Effects of oxygen content on the structural, optical, and electrical properties of NiO films fabricated by radio-frequency magnetron sputtering

Nickel oxide (NiO) films were deposited on Corning glass substrate with variable (0-100%) oxygen content by radio-frequency sputtering. Effects of different oxygen content on the structural, optical, and electrical properties of NiO films were studied. X-ray diffraction showed that the NiO film deposited on substrate with 0% oxygen content resulted in a random polycrystalline structure and small grain size. The introduction of oxygen gas leaded to a (200) preferential orientation and larger grain size. The transmittance decreases with oxygen content due to the increase of oxygen interstitials in NiO films. The 0%-O2 deposited NiO film has a tensile strain and a small band gap. Upon introducing 33%-O2 content, the NiO film exhibits a compressive strain, increasing the bandgap. However, the compressive strain is released and gradually turns into tensile strain, which leads to the narrowing of bandgap with the increase of oxygen content. Hall measurement shows the obtained NiO is p-type and the resistivity decreases from 4.3 × 105 Ω-cm to 5.02 Ω-cm with increasing oxygen content from 0% to 100%. The carrier concentration increases from 6.3 × 1014 cm−3 to 4.6 × 1018 cm−3 and the mobility decreases from 26 cm2/V-s to 0.26 cm2/V-s for the NiO films deposited with oxygen content increasing from 50% to 100%. X-ray photoelectron spectroscopy showed that the Ni+3/Ni+2 ratio is the origin of p-type NiO and the ratio increases from 1.32 to 2.63 by increasing the oxygen content from 0% to 100%, which caused more defects, oxygen interstitials and nickel vacancies.