Aw McKinnon

Spatially resolved electrical measurements of SiO2 gate oxides using atomic force microscopy

Using a modified atomic force microscope (AFM) with a conducting cantilever, we have investigated the dielectric strength of SiO2 gate oxide films. This has been achieved by spatially resolving the prebreakdown tunneling current flowing between the silicon substrate and tip. During AFM imaging a voltage ramp was applied to the tip at each image point so as to determine the local threshold voltage required to generate a small tunneling current in the oxide, without causing an irreversible electrical breakdown. For an oxide 12‐nm thick this voltage was found to vary by more than a factor of 2.7 over an area of 0.14 μm2, with a maximum value of 40.5 V. This suggests that the breakdown strength of conventional metal‐oxide‐silicon capacitors may not be limited by the intrinsic dielectric strength of the oxide, but by imperfections or nonuniformities in the Si/SiO2 structure. By preventing irreversible oxide breakdown during scanning, we can image the dielectric properties of oxide films with a lateral resoluti...

An in-situ high temperature scanning tunnelling microscopy study of the boron-induced √3×√3 reconstruction on the Si(111) surface

Abstract In-situ high temperature scanning tunnelling microscopy (STM) measurements were performed on vacuum-annealed, boron-doped Si(111) wafers. The samples were p-type with an atomic boron concentration of ∼ 10 19 cm −3 which were chosen such that after ultra-high vacuum processing at up to 1250°C and annealing at 600°C they gave rise to co-existence of both 7 × 7 and √3 × √3R30° surface terminations. After the initial “flashing” of the sample to 1250°C, we observe pinning of a variety of reconstructions due to the random out-segregation of boron to the surface. Prolonged annealing at the measurement temperature of 600°C was observed to give rise to the agglomeration of the boron into well-defined √3 × √3 regions. In the measurements, we observe preferential directions for the in-plane boundaries between √3 × √3 and 7 × 7 regions, showing clearly the importance of dimerisation as a strain relief mechanism between these regions. Detailed observations reveal the lateral diffusion of boron and silicon in the √3 × √3 regions as a function of time. The data are discussed in terms of the various controlling processes at the surface.

A one-dimensional piezoelectric-driven inertial micropositioner with vertical capabilities

The authors have developed a novel piezoelectric-based linear inertial slider to achieve remote micropositioning in any orientation. It is particularly suitable for SPM in cryogenic and ultra-high vacuum environments and the simple, coaxial design results in a rigid structure. Speeds of up to 2 mm s-1 have been achieved vertically against gravity, and the ability to lift up to 100 g has been demonstrated. A computer simulation for the system has been developed and used to investigate the effects of different driving waveforms. In particular an exponential ramp was shown to be most effective, this was corroborated by experimental results.

Direct writing of nanostructures from silane on silicon (111)

Silicon based structures were fabricated using scanning tunneling microscope on a Si(111) surface by the localized decomposition of gaseous silane at pressures of 10−5–10−6 Torr. Continuous wires of width 5 nm could be produced while atomically resolved images of the nearby substrate were obtained. We argue that the fabrication process is effected by field‐assisted decomposition of silane on the tip surface, which subsequently migrates to the tunneling region at the tip apex where it field desorbs to the silicon surface.

Thermal desorption of Na from NaSi(111)3 × 1: in situ observation of the 3 × 1-to-7 × 7 structural transformation using a high-temperature scanning tunnelling microscope

Abstract The Na-stabilised Si(111)3 × 1 phase has been investigated upon annealing. At temperatures between 410 and 540°C the Na desorbs gradually, resulting in a structural transformation to the 7 × 7 reconstruction. The use of a high-temperature STM enabled the in situ observation of this morphological transition, revealing a variety of intermediate Si(111) reconstructions such as 2 × 2, c4 × 2, √3 × √3R30°, 5 × 5, 7 × 7 and 9 × 9 on Na-free areas, before the entire surface reconstructed to the 7 × 7 structure at higher temperatures (800°C). The transition is accompanied by a pronounced mass transport and can be explained in terms of the removal of excess Si atoms from local Na-free 3 × 1 formations.

Scanning tunnelling microscopy and atomic force microscopy of carbondiamond films

Abstract We have used both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) to image and measure electrical properties of diamond films. The carbon-diamond films were deposited on {100}, 1–5 W cm, n-type Si substrates from a 13.56 MHz, capacitively coupled CH 4 Ar rf plasma. Film thicknesses ranged from 10 nm to 150 nm and were transferred immediately into UHV for measurement. In the case of the 10 nm films, images were obtained in the STM revealing three types of morphology: (i) large atomically-flat regions with an unpinned Fermi level exhibiting semiconductor-like behaviour with a band gap of ∼4 eV as determined by taking current-voltage (1 V) characteristics at selected sites. Such characteristics also showed a reverse bias stability with no evidence of breakdown up to 8 eV. (ii) Three-dimensional crystalline regions with crystal sizes up to 200 nm. (iii) Amorphous regions occupying spaces between grains and often decorating the surfaces of crystalline regions. The electrical characteristic of these regions showed an essentially metallic-like behaviour with no obvious band structure, indicating that they were probably amorphous carbon. Thicker carbon-diamond films became increasingly difficult to image in the STM indicating that most of the voltage was being dropped within the film, preventing electrons being injected into the conduction band of the silicon substrate for detection. In these cases samples were imaged in the AFM which indicated that the films were essentially flat with a roughness of ∼5 nm. Friction measurements carried out with the AFM gave a value of 0.03 which agrees favourably with previous measurements on pure diamond in air.

Observation of the ordered growth of coronene on Ag(111): a scanning tunnelling microscopy study of vacuum deposited molecules

Abstract The molecular ordering of coronene (C 24 H 12 ) obtained by vacuum-deposition onto predominantly Ag(111) on mica has been investigated using the scanning tunnelling microscope. Real-space topographic images reveal that in certain regions we obtain layer-by-layer ordered growth of the molecules on this substrate which agrees with previous indirect measurements (the growth did not display this ordering in other regions). In our experiments on the ordered regions, we observe the best imaging contrast at a voltage bias of −0.28 V which may correspond to a resonant tunnelling process through the molecules.

Scannint tunnelling microscopy of plasma- and photo-enhanced chemical vapour deposited hydrogenated amorphous silicon

A series of ‘intrinsic’ amorphous silicon (a-Si:H) films produced by photo- and plasma- chemical vapour deposition was studied using scanning tunnelling microscopy (STM). The high resistivity of these films meant that photo-induced enhancement of the carrier concentration in the material was required to enable a stable tunnelling regime to be obtained. This enhancement was achieved using a 3 mW He-Ne laser shone through the back of the sample. Tunnelling I/V spectra showed rectification behaviour typical of metal-insulator-semiconductor tunnelling. In addition, spatially independent photovoltage effects were observed while tunnelling under illumination. A series of I/V data taken as a function of tip/sample separation shows a gradual transition from spectra dominated by weak surface states to a more bulk-like behaviour. The surface roughness of the photo-CVD material (33 A rms) was found to be an order of magnitude greater than that for the plasma-CVD material (6 A rms). This difference may be the result of hydrogen radical etching in the former process.

Tip effects and surface modification in scanning tunnelling microscopy

Abstract In this paper we discuss the effects of tip structure on the behaviour of the scanning tunnelling microscope. The geometry and electronic structure of the tip affect both resolution in imaging and current-voltage spectroscopy. Interactions between tip and sample may lead to modification of the sample. Examples of these effects are drawn from a wide range of systems, including reconstructions on semiconductor surfaces, imaging of nanostructures and low-temperature scanning tunnelling microscopy.