Model for energy transfer from porous silicon nanocrystallites to SiH surface vibrations at low temperature

In contrast to oxidized silicon layers, photoluminescence in fresh porous s ilicon layers is quenched when the temperature decreases. Moreover, the spe ctra become structured below 100 K and display two large holes centered at 630 and 750 nm, respectively. We explain these experimental results by a mo del of electric dipole-dipole coupling between the photocreated carriers in the silicon crystallites and Si-H surface vibrations. We demonstrate here that the Morse potential, which is often employed to represent diatomic mol ecules, is not valid in our case. We compare the energy levels and the osci llator strengths of a Morse well and of a topologically similar well with a square base and adjustable slope: although the energy levels can be matche d quite closely, the oscillator strengths are drastically different. We sho w that only the second well accounts for the experimental results for both energies and oscillator strengths. [S0163-1829(98)00347-6].