We reduce the dimensionless interaction strength α in graphene by adding a water overlayer in ult... more We reduce the dimensionless interaction strength α in graphene by adding a water overlayer in ultra-high vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30 percent, due to the background dielectric constant enhancement leading to reduced interaction of electrons with charged impurities. However, the carrier-densityindependent conductivity due to short range impurities is decreased by almost 40 percent, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron/hole puddle density and longrange impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.
We present a calculation of the force, due to electron scattering, acting upon an adatom at the s... more We present a calculation of the force, due to electron scattering, acting upon an adatom at the surface of a current-carrying metal, i.e., the so-called "wind" force in surface electromigration.
A non-destructive technique for obtaining voltage contrast information with photoelectron emissio... more A non-destructive technique for obtaining voltage contrast information with photoelectron emission microscopy (PEEM) is described. Samples consisting of electrically isolated metal lines were used to quantify voltage contrast in PEEM. The voltage contrast behavior is characterized by comparing measured voltage contrast with calculated voltage contrast from two electrostatic models. Measured voltage contrast was found to agree closely with the calculated voltage contrast, demonstrating that voltage contrast in PEEM can be used to probe local voltage information in microelectronic devices in a non-intrusive fashion.
The thermal deposition and transfer Printing method had been used to produce pentacene thinfilms ... more The thermal deposition and transfer Printing method had been used to produce pentacene thinfilms on SiO2/Si and plastic substrates (PMMA and PVP), respectively. X-ray diffraction patterns of pentacene thin films showed reflections associated with highly ordered polycrystalline films and a coexistence of two polymorphs phases classified by their d-spacing, d(001): 14.4 and 15.4 Å .The dependence of the c-axis correlation length and the phase fraction on the film thickness and printing temperature were measured. A transition from the 15.4 Å phase towards 14.4 Å phase was also observed with increasing film thickness. An increase in the c-axis correlation length of approximately 12∼16% was observed for Pn films transfer printed onto a PMMA coated PET substrate at 100∼ 120 o C as compared to as-grown Pn films on SiO2/Si substrates. The transfer printing method is shown to be an attractive for the fabrication of pentacene thin-film transistors on flexible substrates partly because of the resulting improvement in the quality of the pentacene film.
In photoelectron emission microscopy (PEEM), there are several contrast mechanisms that may be us... more In photoelectron emission microscopy (PEEM), there are several contrast mechanisms that may be useful for dopant profiling, failure analysis, or as a processing diagnostic of Si. In particular, contrast arises from the reduction of the effective photothreshold when Si is heavily to degenerately doped. We report on a quantitative investigation of dopinginduced contrast in PEEM images of Si devices. Additionally, we have performed electron optics simulations of the imaging properties of PEEM in order to understand how contrast mechanisms individually and in combination produce a PEEM image. Device structures were fabricated using standard photolithography and focused ion beam (FIB) writing. The devices consist of ?-type (B) striped of different dopant concentrations (10's-1020 ~m -~) and various line separations written on n-type (P 1014 cm' ) Si(OO1) substrates. Using a near-threshold light source, we find that the signal intensity increases monotonically with doping over a range of doping from lx1017 to 2 ~1 0 ~' ~m -~. The measured intensity ratios are in good agreement with a calculation based on photoemission from the valence band. A numerical simulation program based on the Surface Charge Method is being used to assist in PEEM image interpretation. Model samples with regions of different photothreshold, vertical structure and lateral fields are being constructed. In combination, the results will be used to investigate how doping information may be extracted from PEEM images of simulated device samples.
The co-adsorption of CO and H2 on Rh(111) at low temperature (~ 100 K) has been studied using the... more The co-adsorption of CO and H2 on Rh(111) at low temperature (~ 100 K) has been studied using thermal desorption mass spectrometry (TDS) and Low-Energy Electron Diffraction (LEED). The probability of adsorption of CO on rhodium pretreated with hydrogen has been found to decrease slowly with increasing amounts of hydrogen on the surface. In addition, the effect of surface hydrogen on the CO LEED patterns indicates segregation of hydrogen and CO. These results can be explained in terms of a strong repulsive CO-H interaction and a mobile precursor model of CO adsorption. W
Chlorine adsorption on Ag(100) saturates at 2 monolayer (ML) in a c(2&&2) structure. Desorp- tion... more Chlorine adsorption on Ag(100) saturates at 2 monolayer (ML) in a c(2&&2) structure. Desorp- tion occurs above 650 K without disordering. The sticking coefficient remains constant to within 0.05 ML of saturation, consistent with the existence of a highly mobile precursor to adsorption. The c(2&2) structure forms at a critical coverage of 0.394+0.007 ML at room temperature. This is slightly higher than the critical coverage of the hard-square model (0.368 ML), suggesting the influ- ence of further-neighbor interactions. The transition to an ordered structure as a function of cover- age is second order aud the measured critical exponent P/(1 -ct} is 0. 12+0.04, consistent with the Ising value of 8.
Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material at number... more Pentacenequinone (PnQ) impurities have been introduced into a pentacene source material at number densities from 0.001 to 0.474 to quantify the relative effects of impurity content and grain boundary structure on transport in pentacene thin-film transistors. Atomic force microscopy (AFM) and electrical measurements of top-contact pentacene thin-film transistors have been employed to directly correlate initial structure and final film structures, with the device mobility as a function of added impurity content. The results reveal a factor four decrease in mobility without significant changes in film morphology for source PnQ number fractions below ~0.008. For these low concentrations, the impurity thus directly influences transport, either as homogeneously distributed defects or by concentration at the otherwise-unchanged grain boundaries. For larger impurity concentrations, the continuing strong decrease in mobility is correlated with decreasing grain size, indicating an impurity-induced increase in the nucleation of grains during early stages of film growth.
Equilibrium crystal shapes exhibit ¯at facets and rough vicinal surfaces, with transitions betwee... more Equilibrium crystal shapes exhibit ¯at facets and rough vicinal surfaces, with transitions between them being either continuous or discontinuous, the latter recognizable by a sharp edge. In general, mixed repulsive/attractive step±step interactions may lead to continuous or discontinuous facet-to-vicinal transitions. In can be shown that the contact angle at the facet for a discontinuous transition is directly related to the ratio of the step interaction strengths. Alternatively, surface reconstruction of facets can also be responsible for sharp edges at the facet boundary. In this case the contact angle is related to the dierence between surface free energies of the reconstructed and unreconstructed facet as well as the corresponding dierence of step interaction energies. Fitting the experimental shapes by theoretical expressions can be used to extract the relevant surface and step free energies and also step interaction energies. Experimental examples of Pb and Au equilibrium shapes are evaluated and discussed. Step free energies of vicinal Au(1 1 1) and Au(1 0 0) surfaces, evaluated by both models, are 30 and 10 meV/ A 2 , respectively.
We present a calculation of the force, due to electron scattering, acting upon an adatom at the s... more We present a calculation of the force, due to electron scattering, acting upon an adatom at the surface of a current-carrying metal, i.e., the so-called "wind" force in surface electromigration.
We reduce the dimensionless interaction strength in graphene by adding a water overlayer in ultra... more We reduce the dimensionless interaction strength in graphene by adding a water overlayer in ultrahigh vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30%, due to the background dielectric constant enhancement leading to a reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short-range impurities is decreased by almost 40%, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron-hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.
The relations between electrical and mechanical properties of constrained ferroelectric films are... more The relations between electrical and mechanical properties of constrained ferroelectric films are analyzed. It is shown that the internal stresses and the elastic constants can be determined through the measurement of the electrical response. The change in the polarization is proportional to internal stresses due to film-substrate misfit, whereas the linear electrical and electromechanical responses to external field do not depend on the misfit and are determined by the film constraint. The theoretical results are successfully applied to PbZr 0.2 Ti 0.8 O 3 films on (001) LaAlO 3 substrate which exhibit a considerable increase in the saturation polarization due to epitaxial stresses. Significant recovery in the piezoelectric constant and susceptibility is theoretically predicted and experimentally verified for specific film configurations which reduce the degree of constraint. The concept presented in this Letter can be expanded to constrained ferromagnetic and superconductor films.
