Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2

Time resolved photoluminescence (PL) decays have been measured for Si nanoc rystals embedded in silicon dioxide. The nanocrystals were formed by implan ting 40 keV Si ions into a 1000 Angstrom thick film of thermally grown SiO2 , followed by thermal annealing at 1000-1200 degrees C. The observed lumine scence, peaking at 700-850 nm, is compared to similar measurements performe d on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with c haracteristic decay time tau in the range 10-150 mu s and dispersion factor beta in the range 0.7-0.8. Both parameters are, however, higher for the na nocrystals compared to those of porous Si indicating superior passivation o f the nanocrystals in the SiO2 matrix. Evidence is also presented for a sin gle exponential behavior at the decay end suggesting a remaining fraction o f excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocrystals an d a more curved decay line shape for higher ion doses to a higher crystal d ensity, promoting excitonic migration to nearby nanocrystals. These observa tions provide strong evidence that the origin of the stretched exponential line shape of the PL decay results from migration and trapping of excitons in a system of randomly distributed and interconnected nanocrystals with a dispersion in size. (C) 1999 American Institute of Physics. [S0021-8979(99) 04623-X].