Electrical Characterization of Defects in GaAs Grown on Si by MBE
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Electrical Characterization of Defects in GaAs Grown on Si by MBE
N. CHAND, R. FISCHER, A. M. SERGENT, D. V. LANG and A. Y. CHO AT&T Bell Laboratories, Murray Hill, New Jersey 07974
ABSTRACT We show that MBE-grown GaAs on Si exhibits only a modest increase in the concentrations of the well-known electron traps typical of MBE-GaAs with no evidence for any new electron deep levels in the upper half of the bandgap in spite of the dislocations and other defects in the material. As shown by Au-GaAs Schottky contacts, the defects are unnoticeable when the device is forward biased but become very active in reverse biased condition, causing large leakage current and low breakdown voltage (although the device is still acceptable for many applications, especially FET's). The defects become more active after hydrogenation and more inactive after a post-growth rapid thermal annealing (RTA). Performance of devices made on thermally annealed GaAs on Si is comparable to those of GaAs on GaAs. Also, following the application of a large- current the device behavior improves, indicating a self-annealing action as a result of internal heating. The reverse current in the as-grown material shows a very weak temperature dependence, indicating its origin is not thermionic emission or carrier generation. It is speculated that a large part of the leakage current in the as-grown GaAs on Si is due to the defect assisted tunneling. After RTA, the average spacing between defect clusters increases, thus reducing the tunneling probability and tremendously improving the device characteristics.
INTRODUCTION
Although great progress has been made recently towards achieving device quality growth of GaAs on Si substrates, the high density of threading dislocations 2 (--107 - 108 cm- ) originating at the heterointerface which propagate through the GaAs layer is still the major remaining problem. The dislocations can deteriorate the device performance in several ways. They can act as generation-recombination centers reducing the radiative efficiency in optical devices and increasing the defect current in electrical devices. They may attract impurities towards them causing cross-doping near the GaAs/Si interface. Normally oversized atoms are attracted to the region of tension around a dislocation; undersized substitutional atoms are attracted to the region of compression [1]. Precipitation of impurities around dislocations may also act like metallic pipes which can shunt the current path, enhance the tunneling probability of carriers, or may result in formation of localized pockets of large electric field causing large leakage current and premature breakdown of the device. Mat. Res. Soc. Symp. Proc. Vol. 91. 1987 Materials Research Society
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If GaAs-on-Si technology is to be perfected, it is necessary to examine how these dislocations affect the device performance electrically. Such a study has been lacking so far. We have used avalanche photodiodes (APD's) and Schottky contacts to understand the electrical activity of these defects. The study o
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