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We report a material-dependent critical temperature for ion beam sculpting of nanopores in amorphous materials under keV ion irradiation. At temperatures below the critical temperature, irradiated pores open at a rate that soon saturates with decreasing temperature. At temperatures above the critical temperature, the pore closing rate rises rapidly and eventually saturates with increasing temperature. The observed behavior is well described by a model based on adatom diffusion, but is difficult to reconcile with an ion-stimulated viscous flow model.

We have used the tunneling microscope to measure the lattice relaxation of Ge islands on Si (111) as a function of their height. Lattice constants on the top surfaces of individual Ge islands can be measured with an uncertainty of approximately one percent. The lattice constant is a continuous, Monotonically increasing function of island height up to 20 bilayers. At heights around 5O bilayers, the island-top lattice parameter may exceed that of bulk Ge. Defects can be observed penetrating the top surface of islands which have heights around 90 bilayers.

The desire to modify materials on the smallest possible scale is motivated by goals ranging from high-density information storage to the purposeful transformation of genetic material. Here we report an atomic-scale modification of the surface of a nearly perfect germanium crystal, effected by the tungsten tip of a tunnelling microscope. We believe this to be the smallest spatially controlled, purposeful transformation yet impressed on matter and we argue that the limit set by the discreteness of atomic structure has now essentially been reached.

We demonstrate an ion implantation method for large-scale synthesis of high quality graphene films with controllable thickness. Thermally annealing polycrystalline nickel substrates that have been ion implanted with carbon atoms results in the surface growth of graphene films whose average thickness is controlled by implantation dose. The graphene film quality, as probed with Raman and electrical measurements, is comparable to previously reported synthesis methods. The implantation synthesis method can be generalized to a variety of metallic substrates and growth temperatures, since it does not require a decomposition of chemical precursors or a solvation of carbon into the substrate.

We report the closure of nanopores to single-digit nanometer dimensions by ion sculpting in a range of amorphous materials including insulators (SiO2 and SiN), semiconductors (a-Si), and metallic glasses (Pd80Si20)—the building blocks of a single-digit nanometer electronic device. Ion irradiation of nanopores in crystalline materials (Pt and Ag) does not cause nanopore closure. Ion irradiation of c-Si pores below 100 °C and above 600 °C, straddling the amorphous-crystalline dynamic transition temperature, yields closure at the lower temperature but no mass transport at the higher temperature. Ion beam nanosculpting appears to be restricted to materials that either are or become amorphous during ion irradiation.

The addition of a small number of lead atoms to a germanium(111) surface reduces the energy barrier for activated processes, and with a tunneling microscope it is possible to observe concerted atomic motions and metastable structures on this surface near room temperature. The formation and annihilation of these metastable structural surface excitations is associated with the shift in position of large numbers of germanium surface atoms along a specific row direction like beads on an abacus. The effect provides a mechanism for understanding the transport of atoms on a semiconductor surface.

We argue that it is possible to bind a neutral atom in stable orbits around a wire charged by a time-varying sinusoidal voltage. Both classical and quantum-mechanical theories for this system are discussed, and a unified approach to the Kapitza picture of effective potentials associated with high-frequency fields is presented. It appears that cavities and waveguides for neutral-atomic-matter waves may be fashioned from these considerations

We measure the activation energy for surface diffusion of isolated Pb adatoms on Ge(111) by observing individual atomic interchanges using a scanning tunneling microscope. The diffusion occurs mainly along one direction of the c(2\ifmmode\times\else\texttimes\fi{}8) reconstructed surface with Pb adatoms moving between substitutional Ge adatom sites. Only half of the adatom interchanges are to near-neighbor sites, the rest are ``long jumps.'' The scanning process does not affect the diffusion. For temperatures from 24 to 79 \ifmmode^\circ\else\textdegree\fi{}C we find the diffusion obeys an Arrhenius law with an activation energy of 0.54\ifmmode\pm\else\textpm\fi{}0.03 eV.

