Papers by Author: Efstathios K. Polychroniadis

Abstract: In this research, the possibility of applying multilayer multielement super hard coatings by Cathodic Arc is investigated. More precisely the structure of the coating consisting of quaternary CrAlSiN and ternary AlSiN layers is examined by electron microscopy, X-ray diffraction and X-ray photoelectron microscopy analytical methods. The as-deposited samples were found to have distinguishable layers. The CrAlSiN layer is characterized by an extra sequence of repeated nanolayers. The AlSiN layer consisted of nanosized grains having a preferential orientation. Finally the surface layer was found to contain a solid solution of CrxAl1-xN, while Si3N4was identified only by XPS most probably due to its amorphous structure.

Abstract: The present communication focuses on the bilayer graphene formation on a Ge doped 4H-SiC surface. The 4H-SiC epilayer was grown by CVD with Germane (GeH₄) as the dopant precursor. This easily leads to the formation of Ge islands as well as graphene on the 4H-SiC surface. The Ge island decorated surface was studied by Raman spectroscopy, XPS, and TEM. It was found that the bilayer graphene is free standing and that the native oxides act as a buffer layer on the surface, covering the Ge islands. The intensity variations of the Ge component in the XPS spectra indicate that Ge atoms can be buried in the SiC surface. The TEM analysis revealed that the graphene layers are in the form of flakes, which can also be rived vertically with the support of the Ge islands on SiC surface.

Abstract: This work was performed on 3C-SiC layers grown on 4H-SiC substrates by chemical vapour deposition after a surface pre-treatment using GeH₄ gas. By means of TEM, the effects of different GeH₄ fluxes in the 3C layer quality has been studied and compared. An optimal GeH₄ flux permits to drastically reduce twin boundaries but another type of defect occurs and has been widely studied in this paper.

Abstract: So far, most of the SiC homoepitaxy has been realized on 8-deg-off and 4-deg-off substrates, whereas GaN heteroepitaxy is done on SiC on-axis (up to 0.3 deg off). As 6-inch SiC wafers are being introduced into the market, a decrease of the substrate off-cut to 2 deg for SiC homoepitaxy is desirable to reduce the manufacturing costs. If both, GaN heteroepitaxy and SiC homoepitaxy are successful on 2-deg-off SiC substrates, this would pave the way to monolithic integration of both kinds of devices, as well as to obtain cheap insulating SiC substrates for AlGaN/GaN microwave and THz devices. In this work, we present our present status of AlGaN/GaN growth on SiC 2-deg off. Comparing to the on-axis situation, we obtained similar structural (XRD and TEM data) and electrical characteristics, but not morphological ones. Therefore, we propose two ways of a decrease of AlGaN surface roughness when grown on SiC 2 deg-off: i) by planarization, ii) by lateral patterning.

Abstract: The present work concerns the microstructural characterization of a multi-component (based on GaN and related materials) and multi-layered (5 layers) film, grown on 6H-SiC substrate (with a misorientation of 1 degree off from the (0001) plane), using transmission electron microscopy (TEM). The TEM characterization showed no surface undulation, despite the presence of steps in the SiC/AlN interface.

Abstract: In the present work we report on the polytypic transformations taking place in nanoscale dimensions within 6H-SiC crystals. The examined crystals were grown by Liquid Phase Epitaxy using a mixture of Si and Al as solvents. The study concentrated on the differences from the “correct” stacking order of the Si-C bilayers for 6H-SiC leading to the formation of other polytypes. A great variety of sequences was found, which resulted to the appearance of rare short and 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 present study reports on the propagation of twin boundaries in (111) 3C-SiC by means of conventional (CTEM) and high resolution transmission electron microscopy (HRTEM). The investigated 3C-SiC layers were homoepitaxially grown by Chemical Vapour Deposition (CVD) on layers previously grown by Vapor Liquid Solid (VLS) mechanism on 6H-SiC substrates. At the initial stages of growth the usual twin boundary that occurs is an incoherent {-211} Σ3 one. It transforms to more energetically favorable cases by several ways: (i) The initial {-211} boundary turns 90º, to a fully coherent (111) interface, forming microtwins; (ii) A step-like interface occurs with facets along the (111) and the {-211} planes; (iii) It transforms in a fourfold twin complex propagating to the surface.

Abstract: Multilayered, Gradient Tialsin-Based Nanocomposite Coatings Have Been Developed and Investigated with Respect to their Applicability in the Machining Industry. the Main Coating Layer Was Composed of 5-8 Nm Tin and Aln Nanograins. the Coating Possessed Hardness as High as 40 GPA, which Allows it to Be Classified as Superhard. during Heating up to 900oC in Air in Steps of 100oC for 6 H at each Temperature, the Coating Showed Good Stability up to 700oC. Thermal Treatment over this Temperature Caused a Decrease in the Hardness to Values Characteristic for Tialn Multilayered Coatings, while the Adhesion to the Substrate Remained Steady.

Abstract: Both 3C and 6H stacking faults have been observed in a low doped 4H-SiC epitaxial layer grown in a hot-wall CVD reactor on a heavily doped (off-axis) 4H-SiC substrate. They appear differently on the different parts of sample, with energetic dispersion ranging from 3.01 eV to 2.52 eV. Since they behave as natural type-II quantum wells in the 4H-SiC matrix, the thickness dependence of the excitonic recombination is investigated using the standard effective mass approximation. The results are discussed in terms of built-in electric field and inter-well coupling effects.

Abstract: The current communication focuses on the influence of a post-growth annealing on the evolution of defects inside (111) 3C-SiC layers grown by the Vapour Liquid Solid (VLS) mechanism in SiGe melts on Si-face on- and off axis 6H-SiC substrates. The layers are studied by Transmission Electron Microscopy (TEM) and Low Temperature Photoluminescence (LTPL). It was found that the growth on off-axis substrates results in a 3C-SiC layer containing mainly stacking faults (SFs) and microtwins (MT). The density of MT lamellae and SFs reduces in the layers grown on the on-axis substrate compared to off-axis substrate. In the layers grown on off-axis substrates the annealing strongly reduces the density of SFs inclined to the 3C/6H-SiC interface. Additionally, 3C to 6H polytypic transformation appears only at the interface, most probably starting from substrate step edges. This was only seen on off-axis seed since the step edges are more.