Evidence for fast decay dynamics of the photoluminescence from Ge nanocrystals embedded in SiO2

We have investigated the origin of room temperature photoluminescence from ion-beam synthesized Ge nanocrystals (NCs) embedded in SiO2 using steady state and time-resolved photoluminescence (PL) measurements. Ge NCs of diameter 4-13 nm were grown embedded in a thermally grown SiO2 layer by Ge+ ion implantation and subsequent annealing. Steady state PL spectra show a peak at ∼2.1 eV originating from Ge NCs and another peak at ∼2.3 eV arising from ion-beam induced defects in the SiO2 matrix. Time-resolved PL studies reveal double exponential decay dynamics on the nanoseconds time scale. The faster component of the decay with a time constant τ1∼3.1 ns is attributed to the nonradiative lifetime, since the time constant reduces with increasing defect density. The slower component with time constant τ2∼10 ns is attributed to radiative recombination at the Ge NCs. Our results are in close agreement with the theoretically predicted radiative lifetime for small Ge NCs.