Photoluminescence properties of silicon quantum-well layers

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 ∼35 μs at room temperature and ∼54 μs 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 ∼50–100 μm 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.