Abstract:
This invention relates to an apparatus of catalytic molecule beam epitaxy (cat-MBE) and process for growing Group III nitride materials using thereof, characteristically in that said apparatus is equipped with a hot wire to catalytically decompose gaseous ammonium or nitrogen molecule into activated nitrogen radicals as the nitrogen source for growing epitaxial layers by MBE.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to an epitaxy apparatus of III-nitride, particularly to an apparatus of catalytic molecule beam epitaxy (catalytic MBE), which is characterized in that, said apparatus is equipped with a hot wire to catalytically decompose gaseous ammonium or nitrogen molecule into activated nitrogen radicals as the nitrogen source for growing epitaxy by MBE.  
       DESCRIPTION OF THE RELATED PRIOR ART  
       [0002]     The most common technologies used for conventional growth of Group III-nitride materials are: metal-organic chemical vapor deposition (MOCVD) and molecule beam epitaxy (MBE).  
         [0003]     As to MOCVD technology, the growth rate is fast and the thickness is precisely controlled, so that it is particularly applicable to mass production of LEDs and LDs. Therefore, Emcore Company and Aixtron Company in U.S. and Tomas Swan Company in UK have developed MOCVD apparatuses used for mass production of gallium nitride. However, there are some obvious drawbacks in terms of MOCVD technology including higher growth temperature, higher pressure, and consumption of a large amount of ammonia to maintain the chemical composition of gallium nitride film. Besides, due to higher Reynolds number of ammonia, it is easy for fluid to produce turbulence phenomenon, so that the design of growth reactor and the control on growth uniformity of film are of technical difficulty, and it is not easy to install in-situ analysis elements into the system.  
         [0004]     In contrast to the above MOCVD, to grow gallium nitride with MBE technology is capable to conduct at low temperature and low pressure with high growth uniformity of film and slow growth rate, so that it is possible to control the film thickness more preciously to atomic layer order, and is particularly applicable to material growth technology for production of quantum well layer structure. As molecule beams of each source in MBE technology are transmitted to substrate independently, it is possible to eliminate the homogeneous reaction between the sources in reactor space before they are transmitted to substrate. In addition, due to high vacuum degree in MBE system, normally at 10 −10  torr, the background contamination of film materials originated from contaminants such as carbon and oxygen is low.  
         [0005]     However, the drawback of MBE technology is, since the feature of NH 3  and N 2  is difficult to be decomposed at low temperature, currently MBE epitaxy of gallium nitride can only be enhanced by radio frequency (RF) and electron cyclotron resonance (ECR) plasmas to excite NH 3  and N 2  as nitrogen source. For example, when metal gallium or metal-organic gallium is used as gallium source, it is possible to react on the substrate surface to form gallium nitride; however, it is easy for high energy ion stream generated from RF or ECR plasma to damage film, so that the quality of gallium nitride epitaxial layer is obviously reduced.  
         [0006]     For example, U.S. Pat. No. 6,146,458 discloses a molecule beam epitaxy, to improve present MBE technology, which comprises introducing NH 3  gas via first conduit and Group III gas via second conduit, in which NH 3  gas is introduced by RF as conventional MBE; in addition, U.S. Pat. No. 6,500,258 discloses a growth process for semiconductor crystal layer by MBE technology, which is characterized in that, mainly for production of Group III nitride semiconductor layer, to control temperature of substrate by using time difference, and to introduce NH 3  gas at right time to elevate V/III ratio. However, NH 3  gas is still introduced by RF as conventional MBE technology, so that it is possible for high-energy ion stream to damage film as U.S. Pat. No. 6,146,458. Further, U.S. Pat. No. 5,637,146 discloses a growth process and apparatus for Group III nitride semiconductor layer, which is characterized in that, nitrogen is supplied through RF plasma-excited radical atom technology, but there are still problems regarding epitaxial layer damage present. In the present invention, nitrogen source is supplied through hot wire catalytic decomposition of NH 3 , so that obviously there are no problems regarding film damage by high-energy ion stream present as in conventional high-energy dissociation of nitrogen source by RF or ECR plasmas.  
       SUMMARY OF THE INVENTION  
       [0007]     The main object of the invention is to provide a catalytic molecule beam epitaxy (catalytic MBE) process and apparatus for growth of Group III nitride materials, which solves the problems of high energy ion stream damage in conventional molecule beam epitaxy due to RF or ECR, by supplying a stable activated nitrogen source, so that the quality of GaN epitaxial layer is elevated while maintaining a growth rate comparable to RF or ECR molecule beam epitaxy.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a scheme showing a catalytic molecule beam epitaxy (cat-MBE) apparatus in a preferred embodiment of the present invention;  
         [0009]      FIG. 2  is a TEM image showing the cross-sectional GaN sample grown by a cat-MBE apparatus according to the present invention; and  
         [0010]      FIG. 3  is the x-ray diffraction curve of GaN sample grown by a cat-MBE apparatus according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     The catalytic molecule beam epitaxy apparatus of the present invention includes: 1) a cool-wall stainless steel super ultra-high vacuum system used as environment for growing Group III nitride materials; 2) a hot wire used for catalytically decomposing gases comprising nitrogen; 3) a solid Group III metal source used for providing Group III elements needed in the growth of Group III nitride semiconductor, such as Ga, Al or In, wherein, when ammonia or nitrogen are passed through hot wire, they are catalytically decomposed to produce activated ions comprising nitrogen, and said activated ions and Group III elements arrive at substrate in the form of molecule beam, react thereon to form Group III nitride epitaxial layer.  
