Nanocrystalline formation and optical properties of germanium thin films prepared by physical vapor deposition

Amorphous and nanocrystalline germanium thin films were prepared on glass substrates by physical vapor deposition (PVD). The influence of thermal annealing on the characteristics of the Ge thin films has been investigated. X-ray diffraction (XRD) and SEM show amorphous structure of films deposited at room temperature. After thermal annealing, the crystallinity was improved when the annealing temperature increases. The Ge thin films annealed at different temperatures in air were nanocrystalline, having the face-centered cubic structure with preferred orientation along the 〈1 1 1〉 direction. The nanostructural parameters have been evaluated by using a single-order Voigt profile analysis. Moreover, the analysis of the optical transmission and reflection behavior was carried out. The values of direct and indirect band gap energies for amorphous and nanocrystalline phases are 0.86±0.02, 0.65±0.02 and 0.79±0.02, 0.61±0.02 eV, respectively. In addition, the complex optical functions for the wavelength range 600-2200 nm are reported. The refractive index of the nanocrystalline phase drops from 4.80±0.03 to 2.04±0.02, and amorphous phase changes from 5.18±0.03 to 2.42±0.02 for the whole wavelength range. The dielectric functions ε₁ and ε₂ of the deposited films were recorded as a function of wavelength within the range from 600 to 2200 nm.