Search

We used the mechanically controllable break junction technique to discriminate between the electric field and temperature effects in the process of surface modification. The electric field strength at the surface of electrodes was accurately determined using the field emission resonance spectra and was gradually raised to the point where surface modification starts. We found that only a limited number of metals with large values of the work function, >=4.5–5 eV, and evaporation fields, <=1.5–2 V/Å, can be modified in this way. Adsorption of He on the surface drastically increases the local work function of material and enables the field induced nanostructuring of the electrode surface practically for all metals. [ABSTRACT FROM AUTHOR]

Abstract. We present a novel spectroscopic technique that is based on the ballistic injection of minority carriers from the tip of a scanning-tunneling microscope into a semiconductor heterostructure. By analyzing the resulting electroluminescence spectrum as a function of tip-sample bias, both the injection barrier height and the carrier relaxation rate GAMMA[sub s] alter injection can be determined. At 4.2 K we found GAMMA[sub s] =5 x 10[sup 13] s[sup -1] and at 77 K we found GAMMA[sub s] = 8 x 10[sup 13] s[sup -1]. From current-dependent measurements we find that, at room temperature, a large fraction of the carriers is trapped prior to radiative recombination. At high currents or low temperatures the traps become saturated. We tentatively identify the Be acceptors in the structure as trapping centers. [ABSTRACT FROM AUTHOR]

Describes the design and performance of a low temperature scanning tunneling microscope (STM) setup that can be used in magnetic fields. Sensitivity of the instrument to external vibrations; Influence of external magnetic fields on the performance of the STM; Spectroscopic capability.

A scanning tunneling microscope (STM) has been developed, dedicated for use on artificially fabricated nanostructures at low temperatures. With this STM mesoscopic phenomena can be studied, combining the unique possibilities of scanning tunneling microscopy and artificially fabricated nanostructures for investigating the physics in this regime. The STM is vibration insensitive, has reliable coarse approach mechanisms in three dimensions, and operates at low temperatures. In order to be able to position the tip above the structure of interest, also a searching strategy has been developed. In this article we describe design and operation of the STM and demonstrate searching. [ABSTRACT FROM AUTHOR]

We investigated the behavior of piezoelectric elements used in an STM for fine positioning of the tip. Special attention was paid to the influence of hysteresis. We found that the expansion coefficient of the elements depends on the magnitude of the displacements they produce. If the range is limited to several nanometers, the sensitivity can be taken constant and hysteresis can be neglected. However,if displacements of about a micron are involved, the sensitivity is about 40% higher and the hysteresis will significantly disturb the image. We also compared different calibration methods. An inductive displacement transducer yields results which correspond to those obtained with the known height of steps on a flat gold surface. A Michelson interferometer yields results which are valid for large displacements which cannot be used if small-scale structures are imaged. [ABSTRACT FROM AUTHOR]

We have constructed a scanning tunneling microscope for operation under UHV conditions (10-8 Pa). With this instrument topographic measurements can be made on metal and semiconductor surfaces by means of a scanning tip electrode, driven by piezoelectric ceramic elements. The maximum area covered ranges up to 4000×4000 Å2 with a resolution better than 10 Å laterally and 0.15 Å rms perpendicular to the plane. Because of its compact design, the scan unit is very insensitive to vibrations and has a response time down to 0.3 ms. This allows a high scan rate to be used. In order to minimize temperature effects, special attention is paid to the geometry of the construction and the materials used, resulting in a drift ≤4 Å/min along the surface and 0.5 Å/min perpendicular. [ABSTRACT FROM AUTHOR]

The present letter describes a method to make a spin-polarized scanning tunneling microscopy tip by applying voltage pulses between a W tip and a magnetic sample. This spin-polarized tip has the similar characteristics as an Fe-coated W tip, which was confirmed by observations of antiferromagnetically coupled ferromagnetic Mn(001) layers (> 3 ML) grown on an Fe(001) whisker at 370 K. Furthermore, we demonstrate that these voltage pulses can vary the tip magnetization direction. [ABSTRACT FROM AUTHOR]

Investigates the electronic structure with atomic spatial resolution using scanning tunnelling microscopy. Accounts on the intrinsic property of Cr(001) surface; Types of impurities on the cleaved surface; Visualization of superconducting gap near a zinc atom.

The increase in surface conductivity upon illumination of semi-insulating GaAs was used to enable surface-topography measurements with a scanning tunneling microscope (STM). Images were recorded in ultrahigh vacuum and in air using a HeNe laser and a halogen lamp for the generation of a photoconductive top layer. From the current-voltage characteristics of the tip-sample contact the maximum feedback-controlled tunnel current at a given voltage can be deduced. A calculation of the increase in carrier density and conductance upon illumination is given, which confirms the possibility of using STM on highly resistive photoconductive materials. Further implications and applications are discussed. [ABSTRACT FROM AUTHOR]

We present experimental results and numerical calculations on the detection of picosecond electrical transients by a photoconductively gated scanning tunneling microscope. We show that the transient signal detected in the tunneling regime is coupled by the 5 fF geometric capacitance between tip and sample leading to a correlation current that is linearly proportional to the tunnel conductance. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the sensitivity of the semiconductor scanning tunneling microscope (STM) tips to an incident optical field. Characteristics of the semiconductor STM tip; Representation of the surface photovoltage under steady-state photoexcitation; Adjustment of the STM junction to obtain a photovoltaic or photoamperic behavior.

