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A4.8.1

Bottom-Gate TFTs with Channel Layer Grown by Pulsed PECVD Technique David J. Grant, Czang-Ho Lee, Arokia Nathan, Ujjwal K. Das1 , Arun Madan1 Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada 1 MVSystems Inc., 17301 W. Colfax Avenue Suite 305, Golden, CO 80401, U.S.A. ABSTRACT In this paper, we report on nc-Si:H thin films deposited by the pulsed PECVD technique at a temperature of 150◦ C and TFTs made using this material. RF power and silane flow rate were varied in order to study the effect of different levels of crystallinity on the film. Electrical conductivity, Hall mobility, optical transmittance, and Raman backscattering were measured on films of two different thicknesses. From the Raman data we see that the 50 nm films with hydrogen dilution are mostly amorphous, indicating the presence of a thick incubation layer. The values obtained for the conductivity, mobility, and optical gap varied depending on the processing conditions and these results are discussed. Bottom-gate TFTs were fabricated using a pulsed PECVD channel layer and a SiN gate dielectric. The TFTs’ extracted parameters are µ sat ≤ 0.38 cm2 / (V · s), Vt,sat ≥ 7.3 V, Ion/o f f > 106 , and S < 1 V/decade. The TFT performance and material properties are presented and discussed. INTRODUCTION Hydrogenated amorphous silicon (a-Si) is used in thin-film transistors (TFTs) for flat-panel displays (FPD) and large-area imagers, and it is also a promising photovoltaic material. a-Si TFTs have a low off-current and sufficient on-current for most applications; however, they suffer from poor carrier mobility and threshold voltage (Vt ) shift. Its low mobility (µ ) places a limitation on pixel sizes for display and imaging applications, and its poor hole mobility prohibits a usable p-type device. Its drifting threshold voltage also means that it cannot be used easily or reliably in column multiplexer and row shift register circuits [1]. Polycrystalline silicon can overcome the disadvantages of a-Si TFTs; however, it has some drawbacks, with regards to cost and reliability. Alternatively, nanocrystalline silicon (nc-Si:H) is deposited by plasma-enhanced chemical vapour deposition (PECVD) and can thus be easily integrated into standard PECVD systems with little additional cost. In theory, it can provide equal or increased mobility and improved stability over its amorphous counterpart due to its crystallinity; however, in practice this is not always the case [1]. Recently, nc-Si:H films deposited by pulsed PECVD for use in solar cells have been reported [2]. Pulsed PECVD is the same as conventional PECVD at 13.56 MHz, except the plasma is modulated with a frequency in the kHz range. During the off-cycles, negatively charged particles can be neutralized, thus reducing the density of powder particles [3]. This allows the growth rate to be increased without compromising the material’s quality. Because of these advantages and its success so far as a solar cell material, pulsed PECVD may be a promising depo