On the observation of electron-hole liquid luminescence under low excitation in Al₂O₃-passivated c-Si wafers

We report on low-temperature photoluminescence (PL) from aluminum oxide (Al₂O₃)-passivated c-Si wafers, which surprisingly exhibits clear signature of the formation of the so-called electron-hole liquid (EHL), despite the use of excitation powers for which the condensed phase is not usually observed in bulk Si. The elevated incident photon densities achieved with our micro-PL setup together with the relatively long exciton lifetimes associated with a good quality, indirect band-gap semiconductor such as our float-zone c-Si, are considered the key aspects promoting photogenerated carrier densities above threshold. Interestingly, we observe a good correlation between the intensity of the EHL feature in PL spectra and the passivation performance of the Al₂O₃ layer annealed at different temperatures. The change in the extension of the sub-surface space-charge region that results from the balance between the induced fixed charge in the Al₂O₃ and the defect states at the alumina/Si interface is at the origin of the observed correlation.