Dopant and carrier profiling in modulation-doped GaAs multilayers with cross-sectional scanning tunneling microscopy

We report the direct measurement of dopant and carrier density profiles in GaAs (001) modulation-doped structures using cross-sectional scanning tunneling microscopy. On ultrahigh-vacuum-cleaved cross-sectional (110) GaAs surfaces, individual electrically active dopants are observed as hillocks in the top several surface layers, and the tip-sample separation is found to be sensitive to the carrier concentration. In structures where the conventionally measured p-type dopant concentration varied from 1×1018 to 1×1019 cm-3 the density of such dopant hillocks varies accordingly and the tip-sample separation changes by 0.1 nm.