Crystallization of Ge in ion irradiated amorphous-Ge/Au thin films
Structural, optical, and electrical properties of Au induced crystallization in amorphous germanium (a-Ge) thin films are presented for future solar energy materials applications. The a-Ge/Au thin films on quartz substrate have been prepared by means of e-beam and thermal evaporation techniques, respectively. The as-prepared samples have been irradiated with 100 MeV Ni+7 ions at different fluences of 1×1012 ions-cm-2, 5×1012 ions-cm-2, 1×1013 ions-cm-2 and 5×1013 ions-cm-2. The ion irradiation does not result in the crystallization of Ge; instead, an increasing ion fluence decreases the Au crystallite size, which ultimately deteriorates the Au crystallization. The ion irradiated samples have been post-annealed for 4 hours at a fixed temperature of 300 ºC to achieve the crystallization of Ge. The post-annealing of ion irradiated samples leads to the crystallization of Ge; the crystallization of Ge is better for the samples irradiated at high fluence leading to smaller grain size corresponding to high grain boundary and hence high defect sink density. The energy bandgap of crystalline Ge was determined by reflectance and transmittance spectra, and was found to be low (~0.64 eV) at higher fluence. The field emission scanning electron microscopy and atomic force microscopy analyses show that the surface microstructure changes for post-annealed samples irradiated at different fluences. The role of fractal like surface microstructure in multiple scattering of light on the film surface has been discussed. The lower values of resistivity and sheet resistance as well as good optical properties make it a promising material for future poly-Ge based solar cell applications.