Ab initio study of oxygen vacancy effects on electronic and optical properties of NiO
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Ab initio study of oxygen vacancy effects on electronic and optical properties of NiO John Petersen1, Fidele Twagirayezu1, Pablo D. Borges2, Luisa Scolfaro1 and Wilhelmus Geerts1 1
Department of Physics, Texas State University, San Marcos, TX 78666, U.S.A.
2
Instituto de Ciências Exatas e Tec., Universidade Federal de Viçosa, 38810-000 Rio Paranaíba,
MG, Brazil. ABSTRACT Density Functional Theory calculations of electronic and optical properties of NiO, with and without O vacancies, are the focus of this work. Two bands, one fully occupied and the other unoccupied, induced by an O vacancy, are found in the gap. These energy levels are identified and analyzed by means of a local density of states (LDOS) calculation, and notable crystal field splitting can be seen. The real and imaginary parts of the dielectric function are calculated, and an additional optical transition can be seen at lower energy, which can be attributed to the O vacancy induced state in the band gap. INTRODUCTION Resistive Random Access Memory (RRAM) is a promising candidate for next-generation memory devices due to its high endurance, good retention, low energy use, and high speed. The semiconductor NiO is one of the top contenders for the functional material in a commercial RRAM device. While NiO has been studied extensively, both experimentally and theoretically, little work has been done with O vacancies in NiO. The switching mechanism is thought to be a result of O vacancy diffusion from an applied electric field, yet it is not well understood [1, 2]. Nano-filaments have been observed to pass through the material, connecting the cathodes on either end, reducing the resistance [2]. Switching between a semiconducting high resistance state and a metallic Ni-rich low resistance state is the basis for logic devices based on NiO-RRAM. The most stable defect over the broadest range of Fermi energy in the Ni-rich condition is an O vacancy [3]. By identifying a distinct signature of O vacancies in the optical spectra, near-field optical techniques could provide a method of detecting nano-filaments remotely on devices with transparent electrodes, allowing one to study switching properties on complete devices which is currently not possible. Recently, Gosh et al. found that upon annealing at 500 ÛC in Ultra High Vacuum (UHV), the optical spectrum of NiO changes permanently, creating a broad peak in the H2 spectrum a little below 2 eV [4]. They attribute this peak to the reduction of the oxide at those elevated temperatures. Similarly, Yubo et al. found a small peak slightly above 2 eV in the absorption spectrum of permalloy oxide (NiO alloyed with FeO) [5]. Their analyses are based on a simultaneous fit of the transmission and ellipsometry spectra. Energy Dispersive X-ray Spectroscopy (EDS) measurements, corrected for the thin film structure, suggest their films have a significant amount of oxygen vacancies. Here, our results give insight on the H2 peak at approximately 2 eV, by theoretically investigating the effects of O vacancies on the electroni
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