Polarization-induced charge and electron mobility in AlGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy

Abstract

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ $\text{(x⩽0.31)}$ heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm²/V s $\text{(T=13\hspace{0.05em}}\mathrm{K})$ was obtained in the ${\mathrm{Al}}_{\mathrm{0.09}}{\mathrm{Ga}}_{\mathrm{0.91}}\mathrm{N/GaN}$ structure with a two-dimensional electron gas density of ${\text{2.23×10}}^{12} {\mathrm{cm}}^{\mathrm{-2}}.$