Abstract:
A magnetron sputter source is adapted to fit into a K-cell port in a molecular beam epitaxy apparatus. The MSE source has a protruding cylindrical body for insertion into the K-cell port. The cylindrical body is attached at its proximal end to a flange and has its distal end open. An array of permanent magnets is arranged at the distal end of the cylindrical body. A magnet return piece is mounted behind the permanent magnets. A sputter target is mounted in front of the permanent magnets, and cooling ducts within the cylindrical body carry a cooling medium to the sputter target.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of growing epitaxial layers using a magnetron sputter source in molecular beam epitaxy apparatus. 
     BACKGROUND OF THE INVENTION 
     Magnetron sputter epitaxy has become increasingly popular as a technique for depositing thin layers. For example, in recent years there has been tremendous interest in GaN based III-N materials. A dramatic improvement in the material quality has led to the development of high brightness light emitting diodes, and more recently “blue” laser diodes. There has also been a dramatic improvement in the performance of high power microwave metal-semiconductor field-effect transistors and modulation doped field-effect transistors based on these materials. 
     Crucial to all these applications is the growth of material with high crystalline quality and of high purity. Magnetron sputter epitaxy (MSE) is a technique that has been used to grow GaN. Typically GaN epilayers are grown on sapphire substrates, which are highly lattice mismatched, necessitating the predeposition of a thin (˜500 Å) buffer/nucleation layer of either GaN or AlN. The observed electrical and optical properties of the resulting GaN layers is strongly dependent on the dislocation density and of the overall impurity content. 
     Prior art magnetron sputter sources require special MBE machines. An object of the invention is provide an MSE source that will fit onto a standard MBE machine. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a method wherein a magnetron sputter source is adapted to fit into a K-cell port in a molecular beam epitaxy apparatus, the magnetron sputter source comprising a protruding cylindrical body for insertion into the K-cell port, said cylindrical body being attached at its proximal end to a flange and having its distal end open; an array of permanent magnets arranged at the distal end of said cylindrical body; a magnet return piece mounted behind said permanent magnets; a sputter target mounted in front of said permanent magnets; and cooling ducts within said cylindrical body for carrying a cooling medium to said sputter target. 
     The magnetron sputter source can be used for depositing gallium nitride or like epilayers as disclosed in U.S. Pat. No. 6,291,318. Using this method, silicon doped GaN epilayers having room temperature electron mobilities &gt;550 cm 2 /V s can be grown on (0001) sapphire. Unlike other growth techniques, the initial buffer/nucleation layer, preferably of AlN (aluminum nitride), is grown by MSE. The magnetron sputter source in accordance with the invention can also be used for other applications. 
     The magnetron sputter source can be used in a dual mode MBE/MSE system. The MSE technique differs from conventional MBE in that an ultrahigh vacuum dc magnetron sputter cathode is used as the group III source and deposition of the layers occurs in the pressure range of 1-5 mTorr. 
     Typically, the MSE technique is employed only for the growth of a buffer/nucleation layer. The GaN layer is deposited by ammonia MBE where a conventional dual filament K cell is used for the gallium source, and high purity ammonia is used as the source of nitrogen. 
     Preferably, the deposition system is equipped with a substrate holder capable of heating the 2 in. sapphire(0001) wafers to temperatures in excess of 1000° C. Typical growth temperatures for the GaN layers were in the range of 860-920° C. as measured by an optical pyrometer (emissivity set to 0.3). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which; 
     FIG. 1 is a schematic diagram of an MSE system; 
     FIG. 2 is a plot showing photoluminescent intensity against energy for a 2.2 thick GaN epilayer on (0001) sapphire at 4K and room temperature; 
