First principle study on structural and optoelectronic properties and band-gap modulation in germanium incorporated tin (IV) oxide

Abstract

First principle calculations on pure and germanium (Ge) incorporated tin (IV) oxide (SnO2) are performed to understand their structural, electronic, and optical properties at the microscopic level. The optimized ground state structures exhibited a reduction in their lattice parameters as well as total energy with increasing germanium concentration. From the band structures of the pristine and doped SnO2, we have observed a reduction in the energy gap due to the incorporation of germanium into stannic oxide (SnO2). As conduction band minimum near Fermi level reaching a lower energy level with an increase in doping concentration, the electron affinity is increasing. The calculated band gaps of pure, 6.25 %, 12.5 % and 25 % ‘Ge’ incorporated tin (IV) oxides are 1.76, 1.73, 1.56 and 1.53 eV, respectively, and all are direct in nature. Further, the calculated electron effective mass at the conduction band minimum is lower than the hole effective mass at the valence band maximum. Hence, the mobility of electrons is found to be higher, suggesting the n-type semiconducting property. The contributions of different atoms on valence and conduction band are studied by calculating the density of states spectra. Optical properties, such as real and imaginary dielectric constants, absorption, reflectivity, refraction and extinction coefficient, loss function, real and imaginary optical conductivity are calculated. The calculated optical absorption shows that both un-doped and doped materials preserve the optical transparency in the infra-red region. Our analysis shows that Ge-doped SnO2 has a tunable band gap and work function, which makes it suitable for applications in photovoltaic devices.