Kategória: Publikácie

Interference-enhanced Raman scattering in SiO2/Si structures related to reflectance

Interference-enhanced Raman scattering in SiO2/Si structures related to reflectance

Ľubomír Vančo, Mário Kotlár, Magdaléna Kadlečíková, Viliam Vretenár, Marian Vojs, Jaroslav Kováč

Journal of Raman Spectroscopy, 2019, 0377-0486

doi.org/10.1002/jrs.5666

Abstract

Enhancement of Raman scattering due to optical interference may act as a source of error in the issues necessitating a determination of Raman intensity. Its dependence on thin film thickness is the conventional way how to examine the effects of optical interference in Raman signal. To provide a new platform for evaluation of signal coming from substrate in the presence of a transparent capping, we investigate its relation to reflectance (R) instead of the capping thickness. We derived a theoretical model, which was experimentally tested on simple structures consisting of SiO2 deposited on mono-Si substrates. In agreement between the model and the experiment, interference enhancement is proportional to the product of (1 – R) terms taken at excitation and scattered light wavenumbers. We experimented with two different Raman bands in Si on two different Raman systems. The model was valid regardless of excitation, Raman band, and grating. Constructed for normal incidence, it was in agreement with experiment using objectives with numerical apertures up to 0.25 (0.32). The model was valid also in ultraviolet region, where imaginary part in refractive index of Si considerably rises.

Covalent Diamond–Graphite Bonding: Mechanism of Catalytic Transformation

Covalent Diamond–Graphite Bonding: Mechanism of Catalytic Transformation

Semir Tulić, Thomas Waitz, Mária Čaplovičová, Gerlinde Habler, Marián Varga, Mário Kotlár, Viliam Vretenár, Oleksandr Romanyuk, Alexander Kromka, Bohuslav Rezek, Viera Skákalová

ACS Nano, 2019, 1344621-4630

https://doi.org/10.1021/acsnano.9b00692

Abstract

Aberration-corrected transmission electron microscopy of the atomic structure of diamond–graphite interface after Ni-induced catalytic transformation reveals graphitic planes bound covalently to the diamond in the upright orientation. The covalent attachment, together with a significant volume expansion of graphite transformed from diamond, gives rise to uniaxial stress that is released through plastic deformation. We propose a comprehensive model explaining the Ni-mediated transformation of diamond to graphite and covalent bonding at the interface as well as the mechanism of relaxation of uniaxial stress. We also explain the mechanism of electrical transport through the graphitized surface of diamond. The result may thus provide a foundation for the catalytically driven formation of graphene–diamond nanodevices.

Effect of temperature and carrier gas on the properties of thick InxAl1-xN layer

Effect of temperature and carrier gas on the properties of thick InxAl1-xN layer

Prerna Chauhan, Stanislav Hasenöhrl, Edmund Dobročka, Ľubomír Vančo, Roman Stoklas, Jaroslav Kováč, Peter Šiffalovič, Ján Kuzmík

doi.org/10.1016/j.apsusc.2018.10.231

Abstract

Thick (>150nm) InxAl1-xN layers were grown on GaN/sapphire (0001) by organometallic vapor phase epitaxy. Growth temperature of InxAl1-xN layers was reduced from 790 to 730°C, to examine the effects of growth temperature in InxAl1-xN layers grown under H2 carrier gas. Indium incorporation, surface morphology, electrical, and optical properties of InxAl1-xN layers were examined as a function of growth temperature. Increase in In-molar fraction, as determined by high resolution X-ray diffraction, was observed with decreasing growth temperature of InxAl1-xN layers at the expense of surface roughness. Unstrained InxAl1-xN layer was achieved at 730°C under H2 carrier gas with x=0.18. However, InxAl1-xN layer grown under N2 carrier gas at 730°C to study the effects of carrier gas, was observed with two times higher In-molar fraction (x=0.37) and one order lower carrier concentration. This work shows the essential requirement of a multi-characterization approach to establish a connection between structural, electrical, and optical properties to improve our understanding towards InxAl1-xN. Edge threading dislocations density is found to be the most important parameter in deciding the characteristics of an InxAl1-xN layer.

Adhesive-deformation relationships and mechanical properties of nc-AlCrN/a-SiNx hard coatings deposited at different bias voltages

Adhesive-deformation relationships and mechanical properties of nc-AlCrN/a-SiNx hard coatings deposited at different bias voltages

