Abstract:
Films of gallium manganese nitride are grown on a substrate by molecular beam epitaxy using solid source gallium and manganese and a nitrogen plasma. Hydrogen added to the plasma provides improved uniformity to the film which may be useful in spin-based electronics.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on U.S. provisional application No. 60/364,989 filed Mar. 14, 2002 and entitled “Method and Apparatus for the Production of a Semiconductor Compatible Ferromagnetic Film” and claims the benefit thereof. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0002]    Most present-day, electronic, semiconductor devices measure and manipulate electron charge. Recently, however, there has been considerable interest in semiconductor devices that measure and manipulate the spin of electrons, either alone or in conjunction with electron charges.  
           [0003]    A number of devices are envisioned using electron spin, including Spin-FETs (Field Effect Transistors), Spin-LEDs (Light Emitting Diodes) and Spin-RTDs (Resonant Tunneling Devices), optical switches, modulators, encoders, decoders and quantum bits for quantum computation and communication. An overview of electronic devices based on electron spin is provided in S. A. Wolf, et. al., “Spintronics: A Spin-Based Electronics Vision for the Future”, Science, Vol. 294, pps. 1488-1495, (November 2001).  
           [0004]    Ferromagnetic semiconductors such as GaMnAs and InMnAs are possible candidates for the ferromagnetic film that may be used in electron spin devices. Unfortunately, to date, the highest Curie temperature (the temperature beyond which ferromagnetic properties disappear) for GaMnAs is 110K which is too low for routine semiconductor device applications.  
           [0005]    Gallium nitride, p-doped with five percent manganese, has been predicted to have a Curie temperature above room temperature. However, this concentration of five percent is several orders of magnitude higher than the solubility limit of manganese in gallium nitride. The low solubility results in the formation of stable secondary phases, such as GaMn and Mn 3 N 2 . Recently, there have been reports of ferromagnetic ordering in gallium nitride n-doped with manganese. Nevertheless, phase segregation is still a problem.  
           [0006]    In such phase segregation, the manganese migrates into strips and clusters in the film leaving the remaining areas depleted of manganese. An inability to provide for a uniform film is an obstacle to the production of electronic devices described above.  
           [0007]    Current investigations in the growth of GaMnN use low temperature molecular beam epitaxy to suppress the formation of intermediate compounds such as Mn 3 N 2  and GaMn.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present inventors have determined that the introduction of controlled amounts of hydrogen into a nitrogen plasma used during molecular beam epitaxy substantially suppresses phase segregation and produces a highly homogenous thin film that may be better suited for electronic devices.  
           [0009]    The mechanism as to why phase segregation is suppressed is not clear at the moment. Previous studies have shown that hydrogen can enhance the growth rate of AlN and GaN, as well as the incorporation of indium in GaN, by increasing the number of reactive nitrogen species. 
       
    
    
       [0010]    In the following description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a simplified schematic representation of a molecular beam epitaxy chamber used in the present invention,  
         [0012]    [0012]FIG. 2 is a flow chart of the process of the present invention using the apparatus of FIG. 1 and providing for gallium and manganese molecular beams directed toward a substrate in the presence of a nitrogen plasma to form a GaMnN film on the substrate;  
         [0013]    [0013]FIG. 3 is a scanning electron micrograph of a GaMnN film deposited using molecular beam epitaxy without the addition of hydrogen to the nitrogen plasma showing severe phase segregation.  
         [0014]    [0014]FIG. 4 is a figure similar to that of FIG. 3 showing improved uniformity in the GaMnN film with the addition of hydrogen to the nitrogen plasma; and  
         [0015]    [0015]FIG. 5 is an x-ray diffraction spectrum suggesting the existence of only a single phase in the GaMnN film of FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    Referring now to FIG. 1, the present invention employs the technique of molecular beam epitaxy such as employs a vacuum chamber  10  suitable for providing an ultra-high vacuum within chamber region  12  by means of a multi-stage vacuum pump  11 , of a type well-known in the art. A vacuum gauge  13  allows control of the vacuum within the chamber region  12  to a predetermined desired setting.  
