Optical absorption in array of Ge/Al-shell nanoparticles in an Alumina matrix

The absorption spectra in array of Ge, Al and Ge/Al-shell nanoparticles immersed in alumina (Al₂O₃) matrix is calculated in framework of ab initio macroscopic dielectric model. It is demonstrated that absorption is strongly enhanced when germanium nanospheres are encapsulated by Al-shell. Two absorption peaks, appearing in the spectra, correspond to low energy ω₊ and high energy ω_- plasmons which lie in visible and ultraviolet frequency range, respectively. It is demonstrated that in Ge/Al-shell composite the ω₊ plasmon exists only because quantum confinement effect which provides larger Ge band gap (Δ ~ 1.5 eV) and thus prevent decay of ω₊ plasmon to continuum of interband electron-hole excitation in semiconducting core. Absorption in visible frequency range enhances additional 3 times when alumina is replaced by large dielectric constant insulator, such as SiC, and additional 6 times when Ge core is replaced by wide band-gap insulator, such as Si₃N₄. Strong enhancement of optical absorption in visible frequency range make this composites suitable for optoelectronic application, such as solar cells or light emitting devices. The simulated plasmon peaks are brought in connection with peaks appearing in ellipsometry measurements.