Theoretical and Experimental Determination of Properties of NiO and Fe-doped NiO

Twagirayezu, Fidele J.
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In this era of fast advances in nanotechnology and electronics Ni has become an interesting element because its oxidized form possessing semiconducting properties is enabling several applications such as gas sensors, dyed sensitized photocathodes and electrodes in alkaline batteries. In addition, recently NiO has been utilized as a resistive switching (RS) memory [1], [2], [3], and energy-saving application as an electrochromic (EC) [4], [5], [6] smart window. In particular, NiO has promise for RS memory to replace Flash memory beyond the 14 nm process node. RS memory guarantees rapid speeds in reading and writing, high storage density and non-volatility with lifetimes of ten years or more. [7]The EC property of the NiO is also a driving force behind futuristic technologies such as smart windows and non-volatile displays. However, the exact mechanisms behind certain phenomena such as RS and EC are still not fully understood. In addition, in recent years there has been lots of research in Diluted Magnetic Semiconductor (DMS) materials because of their important utilization in spintronic devices. DMS materials have charge and spin degrees of freedom enabling the realization of devices with magnetic, electronic and optical functionalities. DMS electronic devices can have a higher speed and a lower switching energy than traditional electronic devices. NiO is an antiferromagnetic p-type semiconductor because of the presence of vacancy at Ni2+ sites [8], [9]. [10]The magnetic structure of NiO consists of ferromagnetic sheets of Ni2+ parallel to the (111) plane with opposite spin directions in neighboring planes. The Fe doping of NiO leads to Fe-doped NiO (Permalloy Oxide) which exhibits advantageous DMS properties over NiO. However, the effect of transition metal (TM) ion doping on the magnetic properties of NiO at higher iron concentration has not been fully determined yet. The effect iron doping has on the Neel temperature of NiO is also still unknown. The study of switching properties of NiO or Fe-doped NiO in resistive randomaccess memory (RRAM) devices can be easily possible by using optical or magnetic techniques that is why their physical properties such as optical or magnetic properties are also of interest in this thesis. In this thesis research, the physical properties of NiO, and Fe-Doped NiO were determined via theoretical and experimental methods. a) Theoretical Methods: For the theoretical part of the research, computer simulations were performed, and the physical properties were determined with the Density Functional Theory (DFT) using Vienna ab initio Simulation Package (VASP). Electronic, optical and magnetic properties of 4-atom NiO, 32-atom NiO, and 32-atom Fe-doped NiO supercells with spin polarization in the (111) planes were calculated using (GGA) and GGA+U methods. The theoretical outcome was compared with the experimental one or other findings from the literature. We began with simulations of 4-atom NiO, and 32-atom NiO. For the simulation of 32-atom Fe-doped NiO, we utilized the crystal structure of 32-atom NiO with Ni atoms substituted by Fe atoms (6.25%, and 12.5% Fe concentrations) without altering the initial spin ordering. Crystal structure and Brillouin zone were produced using Vesta software and Bilbao Crystallographic Server respectively, and graphs were produced using p4VASP and Origin Pro software. The results show that NiO is an antiferromagnetic semiconductor with a band gap depending on the Hubbard potential (U). The nature of the magnetism of Fe-doped NiO depends on the amount of Fe atoms. The Fe doping reduces the band gap, and this reduction of the band gap has the effect on the optical properties. With the Hubbard potential U(4eV), the static refractive index is about 2.2 for NiO, and 2.25 for 6.25 at.% Fe-doped NiO respectively. b) Experimental Methods: For the experiment part of the research, different laboratory instruments including an AJA Sputtering System, an X-ray Diffractometer (XRD), an Ellipsometer, and a Physical Property Measurement System (PPMS) were utilized. Data analysis was carried out using specialized software. The samples were manufactured using the AJA Sputtering system. An XRD system was used to gather information about the crystal structure for each sample. The optical properties were determined using Ellipsometer and the CompleteEase software was used for ellipsometric data analysis. Magnetic properties were obtained using the PPMS interfaced with the Multivu program. All graphs were obtained using Microsoft Excel or Origin Pro software. The experiment involved NiO and Ni(0.8)Fe(0.2)O1-δ samples, and one sample for each species was sputtered at low O pressure and another was sputtered at high O pressure. The XRD shows that all samples have NaCl-type structure. Samples sputtered at low O pressure have a higher magnetic moment than those sputtered at high O pressure. Each sample has the static refractive index (n) close to 2.
Density Functional Theory, Experiment, Physical properties, NiO, Fe-doped NiO
Twagirayezu, F. J. (2017). <i>Theoretical and experimental determination of properties of NiO and Fe-doped NiO</i> (Unpublished thesis). Texas State University, San Marcos, Texas.