High electron mobility exceeding 10⁴Cm²v^-1 s^-1 in Mg_xZn_1-x O/ZnO single heterostructures grown by molecular beam epitaxy

Nominally undoped Mg_xZn_1-x O/ZnO (x = 0.05 and 0.08) single heterostructures were prepared on Zn-polar ZnO substrates by using plasma assisted molecular beam epitaxy (MBE). The samples showed a metallic conductivity below 50K and a mobility exceeding 10⁴ cm² V^-1s^-1 at 0.5 K. We observed quantum Hall effect accompanying Shubnikov - de Haas oscillations, in which zero-resistance states were clearly seen above 5 T. Rotation experiments in magnetic field suggest strong two-dimensional carrier confinement at low temperatures. The results indicate that the MBE grown films have much higher quality than the previously reported samples grown by pulsed laser deposition.