Multilayer Si/Ge thin films with quantum confinement effects for photovoltaic applications

Multilayer thin films of Si and Ge were grown by e-beam evaporation and resistive heating techniques. Each Si-layer has a thickness of 15 nm and that of Ge is 20 nm. In this way, three sets of multilayer structures with 2, 4 and 6 layers of Si and Ge in an alternative way were deposited on glass substrates. Structural and electrical properties of these multi-layer films were studied using Raman spectroscopy, Rutherford backscattering, Fourier transform infrared spectroscopy, and electrical resistivity measurements. Raman spectra of these multilayer thin films exhibit peaks shift towards lower wavelength (in comparison with bulk Ge) demonstrating that the films consist of nanostructures and also represent quantum confinement effect in Ge. The quantum confinement effect increases with the increase in number of layers. Raman spectra also reveal the formation of silicon oxide and germanium oxide. FTIR spectroscopy also confirms the presence of oxides in these multilayer films. The layer thickness and composition was determined using Rutherford backscattering spectroscopy. The DC-conductivity measurement of Si/Ge multilayer thin films shows gradual increase in conductivity with the increase in number of layers. These multi-layer Si/Ge films show high electrical conductivity in comparison with pure Ge and Si films due to the SiGe alloy phase formed at the interface during deposition. These investigations suggest that Si/Ge multilayer thin films with quantum confinement effects can be used for solar cells.