Category: Articles

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

Posted on by Lubomír Vančo

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

In Applied Surface Science. Vol.470, (2019

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.

Biomass waste-carbon/reduced graphene oxide composite electrodes for enhanced supercapacitors

Posted on by Viliam Vretenár

GUARDIA, Laura – SUÁREZ, Loreto – QUEREJETA, Nausika – VRETENÁR, Viliam – KOTRUSZ, Peter – SKÁKALOVÁ, Viera – CENTENO, Teresa A.

In Electrochimica Acta. Vol. 298, iss. 3 (2019)

Abstract

We present a simple and effective alternative which optimizes electrodes based on low-cost carbons for high-performance supercapacitors. The combination with reduced graphene oxide (rGO) greatly improves the operation of microporous carbons easily produced by one-pot activation of grape seeds. The use of composite electrodes with rGO lowers the supercapacitor resistance and enables a much higher rate capability.

The mixture of rGO flakes and particles of a highly porous carbon obtained by KOH activation allows retaining the high capacitance of 260 F g−1 of the standard electrodes at 1 mA cm−2 in aqueous H2SO4 whereas the value at 200 mA cm−2 is increased by around 2.4 times. Consequently, at high current density, the capacitor assembled with these composites stores eight times more energy and the power density is multiplied by four.

The synergy between rGO and an ultramicroporous carbon produced by CO2-activation results extremely profitable, the cell assembled with composite electrodes reaching three times more energy and power at 200 mA cm−2 than the best performance of the standard counterpart.

More importantly, the higher density of the composite electrodes leads to a capacitance of around 200 F cm−3 which translates into a remarkable improvement in the supercapacitor operation normalized to volume.

Evidence of relationship between strain and In-incorporation: Growth of N-polar In-rich InAlN buffer layer by OMCVD

Posted on by Mario Kotlár

CHAUHAN, Prerna – HASENÖHRL, Stanislav – DOBROČKA, Edmund – CHAUVAT, Marie-Pierre – MINJ, A. – GUCMANN, Filip – VANČO, Ľubomír – KOVÁČ, Jaroslav jr. – KRET, S. – RUTERANA, Pierre – KUBALL, Martin – ŠIFFALOVIČ, Peter – KUZMÍK, Ján

In Journal of Applied Physics. 125, iss. 10 (2019)

https://doi.org/10.1063/1.5079756

Abstract

Two In𝑥Al1−𝑥NInAl1−N layers were grown simultaneously on different substrates [sapphire (0001) and the Ga-polar GaN template], but under the same reactor conditions, they were employed to investigate the mechanism of strain-driven compositional evolution. The resulting layers on different substrates exhibit different polarities and the layer grown on sapphire is N-polar. Moreover, for the two substrates, the difference in the degree of relaxation of the grown layers was almost 100%, leading to a large In-molar fraction difference of 0.32. Incorporation of In in In𝑥Al1−𝑥NInAl1−N layers was found to be significantly influenced by the strain imposed by the under-layers. With the evolutionary process of In-incorporation during subsequent layer growth along [0001], the direction of growth was investigated in detail by Auger electron spectroscopy. It is discovered that the In0.60Al0.40NIn0.60Al0.40N layer grown directly on sapphire consists of two different regions with different molar fractions: the transition region and the uniform region. According to the detailed cross-sectional transmission electron microscopy, the transition region is formed near the hetero-interface due to the partial strain release caused by the generation of misfit-dislocations. The magnitude of residual strain in the uniform region decides the In-molar fraction. In𝑥Al1−𝑥NInAl1−N layers were analyzed by structural and optical characterization techniques. Our present work also shows that a multi-characterization approach to study In𝑥Al1−𝑥NInAl1−N is a prerequisite for their applications as a buffer layer.

