A. I. Oliva

Electrochemical preparation of tungsten tips for a scanning tunneling microscope

We present experimental results obtained during the electrochemical preparation of tungsten tips for a scanning tunneling microscope. Experiments were done with direct current and two kinds of electrolytes widely reported in the literature: KOH and NaOH. We report the effects of the applied voltage, time of etching, tip length, electrolyte concentration, wire diameter, and immersed portion as relevant parameters in the process. From the images obtained by a metallurgical microscope and a scanning electron microscope as well as from Auger and electron diffraction x‐ray analysis the best conditions for W tip preparation were obtained. We found that KOH is better than NaOH as an electrolyte to prepare tips for scanning tunneling microscopy and that tip quality increases as the wire diameter and the immersed portion increases.

Comparison of properties of CdS thin films grown by two techniques

Abstract Polycrystalline cadmium sulfide (CdS) thin films were deposited on glass substrates by chemical bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The typical deposition temperatures between these techniques are quite different. The CdS thin films deposited by CBD were prepared using two methods of bath agitation: magnetic and ultrasonic agitation. We found that films deposited with ultrasonic agitation presented a cleaner surface with minor presence of contaminants, similar to the obtained for CSS films, as demonstrated by Auger analysis. Properties of the CdS films such as morphology, optical transmission, crystallinity and band gap energy, are discussed in order to compare them in both techniques. We obtain that crystallinity of CdS films depends strongly on the temperature used for deposition. In agreement with several works, films prepared by CBD technique presented a cubic structure, while films grown by CSS technique exhibited an hexagonal symmetry.

Effect of indium tin oxide substrate roughness on the morphology, structural and optical properties of CdS thin films

Abstract Indium tin oxide (ITO) coatings of glass substrates were etched with hydrochloric acid in order to obtain different root-mean-square roughness ( R ITO ) on its surface. The effect of R ITO on the morphology, structural and optical properties of CdS films deposited was investigated. Polycrystalline cadmium sulfide thin films were deposited on ITO/glass substrates by chemical-bath deposition (CBD) at 358 K, and studied by atomic force microscopy (AFM). Roughness of CdS films ( R CdS ) showed a nearly linear increase with R ITO . The thickness of CdS films was investigated by Auger Electron Spectroscopy (AES) and showed an increment with R CdS . X-ray diffraction results showed that CdS films have a cubic zincblende structure with a (111) preferred orientation. The measured residual strain of the CdS films showed an initial increse with R ITO reaching a maximum point at 15±2 nm, and after that exhibited a decreasing dependence. The optical band gap E o of the CdS films obtained from transmittance measurements did not depend on R ITO .

First stages of growth of CdS films on different substrates

Cadmium sulfide films were grown on glass, silicon, and ITO substrates by chemical-bath deposition (CBD) at 358 K, and studied by atomic force microscopy, Auger electron microscopy and X-ray diffraction. CdS film growth initiates immediately and thickness increases with deposition time. The rms roughness of CdS films grown for 1 to 10 min (thicknesses of 10 to 70 nm) was measured by atomic force microscopy. The roughness of films grown on ITO remains similar to that of the substrate regardless of deposition time. On the other hand, the roughness of CdS films grown on glass and silicon increases with deposition time, finally reaching a roughness similar to that of films grown on ITO. Auger profiles show that the CdS/substrate interface is not abrupt and depends on the substrate. Substrate roughness plays an important role during the initial CdS growth process. X-ray studies show that silicon substrates are not appropriate for CdS film growth by CBD. The most appropriate substrate is ITO.

The effect of electrical current (DC) on gold thin films

Abstract We studied changes induced by electrical current (DC) on gold thin films by using a combination of scanning tunnelling microscopy and grazing incidence X-ray diffraction. The results show that the process inducing device failure is based on surface diffusion that produces the growth of microcrystals at the expense of mechanically strained microcrystals. The de-percolation and loss of adherence to the substrate reduce the heat transfer which in turn produce an increase of film temperature. This temperature increase induces a large mechanical stress due to the differential dilatation of substrate and film. The result of these processes is the final failure of current conduction by the film.

Analysis of scanning tunneling microscopy feedback system

A theoretical analysis of the feedback system in the scanning tunneling microscope (STM) is presented. The proposed model includes all the elements involved in the STM loop. The knowledge of its behavior allow one to accurately determine the region where the unstable STM operation could affect the measurements, and also to set the optimal working parameters. Each element of the feedback circuitry is analyzed and discussed as well as their mutual interactions. Thus the stability region of a STM has been obtained analytically, using all the possible elements in the feedback loop and without using simplifications or rare models for the system. The mathematical models for each element of the loop has been used and the problem has been solved using control theory. Different working conditions are simulated and analyzed. Some relations for stability conditions, considering the value of each component involved in the feedback loop, are proposed and analyzed. The good agreement between theory and experimentation ...

Hexagonal Phase of CdS Thin Films Obtained by Oscillating Chemical Bath

Cadmium sulfide CdS thin films were deposited in an oscillating chemical bath using a Teflon vane connected to an electricaldental brush oscillating at 37 Hz. The range of the bath temperature was from 65 to 85°C in intervals of 5°C and differentdeposition times. By this agitation technique we obtained CdS films with a highly oriented hexagonal structure -greenockite inthe 002 direction as measured by X-ray diffraction patterns. Images of atomic force microscopy and stoichiometry valuesobtained by energy-dispersive analysis by X-ray confirm the good quality of deposited films. High stoichiometry on the CdS filmswas measured by elemental analysis with variations less than 3.5% in all samples. The mean value of the gap energy was about2.38 eV, a close value for these films.© 2007 The Electrochemical Society. DOI: 10.1149/1.2820620 All rights reserved.Manuscript submittedApril 12, 2007; revised manuscript received October 15, 2007. Available electronically December 19, 2007.

Current induced effects in aluminum thin films

Aluminum thin films grown on silicon by free evaporation were studied by grazing incidence X-ray diffraction and atomic force microscopy techniques. The behavior of the films during current flowing, the induced changes by large periods of current applied, as well as the temporal evolution of the deformation due to thermal expansion because of the Joule effect, were studied. We explain the degradation of the films by a combined action of thermal effects and mechanical stress between film and substrate. The results could conduce to a new method to measure some thermal properties on metallic thin films.

Electromigration in gold thin films

Abstract We study the modification of gold thin film surface by Scanning Tunnelling Microscopy (STM) and grazing incidence X-ray to understand the process of electromigration. As a result of the applied current we have observed by STM a large movement of matter in the film surface that results in a strong rearrangement and modification of all the surface structures in a matter of minutes. We obtained that the 〈111〉 peak position shift towards the gold standards with elapsed time indicating that the surface strains formed during film growth disappears. The conclusion of the study is that the electrical current induces a rearrangement of the gold film surface.

Vibration isolation analysis for a scanning tunneling microscope

We analyze the efficiency of a vibration isolation system (VIS) for a scanning tunneling microscope as a function of the different parameters involved. The VIS consists of a stack of several metallic plates, separated by rubber elements with known properties. We show three‐dimensional graphs obtained for different values of parameters such as rigidity (spring) constant (K), damping constant (C), mass (M), and the number of stages (n). Analyzing the K dependence of the position of the main peaks, we find a parabolic behavior when the damping constant is small, with a slight deviation for larger values.