Investigation of structural and optical properties of Ge/Al2O3 multilayer thin films fabricated using electron beam evaporation

•Growth of Ge nanocrystals in multilayer Ge/Al2O3 thin films.•Multilayer Ge/Al2O3 samples exhibit photoluminescence in the visible region.•PL intensity increases with annealing temperature.•Average decay lifetime is in nanoseconds and varies with annealing temperature. The present work reports the structural, optical, and luminescence properties of Ge/Al2O3 multilayers prepared using electron beam evaporation followed by conventional furnace annealing at 600 and 700 °C. XRD and Raman spectroscopy results reveal the formation of germanium nanocrystals (Ge NCs) in Al2O3 matrix after annealing. The estimated band gap of the pristine sample is 0.78 eV. The band gap of film annealed at 600 °C is 1.34 eV, which is decreased with an increase in annealing temperature. The obtained band gap confirms the size-dependent quantum confinement effect in Ge/Al2O3 samples. A broad band at 624 nm is observed in photoluminescence (PL) spectra from the multilayer structure. PL spectra show an enhancement in the peak intensity with an increase in annealing temperature. The average decay lifetime of charge carriers for the pristine and annealed samples is in the nanosecond range. The origin of PL emission and variation of band gap with annealing temperature is discussed in detail. The correlation of optical results with structural properties, and the possible use of Ge NCs embedded in Al2O3 matrix in optoelectronic devices are reported.