Energetic deposition of dielectric metal oxide coatings
Creators: name: Murdoch, Billy
Publisher: name: RMIT University
Year: 2024

description: This thesis examines the optical and electronic properties of wide-bandgap metal oxides grown using energetic deposition methods. The films have also been incorporated in metal/oxide/metal devices for use as high-κ gate oxide materials and memristive devices. This study aimed to assess the potential advantages of energetic deposition methods compared to conventional physical vapour deposition techniques for depositing high quality dielectric thin films. In particular, the influence of dopants, native defects and impurities on the performance of optical and electrical devices incorporating energetically deposited metal oxides was explored<br><br>Firstly, thin films of HfO2 were energetically deposited from a filtered cathodic vacuum arc (FCVA) at room temperature and found to exhibit higher density and significantly lower current leakage than HfO2 deposited by conventional, low energy physical vapour deposition techniques. Capacitance-voltage measurements performed on the FCVA HfO2 film revealed a low threshold voltage shift (ΔVFB = 0.60 V) corresponding to low fixed oxide charge density (8.3 × 1011 cm−2). A wide optical bandgap of 6.0 eV and high refractive index of &gt;2.1 in the visible spectrum were also observed. The excellent optical and electrical properties of the FCVA deposited HfO2 are attributed to film densification caused by the energetic depositing flux.<br><br>Secondly, HfO2-xNx films were energetically deposited by high-power impulse magnetron sputtering (HiPIMS). The passivation of oxygen vacancies (Ov) defects by nitrogen and local bonding of the resultant films was investigated by X-ray absorption spectroscopy (XAS). It was found that Ov densities in HfO2-xNx films decrease with increasing N2 partial pressure during deposition. Passivation was achieved by atomic nitrogen substitution and N2 interstitials at the Ov site. The incorporation of low mobility interstitial species inhibited crystallisation. These effects combine to greatly reduce electron leakage paths in the films resulting in high breakdown strengths. The refractive indices of HfO2-xNx films were found to be dependent on N2 pressure during deposition. Increases in the refractive index were attributed to high densities of N2 interstitials. The substitution of atomic nitrogen for Ov did not significantly reduce the optical bandgap, allowing sufficient band offsets for the HfO2-xNx films to be used as an effective gate dielectric.<br><br>The resistive switching mechanisms in memristors formed on monoclinic HfO2 and HfO1.86N0.14 films deposited by HiPIMS were investigated. Gradual conductance modulation, short-term plasticity (STP) and long-term potentiation (LTP) were implemented in HfO2 memristors with high Ov densities using voltage-spike stimulation, suggesting suitability for electronic synaptic applications. The switching dynamics of the HfO2 memristor were explained by the interactions of Ov with grain boundaries. The passivation of Ov defects by nitrogen in the HfO1.86N0.14 film was confirmed by XAS. The HfO1.86N0.14 memristor exhibited threshold switching and current-controlled negative differential resistance (CC-NDR). Numerical modeling showed this behaviour to be Joule heating-induced. These findings suggest simple bilayer selector/memory cells could be fabricated from hafnia heterolayers.<br><br>Lastly, the neuronal properties of a zinc tin oxide/silver oxide memristor fabricated by FCVA were demonstrated. STP, LTP and spike-timing dependent plasticity learning/memory functions have been described and explained by Ag+/O- electromigration across a dielectric interfacial layer. Importantly, CC-NDR induced oscillations were observed in the same device above room temperature. This discovery paves the way for transistor-free neuromorphic architectures based solely on memristors, thereby enabling the next generation of non-von Neumann computing architectures.