Electron mobility in polarization-doped Al_0-0.2GaN with a low concentration near 10¹⁷ cm⁻³

In this letter, carrier transport in graded Al_xGa_1-xN with a polarization-induced n-type doping as low as ∼10¹⁷ cm⁻³ is reported. The graded Al_xGa_1-xN is grown by metal organic chemical vapor deposition on a sapphire substrate, and a uniform n-type doping without any intentional doping is realized by linearly varying the Al composition from 0% to 20% over a thickness of 600 nm. A compensating center concentration of ∼10¹⁷ cm⁻³ was also estimated. A peak mobility of 900 cm²/V·s at room temperature is extracted at an Al composition of ∼7%, which represents the highest mobility achieved in n-Al_0.07GaN with a carrier concentration of ∼10¹⁷ cm⁻³. A comparison between experimental data and theoretical models shows that, at this low doping concentration, both dislocation scattering and alloy scattering are significant in limiting electron mobility and that a dislocation density of <10⁷ cm⁻² is necessary to optimize mobility near 10¹⁶ cm⁻³. The findings in this study provide insights into key elements for achieving high mobility at low doping levels in GaN, a critical parameter in the design of novel power electronics taking advantage of polarization doping.