Analysis of Anomalous Peak Heights in DLTS of MBE-Grown Aluminum Gallium Arsenide
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ANALYSIS OF ANOMALOUS PEAK HEIGHTS IN DLTS OF MBE-GROWN ALUMINUM GALLIUM ARSENIDE W. LIM, L. P. TROMBETTA, AND KEITH JAMISON Space Vacuum Epitaxy Center, University of Houston, Houston, 7X 77204
ABSTRACT DLTS data taken on MBE grown AlGaAs films show peak heights which increase, maximize, and then decrease with increasing sample temperature. This behavior is difficult to explain within the context of conventional DLTS analysis. We suggest that the data can be accounted for by a trap with a temperature dependent capture cross section in conjunction with a model described by Lee and Borrego in which electron and hole emission rates are comparable. Using this analysis, we obtain an effective trap depth Eeaf of 0.35 eV and a capture cross section activation energy E of 0.25 eV. INTRODUCTION In the course of a DLTS survey of MBE grown AlGaAs thin films prepared in our laboratory, data were obtained which could not be readily explained within the context of conventional DLTS analysis. In particular, the DLTS peak heights were found to increase, maximize, and then decrease with increasing temperature (see Figure 1 below). While a temperature dependent capture cross section might be invoked to explain either an increase or a decrease in peak heights, it seems unlikely that this explanation holds for both trends. Recent work by Lee and Borrego [I has shown that a change in peak heights may be observed for a trap for which the electron and hole emission rates are comparable. This is in contrast to standard DLTS analysis where it is assumed that one of the emission rates is dominant. For comparable emission rates, a trap in the upper half bandgap will give decreasing DLTS peak heights with increasing temperature, while one in the lower half will give increasing peak heights. We argue here that the behavior displayed by our samples can be explained by assuming a temperature dependent capture cross section in conjunction with comparable electron and hole emission rates. The analysis described by Lee and Borrego can be summarized as follows. When the electron and hole emission rates are comparable, the DLTS signal is &C(T) = AC 0 (T)[e-(en-ep)tl - e(entep)tZL],
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
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where ti and t2 are the capacitance transient sampling times. ACo = CoNt/2ND where CO is the quiescent reverse bias capacitance, Nt is the trap density, and ND is the donor dopant density. The emission rates e. and ep have the form e = kTke-Et/kT
(2)
Details of the analysis for comparable emission rates are given in reference 1. EXPERIMENTAL DETAILS Sample Preparation Thin films of AI0 .25Ga0 . As were fabricated using a Riber 32 MBE facility at the Space Vacuum Epitaxy Center (SVEC) at the University of Houston. The samples were grown on (100) SI GaAs substrates using In to mount the substrate to the sample holder. The substrate growth temperature was 670 C. The samples were Si doped to 1-2 x 1017 cm-3. Diodes were fabricated using thermally evaporated aluminum as a Schottky contact, and
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