Hexagonal-phase single-crystal Gd2O3 is deposited on GaN in a molecular beam epitaxy system (see image). The dielectric constant is about twice that of its cubic counterpart when deposited on InGaAs or Si. The capacitive effective thickness of 0.5 nm in hexagonal Gd2O3 is perhaps the lowest on GaN-metal-oxide-semiconductor devices. The heterostructure is thermodynamically stable at high temperatures and exhibits low interfacial densities of states after high-temperature annealing.