Heterostructure Doping by Compact Electron Beam Sources
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P.P.CHOW*,J.VAN HOVE*,M.F.ROSAMOND*,G.L.CARPENTER*, and L.A.CHOW** * SVT Associates, Eden Prairie, MN Stanford University, Dept. of Materials Science and Engineering, Palo Alto, CA **
ABSTRACT Performance of many III-V compound heterostructure devices relies on exact control of dopant placement and concentration. This requires the dopant source to be responsive and reproducible. We report a novel electron beam evaporation source for both carbon and silicon dopant incorporation in MBE growth of III-V compounds. Carbon in particular has attracted much interest as a replacement of beryllium for p-type doping in III-V compounds. Not only high doping levels are achievable, its low diffusion coefficient makes abrupt interface possible even after high temperature processing and operation. This source provides a convenient means for generating high carbon flux. It is equipped with a compact flux monitor such that the dopant levels can be adjusted in real time. The GaAs samples show excellent photoluminescence peaks and mobility results. Good carbon doping for GaSb was reported for the first time.
INTRODUCTION Carbon doping in III-V heterostructures has generated much attention because of applications in high temperature and high current devices. Although Be is presently used in MBE grown samples, it has high diffusion coefficient that causes dopant redistribution during growth and subsequent processing, degrading device performance. Therefore there is much activity in search of an alternate dopant material such as carbon. Several carbon sources have been reported in the literature. Gas sources such as trimethylgallium (TMGa) in metalorganic molecular beam epitaxy 3 , carbon tetrachloride (CCI 4 ) in gas source molecular 5 beam epitaxy 4 and carbon tetrabromide (CBr 4 ) in conventional molecular beam epitaxy have been used. Excellent results have been obtained but concerns about short and long term memory effects, gas pumping loads and safety issues still exist. The most common solid carbon source is to employ a directly heated graphite filament for carbon evaporation 6 -8 . A heterojunction bipolar transistor with a 1 x 1019 cm- 3 carbon doped base was found to have current gains greater than 1006. Although the direct heating graphite filament source has demonstrated its utility, it also has severe limitations. They have been found to have relatively short lifetimes, low dopant activation, and surface morphology degradation at high doping levels. Very high input power (around 600 W) 8 is required for the highest doping levels causing system outgassing. In this work, we report on the use of a novel electron beam evaporation source for carbon and silicon doping. The design and operation of the source is described. A unique feature of the source is a sensitive flux monitor that allows precise dopant control in real time. Source 567 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
uniformity over 3" wafers was found to be within +/- 1%. Carrier mobility and concentration values are given for both GaAs
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