Towards an All-Hot-Wire TFT: Silicon Nitride and amorphous Silicon deposited by Hot-Wire Chemical Vapor Deposition
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Towards an All-Hot-Wire TFT: Silicon Nitride and amorphous Silicon deposited by Hot-Wire Chemical Vapor Deposition B. Stannowski, M.K. van Veen, and R.E.I. Schropp Utrecht University, Debye Institute, Physics of Devices, P. O. Box 80000, 3508 TA Utrecht, the Netherlands ABSTRACT We present thin-film transistors with both amorphous silicon and silicon nitride deposited by hot-wire chemical vapor deposition. Hot-wire amorphous silicon with good electrical properties was deposited from the decomposition of silane at a substrate temperature of 250 ◦ C. For Hot-wire silicon nitride we used silane and ammonia at a substrate temperature of 340 ◦ C. In this paper we address structural and electrical properties of this material. A high ammonia flow results in porous films that exhibit post-deposition oxidation. By limiting the ammonia/silane ratio to 30, compact layers with a hydrogen content of only 10 at.% and a refractive index of 1.95 are obtained. Using this layer as gate dielectric results in thin-film transistors with good switching behavior and a field-effect mobility of 0.3 cm2 /Vs. INTRODUCTION Hot-Wire Chemical Vapor Deposition (HWCVD), also referred to as Catalytic (Cat-)CVD, is a promising technique for the deposition of hydrogenated amorphous silicon (a-Si:H) and silicon based alloys [1]. The major advantages compared to the conventionally used Plasma Enhanced (PE) CVD are a high deposition rate [2] and the potential for a cost efficient up scaling to large-area deposition [3]. Furthermore, it was shown that thin-film transistors (TFTs) incorporating HWCVD a-Si:H have a superior stability upon gate-bias stress [4, 5]. For TFTs it is desirable to deposit all layers with one technique. Therefore, HWCVD silicon nitride (SiNx ) has been developed in our group [6] to replace the PECVD SiNx , thus far used as the gate dielectric in our TFTs. Few other groups successfully deposited HWCVD SiN x at low substrate temperatures of 200 - 400◦ C. Stoichiometric layers with a hydrogen content < 10 at.% have been deposited [7, 8]. Recently, the first All-Hot-Wire-TFTs, incorporating SiN x and a-Si:H deposited by HWCVD, were presented [9]. HOT-WIRE AMORPHOUS SILICON Deposition of a-Si:H and SiNx was performed in our multi-chamber deposition system PASTA [10]. Two hot tantalum filaments are mounted few centimeters underneath and parallel to the substrate. In a catalytic process the source gases, fed into the reaction chamber from the side, are decomposed and the deposition precursors diffuse towards the substrate. HWCVD a-Si:H was deposited by the decomposition of undiluted silane (SiH 4 ) at a pressure of 20 µbar and a filament temperature of 1750◦ C. The substrate temperature was only 250◦ C. A deposition rate of 1.0 nm/s was achieved. From the Si–H rocking mode at 630 cm−1 in the Fourier-Transform Infrared (FTIR) absorption spectrum we calculated a hydrogen content of (12 ± 1) at.%. The micro structure parameter was calculated as R∗ = I2090 /(I2000 + I2090 ) = 0.25 ± 0.05, from the integrated absorption at A17.3.1
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