We study a reversible phase transition of one monolayer of Pb on a Ge(l11) surface over a range of temperature from below to above the transition temperature (∼175°C) with the tunneling microscope. The high temperature l×1 phase is a ordered monolayer l×l 2D solid. Below the transition temperature, Pb atoms are displaced from their ideal T1 positions to form trimers with periodicity.

Knowledge about atomic scale motions is essential to the understanding of dynamical phenomena on surfaces, such as diffusion, phase transitions, and epitaxial growth. We demonstrate that the addition of a very small number of Pb atoms to a Ge(111) surface reduces the energy barrier for activated processes, thus allowing one to observe concerted atomic motions and metastable structures on this surface near room temperature using a tunneling microscope. The activation energy for surface diffusion of isolated substitutional Pb atoms in Ge(111)-c(2×8) was measured by observing individual atomic interchanges from 24°C to 79°C. We also observed the formation and annihilation of metastable structural surface excitations, which are associated with large numbers of germanium surface atoms in one row of the c(2×8) reconstruction shifting along that row like beads on an abacus. The effect provides a new mechanism for atomic transport on semiconductor surfaces and can explain a number of other observed phenomena associated with Ge(111) surfaces, including the surface diffusion of Pb atoms.

We present conclusive results on the structures and coverages of the submonolayer phases of Pb on the Si(111) 7\ifmmode\times\else\texttimes\fi{}7 surface based on scanning-tunneling-microscopy, low-energy-electron-diffraction, and Rutherford backscattering measurements. For room-temperature deposition at low coverage on the 7\ifmmode\times\else\texttimes\fi{}7 surface, we find that Pb atoms occupy sites above and between the Si adatoms (with a preference for the faulted half of the unit cell) leaving the 7\ifmmode\times\else\texttimes\fi{}7 structure intact. After annealing to 450 \ifmmode^\circ\else\textdegree\fi{}C, some Pb atoms occupy Si(111) 7\ifmmode\times\else\texttimes\fi{}7 adatom sites, while others have nucleated to form small areas of an unusual \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 phase which has a 1:1 ratio of Si:Pb.

Ways of characterizing single-molecule events are rapidly emerging in the biochemical sciences. One such line of research now shows how measurement of ionic transport through a single, atomic-scale pore in an insulating membrane can distinguish between organic molecules of relative molecular mass as low as 100; remarkably, a single protein channel can be adapted for simultaneous analysis of a mixture of organic molecules.

Electrical current fluctuation studies are reported for coaxial p-type and n-type single-wall carbon nanotube field-effect transistors (FETs). Abrupt discrete switching of the source-drain current is observed at room temperature. The authors attribute these random telegraph signals to charge fluctuating electron traps near the FET conduction channels. Evolution of the current-switching behavior associated with the occupancy of individual electron traps is demonstrated and analyzed statistically. The result strongly indicates room temperature single charge sensitivity in carbon nanotube FETs, which may offer potential applications for single molecule sensors based on suitably prepared FET devices.

We demonstrate that 3 keV ion beams, formed from the common noble gasses, He, Ne, Ar, Kr, and Xe, can controllably “sculpt” nanometer scale pores in silicon nitride films. Single nanometer control of structural dimensions in nanopores can be achieved with all ion species despite a very wide range of sputtering yields and surface energy depositions. Heavy ions shrink pores more efficiently and make thinner pores than lighter ions. The dynamics of nanopore closing is reported for each ion species and the results are fitted to an adatom diffusion model with excellent success. We also present an experimental method for profiling the thickness of the local membrane around the nanopore based on low temperature sputtering and data is presented that provides quantitative measurements of the thickness and its dependence on ion beam species.

