Thermally Stimulated Evolution of Optical and Structural Properties of Germanium-Doped Alumina Films

Abstract

Ge-doped Al2O3 films were deposited on long, non-rotated quarts substrate by magnetron co-sputtering of Ge and Al2O3 space-apart targets. This approach allowed to vary Ge content along the film length in the range 5-85 at.%. Simultaneous annealing treatment of such long samples permitted to investigate the evolution of optical and structural properties of the films with different Ge content. It was observed that all as-deposited films and those annealed at TA=550 °C during 50 min in nitrogen atmosphere were amorphous whatever Ge content. The formation of Ge nanocrystals was observed after treatment at TA=600 °C during 1-15 min. The higher Ge content, the shorter annealing time was required to form Ge nanocrystals. This annealing stimulated also an appearance of photoluminescence in visible-near infrared spectral range. Three specific bands centered at about 590 nm, 620 nm and 700-730 nm were detected. Their relative contribution was depended on Ge content determining the shape and peak position of luminescence spectrum. Two former components were ascribed to the carrier recombination via the defects in Ge oxide shell covered Ge nanocrystals, whereas the latter one was assumed to originate from carrier recombination in amorphous Ge clusters or via defects at Ge-cluster/Al2O3 interface. This statement was supported by the heat treatment of the films at TA=700-800 °C caused significant decrease of Ge content due to Ge out-diffusion from film volume via formation of volatile GeO.