The Role of the Metal-Ion Charge in Mineral Interface Doping

In this paper, we investigate the mineral interface doping (MID) process of silicon substrates, using various minerals to explore the role of the metal ions involved. Specifically, we use metal ions with +1, +2, and +3 charge states to determine whether the observed effects are unique to individual metals or if they reflect broader trends within charge groups. Silicon wafers are coated with different minerals and doped by rapid thermal annealing (RTA). The process is subsequently analyzed as a function of temperature using time-of-flight secondary ion mass spectrometry (ToF-SIMS), infrared spectroscopy (IR), and electrochemical impedance spectroscopy (EIS). To unravel the underlying atomistic mechanisms, we perform density functional theory (DFT) simulations, obtaining the kinetic activation energies of the corresponding doping processes and their dependence on metal valence. Our findings reveal that the required doping temperature decreases with the charge density of metal ions; however, there is a required minimum temperature to achieve the diffusion of phosphorus into the silicon bulk. During the MID process, metal silicates are first formed at the interface, while the atomic phosphorus constituents diffuse into the silicon substrate. Finally, the metal silicates are removed post-doping using non-toxic acids, thus making the process broadly applicable.