Showing papers on "Scanning tunneling spectroscopy published in 1989"

Determination of atom positions at stacking-fault dislocations on Au(111) by scanning tunneling microscopy

Abstract: The determination of atom positions around Shockley-type partial dislocations [Burgers vector \( \frac{1}{6}(1,1, - 2)] \)at the (111) surface of gold has been achieved by operating a scanning tunneling microscope with atomic resolution on regions containing several unit cells of the\( (23 \times \sqrt {3} ) \) stacking-fault reconstruction of this surface. The data show directly the occupation of both hexagonal-closepacked and face-centered-cubic sites in the surface layer, verifying earlier reconstruction models.

Determination of the local electronic structure of atomic‐sized defects on Si(001) by tunneling spectroscopy

Abstract: Tunneling spectroscopy and voltage‐dependent scanning tunneling microscopy have been used to study the geometry and electronic properties of atomic‐sized defects on the Si(001) surface. Individual dimer vacancies are shown to be semiconducting, consistent with the π‐bonded defect model of Pandey. Another type of characteristic defect is found which gives rise to strongly metallic tunneling I–V characteristics, demonstrating that it has a high density of states at the Fermi level and is likely active in Fermi level pinning on Si(001). Spatially dependent I–V measurements and tunneling barrier height measurements also directly reveal the spatial extent of this metallic character and provide direct measures of the ‘‘size’’ of the defects.

Negative Differential Resistance on the Atomic Scale: Implications for Atomic Scale Devices

Abstract: Negative differential resistance (NDR) is the essential property that allows fast switching in certain types of electronic devices. With scanning tunneling microscopy (STM) and scanning tunneling spectroscopy, it is shown that the current-voltage characteristics of a diode configuration consisting of an STM tip over specific sites of a boron-exposed silicon(111) surface exhibit NDR. These NDR-active sites are of atomic dimensions (∼1 nanometer). NDR in this case is the result of tunneling through localized, atomic-like states. Thus, desirable device characteristics can be obtained even on the atomic scale.

Atom-resolved surface chemistry studied by scanning tunneling microscopy and spectroscopy.

Abstract: We have used scanning tunneling microscopy and spectroscopy to study the reaction of Si(111)- (7×7) with NH3. We have found that by use of topographs obtained at different energies, as well as atom-resolved spectra, reacted and unreacted surface sites can be imaged selectively. Thus we have been able to probe the spatial distribution of the surface reaction on an atom-by-atom basis. We find that there are significant differences in reactivity between the various dangling-bond sites on the Si(11l)-(7×7) surface. Specifically, rest-atom sites are more reactive than adatom sites and, moreover, center-adatom sites are more reactive than corner-adatom sites. We ascribe the reduced reactivity at adatom sites to the delocalized nature of their dangling-bond state. We suggest that a bonding interaction between adatoms and the Si atoms directly below them is responsible for this behavior—a suggestion supported by electronic-structure calculations. Thus, while reaction at a rest-atom site can be considered a dangling-bond saturation process, reaction at an adatom site involves the formation of a hypervalent (fivefold-coordinated) adatom. We tentatively ascribe the reactivity differences between center and corner adatoms to differences in the strain they induce upon reaction on the dimer bonds. Atom-resolved spectroscopy allows us to probe interactions and charge transfer between surface sites, and for the first time, we can directly observe how chemisorption affects the substrate electronic structure at neighboring unreacted sites.

Geometric and electronic structure of antimony on the GaAs(110) surface studied by scanning tunneling microscopy

Abstract: Antimony overlayers on the GaAs(110) surface have been studied by scanning tunneling microscopy and spectroscopy. The Sb is observed to grow as a 1\ifmmode\times\else\texttimes\fi{}1 ordered monolayer. The positions of the Sb atoms in the surface unit cell are deduced from voltage-dependent imaging, and compared with previously proposed structural models. A best fit is found for a model in which the Sb atoms occupy positions similar to what would be the positions of Ga and As atoms at an unrelaxed GaAs(110) surface, in agreement with the results from low-energy electron-diffraction experiments. The surface-state density is measured with tunneling spectroscopy. Two filled-state peaks and one empty-state peak are observed in the spectra, as well as a band-gap region with a width nearly equal to the bulk band gap. These observations are compared with previous photoemission results and theoretical calculations.