A bromide-modified silver (111) surface, covered by Jelly-type aggregates of a cyanine dye, is investigated by scanning tunneling microscopy (STM) in air. We show that, under appropriate conditions, light is emitted out of the junction by inelastic electron tunneling. Study of the relation between the light intensity and the tunnel bias clearly reveals a molecular influence on the light emission. Furthermore, through a spectral analysis, we show that sharp resonances appear for dye-covered surfaces by coupling of tip-induced plasmons with molecular excitations. This demonstrates that through STM-induced luminescence local spectroscopic information can be obtained on molecules. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A magneto-optical near-field scanning tunneling microscope is used to image the prewritten magnetic domain structure of a Pt/Co multilayer. A semiconducting tip acts as a local photodetector to measure the magnetic circular dichroism signal coming from the magnetic sample. The resolution of the magnetic imaging is given by the photoelectrically active volume of the tip. Reduction of the laser light wavelength resulted in a factor of 4 improvement of the magnetic resolution. Based on a sound and applicable definition we estimate the resolution to be (60±35) nm for a wavelength of 532 nm. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Intermixing, growth, geometric and electronic structures of gold films grown on antiferromagnetic stacking body-centered-tetragonal manganese (001) films were studied by means of scanning tunneling microscopy/spectroscopy at room temperature in ultra-high vacuum. We found stable ordered c(2×2)-MnAu(001) alloy layers after depositing Au on pure Mn layers. Since at the fourth layer (5×23)-like Au reconstruction appears instead of the c(2×2) structure and local density of states peaks obtained on the c(2×2)-MnAu surface disappear, pure Au layers likely grow from the fourth layer. [Copyright &y& Elsevier]

To elucidate the magnetic properties of a magnetic multilayer film, an understanding of the local geometry, electronic structure, and possible intermixing at the interfaces is of utmost importance. By gradually increasing the Fe coverage on the Mn(0 0 1) film, we obtained this information on the interface region by means of Auger spectroscopy and scanning tunneling microscopy/spectroscopy. Intermixed atoms and various geometries and density of states are found to exist in this system. [Copyright &y& Elsevier]

The growth of ultrathin Mn films on an Fe(0 0 1) whisker at 370 K is studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy at room temperature in ultrahigh vacuum. Atomically and chemically resolved STM images show that the Mn film grows with the same in-plane lattice constant as Fe(0 0 1) and that Fe atoms intermix with the first (14%), the second (4%), and the third Mn layer (2%), while a negligible amount of Fe atoms is found above the third layer. The growth mode changes from layer-by-layer to layer-plus-island at a coverage of 3 ML Mn. dI/dV curves which are normalized by voltage-dependent tunneling probability functions show clear peaks on each Mn layer. These peaks are tentatively ascribed to surface states. On the first Mn layer, peaks are found at +0.35 eV on pure Mn areas and at +0.28 eV on mixed MnFe areas. The second and the third Mn layer show peaks at +0.20 and +0.8 eV, respectively. Mn films thicker than three layers reveal besides a strong peak at +0.8 eV a weaker peak at −0.6 eV. Our apparent step height measurements show that the Mn film relaxes at the third layer: the interlayer spacing is ∼0.16 nm for the first two layers, and it increases to ∼0.18 nm at the third layer. Starting from the fourth layer the interlayer spacings are geometrically equivalent (∼0.165 nm). [Copyright &y& Elsevier]

The submonolayer growth of Mn on Fe(0 0 1) in the temperature range between 50 and 200 °C is studied by scanning tunneling microscopy and scanning tunneling spectroscopy. For growth temperatures above 100 °C, atomically resolved STM images with chemical contrast clearly reveal the incorporation of Mn atoms in the Fe(0 0 1) substrate. The fraction of place exchanged Mn atoms is observed to increase with growth temperature. Although on islands a c(2×2) structure forms locally which is attributed to an ordered MnFe surface alloy, long range order could not be obtained for the growth temperatures and coverages studied. Spectroscopy results are presented for clean Fe(0 0 1), pure Mn ad-islands, single incorporated Mn atoms and c(2×2)-ordered MnFe areas. In contrast to embedded Cr atoms in the Fe(0 0 1) surface [Phys. Rev. Lett. 76 (1996) 4175], isolated embedded Mn atoms do not lead to double peak structures in dI/dV curves. Nevertheless, on the ordered c(2×2) MnFe structure and the pure Mn monoatomic islands surface states are detected as peaks in the dI/dV and (dI/dV)/(I/V) curves. However, due to the strong influence of a tip-dependent background on these peaks, the corresponding surface state energies cannot be found directly from those curves. The real surface state energies were recovered by normalizing the measured dI/dV curves by fitted quadratic backgrounds. Thus, surface state energies of about +0.35 and +0.25 eV are estimated for pure Mn islands and alloyed MnFe areas, respectively. [Copyright &y& Elsevier]