MOVPE growth and characterization of (AlxGa1-x)2O3 thin films for high power devices

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Beta-type gallium oxide (β-Ga2O3) provides promising perspectives for high-power applications outperforming current key technology because for β-Ga2O3 a considerably stronger electrical breakdown field is predicted. In addition, it offers potentially low cost and large substrate size preparation from bulk crystals with controllable n-type doping in comparison to other promising materials. The performance of high-power devices directly depends on the breakdown field to the power of three as well as on the mobility of the charge carriers. The incorporation of aluminum into β-Ga2O3 allows tuning the band gap and consequently the breakdown field. Therefore, a suitable material growth method is required resulting in high-quality binary oxide thin films with optimized band gap and uncompromised materials properties.Hence, in this project we propose to develop a novel approach based on metal-organic vapor phase epitaxy (MOVPE) of β-(AlxGa1-x)2O3 (AlGaO) thin films on lattice-matched (100)-oriented β-Ga2O3 enabling the growth at temperatures above 800°C with an enhanced solubility of aluminum in β-Ga2O3. For this purpose, we will initially grow bulk aluminum-doped β-Ga2O3 single crystals exhibiting a minimal lattice mismatch with the targeted AlGaO films. Subsequently, the quasi-homoepitaxial growth of high-quality AlGaO thin films on these substrates by MOVPE will be engineered and optimized thanks to the detailed insights from sophisticated materials characterization. Our concerted, systematic use of atomic force, electron, and photoemission microscopy, in situ x-ray and electron diffraction, spectroscopic ellipsometry as well as photoelectron spectroscopy techniques will facilitate to unravel the growth mode, morphology, composition as well as the structural, electronic, electrical, and optical properties of the AlGaO thin films.Specifically, we will determine the limiting factors for Al distribution and its maximally possible incorporation into β-Ga2O3 without phase separation. Then, we will explore the possibilities for band gap and strain engineering in the AlGaO system, investigate the surface morphology as well as the interface of the AlGaO on β-Ga2O3 system, and perform electrical and structural analysis to understand the process of defect formation and the role of impurities. Our strategy is threefold: (1) preparation of epitaxy-ready aluminum-doped (up to 15%) bulk β-Ga2O3 crystals (2 cm to 2 inches in diameter) as substrates for the subsequent quasi-homoepitaxial growth of AlGaO thin layers and characterization of the obtained films to (2) optimize the growth and to (3) evaluate the application-relevant properties. Particularly, the project focuses on the preparation and characterization of epitaxial AlGaO with maximum aluminum incorporation resulting in the highest possible increase of the bandgap and the breakdown field.