Wei and Su recently published a letter in this journal on low-temperature homoepitaxial growth of silicon mediated by thin overlayers of Pb. A significant portion of the experimental work reported in their letter was performed in our laboratory ~depositions, Rutherford backscattering analysis, and electron microscopy!. Those experiments were poorly controlled and produced major inconsistencies that made the work unfit for publication. A subsequent, more systematic, investigation revealed the origin of these inconsistencies and established the precise conditions for high-quality growth. Our report on the latter work appeared in this journal several months prior to submission of Ref. 1. The purpose of this comment is to correct the factual errors reported in Ref. 1. Foremost among these is the claim that high-quality Si films can be grown on vicinal Si~111! for Pb coverages of 0.8–1.0 ML (1 ML57.83310 atoms cm) by first depositing the Pb overlayer and subsequently growing the Si film without continuously supplying the sample surface with additional Pb. As reported in Ref. 2, the growth of arbitrarily thick, high-quality Si films requires a Pb coverage of 1.0 60.1 ML which cannot be achieved under the conditions outlined in Ref. 1 because Pb desorbs from the surface of the growing film. We have observed evaporation of Pb from vicinal surfaces including those from the same substrate material used by Wei while working in our laboratory. To maintain a constant Pb coverage at substrate temperatures of 280 °C or higher, one must deposit Pb during the growth of the Si film. For example, at 295 °C a Pb flux of 0.12 ML min is needed to maintain a Pb coverage of 1.0 ML on the sample surface. The Pb coverages reported in Ref. 1 were measured at the end of the deposition of Si. By wrongly assuming that the final Pb coverages they measured represent the actual coverages during growth, Wei and Su have overlooked the effect of Pb desorption on their results. The misinterpretation of ion channeling spectra in Ref. 1 undermines the authors’ claim that high-quality films can be

X‐ray standing wave measurements on single crystals of silicon are used to determine the coverage and position of chemisorbed bromine. Detailed analysis of the position information leads to the conclusion that silicon surface atoms bonded to adsorbed bromine atoms are in extrapolated bulk‐line positions. Direct measurement of the desorption of correlated bromine in air demonstrates the high stability of the Br/Si surface interface.

Some exact solutions to the linear Boltzmann transport equation in a one-dimensional space are presented. These results may be applied to demonstrate the trend towards statistical equilibrium of fast channeled particles in the transverse plane. Statistical mechanics of channeled particle motion is then discussed in terms of the random string approximation and further considerations.

By monitoring the characteristic fluorescent radiation excited by x-ray standing waves we have been able to distinguish the two different kinds of lattice sites in a heteropolar crystal of GaAs. The strong extinction effects that mask the position-dependent fluorescent signal were minimized by adopting a suitable detector geometry.

The momentum profiles from positron annihilation with core electrons in Al have been observed with a recently developed two-detector Doppler-broadening technique. Independent-particle-model (IPM) calculations are in good agreement with the experimental results beyond twice the Fermi momentum and suggest the absence of strong positron-electron correlation effects in this region. Comparisons of calculations with properly normalized momentum profiles indicate that high-momentum core contributions may provide information on the nature of vacancy-type defects and self-trapping effects.

The charge state dependence of channeled ion energy loss has been determined for a series of ions ranging from fluorine to chlorine along the $〈110〉$ direction in a silicon crystal. Energy losses for both bare ions and ions partially clothed with bound electrons at $\frac{E}{A}\ensuremath{\cong}3$ MeV/amu have been measured. The energy-loss rate for bare ions follows a strict ${Z}_{1}^{2}$ scaling and agrees reasonably well with quantal perturbation calculations without the need for polarization or Bloch corrections. An explanation for this result is discussed. The clothed-ion energy losses appear to demonstrate screening effects that agree qualitatively with simple estimates. The angular dependence of the observed energy-loss effects is also presented.

Classical calculations of the intensity of channeling radiation from GeV positrons moving in continuum potentials agree reasonably well with experimental data. However, generally, the peaks of the experimental spectrum appear at 5–10% lower frequencies, and they are broader. Since multiple scattering was expected to cause such changes, the classical formula for the intensity has been extended to include this effect. While the calculated broadening is of the right order, it appears that multiple scattering is not the major cause of the shift.

Interference effects have been observed when coherently related x-ray beams are Compton scattered from an atomic system. We use dynamical diffraction methods to prepare the initial x-ray state and observe the coherent Compton effect in both crosssection and profile measurements.

