Vacuum-cleaved Surfaces Research Articles

Quasiparticle interference in ZrSiS: Strongly band-selective scattering depending on impurity lattice site

Scanning tunneling microscopy visualizations of quasiparticle interference (QPI) enable powerful insights into the k-space properties of superconducting, topological, Rashba and other exotic electronic phases, but their reliance on impurities acting as scattering centers is rarely scrutinized. Here we investigate QPI at the vacuum-cleaved (001) surface of the Dirac semimetal ZrSiS. We find that interference patterns around impurities located on the Zr and S lattice sites appear very different, and can be ascribed to selective scattering of different sub-sets of the predominantly Zr 4d-derived band structure, namely the m = 0 and m = +/-1 components. We show that the selectivity of scattering channels requires an explanation beyond the different bands' orbital characteristics and their respective charge density distributions over Zr and S lattices sites. Importantly, this result shows that the usual assumption of generic scattering centers allowing observations of quasiparticle interference to shed light indiscriminately and isotropically upon the \textit{q}-space of scattering events does not hold, and that the scope and interpretation of QPI observations can therefore be be strongly contingent on the material defect chemistry. This finding promises to spur new investigations into the quasiparticle scattering process itself, to inform future interpretations of quasiparticle interference observations, and ultimately to aid the understanding and engineering of quantum electronic transport properties.

Ab initiostudy of the two-dimensional metallic state at the surface of SrTiO3: Importance of oxygen vacancies

Motivated by recent angle-resolved photoemission spectroscopy (ARPES) observations of a highly metallic two-dimensional electron gas (2DEG) at the (001) vacuum-cleaved surface of SrTiO3 and the subsequent discussion on the possible role of oxygen vacancies for the appearance of such a state (Ref 1), we analyze by means of density functional theory (DFT) the electronic structure of various oxygen-deficient SrTiO3 surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies --independently of the oxygen concentration-- redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a 2DEG. We discuss the underlying model emerging from such observations.

Two-dimensional electron gas with universal subbands at the surface of SrTiO3

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ∼0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

Atomic force microscopy study of the CaF 2(111) surface: from cleavage via island to evaporation topographies

The development of (111) cleavage faces of CaF 2 upon thermal treatment is studied by atomic force microscopy (AFM). Vacuum-cleaved surfaces show the atomic periodicity of the uppermost fluorine layer. Cleavage steps have heights of about 0.32 nm, corresponding to the spacing of neighboring F −–Ca 2+–F − triple layers, or integer multiples thereof. The steps exhibit ‘lightning’-shaped arrangements with tips pointing in the direction of cleavage. Steps run essentially parallel to the 〈110〉, 〈211〉, and 〈321〉 directions. At about 820 K the original cleavage topography transforms into a topography with trigonal and hexagonal islands. The islands develop from the cleavage tips by a melt-off process mediated by step and/or surface diffusion. During further heat treatment at even higher temperatures the surface flattens and the formation of triangular islands with one type of 〈110〉 step is observed. The atomic configuration for this type of step is discussed. Strongly vaporized surfaces studied by an electron microscopic shadowing technique exhibit hexagonal evaporation pits. The exposure to stray electrons from the heater filament at a temperature of 820 K results in a strong surface erosion with deep triangular etch pits with edges parallel to 〈110〉. The formation of these etch pits is probably caused by a combined action of electron-stimulated desorption of fluorine, vaporization of metallic calcium clusters and a partial healing of the surface via step and/or surface diffusion.

Reaction of water with MgO(100) surfaces: Part III. X-ray standing wave studies

Clean MgO(100) surfaces cleaved in vacuum and exposed to water vapor or bulk water were studied using the X-ray standing wave (XSW) technique in back reflection mode and surface sensitive, element specific O KLL and Mg KLL Auger electron yield detection. The effects of surface charging were mitigated, but not entirely eliminated, by using a low-energy electron flood gun. Simulation of the XSW signal showed that the effect of surface charging on the XSW data could be minimized with careful experimental design. We demonstrate that the XSW method can be applied to studies of insulating surfaces, and our results for MgO(100) surfaces exposed to water vapor or bulk water indicate the following: (1) the vacuum-cleaved clean surface undergoes no surface reconstruction or significant relaxation perpendicular to the surface; (2) Mg–OH distances on surfaces exposed to water vapor or bulk water measured perpendicular to the (100) surface are the same as in bulk MgO; and (3) the z-position of the surface Mg atoms does not change within the estimated error [±2% of the (200) spacing] after the surface is fully hydroxylated. Our results for the clean, vacuum-cleaved surface disagree with results from impact collision ion-scattering spectroscopy and surface-extended electron-loss fine structure for MgO(100), which indicate 15 and 17% inward relaxation, respectively, and they support results from low-energy electron diffraction, reflection high-energy electron diffraction, and photoelectron diffraction that show little, if any, relaxation or rumpling of the surface.

