Metastable defects by low-intensity pulsed illumination of hydrogenated amorphous silicon
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Metastable defects by low-intensity pulsed illumination of hydrogenated amorphous silicon Stephan Heck and Howard M. Branz National Renewable Energy Laboratory, Golden, CO, 80401
ABSTRACT Illumination of hydrogenated amorphous silicon (a-Si:H) samples with short (e.g., 40 microsecond) pulses of red light produces a smaller metastable absorption increase in the defect region than continuous illumination of the same intensity for the same integrated exposure time. The defect absorption was measured at 1.3 and 1.4 eV by use of the constant photocurrent method (CPM). This smaller degradation is also observed in the photoconductivity. Careful measurement of the film temperature with several techniques confirm that the film temperature rises by less than 5 ˚C, under continuous illumination. However, several experiments suggest that the suppressed degradation during pulsed illumination is caused primarily by annealing during the dark time between pulses, even at 15 to 26 ˚C.
INTRODUCTION Degradation of the electronic properties of hydrogenated amorphous silicon (a-Si:H) has remained an important and controversial topic since its discovery by Staebler and Wronski in 1977 [1, 2]. The creation of metastable threefold-coordinated Si dangling bonds by electron-hole (e-h) recombination [3] would result in microsecond timescales for the onset of degradation, while other models [4] suggest an intermediate precursor to degradation that may rise more slowly. To test the intermediate-precursor hypothesis we degrade a-Si:H films with light pulses long enough (≥ 40 µs) that we can neglect the rise and fall time of the e-h recombination rate. The resulting changes in photoconductivity (σ) and defect absorption (α) are compared to degradation by continuous (CW) light of same peak intensity and same accumulated exposure time. We find less defect creation for the pulsed compared to the CW exposure. However, several experiments show that room-temperature annealing during the exposure can account for the observed difference between pulsed and CW degradation. Our experiments appear to exclude models in which the onset of degradation is delayed by more than 40 µs.
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EXPERIMENTAL Samples We report detailed results for a device-quality a-Si:H films grown by plasma-enhanced (PE) chemical vapor deposition (CVD) at the Electrotechnical Laboratory (ETL) in Japan and at the University of Chicago. Both samples have coplanar thermally evaporated chromium electrodes on top. We confirmed our main findings on PECVD samples obtained from the University of Marburg, the National Renewable Energy Laboratory (NREL) and on a hot-wire CVD sample from NREL. Procedures For good reproducibility of the data, we anneal, degrade and measure samples in-situ, i.e., once mounted the sample is not moved between experiments. The samples are exposed with the focused light of a red laser diode (665nm) that produces 200 mW/cm2 on the sample. The laser diode is driven by a power supply which can be externally modulated with a function generator to produce light pulses
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