Abstract:
A semi-conductor crystal and method of forming the same. The method includes providing a flow of dopant and column III element containing gases, then stopping flow of dopant and column III element containing gases, reducing the temperature, restarting flow of column III containing gases and then elevating the temperature.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 61/167,820, filed on Apr. 8, 2009, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of gallium nitride (GaN) transistors. In particular, the invention relates to a method and apparatus to trap excess dopants. 
       BACKGROUND OF THE INVENTION 
       [0003]    Gallium nitride (GaN) semiconductor devices are increasingly desirable for power semiconductor devices because of their ability to carry large current and support high voltages. Development of these devices has generally been aimed at high power/high frequency applications. Devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFET), high electron mobility transistors (HEMT), or modulation doped field effect transistors (MODFET). These types of devices can typically withstand high voltages, e.g., 100 Volts, while operating at high frequencies, e.g., 100 kHz-10 GHz. 
         [0004]    A GaN HEMT device includes a nitride semiconductor with at least two nitride layers. Different materials formed on the semiconductor or on a buffer layer causes the layers to have different band gaps. The different material in the adjacent nitride layers also causes polarization, which contributes to a conductive two dimensional electron gas (2DEG) region near the junction of the two layers, specifically in the layer with the narrower band gap. 
         [0005]    The nitride layers that cause polarization typically include a barrier layer of AlGaN adjacent to a layer of GaN to include the 2DEG, which allows charge to flow through the device. This barrier layer may be doped or undoped. Because of the 2DEG region existing under the gate at zero gate bias, most nitride devices are normally on, or depletion mode devices. If the 2DEG region is depleted, i.e. removed, below the gate at zero applied gate bias, the device can be an enhancement mode device. Enhancement mode devices are normally off and are desirable because of the added safety they provide. An enhancement mode device requires a positive bias applied at the gate in order to conduct current. 
         [0006]      FIG. 1  illustrates a conventional GaN transistor device  100 . Device  100  includes substrate  11  composed of silicon (Si), silicon carbide (SiC), sapphire, or other material, transition layers  12  typically composed of aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) that is about 0.1 to about 1.0 μm in thickness, Mg doped GaN layer  10 , buffer layer  13  typically composed of GaN that is about 0.5 to about 3 μm in thickness, current conducting region  14  composed of GaN or indium gallium nitride (InGaN) typically about 0.01 to about 0.5 μm in thickness, contact region  15  typically composed of AlGaN, Al and titanium (Ti) that may have Si, typically about 0.01 to about 0.03 μm in thickness, barrier layer  16  typically composed of AlGaN where the Al to Ga ratio is about 0.1 to about 0.5 with a thickness of about 0.01 to about 0.03 μm, gate structure  17  composed of a nickel (Ni) and gold (Au) metal contact, and ohmic contact metals  18 ,  19  composed of Ti and Al with a capping metal such as Ni and Au. 
         [0007]    During growth of Mg doped GaN material in a conventional GaN transistor device (e.g.,  FIG. 1 ), magnesium (Mg) is added to the growth environment. This Mg accumulates on the surface of the GaN and becomes part of the crystal. In addition, Mg coats the walls of the growth chamber during this part of the growth. Growth of undoped GaN, with the intention of having material without Mg present, following the growth of Mg doped material is difficult due to the presence of Mg still residing on the surface of the GaN and other Mg on the walls of the chamber. This residual Mg will continue to contaminate the crystal for extended lengths of time as Mg moves easily about the growth chamber. 
         [0008]    Conventional GaN transistors have many disadvantages. The breakdown voltage is limited by the width of the gate  17  (as shown in  FIG. 1 ). To reach high voltages, a wide gate and large separation between gate  17  and drain contact  18  is required due to residual n-type doping from oxygen contamination and nitrogen vacancies in the undoped GaN material  13 . In addition, conventional GaN transistors using Mg doping in the buffer layer suffer from changes in the conductivity caused by Mg near the barrier layer. 
         [0009]    It would be desirable to provide a method and apparatus to achieve the improved breakdown of devices utilizing doped buffers, while eliminating the device performance variations caused by dopants near the barrier layer. To achieve this goal, it is desirable to trap excess dopants in order to avoid the above-mentioned disadvantages of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates a cross-sectional view of a conventional GaN transistor device. 
