Abstract:
A semiconductor device comprising: an active layer, which has a composition represented by the formula: Al x M y Ga 1-x-y N, wherein x satisfies 0≦x≦1, wherein y satisfies 0≦y≦1, wherein x+y satisfies 0≦x+y≦1, and wherein M contains at least one of In and B; a substrate containing GaN; and a buffer layer provided between the active layer and the substrate, wherein the semiconductor device is operated by electrical current flowing through the active layer in a direction parallel to a face of the substrate, wherein the buffer layer has a composition represented by the formula: Al p In 1-p N, wherein p satisfies 0≦p&lt;1, and wherein the buffer layer, which has a band gap energy wider than that of the substrate, and which is lattice-matched to the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2009-195136 filed on Aug. 26, 2009, the entire subject matter of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a structure of a semiconductor device including an active region on a hetero junction of a nitride semiconductor. 
         [0004]    2. Description of the Related Art 
         [0005]    As a semiconductor device using a compound semiconductor, specifically, as a device for high power and high frequency, an High Electron Mobility Transistor (HEMT) device using such as GaN has been used. A schematic structure of a cross-section of an HEMT device  90  is shown in  FIG. 4 . In this figure, an electron transit layer  93  and an electron supply layer  94  are formed by an epitaxial growth method over a buffer layer  92  on a substrate  91 . Here, the electron transit layer  93  is made of non-doped GaN, and the electron supply layer  94  is made of non-doped AlGaN (strictly, Al x Ga 1-x N, x=about 0.20). Here, a two-dimensional electron gas layer is formed on a portion of the electron transit layer in an interface between the electron transit layer  93  and the electron supply layer  94 . The two-dimensional electron gas layer is formed between a source electrode  95  and a drain electrode  96 , and electrical current flows through the two-dimensional electron gas layer. Here, a channel of the two-dimensional electron gas layer is controlled on-off by voltage applied to a gate electrode  97 , and a switching operation of the transistor is performed. At this time, because a speed (mobility) of the electrons within the two-dimensional electron gas layer is extremely high, high speed operation is possible. Moreover, GaN has larger band gap energy than GaAs, etc., the HEMT device  90  has high insulation withstand voltage and can be perform high power operation. In this case, in order to obtain better amplification or switching characteristics, it is necessary to enhance an on-off ratio of the current flowing between the source electrode  95  and drain electrode  96 , or a ratio of electrical resistance between the source electrode  95  and the drain electrodes  96  in the turning-off time to electrical resistance therebetween in the turning-on time. Incidentally,  FIG. 4  merely shows the simplest structure of the HEMT device is shown  FIG. 4 . However, in practice, a contacting shape between the source or drain electrodes  95 ,  96  and the electron supply layer  94  or a shape around the gate electrode  97 , etc., may be more optimized. Thus, a practical structure of the HEMT device may be different from the structure of the HEMT device in  FIG. 4 . 
         [0006]    The electron transit layer  93  or the electron supply layer  94  is formed by the epitaxial growth over the substrate  91 , and properties of the HEMT device  90  are considerably affected by a crystalline state of the electron transit layer  93  and the electron supply layer  94 . Since the crystalline state and the manufacturing cost is considerably affected by the substrate  91 , choosing what kind of the substrate  91  is important. For example, sapphire or semi-insulating SiC, etc., may be used as the substrate  91 . However, because it is difficult to form the electron transit layer  93  (non-doped GaN) with better crystalline state directly on the material (wafer) such as sapphire or semi-insulating SiC, it is necessary that forming a buffer layer  92  made of other material between the substrate  91  and the electron transit layer  93 . 
         [0007]    Recently, as GaN wafer, n-GaN (n type GaN) wafer, which has a size easy to operate, can be obtain at a low cost and can be used as the substrate  91 . For example, a fourth embodiment of JP-A-2009-12727 discloses a structure of the HEMT device in which wafer is used as a substrate  91 . In this case, since the material of the substrate  91  is identical with that of the layer  93 , it is comparatively easy to form non-doped GaN with the better crystalline state as the electron transit layer  93  on such the GaN substrate  91 . 
