Exploring Electronic Functionalities of Transition Metal Oxynitrides in Thin-Film Form

Transition metal oxynitrides having both ionic and covalent characters are known to exhibit a wide variety of physical properties such as ferroelectricity, superconductivity and magnetoresistance. Here we present the electronic functionalities unique to oxynitride thin films grown by non-equilibrium gas phase processes.<br/>Amorphous zinc oxynitride (ZnO_xN_y) is an amorphous semiconductor with very high electron mobility exceeding 200 cm²/Vs. In amorphous semiconductors, thermal transport is generally suppressed because phonons are no longer well-defined quasiparticles. Such low thermal conductivity in addition to high carrier mobility is favorable for thermoelectric devices of which figure of merit is expressed as ZT=σS2T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient and κ is the thermal conductivity. We experimentally verified that ZnO_xN_y indeed has low κ, leading to ZT =0.042 at room temperature. We recently found that zinc oxysulfide also behaves as a high mobility amorphous semiconductor. A thin film transistor using amorphous ZnO_xS_y showed clear behavior of an n-type field effect transistor<br/>SrNbO₃, being a good metal with d⁰ configuration, is converted to semiconducting SrNbO₂N, which is known to be a promising photocatalyst, by nitrogen doping. In this nitrogen doping process of SrNbO_2-xN_x, the band structure changed in a non-rigid band manner, which can be explained by a complex combination of several band-shifting effects induced by N substitution. Metal-insulator transition occurred near x~1, where the threshold carrier concentration was more than three orders of magnitude higher than the value predicted by Mott criterion, indicating that the transition was induced by a significant random potential due to anion mixing. Measured magnetoresistance reflected the anisotropy in the band structure, which demonstrates the coexistence of periodicity and randomness.<br/>Colossal negative magnetoresistance (MR &gt; −99%) was reported in bulk perovskite EuNbO₂N. In order to understand the role of nitrogen in the negative MR, we synthesized a series of EuNbO_3-xN_x thin films with different nitrogen contents. The saturation magnetization of the EuNbO_3-xN_x films was ~3.0 μ_B/f.u., independent of x, indicating that nearly half of Eu existed as Eu³⁺. The transport properties were gradually changed from metallic to semiconducting behavior with increasing x, where the semiconducting behavior could be described by the three-dimensional variable-range hopping model. The negative MR ratio at 2 K increased up to 98 % with x. These results suugges that the exchange interaction between Eu²⁺ localized spins and Nb 4d¹ spin, locally modulated by doped nitrogen, plays a key role in the colossal negative MR of EuNbO_3-xN_x.