M. Haršáni, N. Ghafoor, K. Calamba, P. Zacková, M. Sahul, T. Vopát, L. Satrapinskyy, M. Čaplovičová, Ľ. Čaplovič,

Thin Solid Films, Volume 650, 2018, Pages 11-19

doi.org/10.1016/j.tsf.2018.02.006

Abstract

A series of Al-Cr-Si-N hard coatings were deposited on WC-Co substrates with a negative substrate bias voltage ranging from −50 to −200 V using cathodic arc evaporation system. A Rockwell-C adhesion test demonstrated that excellent adhesion was observed at lower bias voltages of −50 V and −80 V, while further increases in bias voltage up to −200 V led to severe delaminationand worsening of the overall adhesion strength. X-ray diffraction and transmission electron microscopy analysis revealed a single phase cubic B1-structure identified as an AlCrN solid solution with a nanocomposite microstructure where cubic AlCrN nanocrystals were embedded in a thin continuous amorphous SiNx matrix. Coatings exhibited a 002-texture evolution that was more pronounced at higher bias voltages (≥−120 V). Stress-induced cracks were observed inside the coatings at high bias voltages (≥−150 V), which resulted in stress relaxation and a decline in the overall residual stresses.
Ir/Al multilayer Gates for High Temperature Operated AlGaN/GaN HEMTs

Ir/Al multilayer Gates for High Temperature Operated AlGaN/GaN HEMTs

 Lalinský, T., Vanko, G., Dobročka, E., Osvald, J., Babchenko, O., Dzuba, J., Veselý, M., Vančo, L., Vogrinčič, P., Vincze, A.

Physica Status Solidi (A) Applications and Materials Science, Volume 214, Issue 12, December 2017, Article number 1700691
DOI: 10.1002/pssa.201700691

Abstract

The fabrication and characterization of the sequentially evaporated Ir/Al multilayer gates of AlGaN/GaN circular high electron mobility transistors formed by high temperature oxidation is reported. Annealing at temperature of 800 °C, for 60 s in O2 ambient makes possible to form a sharp gate interface with a high Schottky barrier height at RT (φb = 1.2 eV). It is also shown that high temperature oxidation can be an effective approach in reducing of both the gate and drain leakage currents of high electron mobility transistors (more than six orders). A comprehensive microstructural, electrical, and electro‐thermal characterization of the Ir/Al gates is carried out to study the thermal stability of the gate interface and high temperature performance of the devices. Stable operation of the devices with multilayer Ir/Al gates in the temperature range up to 500 °C is demonstrated. Here, it is proposed that the thermal stability of the interface is controlled by the formed aluminum oxide interfacial layer. Finally, perfectly clear pinch‐off characteristics and thermally induced threshold voltage (Vth) instability as low as −0.58 mV °C−1 are achieved.

Thermally induced age hardening in tough Ta-Al-N coatings via spinodal decomposition

Thermally induced age hardening in tough Ta-Al-N coatings via spinodal decomposition

Mikula, M., Sangiovanni, D.G., Plašienka, D., Roch, T., Čaplovičová, M., Truchlý, M., Satrapinskyy, L., Bystrický, R., Tonhauzerová, D., Vlčková, D., Kúš, P.

Journal of Applied Physics 121, 2017, 155304

doi.org/10.1063/1.4981534

Abstract

We combine experiments and ab initio density functional theory calculations to investigate the evolution in structural and mechanical properties of TaAlN coatings as a function of the annealing temperature T. Formation of coherent cubic TaN- and AlN-rich nanometer-size domains, occurring during the initial stage of thermally induced phase separation within cubic NaCl-type (B1) TaAlN solid solutions, yields a monotonic increase in hardness from 29 GPa (as deposited coatings) up to a maximum of 35 GPa (+17%) reached after annealing at 1000 °C. Further thermal treatment at T > 1000 °C leads to the transformation of metastable cubic domains into stable hexagonal TaNx and wurtzite AlN phases, thus resulting in hardness reductions. A comparison of our results with those reported in the literature reveals that TaAlN coatings are at least as hard while considerably less stiff (lower elastic moduli) than TiAlN coatings, thus indicating a substantial increase in toughness achieved upon replacing Ti with Ta in the host lattice. Present findings suggest that cubic TaAlN solid solutions are promising candidates for applications in protective coatings possessing both high-temperature hardness and toughness. © 2017 Author(s).

Influence of GaN/AlGaN/GaN (0001) and Si (100) substrates on structural properties of extremely thin MoS2 films grown by pulsed laser deposition

Influence of GaN/AlGaN/GaN (0001) and Si (100) substrates on structural properties of extremely thin MoS2 films grown by pulsed laser deposition

Š. Chromik, M. Sojková, V. Vretenár, A. Rosová, E. Dobročka, M. Hulman

doi.org/10.1016/j.apsusc.2016.06.038

Abstract

Very thin MoS2 films were prepared on hexagonal GaN/AlGaN/GaN (0001) and Si (100) substrates from a stoichiometric target by a pulsed laser deposition. Combined results from Raman and X-ray reflectivity measurements have shown that the thinnest samples are 2–2.5nm thick. The thickness increases with the number of laser pulses applied albeit no simple direct proportion between the two quantities has been observed. Concerning the stoichiometry, the distribution of Mo and S elements within as-deposited films is rather complex. The stoichiometric MoS2 is making-up only a part of the film. In spite of this, selected area electron diffraction studies have clearly confirmed that the films deposited on Si (100) are nanocrystalline and oriented perpendicularly to the substrate surface while an epitaxial growth of MoS2 films was observed on GaN/AlGaN/GaN (0001) substrates.