Abstract:
An N-face GaN HEMT device including a semiconductor substrate, a buffer layer including AlN or AlGaN deposited on the substrate, a barrier layer including AlGaN or AlN deposited on the buffer layer and a GaN channel layer deposited on the barrier layer. The channel layer, the barrier layer and the buffer layer create a two-dimensional electron gas (2-DEG) layer at a transition between the channel layer and the barrier layer.

Description:
GOVERNMENT CONTRACT 
     The U.S. Government may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. HR011-09-C-0132 awarded by DARPA. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to an N-face GaN semiconductor device and, more particularly, to an N-face GaN high electron mobility transistor (HEMT) device that includes an AlN or AlGaN buffer layer. 
     2. Discussion of the Related Art 
     Integrated circuits are typically fabricated by epitaxial fabrication processes that deposit or grow various semiconductor layers on a semiconductor substrate to provide the circuit components of the device. Substrates for integrated circuits include various semiconductor materials, such as silicon, InP, GaAs, etc. As integrated circuit fabrication techniques advance and become more complex, more circuit components are able to be fabricated on the substrate within the same area and be more closely spaced together. Further, these integrated circuit fabrication techniques allow the operating frequencies of the circuit to increase to very high frequencies, well into the GHz range. 
     HEMT devices are popular semiconductor devices that have many applications, especially high frequency or high speed applications. GaN HEMT devices are typically epitaxialy grown on a suitable substrate, such as silicon carbide (SiC), sapphire, silicon, etc., all well known to those skilled in the art. One fabrication process for GaN HEMT devices is referred to in the art as Ga-face fabrication, where the device profile layers are grown having a positive, or Ga polar orientation. For example, a typical HEMT device may have a SiC substrate that includes alternating crystalline layers of silicon and carbon. A nucleation layer, such as an AlN layer, is typically deposited on the SIC substrate to facilitate epitaxial growth, where the nucleation layer is grown on the side of the substrate having a silicon face so that the orientation of the crystalline structure of the nucleation layer, and the subsequent device layers, has a gallium orientation. When the gallium and nitrogen are provided to the vacuum chamber for the epitaxial deposition process of the nucleation layer, the semiconductor elements will be deposited on the substrate based on their crystal orientation so that alternating layers of gallium or aluminum and nitrogen are formed, where a nitrogen layer is formed first. A GaN buffer layer is typically grown on the nucleation layer that provides a crystalline structure having limited defects. An AlGaN barrier layer is deposited on the buffer layer, where the combination of the buffer layer and the barrier layer creates a two-dimensional electron gas (2-DEG) layer for the flow of electrons at the transition between these layers. 
     Various types of Ga-face fabrication processes have typically been employed because of the efficiency with which the epitaxial layers can be grown. It has been proposed in the art to reverse the orientation of the epitaxial growth process so that the opposite side of the substrate is the side on which the other device profile layers are grown, referred to as N-face or N-polar device processes. A typical N-face GaN HEMT device, sometimes referred to as an inverted-HEMT, typically includes an AlN nucleation layer, GaN buffer layer, an AlGaN back-barrier layer and a GaN channel layer. For the example discussed above for the SiC substrate, the device profile layers are grown on the carbon face of the substrate, so that the crystalline orientation of the device profile layers has a nitrogen orientation instead of a gallium orientation. 
     As discussed above, for an N-face device, similar layers are basically deposited as for a Ga-face device, but they have an opposite orientation and polarity so that the orientation of the aluminum, gallium and nitrogen crystals that make up the AlGaN/AlN nucleation layer, GaN buffer layer and GaN channel layer are opposite in crystalline orientation. The GaN channel layer is grown on the AlGaN back-barrier layer, where the 2-DEG channel is then formed between those two layers. The channel electrons in the 2-DEG layer are induced from the piezoelectric/spontaneous polarization effect between the AlGaN/AlN back-barrier and the GaN channel layer. For the N-face device, when the AlGaN back-barrier layer is grown on the GaN buffer layer, the opposite orientation of the crystals does not cause the 2-DEG layer to be formed therebetween. 
     Although N-face fabrication processes are typically more difficult than Ga-face fabrication processes, N-face fabrication processes typically provide more desirable results when forming the 2-DEG layer. Because the channel layer is formed on the AlGaN back-barrier layer in an N-face device, a number of advantages can be realized, such as the ability to make better electrical contact for the source and drain terminals. Also, because the channel layer is between the barrier layer and the contacts, the on/off switching of the device can be performed more quickly and efficiently. Further, the location of the channel layer reduces the buffer leakage current, which saves power and increases performance. 
