Optimizing high-temperature electron mobility in single-crystal Bi$_2$O$_2$Se based on its unconventional dependence on concentration
Quasi-2D Bi$_2$O$_2$Se is part of an intensive materials research effort aimed at finding new semiconductors that outperform silicon-based electronics in terms of speed and power consumption. This material exhibits exceptionally high carrier mobility at low temperatures but mediocre mobility at 300 K. Its high mobility is generally associated with its high permittivity ($\varepsilon_r$~500), which is also associated with metallicity persisting down to very low carrier concentrations. This material exhibits a counterintuitive increase in carrier mobility as the concentration increases. The connection between low both carrier concentration and mobility in Se-rich conditions, and both high carrier concentration and mobility in Se-poor conditions suggests that the increase is related to native defects. We demonstrate that these defects can alter the effective mass of charge carriers. Specifically, substitutional Se(Bi) defects, which appear under Se-rich conditions, destroy the Bi$_2$O$_2$ channel and compromise charge transport properties. These defects increase the effective mass of charge carriers transforming the original semiconductor into a semimetal and introducing holes into charge transport. Additionally, we show that single crystals are generally inhomogeneous, particularly those grown under Se-rich conditions. Unlike Se-poor conditions, Se-rich conditions induce a higher concentration of dislocations and extraneous phases. These findings suggest that the perfection of the Bi$_2$O$_2$ channel is crucial for mobility, particularly at room temperature.