Plasma assisted low temperature electron beam deposited NiO thin films for electro-optic applications


Cosar M. B. , Icli K. C. , ÖZENBAŞ A. M.

JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A, cilt.36, 2018 (SCI İndekslerine Giren Dergi)

  • Cilt numarası: 36 Konu: 3
  • Basım Tarihi: 2018
  • Doi Numarası: 10.1116/1.5013126
  • Dergi Adı: JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A

Özet

This study aims to create high quality nickel oxide (NiO) thin films at low temperatures, which is a prerequisite for coatings on temperature sensitive substrates. NiO chunks were evaporated by electron beam source, and NiO thin films were deposited at a thickness value around 250 nm. Depositions were performed at different experimental conditions: oxygen flow rate, deposition temperature, deposition rate, and plasma assistance. Deposited films were analyzed with regard to the structural, optical, and electrical aspects. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy results reveal that films are grown in cubic nickel oxide phase with preferred orientation of (111) plane. Nonstoichiometry of NiO films increases with increasing oxygen flow rate and plasma assistance leads to stoichiometric NiO films. Needle, spherical, and cuboidal particle formation were seen in scanning electron microscopy (SEM) images. Grain size, lattice parameter, and grain morphology were used to explain the variations in optical and electrical properties. It was seen that the mobility of the films increases with oxygen flow rate because of enhanced grain size revealed by XRD calculations and SEM images. Plasma assistance dramatically lowers the resistivity to 150 Omega cm compared to nonassisted films possessing resistivities on the order of megaohm centimeter values. Although plasma assistance results in low mobility [0.2 cm(2)/(Vs)], enhanced sheet carrier concentration (1.1 x 10 13 cm(-1)) was found to be the major factor leading to high conductivity. This situation is related to denser films with higher crystallinity, which was detected from the refractive index spectrum and confirmed by SEM analysis. Optical absorption studies at 400-600 nm wavelengths revealed that absorption can be minimized by deposition under high oxygen flow rate, high deposition temperature, and low deposition rate conditions. Optical band gaps can be tuned by varying the oxygen flow rate, deposition temperature, and deposition rate. It was seen that the Fermi level and valence band minima of the films highly depend on the oxygen flow rate and can be engineered by manipulating the flow rate of oxygen and deposition conditions. Published by the AVS.