The complexity of a system, in general, makes it difficult to determine some or almost all matrix elements of its operators. The lack of accuracy acts as a source of randomness for the matrix elements which are also subjected to an external potential due to existing system conditions. The fluctuation of accuracy due to varying system conditions leads to a diffusion of the matrix elements. We show that, for single-well potentials, the diffusion can be described by a common mathematical formulation where system information enters through a single parameter. This further leads to a characterization of physical properties by an infinite range of single-parametric universality classes.
Polycrystalline silicon (poly-Si) films were fabricated by very high frequency (VHF)
plasma enhanced (PE) chemical vapor deposition (CVD) from SiF4 and H2 gas mixture with
small amounts of SiH4. Reactions and growth of poly-Si in the SiF4/H2/SiH4 system were
discussed together with the results obtained from in situ plasma diagnostics, and compared
with those obtained by microwave PECVD (MW CVD). As a result, similar relationships
among growth temperature, SiF4/H2 gas flow ratio and film structure to those obtained with
MW CVD were obtained with VHF CVD. For example, growth temperature could be reduced
to 100°C while keeping a high crystal fraction (>80%) when small SiF4/H2 gas flow ratios
were used. In contrast, under large SiF4/H2 gas flow ratios, crystal fraction rapidly decreased
with decreasing temperature. The role of fluorine-related species in the growth of poly-Si was
examined in relation to film microstructure and the results obtained from plasma
diagnostics. Finally, guiding principles to achieve high rate and/or low-temperature growth of
poly-Si by VHF CVD using SiF4/H2 gas mixtures were discussed.
Role of seed crystal layer which played in low temperature
growth of polycrystalline silicon (poly-Si) thin films was
investigated by two-step-growth (TSG) process. The TSG involves two
different deposition processes, which are called as `seed process' and
`growth process'. In order to satisfy the conflicting demands such as
low temperature, high rate and high quality crystal growth, the
deposition conditions in the seed and growth processes were
examined. As a result, it is confirmed that the seeding of high
quality crystal layer is effective to improve crystallinity of growing
poly-Si film, especially for the case grown at lower temperature than
300°C. In order to fully promote crystallinity, some thick
and high crystallinity seed layer is needed. In addition,
epitaxial-like growth on the seed layers can be realized by optimizing
deposition conditions both during the seed and growth processes. When
H2/SiF4 flow ratios larger than those used during the seed
process were used during the growth process, lower growth temperatures
were possible with maintaining a smooth interface between the seed and
the grown poly-Si layers. In essence, the hydrogen mixing
ratio and deposition temperature are complementary parameters, thus it
was possible to reduce deposition temperatures with maintaining large
crystal fraction and oriented structure, if the hydrogen mixing ratio
was increased to an appropriate amount. Furthermore, in situ
ellipsometry analysis indicated that, under optimal conditions, the
interface between the seed and the growing film can indeed be smooth
and epitaxial-like. TSG is a promising technique by which to fabricate
high quality poly-Si thin films on glass substrates at low
temperatures.
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