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0989-A21-07

Evolution of Structural and Electronic Properties in Boron-doped Nanocrystalline Silicon Thin Films Hyun Jung Lee1, Andrei Sazonov1, and Arokia Nathan2 1 Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada 2 London Centre for Nanotechnology, University College Lodon, London, WC1H 0AH, United Kingdom ABSTRACT We report on the boron-doping dependence of the structural and electronic properties in nanocrystalline silicon (nc-Si:H) films directly deposited by plasmaenhanced chemical vapor deposition (PECVD). The crystallinity, micro-structure, and dark conductivity (σd) of the films were investigated by gradually varying a ratio of trimethylboron [B(CH3)3 or TMB] to silane (SiH4) from 0.1 to 2 %. It was found that the low level of boron doping (≤ 0.2 %) first compensated the nc-Si:H material which demonstrates slightly n-type properties. As the doping increased up to 0.5 %, the maximum σd of 1.11 S/cm was obtained while high crystalline fraction (Xc) of the films (over 70 %) was maintained. However, further increase in the TMB-to-SiH4 ratio reduced σd to the order of 10-7 S/cm due to a phase transition of the films from nanocrystalline to amorphous, which was indicated by Raman spectra measurements. P-channel nc-Si:H thin film transistors (TFTs) with top gate and staggered source/drain contacts were fabricated using the developed p+ nc-Si:H layer. The fabricated TFT exhibits a threshold voltage (VTp) of -26.2 V and field effective mobility of holes (µp) of 0.24 cm2/V·s. INTRODUCTION Low-power complementary (n- and p-channel) operation of TFT circuits is of great importance for highly functional and ultra-compact system-on-panel. While laserannealed polycrystalline silicon (poly-Si) TFTs, one of the promising candidates for this purpose, still suffer from relatively high manufacturing cost, complex processing, and non-uniform electrical characteristics over large area [1], there has been considerable interest in nc-Si:H TFTs as a high-performance and low-cost alternative [2]. However, achieving heavily doped p-type layers remains an issue for the high-performance pchannel nc-Si:H TFTs [3], [4]. In this work, we investigate the effect of boron doping on the structural and electronic properties in nc-Si:H thin films and its application to TFTs by using TMB as a doping gas, which has the increased thermal stability over diborane (B2H6) currently used for the TFT fabrication [5].

EXPERIMENTS Film Deposition and Characterization The films were deposited on Corning 1737 glass substrates using a conventional (13.56 MHz) PECVD system (PlasmaTherm 790 Series). The substrate temperature, RF power density, pressure, and hydrogen dilution ratio, [H2] / ([H2] + [SiH4]) × 100 %, were fixed at 260 °C, 70 mW/cm2, 900 mTorr, and 99 %, respectively. The structural and electronic properties of the boron-doped films were studied as a function of a TMB-toSiH4 ratio, [TMB] / [SiH4], which was gradually varied from 0.1 to 2 %. The growth rate (rd) was calculated from the