Continuous Multi-exponential Method for Analyzing Transient Photoconductivity in Amorphous Oxide Semiconductors
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Continuous Multi-exponential Method for Analyzing Transient Photoconductivity in Amorphous Oxide Semiconductors Jiajun Luo1 and Matthew Grayson1 1 Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60201, U.S.A. ABSTRACT Amorphous oxide semiconductors (AOS) are promising candidate materials for thin film transistors in display devices, but one major challenge for mass application is their instability under illumination. In this work, a theoretical method for analyzing transient photoconductivity response in such AOS thin films is reviewed, namely the continuous multi-exponential model. This model can deduce a continuous distribution of decay time constants representing activation energy levels in an AOS, and is shown to reliably reproduce a model of density of states (DOS) of mid-gap traps assumed to be responsible for the transient photoconductivity. Provided the data collection time is sufficiently long, the continuous multi-exponential model was verified to reconstruct the modeled continuous DOS spectrum, thus providing a powerful tool to analyze photoresponse in AOS. This method has the advantage that no prior assumptions about the form of the density of states are needed, but the drawback that long data collection times are required for the transient to be fully relaxed. INTRODUCTION Amorphous oxide semiconductors (AOS), as channel materials for thin film transistors (TFTs), are increasingly applied in the display industry [1-3]. Comparing with TFTs based on amorphous silicon (a-Si) or polycrystalline silicon (poly-Si), AOS-based devices have the advantage of high mobility and ease of manufacturing. A typical AOS-based TFT can reach a mobility over μ = 10 cm2/Vs, over one order of magnitude larger than that of a-Si. This high mobility is essential for making high resolution displays [2,4]. In addition, AOS thin films can be uniformly deposited over a large area with low cost, whereas poly-Si thin films suffer from inhomogeneity over large areas [2,5]. Recent reports also demonstrated solution-base process to fabricate AOS thin films, which can further reduce the cost [6]. However, one major challenge that may limit mass application of AOS thin films is its instability under illumination. Previous reports showed a large and slow photoconductivity response in AOS thin films [7,8]. The slow photoresponse in AOS can be related to deep level density of states around 1 eV below the conduction band mobility edge [9]. An identification of the energetic distribution of the states responsible for photoresponse can be used in future work to optimize the fabrication process and improve device stability. Prior works have used sum of exponential decays with three decay time constants and weighted amplitude [8] and stretched exponential function [7,10,11] to fit the transient response. Here we will demonstrate another method called continuous multiexponential method developed originally for amorphous silicon and polymers that has not been applied yet to the AOS system. THEORY
In AOS, electron
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