Effect of the N2/RE Ratio During Growth on the Properties of Rare Earth Nitrides

Abstract

The rare earth nitride (REN) series is a promising candidate for spintronics applications. It is the only epitaxy-compatible series of intrinsic ferromagnetic semiconductors, with members possessing complementary and contrasting magnetic properties.1 It is established that REN properties display high sensitivity to native defects, mainly the nitrogen vacancies (VN). VN form at the level of a few percent or less during thin film growth even in the most nearly stoichiometric films. The vacancies act as dopants and introduce n-type conductivity. The literature reports on more VN-induced effects in the REN such as the enhanced Curie temperature in GdN (from 50K to 70K) due to the formation of magnetic polarons at VN sites, and superconductivity below 4K in heavily VN-doped ferromagnetic semiconducting SmN thin films 2,3 Here, we show that the rate of N2 flow during growth and thin film properties are intimately linked, suggesting that the growth stoichiometry is key for understanding the structural, transport, and magnetic properties of the REN. X-ray diffraction, magnetic, Hall-effect, and transport measurements are carried out on a series of polycrystalline REN (GdN, SmN, and DyN) thin films. We show remarkable changes to the properties of the films: SmN film resistivity can be controlled over six orders of magnitude by less than one order of magnitude change of the N2/RE growth ratio. Hall-effect measurements correlate this increase in resistivity to a drop in the VN concentration. We also observe the signature of a secondary structural phase of GdN associated with N-deficient regions that develops as the N2 pressure decreases during the growth and has a smaller lattice parameter than stoichiometric GdN. We also observe an enhanced TC (up to 90 K) for the films grown with the lowest N2/Gd ratio.