CARRIER LOCALIZATION IN POROUS SILICON INVESTIGATED BY TIME-RESOLVED LUMINESCENCE ANALYSIS

We analyzed the photoluminescence (PL) mechanisms of porous silicon, a nd in particular, the origin of the PL high quantum efficiency (QE) at room temperature. For this we used postformation treatments. anodic o xidation, and hydrofluoric acid (HF) etching (known for their strong Q E enhancement effect) correlated with a PL time resolved analysis. A t hird parameter was the temperature which, for heating above room tempe rature, gave a reversible quenching of the PL. All three parameters gi ve a similar evolution of thr PL decay shape, which we consider to ori ginate from the same evolution of the carrier dynamics. Porous silicon is described as an undulating wire, The high QE at room temperature i s attributed to carrier localization inside minima of the fluctuating potential along the wire; these considerations are extended to another porous material: amorphous porous silicon. Anodic oxidation and HF di ssolution diminish the wire size, giving a reduction of the localizati on length of the carriers and progressive suppression of the nonradiat ive recombination channel. A simple model permits one to link the chan ges of the PL decay shape to the QE evolution. The nonexponential PL d ecay shape is interpreted as being due to a distribution of nonradiati ve recombination rates, the value of the nonradiative recombination ra te being limited by a tunneling effect. This highly simplified model e xplains the origin of the nonexponential decay shape, its modification and gives a good description of the QE evolution as a function of tem perature, oxidation level. or porosity. (C) 1996 American Institute of Physic.