Modulated photoconductivity in the low frequency regime for semiconductors having multiple species of traps

J. A. SCHMIDT; C. LONGEAUD; R. R. KOROPECKI; R. ARCE; J. P. KLEIDER

Colorado, USA

Conferencia; 22nd International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS22); 2007

NREL

Modulated photoconductivity has proven to be a valuable tool to estimate the density of states within the gap of semiconductors. However, only the high-frequency regime of the method has been extensively studied, while the low frequency modulated photoconductivity (LF-MPC) has been the object of researches only recently. If a single species of states is present within the forbidden gap, it was shown that LF-MPC is able to provide achúrate information on the density of states (DOS) distribution. In this paper we extend our previous theoretical analysis of the LF-MPC method to the case where different species of traps are present. We solve the complete system of equations that describe the experiment, and we apply simplifying assumptions to deduce a simple formula relating the photocurrent phase shift to the DOS at the majority carriers’ quasi-Fermi level. By means of numerical calculations we discuss the accuracy of our development, and of previous aproxímate formulas (the ones introduced by Brüggemann and Kounavis), to reconstruct a given DOS. One of the most important factors that influence the accuracy of the DOS reconstructions, especially when several species of gap states are present, is to extract reliable values for the capture coefficients. We test the different methods proposed for that purpose, applying the proposed criteria to computer-generated data. Finally, we present experimental and theoretical evidence that, under sensitization conditions, the sign of the phase shift of the modulated photocurrent reverses, and the steady-state photoconductivity depends superlinearly on the generation rate.