Characterization of the chips generated by the nanomachining of germanium for X-ray crystal optics

Posted on by Mario Kotlár

 ZÁPRAŽNÝ, Zdenko – KORYTÁR, Dušan – JERGEL, Matej – HALAHOVETS, Yurily – KOTLÁR, Mário – MATKO, Igor – HAGARA, Jakub – ŠIFFALOVIČ, Peter – KECKES, Jozef – MAJKOVÁ, Eva

In International Journal of Advanced Manufacturing Technology. Vol. 102, iss. 9-12 (2019)

https://doi.org/10.1007/s00170-019-03392-z

Abstract

Micro-Raman spectroscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were used to study the effect of cutting speed and cutting depth on the mode of the single-point diamond fly cutting of Ge(110) surface via crystallinity of the chips. Reducing the cutting depth from 15 to 2 μm and concurrently cutting speed from 10 to 2 mm/min at 2000 rpm, the content of amorphous phase in the chips increased at the expense of the crystalline one from 28 to 46%. Simultaneously, the chip morphology visible by SEM suggested transition from a brittle to a mixed brittle-ductile mode of nanomachining. The damage transition line indicates 1/3 portion of the ductile component at 2-μm cutting depth that produced twisted lamellae of a width of 18–20 μm without any signs of a fracture. As the feed rate here was 1 μm/rev, the tool made 18–20 revolutions while passing the same point of the nanomachined surface that was enough to gradually remove the surface region damaged by the brittle cutting component along with the entire amorphous region beneath, both being delaminated by the chips. This explains the dislocation-free single-crystal lattice beneath the Ge(110) surface machined under these conditions. A close relationship between the brittle mode of nanomachining and crystallinity of the chips observed by micro-Raman spectroscopy and SEM was confirmed by HR-TEM showing dense occurrence of nanocrystals in the chips coming from the nanomachinings with 5-μm and 15-μm cutting depths. These results demonstrate potential of the single-point diamond machining for the preparation of high-quality X-ray surfaces with undistorted single-crystal lattice beneath for next-generation X-ray crystal optics.

Generation of hole gas in non-inverted InAl(Ga)N/GaN heterostructures

Posted on by Lubomír Vančo

 HASENÖHRL, Stanislav – CHAUHAN, Prerna – DOBROČKA, Edmund – STOKLAS, Roman – VANČO, Ľubomír – VESELÝ, Marián – BOUAZZAOUI, Farah – CHAUVAT, Marie-Pierre – RUTERANA, Pierre – KUZMÍK, Ján

In Applied Physics Express. Vol. 12, iss. 1 (2019)

https://doi.org/10.7567/1882-0786/aaef41

Abstract

InAlN/GaN structures are grown using organometallic chemical vapor deposition at 730 °C. The sample for which the chamber cleaning step was applied after GaN growth shows a sharp In0.3Al0.7N/GaN transition, free electron density of ∼2 × 1011 cm−2 and mobility of 44 cm2 V−1 s−1. On the other hand, the sample prepared without growth interruption demonstrated In0.4Al0.15Ga0.45N at the interface and compositional grading towards the In0.4Al0.6N surface. In this case a two-dimensional hole gas (2DHG) is created with a density of ∼2 × 1012 cm−2 and mobility of ∼0.6 cm2 V−1 s−1. Ga incorporation in the InAlN barrier is crucial for designing non-inverted 2DHG transistors.

Structure of superconducting MgB2 thin films prepared by vacuum evaporation and ex-situ annealing in Ar and O-2 atmospheres

Posted on by Mária Čaplovičová

ROCH, Tomáš – GREGOR, Maroš – ŠVEC, Peter, Jr. – PLECENÍK, Tomáš – SATRAPINSKYY, Leonid – ČAPLOVIČOVÁ, Mária – BYSTRICKÝ, Roman – KÚŠ, Peter – PLECENÍK, Andrej

