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Title: Characterisation of scandium- and iron-based GaN for high electron mobility transistors (HEMTs)
Author: Pace, Simona
ISNI:       0000 0004 7659 1134
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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III-nitrides have received increasing attention for high electron mobility transistors (HEMTs), due to their structural and electrical properties. The AlGaN/GaN structure shows large band offset and can generate 2D physically-confined channel at its interface. However, the performance of these devices is still limited by defects and impurities present in the structure. The former act as electron-traps in the channel, while the latter generate unwanted background carriers. To overcome this problem, scandium- and iron-based GaN materials have been proposed. When Sc is introduced in GaN, the c/a ratio decreases, and the band gap increases, leading to a new set of latticeparameter/ band-gap ratios. Such that, ScGaN/GaN may show large band offset with lower concentration of defects. Furthermore, the large piezoelectric constant of ScGaN should increase the carrier concentration in the channel. The background conductivity can be improved by introducing a (Fe,Ga)N layer at the GaN/sapphire interface. Fe introduces acceptor-like states in the band gap, leading to semi-insulating behaviour and improved channel-confinement. In this work the properties of GaN, ScGaN and (Fe,Ga)N grown using e-beam physical vapour deposition are investigated. The optimal Ga e-beam currents were found and epitaxial growth of GaN with relatively good quality achieved. ScGaN with increasing Sc concentration was also investigated. Large structural improvement was found for Sc 20%, possibly due to some relation between ScGaN and sapphire lattice parameters that reduces the stress in the thin film. In agreement with theoretical results, the band gap of Sc0.2Ga0.8N increases by 0.2 eV. Finally, the homogeneity and resistivity of (Fe,Ga)N were investigated. When Fe is 0.8% the structural quality is not affected while the resistivity increases to 108cm-2. For higher Fe concentration, Fe-rich nanocrystals become visible. However, no relation between inhomogeneity and electrical properties is observed and when Fe is 4.6% the resistivity further increases by 1 order of magnitude.
Supervisor: Alford, Neil Sponsor: Leverhulme Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral