Size, porosity, and surface-termination dependence of the radiative and nonradiative relaxation processes of porous silicon

The role of porosity and surface-termination on the radiative and the nonradiative relaxation processes of luminescent porous silicon is investigated using temperature-dependent, time-resolved photoluminescence spectroscopy. We show that, for porous silicon having low- to mid-porosity, radiative relaxation times should be associated with the quantum confinement of excitons (the confined photo-excited electron-hole pairs), while nonradiative relaxation processes are related to the state of the surface (e.g., the surface chemistry), in agreement with previous reports. However, for high-porosity films of porous silicon, we have found much faster low-temperature relaxation times, which are associated with radiative transitions from the triplet excitonic state. This state becomes partially allowed due to a strong coupling and mixing with the singlet state in high-porosity films of porous silicon containing fairly small silicon nanocrystallites.