Structural Changes Induced by Heating in Sputtered NiO and Cr2O3 Thin Films as p-Type Transparent Conductive Electrodes

Abstract

NiO and Cr2O3 are transition metal oxides with a partially filled d electron band that supports p-type conduction. Both are transparent to the visible light due to optical absorption beginning at wavelengths below 0.4 μm and the creation of holes by metal vacancy defects. The defect and strain effects on the electronic characteristics of these materials need to be established. For this purpose, NiO and Cr2O3 thin films were deposited on unheated glass substrates by reactive DC sputtering from metallic targets. Their structural, morphological, optical and electrical properties were analyzed comparatively in the as-grown conditions (25 °C) and after heating in air at 300 °C or 500 °C. The cubic NiO structure was identified with some tensile strain in the as-grown conditions and compressive strain after heating. Otherwise, the chromium oxide layers were amorphous as grown at 25 °C and crystallized into hexagonal Cr2O3 at 300 °C or above also with compressive strain after heating. Both materials achieved the highest visible transmittance (72%) and analogous electrical conductivity (~10−4 S/cm) by annealing at 500 °C. The as-grown NiO films showed a higher conductivity (2.5 × 10−2 S/cm) but lower transmittance (34%), which were related to more defects causing tensile strain in these samples.

Effects of sapphire substrate orientation on Sn-doped α-Ga2O3 grown by halide vapor phase epitaxy using α-Cr2O3 buffers

“…Hybridization of p-states of oxygen with unfilled d-states of transition metal can raise the valence band edge and make p-type doping possible in a way reported for p-type (Ir x Ga 1−x ) 2 O 3 solid solutions [12]. The advantage of Cr is the wider bandgap of α-Cr 2 O 3 compared to α-Ir 2 O 3 , allowing a bandgap increase of the solid solution [12,22,23], and the lower lattice mismatch of α-Cr 2 O 3 and α-Ga 2 O 3 compared to α-Ir 2 O 3 , facilitating easy solubility [10,12]. Growth of solid solutions of α-(Cr x Ga 1−x ) 2 O 3 has been reported for the case of mist CVD [19] but electrical properties have not been studied in detail.…”

“…It can be grown on sapphire by magnetron sputtering with subsequent annealing [20,21] or by mist CVD [19]. The attractive feature of α-Cr 2 O 3 is that it is naturally p-type, due to Cr vacancies [22,23]. This opens possibilities for advantageously combining n-type α-Ga 2 O 3 and p-type α-Cr 2 O 3 to form a heterojunction.…”

Engineering NiO Thin Film Properties using Ag9+ Ion Irradiation at Various Fluences

“…Its low toxicity and abundance in nature make it an environmental-friendly alternative of other functional materials. 1 Therefore, recent studies have demonstrated the versatility of NiO as a valuable material for a wide range of uses, encompassing smart windows, 2 gas sensors, 3 solar cells, 4 ptype transparent conducting electrodes, 5 and electrochromic devices. 6 For many of these applications, NiO must be deposited as a thin film with a well-regulated microstructure.…”