Point defects control in Ga2O3 thin films grown via molecular beam epitaxy

• Applicants: Dr. Oliver Bierwagen, since 1/2021; Professor Dr. Manfred Martin; Privatdozent Dr. Markus R. Wagner
• Subject Area: Synthesis and Properties of Functional Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 446185170

Its wide bandgap (Eg≈4.7 eV) with associated large theoretically predicted breakdown field (Ebr ≈ 8 MV/cm) and the simultaneous possibility to control its electrical properties from semi-insulating to conducting (at least p ≈ 10^10 to 10^-3 Ωcm) render monoclinic gallium oxide (ß-Ga2O3) a promising semiconductor material for power electronics. Nonetheless, by now there is still a considerable difference between the theoretically predicted properties of Ga2O3 and the outcome performance obtained in device structures. To this end, point defects, such as oxygen- or gallium-vacancies (VO, VGa), and/or interstitials, can play a detrimental role by (i) compensating intentional doping, (ii) decreasing the electron mobility, and (iii) providing deep levels whose charging/discharging during device operation dissipates power.This project is aiming at giving novel experimental evidence on the point defects in ß-Ga2O3 and their role in determining its functional properties. The novel approach we are proposing will tackle this fundamental problem by cross-linking various experimental characterizations on thin films deposited in well-defined regimes (i.e. metal- or oxygen-rich) via molecular beam epitaxy - MBE.At first, the deposition of thin films with different O-isotopes will allow us to collect unprecedented experimental evidence by Raman spectroscopy on the relative impact of the O- and Ga- sublattices on different Raman modes. Supported by ab-initio calculations this preliminar step will allow us to correlate the variation in ß-Ga2O3 Raman active modes with the formation of Ga- and/or O-related point defects induced by different MBE synthesis conditions or post-growth annealing treatments, e.g. O-rich or Ga-rich deposition regimes are expected to promote the formation of high concentration of VGa or VO respectively. These results will be systematically coupled with secondary ion mass-spectroscopy-based anion and cation tracer diffusion studies and positron annihilation spectroscopy, both of which will provide independent evidence on vacancy defects. The combination with electrical transport measurements of donor-doped layers will characterize the impact of different point defects on the thin films functional properties.Finally, taking advantage of the collected results in the previous steps we will focus on a peculiar MBE deposition mechanism highly promising for Ga2O3 thin film depositions – i.e. In-mediated metal-exchange catalysis – trying to clarify its role on the formation of different point defects in the material in comparison to the standard deposition process.In this way we aim to build up concrete guidelines for the synthesis of ß-Ga2O3 on how to prevent or engineer point defects as a means of controlling the functional properties. This approach can represent a fundamental step in the understanding of (i) ß-Ga2O3 devices functioning and (ii) the material fundamental limits.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Andreas Falkenstein
• Ehemaliger Antragsteller: Dr. Piero Mazzolini, Ph.D., until 12/2020