Abstract:
A method is disclosed for forming a high electron mobility transistor. The method includes the steps of implanting a Group III nitride layer at a defined position with ions that when implanted produce an improved ohmic contact between the layer and contact metals, with the implantation being carried out at a temperature higher than room temperature and hot enough to reduce the amount of damage done to the Group III nitride layer, but below a temperature at which surface problems causing leakage at the gate or epitaxial layer dissociation would occur. An ohmic contact selected from the group consisting of titanium, aluminum, nickel and alloys thereof is added to the implanted defined position on the Group III nitride layer.

Description:
STATEMENT OF GOVERNMENT INTEREST 
       [0001]    This invention was developed, at least in part, under DARPA Contract No. 4400121759. The Government may have certain rights in this invention. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to semiconductor devices and, more particularly, to Group-III nitride based high electron mobility transistors (HEMTs). 
         [0003]    HEMTs are semiconductor devices that can be used for a variety of applications including microwave and millimeter-wave communications, radar, radio astronomy, cell phones, direct broadcast satellite receivers, and electronic warfare systems. 
         [0004]    Traditional semiconductors usually require a conductive layer that is doped with n-type impurities to generate free electrons; however, the electrons within the layer tend to collide with these impurities, which slows them down. HEMTs are field effect transistors that utilize the heterojunction between two materials having different bandgaps to form a conductive channel rather than a doped region. HEMTs typically do not require impurities to form the conductive layer and therefore allow for higher electron mobility. 
         [0005]    HEMTs are regularly fabricated from semiconductor materials such as silicon (Si) and gallium arsenide (GaAs). Si has a low electron mobility, which generates a high source resistance; therefore, Si semiconductor materials may not be well suited for high power, high frequency, and high temperature applications. 
         [0006]    Signal amplification devices in radar, cellular, and satellite communications frequently use GaAs based HEMTs. GaAs semiconductor materials have higher electron mobility and a lower source resistance than Si, which allows them to operate at higher frequencies. GaAs has, however, a relatively small bandgap preventing the use of GaAs HEMTs in high power at high frequency applications. 
         [0007]    Improvements in the manufacturing of gallium nitride (GaN) semi-conductor materials and semi-conductor materials made from an alloy consisting of aluminum nitride and gallium nitride (AlGaN) have focused interest on the use of AlGaN/GaN HEMTs for use in high frequency, high power and high temperature applications. AlGaN and GaN have large bandgaps making them superior to Si and GaAs for these types of applications. 
         [0008]    The use of AlGaN layered upon GaN and the mismatch of the crystal structures of the two materials and their different bandgap energies results in the creation of a two-dimensional electron gas (2DEG) under certain circumstances. The 2DEG layer accumulates in the smaller bandgap material and contains a very high electron concentration. Electrons originating in the wider-bandgap material transfer to the 2DEG allowing for a higher electron mobility. 
         [0009]    The combination of high electron concentration and high electron mobility gives AlGaN/GaN HEMTs greater performance than metal-semiconductor field effect transistors (MESFETs) for high-frequency applications. 
         [0010]    One method of fabricating an AlGaN/GaN HEMT includes forming a layer of GaN on a substrate (typically silicon carbide (SiC)), forming a thin layer of AlGaN on the GaN layer, and providing ohmic contacts and a gate contact on the AlGaN layer. 
         [0011]    Traditionally an alloy of titanium (Ti), aluminum (Al), nickel (Ni), and gold (Au) forms the ohmic contacts. In order to function as an electronic contact the alloy must be ohmic towards the 2DEG layer rather than the AlGaN layer on which it is placed. The addition of Au to this alloy enables the contacts to be ohmic towards the 2DEG; however, the Au also gives the alloy bad morphology. 
         [0012]    Another method of forming low-resistive ohmic contacts on AlGaN/GaN HEMTs generally well-understood within the art employs ion implantation at the ohmic contact region. Implantation in this manner allows the use of a Ti/Ni/Al contact, thus eliminating the poor morphology created by the use of Au. 
         [0013]    Nevertheless, this implantation process creates another problem because of the high doses of implantation ions that must be used in order to get sufficient activation rates within the implanted regions. The high doses of implantation ions create large amounts of crystal destruction. This destruction can be corrected by annealing the device; however, SiC and AlGaN are difficult to recrystallize by annealing. In order to recrystallize SiC and AlGaN properly, longer annealing temperatures can be used. Longer anneal times, however, damage other features of the device. 
         [0014]    Therefore, there is a need for an implantation process that uses high doses of ions and creates less damage to the implanted crystal thus requiring less annealing. 
       SUMMARY 
       [0015]    In one aspect, the invention is a method of forming a high electron mobility transistor. The method includes implanting a Group III nitride layer at a defined position with ions that when implanted produce an improved ohmic contact between the layer and contact metals. 
         [0016]    Implantation is carried out at a temperature higher than room temperature and hot enough to reduce the amount of damage done to the Group III nitride layer, but below a temperature at which surface problems causing leakage at the gate or epitaxial layer dissociation would occur. An ohmic contact s is added to the implanted defined position on the Group III nitride layer. 
         [0017]    In another aspect the method includes carrying out the implantation with an ion beam current high enough to successfully implant ions within the Group III nitride layer, but below a current that would melt or destroy the Group III nitride layer and at a temperature higher than room temperature and hot enough to reduce the amount of damage done to the Group III nitride layer, but below a temperature at which surface problems causing leakage at the gate or epitaxial layer dissociation would occur. 
