Suboxide Molecular-Beam Epitaxy (S-MBE): Toward high-mobility n-type α-(Al,Ga)2O3 and β-(Al,Ga)2O3

• Applicant: Dr. Patrick Vogt, Ph.D.
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548632596

The search for novel materials has always been accompanied by the search for novel synthesis methods. Groundbreaking thin-film deposition techniques have been developed for the synthesis of new functional materials. To a large extent, the synthesis-science of molecular-beam epitaxy (MBE) has enabled the development of a new technological era: The development of solid-state and optoelectronic devices --- which today govern our modern daily lives. The present proposal targets the growth and doping of the ultra-wide bandgap (UWBD) semiconductor (Al,Ga)2O3 by suboxide MBE (S-MBE) --- a new MBE growth technique using suboxides as precursors. The groundbreaking feature of S-MBE is that it converts the complex 2-step kinetics observed for III-O materials into simple and 1-step surface kinetics. Thus, S-MBE allows the growth of III-O materials with higher crystalline perfection, significantly higher growth rates, and lower growth temperatures, than currently demonstrated by any other oxide MBE method. In this project, S-MBE will be extended and developed to grow α-(Al,Ga)2O3 and β-(Al,Ga)2O3 using the suboxides Ga2O and Al2O. For both polymorphs, the miscibility limit of Al2O3 in Ga2O3 will be investigated (on different substrates). In a follow-up study, these optimized (Al,Ga)2O3 alloys will be doped with the suboxides SiO and GeO used as n-type dopants. To date, the highest n-type room temperature mobility (μe) achieved for α-Ga2O3 and β-Ga2O3 are μe = 65 cm-2 V-1 s-1 and μe = 176 cm-2 V-1 s-1, respectively, both grown by chemical vapor deposition techniques. In contrast, no conductive n-type α-(AlxGa1-x)2O3 is reported in the literature, and n-type doped β-(Al0.15Ga0.85)2O3 is limited (to date) to μe = 4 cm-2 V-1s-1. Depending on the dopant (e.g. Si) and the Al concentration in the (AlxGa1-x)2O3 crystal, μe plummets due to the formation of DX centers or other compensating point defects. The applicants now propose that by the surface kinetics and defect formation thermodynamics unleashed by S-MBE, n-type α-(AlxGa1-x)2O3 and β-(AlxGa1-x)2O3 layers with x > 0.2 and μe > 50 cm2 V-1 s-1 may become possible --- and is the scientific goal of this proposal.

• DFG Programme: Research Grants