Atomic-Resolution Surface Spectroscopy with the Scanning Tunneling Microscope

Abstract: The study of surfaces has enjoyed an explosive growth during the last 2 5 years, due largely to the development of new techniques for probing the symmetry, chemical composition, and the electronic and vibrational states of surfaces and of adsorbed atomic and molecular species. Although a veritable arsenal of surface science tools is available, the study of surfaces is often so complex that even when several tools are applied simultaneously, unambiguous results may not be obtained. The study of surfaces has been greatly advanced during the last five years by the newly developed technique of scanning tunneling microscopy (STM) (1-6). In this technique, a very sharp tip (usually of tungsten) is brought to within a few atomic diameters of the surface under investigation without actual physical contact, so that there is a very small overlap of the wavefunctions of the surface with the nearest atom of the tip. When a small bias voltage (10 mV--4V) is applied between the sample and tip, electrons tunnel across this gap with a probability that increases exponentially as the tip approaches the sample. This exponential dependence of the tunneling current on the sample-tip separation provides an extremely sensitive way of detecting the small changes in the surface height due to the individual atoms, thus providing the basis for the scanning tunneling microscope. The images obtained in STM are often strongly dependent on the sample-tip bias voltage in a nontrivial manner. Although early STM stud-

Demonstration of the tunnel-diode effect on an atomic scale

Abstract: THE tunnel diode1, which is widely used in high-speed electronics applications2, depends on the property of negative differential conductivity, that is, a negative slope in the current–voltage curve. The mechanism underlying the tunnel diode's behaviour, namely the existence of a range of biases for which tunnelling is forbidden or suppressed following a bias for which tunnelling is strongly favoured, has been employed subsequently in the design of new devices that also display the conductance anomaly, such as the double-barrier resonant-tunnelling device3. It has been predicted4 that the conductance anomaly could result from a similar mechanism at the tunnel junction of the scanning tunnelling microscope (STM), where localized states on adsorbate and tip atoms give rise to allowed and suppressed energies for tunnelling. The STM has the capability to image regions of negative differential conductivity induced by individual atoms on a surface. Here we report the observation of negative differential conductivity on particular binding sites of a Si (111) surface doped with boron. Specific current–voltage characteristics are shown to be related to the presence or absence of the dopant at individual atomic sites, and negative differential conductivity is observed at –1.4 V tip bias at a specific type of site. Tunnelling spectroscopy indicates that the effect results from a tunnel-diode mechanism.

Model of phonon-associated electron tunneling through a semiconductor double barrier.

Abstract: We propose an approach to study one-dimensional electron tunneling in an arbitrarily shaped barrier in the presence of electron-optical phonon scattering. An independent-boson model is used for the electron-phonon interaction. Our result for a double-barrier structure shows the occurrence of phonon-assisted resonant tunneling.

Mechanisms of current rectification in an STM tunnel junction and the measurement of an operational tunneling time

Abstract: The effects of material, thermal, and geometrical asymmetries on the nonlinearity of the I-V characteristics of a scanning tunneling microscope (STM) tunneling junction are investigated theoretically and experimentally. As a consequence of the nonlinearity, an STM junction can rectify an incident laser beam to produce both DC and AC tunneling current components. Using the dependence of the DC rectified current on the tip-sample distance s and the laser frequency v, an operational definition of a tunneling time is proposed. An experiment using a laser-irradiated STM junction is also performed to measure the rectified current as a function of s, for a fixed v. From the preliminary results of this experiment, a tunneling time on the order of 1.8 fs is determined. This time is in reasonable agreement with those obtained in other experimental and theoretical studies of tunneling junctions of comparable dimensions and energies. It is also shown that the AC current contains components at the laser frequency and its higher harmonics. This implies that the STM junction can be used as a nonlinear device to detect, generate, and measure laser frequency harmonics. >

Resonantly enhanced electron tunneling rates in quantum wells

Abstract: Resonant tunneling of electrons in GaAs-AlGaAs quantum wells is resolved by picosecond pump-and-probe electroabsorption measurements. The temperature-independent tunneling escape times are dramatically affected by the applied electric field, with a pronounced minimum at the field corresponding to the resonance between the n=1 electron level in one quantum well, and the n=2 electron level in the adjacent one. The calculated field dependence of the tunneling times is in qualitative agreement with the data.