         [0018]    In yet another aspect, the invention is a transistor precursor that includes a growth substrate selected from the group consisting of silicon carbide and sapphire, a layer of gallium nitride on the growth substrate, a layer of aluminum gallium nitride on the gallium nitride layer for generating a two dimensional electron gas at the interface between the layers when a current is applied in a HEMT orientation and defined implanted regions in the aluminum gallium nitride and gallium nitride layers for improving the ohmic characteristics of the layers when an ohmic metal is added to the defined implanted regions, with the defined implanted regions of the transistor precursor having a temperature of between about 250° and 900° C. 
         [0019]    The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic cross-sectional view of an AlGaN/GaN HEMT according to an embodiment of the present invention. 
           [0021]      FIGS. 2-4  are photographs of three gallium nitride wafers implanted with silicon ions under different conditions. 
           [0022]      FIG. 5  depicts the transmission spectrums in the visible range of wafers implanted with silicon ions under various conditions. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  illustrates a schematic cross-section of an AlGaN/GaN based HEMT  10  constructed in accordance with an embodiment of the present invention. The HEMT includes a substrate  11  formed from materials generally well-understood within the art (e.g., silicon carbide (SiC) or sapphire (Al 2 O 3 )). A GaN layer  12  is provided on the substrate  11 . The HEMT  10  includes an AlGaN layer  13  provided on top of the GaN layer  12 . 
         [0024]    Preferably, SiC forms the substrate  11 . The crystal lattice structure of SiC matches more closely to the Group-III nitrides than sapphire, resulting in higher quality Group-III nitride films. Furthermore, SiC has a very high thermal conductivity which allows for higher total output power of the device. 
         [0025]    The AlGaN layer  13  has a wider bandgap than the GaN layer  12  which results in a free charge transfer from the AlGaN layer  13  to the GaN layer  12 . Charge accumulates at the interface of the AlGaN layer  13  and the GaN layer  12  forming a two-dimensional electron gas (2DEG) (not shown). The 2DEG has very high electron mobility resulting in a HEMT  10  with very high transconductance at high frequencies. The voltage applied to the gate  14  controls the electron flow in the 2DEG under the gate  14 , allowing for control over the total electron flow. Preferably, the gate  14  is a Schottky gate. 
         [0026]    The source contact  15  and drain contact  16  provided on the AlGaN layer  13  are preferably formed of alloys of titanium (Ti), aluminum (Al), and nickel (Ni). Traditional alloys used for ohmic contacts  15  and  16  are formed of Ti, Al, Ni as well as gold (Au). The addition of Au gives the alloy poor morphology, so the contacts  15  and  16  of the present invention preferably do not include Au. Other candidate compositions for the ohmic contact include titanium-tungsten-nitride (Ti—W—N), titanium-nitride (Ti—N), molybdenum (Mo) and molybdenum silicides. 
         [0027]    High temperature ion implantation using a high ion beam current of an n-type dopant creates the implanted regions  20  and  21 . Preferably, the dopant ions are Si + . These implanted regions  20  and  21  allow the contacts  15  and  16  to be ohmic towards the 2DEG. 
         [0028]    High temperature as used herein should be understood to include temperatures that are below a temperature at which surface problems causing leakage at the gate  14  or epitaxial layer dissociation would occur but higher than room temperature. Preferably, the temperature at which implantation occurs is between about 250° C. and 900° C. 
         [0029]    High ion beam current as used herein should be understood to include a beam current below that which would melt or destroy the crystal, but high enough to successfully implant ions within the AlGaN and GaN layers  13  and  12 . Preferably, the beam current is between 30 μA and 130 μA. 
         [0030]    In one embodiment, the present invention is an AlGaN/GaN HEMT wherein implanted regions  20  and  21  have been implanted at a temperature of 650° C. using an ion beam current of 40 μA. 
         [0031]    In another embodiment, the present invention is an AlGaN/GaN HEMT wherein implanted regions  20  and  21  have been implanted at a temperature of 650° C. using an ion beam current of 120 μA. 
         [0032]    In a third embodiment, the present invention is an AlGaN/GaN HEMT wherein implanted regions  20  and  21  have been implanted at a temperature of 350° C. using an ion beam current of 40 μA. 
         [0033]    In a fourth embodiment, the present invention is an AlGaN/GaN HEMT wherein implanted regions  20  and  21  have been implanted at a temperature of 350° C. using an ion beam current of 120 μA. 
         [0034]    The present invention also includes methods for fabricating HEMTs using high temperature ion implantation. U.S. Pat. No. 7,230,284 discloses a method for forming the GaN layer  12  on the substrate  11  and forming the AlGaN layer  13  on the GaN layer  12 . 
         [0035]    A mask layer protects only the portion of the AlGaN layer that will not be implanted leaving the crystal exposed in the regions to be implanted. The mask layer is a material capable of withstanding the conditions of the high temperature, high ion beam current implantation without otherwise adversely affecting either the implantation step or the device. For example, the mask layer may be formed of an oxide. 
         [0036]    The device is then maintained at a high temperature and a high current beam implants n-type dopant ions (e.g. Si +  ions) through the AlGaN layer  13  into the GaN layer  12  to form implanted regions  20  and  21 . The ions are implanted to a depth that allows the ohmic contacts  15  and  16  to act ohmic towards the 2DEG. 
         [0037]    In a particular embodiment of this method, a protective layer may be placed over the regions to be implanted. In a more particular embodiment, silicon nitride (Si 3 N 4 ) is the protective layer. The protective layer reduces the amount of damage to the AlGaN layer created by the ion beam. The use of a protective layer is preferred because AlGaN can be particularly difficult to recrystallize using annealing methods. 
         [0038]    After implantation, an annealing process improves the damaged implanted regions  20  and  21 . Shorter anneal times are desired because the anneal has the potential to damage other parts of the device. Implantation at high temperature results in less damage to the implanted regions and therefore allows shorter anneal times, which reduces the possibility for anneal-based secondary damage. 
         [0039]    Ohmic contacts  15  and  16  are then formed on the AlGaN layer  13  over the implanted regions  20  and  21 . The ohmic contacts  15  and  16  connect with the 2DEG electronically through the implanted regions  20  and  21 . Preferably, an alloy of Ti, Ni, and Al forms the ohmic contacts  15  and  16 . 
         [0040]    A gate  14  can be formed on the AlGaN layer  13  over the 2DEG. The voltage applied to the gate  14  controls the electron flow in the 2DEG, allowing for control over the total electron flow. Preferably, the gate  14  is a Schottky gate. 
