Hydrogenated Amorphous Silicon Alloys Prepared by CVD of Higher Silanes
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HYDROGENATED AMORPHOUS SILICON ALLOYS PREPARED BY CVD OF HIGHER SILANES MASUD AKHTAR AND HERBERT A. WEAKLIEM Chronar Corp., P.O. Box 177, Princeton,
NJ 08542
ABSTRACT Hydrogenated amorphous silicon may be deposited at relatively low temperatures, where the density of defects may be expected to be low, by the chemical vapor deposition (CVD) of higher silanes. This method is an attractive alternative to plasma deposition techniques. We describe here the preparation of a-Si:H and related alloys incorporating carbon, germanium, and fluorine. a-Si:H films were deposited on heated substrates in the range 365 0 C-445 0 C by CVD of Si 2 H6 and Si3H8 . The optical gap (Eg) ranged from 1.4 to 1.7 eV and the properties of films deposited from either Si 2 H6 or Si 3 H8 were quite similar. Wide band gap (Eg-_2 eV) alloys of a-SiC:H doped with boron were prepared by CVD of disilane, methyl silane, and diborane. We also prepared variable band gap a-SiC:H alloys by substituting F 2 C= CFH for methylsilane, and these films were found to have approximately 1-2% fluorine incorporated. The dark conductivity of the boron doped a-SiC:H alloys dep~sited from either carbon 7 source ranged from ix10to 6x10(ohm-cm)-I. We also prepared low band aap alloys of Si and Ge by CVD of trisilane and germane. The band gap of a film containing 20% Ge was 1.5 eV; however, the photoconductivity of the film was relatively low. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) has been prepared by CVD of disilane (Si 2 H6 ) in the temperature range of 380-473 0 C using static as well as flow deposition conditions (1,2,3). Although a-Si:H thus prepared has about 10 atom % bonded hydrogen, the control of optical band gap from 1.5 eV to 1.68 eV is possible, and the material can be easily doped p- or n-type, however no high efficiency photovoltaic devices have been obtained (2). The photoconductivity of CVD a-Si:H films is an order of magnitude less than that of glow-discharge deposited material (4,5). Since the decomposition temperatures of silanes SinH2n+ 2 decrease with increasing number of Si atoms, we have deposited a-Si:H from trisilane (Si 3 H8 ) employing lower temperatures as compared with those required for monosilane (6-8) and disilane (1,p). In addition, we have prepared several a-Si:H alloys with C,F and Ge by mixing disilane and trisilane with the appropriate feedstock gas. Such alloys could be useful in the fabrication of multijunction solar cells where variable band gap materials are desired (9), as protective coatings, and as diffusion barriers in thin film technology. EXPERIMENTAL Ultra-high purity disilane and trisilane were prepared by silent electric discharge (10) in silane (CCD Airco). Disilane
Mat, Res. Soc. Syup. Proc. Vol. 70.
1986 Materials Research Society
44
used for film deposition consisted of Si 2 H6 95%, Si 3 H8 1% and balance SiH4 . Trisilane used was Si 3 H8 96%, Si 2 H6 2% and the balance being SiH4 . No light hydrocarbon impurities greater than 0.1 ppm and no permanent gas impurities greater than 1 ppm were detec
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