     FIG. 3 shows the Hall effect carrier mobility and carrier density for a 2.3 μm thick GaN epilayer on (0001) sapphire for temperatures in the range 77 to 300° K; 
     FIG. 4 a  is a schematic sectional view of a magnetron sputter source for molecular beam epitaxy; and 
     FIG. 4 b  is a perspective detail of the end of the MSE device shown in FIG. 4 a.   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The dual MBE (molecular beam epitaxy)/MSE system shown in FIG. 1 comprises a vacuum chamber  10  with a nitrogen cryoshroud  12 , and port(s)  14  for K-cells, port  16  for a 3-source alkyl/gas injection cell, and port(s)  18  for modified-UHV magnetron sputter sources, such as aluminum. Cryopump  20  is connected to the chamber  10 . 
     The main chamber  10  is cryo/turbopumped to a base pressure of &lt;5×10 −11  torr. Substrates up to 4″ diameter are introduced into the main chamber via a load-lock and transfer arm. A BN coated graphite heater is used to heat substrates up to 1200° C. Substrates are typically 2″ (0001)sapphire coated on the back surface with molybdenum. Smaller pieces are mounted using indium solder to a 2″ silicon wafer. 
     With this system, a solid MBE-grade aluminum source can be D.C. sputtered using an argon plasma operating at 1˜3 mtorr [target power of 50 Watts, 400 Volts]. High purity ammonia introduced via the gas injection cell is used as the nitrogen source [flow of 15 sccm]. The magnetron plasma is sufficient to provide a source of nitrogen ions for the growth of AlN. Growth rates for the MSE grown AlN buffer layer at 880° are typically 0.15 to 0.25 μm/hr. 
     EXAMPLE 
     Sapphire wafers (backside sputter coated with Mo) were degreased in chloroform vapor followed by a 1 min dip in 10% HF, then rinsed in deionized water and blown dry with nitrogen gas. The substrate was then introduced into the system load-lock where degassing of the wafer was carried out before introduction into the growth chamber  10 . The growth chamber was fully cryoshrouded with a base pressure of &lt;10-9 Torr. The substrate was then heated to a temperature of 1000° C. under 130 sccm of ammonia for 10 min before cooling to the buffer layer growth temperature of 880° C. 
     The buffer or nucleation layer of AlN was used. This layer was deposited by MSE using a high purity Al magnetron sputter cathode and ammonia. The growth of the nucleation and epilayer was monitored using in situ laser reflectance spectrometry. A 200 A nucleation layer was deposited at a growth rate of 34 A/min with argon and ammonia flows of 40 and 15 sccm, respectively. This resulted in a deposition pressure of about 1.4 mTorr. 
     Following the deposition of the AlN nucleation layer, the substrate was then heated to the GaN epilayer growth temperature. The Ga K-cell temperature was adjusted to give a growth rate of from 1-2 μm/h at an ammonia flow rate of 50 sccm. Typical base pressures of 3-10 −6  Torr were observed during growth. Both undoped and silicon doped layers were grown. For the doped layers, silane was used as the dopant source. The formed GaN layers had thicknesses of ˜2 μm. 
     The epilayers were characterized using a triple axis x-ray diffractometer ω and ω-2θ scans were carried out to determine both the mosaicity and crystalline quality of the deposited layers. Hall effect mobilities and carrier densities were measured using a van der Pauw geometry (sample size of ˜0.5×0.5 cm, applied field of 3 kG) with soldered indium dots as the ohmic contacts. The contacts were verified to be ohmic by I-V measurements. Photoluminescence (PL) was performed at room temperature (RT) and 4 K using excitation from a He—Cd laser with an incident power density of &lt;0.25 W/cm2. 
     Table 1 gives the observed x-ray linewidths and electrical data for a number of layers grown at several different temperatures. All data shown in Table I are for layers that were intentionally doped to 1-7×10 17  cm 3 . As shown in Table 1, the minimum linewidths for the ω and ω-2θ scans were 210 and 13.7 arcsec, respectively, with the highest observed mobility of 560 cm 2 /V s for a carrier density of 1.44×10 17 cm −3 . 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 X-ray 
                 X-ray 
                   