         [0041]    Each wafer in  FIGS. 2-4  is GaN that has been implanted with Si ions.  FIGS. 2-4  illustrate that conducting the implantation step of this method at high temperature avoids damaging the implanted crystals as much as implantation at room temperature. In  FIGS. 2-4 , the lighter shades indicate more light passage based upon less crystal damage, while darker shades indicate less light passage resulting from more crystal damage. 
         [0042]    In  FIG. 2 , the top left quarter of the wafer  30  has been implanted using an ion beam current of 120 μA at a temperature of 350° C. The bottom right quarter of the wafer  31  has been implanted using an ion beam current of 120 μA at room temperature. The top left quarter  30  is significantly lighter than the bottom right quarter  31 , indicating significantly less damage because of the higher temperature during implantation. 
         [0043]    Similarly, in  FIG. 3 , the top left quarter of the wafer  32  has been implanted using an ion beam current of 120 μA at a temperature of 650° C. The bottom right quarter of the wafer  33  has been implanted using an ion beam current of 120 μA at room temperature. The top left quarter  32  is significantly lighter than the bottom right quarter  33 , indicating significantly less damage because of the higher temperature during implantation. 
         [0044]    A comparison of the top left quarter of the wafer  30  in  FIG. 2  and the top left quarter of the wafer  32  in  FIG. 3  also indicates that implantation at higher temperatures decreases the amount of crystal damage created by the implantation process. 
         [0045]    The use of a high current ion beam provides two distinct advantages over a lower current ion beam. Using a high current ion beam reduces the time it takes to implant a given dose of dopant ions therefore reducing costs. Additionally, the use of a high current ion beam further increases the temperature of the wafer during implantation, resulting in less damage to the implanted crystal. 
         [0046]    The wafer in  FIG. 4  illustrates the second advantage of using a higher current ion beam. In  FIG. 4  the top left quarter of the wafer  34  has been implanted using an ion beam current of 120 μA at a temperature of 350° C. The bottom right quarter of the wafer  35  has been implanted using an ion beam current of 40 μA at a temperature of 350° C. The top left quarter  34  is significantly lighter than the bottom right quarter  35 , indicating significantly less damage because of the higher ion beam current during implantation. Some of the reduced damage in the top left quarter  34  may be attributable to the fact that it was effectively annealed at 350° C. while the bottom right quarter of the wafer  35  was being implanted. The difference in clarity created by the anneal is believed to be less significant than the difference in clarity created by the higher ion beam current. 
         [0047]      FIG. 5  illustrates the previously mentioned lightness and darkness comparisons in a more concrete manner. The graph plots the percentage of light transmitted through GaN wafers that have been implanted with Si +  versus the wavelength of the light for the visible range. 
         [0048]    The two highest plots, one formed of squares and the other of diamonds, indicate the transmission percentages of two GaN wafers that have not been implanted. As shown in  FIG. 5 , these unimplanted wafers transmit the highest percentage of visible light. 
         [0049]    The lowest plot, formed of downward pointing triangles indicates the transmission percentage of a GaN wafer implanted with Si +  at room temperature with an ion beam current of 120 μA over the visible range. As shown in  FIG. 5 , this wafer transmits the lowest percentage of visible light. 
         [0050]    The mid-level plot formed of circles indicates the transmission percentage of a GaN wafer implanted with Si +  at a temperature of 650° C. with an ion beam current of 120 μA over the visible range. The mid-level plot formed of diamonds indicates the transmission percentage of a GaN wafer implanted with Si +  at a temperature of 350° C. with an ion beam current of 120 μA over the visible range. The mid-level plot formed of downward pointing triangles indicates the transmission percentage of a GaN wafer implanted with Si +  at a temperature of 350° C. with an ion beam current of 40 μA over the visible range. These three mid-level plots indicate the transmission spectrums of three different wafers that are embodiments of the present invention. As shown in  FIG. 5 , these three wafers transmit more light over the visible range than the wafer implanted at room temperature.  FIG. 5  also indicates that these three wafers transmit less light over the visible range than the unimplanted wafers. 
         [0051]    It should also be noted that the mid-level plot formed of diamonds (i.e., the wafer implanted at 350° C. with a current of 120 μA) has higher values over the visible range than the mid-level plot formed of downward pointing triangles (i.e, the wafer implanted at 350° C. with a current of 40 μA). This comparison indicates that wafers implanted at higher beam currents typically transmit a greater percentage of light than wafers implanted at lower beam currents. 
         [0052]    The plots show that implantation at high temperatures (e.g., the three mid-level plots) produces a wafer that transmits a much higher percentage of visible light than implantation at room temperature (e.g., the lowest plot). The transmission percentage correlates with less damage to the crystal structure of the wafer. Therefore, implantation at high temperatures creates less damage to the crystal structure of the wafer than implantation at room temperature. 
         [0053]    The plot also shows that a wafer implanted using a higher ion beam current (e.g., the mid-level plot formed of diamonds) transmits a higher percentage of visible light than a wafer implanted using a lower ion beam current (e.g., the mid-level plot formed of downward pointing triangles). Once again, the transmission percentage correlates with less damage to the crystal structure of the wafer. Therefore, implantation using higher ion beam currents creates less damage to the crystal structure of the wafer than implantation at lower ion beam currents. 
         [0054]    The method of the present invention produces HEMTs with significantly less damage to the crystal than previously disclosed methods. As a result, HEMTs produced using the method of the present invention perform superiorly to prior HEMTs. 
         [0055]    The following table contains data on the performance of a HEMT device implanted at room temperature and a HEMT device implanted at high temperature in accordance with the present invention: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 RF 
                 Gate- 
                 Drain- 
                   
