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

Cecilia Guillén,José Herrero

doi:10.3390/electronicmat2020005

Cecilia Guillén, José Herrero

Open Access

https://doi.org/10.3390/electronicmat2020005

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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.

Highlights

Transparent conducting oxides (TCOs) are critical to numerous technological applications, ranging from flat panel displays or light emitting diodes to smart windows and photovoltaic cells [1,2]
No diffraction peaks were observed for the as-grown chromium oxide samples, indicating that they were amorphous as reported for analogous layers prepared at room temperature [25,26]
NiO and Cr2O3 thin films were deposited by reactive DC sputtering on unheated glass substrates and subsequently annealed in air at 300 °C or 500 °C

Summary

Introduction

Transparent conducting oxides (TCOs) are critical to numerous technological applications, ranging from flat panel displays or light emitting diodes to smart windows and photovoltaic cells [1,2]. Another approximation is to utilize the electron correlation to promote VB modifications that favor p-type conduction [7] In this sense, many transition metal oxides with a partially filled d electron band are described by extended Hubbard models [8,9] where the Coulomb interaction between the electrons (U) splits the d band into an upper Hubbard band (UHB) and a lower Hubbard band (LHB) with a separation of U [10]. Many transition metal oxides with a partially filled d electron band are described by extended Hubbard models [8,9] where the Coulomb interaction between the electrons (U) splits the d band into an upper Hubbard band (UHB) and a lower Hubbard band (LHB) with a separation of U [10] These compounds can support p-type conduction when the VB is composed of O 2p orbitals and metal d orbitals (LHB) driven by the electron correlation

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