                 Electron 
               
               
                 Layer 
                 ω scan 
                 ω 2θ scan 
                 Carrier density 
                 mobility 
               
               
                 No. 
                 (arcsec) 
                 (arcsec) 
                 n (×10 17 cm −3 ) 
                 μ(cm 2 /Vs) 
               
               
                   
               
             
             
               
                 1 
                 210 
                 13.7 
                 X.5 
                 356 
               
               
                 2 
                 271 
                 17.0 
                 7.0 
                 400 
               
               
                 3 
                 299 
                 14.8 
                 7.0 
                 340 
               
               
                 4 
                 377 
                 15.8 
                 3.5 
                 407 
               
               
                 5 
                 303 
                 14.2 
                 1.4 
                 560 
               
               
                 6 
                 324 
                 14.5 
                 1.6 
                 547 
               
               
                 7 
                 323 
                 14.9 
                 1.3 
                 544 
               
               
                   
               
             
          
         
       
     
     FIG. 2 gives a plot of the observed PL spectrum for a representative layer of GaN. As shown in FIG. 2, even at very low excitation intensities the yellow luminescence band is very weak at room temperature and virtually absent at 4° K. The spectra are dominated by strong donor-bound exciton emissions at 3.48 and 3.42 eV with FWHM of 4.9 and 47 meV for temperatures of 4 K and room temperature, respectively. At 4 K only very weak emission from donor-acceptor transitions are observed, which would indicate a low density of defects and/or impurities. This is consistent with the observed high mobilities and correspondingly low compensation for these layers. 
     FIG. 3 shows the measured mobility and carrier density of a representative layer. As shown in FIG. 3, a peak mobility of 952 cm2/V s is observed at a temperature of 145 K. The corresponding carrier density shows a single activation energy of about 13 meV, which is similar to that observed previously for silicon doped GaN. 
     By the described technique, MBE, GaN layers with electron mobilities of up to 560 cm2/V s can be successfully grown with good reproducibility. This indicates that using the MBE technique with an MSE grown AlN buffer layer, the type of defects affecting the electron mobility can be reduced significantly as they have been using MOVPE but without the attendant disadvantages of MOVPE. 
     The magnetron sputter epitaxy source assembly in accordance with the principles of the invention will now be described in more detail with reference to FIGS. 4 a  and  4   b.    
     As will be appreciated by one skilled in the art, in molecular beam epitaxy systems, such as illustrated in FIG. 1, K-cells are typically used as the MBE source. K-cells are effusion cells where the material to be deposited is heated in a crucible to the vaporization temperature. The problem with K-cells is that the flux is controlled by the temperature of the crucible. Since the vapor pressure is exponentially dependent on temperature, control of the flux is difficult. By contrast, a magnetron sputter source provides a highly stable flux, which is easily controlled. 
     It is desirable simply to be able to replace a K-cell with an MSE sputter cell in order to perform the initial magnetron sputter epitaxy. The design shown in FIGS. 4 a  and  4   b  is compatible with an existing K-cell. Furthermore, it uses the conventional DC power supply of conventional magnetron sources. The design is UHV (Ultra High Vacuum) compatible and serves as a direct replacement for existing K-cells. 
     As shown in FIGS. 4 a  and  4   b , the MSE sputter source comprises a gun assembly  40  with a cylindrical anode shield  41  protruding from an annular support  42  attached to a flange  43 . 
     Within the anode shield  41  is mounted a cylindrical cathode body  44  containing feed and return pipes for cooling water. 
     A target  46 , for example of aluminum, is mounted on the end of the gun assembly  44 . Behind the target  46  are located permanent magnets  47  and magnet return piece  48 . The magnets are configured so that the magnetic field lines radiate angularly from the center, extending through the target surface. The magnets are typically high field ceramic magnets with sufficient field strength for use with targets having thicknesses in the range of 0.125 to 0.25 inches. Typically, the magnets could be high field samarium cobalt magnets, such as might, for example, be obtained from Jean-Ell Enterprise Co. Ltd., No. 61, Sec. 1, Chung Cheng Road, Shih-Lin, Taipei, Taiwan, R.O.C. All components are UHV compatible. Metal UHV seals are used in the device. The length of the assembly is designed to fit commercially produced K-cell ports. The flange  43  is similar to a flange on a typical K-cell so that the assembly can be inserted as a replacement for a conventional K-cell. 
     The targets  46  are water-cooled with the water flowing through the cooling ducts  45 . The cooling water is isolated from the vacuum system. The magnets and cooling lines are removable to permit bake-out at the source. To remove the magnets and cooling plates, retaining screws that hold the plate  48  to the gun assembly  44  are first removed. The plate  48  is sealed using a standard Viton “O” ring seal. This is a water seal only, which is not exposed to a vacuum. 
     The gun assembly  44  is sealed via metal “C” ring seals to the ceramic insulator  50  and then to the UHV flange  43  which connects to the main chamber. The arrangement forms a UHV seal. As a result, when the magnet assembly is removed, the gun can be baked out since no magnets or cooling liquid is present. 
     The device is designed for DC operation. Typical operation occurs at Argon pressures down to 1.8 mTorr. Typical cathode voltage is 394 volts for powers of 10 to 50 watts. The target  46  can of course be made of materials other than aluminum. 
     The device operates in a manner similar to a conventional magnetron source, but the novel design permits it to be substituted for a K-cell in a conventional molecular beam epitaxy apparatus.