Low dark current and high dynamic range a-Si:H MSM photodetector for large area medical imaging Sina Ghanbarzadeh *a , Shiva Abbaszadeh a , Michael Adachi †a , Karim S. Karim a a Dept. of Electrical and Computer Engineering, University of Waterloo, Ontario N2L 3G1, Canada ABSTRACT Metal-Semiconductor-Metal (MSM) photodetectors are attractive as sensors due to their ease of fabrication and compatibility with thin film transistor fabrication process primarily because there is no P + doped layer. We report an a- Si:H MSM lateral structure with low dark current, high dynamic range and comparable sensitivity to conventional p-i-n photodiodes. These improvements are achieved by the introduction of a thin polymer layer as a blocking contact. The fabricated amorphous silicon based MSM detector exhibits a photo-response of more than 3 orders of magnitude to a green light source (Ȝ = 525nm) with intensity of 73μW/cm 2 . In comparison to vertical p-i-n structures, the reported MSM lateral devices show gains in terms of dynamic range, ease of fabrication (no p+ layer), and faster speed at the cost of slightly reduced responsivity. The experimental results of dark and photocurrent measurements as well as the responsivity for two in-house fabricated MSM structures at different bias voltages and light intensity are presented. This results are promising and encourage the development of a-Si:H lateral MSM devices for indirect conversion large area medical imaging applications and especially low cost flat panel computed tomography. Keywords: MSM photodetectors – Amorphous Si – indirect conversion X-ray imaging – dark current 1. INTRODUCTION Amorphous silicon technology has already found its place in X-ray imaging, especially in indirect detection [1]. Radiation resistivity of amorphous silicon and its suitability in fabrication and integration with large area flat panels resulted in widely use of this material in X-ray imaging markets. The most common technique in X-ray detection is indirect conversion which utilizes scintillator to convert X-ray to visible light. In this type of detection the total conversion gain is product of scintillator conversion efficiency and the effective quantum efficiency [2]. Hence in order to achieve maximum efficiency, the peak of emission spectra of scintillator should match the absorption spectra of detector. The advantage of indirect conversion technique in comparison with direct technique is high absorption of X- ray. However, the resolution of the detector is limited due to light scattering of scintillator. Metal-Semiconductor-Metal (MSM) photodetectors are attractive due to their ease of fabrication primarily because there is no p+ doped semiconductor layer, thus making it compatible with industry standard amorphous silicon thin film transistor electronics processing. Recently a-Si:H MSM lateral detectors for indirect medical imaging application has been reported [3]. However the earlier devices exhibited high dark current which is problematic for integration mode imaging. In the other words, they were limited in term of dynamic range. In this study we demonstrate an a-Si:H MSM lateral structure with low dark current, high dynamic range and comparable sensitivity to conventional p-i-n photodiodes. It has been understood that dark current in MSM diodes stems from two mechanisms: thermionic emission and tunneling, which both strongly depend on width and height of the barrier at metal/semiconductor interface. It had been shown before that the height of barrier at metal/amorphous Si is fairly dependent on metal work function due to similar densities and energy distribution of surface state at metal/a-Si interface for different metals [4]. However the width of the barrier is strongly dependent to electrical field and trapped charges at interface and their releases over time. These charges trapped at interface arise from dangling bond and defect state in mid-gap of amorphous Si [5]. The improvements of proposed structure in terms of dark current and stability are achieved by the introduction of thin polymer layer as blocking layer. The primary objective is to investigate the effect of thin Polyimide (as blocking layer) * sghanbar@uwaterloo.ca † Current affiliation: Postdoc, University of Toronto Medical Imaging 2013: Physics of Medical Imaging, edited by Robert M. Nishikawa, Bruce R. Whiting, Christoph Hoeschen, Proc. of SPIE Vol. 8668, 86683U · © 2013 SPIE · CCC code: 1605-7422/13/$18 · doi: 10.1117/12.2007973 Proc. of SPIE Vol. 8668 86683U-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/26/2014 Terms of Use: http://spiedl.org/terms