Study of Transport Through Low-Temperature GaAs Surface Insulator Layers
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STUDY OF TRANSPORT THROUGH LOW-TEMPERATURE GaAs SURFACE INSULATOR LAYERS
J.P. Ibbetson*, L.-W. Yin+, M. Hashemi+, A.C. Gossard*+, U.K. Mishra+, Materials Dept.* and Dept. of Electrical and Computer Engineering+, University of California at Santa Barbara, Santa Barbara, CA 93106.
INTRODUCTION Since epilayers of GaAs grown at low substrate temperature (LTGaAs) and annealed at 600'C were first demonstrated to be an effective buffer layer for eliminating backgating effects, the material properties and electronic characteristics of bulk LTGaAs have been actively investigated'- 3 . Less attention has been paid to thin layers of LTGaAs (-2000A), although these have been shown to improve gate-to-drain breakdown characteristics when incorporated as the surface insulator layer in GaAs MISFET's 4 . In bulk LTGaAs that has been annealed for 10 minutes at 600 0 C, the formation of arsenic precipitates with a density of 1018 cm- 3 has been observed. These are considered to be at least partially responsible for the high resistivity of LTGaAs 2 . While the exact mechanism of precipitate formation is currently unknown, it would seem reasonable to expect the availability of the growth surface to have a significant effect on any defect redistribution during the anneal. This surface effect would become increasingly apparent as the LTGaAs layer thickness was decreased. It is desirable for MISFET applications that the LTGaAs gate insulator layer be as thin as possible, whilst maintaining high breakdown, in order to maximize device transconductance5 . To achieve this, it is important to understand how the observed bulk features (such as -60A size arsenic precipitates) are affected in thin LTGaAs layers. In this work, we report on the investigation of current transport through 100•A-5000oA thick LTGaAs surface layers on an n-type GaAs channel. Room temperature I-V characteristics of schottky diodes incorporating a LTGaAs surface layer are presented. The current mechanism through the LTGaAs is found to be space-charge limited due to the high density of deep traps present. Comparison of the measured trap-filled limit voltage for different thicknesses of LTGaAs suggests that the trap spatial distribution is non-uniform with respect to the growth direction, with a significant reduction in the trap density close (
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