Lateral tunneling, ballistic transport, and spectroscopy in a two-dimensional electron gas.

Abstract: We report a direct observation, via electron energy spectroscopy, of lateral tunneling and lateral ballistic-electron transport in a two-dimensional electron gas (2D EG). This was accomplished through the use of a novel transistor structure employing two potential barriers, induced by 50-nm wide metal gates deposited on a GaAs/AlGaAs selectively doped heterostructure. Hot electrons with very narrow energy distributions (\ensuremath{\simeq}5 meV wide) have been observed to ballistically traverse 2D EG regions \ensuremath{\simeq}170 nm wide with a mean free path of about 480 nm.

Scanning tunneling microscopy of insulators: CaF2 epitaxy on Si (111)

Abstract: Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) have been used to study the epitaxy of CaF2 on Si (111). Energy‐resolved images of the submonolayer structures produced at the initial stages of CaF2 deposition were obtained. We found that in these structures and also at the 1×1 interface, bonding involves the Ca atom in a reduced, Ca+‐like state. Using STS we were able to measure the CaSi bonding‐antibonding level splitting at the interface. The distribution of charged defects was also imaged by the STM. More important, we found that we can image strongly insulating CaF2 multilayers by tunneling into their conduction band.

Noise in vacuum tunneling: Application for a novel scanning microscope

Abstract: The noise in the tunneling current of a scanning tunneling microscope (STM) has been investigated. At gap voltages above a few mV the current noise shows a 1/f spectral distribution over a range of 1 Hz to 100 kHz. However, at zero bias white noise scaling like thermal noise for the equivalent gap resistance is found. Due to its exponential distance dependence, this noise at zero bias can be used to adjust the tip‐to‐sample distance with an accuracy similar to the conventional STM method. The performance of a scanning microscope based on this principle is demonstrated.

Energy gap of Tl-Ba-Ca-Cu-O compounds by tunneling

Abstract: Tunneling spectroscopy measurements were carried out on surfaces of polycrystal Tl-Ba-Ca-Cu-O superconductors of two stoichiometric compositions (2212 and 2223). Broad gap-like structures were routinely observed on differential conductance curves, and taking their peak positions and resistive T c 's, we obtain the estimated 2Δ k B T c of ∼ 6 for both compositions which is in good agreement with our earlier tunneling results on oriented film surfaces of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O compounds.

Time‐resolved measurement of tunneling and energy relaxation of hot electrons in GaAs/AlGaAs double quantum well structures

Abstract: The tunneling and cooling times of photoexcited hot electrons in AlGaAs/GaAs double (one narrow and the other wide) quantum well structures have been measured using photoluminescence excitation correlation spectroscopy. The tunneling time was of the order of 200 ps for a 60 A barrier. The tunneling is the indirect process assisted by the emission of optical phonons. The relaxation time of electrons as a function of the kinetic energy shows a threshold for cooling via the emission of optical phonons.

Observation of electron resonant tunneling in a lateral dual-gate resonant tunneling field-effect transistor

Abstract: A new lateral resonant tunneling field‐effect transistor (LARTFET) has been fabricated using molecular beam epitaxy and ultrahigh‐resolution electron beam lithography. The LARTFET has two 80‐nm‐long gate electrodes separated by 100 nm. The dual gates create double potential barriers in the channel and a quantum well in between. Conductance oscillations are observed, which, for the first time, indicate electron resonant tunneling through the energy states in a lateral double‐barrier quantum well formed electrostatically. Furthermore, after illumination, two additional negative transconductance peaks are observed. These additional peaks may be related to electron resonant tunneling through the donor‐related deep levels in silicon‐doped Al0.35Ga0.65As .

Spin‐polarized secondary electrons from a scanning tunneling microscope in field emission mode

Abstract: A new technique has been developed which opens the way to magnetic imaging with nm resolution A narrow electron beam produced with a scanning tunneling microscope operating in field emission mode impinges on the magnetic surface, and the spin polarization of the emitted secondary electrons is monitored As a first result, a hysteresis loop from an Fe‐based metallic glass shows that the low‐energy secondary electrons excited with this technique are spin polarized

