Growth and characterisation of Cu-doped NiO thin films by molecular beam epitaxy for optoelectronic applications

This thesis investigates the role of Cu concentration on the structural, electrical, and optical properties of single-crystal NiO thin films deposited on single-crystal MgO(001) substrates via the molecular beam epitaxy (MBE) growth technique, as well as the use of MBE-grown NiO thin films in optoelectronic applications. This study is the first to utilise the MBE technique and MgO(001) substrates to examine Cu doping in NiO films, providing new insights into material behaviour and potential applications. This project comprises three parts. In the first part, the impact of Cu concentrations (0, 3, 6, 10, 24 and 42 at.%) on the properties of NiO films has been investigated. The findings indicate a shift from 2D to 3D growth modes as Cu content increases, with single-crystal cubic structures developing into polycrystalline films beyond 10 at.% Cu due to Cu phase segregation. A transition from p-type to n-type conductivity is observed at 6 at.% Cu doping concentrations, alongside with a decrease in the band gap (from 3.61 eV to 3.36 eV) and resistivity (from 3.5 ×10^4 Ω.cm to 2.39 Ω.cm) with increasing Cu content. These findings demonstrate the potential of Cu-doped NiO films for use in devices requiring p-type or n-type materials, provided doping levels are carefully controlled. The next part focuses on low Cu doping (0, 0.7, 1.2, and 2.4 at.%) in forming p-type single-crystal NiO films, showing that doping maintains high crystallinity and homogeneous Cu distribution without phase segregation. Electrical and optical measurements reveal enhanced conductivity and a slight reduction in the band gap, suggesting the capability of low Cu doping to adjust NiO film properties while maintaining structural quality. In the final part, the influence of NiO thin film thickness on its properties using polycrystalline ITO substrates has been examined. Increasing the NiO films thickness results in higher surface roughness, reduced transparency, and band gap. Finally, MBE-grown NiO films have been incorporated into optoelectronic devices, including a perovskite solar cell (PSC) and a novel homojunction p-i-n (p-type NiO:Mg/intrinsic NiO/n-type NiO:Cu) photodiode. The use of undoped NiO film as a hole transport layer in PSC resulted in a 5.66% power conversion efficiency. MBE-grown NiO thin films have also been successfully employed in a novel homojunction p-i-n (p-type NiO:Mg/i-NiO/n-type NiO:Cu) UV photodiode, demonstrating good diode characteristics and performance, with a forward current of 38 mA and a reverse current of 1 mA at ±2 V.