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

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1-xN/GaN (x less than or equal to 0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperatu re electrical-transport measurements revealed that the two-dimensional elec tron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the th eoretical estimates of the polarization-induced 2DEG concentrations. Low-te mperature electron mobility was found to be much higher in the structures w ith lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maxim um mobility of 51 700 cm(2)/V s (T=13 K) was obtained in the Al0.09Ga0.91N/ GaN structure with a two-dimensional electron gas density of 2.23x10(12) cm (-2). (C) 1999 American Institute of Physics. [S0021-8979(99)08020-2].