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Topological electronic phases exist in a variety of naturally occurring materials but can also be created artificially. We used a cryogenic scanning tunneling microscope to create dimerized chains of identical quantum dots on a semiconductor surface and to demonstrate that these chains give rise to one-dimensional topological phases. The dots were assembled from charged adatoms, creating a confining potential with single-atom precision acting on electrons in surface states of the semiconductor. Quantum coupling between the dots leads to electronic states localized at the ends of the chains, as well as at deliberately created internal domain walls, in agreement with the predictions of the Su-Schrieffer-Heeger model. Scanning tunneling spectroscopy also reveals deviations from this well-established model manifested in an asymmetric level spectrum and energy shifts of the boundary states. The deviations arise because the dots are charged and hence lead to an onsite potential that varies along the chain. We show that this variation can be mitigated by electrostatic gating using auxiliary charged adatoms, enabling fine-tuning of the boundary states and control of their quantum superposition. The experimental data, which are complemented by theoretical modeling of the potential and the resulting eigenstates, reveal the important role of electrostatics in these engineered quantum structures., 16 pages double-line text plus appendix, 7 figures

Scanning tunneling spectroscopy was used to examine the cross-sectional surface of an InAs p–n junction at low temperature. The depletion layer only in the p-type region was studied by employing a substrate that was doped with large amounts of donor and acceptor impurities as the n-type region. The energy band profile for the p–n junction reveals that the width of the depletion layer in the n-type region is negligibly small. Compared with the expected width of the depletion layer in the p-type region on the basis of the doped acceptors, the observed width is much wider, indicating the low ionization ratio of the acceptors at low temperature. Owing to the small amount of tip-induced band bending (TIBB) for the conduction band in the p-type region, the observed conduction band edge is fitted well with a simple calculation. In contrast, the observed valence band edge is modified by the TIBB.

Indium atom manipulation was achieved on the (110) cross-sectional surface of a quantum structure consisting of InAs/GaSb multilayer using a scanning tunneling microscope (STM) at low temperature. Positioning, removing, and lateral displacement by means of tip-induced hopping was performed on both InAs and GaSb layers. Electron tunneling via the electron orbital of the manipulated atom (adatom) was observed not only on the InAs layer but also on the GaSb layer, even when the energy of the orbital was in the GaSb band gap. The tunneling process from the STM tip to the InAs conduction band via the adatom and the GaSb band gap is explained well by the single electron tunneling model through the double-barrier tunneling junction. In the case of the adatom being very close to the InAs/GaSb heterointerface, interference with the electron tunneling to the InAs conduction electronic states through the GaSb barrier with/without passing the adatom was observed. The peak energy in the tunneling spectrum corresponding to the orbital of the adatom depends on the adatom position on the GaSb, demonstrating the fundamental interactions between the surface atomic-scale structures and semiconductor heterostructures.

We created hexagonal rings on a semiconductor surface by atom manipulation in a scanning tunneling microscope (STM). Our measurements reveal the generic level structure of a quantum ring, including its single ground state and doubly degenerate excited states. The ring shape leads to a periodic potential modulation and thereby a perturbation of the level structure that can be understood in analogy to band gap formation in a one-dimensional periodic potential. The modulation can be enhanced or inverted by further adding or removing atoms with the STM tip. Our results demonstrate the possibility of designing and controlling electron dynamics in a tunable periodic potential, holding promise for the construction of two-dimensional artificial lattices on a semiconductor surface.

Transistors, regardless of their size, rely on electrical gates to control the conductance between source and drain contacts. In atomic-scale transistors, this conductance is exquisitely sensitive to single electrons hopping via individual orbitals. Single-electron transport in molecular transistors has been previously studied using top-down approaches to gating, such as lithography and break junctions. But atomically precise control of the gate - which is crucial to transistor action at the smallest size scales - is not possible with these approaches. Here, we used individual charged atoms, manipulated by a scanning tunnelling microscope, to create the electrical gates for a single-molecule transistor. This degree of control allowed us to tune the molecule into the regime of sequential single-electron tunnelling, albeit with a conductance gap more than one order of magnitude larger than observed previously. This unexpected behaviour arises from the existence of two different orientational conformations of the molecule, depending on its charge state. Our results show that strong coupling between these charge and conformational degrees of freedom leads to new behaviour beyond the established picture of single-electron transport in atomic-scale transistors., 15 pages, 4 figures

