Theory of the radiative and non radiative processes in silicon nanocrystallites
Résumé
This chapter describes the theory of radiative and nonradiative processes in Silicon (Si) nanocrystallites. Near threshold optical transitions in an indirect bandgap material like silicon have very small oscillator strength preventing its use in optoelectronics. An interesting field of research is thus to devise Si-based materials where this selection rule is broken, leading to efficient luminescence. A major step in this direction has been the observation of intense photoluminescence (PL) from porous silicon (π-Si). This has stimulated considerable theoretical activity on the optical properties of Si nanocrystallites and quantum wires. Quantum confinement is characterized by several consequences—(i) the fundamental gap exhibits a blue shift, (ii) in nanocrystallites the filled and the empty states become quantized, and (iii) optical dipole transitions across the fundamental gap become allowed. All three effects increase with decreasing size of the crystallites. Although π-Si is a very heterogeneous material on a microscopic scale, some fine structures clearly appear in the excitation spectrum of the visible luminescence at 2 K of some π-Si samples.