Hydrogenated Nanocrystalline Silicon Thin Film Transistor Array for X-ray Detector Application
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Hydrogenated Nanocrystalline Silicon Thin Film Transistor Array for X-ray Detector Application Kyung-Wook Shin1, Mohammad R. Esmaeili-Rad1, Andrei Sazonov1, and Arokia Nathan2 1 Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada 2 London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH, United Kingdom ABSTRACT Hydrogenated nanocrystalline silicon (nc-Si:H) has strong potential to replace the hydrogenated amorphous silicon (a-Si:H) in thin film transistors (TFTs) due to its compatibility with the current industrial a-Si:H processes, and its better threshold voltage stability [1]. In this paper, we present an experimental TFT array backplane for direct conversion X-ray detector, using inverted staggered bottom gate nc-Si:H TFT as switching element. The TFTs employed a nc-Si:H/a-Si:H bilayer as the channel layer and hydrogenated amorphous silicon nitride (a-SiNx) as the gate dielectric; both layers were deposited by plasma enhanced chemical vapor deposition (PECVD) at 280oC. Each pixel consists of a switching TFT, a charge storage capacitor (Cpx), and a mushroom electrode which serves as the bottom contact for X-ray detector such as amorphous selenium photoconductor. The chemical composition of the a-SiNx was studied by Fourier transform infrared spectroscopy. Current-voltage measurements of the a-SiNx film demonstrate a breakdown field of 4.3 MV/cm. TFTs in the array exhibit a field effect mobility (μEF) of 0.15 cm2/V·s, a threshold voltage (VTh) of 5.71 V, and a subthreshold leakage current (Isub) of 10-10 A. The fabrication sequence and TFT characteristics will be discussed in details. INTRODUCTION The commercialization of the direct conversion flat panel X-ray detector is in progress with the a-Si:H TFT technology [2, 3]. The direct conversion flat panel X-ray detector has advantages over the indirect conversion counterpart in the image quality, due to lack of lateral optical scattering in photodiode, and simple fabrication of photoconductor. On the other hand, the inverted staggered bottom gate nc-Si:H TFT with stable VTh over 3 to 5 hours of stress has been reported by our group [1]. Compatibility with a-Si:H production technology, along with the stability in VTh allow simpler readout scheme compared to that with a-Si:H counterpart. Therefore, we have fabricated a direct conversion X-ray detector backplane to evaluate the suitability of the nc-Si:H TFT for this application.
EXPERIMENT The TFT cross section and an optical micrograph of a single pixel are shown in Figs. 1 (a) and (b), respectively. We used 100 nm molybdenum (Mo) as the gate, and 300 nm a-SiNx as the gate dielectric. The bilayer channel comprised of 15 nm nc-Si:H capped with 35 nm a-Si:H. Subsequently, n+ doped layer and aluminum source/drain contacts are formed. As shown in Figure 1 (b), each pixel consists of a TFT (In-pixel nc-Si:H TFT) and the Cpx. Here, the aspect ratio (WL) of the In-pixel TFT is 300
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