THEORY OF RADIATIVE AND NONRADIATIVE-TRANSITIONS FOR SEMICONDUCTOR NANOCRYSTALS

Existing calculations on the radiative and nonradiative transitions in semiconductor crystallites are reviewed with particular emphasis on i ndirect band-gap materials like silicon for which the quantum confinem ent effects are more spectacular. It is shown that the crystallite gap s and radiative recombination rates can be predicted with fair accurac y. Effects related to atomic relaxation in the excited state (Stokes s hift) are calculated and it is shown that small enough crystallites le ad to self-trapped excitons which provide another source of luminescen ce, much less dependent on size effects. Nonradiative processes are th en examined: intrinsic, due to Auger recombination, and extrinsic, due to dangling bond surface states. Both are found to play an essential role in the interpretation of experimental data. Finally, dielectric s creening is studied, justifying the use of a reduced internal dielectr ic constant and providing an estimate of the Coulomb shift due to char ging effects.