Scanning tunneling microscopy and spectroscopy of gold on the GaAs(110) surface

Abstract: The geometric and electronic structure of Au on the GaAs(110) surface is studied with the scanning tunneling microscope. At low metal coverage, the binding site for the Au adsorbates is found to be beside a surface Ga atom. The adsorbates usually occupy every second unit cell on the surface. In tunneling spectroscopy, a characteristic band gap state is observed for the adsorbed Au atoms, located ∼1.0 eV above the valence‐band maximum. A resonant state in the valence band is also observed. The valence‐band resonance and band gap state are interpreted as the first and second electron states of the Au–Ga bond, respectively. Both states form bands, and the surface Fermi level is pinned between these bands. At higher metal coverage, the Au atoms form clusters, which have a distinct epitaxial relationship with the substrate. Spectroscopy at high metal coverage reveals a continuum of states throughout the band gap, associated with the metallic nature of the clusters.

Atomic theory of scanning tunneling microscopy

Abstract: We present a quantitative analysis of the modifications of the scanning-tunneling-microscopy images due to the local perturbations of the electronic states induced by the tip in close proximity to the sample surface. Using an empirical tight-binding method, we have calculated the electronic states of a prototype tip-sample system consisting of a single-atom tip and the graphite surface, as a function of the tip-sample distance. We find that as the tip approaches the sample, their states start to interact and become laterally confined in the vicinity of the tip at small tip-sample separation. These states influence the tunneling phenomenon by connecting the tip and sample surface electronically. The effect of the tip-induced localized states is discussed, and the expression for the tunneling current is reformulated by incorporating the tip-induced states. Calculations using this expression show that the corrugation amplitude obtained from scanning tunneling microscopy is enhanced and deviates from the proportionality to the local density of states of the bare sample at the Fermi level evaluated at the center of the tip.

Scanning Tunneling Microscopy and Nanolithography on a Conducting Oxide, Rb0.3MoO3

Abstract: The scanning tunneling microscope has been used to image and modify the surface of a conducting oxide (Rb0.3MoO3)in ambient atmosphere. Individual octahedral MoO6 units of the oxide can be imaged, and under certain conditions defects can be created in the surface that are stable in air. The ability to produce nanometer-sized structures on the surface of an oxide is demonstrated and discussed with reference to nanolithographic applications.

Voltage shifts of Fowler-Nordheim tunneling J-V plots in thin gate oxide MOS structures due to trapped charges

Abstract: This paper describes a novel simulation of Fowler-Nordheim (F-N) tunneling of electrons from either tunneling interface, i.e. from the gate or the inversion layer of a p-type substrate into the oxide. It accounts for the effects of finite electron-hole pairs generation in the substrate and shapes of tunneling barrier created by charge trapped in the oxide for F-N tunneling.

Time‐resolved measurements of tunneling between double quantum wells in In0.53Ga0.47As/InP

Abstract: Tunneling of electrons between double quantum wells is investigated by time‐resolved photoluminescence in the picosecond regime. The samples contain two In0.53Ga0.47As quantum wells with different widths, separated by various InP barriers. At low excitation density, the luminescence decay time of the narrower quantum well depends strongly on the thickness of the barrier, revealing the lifetimes to be tunneling controlled. A semiclassical model explains the observed nonresonant tunneling escape times. With increasing density, the luminescence decay time of the narrower quantum well strongly increases and finally saturates due to effective mass filtering, which leads to a lineup of the electron levels in both wells and resonant tunneling.

Magnetotunneling spectroscopy in a double-barrier heterostructure: Observation of incoherent resonant-tunneling processes.

Abstract: We report striking magneto-oscillations in the current-voltage characteristics of a double-barrier resonant-tunneling structure under longitudinal magnetic field. We have identified, for the first time, inter-Landau-level resonant tunneling with Landau-level indices changed by as large as six, two distinct LO-phonon-emission-assisted resonant tunneling, and a combination of the two. The voltage-to-energy conversion is experimentally calibrated, which makes tunneling spectroscopy possible. Our experiment is a manifestation of the two-dimensionality of the source electrons.

In-Situ Scanning Tunneling Microscopy of Semiconductor(n-TiO2)/Liquid Interfaces. A Role of Band Bending in Semiconductors

Abstract: It is found in this study that electron tunneling from the conduction band in the semiconductor to the metal tip is possible if the electrode potential of the semiconductor is negative with respect to the flat band potential. The result clearly reveals the role of band bending in semiconductors for electron tunneling at the semiconductor/liquid interface.