The Nature of Native Defects in LEC grown Semi-Insulating GaAs by Thermally Stimulated Current Spectroscopy
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THE NATURE OF NATIVE DEFECTS IN LEC GROWN SEMI-INSULATING GaAs BY THERMALLY STIMULATED CURRENT SPECTROSCOPY
ZHAOQIANG FANG, LEI SHAN, T. E. SCHLESINGER AND A.G. MILNES Department of Electrical and Computer Engineering Carnegie Mellon University, Pittsburgh, PA 15213
ABSTRACT Traps have been studied by thermally stimulated current spectroscopy (TSC) with intrinsic (1.96 eV) and extrinsic (1.15 eV) light for both In doped and undoped LEC materials grown under various non-stoichiometric conditions. Significant differences are seen in the bulk trap spectra associated with Ga-rich and As-rich material and with isoelectronic In doping. Proximity wafer-annealing at 9500 C has been shown to improve minority carrier lifetime in n-type GaAs and we show that in semi-insulating GaAs this causes changes in trap structure. From such thermal studies and the effects of non-stoichiometric growth, the probable nature of the traps commonly seen is inferred. TYPICAL TSC SPECTRA Five kinds of SI-GaAs were used in the study. Four are as-grown SI-GaAs: Asrich melt grown, Ga-rich melt grown, In doped and undoped. The other is a SI-GaAs wafer proximity-annealed at 9500 C for 16 hr. in a sealed pure quartz ampoule. Both 1.96 eV light from a He-Ne laser and 1.15 eV light from a tungsten lamp via a monochrometer were used for the excitation at 90 K.The details of sample preparation, experimental set-up and measurement conditions have been described elsewhere [1]. From typical TSC spectra (1.96 eV) for as-grown undoped SI-GaAs, shown in Fig. 1 and 2, it can be found that 1) at least six traps designated T1 to T6 can be observed in the temperature range from 90 K to 250 K,2) T2 dominates T3 in the SI-GaAs with As-rich stoichiometry and T3 dominates T2 in the SI-GaAs with Ga-rich stoichiometry and 3) the peak T3 in the 'Ga-rich" sample appears to be composed of several traps with a main one at Tm = 190 K.But from the comparison of spectral structure between In doped and undoped SI-GaAs (Fig. 2), it can be seen that 1) in addition to the six traps observed in undoped SI-GaAs the other traps TA and TD can be clearly observed in In-doped SIGaAs, 2) the TA at 168 K dominates T2 and T3 and becomes the main trap in In-doped SI-GaAs and 3) although the etch pit density (EPD) in the In-doped sample (2x10 3 cm-2) is much lower than the EPD in undoped sample. The estimated density of TA is in 1017 cm- 3 range, which is higher than for T2 and T3 in undoped SI-GaAs. Three methods were used to determine the trap depths for main traps. They are 1) calculation by an approximate equation, Ei = kTm ,n T4 /P [2], where Ei is the trap depth of a given trap, k is BoItzmann's constant, Tm is the temperature at the TSC peak and 0 is the heating rate during the thermal scan, 2) determination from Arrhenius plots of , n T•/3 vs. 1/Tm (31 and 3) determination from an initial rise slope or low temperature exponential edge for a given trap, which is measured after removing all emissions from traps shallower than the test one by raising the temperature to T (T < Tm for a
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