Predictive and Descriptive Models for Transient Photoconductivity in Amorphous Oxide Semiconductors
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Predictive and Descriptive Models for Transient Photoconductivity in Amorphous Oxide Semiconductors Jiajun Luo1 and Matthew Grayson1 1 Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, U.S.A. ABSTRACT Amorphous oxide semiconductors (AOS) are important candidates for next generation display transistors, but instability under illumination with month-long transients is a significant drawback and may limit broader use. Several models have been developed to fit transient photoconductivity observed in AOS and relate it to a spectrum of weighted time constants, equivalent to either a density of states distribution of deep traps within the activated energy model, or to a time-dependent relaxation time constant in other models. In this work, we classify fits of the time constant spectrum to the transient data as either “descriptive” if they make no presumption about the spectral shape, or “predictive” if they assume a spectral shape a priori characterized by a few simple parameters. By fitting both descriptive and predictive models to simulated transients, it is observed that the best fit converges for the descriptive model if the measurement duration exceeds the mode (“peak”) value of the time constant distribution. The predictive models can converge orders of magnitude faster, but rely on a proper identification of the correct lineshape a priori. Therefore, it is recommended that first an unbiased descriptive model of sufficient measurement duration be performed. Then the known lineshape can be applied as a predictive model for future measurements, reducing subsequent measurement durations by orders of magnitude. INTRODUCTION Amorphous oxide semiconductor (AOS) materials have been widely applied in flat panel displays due to their superior electrical properties over amorphous silicon. [1, 2, 3] However, many works report a large and slow non-exponential transient photoconductivity in AOS, presumably due to deep level defects. [4, 5, 6] The simplest fit to this non-exponential transient is as a summation of simple exponential transients with a weight spectrum for the different time constants. [7, 8] Alternatively, other models attempt to reduce the number of fit parameters by assuming a specific spectral lineshape a priori. Of those, some studies assumed the spectrum to have a Gaussian lineshape arising from the statistical nature of an amorphous systems, [9, 10] while other experimental reports showed that the transient photoconductivity measured in AOS fits a stretched exponential function. [4, 11] Regardless of the fit method, the spectrum can be physically interpreted either as a continuous density of trap states (DOS) under the assumption of an activated energy model, [4, 12] or as a time-dependent relaxation time constant, whereby a power-law time dependence in the time-constant yields the frequently observed stretched exponential transient. [13] In any case, the time constant spectrum serves as an essential fingerprint, so this work will focus on the effectiveness of various fi
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