Electrical Characterization of Low Temperature GaAs Layers, and Observation of the Extremely Large Carrier Concentration
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ELECTRICAL CHARACTERIZATION OF LOW TEMPERATURE GaAs LAYERS, AND OBSERVATION OF THE EXTREMELY LARGE CARRIER CONCENTRATIONS INUNDOPED MATERIAL BIJAN TADAYON,* MOHAMMAD FATEMI, SAIED TADAYON, F. MOORE, AND HARRY DIETRICH *Code 6856, Naval Research Laboratory, Washington, DC 20375 **COMSAT Lab, 22300 COMSAT Dr., Clarksburg, MD 20871 ABSTRACT We present here the results of a study on the effect of substrate temperature, Ts, on the electrical and physical characteristics of low temperature (LT) molecular beam epitaxy GaAs layers. Hall measurements have been performed on the asgrown samples and on samples annealed at 610 OC and 850 °C. Si implantation into these layers has also been investigated. INTRODUCTION It has been shown that GaAs buffers grown at low substrate temperatures, commonly known as low temperature (LT) buffers, can be used to eliminate the backgating problem in metalsemiconductor field-effect transistors [1]. LT buffers are of interest because of their electrical, optical, and compositional characteristics [1 -4]. In this paper, we present the results of a study on the effect of the growth temperature (substrate temperature, Ts) on the electrical and physical characteristics of LT GaAs layers. EXPERIMENT GaAs epilayers were grown on semi-insulating (100) GaAs substrates at a rate of one micron per hour, in a VG V80H molecular beam epitaxy (MBE) system. The substrates were prepared for growth by a standard procedure of degreasing and etching in 7 H2 SO4 :1 H2 0 :1 H2 0 2 . The As (As4 ) to Ga beam equivalent pressure ratio was 18. After the oxide was desorbed from the substrate surface at a temperature of 580 °C, Ts was adjusted to the growth temperature, the sample was stablized under an As overpressure, and the growth of the GaAs buffer was Mat. Res. Soc. Symp. Proc. Vol. 241. 01992 Materials Research Society
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started. No post-growth annealing was performed in the MBE chamber. Sixteen samples were grown at different substrate temperatures (between 100 and 600 °C). Before growing samples 1 (Ts = 600 °C) and 2 (Ts = 500 °C) the thermocouple used to monitor Ts was calibrated against the congruent sublimation temperature of GaAs. This assures that the quoted Ts values for samples 1 and 2 are quite accurate. However, with decreasing Ts, this calibration becomes increasingly inaccurate. Similarly, pyrometric measurements are inherently inaccurate for low temperatures. Therefore, for samples 3-16, which were grown in a temperature range where no direct calibration is available for the thermocouple, the accuracy of Ts is unknown. However, as described on the next page, the relative values of Ts can be compared using the X-ray calibrations. Double-crystal X-ray diffraction was performed on all samples. For samples 1-4 (Ts >_460 °C) and samples 12-16 (Ts< 250 °C), only one peak was observed in the rocking curves. For samples 5-11 (260 _ 460 °C, 2) intermediate range, 260 _
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