Thermal desorption flux of cation adatoms is modeled in the context of the transition state theory. The model, on the basis of thermally excited electron tunneling to surface adatoms, suggests that the rate of phonon-stimulated electronic transition (In+ + e− → In0) by tunneling is essential to explain the desorption flux at the InAs surface. The adatom thermal desorption is electronic in nature. The importance of an electronic process as one of elemental dynamics of epitaxy, as well as collisional kinetic energy transfer among heavy nuclei of atoms and surface, is emphasized. By means of verifying the equivalence of electrons and phonons as excitation sources in the case of the electronic transition, usefulness of low-temperature scanning tunneling microscopy (LT-STM) as a tool for studying the crystal growth mechanism is explained. A molecular beam epitaxy (MBE) system equipped with the LT-STM provides a promising complement to in situ STM observation during MBE.

Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantum dots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. Electrostatic gates can be used to mitigate these variations by adjusting the electron energy levels, but the more ambitious goal of creating quantum dots with intrinsically digital fidelity by eliminating statistical variations in their size, shape and arrangement remains elusive. We used a scanning tunnelling microscope to create quantum dots with identical, deterministic sizes. By using the lattice of a reconstructed semiconductor surface to fix the position of each atom, we controlled the shape and location of the dots with effectively zero error. This allowed us to construct quantum dot molecules whose coupling has no intrinsic variation but could nonetheless be tuned with arbitrary precision over a wide range. Digital fidelity opens the door to quantum dot architectures free of intrinsic broadening-an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies of confined electrons.

Indium adatoms remaining at the InAs(111)A surface after molecular beam epitaxy were characterized by a low-temperature scanning tunneling microscope at 5 K. The resonance spectrum of the adatom orbital was analyzed including the tip-induced line width broadening effect. The energy difference between the resonance level and surface Fermi level was estimated to be 0.79 eV at 420 °C, which is equivalent to the adatom desorption energy of 0.8 eV previously reported. The adatom desorption dynamics was modeled on the basis of the comparison between the period of the phonon-assisted inelastic electron tunneling and the electron lifetime at the adatom resonance level. The calculated temperature range of the adatom desorption excellently agrees with the reported experimental one. It is suggested that the adatom density was almost completely maintained and observed at 5 K as it was at 420 °C (693 K).

Scanning tunneling microscopy (STM) at 5 K reveals that native atoms in the surface layer of a semiconductor crystal become bistable in vertical height when a nanostructure is assembled nearby. The binary switching of surface atoms, driven by the STM tip, changes their charge state. Coupling is facilitated by assembling adatom chains, allowing us to explore the emergence of complex multiple switching. Density-functional theory calculations rationalize the observations and a lattice-gas model predicts the cooperative behavior from first principles.

We measure local density of states (LDOS) for GaSb/InAs heterostructures with quantum wells in the valence band by scanning tunneling spectroscopy (STS) on the cleaved surface. Clear standingwave patterns of LDOS corresponding to the holes confined in the quantum wells are observed.

Low-temperature scanning tunnelling microscopy (LT-STM), surface acoustic wave (SAW), and mechanical cantilever are used to measure and manipulate quantum characteristics in semiconductor heterostructures. A nanoscale detection of local density of states is demonstrated for electrons confined in semiconductor quantum wells and dots by a combination of InAs-based heterostructures and LT-STM. The SAW is used to realize moving quantum wires and dots in semiconductor heterostructures. A synchronized optical measurement confirms two types of modulation, i.e. bandgap and potential modulations. A combination of mechanical cantilever and quantum heterostructures gives a new mechanical functionality and a novel tool to analyse quantum transport characteristics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We measure the spatial variation of the local density of states normal to an InAs/GaSb heterointerface on a cleaved cross-sectional surface by low-temperature scanning tunneling spectroscopy. Clear standing waves originating from the interference between the incident and reflected electron waves at the interface are observed.