         [0008]    Further, in view of reducing on-resistance of the HEMT device  90 , for example, FIG. 13 and FIG. 14 in JP-A-2006-216671 discloses that the through-hole electrode penetrating from the source electrode  95  to the substrate  91  is formed. Thus, a potential of the source electrode  95  intentionally comes to equal to the potential of the substrate  91  by the through-hole electrode. Accordingly, a reverse face electrode formed with a large size on the reverse face of the substrate  91  can be used as the source electrode. Therefore, as described in the paragraph [0046] of JP-A-2006-216671, it is not necessary to form a source electrode pad on the front surface (upper face) of the HEMT device  90 , the entire area of the chip can be use effectively. 
         [0009]    However, in the above-described arts, there is a problem with respect to the insulation withstand voltage or a leak current between the source electrode  95  and drain electrode  96 , when high voltage is applied therebetween. That is, an insulation resistance between the source electrode  95  and drain electrode  96  becomes lower, or the leak current also flow even at pinch-off operation in the turning-on time, and then it may trouble in operation of the device. 
         [0010]    As described above, the two-dimensional electron gas (channel), which is a main body of the current flowing in the turning-on time, pass through beneath the gate electrode  97  and is on-off controlled by the voltage applied to the gate electrode  97 . However, electrical currents flow between the source electrode  95  and drain electrode  96  through paths other than the channel controlled by the gate voltage. The currents flowing through the other paths may become a main body of the leak current. As examples of the other paths, as indicated in  FIG. 4  by arrows, there are paths passing through the buffer layer  92  and the substrate  91 . If above described n-GaN wafer is used as the substrate  91 , the n-GaN substrate  91  in itself is conductive, so that the leak current flows in the substrate  91 . 
         [0011]    In order to improve such a problem, it may be considered that MN layer, which is known to have larger band gap energy than that of GaN and has better insulation property, or non-doped AlGaN, which is mixed crystal of MN and GaN, is formed as the buffer layer  92  on n-GaN wafer (the substrate  91 ) and the electron transit layer  93  is formed on the buffer layer  92 . However, in this case, it occurs great lattice mismatch (difference between lattice constants) between the buffer layer  92  made of AlN or AlGaN and the electron transit layer  93  made of GaN. Therefore, many crystal defects such as dislocations, etc., are generated in the interface between the buffer layer  92  and the electron transit layer  93 , so that those crystal defects cause the electric conduction through thereof. Accordingly, the leak current (buffer leak) flow between the source electrode  95  and drain electrodes  96 . This phenomenon also occurs when the sapphire or SiC, etc., is used as the substrate. 
         [0012]    This problem is not limited to only the HEMT device also other devices, which include an substrate including GaN and an active layer having a hetero structure on the substrate, and in which electric current flows in a lateral direction (i.e. a direction parallel to the surface of the substrate) for operating the device. Examples of such devices may be Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or Schottky Barrier Diode (SBD), etc. 
         [0013]    Accordingly, in a prior art, it is difficult to manufacture the device in which the buffer leak is reduced on the substrate containing GaN. 
         [0014]    The present invention is made with considering the above problem, and provides the device solved the problem. 
       SUMMARY 
       [0015]    Accordingly, a purpose of the present invention is to provide a device in which the leak current (buffer leak) reduces on the GaN substrate. The present invention includes following configuration. A semiconductor device comprising: an active layer, which has a composition represented by the formula: Al x M y Ga 1-x-y N, wherein x satisfies 0≦x≦1, wherein y satisfies 0≦y≦1, wherein x+y satisfies 0≦x+y≦1, and wherein M contains at least one of In and B; a substrate containing GaN; and a buffer layer provided between the active layer and the substrate, wherein the semiconductor device is operated by electrical current flowing through the active layer in a direction parallel to a face of the substrate, wherein the buffer layer has a composition represented by the formula: Al p In 1-p N, wherein p satisfies 0≦p≦1, and wherein the buffer layer, which has a band gap energy wider than that of the substrate, and which is lattice-matched to the substrate. 
