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
Defects create energy levels in the bandgap. If carriers are
trapped, their motion is impeded and there is a delay in the
capacitance response.
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.
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.
•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.