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In this study low temperature
photoconductivity of undoped hydrogenated amorphous silicon(a-Si:H)
thin films have been studied to investigate the effect of native and
Staebler-Wronski defects. The study covers undoped a-Si:H films
prepared by various deposition techniques such as DC glow discharge,
RF-PECVD with and without H-dilution, RF magnetron sputtering and
hot-wire(HW) CVD.
In the annealed state, the
samples were characterized using temperature dependence of dark
conductivity, steady-state photoconductivity, s ph,
versus light intensity at room temperature and steady-state
photoconductivity versus temperature down to 90 0K at
three different intensities. Activation energy ,EF, of
the samples changes from 0.60 eV to 1.0 eV. s ph shows a
few orders of magnitude higher values from the dark conductivity and
its magnitude is sample dependent due to differences in deposition
conditions. The intensity dependence of s ph ,g , (s ph
a Fg ) is close to unity and varies between 0.70 to 0.90,
indicating recombination kinetics through the midgap defect states
in the bandgap of a-Si:H. Low temperature photoconductivity versus
1000/T spectrum shows three distinctly different regions. In Region
I, s ph decreases with temperature until a transition
temperature. Then Region II begins, where s ph begins to
increase resulting a peak in spectrum or remains to be unchanged
until a second transition temperature to Region III, where s ph
continuously decreases with T. Transition temperatures and the
degree of increase in s ph in Region II is sample
dependent. These results indicate the presence of at least two
different types of midgap defect states in the bandgap and
exponential tail state present in the annealed state.
In the light soaked state, Staebler-Wronski
effect (SWE) was investigated after exposing the samples to white
light illumination of a few suns intensity. The characterization
involves dark conductivity and steady-state photoconductivity at
room temperature and s ph versus temperature down to 90 0K
for different intensities. Dark conductivity values decreased a
certain factor indicating a slight shift in EF through
midgap. s ph values decreased substantially from its
annealed values due to creation of Steabler-Wronski defects in the
bandgap. The intensity dependence of s ph become almost
equal and close to unity for all the films even it shows slight
variation in the annealed state. The shape of low temperature
photoconductivity spectra becomes almost the same for all samples
even drastic differences were observed in the annealed state. The
spectrum is mainly dominated by only two regions. Region I dominates
from room temperature down to 170 0K, where s ph
decreases with a constant slope as T decreases. After that
temperature, Region II sets in. s ph remains to be
constant until temperature used in this study. Region III can only
be detected at higher intensity and temperatures lower than 90 0K.
Results indicate that more defects around the midgap are created by
light, which decrease s ph and relatively less defects
are created away from midgap and closer to band edge, which improve
s ph instead of decreasing it as temperature decreases.
The defect states in Region I responsible for decreasing s ph
are more likely that they are neutral silicon dangling bond defects
,D0, and those in Region II responsible for increasing s ph
are non-D0 defect states. They act as photosensitising
defects with a very low capture cross-sections for electrons. They
could be charged silicon dangling bonds ,D+ and D-,
or floating bonds results in defect models proposed for a-Si:H. |