         [0016]    According to the present invention, it is possible to manufacture the device in which the buffer leak is reduced on the substrate including GaN. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a view showing a structure of a cross-section of an HEMT device according to a first exemplary embodiment of the present invention; 
           [0018]      FIG. 2  is a view showing a structure of a cross-section of the HEMT device according to a modification of the first exemplary embodiment of the present invention and a composition distribution of a buffer layer thereof; 
           [0019]      FIG. 3  is a view showing a structure of a cross-section of the HEMT device according to a second exemplary embodiment of the present invention; and 
           [0020]      FIG. 4  is a view showing a cross-section of the HEMT device using a nitride semiconductor in a prior-art. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]    A semiconductor device according to an embodiment of the present invention, specifically, a High Electron Mobility Transistor (HEMT) device will be described in detail. In the HMET device, a hetero structure is formed on a GaN substrate, and the HEMT device is operated by the electric current flowing in a lateral direction (in a direction parallel to the surface of the substrate). At this time, the present device includes a configuration by which the buffer leak or leak current flowing in a vertical direction is suppressed. 
       First Embodiment 
       [0022]      FIG. 1  is a view showing a structure of a cross-section of the HEMT device  10  according to a first exemplary embodiment of the present invention. The HEMT device  10  uses n-GaN (n type GaN wafer) as a substrate  11 . Non-doped (semi-insulating) Al p In 1-p N (0&lt;p&lt;1) is used as a buffer layer  12  on the substrate  11 . Here, the term “non-doped’ means that an impurity is not intentionally introduced in order to control conductance level. On the buffer layer  12 , both an electron transit layer  13  made of non-doped GaN and an electron supply layer  14  made of non-doped AlGaN on the layer  13  are formed by a Molecular Beam Epitaxy (MBE) method or a Metal Organic Vapor Phase Epitaxy method (MOVPE) in order. Both the electron transit layer  13  and the electron supply layer  14  function as an active layer in the HEMT device  10 . Precisely, the non-doped AlGaN forming the electron supply layer  14  is Al x Ga 1-x N (preferably 0.1&lt;x&lt;0.3, specifically x=about 0.20). A source electrode  15  and a drain electrode  16  are formed on the electron supply layer  14 , two-dimensional electron gas layer is formed at a portion of the electron transit layer  13  in an interface between the electron transit layer  13  and the electron supply layer  14 . Turning on/off operation of a channel made of the two-dimensional electron gas layer is controlled by voltage applied through a depletion layer to a gate electrode  17 . Both the source electrode  15  and drain electrodes  16  are made of a material, such as Ti/Au, contacting with the two-dimensional electron gas layer in an ohmic manner. The gate electrode  17  is made of a material, such as Ni/Au, contacting with the non-doped AlGaN layer in a schottky manner and form a depletion layer in the non-doped AlGaN layer. Meanwhile, the electron supply layer  14  may be made of AlGaN layer having n type conductance. 
         [0023]    The HEMT device  10  is characterized in that the buffer layer  12  is made of non-doped Al p In 1-p N. AlInN may be mixed crystal of aluminum nitride (AlN) and indium nitride (InN), and physical properties of AlInN may be controlled by molar fraction between MN and InN. On the other hand, hereinafter, the molar fraction between MN and InN is represented by composition ratio (namely, p) of Al in Al p In 1-p N. For example, band gap energy of AlInN may take a value in a range of 0.7 eV to 6.2 eV by gradually increasing the composition ration p of Al. Moreover, a lattice constant of AlInN can be controlled in the same way, thus, the composition ration p of Al can be adjusted in order to acquire a desired lattice constant of AlInN. Further, it is well known that a lattice constant of GaN (at an “a” axis) is approximately 3.18 and a lattice constant of AlInN (at an “a” axis) may take a value in a range of 3.11 to 3.54. In the present example, it is preferable that the composition ration p of Al is as follows: 0.6≦p≦0.9 so that a lattice of Al p In 1-p N layer (the buffer layer  12 ) matches with a lattice of GaN layer (so that the lattice constants of the two layers are the same). It is more preferable that the composition ration p of Al is as follows: 0.7≦p≦0.85. 
