Incorporation of Excess Arsenic in GaAs and AlGaAs Epilayers Grown at Low Substrate Temperatures by Molecular Beam Epita
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INCORPORATION OF EXCESS ARSENIC IN GaAs AND AIGaAs EPILAYERS GROWN AT LOW SUBSTRATE TEMPERATURES BY MOLECULAR BEAM EPITAXY M.R. MELLOCH*, N. OTSUKA**, K. MAHALINGAM**, A.C. WARREW**, J.M. WOODALL***, and P.D. KIRCHNER*** * School of Electrical Engineering, Purdue University, West Lafayette, IN 47907 **School of Materials Engineering, Purdue University, West Lafayette, IN 47907 ***IBM Research Division, P.O. Box 218, Yorktown Heights, NY 10598 ABSTRACT Excess arsenic can be incorporated in GaAs and AIGaAs epilayers by growing at low substrate temperatures (LT-GaAs and LT-AIGaAs) by molecular beam epitaxy (MBE). Upon annealing these epilayers, the excess As precipitates forming GaAs:As and AIGaAs:As. Using transmission electron microscopy (TEM), we have measured the densities and sizes of the As precipitates and thereby determined the amount of excess As incorporated in these epilayers. The volume fraction of excess As as a function of inverse substrate growth temperature follows an Arrhenius-type behavior with an activation energy of 0.87 eV. The sizes of the As precipitates increase and the densities decrease with increase anneal temperatures; for Si-doped GaAs:As this results in n-type material when the densities become small enough that the depletion regions around the As precipitates no longer overlap. Also investigated is the formation of As precipitates at GaAs/AIGaAs heterojunctions and superlattices, and our attempts to tailor the As precipitate distribution. INTRODUCTION Typical MBE of GaAs is at substrate temperatures of about 6000 C. At this substrate temperature, for high quality stoichiometric films an As4/Ga flux of somewhat greater than 10 is necessary [1]. Ifone maintains such an excess flux of As but reduces the substrate temperature to 200-250oC, as much as 1% excess As can be incorporated into the film [2]. This excess As is incorporated in the form of As anti-sites and interstitials; the interstitial As results in an increase in the lattice constant of the film [2-5]. Upon annealing this highly non-stoichiometric material at temperatures of 600 0 C, the lattice constant approaches that of highly stoichiometric GaAs [2-5]. Furthermore, as determined by electron paramagnetic resonance, the amount of As anti-site defects falls below the detectability limit after such anneals [2,6]. Original TEM studies of such annealed films indicated no difference in contrast between the substrate and epilayer, similar dislocation densities, and no additional defects [2,6]! This was indeed a puzzle because where had all the excess As gone? When we performed TEM studies of our GaAs films that had been grown at 250 0C we found that upon annealing at 6000 C our excess As was forming precipitates with an average diameter of 50 A and a density of 1x10 17 cm- 3 . This is what happens to the excess As in all such annealed material [7-10] including AIGaAs [11]. Also, very similar behavior is observed both structurally [12] and electronically [13] if the dimer species As2 is used for MBE instead of the tetramer As4 .
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