Delta-doped β-(Al_xGa_1−x)₂O₃/Ga₂O₃ heterostructure field-effect transistors by ozone molecular beam epitaxy

Heterojunction field-effect transistors based on the β-(Al_xGa_1−x)₂O₃/Ga₂O₃ heterostructure grown by ozone-assisted molecular beam epitaxy were demonstrated for the first time. Al composition ratios in the 14%–23% range were validated using x-ray diffraction on the three samples grown for this study. Electrochemical capacitance-voltage (ECV) measurements showed the presence of a charge sheet in the delta-doped (Al_xGa_1−x)₂O₃ barrier layer. Secondary ion mass spectroscopy and ECV measurements also revealed an unintentional Si peak at the (Al_xGa_1−x)₂O₃/Ga₂O₃ interface. Direct current (I_DS-V_GS) and transconductance (G_m-V_GS) measurements demonstrated depletion-mode transistor operation as well as the presence of a parallel conduction channel. A one-dimensional Poisson model suggested that dopant redistribution in the delta-doped region could cause a secondary channel to form in the barrier in addition to the primary channel near the (Al_xGa_1−x)₂O₃/Ga₂O₃ interface under certain conditions met in these samples. Fabricated devices on sample A did not exhibit breakdown up to the measurement limit of 1100 V, with stability after ten cycles. A maximum output drain current density of 22 mA/mm was measured on sample B. Room temperature Hall measurements yielded a sheet carrier density of 1.12 × 10¹³ cm⁻² with corresponding Hall mobility of 95 cm²/V s in sample C.