The spatial distribution of the electron local density of states (LDOS) in InAs/GaSb double quantum wells (DQWs) was investigated by low-temperature scanning tunneling spectroscopy on cleaved surfaces. For DQW with a thick central barrier, clear standing wave patterns corresponding to subbands confined to each InAs single quantum well appeared in the spatial variation of LDOS spectra. In contrast, for the DQW with a thin central barrier, the standing wave patterns extended over both quantum wells. The deviation of the pattern arising from the asymmetry due to a slight difference of the well thickness appeared clearly. The observed spectra are well explained by the calculated LDOS taken to be the sum of LDOS contributed from all energetically accessible subbands.

Low-temperature scanning tunneling spectroscopy under ultra-high vacuum was employed to investigate the two-dimensional electron system at the epitaxial surface of Sidoped In0.53Ga0.47As/In0.52Al0.48As(111)A quantum-well structures. The electron density in the near-surface region of the quantum well could be controlled through modulation doping. Spectra of the electronic local density of states in the conduction band showed a clear step-like energy dependence that reveals the subband states. In spectra acquired at some areas of nanometer size, peaks were observed near subband minima, indicating the existence of bound states.

The structural stability of InAs stacking-fault tetrahedron (SFT) in InAs/GaAs (1 1 1) is theoretically investigated. Using an empirical interatomic potential, cohesive energies are calculated for the three types of InAs/GaAs(1 1 1) system where coherent InAs and relaxed InAs with the SFT and misfit dislocations (MDs). The calculated results reveal that InAs with the SFT is more favorable beyond the film thickness of 21 monolayers (MLs) than coherent InAs. The critical film thickness of 21 ML is comparable with that of 8 ML for the MDs generation. This suggests that the SFT appears in InAs thin film layers instead of MDs resulting from lowering the strain energy accumulated in InAs thin film layers.

The electronic density of states at clean surfaces of n-type In 0.53 Ga 0.47 As, grown by molecular beam epitaxy on lattice-matched (0 0 1)- and (1 1 1)A-oriented InP substrates, was measured by low-temperature scanning tunneling spectroscopy under ultra-high vacuum. It was found that the surface Fermi level (FL) pinning strength dramatically depends on crystal orientation. The FL at the (0 0 1)-(2×4) surface is pinned near midgap, independently of the dopant concentration in the bulk. In contrast, the FL at the (1 1 1)A-(2×2) surface lies in the conduction band, close to the bulk FL, and increases with dopant concentration.

Quantum-dot molecules were constructed on a semiconductor surface using atom manipulation by scanning tunneling microscopy (STM) at 5 K. The molecules consist of several coupled quantum dots, each of which comprises a chain of charged adatoms that electrostatically confines intrinsic surface-state electrons. The coupling takes place across tunnel barriers created reversibly using the STM tip. These barriers have an invariant, reproducible atomic structure and can be positioned---and repeatedly repositioned---to create a series of reconfigurable quantum-dot molecules with atomic precision.

By means of scanning tunneling spectroscopy (STS) we study the electronic structure of the III-V semiconductor surface InAs(111)A in the field emission regime (above the vacuum level). At high sample bias voltages (approaching +10 V), a series of well defined resonances are identified as the typical Stark shifted image states that are commonly found on metallic surfaces in the form of field emission resonances (FER). At lower bias voltages, a more complex situation arises. Up to three double peaks are identified as the first three FERs that are split due to their interaction with the periodic surface potential. The high corrugation of this potential is also quantified by means of density functional theory (DFT) calculations. Another sharp resonance not belonging to the FER series is associated with an unoccupied surface state., 6 pages, 4 figures

We investigated the surface properties of InAs(1 1 1)A by low-temperature scanning tunneling microscopy (LT-STM) with atomic resolution and first-principles calculation. Very clear atom image was observed, showing that the surface reconstruction is an In-vacancy structure. We also observed two kinds of adsorbates on the surface. The first-principles calculations indicate that the In-vacancy structure is the most stable surface reconstruction under any experimental conditions, which is consistent with the LT-STM observation. Investigations of adsorption properties of an In atom, an As atom, and an As2 molecule by the first-principles calculations imply that the observed adsorbates are an In atom and an As2 molecule.

The local density of states at cleaved cross-sectional surfaces of InAs/GaSb superlattices were investigated by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) with atomic resolution. Broken-gap energy band profiles (the overlap of the conduction band of InAs and the valence band of GaSb) were clearly seen in the LT-STS spectra as a function of growth direction. Clear images of electron standing waves confined in the superlattice were also obtained. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-dimensional (2 D) electron systems in compound semiconductors are markedly important not only for studies of low-dimensional electron physics but also for application in recent information-oriented society. To investigate behavior of such 2 D electrons in nanometer-scale regions, the local density of states (LDOS) was characterized at the clean surfaces in ultra-high vacuum using a low-temperature scanning tunneling microscope (LT-STM). This LT-STM was equipped with a chamber for molecular beam epitaxy (MBE) of III-V compound semiconductors. At the InAs(111)A surfaces, LDOS standing waves were clearly imaged around the defects. The wave features were confirmed to originate from the Friedel oscillation of naturally formed 2 D states in the surface electron accumulation layer. Also, the electronic states of highly Si-doped InGaAs surface quantum wells (QWs), grown on the GaAs(111)A substrates, were characterized. The LDOS spectra revealed the formation of artificial 2 D subbands quantized in the QWs. The effects related to phase coherence in the QWs are discussed. It is shown that nanoprobing the clean surfaces is very useful for microscopic understanding of quantum mechanical phenomena aiming at advanced electronic devices for future communication technologies.

We studied the structural, electrical, and mechanical properties of an InAs thin film grown on GaAs (1 1 1)A substrates by molecular beam epitaxy. In contrast to conventionally used (0 0 1) surfaces, where Stranski–Krastanov growth dominates the highly mismatched heteroepitaxy, layer-by-layer growth of InAs can be established. One of the largest advantages of this unique heteroepitaxial system is that it provides a two-dimensional electron gas system in the near-surface region without the problem of electron depletion. We review the fundamental properties and applications of this unique heteroepitaxial system.

A scanning tunnelling microscope (STM) operated at 5 K was used to study dysprosium biphthalocyanine (DyPc2) molecules adsorbed on the inert III–V semiconductor surface InAs(1 1 1)A. Orbital imaging and scanning tunnelling spectroscopy measurements reveal that the molecular electronic structure remains largely unperturbed, indicating a weak molecule-surface binding. The molecule adsorbs at the In vacancy site of the (2 × 2)-reconstructed surface and is highly sensitive to current-induced excitations leading to random rotational fluctuations. Owing to the weak surface binding, individual molecules can be precisely repositioned and arranged by the STM tip via attractive tip-molecule interaction. In this way, DyPc2 dimers of well-defined internal structure can be assembled which exist in two conformations of equivalent appearance. A binary switching between these two conformers can be induced by injecting electrons into one of the two molecules. The conformational change of the dimer proceeds via a concerted molecular rotation and minor lateral displacement. The synchronised switching observed here is attributed to steric interactions between the two molecules constituting the dimer.

A scanning tunneling microscope (STM) operated at 5 K was used to reposition positively charged In adatoms on the InAs(111)A-(2 × 2) surface with atomic-scale precision. Vertical atom manipulation allows for a reversible and highly reliable exchange of individual In atoms between the surface and the apex of the STM tip. Chains of In adatoms assembled in this way create an attractive potential for surface-state electrons. The resulting chain-confined states have wavefunctions with n lobes and n – 1 nodes, establishing the generic properties of a quantum dot (QD) with a perfectly defined level structure. In a second step, QD assemblies are constructed whose quantum coupling has no intrinsic variation but can nonetheless be tuned over a wide range. The states of these QD molecules can be described by standard molecular-orbital theory in terms of their bonding–antibonding character and degeneracy, providing a simple and transparent means to predict the level structure of coupled QDs assembled one atom at a time.

Low-temperature scanning tunneling spectroscopy under ultrahigh vacuum was used to investigate an In 0.53 Ga 0.47 As surface quantum well (QW) grown by molecular beam epitaxy. The electronic local density of states (LDOS) was probed with nanometer-scale resolution around native point defects located at the QW surface. In LDOS spectra acquired in the vicinity of a single point defect, a sharp peak was observed near each subband minimum, which indicates the formation of donor bound states. The LDOS peak intensity was measured as a function of distance from the point defect in order to estimate the Bohr radius of the donor bound states.

