Growth and Characterization of Hydrogenated Amorphous Silicon using Liquid Organic Sources
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GROWTH AND CHARACTERIZATION OF HYDROGENATED AMORPHOUS SILICON USING LIQUID ORGANIC SOURCES K. GAUGHAN, S. HERSHGOLD, J.M. VINER AND P.C. TAYLOR Department of Physics, University of Utah, Salt Lake City, Utah 84112
ABSTRACT The uses of liquid sources such as tertiarybutylphosphine (TBP) for n-type doping in hydrogenated amorphous silicon (a-Si:H) and ditertiarybutylsilane (DTBS) and n-butylsilane (NBS) for hydrogenated amorphous silicon-carbon alloys (a-SiC:H) are described. A rf glow discharge process is employed to produce the doped a-Si:H and a-SiC:H thin films. Tertiarybutylphosphine (TBP) may ultimately be preferred over phosphine because TBP is less toxic, less pyrophoric and safer to implement. The gross doping properties of a-Si:H doped with TBP are the same as those obtained with phosphine, but there are some differences. N-butylsilane (NBS) and DTBS have been used to produce wide band gap (E0 4 2- 3 eV) a-SiC:H. INTRODUCTION The electronic properties of hydrogenated amorphous silicon (a-Si:H) can be controlled by substitutional doping in the gas phase by adding phosphine and diborane to silane to produce n-type and p-type films, respectively [1,2]. Unfortunately, phosphine and diborane are extremely hazardous gases. A less toxic liquid source, trimethylboron (B(CH.) 5 ), has been used to improve the electronic and optical properties of p-type films [3]. We have recently reported successful ntype doping of a-Si:H using tertiarybutylphosphine (TBP:C 4HlP) [4]. The fabrication of high quality doped a-Si:H films is desirable for many technical applications, in particular, amorphous silicon solar cells and thin-film transistors. The advantages of the a-Si:H-based devices are the ease and low cost with which they can be produced. Amorphous silicon-carbon alloys (a-SiC:H) have been used in solar cells as the window layer [5] and in light emitting diodes [6]. Mixtures of silane and various hydrocarbons such as methane and ethylene have been employed in a rf glow discharge process to produce a-SiC:H [5]. Liquid organic sources such as tetramethylsilane (TMS) and tetramethyldisilane (TMDS: Si 2H2(CH 3 )4 ) [7] have been employed to produce a-SiC:H. In this paper, we investigate ditertiarybutylsilane (SiH 2(C4 H,) 2) and n-butylsilane (CH,(CH2 ),SiH3 ) as potential sources for the deposition of a-SiC:H. The boiling points of DTBS and NBS are 128"C and 57"C, respectively. We are investigating these liquid organic sources, because they are generally less toxic, less pyrophoric and safer to implement than phosphine and silane. For example, exposure to a concentration of 2000 ppm of phosphine is lethal within a few minutes, but exposure to an equal concentration of TBP has only shown ocular irritation in animals [8]. N-butylsilane and DTBS are less pyrophoric than silane. EXPERIMENTAL A plasma lab pP glow discharge system was used to deposit a-Si:H. The reactor has a capacitively coupled electrode configuration and was operated at a frequency of 13.56 MHz. The2 electrodes are 24 cm in diameter and were supplied with a c
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