Presentation on theme: "Charge carriers must move to accommodate the bias-dependent depletion layer edge in a diode. Meanwhile, the capacitance depends on the position of the."— Presentation transcript:
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. Defects create energy levels in the bandgap. If carriers are trapped, their motion is impeded and there is a delay in the capacitance response. 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 Ga 0.72 AsP 0.28 P 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. Thermally-activated reciprocal capacitance transients in a GaAsP diode Peter Simov and Tim Gfroerer, Davidson College Mark Wanlass, National Renewable Energy Laboratory Arrhenius plot of the fast and slow escape rates R F and R S 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. Capacitance transients plotted on a reciprocal scale. The results are surprisingly linear, indicating that a different transport mechanism is operating. 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. 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. 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. 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.