           [0011]      FIG. 2  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a first embodiment of the present invention. 
           [0012]      FIG. 3  is a graph of Mg concentration in a buffer layer for single and multiple row interrupts in comparison to non-interrupted or standard growth. 
           [0013]      FIG. 4  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    In the following detailed description, reference is made to certain embodiments. These embodiments are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed and that various structural, logical, and electrical changes may be made. 
         [0015]    The present invention is a GaN transistor device with a Mg growth interruption layer to trap excess or residual dopants, and a method for making such a device. The invention is designed to force reaction of Mg with nitrogen, for example, to form a less volatile material, i.e., magnesium nitride. This material is then covered by a layer of either GaN or AlGaN. The coating step can be done at lower temperatures as well to assist the coating. By lowering the temperature, less reaction between the MgN and Al or Ga will occur. The reaction of Al and MgN is such that AlN is formed and MgN is reduced to Mg. This reaction is in competition with the desired coating and entrapment of MgN. Thus, Mg will more readily remain in the MgN form if the reaction can be suppressed by reducing the temperature. 
         [0016]    Referring to  FIG. 2 , a first embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 2  illustrates a cross-sectional view of the device  200 . Device  200  includes, from bottom up, substrate  31 , transition layers  32 , Mg doped layer  33 , growth interrupt layers  39 , buffer layer  34 , barrier layer  35 , ohmic contact metals  36 ,  37 , and gate structure  38 . The growth interrupt layers (Mg diffusion barrier)  39  may be composed of one or more layers of highly Mg doped GaN. They are formed by interruption of growth and exposing the surface to ammonia. Other suitable dopants beside Mg may be used, including iron (Fe), nickel (Ni), manganese (Mn), calcium (Ca), vanadium (V), or other transition metals. 
         [0017]    Formation of the structure of  FIG. 2  will now be described with reference to Mg as the dopant, as an example. The transition layers  32  are formed by nucleation and growth on the substrate  31 . Substrate  31  may be composed of silicon (Si), silicon carbide (SiC), sapphire, lithium gallium oxide (LiGaO 2 ), gallium nitride (GaN), or other suitable material. Transition layers  32  may be composed of AlN, AlGaN, InAlGaN, SiO 2 , SiN, MgO, Al 2 O 3 , or combinations of these, preferably about 0.1 to about 1.0 μm in thickness. The transition layers  32  are typically less than about 1000 Å in thickness. Then, the Mg doped layer  33  is grown. The Mg doped layer  33  may be composed of GaN that is about 0.1 to about 1.0 μm in thickness with a Mg concentration between  10   16  atoms per cm 3  and  10   19  atoms per cm 3 . Next, the Mg blocking growth interrupt layers  39  are grown. The formation of the growth interrupt layers  39  consists of growing GaN without Mg-containing material, stopping a supply of Ga-containing materials while maintaining a supply of ammonia or other activated nitrogen source (e.g., plasma N 2 ) to form a layer of magnesium nitride, beginning a supply of Ga to seal in the magnesium nitride layer by growing a layer of GaN, and interrupting growth again and repeating the above sequence until a target level of Mg in the final layer is reached. Next, buffer layer  34 , barrier layer  35  and gate structure  38  are grown and material processing occurs to form gate contacts. Buffer layer  34  may be composed of GaN, preferably with a thickness of about 0.5 to about 3.0 μm. Barrier layer  35  may be composed of AlGaN where the Al fraction is about 0.1 to about 0.5, preferably with a thickness between about 0.01 and about 0.03 μm. The Al fraction is the content of Al such that Al fraction plus Ga fraction equals 1. Gate structure  38  may be composed of p-type GaN with a refractory metal contact such as tantalum (Ta), titanium (Ti), titanium nitride (TiN), tungsten (W), or tungsten silicide (WSi 2 ). The gate structure can be simple metal, such as Ni under Au, or a semiconductor with metal such as GaN under TiN, or a semiconductor under insulator under metal such as GaN under SiN under TiN. Other semiconductors could be Si, GaAs, or InAlGaN. Other insulators could be AlGaN, InAlGaN, SiO 2 ,SiN, MgO, Al 2 O 3 . Other metals could be Al, Ni, Au, Pt, etc. Also, in place of a metal, polysilicon could be used. The metal and gate layer are each preferably about 0.01 to about 1.0 μm in thickness. Total thickness for the gate structure would preferably be under 1 μm. Next, gate structure  38  is etched in other regions of the device and ohmic contacts  36 ,  37  are produced. Ohmic contact metals  36 ,  37  may be composed of titanium (Ti) and aluminum (Al) with a capping metal such as nickel (Ni) and gold (Au) or titanium (Ti) and titanium nitride (TiN). There can also be an implanted highly doped region associate d with the contact region. The primary channel region can be n-type doped GaN, or undoped or intrinsic InAlGaN. 