         [0024]    The buffer layer  12  made of the above-described Al p In 1-p N is formed on the substrate  11  made of GaN, and the electron transit layer  13  made of GaN is formed on the buffer layer  12 . Further, the composition ratio of Al in the buffer layer  12  is selected so that the buffer layer  12  lattice-matched with GaN layer as described above. Accordingly, in the semiconductor device according to the first embodiment of the present invention, it can be suppressed the lattice mismatches at the interface between the buffer layer  12  and the electron transit layer  13  and at the interface between the electron transit layer  13  and the electron supply layer  14 . As a result, the crystal defects including the dislocations caused by the lattice mismatches can be suppressed, so that the crystal quality is improved. 
         [0025]    Further, it is possible that the band gap energy of the buffer layer  12  made of the above Al p In 1-p N is equal to band gap energy of MN layer by increasing the composition ratio p of Al. Therefore, the semiconductor device according to the first embodiment of the present invention can be configured so that the semi-insulating buffer layer  12  is interposed between the conductive substrate  11  and the semi-insulating electron transit layer  13 . 
         [0026]    Accordingly, the buffer leak through the buffer layer  12  is to be reduced, so that the HEMT device  10  with better characteristics can be acquired. 
         [0027]    When the buffer layer  12  is formed on the substrate (n type GaN)  11  by the MOVPE method, both a flow rate of source gas (for example, trimethylaluminum: TMA) of Al and a flow rate of source gas (for example, trimethylindium: TMI) may be adjusted to control the composition ratio p. When the buffer layer  12  is formed on the substrate (n type GaN)  11  by the MBE method, Al molecular beam intensity and In molecular beam intensity may be adjusted to control the composition ratio p. 
         [0028]      FIG. 2  is a view showing a structure of a cross-section (left side) of an HEMT device  10 ′ according to a modification of the first embodiment and composition distribution (right side) in a buffer layer  12 ′ of the HEMT device  10 ′. Although the electron transit layer  13  is made of the non-doped GaN layer in the first embodiment, an electron transit layer  13 ′ in the modification may be made of a non-doped (semi-insulating) Al x Ga 1-x N (0&lt;x&lt;1, preferably 0&lt;x&lt;0.2) layer, for example. In this case, a lattice constant of the substrate  11  is different from a lattice constant of the electron transit layer  13 ′. Therefore, as shown in  FIG. 2 , the composition ratio p of Al in the buffer layer  12 ′ changes in a thickness direction. In the modification, when the electron transit layer  13 ′ is made of the non-doped Al x Ga 1-x N layer, it is preferable that the composition ratio p of Al in the buffer layer  12 ′ made of Al p In 1-p N is set so that the composition ratio p at the region adjacent to the electron transit layer  13 ′ is larger than that at the region adjacent to the substrate  11 . On the other hand, a changing pattern of the composition ratio p of Al is not limited to a linear changing pattern as shown in  FIG. 2 , and a step-like changing pattern may be used. Incidentally, the composition ratio p of Al may be appropriately changed by ratio between the lattice constant of the substrate and the lattice constant of the electron transit layer. 
         [0029]    Although the buffer layer  12  is made of the non-doped Al p In 1-p N layer in the first embodiment, any material of the buffer layer  12  may be used which makes average resistivity of the buffer layer  12  larger than the substrate  11  and the electron transit layer  13 . Further, impurities such as C, Mg, Fe, etc can be appropriately doped thereto in order to control a resistivity, a stress or a lattice constant of the buffer layer  12 . 
