Ultrathin GaN Crystal Realized Through Nitrogen Substitution of Layered GaS

GaN has been demonstrated as an important wide-bandgap semiconductor in many applications, especially in optoelectronic and high-power electronics. Two-dimensional (2D) GaN, with increased bandgap compared to the bulk counterpart, not only amplifies existing functionalities but also opens up fresh possibilities for compact electronics. Although several methods have recently been developed to synthesize 2D GaN, their practical application is hampered by either harsh growth conditions (e.g., high temperature and ultrahigh vacuum) or unsatisfactory performance due to grain boundaries. Here, we report the realization of few-nanometer-thick GaN crystals via in situ atomic substitution of layered GaS flakes at a relatively low temperature (590°C). GaN with tunable thickness from 50 nm down to 0.9 nm (~2 atomic layers) is achieved by applying the atomic substitution reaction to GaS with different numbers of layers. The obtained ultrathin GaN flakes retain the morphology inherited from the GaS flakes and show high crystallinity by transmission electron microscopy (TEM) characterization, while the thickness of GaN decreases to about 72% of the corresponding GaS flakes from the atomic force microscopy characterization. A time-dependent mechanism study reveals both horizontal and vertical conversion paths, with Ga 2 S 3 as intermediate. Photoluminescence (PL) spectroscopy measurements show that the band edge PL of 2D ultrathin GaN is blue-shifted as compared with bulk GaN, suggesting that the bandgap increases with the decrease in thickness. This study provides a promising method for obtaining ultrathin, high-crystallinity GaN with tunable thicknesses, utilizing a minimal thermal budget. This breakthrough lays a solid foundation for future investigations into fundamental physics and potential device applications.