Hole mobility behavior in Al-gradient polarization-induced p-type AlGaN grown on GaN template

A series of samples with their AlGaN layers of decreasing Al contents along the c-axis are grown on GaN templates with molecular beam epitaxy for producing polarization-induced p-type behaviors. By fixing the Al-gradient rate at 0.15%/nm and AlGaN layer thickness at 100 nm while changing the central Al content in those samples, we study the dependence of hole mobility on the central Al content. As the central Al content increases, hole mobility decreases monotonically with two different decreasing slopes in the two Al-content ranges divided at ∼40%. Tensile strain relaxation is observed when the Al content is higher than ∼40%, resulting in the increase in overall polarization gradient and, hence, hole concentration. Although the variation of the alloy scattering strength of hole in AlGaN can support the smaller decreasing slope of hole mobility in the Al-content range of >40%, other factors, such as piezoelectric scattering and bound charge distribution, may need to be considered for a complete interpretation. Compared with Mg-doped p-type AlGaN of 25% in an Al content, the polarization-induced p-type AlGaN can provide us with a higher hole mobility level at about the same hole concentration.