A double crystal x‐ray monochromator system has been developed for use on synchrotron radiation sources. The system consists of two separate Bragg reflecting crystals constrained by a mechanical linkage system enabling it to tune continuously in x‐ray energy, always maintaining an exit beam of constant offset and direction relative to the incident beam. Accurate parallelism between the two crystals is maintained by a piezoelectrically‐controlled analog feedback circuit.

The time‐resolved reflectivity at 0.63 μm from arsenic‐implanted silicon crystals has been measured during annealing by a 1.06‐μm laser pulse of 50‐ns duration. The reflectivity was observed to change abruptly to the value consistent with liquid silicon and to remain at that value for a period of time which ranged from a few tens of nanoseconds to several hundreds of nanoseconds, depending on the annealing pulse intensity. Concurrently, the transmission of the primary annealing beam dropped abruptly. These observations confirm the formation of a metallic liquid phase at the crystal surface during the annealing process.

The presence and properties of x-ray evanescent waves are demonstrated detecting their absorption and their excitation in the vicinity of a target surface. The role of microscopic reversibility in relating the two cases is discussed and practical applications of the phenomena to probe interfaces are suggested.

With use of bromine atoms, deposited on the surface of a silicon single crystal, the presence of x-ray standing waves above the crystal surface has been detected under conditions of strong Bragg diffraction.

The channeling effect has been observed to reduce the ionization energy loss by about a factor of 3 for 1.35-GeV/c protons and ..pi../sup +/ traversing a 0.67-mm germanium crystal. (AIP)

An x-ray standing-wave-field apparatus has been constructed that demonstrates an impurity atomic position resolution of \ensuremath{\sim}0.02 \AA{}. The method is applied to the measurement of ion-implantation-induced lattice relaxation. It is pointed out that the method is also suited to determine surface impurity locations perpendicular to crystal surfaces.

Tunneling images of single atomic steps on Si(111) have been obtained with a tunneling microscope. The 7\ifmmode\times\else\texttimes\fi{}7 reconstruction is observed to persist essentially undistorted right up to the steps. The position of each step coincides with a 7\ifmmode\times\else\texttimes\fi{}7 unit-mesh edge in both the upper and lower terraces adjacent to it. Asymmetry within the 7\ifmmode\times\else\texttimes\fi{}7 unit mesh is observed and correlated with faulting of terraces adjacent to the step. The results are described by an extension of the dimer adatom stacking-fault model of Takayanagi et al.

CF3Br plasma discharges exhibit a number of unusual phenomena. Under some conditions silicon and SiO2 can be concurrently etched, while in other circumstances only SiO2 is etched and silicon is coated with a uniform, cross‐linked fluorocarbon film. Single crystal n‐doped silicon is etched more rapidly than undoped silicon. The fluorocarbon polymer overlays a monolayer of bromine at the silicon interface, but negligible bromine is incorporated into the fluorocarbon. Some chemical mechanisms which account for these observations are presented.

The azimuthal distributions of protons transmitted through thin silicon single crystals near the axis were measured using a two-dimensional position-sensitive detector. The data are composed of ringlike distributions with strong azimuthal and transverse energy dependence. The azimuthal distributions are compared with theoretical predictions based on the random string approximation using different forms of the interatomic potential. ''Blocking'' in the transverse plane is also observed. In addition, from an analysis of the radial spreading of the distribution the effects of inelastic scattering in the transverse plane are clearly seen.

A new instrument called the tunneling microscope has recently been developed that is capable of generating real-space images of surfaces showing atomic structure. These images offer a new view of matter on an atomic scale. The current capabilities and limitations and the physics involved in the technique are discussed along with specific results from a study of silicon crystal surfaces.

A critical analysis of channeled-ion energy-loss experiments is presented with the goal of commenting on the presence of the Barkas effect of Z/sub 1//sup 3/ corrections to the stopping power. Accurate charge-density values are obtained for silicon and used to evaluate Bloch and straggling effects in the data. The remaining contributions to the data show a clear Z/sub 1//sup 3/ dependence that can be explained with an electron-gas model for the Barkas effect.