                   
                   
                 Power 
                 Average 
                 Drain 
               
               
                 Implant 
                 Power 
                 Source 
                 Source 
                 Current 
                   
                 Output 
                 Added 
                 On- 
                 Source 
               
               
                 Temperature 
                 (GHz) 
                 Voltage 
                 Voltage 
                 (mA/mm) 
                 Gain 
                 Power 
                 Efficiency 
                 Resistance 
                 Current 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 350° C. 
                 10 
                 −2.8 
                 48 
                 43.0 
                 13.7 
                 9.13 
                 51.6 
                 3.54 
                 1007.8 
               
               
                 Room 
                 10 
                 −2.8 
                 48 
                 43.0 
                 11.7 
                 8.42 
                 48.4 
                 3.90 
                 936.4 
               
               
                 Temperature 
               
               
                   
               
             
          
         
       
     
         [0056]    The wafers of the table received the same implant dose of 1.00×10 16  atoms/cm 2  of Si +  and the same temperature anneal (1080° C.). The data shows that both wafers were supplied with the same RF power, gate-source voltage, drain-source voltage, and current. The high temperature implanted HEMT had a higher gain, output power, power added efficiency and drain source current as well as a lower average on-resistance. This data indicates that devices formed according to the present invention have better device performance than devices formed using ion implantation at room temperature. 
         [0057]    In the drawings and specification there has been set forth a preferred embodiment of the invention, and although specific terms have been employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.