Graphene is a model system for the study of electrons confined to a strictly two-dimensional laye... more Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer 1 and a large number of electronic phenomena have been demonstrated in graphene, from the fractional 2,3 quantum Hall effect to superconductivity 4 . However, the coupling of conduction electrons to local magnetic moments 5,6 , a central problem of condensed-matter physics, has not been realized in graphene, and, given carbon's lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported 7-9 , with explanations ranging from lattice defects 10 to edge structures 11 to negative curvature regions of the graphene sheet 12 . Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG), induced by proton irradiation 8 or native to grain boundaries 7 , can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene 13 are local moments which interact strongly with the conduction electrons through the Kondo effect 6,14-16 , providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature T K is tunable with carrier density from 30 to 90 K; the high T K is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material . We previously reported the resistivity of graphene with vacancies induced by ion irradiation in ultra-high vacuum (UHV; ref. ). Here we present a detailed study of the gate voltage (V g ) and temperature (T ) dependence of the resistivity ρ(V g , T ) in similar graphene with vacancies over a wider temperature range 300 mK < T < 290 K. Apart from weak-localization (WL) corrections , we find that ρ(V g ,T ) is explained by a temperatureindependent contribution ρ c (V g ) due to non-magnetic disorder plus a temperature-dependent contribution ρ K (V g ,T ), not present in as-prepared graphene 13 , which follows the universal temperature dependence expected for Kondo scattering from a localized 1/2spin with a single scaling parameter T K . Graphene with vacancies is prepared as described in ref. 13. After irradiation, the devices were annealed overnight at 490 K in UHV, and then exposed to air during transfer to a 3 He samplein-vacuum cryostat. Figure shows σ (V g ) measured at 17 K for a graphene device (sample Q6) before irradiation, immediately after irradiation, and measured at 300 mK after annealing and transfer to the 3 He cryostat. V g is applied to the Si substrate to tune the carrier density n = c g V g /e, where c g = 1.15×10 -8 F cm -2 is the gate capacitance, and e the elementary charge. The mobility of the device is approximately 4000, 300, and 2000 cm 2 V -1 s -1 , respectively, for these three measurements; the conductivity and mobility recover significantly after annealing and air exposure, consistent with our previous study 13 . From the post-annealing mobility we estimate that this device has a defect density, n imp , of approximately 3×10 11 cm -2 ,
The utility of vertical propagation by the Green’s function to test response linearity has been e... more The utility of vertical propagation by the Green’s function to test response linearity has been explored for magnetic force microscopy (MFM) data from current-carrying wires, by comparing the measured signal at various tip heights to the corresponding propagated MFM signals. Application of a one-dimensional Green’s function was found to be sufficient to predict signal height variation for sample regions of high to moderate field symmetry. For regions of high field asymmetry, the two-dimensional Green’s function was required to obtain good prediction of the height variation. Agreement between the measured and propagated signals was generally within 5%, except at the tails where the signal is not well behaved. The quality of agreement deteriorates gradually with the size of the height propagation. The good agreement spanning a decade of tip and sample separation suggests that the MFM signal is not significantly affected by nonlinearities and can thus be interpreted in terms of classic...
A full characterization of the electrical contact between conductive atomic force microscope (AFM... more A full characterization of the electrical contact between conductive atomic force microscope (AFM) cantilevers and carbon nanotubes (CNTs) is presented. The dependence of current through the contact on loading force, geometric parameters, bias conditions, and time is studied in a two-terminal configuration, where a gold coated AFM cantilever serves as a movable electrode. We find that for an optimized placement of the cantilever relative to the CNT, the current through the contact becomes independent of the loading force beyond a certain limit, and this behavior is also independent of bias conditions. In that load-independent regime, the contact is stable in time to within the current fluctuations imposed by 1/f and telegraph noise in the CNT channel. Under certain conditions of tip placement, the current through the contact exhibits a non-monotonic behavior with loading, which is well explained by the parasitic planar motion of the cantilever.
The characteristics of the Si–vacuum interface were compared with the characteristics of the oxid... more The characteristics of the Si–vacuum interface were compared with the characteristics of the oxide–air interface formed following room temperature oxidation for a variety of samples. Scanning tunneling microscopy was used to measure the surface structure following vacuum preparation, and atomic force microscopy was used to measure the oxide surface on the same samples following exposure to air. Samples investigated included nominally flat Si(111) with equilibrated and quenched surface configurations, Si(111) miscut by 1.25° toward the [2̄11] and equilibrated to yield the faceted structure, and nominally flat Si(001) wafers. In all cases, the step morphology of the clean surfaces was duplicated on the surface of the oxide.
Distinctive Fluctuations of Facet Edges 1 M. DEGAWA 2 , T. J. STASEVICH 3 , W. G. CULLEN, ALBERTO... more Distinctive Fluctuations of Facet Edges 1 M. DEGAWA 2 , T. J. STASEVICH 3 , W. G. CULLEN, ALBERTO PIMPINELLI 4 , T. L. EINSTEIN, E. D. WILLIAMS, U. of Maryland, College Park -Spurred by theoretical predictions of distinctive static scaling of the step bounding a facet, 5 we extend the results to dynamic scaling, also rederiving the static results heuristically 6 and we measure this behavior using STM line scans. 7 The correlation functions go as t 0.15±0.03 decidedly different from the t 0.26±0.02 behavior for fluctuations of isolated steps. From the exponents, we categorize the universality, confirming the prediction that the non-linear term of the KPZ equation, long known to play a central role in nonequilibrium phenomena, can also arise from the curvature or potential-asymmetry contribution to the step free energy. We study a simple model with Monte Carlo simulations to illustrate the novel scaling of fluctuations in an asymmetric potential.
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Papers by Ellen Williams