Dicks, Oliver A.; (2018) Computational modelling of defects and charge trapping in amorphous and crystalline metal oxides. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

Thin films of metal oxides, like Al₂O₃ and LaAlO₃, play a crucial role in emerging nanoelectronic devices. Using density functional theory (DFT) and other computational methods, the properties of defects and intrinsic polaron trapping have been calculated in LaAlO₃ and amorphous Al₂O₃. The spectroscopic properties of neutral (Vₒ⁰) and charged (Vₒ⁺) oxygen vacancies in cubic and rhombohedral LaAlO₃ have been investigated using Time Dependent DFT and the embedded cluster method. The peaks of the optical absorption spectra are predicted at 3.5 and 4.2 eV for Vₒ⁰ and 3.6 eV for Vₒ⁺ in rhombohedral LaAlO₃. The calculated electron paramagnetic resonance (EPR) parameters of Vₒ⁺ accurately predict the width (3 mT) and position of its EPR spectrum. Amorphous Al₂O₃ is then investigated, which has applications in non-volatile memory and a-IGZO (amorphous indium-gallium-zinc oxide) thin film transistors. Amorphous Al₂O₃ structures were generated using a molecular dynamics melt-quench approach and found to be in good agreement with experiment. DFT calculations, using a tuned hybrid functional, determined that the a-Al₂O₃ band gap decreases to 5.5 eV, compared to 8.6 eV in α-Al₂O₃, because of the reduction in Al coordination number in the amorphous phase. This causes a shift in the electrostatic potential that lowers the conduction band minimum, adding support to experimental measurements of band offsets. Then intrinsic polaron and bipolaron trapping in a-Al₂O₃ is modelled. The average trapping energy of hole polarons in a-Al₂O₃ was calculated to be 1.26 eV, much higher than the 0.38 eV calculated for α-Al₂O₃. Electrons were found not to trap in both crystalline and amorphous Al₂O₃. To explain the negative charging of Al₂O₃ films the properties of oxygen, hydrogen and aluminium defects were calculated. A mechanism is proposed to explain experimental trap spectroscopy measurements, whereby negatively charge defects are compensated by positively charged defects that have unoccupied states in the band gap. These predictions will facilitate experimental identification of defect states in LaAlO₃ and Al₂O₃ and their effect on nanodevices.

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