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Title: Characterisation of scandium-based III-nitride thin films
Author: Tsui, Hei Chit Leo
ISNI:       0000 0004 5994 2684
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Wurtzite III-nitrides are widely used in optoelectronic applications. However, the external quantum efficiency of III-nitride-based light emitting diodes in the deep-UV region is extremely low compared to those emitting in the visible region. This problem has motivated the search for better materials that can help. Theoretical calculations predict that alloying ScN and GaN can produce wurtzite-structure semiconducting ScxGa1-xN films with direct band gaps in the UV region and a lattice parameter – band gap relationship that differs significantly from that of the AlxGa1-xN alloys used conventionally. Therefore, this thesis investigates the growth, composition, microstructure and optical properties of ScxGa1-xN thin films. Epitaxial ScxGa1-xN (0 ≤ x ≤ 0.5) thin films were grown using molecular beam epitaxy under metal-rich conditions. The alloy composition was determined by four different techniques. A linear relationship was established between the Sc flux measured in the growth chamber and the Sc content measured by Rutherford backscattering, whereas the compositions measured by X-ray photoelectron spectroscopy were 5–8% lower than that by Rutherford backscattering and information obtained from X-ray and electron diffraction (i.e. the a and c lattice parameters and the c/a ratio) cannot provide a reliable estimation of the Sc content. Structural analysis confirmed that ScxGa1-xN can be stabilised in the wurtzite structure up to x = 0.26 using metal-rich growth conditions, which is in line with theoretical predictions and is significantly greater than the value of x = 0.17 for the ScxGa1-xN films grown under N-rich conditions reported previously. UV-Vis absorption measurements showed that the direct optical band gap of ScxGa1-xN increases from 3.33 eV to 3.89 eV as Sc content increases from x = 0 to x = 0.26. This trend is consistent with theoretical predictions but contradicts the observations reported by other groups. Instead, nanoscale cubic inclusions, revealed by aberration-corrected scanning transmission electron microscopy, are responsible for the observed decrease in band gap as the Sc content increases. Finally, valence band offset measurements indicated that type I and type II heterojunctions can be formed by depositing ScxGa1-xN on AlN and on GaN respectively.
Supervisor: Moram, Michelle Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral