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Hydrogen incorporation in n-type Si-doped GaAs layers results in the neutralization of the active dopants and a change of the conductivity along the growth direction. To characterize the active dopant concentration of doped GaAs layers containing hydrogen, we have used secondary ion mass spectroscopy and cross-sectional scanning tunneling microscopy. Spectroscopic measurements are performed as well as conductance images to visualize the variation of the conduction band-edge position. Such a variation, which is related to the concentration of Si–H complexes, allows the determination of the doping profile. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies low temperature grown inGaAs with and without Be doping. Antisite defects in the Be-doped material; Precipitates formed by the annealing of the InGaAs; Dependence of the optical response time on Be doping and annealing.

We present cross-sectional scanning tunneling microscopy images and scanning tunneling spectroscopy results of InAs quantum dots grown on GaAs. The samples contain 12 arrays of quantum dots. The analysis of the scanning tunneling microscope images reveals the self-alignment of the dots as well as the different dot interfaces with the under- and overgrown GaAs layers. We measure the strain distribution along the [001] direction in the (110) plane. The roughness of the dot interfaces along the [1¯10] direction is also estimated and local spectroscopy of the dots evidences the electronic confinement (measured gap of 1.25 eV compared with 0.4 eV for bulk InAs). © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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]

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]

Scanning tunneling microscopy (STM) is used to study low temperature grown GaAs layers. Excess As gives rise to a high concentration of As antisites (As[sub Ga]). On these point defects, tunneling spectroscopy reveals a band of donor states. In fact, the measured tunneling current results from a pure tunneling current between the energy levels of the STM tip and the As[sub Ga] energy level E[sub t] followed by an exchange of carriers between E[sub t] and the bands or between E[sub t] and donor states of neighbor point defects. We determine the influence of both contributions on the current. We show that hopping conduction is required to explain the observation of the midgap states in tunneling spectroscopy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The knowledge of the band alignment in semiconductor heterostructures is crucial, as it governs carrier confinement with many impacts on the performances of devices. By controlling the direction of the current flow in in-plane In0.53Ga0.47As/InP heterostructure nanowires, either horizontally along the nanowires or vertically into the InP substrate with low temperature multiple-probe tunneling spectroscopy, a direct measurement of the band offsets at the buried In0.53Ga0.47As/InP heterointerface is performed. Despite the unavoidable processing steps involved in selective area epitaxy, conduction and valence band offsets of 0.21 ± 0.01 and 0.40 ± 0.01 eV are, respectively, found, indicating the formation of an interface with a quality comparable to two-dimensional In0.53Ga0.47As/InP heterostructures. [ABSTRACT FROM AUTHOR]

Silicon δ-doped layers in GaAs have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy on cleaved (110) surfaces. The samples were grown using molecular beam epitaxy with two growth temperatures: 480 and 580 °C. The concentration of the δ-doped layer was 0.03 monolayer. We show that, as in the case of bulk doping, the silicon has an amphoteric character: both Si[sub Ga] and Si[sub As] are observed at 480 °C and donors are formed before acceptors. At 580 °C, the spatial repartition of silicon evidences the segregation of silicon. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Scanning tunneling spectroscopy allows the determination of the density of states of the involved electrodes as well as the associated barrier height. We have measured the barrier height between tungsten tip and the GaAs (110) surface with a scanning tunneling microscope (STM) in ultrahigh vacuum. When the tunneling junction exhibited a low barrier height, we observed and explained an apparent widening of the GaAs gap which can mislead the real doping concentration of the sample. A theoretical model based on the self-consistent Keldysh–Green functions formalism is used to calculate the STM current. It shows a possible lowering of the barrier height according to the adsorption of As or Ga atoms on the apex of the tungsten tip. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract. We have studied barrier heights on Si(111)-7 x 7 and GaAs(110) cleaved surfaces using an ultrahigh vacuum (UHV) STM with a base pressure of 5 x 10[sup -11] Tort. As the barrier height measurement is sensitive to the nature of the apex, we first focus on the quality and the reproducibility of the tips. Polycrystalline tungsten tips am first prepared by electrochemical etching. Then the tips are cleaned and sharpened with combined UHV techniques: heating, field emission and self-sputtering process. Three techniques are used to measure the barrier height: l(z), z(V) and I(V) at variable z, called l(z(V), V). In most cases, the z(V) technique gives an average barrier height higher than 4 eV for both silicon and GaAs surfaces, whereas l(z) and l(z(V), V) lead to lower values. We explain these apparent contradictions by a theoretical model based on a self-consistent Keldysh Green function formalism. The adsorption of a non-W atom (As, Ga) on the W tip causes a change of the local density of states on the apex which modifies the weight of each exponential in the integral of the current. This may explain, with the formation of an electric dipole, the changes in the barrier height. [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: The ground state of the Ag/Si(111)-(3×1) has been investigated by low temperature scanning tunneling microscopy (STM) and density-functional theory. The Fourier transform of the STM image reveals a (6×2) reconstruction, which is theoretically found to yield a reconstruction with lower energy than the (3×1). The most stable (6×2) structural model leads to excellent correspondence between experimental and simulated STM images, and reveals a dimerization of the silver atoms in the channels formed by neighbouring honeycomb Si chains. [Copyright &y& Elsevier]

