Photoluminescence properties of silicon quantum-well layers

Abstract

Nanometer-scale crystal silicon films surrounded by ${\mathrm{SiO}}_{2}$ were prepared by oxidizing silicon-on-insulator substrates prepared from SIMOX (separation by implantation of oxygen) and crystallized hydrogenated amorphous silicon films. Average silicon layer thickness was determined from reflection spectra. When sufficiently thin (2 nm), all layers emitted red photoluminescence under blue and UV cw excitation, with a spectrum that did not depend on the mean layer thickness. The spectrum was roughly Gaussian with a peak energy of 1.65 eV, which is lower than for most porous silicon spectra. The time scale for the luminescence decay was \ensuremath{\sim}35 \ensuremath{\mu}s at room temperature and \ensuremath{\sim}54 \ensuremath{\mu}s at 88 K; the decay was nonexponential and did not exhibit spectral diffusion. Atomic force microscope images of the silicon layers showed that luminescing layers were broken apart into regions \ensuremath{\sim}50--100 \ensuremath{\mu}m in diameter, suggesting that luminescence comes only from regions small enough to have no nonradiative recombination centers in the band gap. These results are inconsistent with a simple quantum-confinement model for luminescence in two-dimensional silicon and suggest the importance of radiation from surface states.