The termination of the α-Al 2O 3 (0001) surface: a LEED crystallography determination

We have performed a tensor low-energy electron diffraction (TLEED) determination of 10 diffracted beams at off-normal incidence from the (0001) surface of α-Al 2O 3 produced by oxygen plasma cleaning/annealing cycles. Structures with single domain O- or Al-terminated surface layers give a poor agreement (Pendry R-factor=0.42). Such domains are related by steps in the (11 2 ̄ 0) direction that produce an extra plane of symmetry in the LEED pattern. Allowing a mixture of domains due to steps produces a good agreement. An optimized mixture of 2:1 O/Al-terminated domains lowers the R-factor to 0.26 with an inner potential of 19.2 eV. At the R-factor minimum, we find that in both domains, the outermost atomic layer relaxes outward, with some twisting and relief of buckling in deeper layers. An outward relaxation of the top layer is in disagreement with theoretical predictions. However, such predictions are for vacuum-cleaved surfaces that are not realistic descriptions of the surface used in our experiment.

Friction force microscopy investigations of potassium halide surfaces in ultrahigh vacuum: structure, friction and surface modification

Friction force microscopy measurements on the vacuum-cleaved (001) surfaces of KF, KCl and KBr have been carried out. All surfaces exhibit atomically flat terraces with monatomic steps aligned preferentially along low-index lattice directions. Stick-slip lateral forces with the lattice periodicity are observed on all surfaces. Tip-sample contact creates higher friction domains on the terraces of all three materials. The structure, topography and degree of friction force contrast of these domains is material dependent. The dependence of friction upon load generally does not coincide with the behavior expected for an elastic contact. We propose that the observed domains result from surface structural changes created by low load tip-sample contact on these relatively soft materials.

Charge transfer from Cu in YBa2Cu3O7−δ and Ba2Cu3O4Cl2 crystals determined by Auger electron spectroscopy

A formulation is proposed to estimate the charge transfer ΔqCu from Cu to some anions in metal-oxide crystals, using Auger electron spectroscopy. In the formulation, a parameter, the Auger transition probability ω, is introduced, which is defined as the ratio of Auger intensity due to a certain transition to the total Auger intensity due to other transitions when the same core electrons are primarily excited. By applying the formulation to vacuum-cleaved surfaces of YBa2Cu3O7−δ (YBCO) and Ba2Cu3O4Cl2 (BCOC) crystals, we obtained the following results: (1) YBCO cleaves between CuO2 and BaO layers, and the amount of ΔqCu is 2.3 in units of elementary electric charge. The deviation of ΔqCu from 2.0 suggests the existence of Cu3+-like atoms; (2) for BCOC, ΔqCu varies from 3.3 to 1.7 depending on the Ar ion beam sputtering time.

Low-energy positron-diffraction study of NaF and LiF.

Using a magnetically transported beam of slow positrons, we have measured the elastic backscattering probability versus energy for positrons normally incident on air-cleaved and vacuum-cleaved (100) surfaces of NaF and LiF. We observe enormous first-order Bragg peaks and several higher-order peaks, the positions of which imply an inner potential of -0.5\ifmmode\pm\else\textpm\fi{}0.2 and -0.1\ifmmode\pm\else\textpm\fi{}0.2 eV for NaF and LiF, respectively. The atomic plane spacings are equal to the bulk values within 1/2%. There are only slight differences between the data obtained using the freshly air-cleaved and the vacuum-cleaved surfaces. Surface disorder may be responsible for the presence of some weak Bragg reflections corresponding to directions that would otherwise be inaccessible to the incident positron beam. The temperature dependence indicates that the disorder would not have a thermal origin.

Ion and Electron Spectroscopy During Pulsed Laser Irradiation of Silicon

ABSTRACTIon mass spectrometry, charged particle yields, and kinetic energy distributions of electrons and ions are used to characterize silicon wafers and vacuum-cleaved silicon surfaces under conditions related to laser annealing. We find that alkali metals dominate the positive ion emission from chemically-cleaned wafers, a mass-72 peak tentatively identified as Si2O+ comprises the main ion emission from the cleaved surface, and ion and electron temperatures can be derived from the energy distribution curves although the Si2O+ emission implies more than a simple thermal evaporation process.

