Real-time first-principles calculations of ultrafast carrier dynamics of SnSe/TiO2 heterojunction under Li+ implantation

Abstract

Ion implantation has been widely used in biomaterials, alloys, and semiconductors modification. Basing on the studying of trapping states in the equilibrium state, we investigate the ultrafast carrier dynamics of SnSe/TiO2 and SnSe/Li/TiO2 heterojunctions under Li+ implantation by the real-time time-dependent density functional theory. The special type II band alignment and Li+ interfacial states in SnSe/TiO2 heterojunction effectively facilitate the exciton dissociation in a benign process and suppresses the interfacial nonradiative recombination. By monitoring the instantaneous ion–solid interaction energy, electronic stropping power and the excitation electron evolution, we find that atomic reconstruction introduced by the Li inserting layer changes the charge density and crystal potential field in the injection channel, and thus weakens the violent oscillation force and electron excitation on the Ti and O atoms. There exists a weaker and shorter charge excitation at the interface for SnSe/Li/TiO2 implantation system, which suggests that the Li ion layer weakens the e–ph coupling between the interface electrons and the moving ion. Meanwhile, only the hot electrons are produced in the interface region, reducing the probability of carrier recombination. These results provide an understanding for the behavior of carriers in SnSe based heterojunctions and the electron–phonon coupling mechanism at the phase/grain boundary under ion implantation.