OPTICAL-PROPERTIES OF AMORPHOUS FILMS ASSEMBLED FROM THE NANOCLUSTERSOF GE-AL MIXTURES

Two samples of nanocluster-assembled Ge-Al thin films on quartz substr ates have been fabricated by co-evaporation and inert-gas condensation . The thin films are amorphous and single phase, and the mean diameter s of the nanoclusters are similar to 8 nm and similar to 45 nm, respec tively. The composition of the nanoclusters in the two films is about 98.7 at.% Ge and 1.3 at.% Al, and the ratio of Ge and oxygen atoms is 1:1.8 (O). From the absorption spectra of the two samples, we estimate the optical gaps to be about 2.8 eV and 1.6 eV from Tauc plots, respe ctively; these are dependent on the nanocluster sizes and are much lar ger than those of vacuum-evaporated amorphous Ge and amorphous Ge-Al t hin films. Under 3.32 eV (374 nm) excitation; photoluminescence (PL) p eaks at 2.80 and 3.00 eV appear for the thin film with the mean diamet er of the nanoclusters similar to 8 nm; these can be interpreted as ma nifesting electron transitions from the optical gap (2.8 eV) and mobil ity gap (3.0 eV). In the absorption spectra of the two samples, a shou lder peak appears at 5.06 eV (245 nm), corresponding to the absorption band of one of the germanium oxygen-deficient centres (GODCs), namely GODC-1. The PL peak at 3.10 eV (400 nm), which arises from GODC-1, is also observed for the two samples.