Photoluminescence properties of size-controlled silicon nanocrystals at low temperatures

This study attempts to clarify the origin of the temperature dependence of the photoluminescence (PL) spectra of silicon nanocrystals (Si-ncs) embedded in SiO2 from 5 to 300 K. For this purpose, size-controlled Si-ncs with a narrow size distribution were fabricated, using the SiO/SiO2 multilayer structure. The PL intensity is strongly temperature dependent and presents a maximum at around 70 K, depending on the Si-nc size and on the excitation power. The origin of this maximum is first discussed thanks to PL dynamics study and power dependence study. The evolution of the PL energy with temperature is also discussed. In bulk semiconductors the temperature dependence of the gap is generally well represented by Varshni's law. Taking into account the quantum confinement energy, the PL energy of Si-ncs follows very well this law in the range 50-300 K. Below 50 K, a strong discrepancy to this law is observed characterized by a strong increase in the PL energy at low temperature, which is dependent on the Si-nc size distribution. This temperature dependence of the PL energy is correlated with a decrease in the radiative rate at low temperature and is explained by a preferential saturation effect of the bigger Si-ncs. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3169513]