Interpretation of the a-Si:H DLTS and ICTS Experimental Data
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INTERPRETATION OF THE a-Si:H DLTS AND ICTS EXPERIMENTAL DATA RU-QI HAN-, K. L. NGAI** AND J. RUVALDS* *Physics Dept., University of Virginia, Charlottesville, VA 22901 Research Laboratory, Washington, D. C. 20375-5000
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ABSTRACT The isothermal capacitance transient spectroscopy (ICTS) data of aSi:H is consistently analyzed in terms of a relaxation function with a fractional exponential time decay of the form *(t)=exp[-(t/r*) ] with A0.8. The anomalous variation of the effective relaxation time x* (and hence that of the attempt-to-escape frequency v* and the capture cross section 0* observed by Okushi and coworkers) with energy and temperature is shown to follow an additional prediction of the time dependent relaxation rate coupling model developed by Ngai and coworkers for relaxations in many complex systems. A modified energy scale is extended from the ICTS analysis which brings its electronic density of states structure in closer agreement with results obtained by DLTS experiments. INTRODUCTION The position of the doubly-occupied dangling bond D state in a-Si:H within the mobility gap remains a controversial problem. Even within transient capacitance techniques of measurement, rate-windows deep level transient spectroscopy (DLTS) using temperature scanning places the D center at about 0.85eV below Ec [1] while isothermal capacitance transient spectroscopy ICTS [2] which involves isothermal time domain measurement of junction capacitance locates D at 0.52eV below E c . Efforts have been made to resolve this energy scale difference. For example, Tanaka and Okushi [2] suggested the origin of this difference is the energy E and temperature T dependences of the attempt-to-escape frequency v* and the electron capture cross-section 0* revealed in their measurements. Lang et al attributed this disagreement to leakage currents in the ICTS diodes. The problem has not been resolved [3]. Conventionaily, emission and capture processes in DLTS and ICTS are assumed to be exponentially decaying functions of time, e.g. exp(-e n t) and exp(-t/r ). In this paper we relax this assumption and generalize these time Adependences to ýhe fractional exponential (Kohlrausch form: exp(e't) , and exp-(t/T*) , 0l, the primitive rate e is slowed down to c n Mat. Res. Sa.
Symp. Pro.
Vol 70.
1986 Materials Rtesarch Saiely
162
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