A comparative study of Ge nanocrystals in SixGeyOz alloys and SiOx/GeOy multilayers

Amorphous ${\mathrm{Si}}_x{\mathrm{Ge}}_y{\mathrm{O}}_z$ alloy films of different chemical composition and ${\mathrm{SiO}}_x{\mathrm{/GeO}}_y$ multilayers are prepared by dc magnetron sputtering. The formation of Ge nanocrystals in the alloy matrix is induced by annealing at temperatures ranging from 700 to 900 °C. In contrast, the crystallization in the Ge sublayers in the multilayer structures occurs at 500 °C. Using high resolution transmission electron microscopy, we can directly image the Ge nanoclusters in the as-prepared films as well as the Ge nanocrystals after crystallization in the annealed alloy and multilayer samples. Raman spectra of the as-prepared and annealed films demonstrate the strong similarity between the alloy and the multilayer films. In both cases, the crystallization of the Ge phase is observed by the narrowing and disappearing of the amorphous mode around 270 cm⁻¹ and the appearance and increase of the mode below 300 cm⁻¹, typical for nanocrystallized Ge. The Ge cluster formation is stimulated by a high oxygen content of the alloy films. If the Si atoms are not saturated with oxygen, Si–Ge alloy regions are formed, resulting in an increase of the crystallization temperature of the films. A weak broad Raman mode at 390 cm⁻¹ is found as a signature of these regions. Films containing small Ge nanocrystals show visible room temperature photoluminescence around 3.03 and 2.34 eV. Quantum confinement or the presence of luminescence centers (Si₂⁰, Ge₂⁰) at the surface of the Ge nanocrystals are discussed as possible origins of the bright blue luminescence. However, a peak shift to higher energies with decreasing crystal size is not detectable, contrary to the predictions of the quantum confinement model.