Electronic disorder of P- and B-doped Si at the metal-insulator transition investigated by scanning tunnelling microscopy and electronic transport

The (111)-2 × 1 surface of in situ cleaved heavily P- or B-doped Si is investigated by scanning tunnelling microscopy and spectroscopy at room temperature and at low temperature. P atoms have been identified on different sites of the Si(111)-2 × 1 surface by their characteristic voltage-dependent contrast for positive as well as negative buckling of the π-bonded chains. The distributions of dopants per surface area and of nearest-neighbour distances are found to be in agreement with a random arrangement of dopants in Si up to doping levels well above the metal–insulator transition. In addition, P atoms have been identified by their depth-dependent contrast down to the third layer beneath the surface with a volume density in agreement with the bulk doping density. The random electronic disorder supports the view of an Anderson transition driven by disorder close to the critical concentration or critical uniaxial stress

GaSb/GaAs quantum dot formation and demolition studied with cross-sectional scanning tunneling microscopy

We present a cross-sectional scanning tunneling microscopy study of GaSb/GaAs quantum dots grown by molecular beam epitaxy. Various nanostructures are observed as a function of the growth parameters. During growth, relaxation of the high local strain fields of the nanostructures plays an important role in their formation. Pyramidal dots with a high Sb content are often accompanied by threading dislocations above them. GaSb ring formation is favored by the use of a thin GaAs first cap layer and a high growth temperature of the second cap layer. At these capping conditions, strain-driven Sb diffusion combined with As/Sb exchange and Sb segregation remove the center of a nanostructure, creating a ring. Clusters of GaSb without a well defined morphology also appear regularly, often with a highly inhomogeneous structure which is sometimes divided up in fragments. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3701614

Linking structural and electronic properties of high-purity self-assembled GaSb/GaAs quantum dots

We present structural, electrical, and theoretical investigations of self-assembled type-II GaSb/GaAs quantum dots (QDs) grown by molecular beam epitaxy. Using cross-sectional scanning tunneling microscopy (X-STM) the morphology of the QDs is determined. The QDs are of high purity (similar to 100% GaSb content) and have most likely the shape of a truncated pyramid. The average heights of the QDs are 4-6 nm with average base lengths between 9 and 14 nm. Samples with a QD layer embedded into a pn-diode structure are studied with deep-level transient spectroscopy (DLTS), yielding a hole localization energy in the QDs of 609 meV. Based on the X-STM results the electronic structure of the QDs is calculated using 8-band k.p theory. The theoretical localization energies are found to be in good agreement with the DLTS results. Our results also allow us to estimate how variations in size and shape of the dots influence the hole localization energy

Atomically precise impurity identification and modification on the manganese doped GaAs(110) surface with scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy (STM) measurements on molecular beam epitaxy grown Mn doped GaAs(110) at 5 and 77 K are presented. The enhanced mechanical stability of the STM at low temperature allows a detailed study of the electronic contrast of Mn atoms in the GaAs(110) surface. According to reproducible and distinguishable contrast patterns of single Mn atoms, we present statistical evidence for a layer by layer identification of Mn atoms embedded in the first few monatomic layers of the crystal. A comparison with a bulklike theoretical approach reveals a semiquantitative agreement with the measurements. Remaining differences between theory and experiment indicate the influence of the surface as an important factor to understand the contrast of impurities close to the surface. Furthermore, we report the injection of transition-metal atoms into the surface. Finally, reproducible complexes consisting of a surface Mn and an adsorbate atom are found and manipulated

The structural, electronic and optical properties of GaSb/GaAs nanostructures for charge-based memory

The potential for GaSb nanostructures embedded in GaAs to operate as charge-based memory elements at room temperature is introduced and explored. Cross-sectional scanning-tunnelling microscopy is employed to directly probe and optimize the growth of nanostructures by molecular beam epitaxy. The results of structural analysis are combined with electrical measurements made with deep-level transient spectroscopy, showing excellent agreement with theoretical calculations which model the electronic structure of the nanostructures using 8-band k.p theory. Hole-localization energies exceeding 600 meV in quantum dots and near-100% material contrast between GaSb-rich quantum rings (QRs) and the surrounding GaAs matrix are revealed (no intermixing). Optical measurements confirm the depth of the hole localization, and demonstrate substantially lower inhomogeneous broadening than has previously been reported. Multiple peaks are partially resolved in ensemble photoluminescence of GaSb/GaAs QRs, and are attributed to charge states from discrete numbers of confined holes

Enhanced binding energy of manganese acceptors close to the GaAs(110) surface

Scanning tunneling spectroscopy was performed at low temperature on buried manganese (Mn) acceptors below the (110) surface of gallium arsenide. The main Mn-induced features consisted of a number of dI/dV peaks in the band gap of the host material. The peaks in the band gap are followed by negative differential conductivity, which can be understood in terms of an energy-filter mechanism. The spectroscopic features detected on the Mn atoms clearly depend on the depth of the addressed acceptor below the surface. Combining the depth dependence of the positions of the Mn-induced peaks and using the energy-filter model to explain the negative resistance qualitatively proves that the binding energy of the hole bound to the Mn atom increases for Mn acceptors closer to the surface