         [0030]    Meanwhile, although the present invention is applied to the HEMT device in the first embodiment, it is obvious that the present invention is applied to electronic devices other than the HEMT device. According to the present invention, the leak current (buffer leak) through the buffer layer may reduce, so that the insulation withstand voltage may enhance in the electronic devices, in which the buffer layer is disposed between GaN substrate and the active layer, and in which the current flows in the lateral direction (in the direction parallel to the face of GaN substrate). 
       Second Embodiment 
       [0031]      FIG. 3  is a view showing a structure of a cross-section of an HEMT device  20  according to a second embodiment of the present invention. The HEMT device  20  includes a substrate  21  made of n-GaN and a buffer layer  22  formed on the substrate  21 . The buffer layer  22  according to this embodiment is formed as a laminated structure in which AlqIn1-qN (0&lt;q&lt;1) layers  22   a  and p-GaN (p type GaN) layers  22   b  are alternately laminated. An electron transit layer  23  made of non-doped GaN is formed on the buffer layer  22 , and an electron supply layer  24  made of non-doped GaN is formed on the layer electron transit  23 . Accordingly, the laminated structure of HEMT device  20  is different in the laminated structure of the buffer layer  22  from the HEMT device  10  in the first embodiment. In addition, configurations of a source electrode  25 , a drain electrode  26 , and a gate electrode  27  in the second embodiment are similarly to the configurations in the first embodiment. 
         [0032]    Although it is possible to lattice-match between Al q In 1-q N layer  22   a  and p-GaN layer  22   b  as the first embodiment, a composition ratio q of Al usually is restricted. However, in the structure according to the second embodiment, because each thin Al q In 1-q N layers  22   a  and thin p-GaN layers  22   b  (p-type GaN) are alternately laminated, it is possible to reduce the lattice-mismatch generated between Al q In 1-q N layer  22   a  and GaN layer. The range of options in the composition ratio q of Al, which is considered both the crystal quality and the buffer leak, may increase. 
         [0033]    A bottom end of the buffer layer  22  is adjacent to the substrate  21  and is called as a substrate connection region. The p-GaN layer  22   b  is formed at the bottom end of the buffer layer. According this structure, a p-n junction is formed between the substrate  21  made of n-GaN and the buffer layer  22 . Therefore, the buffer leak may reduce, and the HEMT device  20  has better characteristics. Moreover, the top end of the buffer layer  22  is adjacent to the active layer (the electron transit layer  23  and the electron supply layer  24 ) and is called as an active layer connection region. Al q In 1-q N layer  22   a  is formed at the bottom end of the buffer layer. According to this structure, in forming process of an HEMT device  20 , Mg or Zn that is impurities within the p-type GaN layer  22   b  is prevented from diffusing into the electron transit layer  23  of the active layer. 
         [0034]    Although the buffer layer  22  shown in  FIG. 3  is formed as the laminated structure in which two of Al q In 1-q N layers  22   a  and two of p-GaN (p type GaN) layers  22   b  are alternately laminated with uniform thicknesses, the number and thickness of the laminated layers are not limited as long as the effect of the present invention is accomplished. Further, it may be contemplated that composition ratio of Al in the buffer layer  22  changes in the thickness direction in the same way as in the buffer layer  12 ′ of the modification of the first embodiment. 
         [0035]    In the HEMT device  20  according to the second embodiment of the present invention, the material of the substrate  21  is not limited to the substrate made of n-GaN. For example, the substrate  21  may be made of SI-GaN (Semi Insulated-GaN). In the latter case, p-GaN layer  22   b  functions as an electric-field relaxation layer, which advance the insulation withstand voltage of the HEMT device  20 . 
         [0036]    As described above, although certain exemplary embodiments of the present invention have been described in detail, the present invention is not limited to the certain exemplary embodiments, and it will be apparent for those skilled in the art. Certain changes, combinations and modifications within a scope of the claimed invention may be possible. For example, it is possible that AlGaN is used as a material of the substrate, or a semiconductor layer forming the electron supply layer is made of InGaN or AlInGaN, or boron (B) is used instead of indium (In).