Systematic Density Functional Theory Investigation of Stability of Dopant Atoms in Ge Ultra-Thin Film Grown on Si Substrate

Due to the higher carrier mobility of Ge, there are many reports focused on the Ge thin film grown on Si substrate for device application i.e. LSI channel region. In this study, the physical properties of Ge (001) ultra-thin film grown on Si substrate was studied through systematic density functional theory (DFT) investigation of dopant atoms. The depth profile of formation energy of eight dopant atoms (B, Ga, In, C, Sn, P, As, Sb) was obtained in comparison to that of in Ge and Si bulk. In the case of larger acceptor (Ga, In) and larger neutral (Sn) atoms than Ge atom, the formation energy (Ef) was increased in the Ge ultra-thin film from the value of Ge bulk. Conversely, Efof smaller acceptor (B) and smaller neutral (C) atoms than Ge atom, and of donor (P, As, Sb) atoms was decreased in the Ge ultra-thin film from the value of Ge bulk. There are two factors to affect Ef of dopant atom in the Ge ultra-thin film; (1) the positive charge distribution just below the surface affects Ef of acceptor and donor atoms, and (2) the compressive plane stress in the Ge film on the Si substrate affects Ef of atoms with different size from Ge atom. Both factor (1) and (2) are the reasons for Ga, In, P, and As atoms, factor (1) is the reason for Sb atom, and factor (2) is the reason for B, C, and Sn atoms. The thermal-equilibrium concentrations of dopant atoms in each atomic layer of the Ge ultra-thin film were evaluated in consideration of the degeneracy of the atomic configurations. The data of the thermal-equilibrium concentrations of dopant atoms will be useful in the LSI and other device applications to control the depth profile of dopant concentrations.