Frequency-resolved photoluminescence spectroscopy of a-Si:H, a-P₂Se, a-P₃Se, and c-P₄Se₃

Conventionally, frequency-resolved spectroscopy (FRS) measures the photo-luminescence (PL) intensity in exact quadrature to the modulated excitation and gives directly the lifetime distribution G(τ) of the PL over the time-scale 10^-1 to 10^-6s. The first measurement of G(τ) for the midgap PL in a-P₂Se and c-P₄Se₃ has been obtained by this method. G(τ) is found to be extremely narrow for both glass and crystal; this is taken to be evidence for a common excitonic recombination mechanism. The phase shift of the PL with respect to the excitation is usually not presented. We show that the phase is very sensitive to processes faster than 10^-5 s, and can contain significant structure not observed in the quadrature signal. For a-P₂Se, a model with two lifetimes in the nanosecond and millisecond regime reproduces the experimentally observed structure. Amorphous silicon displays similar structure arising from a broad distribution of lifetimes, with a varying component outside the experimental time scale.