A thermally compensated tube scanner scanning tunneling microscope (STM) has been constructed and successfully tested. This design utilizes two concentric piezoelectric tubes, one for scanning and one for thermal compensation and inertial sample translation (over several mm), as well as fine adjustment of sample position while in tunneling range. This design eliminates the need for mechanical components such as springs, levers, gears, or stepper motors that are known to result in considerable vibration sensitivity, thermal drift, and low-resonance frequencies. This new design demonstrates continuously variable-temperature operation as well as atomic resolution without vibration isolation for the first time in a STM. Thermal drift of less than 1 Å/h and less than 10 Å/K have been determined. Also, the lowest mechanical resonance frequency of 21 kHz makes this new design suitable for high-speed applications such as video rate scanning.

The scanning tunneling microscope (STM) has been used to study the c(8X2) reconstruction of GaAs(100). High-resolution STM images imply this surface is composed of equal numbers of arsenic and gallium atoms, resolving disagreements in the interpretation of several previous experiments. The c(8X2) structure arises from an ordered arrangement of (4X2) subunits, with each subunit containing two As dimers and two Ga dimers.The GaAs(100) surface is known to exhibit a large number of surface reconstructions depending on processing conditions. In order of increased annealing temperature in ultrahigh vacuum (UHV), the primary reconstructions are c(4X4), (2X4)/c(2X8), (1X6), (4X6), and (4 X 2 ) /c ( 8 X 2). Of these reconstructions, the (2X4)/c(2X8) has been studied most extensively, and high-resolution scanning tunneling microscopy (STM) images obtained by a number of groups have verified the missing As dimer model for this reconstruction. ' The (4X2)/c(8X2) reconstruction, however, has not been studied as extensively, and its atomic arrangement has not been conclusively determined. Frankel et al. have postulated a missing Ga dimer model for the (4X2) reconstruction, analogous to the missing As dimer model for the (2X4) reconstruction. Biegelsen et al. studied the GaAs(100) c (8X2) reconstruction with STM and obtained images which appeared to support a missing Ga dimer model. We have obtained especially highresolution STM images of the c (8X2) reconstruction and propose a different model for this surface, one which consists of uninterrupted rows of As dimers separated by two rows containing perpendicular Ga dimers. GaAs(100) is a technologically important surface and many studies have attempted to characterize its various reconstructions.Previous work has shown a close relationship between surface stoichiometry and the various GaAs(100) reconstructions. Most studies have concluded that the reconstructions are ordered from highest As surface concentration to lowest with increased annealing temperature as c (4 X 4), (2 X 4) /c (2 X 8), ( 1 X 6), (4 X 6), and (4X2)/c(8X2), with the (1X6), (4X6), and (4X2)/c(8X2) having nearly equal As surface concen-tration.However, attempts to quantify the As/Ga surface ratio have been inconclusive, with different groups reporting widely varying As/Ga ratios for the various constructions. ' Surface Ga concentrations for the (4X2)/c(8X2) reconstruction have been reported from 0.48 (Ref. 6) to greater than 1.0 (Ref. 9) fractions of a monolayer. Previously obtained STM images of (4X2)/c(8X2) reconstructions on GaAs and other III-V semiconductors have consistently shown the presence of well-ordered rows 16 A apart running parallel to the [110] direction. The 16-A period, four times the lattice period along the I 110] direction, strongly implies that the c(8X2) reconstruction is composed of ordered (4X2) subunits. The missing Ga dimer model of the (4X2) subunit advocated by Biegelsen et al. on the basis of relatively low resolution STM images contains two adjacent Ga dimers and two missing dime...

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A new variable-temperature STM has been used to perform studies of the adsorption and ordering of simple polar molecules such as acetone and dimethylsulphoxide (DMSO) on graphite. The primary objective of this work is to obtain fundamental information which can serve as a basis for studies of complex systems. Following exposure of graphite to acetone vapour, STM images show cloudy patches of acetone drifting across the surface, with individual molecules being indistinguishable. Following prolonged exposure, however, an ordered monolayer forms, having triangular symmetry and 2x2 commensurate periodicity relative to the substrate. Similar results are obtained for DMSO on graphite; however, the monolayer is more closely packed and probably incommensurate with the substrate. In a related effort, nitric acid is shown to be an effective chemical etch for creating steps in basal plane graphite. It is hoped that this, and related techniques, will enable the use of STM to study important chemical systems, such as treated fuel-cell electrodes, which utilize non-basal plane graphite.

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