Abstract
Scanning tunneling microscopy (STM) was used to study the (NH 4 ) 2 S-passivated (110) cross-sectional surfaces of both doped and undoped Al 0.3 Ga 0.7 As/GaAs heterostructures on n + -substrates. The ex situ (NH 4 ) 2 S treatment of the cross-sectional surfaces of heterostructures was found to be very stable against oxidation. STM images showed no appreciable deterioration of surface quality in vacuum after more than 40 days. The spectroscopic results on the undoped epilayer showed diodelike behavior, confirming that an undoped large band gap region can be imaged by STM through carrier injection from the conductive regions.