     Vertically scaling of an N-face HEMT device can be performed to reduce the gate-to-channel distance, which causes a natural surface depletion to occur that reduces the charge of the 2-DEG layer. This requires a larger polarization charge to provide compensation. The thickness and/or aluminum composition of the AlGaN/AlN back-barrier layer must be increased to maintain sufficient carrier density of the 2-DEG layer. With a standard GaN buffer layer profile, increasing the aluminum composition or back-barrier thickness creates a large stress in the device as a result of differences in the atomic spacing. As a result, the wafer can crack or have severe bowing. This cracking can deplete the charge in the 2-DEG layer. Further, wafer bowing causes a low yield for high resolution lithography. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a device profile of a known N-face HEMT device; 
         FIG. 2  is a device profile of an N-face HEMT device including an AlN buffer layer; 
         FIG. 3  is a device profile of an N-face HEMT device including an AlGaN buffer layer; and 
         FIG. 4  is a graph with depth on the horizontal axis, energy on the left vertical axis and volume on the right vertical axis showing the electron energy of the device shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to an N-face GaN HEMT device including an AlN or AlGaN buffer layer is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a device profile of a known N-face HEMT device  10  including a substrate  12  on which the various epitaxial or device profile layers of the HEMT device  10  are deposited using known epitaxial growth techniques. The substrate  12  can be any substrate suitable for the purposes discussed herein, such as SiC, sapphire, GaN, AlN, Si, etc. Because the device  10  is an N-face device, the orientation of the substrate  12  is such that the top face of the substrate  12  facing the other profile layers of the device  10  causes the device layers to have a nitrogen orientation, as is well understood by those skilled in the art. 
     In this embodiment, an AlN nucleation layer  14  is grown on the substrate  12  to provide a base layer for proper epitaxial growth of the device profile layers. Next, a GaN buffer layer  16  is grown on the nucleation layer  14 , an AlGaN barrier layer  18  is grown on the buffer layer  16  and an optional AlN barrier layer  20  is grown on the barrier layer  18 , where the combination of the layers  18  and  20  form a back-barrier for the electron channel. A GaN channel layer  22  is deposited on the barrier layer  20 , where the piezoelectric/spontaneous polarization effect between the AlGaN/AlN back-barrier and the GaN channel layer  22  generates a 2-DEG layer  24  between the barrier layer  20  and the channel layer  22 . Suitable patterning and metal deposition steps are then performed to deposit the source, drain and gate terminals (not shown) on the channel layer  22 . 
     In a Ga-face device that may have the same or similar device layers as shown in the device  10 , the opposite crystal orientation of the elements and layers that occurs as a result of the epitaxial growth process creates a 2-DEG layer between the buffer layer  16  and the barrier layer  18 , where the GaN channel layer  22  would not be included in that type of Ga-face device. The AlGaN barrier layer  18  would be a source of electrons between the contacts and the electron gas within the electron channel. 
     For the N-face device  10 , the orientation of the various elements and layers causes the 2-DEG layer  24  to be on top of the barrier layer  20  allowing it to make better electrical contact with the terminals or contacts (not shown) of the device  10 . However, this configuration and orientation of layers creates a free hole charge layer  26  between the layers  16  and  18  as a result of the piezoelectric effect and conservation of charge. The free hole charge layer  26  adversely effects device performance because it creates a parasitic capacitance, can cause leakage currents and reduces the frequency response of the current flow through the device  10 . Further, the free hole charge layer  26  causes a flow of holes that creates trap charges, which also limits the speed of the device  10 . Also, the transition between the GaN buffer layer  16  and the AlGaN barrier layer  18  creates a lateral tensile strain between the layers  16  and  18  as a result of the difference of the atomic spacing between the lattice orientation of the layers  16  and  18 . Particularly, the atomic spacing of the AlGaN and AlN barrier layers  18  and  20  is narrower than the atomic spacing of the layer  16  such that if the concentration of aluminum in the layer  18  or the thickness of the layers  18  and  20  is too high, the lateral tensile strain will cause the layers  18  and  20  to crack or the wafer will have significant curvature. Because this cracking and curvature problem limits the amount of aluminum in the layer  18  and/or the thickness of the layers  18  and  20  that is necessary to prevent the cracking, the amount of charge or electrons available to conduct is also limited, which limits the speed and performance of the device  10 . 
