Effects of low temperature anneals on the photovoltage in Si nanocrystals

Abstract

We report on the time decays of surface photovoltage (SPV) and SPV spectra for Si nanocrystals (nc-Si) embedded into a SiO2 matrix. After precipitation at 1150 °C anneal in Ar the SPV increases by a factor of ≈30 compared with the value observed in an oxidized Si substrate. An increase in the signal is accompanied by longer time decays in the SPV transients (roughly from tens to hundreds of microseconds). The separation of photoexcited electrons and holes at the nc-Si/SiO2 interface is expected to play a major role in increasing the SPV signal. We emphasize that annealing of nc-Si at 450 °C in either N2 + O2 or H2 results in a remarkable increase (up to 10-fold) in photoluminescence intensity, which is accompanied by a concomitant decrease in the SPV signal and modification of the SPV decay transients. Anneal in N2 + O2 ambient slightly accelerates the SPV decay, whereas anneal in H2 dramatically speeds it up. Employment of Fourier transform infrared absorption and x-ray photoelectron spectroscopy techniques allows us to gain insight about the nature of chemical bonds into the oxidized matrix. A hypothesis is suggested that the observed effects are attributable to different passivating abilities of H2 and N2 + O2 ambients. A simple model, which takes into account the capture of photoexcited carriers at the nc-Si/SiO2 interface and the varying passivation ability of the interface traps, is capable of explaining the observed changes in the SPV transients, even quantitatively. The results may be of interest for studying the passivation of dangling bonds at the nc-Si/SiO2 interface by SPV techniques and can be used in advancing the development of silicon based photovoltaic materials with high efficiency.