We have investigated the incorporation of boron into the Si(111) (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} surface from low boron concentration up to (1/3 monolayer, using tunneling microscopy and spectroscopy and first-principles total-energy calculations. Surprisingly, we find that a (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} structure composed almost entirely of Si adatoms on a Si double layer can be stabilized by boron doping in near surface layers. Moreover, when boron atoms are in the surface layers, the adatom site is unfavorable compared with the site underneath the adatom, unlike other group-III elements adsorbed on the Si(111) surface.

We describe experiments and modeling results that reveal and explain the distribution of times that identical double-stranded DNA (dsDNA) molecules take to pass through a voltage-biased solid-state nanopore. We show that the observed spread in this distribution is caused by viscous-drag-induced velocity fluctuations that are correlated with the initial conformation of nanopore-captured molecules. This contribution exceeds that due to diffusional Brownian motion during the passage. Nevertheless, and somewhat counterintuitively, the diffusional Brownian motion determines the fundamental limitations of rapid DNA strand sequencing with a nanopore. We model both diffusional and conformational fluctuations in a Langevin description. It accounts well for passage time variations for DNA molecules of different lengths, and predicts conditions required for low-error-rate nanopore-strand DNA sequencing with nanopores.

The surface crystallinity and stoichiometry of tellurium‐implanted GaAs annealed with a frequency‐doubled Nd:glass laser using time‐resolved reflectivity and channeling measurements has been studied. By optimizing the duration of the liquid melt, depending on the implant dose, it has been possible to laser anneal implanted layers in uncapped GaAs with good surface crystallinity and minimal loss of arsenic due to surface decomposition. We propose a qualitative model of arsenic evaporation at the surface and subsequent replacement by arsenic displaced from the bulk by the tellurium which explains the dependence of the optimum melt duration on the dose of the implant.

We report on the annealing behavior of ion‐implanted silicon exposed to cw argon laser radiation. The quality of the annealing was studied using channeling, in situ reflectivity, phase‐contrast microscopy, and electrical resistivity. Strong dependence of anneal quality on laser power, crystal orientation, and implant dose is reported. Reflectivity measurements with a focused probing laser show that the onset of annealing occurs just below the melting point, and the annealing extends into the region where a thin layer of liquid silicon is formed at the crystal surface. We conclude that the physical mechanism responsible for crystal recovery is similar to the solid‐state epitaxial regrowth observed for over‐annealed samples.

The desire to modify materials on the smallest possible scale is motivated by goals ranging from high-density information storage to the purposeful transformation of genetic material. Here we report an atomic-scale modification of the surface of a nearly perfect germanium crystal, effected by the tungsten tip of a tunnelling microscope. We believe this to be the smallest spatially controlled, purposeful transformation yet impressed on matter and we argue that the limit set by the discreteness of atomic structure has now essentially been reached.

The position of germanium atoms on a Si(111) (7\ifmmode\times\else\texttimes\fi{}7) surface prepared in a molecular-beam-epitaxy system have been established using the characteristic fluorescence generated by x-ray standing waves. For submonolayer coverages germanium occupies both types of (111) lattice sites. Measurements of the germanium position relative to planes inclined to the surface do not support recent stacking-fault models for the Si(111) (7\ifmmode\times\else\texttimes\fi{}7) surface.

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.

Channeling effects have been observed for 1.1 GeV/ c protons and pions transmitted through a 1 mm thick germanium single crystal.

A single pulse of ruby laser radiation is shown to cause significant regrowth in the amorphous region of heavily ion‐implanted GaAs. The implanted Te atoms and the host material both show good channeling dips, suggesting essentially complete substitutionality of the Te. There has been only a minor redistribution of the tellurium atoms. The resulting local Te concentration in the laser‐irradiated sample is more than ten times the known maximum solubility of Te in GaAs.

In the laser annealing of silicon by Nd : YAG laser irradiation a dose dependence of the annealing threshold and final impurity depth distribution has been observed. Impurity depth distributions and damage profiles observed by Rutherford backscattering are consistent with the notion of the formation of a thin liquid layer and subsequent regrowth of the crystal upon cooling. We conclude that the annealing behavior is very sensitive to the time of formation of the liquid layer during the laser pulse.