Electronic states of InAs/GaAs quantum dots by scanning tunneling spectroscopy

Abstract

InAs/GaAs quantum-dot heterostructures grown by molecular-beam epitaxy are studied using cross-sectional scanning tunneling microscopy and spectroscopy. Individual InAs quantum dots (QDs) are resolved in the images. Tunneling spectra acquired 3-4 nm from the QDs show a peak located in the upper part of the GaAs bandgap originating from the lowest electron confined state, together with a tail extending out from the valence band from hole confined states. A line-shape analysis is used to deduce the binding energies of the electron and hole QD states.Published in Appl. Phys. Lett. 97, 123110 (2010).

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

“…1 of our prior publication. 20 We find a cross-sectional shape consistent with either a truncated pyramid, a truncated cone, or a lens (section of a sphere), similar to that found by prior workers. 4,6 Further definition of the QD shape can be obtained using the methodology of Bruls et al 4 in which the measured cross-sectional heights are plotted as a function of the measured cross-sectional base length, as shown in Fig.…”

“…As discussed in our prior work, we attribute this dramatic change in the spectra to the large currents passing through the dots which produce significant occupation of the (normally empty) electron confined states. 20 Similar type charging effects have been seen on other surfaces for which the transport of STM-injected current through the semiconductor is limited, such as neighboring dangling bonds on SiC(0001)√3×√3 or isolated dangling bonds on boron-doped Si(111)√3×√3. 26,27 In the present case, the substantial separation between the neighboring QDs produces this limitation in the transport of the electrons.…”

“…First, an atomic step is seen located about 6 nm away from the centers of both of the QDs, as marked on the images (see Fig. 1 of our prior work for a larger-scale view of such a step 20 ). For spectra acquired near the step, L and X, a distribution of states extending throughout the band gap (about 1 .…”

“…The QDs appear bright in the images because, after cleavage, they relax outwards due to the strain arising from the 7% lattice mismatch between InAs and GaAs. High-pass filtering of such images yields a more direct view of the cross-sectional QD shape, 20 as shown in Fig. 1 of our prior publication.…”

“…Charging effects in the spectra are also apparent, as discussed in our prior publication. 20 We deal with these effects by employing low-current spectra, and modeling them with a theory that includes 3D electrostatic simulation of the tip-vacuum-semiconductor geometry 21 together with computation of tunnel currents using a plane-wave solution of Schrödinger's equation in a supercell geometry. With this method, we are able to extract the energies of the lowest electron and highest hole confined states.…”