The phase transitions between $(2\ifmmode\times\else\texttimes\fi{}4)/c(2\ifmmode\times\else\texttimes\fi{}8)$ and $c(4\ifmmode\times\else\texttimes\fi{}4)$ on GaAs(001) surfaces are studied by ultrahigh-vacuum scanning tunneling microscopy. It is evident that the Ga diffusion is remarkable for length up to the micrometer range at the beginning of the transition, but the diffusion is prevented by nucleation at the final stage of the transition. We found that the size of the minimum nucleus depends on reconstruction. The structural stability is discussed, taking the surface electronic properties into account.

The electronic properties of monolayer steps on (2 x 4)/c(2 x 8) reconstructed GaAs (0 0 1) surfaces are studied by ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). We quantitatively demonstrate that steps play the role of acceptor arrays by applying electron counting consideration to proposed structure models for monolayer steps. This suggestion is consistent with experimental results on vicinal GaAs (0 0 1) surfaces measured by STM. Energetical stability of the proposed monolayer step structures is theoretically explained. We found that the acceptors at the steps are equivalent to those at the kinks. We confirmed that the surface states responsible for surface Fermi level pinning are located at breaking points of coherent arrangement of semiconducting (2 x 4) unit cells on GaAs (0 0 1) surfaces.

Single free-base phthalocyanine (H2Pc), copper phthalocyanine (CuPc), and naphthalocyanine (NPc) molecules on the III-V semiconductor surface InAs(111)A-(2 × 2) were investigated by cryogenic scanning tunneling microscopy (STM) at 5 K. STM imaging of largely unperturbed frontier orbitals of NPc on InAs(111)A reveals that the molecule is physisorbed. In the adsorbed state, the molecular electronic structure of NPc is preserved to a large extent, indicating a weak electronic coupling to the underlying substrate surface. As a free molecule, H2Pc is bistable because of an internal hydrogen transfer reaction (tautomerization). When adsorbed on the InAs(111)A surface, H2Pc experiences rotational fluctuations about its center because of excitations induced by inelastic electron tunneling (IET). STM-based atom and molecule manipulation techniques were used to sterically hinder the molecular rotation by assembling In adatom-molecule complexes, facilitating to probe the hydrogen transfer reaction in a controlled way. STM imaging of the Inad-H2Pc-Inad complex clearly reveals the presence of a left-handed and a right-handed conformer, indicating that the H2Pc tautomerization is left unperturbed by the substrate and the pinning adatoms, and that it can be triggered by the tunneling electrons.

The electronic properties of monolayer steps on (2\ifmmode\times\else\texttimes\fi{}4)/c(2\ifmmode\times\else\texttimes\fi{}8) reconstructed GaAs(001) surfaces are measured using ultrahigh vacuum scanning tunneling microscopy. We propose unit structures for steps and demonstrate that steps play the role of acceptor arrays. An electron-counting consideration on the steps explains the densities of kinks in the As dimer rows on vicinal surfaces. We found that the acceptors at the steps are identical to those at the kinks. We confirmed that the surface states related to surface Fermi-level pinning are located at breaking points of a coherent arrangement of semiconducting (2\ifmmode\times\else\texttimes\fi{}4) unit cells. \textcopyright{} 1996 The American Physical Society.

Individual tin phthalocyanine (SnPc) molecules adsorbed on the InAs(111)A surface were studied by low-temperature scanning tunnelling microscopy (STM) at 5 K. Consistently with the nonplanar molecular structure, SnPc adopts two in-plane adsorption geometries with the centre Sn atom either above (SnPc(up)) or below (SnPc(down)) the molecular plane. Depending on the current and bias applied to the tunnel junction, the molecule can be reversibly switched between the two conformations, implying a controlled transfer of the Sn atom through the molecular plane. The SnPc(down) conformer is characterized by an enhanced surface bonding as compared to the SnPc(up) conformer. SnPc(up) molecules can be repositioned by the STM tip by means of lateral manipulation, whereas this is not feasible for SnPc(down) molecules. The reversible switching process thus enables one to either laterally move the molecule or anchor it to the semiconductor surface.

A weak perturbation of a single molecule by the supporting substrate is a key ingredient to molecular electronics. Here, we show that individual phthalocyanine molecules adsorbed on GaAs(110) and InAs(111)A surfaces represent prototypes for weakly coupled single-molecule/semiconductor hybrid systems. This is demonstrated by scanning tunneling spectroscopy and bias-dependent images that closely resemble orbital densities of the free molecule. This is in analogy to results for molecules decoupled from a metal substrate by an ultrathin insulating layer and proves a weak electronic molecule-substrate coupling. Therefore, such systems will allow single-molecule functionality to be combined with the versatility of semiconductor physics.

Terraces, steps and growth islands on the surfaces of MBE heteroepitaxial AlGaAs and NiAl on GaAs are observed by an MBE-STM hybrid system. The nucleation-and-growth model was confirmed and multi-layer growth process was revealed on AlGaAs. Extended terraces and trenches between the terraces were revealed on NiAl, the latter being undetected by conventional reflection high energy electron diffraction (RHEED). A UHV high-resolution (HR) SEM was modified to perform real-time observation of GaAs MBE growth. Monolayer steps and 10 nm wide growth islands are observed during growth.

Molecular-beam-epitaxy(MBE)-grown GaAs(001) As-stabilized surface morphology has been in-situ observed by scanning tunneling microscopy (STM). A multichamber S(R)EM-MBE-STM system has been developed. The correspondence between reflection high-energy electron diffraction (RHEED) oscillation and two-dimensional nucleation has first been verified by STM on vicinal GaAs surfaces tilted from (001) toward [110] direction. Moreover, STM observations of GaAs surfaces grown at high temperature with a slow growth rate on exactly oriented substrates reveal terraces with an area of more than 200 nm 2 , which is the largest reported to date. STM has manifested itself to be indispensable in the development of atomic-level controlled-growth technologies for quantum-effect devices.

Heteroepitaxy of high-quality InSb films was performed directly on GaAs surfaces by using molecular beam epitaxy. Despite the 14.6% lattice mismatch, two-dimensionally grown InSb on GaAs(111)A substrates were obtained from the initial stage, but not on (001) substrates. A conductive layer was formed from the early stage of the growth on the (111)A surface, and the mobilities and carrier concentrations of InSb on (111)A substrates suggested a low defect density due to confinement of the dislocations to the interface.

The vertical manipulation of native adatoms on a III-V semiconductor surface was achieved in a scanning tunneling microscope at 5 K. Reversible repositioning of individual In atoms on InAs(111)A allows us to construct one-atom-wide In chains. Tunneling spectroscopy reveals that these chains host quantum states deriving from an adatom-induced electronic state and substantial substrate-mediated coupling. Our results show that the combined approach of atom manipulation and local spectroscopy is capable to explore atomic-scale quantum structures on semiconductor platform.

Low-temperature scanning tunneling spectroscopy under ultrahigh vacuum was used to study donor point defects located at the epitaxial surface of an ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ quantum well. The electronic local density of states was measured with nanoscale resolution in the vicinity of single defects. In this way, both the binding energy and the Bohr radius of the defects could be determined. The binding energy and the Bohr radius were found to be functions of the quantum well thickness, in quantitative agreement with variational calculations of hydrogenic impurity states.

We measure the spatial distribution of the local density of states (LDOS) at cleaved surfaces of InAs/GaSb isolated quantum wells and double quantum wells (DQWs) by low-temperature scanning tunneling spectroscopy. Distinct standing wave patterns of LDOS corresponding to subbands are observed. These LDOS patterns and subband energies agree remarkably well with simple calculations with tip-induced band bending. Furthermore, for the DQWs, coupling of electronic states between the quantum wells is also clearly observed.

The local tunneling barrier height (LTBH) has been measured using low‐temperature scanning tunneling microscopy to examine the local potential profile of an InAs nanostructure. The nanostructure is a faultily‐stacked nanocrystal in epitaxial InAs thin film grown on GaAs(111)A substrate. It is found that averaged LTBH is consistent with the workfunction or electron affinity of InAs. The nanostructure boundary is found to have a higher LTBH than the surroundings. The resonance peak calculated using this potential wall is comparable to that measured by spectroscopy of local density of states (LDOS) in the nanostructure. A gradual LTBH decrease is additionally observed at negative sample bias voltage near the boundary indicating downward band bending, which is consistent with LDOS there.