The performance of many semiconductor quantum-based structures is governed by the dynamics of charge carriers between a LocaLized state and a band of electronic states. Using scanning tunneling spectroscopy, we studied the transport of inelastic tunneling electrons through a prototypical localized state: an isolated dangling-bond state on a Si(111) surface. From the saturation of the current at an energy resonant with this state, the hole capture rate by the dangling bond was determined. By further mapping the spatial extension of its wave function, the localized nature of the level was found to be consistent with the small magnitude of its cross section. This approach illustrates how the microscopic environment of a single defect critically affects its carrier dynamics. [ABSTRACT FROM AUTHOR]

Ga[sub 1-x]Mn[sub x]As layers with Mn composition of up to 6.2% are investigated by cross-sectional scanning tunneling microscopy and spectroscopy. We identify in the tunneling spectra contributions from Mn[sup -, sub Ga] acceptor states, compensating As[sup 2+, sub Ga] donor states, and additional compensating donor states, which we suggest to be Mn[sup 2+, sub i] interstitials. On basis of the observed Fermi level shift and a charge carrier compensation analysis, we deduce the concentration of Mn[sup 2+, sub i] interstitials. Furthermore, scanning tunneling microscopy images suggest an inhomogeneous distribution of Mn dopant atoms. [ABSTRACT FROM AUTHOR]

We investigated the growth of silicon nanowires from Au-rich catalyst droplets by two different methods: chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). The growth rate is found to be diameter-dependent and increases with increasing precursor partial pressures. The comparison of the experimental results with models shows that the contribution of Si atoms that diffuses from the substrate and the NW sidewalls toward the catalyst droplet can be neglected in CVD for the different pressures used in this study, whereas it is the major source of Si supply for the MBE growth. In addition, by decreasing the number density of catalyst droplet prior to the NW growth in CVD, it is also found that this parameter affects the NWs morphology, increasing the tapering effect when the silane partial pressure is small enough to allow gold atom diffusion from the catalyst droplet. [ABSTRACT FROM AUTHOR]

The linewidth of the resonances in the single-electron tunneling spectra has been investigated for PbSe semiconductor nanocrystals (NCs) with scanning tunneling spectroscopy at low temperature. The linewidth of the resonances corresponding to tunneling through the first conduction and valence levels is found to increase with decreasing size of the NCs. Based on theoretical calculations, this broadening is mainly induced by the coupling between the tunneling electrons and the longitudinal optical phonon mode of the NC, and by the splitting of the degenerate electronic levels between the different L-valleys in the Brillouin zone. For the smallest sizes, it is shown that the intervalley splitting is the major source of broadening. [ABSTRACT FROM AUTHOR]

Studies samples containing multiple arrays of quantum boxes using cross-sectional scanning tunneling microscopy. Behavior of quantum boxes separated by spacer layers; Factor that affects the alignment probability between two sets of quantum boxes; Factor that poses limits on semiconductor applications development.

We investigate the photoconductivity of a n + -ZnO/ n -Ge NCs/ p + -GaAs junction where the active layer consists of heavily n-doped Ge NCs synthesized in the gas phase. Measurement of a significant current at energies smaller than the band gap of GaAs demonstrates the photogeneration of charge carriers by the Ge NCs. From the correlation of the NC size with the absorption threshold, a narrowing of the direct band gap in the Ge NC thin film is obtained and attributed to the heavy doping of the Ge NCs. A remarkably high electrical activation of ~15% is found for the incorporated P impurities in the NCs. [ABSTRACT FROM AUTHOR]