         [0017]    Positioned within the chamber region  12  is a wafer carrier  14  that may hold a wafer  22  as will be described and which provides electrical leads  20  for resistive heating of the wafer  22 .  
         [0018]    The wafer carrier  14  is positioned opposite to a gallium effusion cell  16  that may produce a gallium beam  24  and manganese effusion cell  18  produces a manganese beam  26 , each directed along an unobstructed path to wafer  22 . As is understood in the art, the effusion cells  16  and  18  include an internal temperature controlled oven (holding a gallium or manganese source, respectively) and a front shutter  28  and  30  that may be opened or closed to control the gallium beam  24  and manganese beam  26 .  
         [0019]    The chamber region  12  may receive set volume-rate streams of nitrogen  38  or hydrogen  40  as controlled by metering devices  42  and  44 , respectively, of a type well known in the art. The streams of gas are received by an electron cyclotron resonance (ECR) plasma source  36 , converting the streams to a plasma state. The ECR plasma source  36  is an MPDR 610i device commercially available from Wavemat, Inc. of Plymouth, Mich.  
         [0020]    Positioned within the chamber region  12 , approximately two centimeters from the wafer  22 , is a silicon wafer  46  that may be heated resistively to produce a silicon vapor as will be described.  
         [0021]    The vacuum chamber  10  provides ports  29 ,  32  and  34  aligned with the wafer carrier  14  allowing observation of the wafer  22  during the molecular beam epitaxy and monitoring of the wafer  22  using a reflection high-energy electron diffraction device (RHEED) and an infrared pyrometer (not shown).  
         [0022]    The method of the present invention employs wafer  22  as a substrate for epitaxial growth of a GaMnN film. The wafer  22  is preferably a silicon carbide wafer with hexagonal structure (6H—SiC(0001)) nitrogen doped with a dopant concentration of approximately 10 18  nitrogen atoms per cubic centimeter as obtained from Cree Research, Inc. of Durham, N.C. Other substrates may also be used including sapphire.  
         [0023]    Referring now also to FIG. 2, at process block  50 , the wafer  22  is cleaned with acetone and methanol, and then dried with flowing nitrogen first, then it is introduced into vacuum chamber  10  and placed in the wafer carrier  14  where it is resistively heated it to 850-950 degrees centigrade by a direct current through the wafer  22 . During the heating of the wafer  22 , the pressure of the chamber region  12  is reduced to approximately 1×10 −9  Torr.  
         [0024]    As indicated by process block  52 , at a next step, a flux of silicon vapor generated by heating the silicon wafer  46  is directed over the wafer  22 . The silicon atoms of the vapor react to the SiO 2  of the wafer  22  and produce a 3×3 reconstruction of the silicon-rich surface of the wafer  22  as may be observed by RHEED and as has been subsequently verified with a scanning tunnel microscope (STM).  
         [0025]    As indicated by process blocks  54  and  56 , during buffer layer formation stage  53 , a layer of gallium nitride is grown on the surface of the wafer  22  to a thickness of approximately 80 nanometers, at a growth rate of forty nanometers per hour. First, the gallium within the gallium effusion cell  16  is raised to a temperature of 950 degrees centigrade. Then, as indicated by process block  56 , nitrogen is introduced at a flow rate of six standard cubic centimeters per minute (sccm) and reactive nitrogen species are generated by the plasma source  36  operating at a power of 30 watts. The temperature of the wafer  22  is brought to approximately 500 degrees centigrade and the total pressure in the chamber region adjusted to 1×10 −4  Torr. Finally, as indicated by process block  54 , the shutter  28  of the gallium effusion cell  16  is opened so that a beam  24  of gallium passes through reactive nitrogen species and is deposited on the wafer  22  as gallium nitride.  
         [0026]    With sapphire, before the gallium nitride buffer layer is formed, an aluminum nitride layer of less than two nanometers is formed by treating the sapphire surface with nitrogen plasma for thirty minutes while the sapphire is heated to 850 degrees C.  
