Research on silicon-carbon alloys and interfaces. Final subcontract report, 15 February 1991--31 July 1994 Page: 6 of 83
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p-type Doping of a-Si,C:H
We deposit p' a-Si,C:H films using a silicon target sintered with 1 atomic % boron. For a Tauc
bandgap of 1.85 eV, the dark conductivity is 8x10-4 /-cm, the thermal activation energy is 0.28 eV,
and the photoconductivity mobility-lifetime product is 3x108 cm2/V. These results are slightly better
than for p+ a-Si,C:H films grown by plasma-assisted chemical vapor deposition. We discuss the film
microstructure, as reflected in infra-red absorption and hydrogen thermal evolution spectra, as a
function of the absolute composition.
Chemical Reduction of Transparent Conductive Oxide Substrates
Very thin layers of hydrogenated or unhydrogenated amorphous silicon are deposited on tin oxide
(SnO2) or zinc oxide (ZnO) using different substrate temperatures and H2 partial pressures. The
deposition processes are monitored by spectroscopic ellipsometry and X-ray photoelectron
spectroscopy. We determine that either H or Si in the deposition flux leads to SnO2 reduction in the
early stages of film growth. However, the contribution of Si to the oxide reduction is secondary to
that of H atoms. No reduction of ZnO is found, even under high hydrogen fluxes.
Sub-surface Transformation of a-Si:H to puc-Si:H
We use spectroscopic ellipsometry to monitor the nucleation and growth of gc-Si:H on a-Si:H. This
interface is important for multi-junction devices which incorporate microcrystalline doped layers. We
increase the hydrogen pressure to promote gc-Si:H formation and observe several effects: initially, H
penetrates 45 A into the a-Si:H substrate and increases its hydrogen content; then 55 A of hydrogen-
rich a-Si:H deposits; finally, pc-Si:H nucleates on top of this 100 A thick, high H-content a-Si:H
interface layer. As pc-Si:H grows, the thickness of the amorphous interface layer decreases by 40A.
These results demonstrate that a-Si:H can be transformed into gc-Si:H in a sub-surface region under
appropriate conditions. The technological implication is that the phase boundary can be made
relatively sharp.
Improved Film Adhesion by UV-ozone Substrate Cleaning
Substrate cleaning and ion bombardment play important roles in the adhesion, morphology and
structure of hydrogenated amorphous silicon and silicon-carbon alloys. We examine differences in
film adhesion as a function of substrate cleaning method and ion bombardment energy during
growth. The UV-ozone cleaning technique, as used industrially for various oxide substrates, is found
to increase film adhesion. Lowering the ion bombardment energy by electrically floating the
substrate also improves adhesion, presumably through a reduction in the stress of the film.v
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Abelson, J. R. Research on silicon-carbon alloys and interfaces. Final subcontract report, 15 February 1991--31 July 1994, report, July 1, 1995; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc621586/m1/6/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.