Quantum size effects in germanium nanocrystals and amorphous nanoclusters in GeSi$_x$O$_y$ films

Films of nonstoichiometric germanium–silicon glasses of two types–GeO$_{x}$[SiO]$_{(1-x)}$ and GeO$_{x}$[SiO$_{2}$]$_{(1-x)}$–are deposited onto cold Si (001) substrates by evaporating GeO$_{2}$ and SiO (or SiO$_{2}$) powders simultaneously in high vacuum. Film samples in their initial (as-deposited) state and after being annealed at 550 and 650$^\circ$C for 1 h are investigated using IR and Raman spectroscopies and electron microscopy, and their photoluminescence (PL) is studied as well. Raman spectroscopy shows that, in contrast to the initial GeO[SiO$_{2}$] film, the initial GeO[SiO] one contains clusters of amorphous germanium, their size being $\sim$3 nm, as found by electron microscopy. The presence of Si–O, Ge–O, and Si–O–Ge bonds in the films is established by IR spectroscopy. Clusters of amorphous germanium are found in both films after annealing at 550$^\circ$C, while germanium nanocrystals are formed in the films subjected to annealing at 650$^\circ$C. For the initial films, a broad band with a maximum at 1050 nm is registered in their low-temperature PL spectra, which may originate from such defects as oxygen vacancies and overstoichiometric germanium atoms. Annealing causes structural changes in the films and affects their PL behavior. The films containing germanium nanoclusters give rise to PL with a maximum at 1400–1600 nm, with the defect-related signal being diminished. The temperature dependence of PL intensity exhibits a decreasing behavior, but PL is observed to temperatures as high as 200 K. The contribution of germanium nanocrystals formed at the annealing stage to PL is discussed.