Fluorine-based plasma treatment for hetero-epitaxial β-Ga₂O₃ MOSFETs

In this work, we demonstrate the lateral metal–oxidesemiconductor field-effect transistors (MOSFETs) with β-Ga₂O₃ film (thickness = 150 nm) grown on a c-plane sapphire substrate by a hydride vapor phase epitaxy (HVPE) method using a CF₄-based plasma treatment. By incorporating fluorine (F) the electrical characteristics of the surface and bulk regions of the resistive β-Ga₂O₃ film could be significantly improved. The analyses by secondary ion mass spectrometry and x-ray photoemission spectroscopy confirmed the formation of the substitutional fluorine, F_O, with its peak concentration of 2.3 × 10¹⁸ cm⁻³ at 5 ~ 10 nm deep from the surface and about 10¹⁵ ~ 10¹⁶ cm⁻³ in the bulk region. While the argon-plasma treatment was insufficient for making a Ti/Au/Ti metal contact ohmic, the same contact formed on the CF₄-treated surface showed an ohmic behavior with specific resistivity of 2.0 × 10⁻³ Ω·cm² and contact resistance of 0.8 Ω·m without any post thermal annealing. With the improvement of the electrical properties in both the surface and bulk regions, the HVEP-grown hetero-epitaxial β-Ga₂O₃ MOSFET exhibited the field-effect mobility of up to 5.3 cm²/V⋅s and on/off ratio of 10⁶.