Bogdan Ranguelov

High temperature electrodeposition of CdS thin films on conductive glass substrates

We study the possibility of preparing thin CdS films on conductive glasses by high temperature molten salt electrocrystallization. The CdS films have been electrodeposited on ZnO or ITO coated glass substrates in solution of CdCl2 and Na2SO3 in fused LiCl-KCl eutectic. They consist only of hexagonal phase -CdS irrespective of the crystal structure of the substrates (hexagonal for ZnO and cubic for ITO). CdS films deposited on ZnO coated glass consist of single platelets oriented with the (0001) face parallel to the substrate. They are yellow in colour and show good adhesion to the substrates at higher current density (>0.4 mA cm-2) and long deposition time (>30 min). The films of CdS deposited on ITO/glass are more dense under identical conditions which is explained by the lower resistivity of the ITO layers in comparison with ZnO layers.

Instabilities at vicinal crystal surfaces: Competition between electromigration of adatoms and kinetic memory effect

We studied the step dynamics during sublimation and growth in the presence of electromigration force acting on the adatoms. In the limit of fast surface diffusion and slow kinetics of atom attachment-detachment at the steps we formulate a model free of the quasi-static approximation in the calculation of the adatom concentration on the terraces. Numerical integration of the equations for the time evolution of the adatom concentrations and the equations of step motion reveals two different step bunching instabilities: 1) step density waves (small bunches which do not manifest any coarsening) induced by the kinetic memory effect and 2) step bunching with coarsening when the dynamics is dominated by the electromigration. The model developed in this paper also provides very instructive illustrations of the Popkov-Krug dynamical phase transition during sublimation and growth of a vicinal crystal surface.

Spirals on Si(1 1 1) at sublimation and growth: REM and LODREM observations

Using recently proposed improvements of Reflection Electron Microsopy (REM) we study in perfectly controlled thermodynamics conditions spiral shapes and spirals on Si(111) surface. It is shown that the new method named low distortion reflection electron microscopy (LODREM) is a powerful instrument, resolving in much more details (compared with REM) growth or evaporation spirals at the crystal surface. More precisely, we examine the distance between two successive steps of a spiral at growth (or evaporation) with respect to the supersaturation (or undersaturation). It is found that this distance scales with an exponent close to -1/2. This result, which deviates from the BCF theory originates from a non local behavior with a slow kinetic of attachment of the adatoms at the steps.

Evaporation and growth of crystals : Propagation of step-density compression waves at vicinal surfaces

We studied the step dynamics during crystal sublimation and growth in the limit of fast surface diffusion and slow kinetics of atom attachment-detachment at the steps. For this limit we formulate a model free of the quasi-static approximation in the calculation of the adatom concentration on the terraces at the crystal surface. Such a model provides a relatively simple way to study the linear stability of a step train in a presence of step-step repulsion and an absence of destabilizing factors (as Schwoebel effect, surface electromigration etc.). The central result is that a critical velocity of the steps in the train exists which separates the stability and instability regimes. When the step velocity exceeds its critical value the plot of these trajectories manifests clear space and time periodicity (step density compression waves propagate on the vicinal surface). This ordered motion of the steps is preceded by a relatively short transition period of disordered step dynamics.

Adatom diffusion on vicinal surfaces with permeable steps

We study the behavior of single atoms on an infinite vicinal surface assuming certain degree of step permeability. Assuming complete lack of re-evaporation and ruling out nucleation the atoms will inevitably join kink sites at the steps but will do many attempts before that. Increasing the probability for step permeability or the kink spacing lead to increase of the number of steps crossed before incorporation of the atoms into kink sites. The asymmetry of the attachment-detachment kinetics (Ehrlich-Schwoebel effect) suppresses the step permeability and completely eliminates it in the extreme case of the infinite Ehrlich-Schwoebel barrier. A negligibly small drift of the adatoms in a direction perpendicular to the steps leads to a significant asymmetry of the distribution of the permeability events, the atoms thus visiting more distant steps in the direction of the drift. The curves are fitted with an exponential function containing a constant which can be considered as a length scale of the effect of the drift. Some conclusions concerning the stability of the vicinals are drawn.

Characterization of Nanoscaled TiO2 Produced by Simplified Sol–Gel Method Using Organometallic Precursor

This work is concerned with development of sol–gel method for preparation of nanoscaled TiO2 using organometallic precursor—titanium tetraisopropoxide (TTIP) and determination of the present crystalline phases depending on the temperature of further thermal treatment. The characteristic processes and transformations during the thermal treatment were determined by means of thermal gravimetric analysis and/or differential thermal analysis (TGA/DTA) method. The crystalline structure and size of the TiO2 crystallites were analyzed by means of Raman spectroscopy and X-ray powder diffraction (XRPD) method. At 250 °C, cryptocrystalline structure was detected, where amorphous TiO2 is accompanied with crystalline anatase. The anatase crystallite phase is stable up to 650 °C, whereas at higher temperature rutile transformation begins. It was observed that at 800 °C, almost the whole TiO2 is transformed to rutile phase. According to XRPD analysis, the increase of the temperature influences on the increase of the size of the crystalline particles ranging from 6 nm at 250 °C to less than 100 nm at 800 °C. The size and shape of the TiO2 crystalline particles were observed by transmission electron microscopy (TEM). The shape of the studied samples changes from nanospheres (250, 380, and 550 °C) to nanorods (650 and 800 °C). Morphology of the formed TiO2 aggregates was observed by scanning electron microscopy (SEM).

Ti- and Cr-based hard coatings obtained at low temperatures by unbalanced magnetron sputtering

We prepared a set of Ti- and Cr-based ternary and quaternary hard coatings at low deposition temperatures (< 200 °C) by closed-field unbalanced magnetron sputtering in a gas mixture of Ar + N2. The morphological, structural and mechanical properties of the coatings were characterized by means of atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectrometry, a micro-scratch tester and a nanoindentation tester. The results of the analyses performed revealed that the surface of the deposited ternary Ti-Al-N coatings was rougher than that of the CrTiAlN coatings. The metallic elements in the coatings react with nitrogen to form CrN, TiN and AlN nitride phases, as no Cr2N phase was detected in the Cr-based coatings. The optimized quaternary coating demonstrated mechanical properties superior to those of the ternary ones in terms of hardness; adherence strength of 32 GPa and elastic modulus of 418 GPa were measured for the multicomponent coating of composition Cr0.68Ti0.19Al0.13N.