Low-temperature scanning tunneling microscopy at $7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is used to study the adsorption and manipulation of single pentacene molecules on Cu(111). Controlled lateral translations of the molecule are performed along different high-symmetry directions of the substrate via attractive tip-molecule interactions. By means of a detailed site-mapping technique combining lateral manipulations of the molecule and of native substrate adatoms we determine the adsorptive configuration of the aromatic molecule. We find a planar adsorption geometry with the long molecular axis aligned with the close-packed Cu atom rows and the benzene units centered over hexagonal close-packed hollow sites of the substrate.

The electronic features of semiconductor nanostructures, such as zero-dimensional states, are usually inferred from macroscopic optical and transport experiments. Although, direct probing of electrical features in semiconductor nanostructures looks very attractive, it is very difficult for a conventional semiconductor structure. However, direct probing becomes possible through a combination of low-temperature scanning tunneling microscopy and InAs(111)A surface in an ultra-high vacuum, where conductive electrons automatically accumulate near the clean surface. The clear observation of a Friedel oscillation pattern around a dislocation demonstrates successful mapping of the local-density-of-states (LDOS) of the conductive electrons. Inverted pyramidal defects are naturally formed during molecular beam epitaxial growth of InAs thin films on GaAs(111)A substrates and they operate as well-defined quantum dots. The measured LDOS pattern inside the quantum dots clearly changes as a function of energy, i.e. a sample bias, reflecting the LDOS pattern of each zero-dimensional state. A resonant concentration of the LDOS to the zero-dimensional energy levels is also demonstrated in these experiments. The LDOS measurements of a series of inverted pyramidal quantum dots with different side lengths and their comparison with theoretical calculations suggest a unique feature of the quantum dot system examined in this study.

The local density of states (LDOS) was characterized at the surface of the InAs thin film epitaxially grown on a GaAs(111)A substrate using low-temperature Scanning tunneling microscopy (LT-STM). The differential conductance (dI/dV) images show bias-dependent standing waves of the two-dimensional (2D) electronic states. Friedel oscillations, caused by the scattering and interference of 2D electrons in the surface accumulation layer, were observed. The dI/dV images also show a LDOS distribution in small InAs nanostructures. Calculations reveal that the LDOS distribution in each dI/dV image corresponds to a discrete zero-dimensional (0D) state in the nanostructures.

The local density of states (LDOS) within tetrahedral InAs structures, formed at the surface of InAs/GaAs(111)A, has been characterized using low-temperature scanning tunneling microscopy. The LDOS of the lowest four zero-dimensional (0D) discrete levels have been imaged in structures with a comparable size to the electron wavelength. The LDOS inside the structures is observed to be higher than that of the surrounding area at intervals of the level separation. This feature indicates the singularity of the LDOS close to the 0D resonant levels.

The local density of states (LDOS) at the epitaxially grown InAs surface on a GaAs(111) A substrate were characterized using low-temperature scanning tunneling microscopy. Using dI/dV signal mapping, LDOS standing waves were clearly imaged at point defects and within nanostructures. Measurement of the wavelength as a function of bias voltage showed a nonparabolic dispersion relation for the conduction band. The observed wave features originate from the Friedel oscillations of the two-dimensional electron gas in the semiconductor surface accumulation layer.

The characteristics of an Al‐Ga top layer on AlGaAs and GaAs surfaces during alternate supply molecular beam epitaxy (MBE) growth are studied by in situ observation using a MBE scanning electron microscope (SEM) hybrid system. It is found that an Al‐Ga alloy top layer has a pseudo‐self‐limiting nature. It is also found that the migration distance of Al‐Ga atoms on an Al‐Ga alloy top layer is as large as 10 μm. By utilizing these characteristics, a μm‐scale Al‐Ga monolayer lateral growth process is realized, and the lateral growth rate, about 1 μm/s, is observed. Comparison of this growth with migration‐enhanced epitaxy is discussed.