Abstract

The MBE growth of ultra-high-quality GaAs/AlGaAs epilayer structures has enabled the discovery of many novel physical phenomena, for example the fractional quantum Hall effect (FQHE) in two- dimensional electron systems (2DESs). In order to attain the highest electron mobilities (in excess of 107 cm2V-1s-1), it is necessary to understand and limit the dominant sources of disorder during the wafer growth. These include both intentional (i.e. doping) and unintentional charged impurities, interface roughness and surface defect states, all of which cause electron scattering.

The electron mobility in a modulation-doped 2DES is dominated by doping impurities. By contrast the electron mobility in an undoped is dominated by unintentional background impurities and interface roughness. By measurement and numerical modelling of the mobility versus density characteristics of an undoped wafer, it is possible to determine the concentration of background impurities and the characteristic length and height scales of the roughness. Therefore these systems are a powerful tool for assessment of the cleanliness of an MBE chamber and optimisation of growth conditions.