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Using Hot Wire Chemical Vapor Deposition (HWCVD), also known as
thermocatalytic
decomposition, heterogeneous silicon thin films can be obtained
with a widely varying degree of order and
crystallinity. Specific parameter regimes have been identified
which allow the deposition of films with a
structure ranging from purely amorphous to fully polycrystalline.
In polycrystalline Si, all hydrogen appears
in isolated, compact monohydride bonds. A comparison of XTEM and
Raman studies confirmed that the
2000 cm-1 infrared mode indeed originates from a completely crystalline
region and that there is no amorphous
tissue in these films. The 2000 cm-1 vibration is due to Si-H bonds at
completely coalescent crystal faces
(between adjacent crystals). In films with this type of crystallinity,
oxygen incorporation is greatly reduced,
both during growth and after completion. The heterogeneous growth has
been utilized in two types of
devices, thin film transistors (TFTs) and thin film solar cells.
TFTs have been made exhibiting excellent
stability. The field-effect channel of these transistors consists of
amorphous silicon hosting nanocrystalline
domains which yields TFTs with a high mobility of 1.5 cm 2 V-1 s-1,
virtually without the usual threshold
voltage instabilities. Solar cells with an intrinsic poly-Si absorber
layer have also been further optimized by
deliberately profiling the active layer. A stepwise profiling sequence
has been developed, starting from
immediate-nucleation growth of small random crystallites to continued
singly oriented growth of columnar
polycrystalline material at a deposition rate of 5 Å/s. These n-i-p solar
cells on stainless steel substrates
presently have 4.41 % conversion efficiency. The short circuit current
density is as high as 19.95 mA/cm2
while the light absorbing i-layer is only 1.2 µm thick and no enhanced
back reflector is used.
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