Growth and Carrier Transport in Gallium Oxide Polymorphs

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Since 2012, when the first beta-Ga2O3 transistor was demonstrated in the seminal work by Higashiwaki, et al. the field has experienced exponential growth. The promise of a ~4.7 eV bandgap, bulk large-area substrates, and n-type electrical conductivity with doping control over four orders of magnitude -- has proven too much to resist. In the intervening years, the number of papers, the amount of funding, and the number of research groups working on beta-Ga2O3 has continued to rise. The less studied polymorph, alpha-Ga2O3, has a larger intrinsic bandgap, Eg ~ 5.3 eV, and unlike beta-Ga2O3, alpha-Ga2O3 can be alloyed with Al over the entire compositional range, reaching bandgaps of 8.8 eV. Moreover, these high mole fraction alpha-(Al,Ga)2O3 films have bulk, large-area substrates: sapphire. While doping of alpha-Ga2O3 has been achieved, the transport properties and capabilities of alpha-Ga2O3 and alpha-(Al,Ga)2O3 may be better realized through the use of molecular beam epitaxy (MBE) growth. The work presented here is devoted to solving problems which are associated with the realization of conductive alpha-Ga2O3 by MBE. (i) Annealing is a critical step in device fabrication and is used to activate ion implanted donors. Because the alpha-Ga2O3 is meta-stable, the alpha-phase reverts to the beta-phase upon moderate annealing. (ii) Due to the oxygen rich environment in which MBE Ga2O3 growth occurs, controllable and repeatable doping has been a challenge for all polymorphs. (iii) Due to the kinetics and thermodynamics of MBE, and due to the complex mechanism which governs the formation and growth of Ga2O3, achieving conductivity in MBE grown Ga2O3 on sapphire has remained elusive. (iv) Achieving conductivity in Ga2O3 is only one-half of the problem. The other half is to ensure that there is no conductivity where it is undesired. The surface accumulation of Si impurities has been a barrier to achieving some high-performance devices regardless of growth technique. This thesis aims to solve each of these aforementioned issues, and make progress toward achieving high-performance alpha-Ga2O3 devices.

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191 pages


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Gallium oxide; molecular beam epitaxy; semiconductors; ultra wide bandgap


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Jena, Debdeep

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Xing, Huili
Schlom, Darrell

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Electrical and Computer Engineering

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Ph. D., Electrical and Computer Engineering

Degree Level

Doctor of Philosophy

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Government Document




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Attribution-NonCommercial-ShareAlike 4.0 International


dissertation or thesis

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