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Title: Computational modelling of defects and charge trapping in amorphous and crystalline metal oxides
Author: Dicks, Oliver A.
ISNI:       0000 0004 7230 437X
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2018
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available