Plasma-assisted Molecular Beam Epitaxy of Beta-Ga2O3: Growth, Doping, and Heterostructures
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Plasma-assisted Molecular Beam Epitaxy of Beta-Ga2O3: Growth, Doping, and Heterostructures


As conventional semiconductors reach their materials limits for modern high power switching applications, we must look towards new materials systems. Ultrawide bandgap semiconductors provide opportunities for future efficient high voltage switches due to their ability to withstand high electric fields. In particular, β-Ga2O3 shows promise due to its high critical electric field (6-8 MV/cm), availability of high-quality melt grown bulk substrates, and donor and deep acceptor doping possibilities. This work focuses on growth and doping of β-Ga2O3 and its alloys via plasma-assisted molecular beam epitaxy (PAMBE). Conventional PAMBE shows promise for (010) β-Ga2O3 growth, however other crystallographic orientations have lower growth rates and poor film quality due to significant suboxide desorption during growth. An indium catalyzed growth mechanism using an additional indium flux during PAMBE growth of β-Ga2O3 is demonstrated, allowing for significantly improved growth rates across various crystallographic orientations. This metal oxide catalyzed epitaxy (MOCATAXY) allows for improved film quality, demonstrated by minimal extended defects and smoother surface morphologies, particularly for (001) β-Ga2O3. The supplied In flux during MOCATAXY growth acts as a catalyst, allowing for growth at high growth temperatures and Ga fluxes for which growth would not occur for conventional PAMBE. This In limits suboxide desorption during growth and does not incorporate into the film for sufficiently Ga rich growth conditions. Donor doping is necessary for achieving a variety of device designs, with its use in contacts, channels, modulation doping, and drift regions. Donor doping with Ge, Sn, and Si is demonstrated for β-Ga2O3 grown on various orientations. While Ge doping can be used for a range of concentrations for conventional PAMBE, at higher growth temperatures and Ga fluxes its incorporation decreases, limiting its use for MOCATAXY. Sn shows the ability to achieve high doping concentrations in conventional PAMBE, however surface segregation during growth and a delay in incorporation into the film is observed for lower concentrations. Sn doping during MOCATAXY growth, however, allows for sharp, controllable doping profiles for a variety of Sn concentrations across various orientations. Furthermore, Sn doping of (010) β-Ga2O3 via MOCATAXY demonstrates the highest electron mobility for continuously doped β-Ga2O3 grown via MBE. Sn doping of (001) β-Ga2O3 via MOCATAXY shows significantly higher electron mobility than conventional PAMBE. Si doping is also investigated, showing degradation of (010) β-Ga2O3 film quality, however promising electron mobility and high doping concentrations were achievable for (001) oriented growth via MOCATAXY. Deep acceptor doping allows for realization of potential barriers in β-Ga2O3, as well semi-insulating regions of the device, such as current blocking layers for vertical structures and an intentionally compensated film-substrate interface for lateral devices. Mg is investigated as an intentional dopant in conventional PAMBE growth of (010) β-Ga2O3. While sharp doping profiles and a range of doping concentrations are achievable, annealing at high temperatures (≥ 925 °C) allows for diffusion of Mg, limiting its application to lower growth temperature epitaxial techniques and processing steps. A mechanism of Mg diffusion via the mobile Mg interstitial species is proposed, involving interactions of point defects in the film during annealing. Additionally, Fe incorporation into β-Ga2O3 films grown on Fe doped substrates is shown to be the result of surface segregation, rather than diffusion. This incorporation can be limited using a low temperature Fe trapping buffer layer prior to growth of critical regions of the film structure. Finally, growth of heterostructures with β-(AlxGa1-x)2O3 via MOCATAXY is investigated. Maximum Al contents for (010) β-(AlxGa1-x)2O3 of 22% are achieved with high quality, coherently strained films. (001) β-(AlxGa1-x)2O3 films with Al contents up to 15% are also grown with smooth surface morphology and no evidence of extended defects or relaxation. A relationship between out of plane lattice parameter is derived using the fundamental stiffness tensor and stress and strain expressions for (001) β-(AlxGa1-x)2O3 coherently strained to the β-Ga2O3. Confirmation of the Al content in the films confirms the validity of the derived relationship. This demonstration of high quality (001) β-(AlxGa1-x)2O3 shows promise for future heterostructure based devices in this orientation.

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