Characterisation of scandium- and iron-based GaN for high electron mobility transistors (HEMTs)
File(s)
Author(s)
Pace, Simona
Type
Thesis
Abstract
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 108cm-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.
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 108cm-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.
Version
Open Access
Date Issued
2018-10
Date Awarded
2019-03
Copyright Statement
Creative Commons Attribution NonCommercial Licence
Advisor
Alford, Neil
Sponsor
Leverhulme Trust
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)