Nanoscale visualization of electronic properties of Al_xGa_(1-x)N/Al_yGa_(1-y)N multiple quantum-well heterostructure by spreading resistance microscopy

摘要

Cross-sectional spreading resistance microscopy has been used to investigate nanoscale variations in electronic properties of an undoped Al_(0.75)Ga_(0.25)N/Al_(0.95)Ga_(0.05)N multiple quantum well (MQW) heterostructure grown by plasma-assisted molecular beam epitaxy on an AlN/c-sapphire template, prepared by metalorganic vapor phase epitaxy. It is found that a current signal from the MQWs can be detected only at a negative sample bias. Moreover, its value changes periodically from one quantum well (QW) to another. Analysis of the current-voltage characteristics of the contacts of a tip with the structure layers showed that periodic contrast of MQWs is the result of fluctuations of the chemical composition of the QWs and the concentration of electrons accumulated in them. Mathematical simulations indicate that this modulation is associated with the periodic fluctuations of an Al-mole fraction in the barrier layers of the structure due to counter gradients of the intensity of Al and Ga molecular fluxes across the surface of a substrate rotating slowly during growth. The nanoscale fluctuations of the current contrast observed along the QW layers are caused, most likely, by the presence of the areas of lateral carrier localization, which originate during the formation of QWs by sub-monolayer digital alloying technique.