         [0012]     In the preferred embodiments of the present invention, said ammonia can be replaced by other gases of compounds comprising nitrogen, such as N 2 , N x Cl y  etc.; the N activated ions produced when ammonia is passed through hot wire may be N* or NH* ion or other activated N component ions. The solid Group III source in the invention comprises high purity metals like Ga, Al and In.  
         [0013]     The molecule beam epitaxy apparatus of the present invention includes a hot wire, a main reactor, a loading chamber, a heater, an entrance and exit for wafer loading in and out, shutters, a molecule source crucible set, and a pump system for maintaining vacuum. It is characterized in that a stable and activated catalytic hot wire is provided to produce activated ions comprising nitrogen such as N* or NH* ion or other activated N component ions, when, for example, ammonia, are passed therethrough. The materials of the hot wire comprise high melting-point metals like tungsten (W), tantalum (Ta), molybdenum (Mo), rhenium (Re), niobium, (Nb), platinum (Pt), titanium (Ti) etc., with tungsten (W) being the most preferred. The temperature of the hot wire depends on needed nitrogen sources and materials, and the range is between 1000° C.˜2500° C., with 1200° C.˜1700° C. being the most preferred.  
       EMBODIMENT  
       [0014]     In order to clearly demonstrate the above and other objects, features and advantages of the present invention, a preferred embodiment is presented in connection with accompanied figures for the explanation thereof, however, the content and scope of the present invention is not limited thereto.  
         [0015]      FIG. 1  is a scheme showing a preferred embodiment of the present invention. Main reactor  20  of catalytic molecule beam epitaxy (catalytic MBE) apparatus  120  is made of stainless steel, and the wall is water-cooled. The heater  40  is capable to heat up to 1200° C., rotate, and carry 1˜2-inch wafers. Molecule source crucible set provides Group III elements like Ga, Al, etc., and solid Mg and Si sources for use as P and N types dopant sources. Nitrogen source is consisted of activated N or NH ions, which are produced by catalytic decomposition of high purity NH 3  gas by passing through hot wire  10 . This is the core of the present invention. The vacuum states of main reactor  20  and loading chamber  30  are maintained by a 1300 l/s and a 600 l/s molecular pump respectively, and the highest vacuum can be reached up to 3×10 −9  torr and 5×10 −6  torr respectively. There is a reflective high-energy electron diffraction (RHEED) analyzer  50  installed in main reactor  20 , in order to conduct an in-situ observation on film growth surface in this preferred embodiment. Entrance and exit for chip web  60  is used for loading and removing of wafers.  
         [0016]     The general steps for growing GaN epitaxial film by using the present apparatus are:  
         [0000]     (1) Firstly, a 1-inch sapphire (0001) substrate is cleaned with acetone and methanol, etched by a mixed solution formulated with H 2 SO 4 :H 3 PO 4  of 1:3, and rinsed with DI water and dried with N 2 ;  
         [0017]     (2) After clean pretreatment, the substrate is immediately loaded into loading chamber  30 , and passed to main reactor  20  when the vacuum degree in loading chamber 30&lt;2×10 −6  torr; the temperature of main reactor is lowered to 500° C. for nitridation treatment for 5 minutes after the substrate is annealed at 900° C. for 10 minutes, and a low-temperature GaN epitaxial buffer layer of thickness of 25 nm is grown at 500° C., finally a GaN epitaxial layer of thickness of 3.5 μm is grown after elevating the temperature to 760° C. In which, NH 3  gas flow rate is controlled at 50 sccm, wire temperature is 1500° C., temperature of Ga source is controlled at 980° C., and growth pressure is 10 −4  torr during the growth process.  
         [0018]      FIG. 2  is a TEM image showing the cross-sectional GaN sample grown by cat-MBE apparatus  120  according to the present invention; and  FIG. 3  is the x-ray diffraction curve of GaN sample grown by cat-MBE apparatus  120  according to the present invention. The above results show the crystal quality of GaN samples grown by cat-MBE apparatus  120  used in the preferred embodiment is very good.  
       DESIGNATION OF MAIN COMPONENTS  
       [0000]    
       
           01  Inlet of cooling water  
           02  Outlet of cooling water  
           10  Hot wire  
           20  Main reactor  
           30  Loading chamber  
           40  Heater  
           50  Reflective high-energy electron diffraction analyzer (RHEED)  
           60  Entrance and exit for chip wafers  
           70  Shutter  
           80  Molecule source crucible set  
           90  Turbo pump  
           100  Mechanical pump  
           110  High purity ammonia  
           120  Catalytic molecule beam epitaxy apparutus