         [0027]    After completion of buffer layer of GaN at buffer layer formation stage  53 , a layer of GaMnN is deposited as indicated by process blocks  59 ,  60 ,  54 ′ and  56 ′ during a GaMnN layer formation stage  58 . Generally, no change is made in the gallium beam  24  or the flow of nitrogen indicated now by process blocks  54 ′ and  56 ′. However, at process block  59 , hydrogen  40  is introduced into the chamber regions  12  through the plasma source  36  at a flow rate of two sccm to generate hydrogen reactive species. The manganese within the manganese effusion cell  18  is raised to a temperature of between 750 and 880 degrees, and the shutter  30  opened so that a beam  26  of manganese passes through the reactive nitrogen and hydrogen species to deposit on the wafer  22  a layer of GaMnN. Growth of the GaMnN layer is monitored by RHEED. A thickness of 200 nanometers may be achieved at a growth rate of 50 nanometers per hour.  
       EXAMPLE I  
       [0028]    Referring now to FIG. 3, a scanning electron micrograph image of GaMnN film using the above-described technique, but without the introduction of the hydrogen per process block  59 , shows a surface characterized by two distinct domains  61  and  62 . Domain  61  is part of a flat terrace and the domain  62  is located on one of a set of randomly distributed strips and clusters.  
         [0029]    Analysis of energy dispersive spectroscopy (EDS) spectra  64  and  66  for domains  61  and  62 , respectively, indicate that terrace domains  61  contain no manganese, while high concentrations of manganese, more than forty percent, are found in the strips and clusters domain  62 . Manganese content was calculated using the K α  peak ratios between Mn and Ga. Since the x-ray has an escape length larger than 1 μm, the Mn concentration obtained is a good indication of its composition in the film. Scanning tunneling microscope pictures of domain  60  indicate spiral mounds characteristic of GaN films grown under gallium-rich conditions. X-ray diffraction studies of the films indicate the preferential formation of a second phase Mn 3 N 2 . The population and size of the clusters and strips increases with increasing manganese effusion cell temperatures suggesting they are related to manganese. These observations indicate that the film grown has phase segregated into two phases GaN and secondary phases that contain Mn. The preferential formation of secondary phases such as Mn 3 N 2  has been confirmed with X-ray diffraction (XRD) studies.  
       EXAMPLE II  
       [0030]    Referring now to FIG. 4, a scanning electron micrograph image of GaMnN film using the above-described technique including the introduction of hydrogen per process block  59 , shows a far more homogenous surface including larger terrace domains  68  and few cluster domains  70 . Energy dispersive spectroscopy spectra taken in the domains  68  and  70  indicate a uniform 6.7% manganese concentration in the film. On the other hand, a slightly lower Mn concentration is found for the clusters of domain  70 , indicating that these clusters of domain  70  are different from the clusters of domain  62  observed for the pure nitrogen growth.  
         [0031]    Referring now to FIG. 5, x-ray diffraction (XRD) was used to assess the crystallinity and structure of the GaMnN film of Example 2. A single phase GaMnN was detected with no secondary phase formation. The shown XRD spectra is for a Ga 1-x Mn x N film with x=0.06. Two peaks are evident located at 34.65 and 34.71 degrees, with a separation of 216 arc seconds. The 34.71 degree peak belongs to the GaN (0002) reflection and is caused by the 80 nm thick buffer layer; while the peak at 34.65 degrees is due to the GaMnN film. These results clearly show that single phase GaMnN containing about 6.0% Mn has been grown by MBE using N 2 /H 2  plasma Note, in FIG. 5, the second substrate peak at 35.8 degrees and the shoulders at 34.8 degrees are due to the K α2  emission of the x-ray source.  
         [0032]    These results clearly show that single-phase gallium manganese nitride containing more that six percent of manganese can be grown by molecular beam epitaxy using the present invention. Films grown without the presence of hydrogen are phase segregated into GaN and manganese containing alloys, while single phase Ga 1-x Mn x N, films with x as high as 0.06, is obtained for films grown with nitrogen-hydrogen plasma.  
         [0033]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.