Photothermal deflection investigation of thermally oxidized mesoporous silicon

In this paper we investigate the thermal oxidation effect on optical and thermal properties of meso-porous silicon (meso-PS) layers using photoluminescence (PL) and photothermal deflection techniques (PDS, PTD). Samples have been successfully prepared by electrochemical anodization process. After a pre-oxidation for 1 h 30 min at 300 °C, layers are thermally oxidized in dry oxygen at different temperatures (800, 1000 °C) and durations. PL measurements show that the annealed layers have comparable spectra with a peak position focused on 2.1 eV. This behavior indicates the formation of luminescent silicon (Si) nanocrystallites with comparable average sizes. From the effective mass theory, the diameter of those nanocrystallites is estimated to be around 2.2 nm. Another estimation of the mean size of the silicon nanocrystallites obtained from the evolution of the thermal conductivity of the meso-PS layers based on photothermal deflection technique (PTD) data was close to the values obtained from the PL results. Photothermal deflection spectroscopy (PDS) measurements show that the thermal oxidation affects the absorption edge of the Si substrate. The optical band gap energy of the started substrates determined from the Tauc's relation is observed to increase with the thermal temperature and duration. The possible origins of this shift are discussed.