the Al composition ratio is smaller than 0.3. It is thus possible that the Al composition ratio of the AlGaN layer  14  is equal to or greater than 0.3. When a sufficient production margin is ensured, it is preferable that the Al composition ratio of the AlGaN layer  14  is equal to or greater than 0.4. 
         [0019]      FIG. 3  shows the quantity of warp (μm) of the semiconductor substrate for different Al compositions of the AlGaN layer  14 . The quantity of warp abruptly increases when the Al composition ratio exceeds 0.6. It is thus preferable that the Al composition ratio of AlGaN layer  14  is equal to or smaller than 0.6. 
         [0020]    It is thus preferable that the Al composition ratio of the AlGaN layer  14  is equal to or greater than 0.3 and is equal to or smaller than 0.6. Through the above-mentioned consideration of the first embodiment, the inventors were able to obtain data shown in  FIGS. 2 and 3  that were unknown in the past. Therefore, the inventors were able to realize the semiconductor substrate having improved crystallinity and reduced warp and semiconductor devices using the above semiconductor substrate. 
         [0021]    It is well known that the growth rate of the AlGaN layer abruptly decreases when the Al composition ratio thereof increases. Thus, the conventional Al composition ratio of the AlGaN layer  14  formed between the AlN layer  12  and the GaN layer  16  was approximately equal to 0.2. However, according to  FIG. 2 , the XRC-FWHM of the GaN layer  16  with the Al composition ratio being approximately equal to 0.2 is about 700 seconds. Further, there is no report that shows the XRC-FWHM is equal to or less than 600 seconds when the Al composition of the AlGaN layer  14  is continuously or stepwise changed as described in the above-mentioned publications. The inventors found out that the crystallinity of GaN layer  16  can be improved by setting the Al composition ratio of the AlGaN layer  14  that is a single layer equal to or greater than 0.3. 
         [0022]    When the Al composition ratio of the AlGaN layer  14  is set equal to or smaller than 0.6, warp of the wafer can be improved. When the Al composition ratio of the AlGaN layer  14  is set equal to or greater than 0.3, the crystallinity can be improved. These improvements may result from the following. 
         [0023]    Table 1 show the linear thermal expansion coefficients (ppm/° C.) and lattice constants (nm) of Si, AlN, AlGaN (the Al composition ratio is 0.5), and GaN. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Linear thermal expansion 
                 Lattice constants 
               
               
                   
                 coefficient (ppm/° C.) 
                 (nm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 GaN 
                 5.6 
                 0.319 
               
               
                   
                 AlGaN 
                 5.0 
                 0.315 
               
               
                   
                 AlN 
                 4.6 
                 0.311 
               
               
                   
                 Si 
                 3.6 
                 0.384 
               
               
                   
                   
               
             
          
         
       
     
         [0024]    Each layer is grown at a temperature approximately equal to 1100° C., and the semiconductor substrate is warped due to compressive stress resulting from the linear thermal expansion coefficient. Further, stress resulting from the difference in lattice constant is applied to each layer. As has been described, the AlN layer  12  is almost lattice-relaxed. Thus, tensile stress applied to the AlGaN layer  14  and the GaN layer  16  can compensate for compressive stress resulting from the thermal expansion coefficient. When the AlGaN layer has a relatively small Al composition ratio, there is great stress resulting from the difference in lattice constant between the AlN layer  12  and the AlGaN layer  14 , and compressive stress resulting from the thermal expansion coefficient can be considerably compensated for. In contrast, when the AlGaN layer has a relatively large Al composition ratio, there is small stress resulting from the difference in lattice constant between the AlN layer  12  and the AlGaN layer  14 , and compressive stress resulting from the thermal expansion coefficient cannot be considerably compensated for. It can be seen from the above that the semiconductor substrate may be warped when the Al composition ratio of the AlGaN layer  14  is great. Consequently, the Al composition ratio of the AlGaN layer  14  is preferably equal to or greater than 0.3 and is equal to or smaller than 0.6. 
         [0025]    The crystallinity is improved as stress during growth is small (that is, lattice-relaxed). Thus, it is better that stress resulting from the difference in lattice constant is small. As has been described, lattice deformation in the AlN layer  12  is almost relaxed. Thus, the lattice constant of the upper surface of the AlN layer  12  is approximately equal to the lattice constant of AlN. Thus, when the Al composition ratio of the AlGaN layer  14  is small, large stress resulting from the difference in lattice constant is applied to the AlGaN layer  14  during growth, and crystallinity cannot be improved. In contrast, when the Al composition ratio of the AlGaN layer  14  is large, small stress resulting from the difference in lattice constant is applied to the AlGaN layer  14  during growth, and crystallinity can be improved. 
         [0026]    The AlN layer  12  and the AlGaN layer  14  are preferably thick in terms of crystallinity. However, these layer are excessively thick, a crack may take place due to internal stress. Thus, it is preferable that each of the AlN layer  12  and the AlGaN layer  14  falls within a range of 250 nm+125 nm. Each of the AlN layer  12  and the AlGaN layer  14  may contain another atom with a composition ratio of about 1%. For example, In may be contained with an In composition ratio of 1% or less. 
         [0027]    Preferably, the AlN layer  12  is a layer in which lattice deformation has been relaxed. As shown in Table 1, the AlN layer and the Si layer have quite different lattice constants. Thus, if lattice deformation of the AlN layer  12  is not relaxed, a crack may be produced in the AlN layer  12  due to internal stress. The lattice deformation of the AlN layer  12  may be relaxed by a method of forming the AlN layer  12  at a temperature equal to or higher than 1000° C., as in the case of the first embodiment. Other than the above method, there may be another method of raising or falling the temperature at which the AlN layer  12  is grown or yet another method of providing a buffer layer between the AlN layer  12  and the Si substrate  10 . Any of the above methods may be used. 
       Second Embodiment 
       [0028]    A second embodiment is a HEMT using the semiconductor substrate of the first embodiment. As shown in  FIG. 4 , a source electrode  22  and a drain electrode  24 , which are ohmic electrodes having a Ti/Au structure, are formed on the electron supply layer  18  of the semiconductor substrate of the first embodiment by evaporation. A gate electrode  26 , which has a Ni/Au is formed on the electron supply layer  18  by evaporation. Through the above steps, the HEMT is completed. In the second embodiment, two-dimensional gas generated at an interface of the GaN layer  16  with the electron supply layer  18  and the electron supply layer  18  function as an operating layer through which carriers run. 
         [0029]    The semiconductor device of the second embodiment has improved performance because it has the GaN layer having improved crystallinity and reduced warp. The semiconductor device may be optical semiconductor devices such as LD, LED and VCSEL other than FETs such as HEMT. These devices have improved performance because the crystallinity of the GaN is improved. In LD, LED or VCSEL, an active layer and a clad layer operate as an operating layer through which carriers run. 
         [0030]    The present invention is not limited to the specifically disclosed embodiments, but other embodiments and variations may be made without departing from the scope of the present invention. 
         [0031]    The present application is based on Japanese Patent Application No. 2006-351436 filed Dec. 27, 2006, the entire disclosure of which is hereby incorporated by reference.