Surface effects on core-level binding energies and valence in thulium chalcogenides

Vacuum-cleaved (100) surfaces of mixed-valent TmSe, divalent TmTe, and trivalent TmS were studied by high-resolution, angle-integrated photoelectron spectroscopy with the use of synchrotron radiation in the energy range $30\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}110$ eV. In the topmost surface layers of TmSe and TmTe the $4f$ levels are found to be shifted to higher binding energies by 0.32\ifmmode\pm\else\textpm\fi{}0.04 and 0.41\ifmmode\pm\else\textpm\fi{}0.05 eV, respectively. In both TmSe and TmS the topmost surface layers are divalent. In the case of TmSe a separation of the ${\mathrm{Tm}}^{2+}$ $4{f}^{12}$ spectral feature into surface and bulk contributions allows a determination of the bulk mean valence $\overline{v}=2.55\ifmmode\pm\else\textpm\fi{}0.05$. While a shift of the Se $3d$ levels to lower binding energy is observed for Se atoms in the topmost surface layer of TmSe, no such shift can be resolved for the Te $4d$ levels of TmTe. The surface-derived divalent spectral features can be quenched completely in all three cases by exposure of the surfaces to submonolayer amounts of oxygen, resulting in the formation of trivalent surface oxides. Values for the electron mean free path $l$ are derived from the observed surface- and bulk-spectral intensities, with $l$ decreasing with decreasing electron kinetic energy down to \ensuremath{\cong}45 eV. Smaller singularity indices $\ensuremath{\alpha}$ of the Doniach-Sunji\ifmmode \acute{c}\else \'{c}\fi{} line shapes as well as smaller extrinsic losses are observed for photoemission from the divalent surface layers as compared to the bulk.

Epitaxy of Bismuth on the surfaces of long-chain molecular single crystals

Tansmission electron microscope Observation was made of the epitaxy of Bi deposited onto the surfaces of long-chain stearic acid molecular crystals, Vacuum-depositions of Bi onto stearic acid substrates, cleaved in air and vacuum, carried out at deposttion rates ranging 13A/min to 3000A/min. The residual gas pressures were 10-6 Torr and 10-4 Torr. The temprature of the substrate during deposition was 15°C. The epitaxy of Bi did not depond upon the lattice misfit between the substrate and the Bi thin film. The morphologies of Bi deposited onto the air-cleaved and vacuum-cleaved surfaces showed similar features at a film thickness ot about 100A. There was also no significant difference in the epitaxial structures of Bi deposited in residual gas pressures of 10-6Torr or 10 -4Torr. Typical morphology of the deposited Bi was observed to bc triangular and hexagonal-shaped islands. The number of the triangular shapes of Bi particles becomes larger as the deposition rate increases; the hexagonal shapes remarkably appeard as the deposition rate decreased. The three-dimensional shape of the triangular particles were those of trigonal pyramids, eud the hexagonal particles were found to be flat. The mechanism of Bi thin films on surfaces of molecular crystals has been discussed prviously.

An experimental assessment of proposed universal yield curves for secondary electron emission

A variety of 'Universal Yield Curves' for the secondary emission process have been proposed. A series of precise measurements of the secondary emission properties of a range of related amorphous semiconducting materials, made under UHV on freshly vacuum-cleaved surfaces, and covering a wide range of primary energies, have recently made possible an accurate assessment of the validity of the various UYCs suggested. It is found that no truly universal curve exists; the atomic number of the target materials plays an important part in determining the secondary emission properties. Agarwal's semi-empirical expression, which takes account of the atomic number and weight, is found to give good agreement for all the materials studied. Further theoretical investigation is required.

Angular dependence of auger electron emission from MgO and NiO

Angular Auger profiles have been obtained from the vacuum-cleaved (100) surfaces of MgO and NiO using LEED optics and a spot photometer. Data for Mg L 2, 3VV, Ni M 2, 3VV and Ni M 2, 3V in the [001] and [011] azimuths are presented. The results are compared with profiles expected from s, p, d, f and g wave sources with LEED final states. The g wave ( l = 4) source provides the best fit for the Ni M 2, 3VV peak profiles but the quality of agreement is not good and theory predicts that l = 3 should dominate. For Mg L 2, 3VV the “best fit” is for l = 3 in the [001] azimuth and l = 4 in the [011] azimuth where theory predicts that l = 1 should dominate in both azimuths. These discrepancies are discussed and the possibility of non-localised Auger sources and effects due to anion polarization are suggested.

Some leed and electron spectroscopic observations on disordered EuO(100) surfaces

Three vacuum-cleaved surfaces of EuO have been studied using LEED, Auger electron spectroscopy, electron loss spectrocopy and mass spectroscopy. The (100) surface is stable in the electron beam but is disordered after cleavage at room temperature. The Auger spectrum below 150 eV is assigned to initial state holes in the 4d shell of europium and to autoionisation processes. The consequences of the observation of disorder upon the interpretation of various magnetic and optical experiments are discussed.