In Applied Surface Science. Vol. 461, (2018)

https://doi.org/10.1016/j.apsusc.2018.05.038

Abstract

Superconducting MgB2 thin films were fabricated on c-cut sapphire substrates by vacuum co-evaporation of magnesium and boron followed by an ex situ post-annealing. We show that annealing in oxygen atmosphere can significantly improve the superconducting properties of the MgB2 thin films compared to the typical annealing in argon atmosphere. In this work, influence of the ex-situ annealing atmosphere on the structure, texture and morphology of the superconducting MgB2 thin films has been studied by transmission electron microscopy, X-ray diffraction and pole figure measurements. Samples annealed at 800 °C in Ar, at 800 °C in O2 and at 500 °C in O2 have been compared. The annealing in O2 at 800 °C produces MgB2 thin films with the highest superconducting transition temperature and critical current density. We show that this is thanks to the thickest MgO layer at the surface produced in this case, which acts as a protecting barrier against out-diffusion of Mg during the annealing and leads to better stoichiometry and larger MgB2 grains compared to the samples annealed in Ar. Our method can be alternative to the customary ex-situ post annealing of Mg-B precursor in sealed vapor cell. In all samples, the MgB2 phase showed single axis texture with the (0 0 0 1) planes slightly inclined by 0° to 15° with respect to the Al2O3(0 0 0 1) substrate surface incurred with the initial precursor evaporation geometry. During the annealing, the excess Mg also reacts with the Al2O3 substrate and minor MgAl2O4 and MgO phases are produced at the substrate-layer interface.

Raman spectroscopy of porous silicon substrates

Posted on by Lubomír Vančo

KADLEČÍKOVÁ, Magdaléna – BREZA, Juraj – VANČO, Ľubomír – MIKOLÁŠEK, Miroslav – HUBEŇÁK, Michal – RACKO, Juraj – GREGUŠ, Ján

In Optik. Vol. 174, (2018)

https://doi.org/10.1016/j.ijleo.2018.08.084

Abstract

We have investigated the effect of the etching time on the Raman spectra of porous silicon prepared by anodic etching. Electrochemical destruction of the substrate increasing with the etching time and the correlation between the microstructure of the silicon wafer and the shape and position of their Raman spectra have been observed. Raman analysis has shown that the intensity of the Raman dominant silicon band decreases and the bandwidth is shifted to lower frequencies, depending on the morphology of the sample. Therefore we believe that the electrochemical destruction of the surface of Si substrates leads to surface amorphization.

Degradation of Al4C3 Due to Atmospheric Humidity

Posted on by Mária Čaplovičová

NÝBLOVÁ, Daniela – BILLIK, Peter – NOGA, Jozef – ŠIMON, Erik – BYSTRICKÝ, Roman – ČAPLOVIČOVÁ, Mária – NOSKO, Martin

In JOM. Vol. 70, iss. 10 (2018)

https://doi.org/10.1007/s11837-018-3053-3

Abstract

The degradation of Al4C3 was investigated by exposing powder samples to atmospheric humidity at laboratory temperature for 150 days. Samples were monitored using powder x-ray diffraction analysis, scanning and transmission electron microscopy, thermal analysis, and energy-dispersive x-ray spectroscopy. Initially, the degradation resulted in an amorphous Al–O–OH network, which gradually recrystallized as low-crystalline boehmite (γ-AlOOH), and, later, partially, as Al(OH)3. Low-crystalline γ-AlOOH consisted of plate-like particles with diameter of 5 µm to 40 µm. The 020 diffraction position of γ-AlOOH was close to that of well-crystalline γ-AlOOH, which may be explained by accumulation of compressive stress along the b-axis of the γ-AlOOH structure.

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications

Posted on by Lubomír Vančo

TIAN, Yilei – VISMARA, Elena – VAN DOORENE, Steve – ŠUTTA, Pavol – VANČO, Ľubomír – VESELÝ, Marián – VOGRINČIČ, Peter – ISABELLA, Olindo – ZEMAN, Miro

In ACS Applied Energy Materials. Vol. 1, iss. 7 (2018)

https://doi.org/10.1021/acsaem.8b00486

Abstract

Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.