On the formation of IR-light-emitting Ge nanocrystals in Ge:SiO$_{2}$ films

The study is concerned with light-emitting Ge nanocrystals formed during the annealing of Ge$_{x}$[SiO$_{2}$]$_{1-x}$ films produced by the high-vacuum cosputtering of germanium and quartz targets onto substrates at a temperature of 100$^{\circ}$C. In accordance with the conditions of growth, the Ge molar fraction was varied from 10 to 40%. By means of electron microscopy and Raman spectroscopy, amorphous Ge nanoclusters $\sim$4–5 nm in dimensions are detected in as-deposited films with a Ge content higher than 20 mol %. To crystallize amorphous nanoclusters, annealing at temperatures of up to 650$^{\circ}$C is used. The kinetics of the crystallization of Ge nanoclusters is studied, and it is established that up to $\sim$1/3 of the amorphous phase is retained in the system, supposedly at the interfaces between nanocrystals and the surrounding amorphous SiO$_2$ matrix. It is found that, upon annealing in normal atmosphere, germanium nanoclusters are partially or completely oxidized (at a Ge molar fraction of 30% and smaller). An intense infrared photoluminescence signal from quantum-confined Ge nanocrystals and a visible photoluminescence signal defined by defect complexes (oxygen vacancy + excess Ge atoms) are observed.