Electron Microscopy of Precipitates of NaCl: FeCl2 Crystals

Suzuki phase precipitates1 produced by divalent impurities in alkali halide crystals can act as preferential gucleation sites for vapor deposited metals using the gold decoration method2. The present work applies this technique to the study of NaCl: FeCl2. Crystals with several FeCl2 concentrations were grown by the Kyropoulos method in a controlled atmosphere and thermally homogenized. The final impurity concentration Cs was determined by atomic absorption spectroscopy. Precipitates were inducel bg linear cooling from 550°C to room temperature at rates of 7.64, 3.82, and 23°C/hr. Gold decoration was performed at 10 Torr on vacuum-cleaved surfaces at 100°C. Precipitates were visible because of an enhanced nucleation density: Figs. la - c. The volume fraction Vσ of precipitates was determined from the total fractional area of the precipitates in micrographs taken from several areas of the replicas. Vσ is linearly related to Cs (Fig. 2) with a slope of 8.1 ± 1.0.

LEED, auger electron spectroscopy (AES) and photoemission studies of the adsorption of cesium on the epitaxially grown GaAs(110) surface

Epitaxially grown GaAs(110) surfaces which are cleaned by argon-ion bombardment followed by annealing show the same behaviour on Cs adsorption in both the LEED, AES and photoemission results, as the vacuum-cleaved GaAs(110) surface. Cs adsorbs in a c(4 × 4) structure up to a coverage of 5.5 × 10 14 atoms cm −2. The structure is enhanced by heat treatment at 260°C and then has a coverage of 4.7 × 10 14 atoms cm −2. Further heat treatment at 320°C gives a GaAs(110)- c(2 × 6) disordered pattern at a coverage of 2.7 × 10 14 atoms cm −2. The Cs coverage is linear with the dosage and shows a sharp break point at 5.5 × 10 14 atoms cm −2, which coincides with the maximum photoemission. NEA can be reached by Cs adsorption only, when the specimen is heated to just below its decomposition temperature (660°C). The photoemission then reaches a value as high as 320 μA lm −1 comparable to the value for vacuum-cleaved surfaces. In other cases the photoemission maximum is only 30 μA lm −1. Activation of the surface with Cs + O 2 leads to photoemission values as high as 1200 μA 1m −1 if in the initial activation with Cs alone NEA is achieved, otherwise the photoemission value is not higher than 650 μA 1m −1.

Schottky barriers on InSb

Schottky barriers formed on air-cleaved InSb are found to be more reproducible and have less leakage than diodes fabricated by chemical processing. Etched surfaces yield diodes with large leakage currents and high series resistances whose values are correlated, i. e. , high series re-sistances have less leakage and vice versa. With air-cleaved surfaces the lowest leakage is observed when the surfaces adjoining the contact are covered with SiO2. From C-2 versus V data, barrier heights of 50 mV and 175 mV at 77K are measured for Au contacts to n- and p-type InSb, respectively. The current-voltage characteristics of Au contacts are consistent with these barriers, but the low-temperature field-emission behavior appears to be dom-inated by tunneling via traps. For Pb contacts, deep-level traps are in evidence that can lead to erroneous in-terpretations of C-2 versus V voltage intercepts. The I-V data for Pb contacts suggest VBn ≈0 and VBp ≈Eg/q. Diodes fabricated on vacuum-cleaved surfaces have large leakage currents which result from the etching procedures introduced to isolate individual diodes.

Metal/non-metal interfaces: adhesion of gold on mica

The adhesion of small gold particles and evaporated gold films to air-cleaved and ultra-high-vacuum-cleaved mica surfaces has been investigated. A vacuum microbalance revealed large variations of particle adhesion for different regions of a vacuum-cleaved surface. A potential probe available in the same system similarly showed large variations in the surface charge. The potential variation and large adhesion disappear on exposure of the surface to contaminants or upon irradiation by an electron beam. These regional variations are believed to be due to local excess and deficiency of potassium ions, created during cleavage. For evaporated gold films on mica, adhesive strengths were compared using a simple cantilever method. Films deposited on vacuum-cleaved faces adhere much more strongly than those on air-cleaved surfaces.

Études de spirales d'évaporation sur des cristaux d'anthracène

The imaging of evaporation structures on vacuum-cleaved surfaces of anthracene single-crystals is obtained by means of nucleation-decoration method first developed by Bassett. From the micrographs of decorated steps some information is available on the obtained structures: step heights, anisotropy of the steps moving. The shape of the evaporation spirals reveals an asymmetrical dissolution kinetic caused by the presence of two kinds of opposite steps along [100].