1,721,002 research outputs found
Plasma-Assisted Molecular Beam Epitaxy 2
No full textThis chapter sheds light on various fundamental aspects of the O plasma-assisted molecular beam epitaxy of Ga2O3. It discusses the volatile suboxide-related growth kinetics of Ga2O3 explaining the observed growth rate behavior as function of all growth parameters. The binary growth kinetics of Ga2O3 is then compared to that of its related oxides In2O3 and SnO2. During the ternary growth of (InxGa1-x)2O3, thermodynamic aspects based on different metal-oxygen bond strengths become important, which will be shown to govern the Ga- versus In-incorporation into the (InxGa1-x)2O3 thin film. More importantly, we describe how the collaborative effect of the different growth kinetics and thermodynamics of the binary oxides can lead to a strong growth rate enhancement of Ga2O3 by metal-exchange catalysis (MEXCAT) using In2O3 or SnO2 as catalyst. A brief overview of the polymorphs of Ga2O3 stabilized by different substrates is given. The surface morphology obtained by homoepitaxy will be discussed in relation to thermodynamically induced faceting. Finally, open questions will be identified that require further research
Thermodynamic and Kinetic Effects on the Nucleation and Growth of ε/κ- or β-Ga2O3 by Metal–Organic Vapor Phase Epitaxy
Full text linkIn this paper, we focus on the growth of β- and ε/κ-Ga2O3 thin films via metal–organic vapor phase epitaxy on c-plane sapphire using water and trimethyl-gallium at temperatures between 610 and 650 °C. Using these precursors, the monoclinic β-phase is usually obtained only at temperatures higher than 700 °C. We show here, for the first time, that both β- and ε-Ga2O3 can also be obtained by tuning the growth rate of the film, that is, by controlling the supersaturation of the vapor phase. The experimental findings are discussed in the framework of classical nucleation theory and Ostwald’s step rule, showing the interplay of thermodynamic (related to different chemical potentials for the metastable ε/κ phase and the stable β phase) and kinetic effects (mainly related to different surface energy barriers for nuclei of different crystallographic phases/planes). The experimental conditions that permit the nucleation and growth of the desired Ga2O3 polymorph are identified and thoroughly explained, giving to this work a fundamental as well as a technological relevance
Efficient suboxide sources in oxide molecular beam epitaxy using mixed metal + oxide charges: The examples of SnO and Ga2O
Full text linkSources of suboxides, providing several advantages over metal sources for the molecular beam epitaxy (MBE) of oxides, are conventionally realized by decomposing the corresponding oxide charge at extreme temperatures. By quadrupole mass spectrometry of the direct flux from an effusion cell, we compare this conventional approach to the reaction of a mixed oxide + metal charge as a source for suboxides with the examples of SnO2 + Sn → 2 SnO and Ga2O3 + 4 Ga → 3 Ga2O. The high decomposition temperatures of the pure oxide charge were found to produce a high parasitic oxygen background. In contrast, the mixed charges reacted at significantly lower temperatures, providing high suboxide fluxes without additional parasitic oxygen. For the SnO source, we found a significant fraction of Sn2O2 in the flux from the mixed charge that was basically absent in the flux from the pure oxide charge. We demonstrate the plasma-assisted MBE growth of SnO2 using the mixed Sn + SnO2 charge to require less activated oxygen and a significantly lower source temperature than the corresponding growth from a pure Sn charge. Thus, the sublimation of mixed metal + oxide charges provides an efficient suboxide source for the growth of oxides by MBE. Thermodynamic calculations predict this advantage for further oxides as well, e.g., SiO2, GeO2, Al2O3, In2O3, La2O3, and Pr2O3 © 2020 Author(s)
Offcut-related step-flow and growth rate enhancement during (100) β-Ga2O3 homoepitaxy by metal-exchange catalyzed molecular beam epitaxy (MEXCAT-MBE)
Full text linkHomoepitaxial β-Ga2O3 layers grown via molecular beam epitaxy (MBE) have exhibited prohibitively low growth rates on (100) oriented substrates in the past. In this work, we investigate the possibility to employ indium-assisted metal exchange catalyzed (MEXCAT) MBE to overcome this limit. We demonstrate that the fine tuning of the MEXCAT growth parameters and the choice of a proper substrate offcut allow for the deposition of thin films with high structural quality via the step-flow growth mechanism at relatively high growth rates for β-Ga2O3 homoepitaxy (i.e., around 1.5 nm/min, ≈ 45% incorporation of the incoming Ga flux), making MBE growth in this orientation feasible. Moreover, through the employment of the four investigated different (100) substrate offcuts along the [00-1] direction (i.e., 0°, 2°, 4°, and 6°), we give experimental evidence on the fundamental role of the (-201) step edges as nucleation sites in the growth of (100)-oriented Ga2O3 films by MBE
Vibrational - Electrical Properties Relationship in Donor Doped TiO2 by Raman Spectroscopy
No full textTransparent conducting TiO2, obtained by Nb or Ta doping of the anatase structure, is gaining increasing attention for the development of transparent electrodes. Usually, regardless the deposition technique, a crystallization process in reducing atmosphere is necessary to achieve large mobility; in addition, electrical and optical properties are also strongly sensitive to the oxygen deposition pressure. These facts reveal that the defect chemistry of donor doped TiO2 is not trivial and involves a strict interplay among extrinsic dopant atoms, oxygen vacancies and ‘electron killer’ defects such as Ti vacancies and O interstitials. We here present a Raman characterization of donor-doped TiO2 films synthesized under several deposition and post-annealing conditions, employing different doping levels and dopant elements (i.e.
Ta and Nb). Correlations between structure, crystallinity, shift and width of Raman peaks and electrical properties are shown and discussed. In particular, a clear relationship between the shift of the Eg(1) anatase Raman mode and the charge carrier density is found, while the B1g(1) mode connected to Ti-Ti vibrations is significantly affected by the extrinsic doping level. In this complex framework Raman
spectroscopy can provide an invaluable contribution towards understanding the material structure and its influence on the functional properties
Molecular Beam Epitaxy of β-(InxGa1–x)2O3 on β-Ga2O3 (010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement
Full text linkIn this work, we investigate the growth of monoclinic β-(InxGa1–x)2O3 alloys on top of (010) β-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(InxGa1–x)2O3 layers on β-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga2O3 substrate can be preserved in the β-(InxGa1–x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (10-2) oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(InxGa1–x)2O3/(AlyGa1–y)2O3 heterostructures
Quasi-1D hydrogen-treated titanium oxide nanostructures for photoelectrochemical water splitting
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