The mechanism of laser-assisted CVD of germanium
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I. INTRODUCTION Several years ago, Eden and co-workers1"3 examined the effect of laser irradiation on chemical vapor deposition (CVD) of silicon and germanium from the decomposition of silane and germane. With germane, they found that no film would grow on a quartz substrate below 700 K in the absence of irradiation. However, if they irradiated the substrate with an ArF or KrF laser, they could get films to grow at temperatures down to 610 K. Later experiments showed that films would also grow when the laser beam is focused parallel to the substrate.4 Eden and co-workers showed that the growth process was photolytic; no growth was observed when they irradiated with a XeCl laser.1 Eden et al. also used emission spectroscopy to identify several species in the gas phase above the growing film.3 They proposed a mechanism for the gas phase photolysis of germane. However, it is unclear how to relate what they observed in the gas phase to film growth. The objective of the work here was to examine how films actually grow during laser-assisted CVD (LCVD) of germane. Three kinds of measurements were done: sampling of the species impinging on the substrate, laser surface photochemistry, and film growth studies. The results "' Address correspondence to this author. 634
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J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
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of the measurements were then used to infer a plausible route for film growth. The sampling measurements will be described first. The objective of these measurements was to try to identify the major species impinging on the substrate during laser irradiation. The work came about because one possible mode of growth would be for the laser to convert some of the source gas to an unstable intermediate. The intermediate could then condense and/or pyrolyze on the substrate to yield a film. Previous work by Eden et al.3 had identified several species in the gas phase during film growth. However, the flux of these species onto the substrate was largely unknown. In the work reported here, the flux of the species impinging on the substrate was measured. These data were then used to identify species which were arriving at the substrate rapidly enough to condense and/or pyrolyze on the substrate at an appreciable rate. The second set of experiments was an attempt to look for laser-induced surface photochemistry. The work came about because a second mode of growth would be for the laser to decompose some of the germane adsorbed on the surface of the substrate. In Eden's initial work,1'2 film growth was observed only where the substrate was irradiated. Later data showed that films will grow even when the laser does not hit the substrate.4 However, the growth rates are lower when the substrate is not irradiated. There is the issue of sample heating, which could account for © 1989 Materials Research Society
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T. R. Gow, D.G. Coronell, and R. I. Masel: Germanium LCVD
some of the effects. Nonetheless, laser surface photochemistry could have a major contr
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