Resonantly excited photoluminescence spectra of porous silicon

Resonantly excited low-temperature photoluminescence spectra of porous silicon (por-Si) samples show a steplike structure with at least four phonon-related steps. For different n- and p-doped por-Si, we examine this phonon structure in spectra taken both during laser excitation and after different detection delay times following laser shutoff, ranging from 10 μs to 1.2 ms. The luminescence efficiency decreases and the phonon structure becomes more pronounced when lowering the excitation energy or when increasing the delay time between the detection window and the excitation. This observation suggests two separate luminescence mechanisms in por-Si: a very efficient one that is usually observed (even at room temperature) when exciting at higher energies (3 eV), and one that shows a phonon structure and can be seen only at low temperatures and when the efficient mechanism is suppressed by excitation at energies lower than 2 eV.