Ge and Si nanocrystals—New ways to luminescence structures

Abstract

Ge nanocrystals with a radius from 25 down to 1.5 nm are formed in a SiOx matrix. A luminescence is observed around 3.1 eV with a subnanosecond photoluminescence (PL) dynamic. The strongest PL is found for films containing the largest nanocrystals. These results are a clear indication that the blue luminescence is not produced by radiative recombination of excitons confined in the Ge nanocrystals. The investigations on Ge nanocrystals are summarized and the PL mechanisms are clarified. Ordered Si nanocrystals are prepared in a Si/SiO2 superlattice structure. Decreasing the a-Si layer thickness down to 1.9 nm increases the inhomogeneous strain by one order of magnitude. The exponential increase of the crystallization temperature with decreased thickness is described with a semi-empirical model using the melting point and the bulk amorphous crystallization temperature. The validity of the model to Ge/SiO2 superlattices is demonstrated. Enhancement in band gap luminescence is discussed for films with nanocrystal sizes near the Bohr radius. First results of investigations on Er doped superlattices are presented.