Operation Of Scanning Tunneling Microscope Research Articles

Selective-area epitaxial growth of gallium arsenide on silicon substrates patterned using a scanning tunneling microscope operating in air

Selective-area epitaxial growth of gallium arsenide on n-Si(100) substrates is reported, where the oxide (SiOx) mask consists of 1–2 monolayer-thick features patterned onto a silicon substrate using a scanning tunneling microscope (STM) operating in air. The technique for generating the STM patterns on hydrogen-passivated silicon was reported recently [J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek, and J. Bennett, Appl. Phys. Lett. 56, 2001 (1990)]. The GaAs epilayer was grown by migration-enhanced epitaxy at 580 °C and its morphology was investigated by scanning electron microscopy. The chemical selectivity of the STM-patterned regions was verified by imaging time-of-flight secondary-ion mass spectrometry. The implications of these results for the development of a unique, STM-based nanostructure fabrication technology are discussed.

Observation of the graphite surface by reflection electron microscopy during STM operation

The basal plane of highly oriented pyrolytic graphite (HOPG) has been studied by reflection electron microscopy (REM) before, during, and after tunnelling with a scanning tunnelling microscope (STM). The STM was designed to replace the standard specimen holder of a commercially available transmission electron microscope (TEM). Three types of contrast are found to be causally linked to tunnelling: (1) A dark feature which follows the tip during scanning; we attribute this to reversible elastic shear strain due to repulsion between the surface and tip. This extends over tens of nanometers. (2) A permanent bright region which forms under the tunnelling tip with repeated scanning; we attribute this to the increase in RHEED reflectivity which results from cleaning of the surface by abrasion and/or oxidation, possibly catalysed by the W tip. WO 3 particles from the tip may play a role in the oxidation reaction at the surface, as observed in independent HREM studies. (3) A permanent bright band of contrast extending beyond the tunnelling tip; further study of this defect is required.

Modification of hydrogen-passivated silicon by a scanning tunneling microscope operating in air

The chemical modification of hydrogen-passivated n-Si (111) surfaces by a scanning tunneling microscope (STM) operating in air is reported. The modified surface regions have been characterized by STM spectroscopy, scanning electron microscopy (SEM), time-of-flight secondary-ion mass spectrometry (TOF SIMS), and chemical etch/Nomarski microscopy. Comparison of STM images with SEM, TOF SIMS, and optical information indicates that the STM contrast mechanism of these features arises entirely from electronic structure effects rather than from topographical differences between the modified and unmodified substrate. No surface modification was observed in a nitrogen ambient. Direct writing of features with 100 nm resolution was demonstrated. The permanence of these features was verified by SEM imaging after three months storage in air. The results suggest that field-enhanced oxidation/diffusion occurs at the tip-substrate interface in the presence of oxygen.

Deoxyribonucleic acid structures visualized by scanning tunneling microscopy

The capability of the scanning tunneling microscope (STM) to produce atomic-resolution images makes it a powerful tool in both basic and applied science. The main application of STM has been for the analysis of structural features and electronic properties of a wide spectrum of metals and semiconductors, while only a few biological items have been investigated. With a STM operating in air at ambient pressure, we present images showing the molecular structure of naked DNA. High-resolution topographic images of linear DNA fragments reveal structures with a periodicity of ∼38±6 Å and a width of 28±4 Å, consistent with the molecular dimensions of the helical repeat of double-stranded DNA. In addition, the supreme resolving power of the STM permits visualization of the major and minor grooves.

Reflection electron microscope imaging of an operating scanning tunneling microscope

A scanning tunneling microscope (STM) has been operated inside the Philips EM 400T transmission electron microscope (TEM). The possibility of combining STM with dark-field reflection electron microscopy (REM) has been confirmed. Preliminary results using the STM for REM in a TEM indicate that this combination has the potential to be a powerful new technique for studying surfaces and for studying the basic physics of STM. The STM holder has been successfully used to obtain REM images while tunneling from the same region. The incident electron beam and the STM instrument do not appear to influence the imaging modes of each other appreciably. The tunneling gap and surface modifications due to tip contact have also been observed by REM. The calibration of the deflection of the tube type STM scanner by electron microscope images is reported. The effects of hydrocarbon contamination on STM operation in air or conventional vacuum have been studied.

Combination of a scanning tunneling microscope with a scanning electron microscope

We have placed a scanning tunneling microscope (STM) in the sample stage of a scanning electron microscope (SEM). This allows us to focus the tip on a preselected spot for STM operation with a precision of 0.5 μm. We are able to check the state of the sample and the tip during STM operation by simultaneous use of the SEM. We are also able to correlate the images given by both techniques.

A scanning tunneling microscope for use in the philips EM400 electron microscope in the reflection mode

A scanning tunneling microscope (STM) has been built in the form of a side-entry specimen holder for the Philips EM400T (9mm pole piece gap). The instrument is intended to provide STM and Reflection Electron Microscope (REM) images of the same region, possibly simultaneously (STM operation with tunneling gaps of up to l.5nm is possible while REM resolution is about 0.9nm). The purpose is to study the basic physics of the STM. The great sensitivity of REM diffraction-contrast imaging to strain will be used to determine if strains, or other surface modifications, accompany tunneling on graphite and gold. Figure 1 shows a typical REM image of the GaP[110] surface obtained on the EM400.Figure 2 shows a schematic diagram of the STM while figure 3 shows the completed device, now undergoing trials. A single tube scanner is used5, as in certain stereo record- player cartridges.

Laser-frequency mixing in the junction of a scanning tunneling microscope

The frequency response and the frequency-mixing properties of the tunneling junction of a scanning tunneling microscope (STM) were investigated using the radiation of two CO2 lasers. Difference frequencies of up to 90 MHz were detected over a wide range of bias voltages and tunneling currents. At tunneling currents typical of STM operation the beat signal decreases as the bias voltage is increased. However, this dependence is reversed at higher tunneling currents, where the STM begins to resemble a point-contact metal-insulator-metal diode. For low tunneling currents the generation of the beat signal is attributed mainly to thermally assisted tunneling.