Size, shape, composition, and electronic properties of InAs/GaAs quantum dots by scanning tunneling microscopy and spectroscopy

InAs/GaAs quantum dot (QD) heterostructures grown by molecular beam epitaxy are studied using cross-sectional scanning tunneling microscopy and spectroscopy. The images reveal individual InAs QDs having a lens shape with maximum base diameter of 10.5 nm and height of 2.9 nm. Analysis of strain relaxation of the QDs reveals an indium composition varying from 65% at the base of the QD, to 95% at its center, and back to 65% at its apex. Room-temperature tunneling spectra acquired 3-4 nm from the center of a dot show a peak located in the upper part of the GaAs band gap originating from the lowest electron confined state of the QD, along with a tail in the conductance extending out from the valence band and originating from QD hole states. A computational method is developed for simulating the tunneling spectra using effective-mass bands treated in an envelope function approximation. By comparison of the computations to low-current spectra, the energy of the lowest electron, and highest hole QD states are determined. These energies are found to be in reasonably good agreement both with optical measurements and prior theoretical predictions of Wang [ Phys. Rev. B 59 , 5678 ( 1999 )] .