Low temperature scanning tunnelling spectroscopy and Raman spectroscopy were used to study the electronic and vibrational properties of silicene formed on the Ag(111) surface for coverage up to one monolayer in the temperature range 230–250 °C. The tunnelling spectra reveal the strong contribution of silver states in the measured density of states around the Fermi level. The Raman spectra are found to evolve as a function of the submonolayer coverages, giving rise at one monolayer coverage to peaks that are characteristic of chemical bonds with distorted sp 3 hybrid orbitals. Such properties account for the electronic transparency of the silicene/Ag(111) interface. [ABSTRACT FROM AUTHOR]

Scanning tunneling spectroscopy was used to investigate surface doping in silicon, based on the grafting of a boron-containing molecular layer and the subsequent thermal diffusion of boron into silicon. Curve fitting of the experimental I(V) characteristics with a planar computation of the tunnel current yields a dopant concentration that is consistent with secondary ion mass spectrometry analyses in the subsurface region. Additional two-point probe electrical measurements performed at variable tip separations indicate a bulk-like transport, that corresponds to a significant diffusion of the boron impurities below the surface of low doped n-type Si wafers. Such results show the interest of multiple-probe scanning tunneling microscopy as a non-invasive technique to determine the electrically active content of doped layers during the fabrication of advanced integrated circuits. [ABSTRACT FROM AUTHOR]

The composition and band gap of the shell that formed during the growth of axial GaAs/GaAs81Sb19/ GaAs heterostructure nanowires have been investigated by transmission electron microscopy combined with energy dispersion spectroscopy, scanning tunneling spectroscopy, and density functional theory calculations. On the GaAs81Sb19 intermediate segment, the shell is found to be free of Sb (pure GaAs shell) and transparent to the tunneling electrons, despite the (110) biaxial strain that affects its band gap. As a result, a direct measurement of the core band gap allows the quantitative determination of the band offset between the GaAs81Sb19 core and the GaAs shell and identifies it as a type I band alignment. [ABSTRACT FROM AUTHOR]

The epitaxial growth and the electrical resistance of multilayer silicene on the Ag(111) surface has been investigated. We show that the atomic structure of the first silicene layer differs from the next layers and that the adsorption of Si induces the formation of extended silicene terraces surrounded by step bunching. Thanks to the controlled contact formation between the tips of a multiple probe scanning tunneling microscope and these extended terraces, a low sheet resistance, albeit much higher than the electrical resistance of the underlying silver substrate, has been measured, advocating for the electrical viability of multilayer silicene. [ABSTRACT FROM AUTHOR]

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm-1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at V in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement. [ABSTRACT FROM AUTHOR]

We investigated the spectroscopic photocurrent response of photovoltaic devices versus an increasing number of drop-casted CdSe nanoparticles onto planar and nanocones silicon p-i-n junctions. For all samples, a strong enhancement of the photocurrent in the UV range was detected as well as a constant increase of the photocurrent up to 20% (16%) for a planar (nanocones) junction in the range 600-800 nm. The analysis of the photocurrent versus the number of drop casted nanoparticles layers allows us to evidence a down-shifting mechanism in the U-V range and an adaptative index effect below the threshold of absorption. [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]

Semiconductor PbSe/CdSe core-shell nanocrystals (NCs) in a double barrier tunnel junction have been investigated by means of scanning tunneling spectroscopy at low temperature. From the analysis of the differential conductance peak position as a function of the potential distribution in both potential barriers, we demonstrate a unipolar transport regime for a large amount of NCs. The same charge carriers are injected on both sides of the zero-conductance gap, and the peaks observed at higher energy arise from the charging of the NCs. Similar results are obtained for CdSe/CdS dot-in-rod NCs, indicating that the addition of a shell favors transitions between different charge states rather than single particle excited states. Further characterization of the PbSe/CdSe core-shell NCs by x-ray photoemission spectroscopy reveals that the variations in the transport properties from NC to NC are explained by the occurrence of unprotected PbSe facets that have different orientations in the junction. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate the organization of nearly monodisperse colloidal InP quantum dots at the air/water interface in Langmuir monolayers. The organization of the particles is monitored in situ by surface pressure–surface area measurements and ex situ by AFM measurements on films transferred to mica by Langmuir–Blodgett deposition. The influence of different ligands on the quality of the monolayer formed has been studied. We show that densely packed monolayers with little holes can be formed using short chain ligands like pyridine and pentamethylene sulfide. The advantage of using short chain ligands for electron tunneling to or from the quantum dots is demonstrated using scanning tunneling spectroscopy. [Copyright &y& Elsevier]