       FIG. 2  is a device profile of an N-face HEMT device  30  that addresses and corrects the problems with the lateral tensile strain in the device  10 , as discussed above. The device  30  includes a substrate  32  similar to the substrate  12  and being made of any suitable material. Instead of the AlN nucleation layer  14  and the GaN buffer layer  16 , the device  30  includes an AlN buffer layer  34  grown directly on the substrate  32 , which eliminates the need for the nucleation layer  14 . Therefore, the buffer layer  34  provides both a structure for facilitating epitaxial growth and a layer that reduces defects that may occur in the substrate  32 . An optional AlGaN back-barrier layer  36  is grown on the buffer layer  34  and is similar to the barrier layer  18  which can be introduced wherever necessary, or can be eliminated because the AlN buffer layer  34  itself would provide enough charge into the GaN channel. A GaN channel layer  38  is then grown on the back-barrier layer  36 , where the piezoelectric/spontaneous polarization effect between the AlN buffer layer  34  and the GaN channel layer  38  causes a 2-DEG layer  40  to form that provides an electron flow channel. In one embodiment, the optional back-barrier layer  36  is graded where the concentration of aluminum and gallium changes as the layer  36  is deposited. Typically, the grading of the optional layer  36  provides a lower concentration of aluminum and a higher concentration of gallium near the transition between the buffer layer  34  and the back-barrier layer  36  and a higher concentration of aluminum and a lower concentration of gallium near the transition between the layer  36  and the channel layer  38 . If desired, an optional AlN cap layer  42  can be deposited above the channel layer  38 . 
     Because the gallium is replaced with aluminum in the buffer layer  34 , the free hole charge layer will no longer exist. The AlN buffer to substrate interface between the layers  32  and  34  includes a significant number of defects relative to the grown layers, the free hole charge layer is unable to significantly develop, and is not able to carry significant charge. The tensile strain will no longer exist if the AlN buffer layer  34  is used, and when the optional back-barrier layer  36  is used, the strain will be significantly reduced, but in a compressive nature as a result of the addition of aluminum in the buffer layer  34  by reducing the difference in the atomic spacing between the layers  34  and  36 . Further, more aluminum in the buffer layer  34  than the back-barrier layer  36  creates a compressive strain that prevents the device  30  from cracking. 
     There is a strong piezoelectric field between the AlN buffer layer  34  and the GaN channel layer  38  that induces electrons in the 2-DEG layer  40 , and thus, the AlGaN back-barrier layer  36  is optional. The wideband gap of the AlN in the buffer layer  34  provides an ideal insulation between the 2-DEG layer  40  and the substrate  32 . This eliminates the free hole charge below the back-barrier, which can cause RF performance degradation. 
     Multiple design options are available to control the electron density of the 2-DEG layer  40 , such as controlling the thickness of the GaN channel layer  38 , providing the AlN cap layer  42  and/or grading the AlGaN layer  36 . Various techniques can be employed to control the number of electrons in the GaN channel layer  38  for a particular device or device application, including selecting the thickness and composition of the back-barrier layer  36 . Further, other design considerations can be employed to control the location of the 2-DEG layer  40 . 
       FIG. 3  is a device profile of another N-face HEMT device  50  that also addresses the cracking problems discussed above with the device  10 . The device  50  includes a substrate  52  similar to the substrate  32  and a GaN channel layer  54  similar to the channel layer  38 . In this embodiment, the AlN buffer layer  34  is replaced with an AlGaN buffer layer  56  that provides the same or similar advantages, for example, reducing or eliminating strain. An optional AlN back-barrier layer  58  is deposited on the buffer layer  56 , where the piezoelectric/spontaneous polarization effect between the back-barrier layer  58  and the channel layer  54  creates a 2-DEG layer  60 . The device  50  provides advantages for strain balancing and lattice mismatch while still eliminating the free hole charge layer  28 , as discussed above, and the tensile strain between the buffer layer  56  and the back-barrier layer  58 . For the device  50 , in one non-limiting embodiment, the back-barrier layer  58  may be 2 nm and the channel layer  54  may be 6 nm in thickness. 
       FIG. 4  is a graph with depth on the horizontal axis, energy on the left vertical axis and volume on the right vertical axis. Graph lines  62  and  64  identify the energy of the conduction band and the valence band, respectively, in the buffer layer  56 , and graph line  66  is the concentration of electrons in the 2-DEG layer  60 . 
     The HEMT devices  30  and  50  offer a number of advantages over the known N-face HEMT profile shown in the device  10 . For example, the devices  30  and  50  allow aggressive device scaling due to the strong piezoelectric field between the GaN channel layer and the AlN or AlGaN buffer layer. Further, the large band gap of the AlN or AlGaN buffer layer reduces the buffer leakage and increases channel confinement. Also, epi-layer cracking is eliminated by reducing the tensile strength in the AlGaN back-barrier layer. Also, the devices  30  and  50  eliminate the free hole of charge behind the back-barrier layer and minimize wafer curvature by eliminating tensile stress or balancing overall epi-layer stress. Further, the devices  30  and  50  increase HEMT device performance by enabling increased scaling of channel thickness and 2-DEG charge, reducing access resistance with higher channel charge, providing higher device yield due to reduced wafer bow, providing higher thermal dissipation for the AlN buffer profile with higher thermal conductivity of the AlN layer versus GaN and AlGaN, providing a lower leakage current in the buffer layer and reduce the RF dispersion by hole charge elimination. 
     The foregoing discussion discloses and describes merely exemplary embodiments. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.