Enhancement of doping efficiency via polarization engineering in nitrogen-polar graded AlGaN grown by plasma-assisted molecular beam epitaxy

Efficient n-type doping in ultra-wide bandgap Al-rich AlGaN remains challenging due to deep silicon donor levels and strong self-compensation, which limit carrier activation. This article reports the demonstration of enhanced doping efficiency in nitrogen (N)-polar graded Al-rich AlxGa1−xN films grown on single-crystal AlN substrates by plasma-assisted molecular beam epitaxy, enabled by polarization induced doping in combination with Si doping. The optimized N-polar graded AlGaN films exhibited atomically smooth morphology with well-defined step edges. Room-temperature Hall measurements revealed a three-dimensional electron gas with a sheet density of 6.9 × 1012 cm−2 and a mobility of 37 cm2/(V s), with almost zero freeze-out down to 50 K. Upon 4 × 1018 cm−3 Si doping, the sheet and bulk electron concentrations increased to 2.3 × 1013 cm−2 and 4.42 × 1018 cm−3, respectively, at room temperature. No carrier freeze-out was observed in these films in the temperature range from 50 to 400 K. A low specific contact resistivity of 4.5 × 10−5 Ω cm2 was achieved using higher Si doping (1 × 1020 cm−3). This approach offers a promising path toward high-performance Al-rich AlGaN transistors.