S. Gangopadhyay

A reverse electrochemical floating-layer technique of SPM tip preparation

An experimental set-up involving a combination of reverse electrochemical etching and the floating-layer technique of tip preparation is presented. In the model only dc bias is used, avoiding the shortcomings of ac bias. The collection of two tips in a single etching process and mechanical auto-breaking of the circuit are emphasized. This cost and time effective, easy in-laboratory approach yields two sharp tips without paying attention to switching off the circuit. The nature of the tip shape arising from the floating-layer technique has been explained in terms of the electrical field distribution between the electrodes. The addition of some new information together with a few established facts will enrich and enlighten the art of tip preparation.

Self-organized 2D nanopatterns after low-coverage Ga adsorption on Si (1 1 1)

The evolution of the Si(1 1 1) surface after submonolayer deposition of Ga has been observed in situ by low-energy electron microscopy and scanning tunnelling microscopy. A phase separation of Ga-terminated -R 30° reconstructed areas and bare Si(1 1 1)-7 × 7 regions leads to the formation of a two-dimensional nanopattern. The shape of this pattern can be controlled by the choice of the surface miscut direction, which is explained in terms of the anisotropy of the domain boundary line energy and a high kink-formation energy. A general scheme for the nanopattern formation, based on intrinsic properties of the Si(1 1 1) surface, is presented. Experiments performed with In instead of Ga support this scheme.

Interfacial interactions at Au/Si3N4/Si(111) and Ni/Si3N4/Si(111) structures with ultrathin nitride films

Synchrotron photoemission spectromicroscopy has been used to study the interfacial interactions, metal diffusivity, and electronic barriers of Au and Ni contacts on ultrathin silicon nitride films. The Au/Si3N4 interface was found to be nonreactive, and only in the case of a very thin nitride film and elevated temperatures, Si can segregate from the Si(111) substrate and interact with Au. In the case of Ni/Si3N4/Si(111) structures, Ni diffusion and degradation of the Si3N4 lattice are evidenced even at room temperature and strongly enhanced at elevated temperatures, leading to formation of a Ni silicide interlayer.

Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications

The cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.

Imaging and manipulation of the Si(100) surface by small-amplitude NC-AFM at zero and very low applied bias

We use a noncontact atomic force microscope in the qPlus configuration to investigate the structure and influence of defects on the Si(100) surface. By applying millivolt biases, simultaneous tunnel current data is acquired, providing information about the electronic properties of the surface at biases often inaccessible during conventional STM imaging, and highlighting the difference between the contrast observed in NC-AFM and tunnel current images. We also show how NC-AFM (in the absence of tunnel current) can be used to manipulate both the clean c(4 × 2) surface and dopant-related defects.

Ultra-thin high-quality silicon nitride films on Si(111)

Ultra-thin silicon nitride films grown by exposure of Si(111) substrates to a flux of atomic nitrogen at temperatures between 700 °C and 1050 °C have been investigated by means of X-ray spectromicroscopy, atomic force microscopy, X-ray reflectivity, and X-ray photoemission spectroscopy. The films show a Si3N4 stoichiometry. For reactive nitride growth at temperatures below 800 °C, a smooth surface and interface morphology is found. Higher temperatures lead to the formation of rough films with holes and grooves of increasing size, approaching a lateral size of several hundred nanometers for growth temperatures above 900 °C. Nonetheless, X-ray spectromicroscopy shows that the bottom of the holes consists of Si3N4.

qPlus atomic force microscopy of the Si(100) surface: Buckled, split-off, and added dimers

Dimer configurations at the Si(100) surface have been studied with noncontact atomic force microscopy in the qPlus mode at 77 K, using both large (10 nm peak to peak) and small (0.5 nm peak to peak) oscillation amplitudes. In addition to the p(2×1), p(2×2), and c(4×2) reconstructions of the pristine surface, a variety of defect types including ad-dimers, vacancies, and split-off dimers have been imaged. Our data appear at odds with the currently accepted structural model for split-off dimers. At low oscillation amplitudes the degree of apparent dimer buckling can be “tuned” by varying the frequency shift set point.

An inexpensive up-gradation of scanning tunneling microscope for ballistic electron emission microscopy and spectroscopy

Abstract Ballistic electron emission microscopy (BEEM) and Ballistic electron emission spectroscopy (BEES) have become highly useful tools for studying the transport property of electrons and holes across metals, metal–semiconductor interfaces and in semiconductors. Although the techniques are developed, but there is hardly any detailed description of experimental set-ups in this regard. We have carried out an inexpensive upgradation of commercial STM (model STM635 of RHK Technologies) for BEEM and BEES studies on metal–semiconductor interfaces. Along with the associated electronics and a suitable sample holder, we have also developed a technique to produce high quality STM tips necessary for these studies. We have carried out BEEM studies on gold–silicon interface and the results are in conformity with those reported earlier. The minimum threshold of obtaining ballistic current is found to be 0.7 V for Au–n-Si interface whereas it may rise up to 4 V if a layer of oxide is permitted to grow on silicon before deposition of gold film. In the case of a 100 A gold film over n-Si(100), the BEEM current is more intense at the grain boundaries and relatively less over the grains of greater heights.

Shape of field-induced nanostructures formed by STM

Creation of controlled and reproducible nanostructures on material surfaces using scanning tunneling microscope is a novel technique, which can be used for a variety of applications. We have examined the shape of the nanostructures so formed on the gold film using tungsten tip and examined the formation parameters, which govern their shape and size. During our investigations it is found that the reproducibility of mound formation can reach up to 90% under optimum operating conditions, whereas the pit formation can be made with almost 100% reproducibility. Formation mechanism of such nanostructures is also discussed.