Optical properties of amorphous films assembled from the nanoclusters of Ge - Al mixtures

Abstract

Two samples of nanocluster-assembled Ge - Al thin films on quartz substrates have been fabricated by co-evaporation and inert-gas condensation. The thin films are amorphous and single phase, and the mean diameters of the nanoclusters are and , respectively. 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, respectively; these are dependent on the nanocluster sizes and are much larger than those of vacuum-evaporated amorphous Ge and amorphous Ge - Al thin films. Under 3.32 eV (374 nm) excitation, photoluminescence (PL) peaks at 2.80 and 3.00 eV appear for the thin film with the mean diameter of the nanoclusters ; these can be interpreted as manifesting electron transitions from the optical gap (2.8 eV) and mobility gap (3.0 eV). In the absorption spectra of the two samples, a shoulder 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.