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The nanoscale intrinsic electrical properties of in-plane InAs nanowires grown by selective area epitaxy are investigated using a process-free method involving a multi-probe scanning tunneling microscope. The resistance of oxide-free InAs nanowires grown on an InP(111)B substrate and the resistance of InAs/GaSb core–shell nanowires grown on an InP(001) substrate are measured using a collinear four-point probe arrangement in ultrahigh vacuum. They are compared with the resistance of two-dimensional electron gas reference samples measured using the same method and with the Van der Pauw geometry for validation. A significant improvement of the conductance is achieved when the InAs nanowires are fully embedded in GaSb, exhibiting an intrinsic sheet conductance close to the one of the quantum well counterpart. [ABSTRACT FROM AUTHOR]

We have studied electronic transport in undoped GaAs/SrTiO3 coreâ€"shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/ ÎĽ m. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm thick SrTiO3 shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO3 shells as gate insulators. [ABSTRACT FROM AUTHOR]

Abstract: Using molecular beam epitaxy, we study self-assembled InAs quantum dots (QDs) grown on GaAs (001) substrates by lowtemperature scanning tunneling microscopy. We compare two different growth processes, the well-known Stranski-Krastanow mode and the more recently studied droplet epitaxy mode. We show that, for the same amount of deposited indium, the shape of the dots shows similar anisotropic tendency, but the InAs coverage dependence of the density shows that the growth mechanisms are different at the initial stage of the QD formation. By scanning tunneling spectroscopy at low temperature, we succeed in mapping the local density of states (LDOS) of a single quantum dot grown by the droplet epitaxy. Maps of LDOS are consistent with those previously reported for the Stranski–Krastanow mode; however, energy spectrum of our QDs shows different peak structures. [Copyright &y& Elsevier]

Cross-sectional scanning tunneling microscopy was used to study GaMnAs diluted magnetic semiconductors grown by low temperature molecular beam epitaxy. The Ga[sub 1-x]Mn[sub x]As layer, containing a concentration of x=0.005, shows that the dominant defect in the material is the arsenic antisite. Mn ions can also be resolved and show a signature distinct from the arsenic antisites. Spectroscopic measurements are perfomed to study the variation of the Fermi level between the Ga[sub 0.995]Mn[sub 0.005]As and GaAs layers. The Mn ions act as acceptor dopants. However, for x=0.005, the Mn concentration in comparison with the As antisite concentration is too small to induce a significant change of the Fermi level from the midgap position, preventing the layer from being ferromagnetic. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Synthesis of nanostructures of uniform size is fundamental because the size distribution directly affects their physical properties. We present experimental data demonstrating a narrowing effect on the length distribution of Ge nanowires synthesized by the Au-catalyzed molecular beam epitaxy on Si substrates. A theoretical model is developed that is capable of describing this puzzling behavior. It is demonstrated that the direction of the diffusion flux of sidewall adatoms is size dependent and has a major effect on the growth rate of differently sized nanowires. We also show that there exists a fundamental limitation on the maximum nanowire length that can be achieved by molecular beam epitaxy where the direction of the beam is close to the growth axis. [ABSTRACT FROM AUTHOR]