Abstract: The interactions of PTCDA molecules with the Si(100) surface have been investigated by means of X-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). Among the large distribution of the adsorbates observed by STM for submonolayer coverages, around 20% show a distinct feature, which is interpreted as the product of the covalently linkage between PTCDA and Si atoms. The formation of this product through the anhydride groups is further supported by the XPS measurements. The occurrence of different adsorption states results eventually in the formation of a disordered initial layer, which nevertheless acts as a passivation layer and leads to the formation of islands for coverage above 1ML. These islands have a common layer structure, similar to the molecular arrangement found in the (102) plane of PTCDA bulk phase. [Copyright &y& Elsevier]

The authors studied the transport in n- and p-type silicon nanowires. When the temperature decreases from 325 to 75 K, they observed a variation of the nanowire resistance, consistent with a transport governed by electron-phonon scattering. The lateral size of the nanowire down to 25 nm is not found to cause further surface scattering, due to the presence of interface states which create a depleted region in the nanowires. Such depleted region allows thus to keep a carrier mobility in the nanowires similar to the bulk one. [ABSTRACT FROM AUTHOR]

The authors have performed electrical measurements at variable temperatures on self-assembled titanium silicide nanowires (NWs) grown on a Si(111) surface. The authors find a metallic I(V) characteristic for the NWs at a temperature of 77 K, whereas scanning tunneling spectroscopic measurements obtained at temperatures below 25 K yield a rectifying behavior. This behavior indicates that the NWs are electronically decoupled from the Si surface on a voltage range of several hundreds of meV at low temperatures. From these measurements, the authors precisely determine the Schottky barrier height between the NWs and the Si surface. [ABSTRACT FROM AUTHOR]

We report on simulations and measurements of the optical absorption of silicon nanowires (NWs) versus their diameter. We first address the simulation of the optical absorption based on two different theoretical methods: the first one, based on the Green function formalism, is useful to calculate the scattering and absorption properties of a single or a finite set of NWs. The second one, based on the finite difference time domain (FDTD) method, is well-adapted to deal with a periodic set of NWs. In both cases, an increase of the onset energy for the absorption is found with increasing diameter. Such effect is experimentally illustrated, when photoconductivity measurements are performed on single tapered Si nanowires connected between a set of several electrodes. An increase of the nanowire diameter reveals a spectral shift of the photocurrent intensity peak towards lower photon energies that allow to tune the absorption onset from the ultraviolet radiations to the visible light spectrum. [ABSTRACT FROM AUTHOR]

Stabilizing ordering instead of randomness in alloy semiconductor materials is a powerful means to change their physical properties. We used scanning tunneling and transmission electron microscopies to reveal the existence of an unrecognized ordering in ternary III-V materials. The lazarevicite short-range order, found in the shell of InAs1-xSbx nanowires, is driven by the strong Sb-Sb repulsion along ⟨110⟩ atomic chains during their incorporation on unreconstructed {110} sidewalls. Its spontaneous formation under group-III-rich conditions of growth offers the prospect to broaden the limited classes of ordered structures occurring in III-V semiconductor alloys. [ABSTRACT FROM AUTHOR]

We investigate the electronic structure of the [formula] prototype surface for GaN nanowire sidewalls. We find a paradoxical situation that a surface state at all k points in the bandgap cannot be probed by conventional scanning tunneling microscopy, due to a dispersion characterized by a steep minimum with low density of states (DOS) and an extremely flat maximum with high DOS. Based on an analysis of the decay behavior into the vacuum, we identify experimentally the surface state minimum 0.6 ± 0.2 eV below the bulk conduction band in the gap. Hence, GaN nanowires with clean [formula] sidewall facets are intrinsically pinned. [ABSTRACT FROM AUTHOR]

The band structure and the Fermi level pinning at clean and well-ordered sidewall surfaces of zincblende (ZB)-wurtzite (WZ) GaAs nanowires are investigated by scanning tunneling spectroscopy and density functional theory calculations. The WZ-ZB phase transition in GaAs nanowires introduces p-i junctions at the sidewall surfaces. This is caused by the presence of numerous steps, which induce a Fermi level pinning at different energies on the non-polar WZ and ZB sidewall facets. [ABSTRACT FROM AUTHOR]

The existence of intrinsic surface states, the position of the Fermi level, and the size of the surface bandgap of the non-polar ZnO[formula] cleavage surfaces were investigated by scanning tunneling microscopy and spectroscopy. The comparison of spectroscopic measurements performed on atomically flat and stepped surfaces reveals the absence of intrinsic surface states within the fundamental bulk bandgap, but shows the occurrence of step-induced gap states. These states lead to a pinning of the Fermi level at the surface within the bandgap and generate a significant defect-related tunnel current, narrowing the measured apparent bandgap. [ABSTRACT FROM AUTHOR]