         [0018]    In accordance with the above-described method, the addition of a p-type GaN layer  33  below the gate and a series of growth interrupt layers  39  reduces the level of Mg in the GaN buffer layer  34 . The Mg doping of layer  33 , in  FIG. 2 , increases the breakdown voltage of the device. The gate length of the device can be greatly reduced without reducing the breakdown voltage of the device. Gate capacitance of the device is reduced due to smaller gate length. Switching speed of the device is improved due to smaller gate capacitance. Growth interrupt layers  39  reduce Mg concentration in layer  34  and near the barrier layer  35 . 
         [0019]      FIG. 3  is a graph comparing the Mg concentration in a buffer layer without growth interrupt layers, a buffer layer with a single growth interrupt layer, and a buffer layer with six growth interrupt layers. As can be seen from the multiple growth interrupt curve, each growth interrupt produces a higher level of Mg at the position of the interrupt, followed by a lower level of Mg in the following layer. Each growth interrupt layer decreases the Mg, and by application of multiple layers, a low level of Mg can be obtained in a smaller distance. 
         [0020]    Reduced Mg in layer  34  increases device conductivity. Reduced Mg in layer  34  also allows placement of layer  33  in close proximity to layer  35  without reduction in device conductivity. In addition, close proximity of layer  33  to  35  results in improved device breakdown and lower gate leakage currents. The structure of  FIG. 2 , however, has some drawbacks. The time required to produce growth interrupt layers  39  can be large, leading to increased cost of fabrication. Additionally, some Mg is still present in layer  34  due to contamination from reactor parts. 
         [0021]    Referring to  FIG. 4 , a second embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 4  illustrates a cross-sectional view of the device  300  formed by the method described below. This embodiment of the invention differs from the first embodiment in that the growth interrupt layers  39  of  FIG. 2  are now replaced by AlGaN layers  49 . The AlGaN layers (doped diffusion barrier)  49  may be composed of one or more layers of AlGaN. They are formed by interruption of growth and exposing the GaN surface to ammonia, similar to the first embodiment, followed by deposition of AlGaN and then GaN. The Al fraction of the AlGaN layers is between about 0.3 and about 1. The thickness of the AlGaN layers is preferably about 0.005 to about 0.03 μm. 
         [0022]    Formation of the structure of  FIG. 4  is similar to that described above with respect to the first embodiment ( FIG. 2 ), with Mg as the dopant, as an example. The dimensions and compositions of the various layers are similar to that of the first embodiment as well. However, instead of forming growth interrupt layers  39  ( FIG. 2 ), AlGaN layers  49  are formed. The formation of AlGaN layers  49  consists of growing GaN without Mg-containing material, stopping a supply of Ga-containing materials while maintaining a supply of ammonia or other activated nitrogen source (e.g., plasma N 2 ) to form a layer of magnesium nitride, reducing growth temperature, beginning a supply of Al and/or Ga to seal in the magnesium nitride layer by growing a layer of GaN, returning growth temperature to the initial temperature to grow GaN, and interrupting growth again and repeating the above sequence until a target level of Mg in the final layer is reached. The step of reducing the growth temperature and returning to the initial growth temperature is optional. 
         [0023]    In accordance with the above-described method, the addition of a p-type GaN layer  43  below the gate and a series of growth interrupt and AlGaN layers  49  reduce the level of Mg in the GaN buffer layer  44 . The second embodiment shares the same advantages of the first embodiment. Furthermore, the addition of the AlGaN layers to the diffusion barrier  49  improves the efficiency of each growth interrupt step, reducing the number of steps to achieve a desired level of Mg doping in the buffer layer  44 . 
         [0024]    The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modifications and substitutions to specific process conditions can be made. Accordingly, the embodiments of the invention are not considered as being limited by the foregoing description and drawings.