Simov SESAPS 2006 poster

1
Thermally-activated reciprocal capacitance
transients in a GaAsP diode Peter Simov and Tim
Gfroerer, Davidson College Mark Wanlass, National
Renewable Energy Laboratory
Abstract When the reverse bias across a
semiconductor diode is changed, charge carriers
move to accommodate the appropriate depletion
thickness, producing a simultaneous change in the
device capacitance. Transient capacitance
measurements can reveal inhibited carrier motion
due to trapping, where the depth of the trap can
be evaluated using the temperature-dependent
escape rate. However, when we employ this
technique on a Ga0.72AsP0.28P n/p diode (which
is a candidate for incorporation in
multi-junction solar cells), we observe a highly
non-exponential response under a broad range of
experimental conditions. Double exponential
functions give good fits, but lead to
non-physical results. The deduced rates depend
on the observation time window and fast and slow
rates, which presumably correspond to deep and
shallow levels, have identical activation
energies. Meanwhile, we have discovered a
universal linear relationship between the inverse
of the capacitance and time. An Arrhenius plot
of the slope of the reciprocal of the transient
yields an activation energy of approximately 0.4
eV, independent of the observation window and
other experimental conditions. We note that in
some cases a temperature-independent transport
mechanism also appears to be present. The
reciprocal behavior leads us to hypothesize that
hopping, rather than escape into high-mobility
bands, may govern the transport of trapped holes
in this system.
Stacking layers of materials, each of which
absorbs a specific energy, can increase the
efficiency of a solar cell. But layer mismatch
creates more defects within the diode, which
affects the semiconductor properties as described
below.
Capacitance transients plotted on a reciprocal
scale. The results are surprisingly linear,
indicating that a different transport mechanism
is operating.
Here is the raw data plotted on a linear vertical
scale. Before numerical analysis, it appears to
follow thermally-activated exponential behavior.
Numerical analysis was conducted using single and
double exponential functions. We studied the
response in the fast and slow time windows shown
above.
Charge carriers must move to accommodate the
bias-dependent depletion layer edge in a diode.
Meanwhile, the capacitance depends on the
position of the charge carriers.
Arrhenius plot of the slope of the reciprocal
capacitance transients for the observation
windows indicated in the legend. The linearity
and time window independence of the activation
energy give credence to this analysis.

• Conclusions
• Capacitance transients are non-exponential and
  rates are incompatible with conventional thermal
  activation analysis
• The reciprocal of the capacitance varies linearly
  with time, and the slope yields a single thermal
  activation energy of 0.38eV.
• Thermally-activated reciprocal behavior is a
  characteristic of hopping transport.
• Acknowledgement
• This work was supported by the American Chemical
  Society Petroleum Research Fund.

Arrhenius plot of the fast and slow escape rates
RF and RS deduced from double exponential fits
for the observation windows specified in the
legend. The time window dependence and the
presence of two distinguishable rates with the
same activation energy is not plausible.
The transient capacitance plotted on a
logarithmic scale, on which exponential functions
would give straight lines. The response is
strongly non-exponential. Measurement
temperatures are given in the legend.
Defects create energy levels in the bandgap. If
carriers are trapped, their motion is impeded
and there is a delay in the capacitance response.