Papers by Author: Efstathios K. Polychroniadis

Abstract: The current communication focuses on the investigation of 3C-SiC layers grown by the Vapour-Liquid-Solid mechanism on on-axis Si-face 6H-SiC substrates in SiSn melts with different compositions and at different growth temperatures. The layers are studied by Transmission Electron Microscopy and Low Temperature Photoluminescence. It was found that for melts with Sn concentration higher than 60 at% large Sn-related precipitates are formed. The depth distribution of the Sn precipitates strongly depends not only on the melt composition but also on the growth temperature. Their formation strongly influences the stacking fault density and the dopant incorporation in the layers. Lower Sn concentrations combined with higher growth temperatures should result in 3C-SiC layer with enhanced structural quality.

Abstract: This paper deals with the formation and propagation of twin boundaries (TBs) inside 3C-SiC layers grown heteroepitaxially on -SiC substrate. The equivalent probability of nucleating 60° rotated 3C islands on such substrate lead to the systematic formation of TB upon coalescence of these islands. Elimination of these defects should occur by bending of the propagation direction. Bending through incoherent TBs is usually encountered during both VLS and CVD growth and it generates crystalline defects due to high built-in energy. One would prefer coherent TBs, formed by two-by-two annihilation of neighbouring TBs, which do not form new defect except microtwin inclusion at the interface. Such TB annihilation seems to be a specificity of growth by VLS mechanism. The mechanism of such bending is discussed

Abstract: This work presents the crystalline quality investigation of 3C-SiC unseeded crystals grown from vapor phase. Samples were polished after different crystallographic planes from crystals grown with or without nitrogen flow. The structural and optical investigation showed that the central part of the samples exhibited a very good crystalline quality. The best samples proved to be the {100} growth sectors where the only defects found were stacking faults with a defect density under 103 cm-1. At the edges, i.e. between two adjacent growth sectors, structural investigation by transmission electron microscopy revealed stacking faults and hexagonal polytype inclusions. The nitrogen doping was found not to have an influence on the crystalline quality.

Abstract: The present work deals with the structural properties of silicon carbide in nanoscale dimensions. The examined crystals were 6H-SiC grown by Liquid Phase Epitaxy. The study was concentrated on the stacking faults and any other differences from the “correct” stacking order of the Si-C bilayers for this polytype. Three main types of stacking faults were observed: (i) Cubic lamellae with thickness of four and two Si-C bilayers, always occurring in reverse stacking with respect to each other and separated by at least one unit cell of 6H-SiC; (ii) “twinned” 6H-SiC lamellae separated by a two-bilayer thick cubic inclusion. As a result the sequence in the “twinned” 6H-SiC changes from (3+3-) to (3-3+). (iii) Lamellae showing fringes, the interrelated distance of which suggests inclusion with sequence (22). Further, a high variety of sequences was found, leading to the appearance of rare long period polytypes or individual lamellae having their “own” stacking inside the 6H-SiC matrix. These nanostructured faults which deteriorate the quality of the grown material indicate also their “sensitivity” to any small or even infinitesimal change of the growth conditions, due to the very small energy among them.

Abstract: The dependence of the structure and composition of nanolayered Au/Ti/Al ohmic contacts to p-type 4H-SiC on the initial Ti:Al ratio has been investigated. Two contact compositions, Au/Ti(70%)/Al(30%) and Au/Ti(30%)/Al(70%), have been studied regarding the electrical properties, structure, composition and annealing temperature in the interval 850 – 1000o C. The correlation between the electrical behaviour and structure of the annealed contacts is discussed. Very low resistivity of 1.42x10-5 .cm2 after annealing at 900o C has been obtained for the contact having an initial composition Ti:Al (30:70), while the lowest resistivity of 1.21x10-5 .cm2 has been measured for the contact with a composition Ti:Al (70:30) after annealing at 1000o C. Strong dependence of the contact structure on the Ti:Al ratio and annealing temperature, respectively, has been found out. A presence of two phases, Au2Ti and Al3Ti, in all contacts has been determined after annealing, despite the temperature value and Ti:Al ratio. The TEM analysis reveals that titanium and aluminum silicides and carbides are formed after annealing as the Ti:Al ratio affects the kind of silicides and carbides created. It is obtained that the initial composition of the deposited metal layers influences only the phase composition of the annealed contact but not the grain sizes of the dominant phases formed. The origin of the ohmic properties improvement is explained by the formation of Ti3SiC2 compound and/or enhanced carrier transport by the presence of metal spikes into SiC depending on the initial contact composition and as consequence the optimal annealing temperature.

Abstract: In the present work Conventional and High Resolution Transmission Electron Microscopy has been used to examine the structure and types of interfaces between 3C-SiC and 6H-SiC for samples grown by Sublimation Epitaxy. The layers were grown on on-axis 6H-SiC substrates at different temperature gradients. The changed growth conditions influence on the nucleation of 3C-SiC on the 6H-SiC substrates and their competition with nucleation of 6H-SiC islands. Three specific types of 3C/6H-SiC interfaces were observed and the implications of these observations are discussed.

Abstract: In the present work the phases of the zinc coatings deposited with hot-dip galvanizing, pack cementation and wire flame spraying are examined with Scanning Electron Microscopy and Transmission Electron Microscopy. The different phases which are observed are identified with the combined results of electron and X-Ray diffraction. From the results it is concluded that pack cementation coatings are consisted by two different layers while hot dip galvanized coatings are composed by the same phases and additionally two extra phases of the Fe-Zn phase diagram. Flame sprayed coatings are composed by pure zinc, in the form of thin lamellae, together with nanocrystaline zinc oxide which is formed from the oxidation of liquid metallic droplets during the spray procedure.

Abstract: In the herein work coatings containing NiCrBSi alloy were deposited on low carbon steel by flame spray technique. The microstructure and morphology of the coatings were examined by electron microscopy and X-ray diffraction analysis. The as sprayed coatings are extremely rough with characteristic lamellic formations and porosity while a nanometer oxide film was formed on the top of the coating. High temperature oxidation tests revealed the high resistance of these coatings when exposed in such environment. This is mostly attributed to the existence of the temperature resistant oxide scale on the surface of the coupons.

Abstract: In the present work the defects appearing in layers grown by liquid phase epitaxy on different substrates are compared. The used seeds were (i) 3C-SiC with (111) orientation, grown heteroepitaxially on (0001) 4H-SiC or 6H-SiC substrates by continuous feed physical vapour transport process and the vapour-liquid-solid mechanism, respectively, and (ii) 3C-SiC wafer with (100) orientation from HOYA. The structural and optical investigation showed that (i) on the (111) substrates, due to the appearance of silicon and 6H-SiC inclusions, a layer which consisted of a sequence of long period polytypes was formed. The dominant polytype formed was 21R-SiC, which after successive transformation to 39R- and 57R- SiC led to the formation of 6H-SiC on the top of the layer. (ii) On the (100) substrates, a 3C-SiC layer with comparatively uniform defect density was formed. The main defects were stacking faults and their density was reducing during the process.

Abstract: In the present work the structural quality of 3C-SiC layers grown by sublimation epitaxy is studied by means of conventional and high resolution transmission electron microscopy. The layers were grown on Si-face 6H-SiC nominally on-axis substrates at a temperature of 2000°C and different temperature gradients, ranging from 5 to 8 °C /mm. The influence of the temperature gradient on the structural quality of the layers is discussed. The formation of specific twin complexes and conditions for lower stacking fault density are investigated.