Transition from Amorphous to Crystalline Silicon: Effect of Hydrogen on Film Growth
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TRANSITION FROM AMORPHOUS TO CRYSTALLINE SILICON: EFFECT OF HYDROGEN ON FILM GROWTH C. C. TSAI, R. THOMPSON, C. DOLAND, F. A. PONCE, G. B. ANDERSON &B. WACKER Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT The transition from amorphous to crystalline silicon in films prepared in a hydrogen-diluted silane plasma has been studied. Emphasis was placed on the role played by hydrogen during film growth. Hydrogen is found to govern the film formation process by promoting the reverse process, 'etching'. By preferentially eliminating energetically unfavorable configurations during the film growth, hydrogen controls the atomic structure, hydrogen incorporation, and grain growth of the film. It affects the total hydrogen content in the film, as well as the way hydrogen is bonded to the silicon. Excessive hydrogen dilution, however, reduces the grain size by changing the columnar grain growth to a more spherical-like grain growth, and eventually eliminates film growth. With appropriate hydrogen dilution, plasma deposition was found to yield 'polycrystalline' silicon films which has a large distribution of grain sizes, with the largest grains being about 2000 A. INTRODUCTION While hydrogenated amorphous silicon (a-Si:H) has recently been shown to be a viable large 2area microelectronics technology [1], its low carrier mobility of about 1 V sec/cm can be a limiting factor in high speed device applications. It is therefore of great interest to see if its carrier mobility can be improved without the necessity of high temperature processing. Hydrogen dilution is known to produce microcrystalline silicon (mc-Si) in a glow-discharge silane plasma at relatively low temperature [2-5]. Thus the aim of this work was to investigate in detail the transition from amorphous to crystalline structure in silicon films prepared by glowdischarge plasma deposition employing hydrogen dilution. In particular, the role played by hydrogen during film growth was the focus of our study. With appropriate hydrogen dilution, plasma deposition was found to yield 'polycrystalline' silicon films which has a large distribution of grain sizes, with the largest grains being about 2000 A. EXPERIMENTAL Plasma deposition of phosphorus-doped silicon films was carried out in a conventional capacitively-coupled rf glow discharge system which has electrodes of 9 cm in diameter. Films ranging from 500A to 1 pm in thickness were deposited on Corning 7059 glass, Cr-coated glass and crystalline silicon substrates clamped to the heated anode. Reactant gas of SiH 4 mixed with 1% PH3 was diluted by H2 with the H2 concentration varied from 0% to 99%. In some cases H2 was replaced by Ar as a comparison. A range of plasma parameters was scanned to study the amorphousto-crystalline transition. Most results presented here correspond to a total gas flow rate of 200-400 sccm, a total gas pressure of 0.4 Torr, a substrate temperature of 200'C, and an rf power of 1OW. The films so obtained have been characterized by a wide range of techniques inc
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