Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a [Formula: see text]-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.
Oxygen vacancies (VOs) and their distribution can affect oxides' properties from various aspects. In this work, we present a dislocation-related surface-layer effect in single crystal SrTiO3 (STO). Our results from the first principles calculations based on density functional theory along with our experimental research based on angle-resolved x-ray photoelectron spectroscopy indicate that, in contrast with bulk STO where VOs tend to cluster in a line, as depth increases from surface region, the concentration of VOs increases first, reaches a maximum value, and then decreases to a saturation value. This effect was argued to be the combinative result of the oxygen-vacancy diffusion along the dislocation lines and the ambient oxygen-atom incorporation into the crystal.
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