Demonstration of β-(AlxGa1−x)2O3/β-Ga2O3modulation doped field-effect transistors with Ge as dopant grown via plasma-assisted molecular beam epitaxy

β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures were grown via plasma-assisted molecular beam epitaxy. The β-(AlxGa1−x)2O3 barrier was partially doped by Ge to achieve a two-dimensional electron gas (2DEG) in Ga2O3. The formation of the 2DEG was confirmed by capacitance–voltage measurements. The impact of Ga-polishing on both the surface morphology and the reduction of the unintentionally incorporated Si at the growth interface was investigated using atomic force microscopy and secondary-ion mass spectrometry. Modulation doped field-effect transistors were fabricated. A maximum current density of 20 mA/mm with a pinch-off voltage of −6 V was achieved on a sample with a 2DEG sheet charge density of 1.2 × 1013 cm−2.