Autor Lubomír Vančo

Growth of lithium hydride thin films from solutions: Towards solution atomic layer deposition of lithiated films

Posted on by Lubomír Vančo

KUNDRATA, Ivan – FRÖHLICH, Karol – VANČO, Ľubomír – MICUŠÍK, Matej – BACHMANN, Julien

In Beilstein Journal of Nanotechnology. Vol. 10, (2019)

https://doi.org/10.3762/bjnano.10.142

Abstract

Lithiated thin films are necessary for the fabrication of novel solid-state batteries, including the electrodes and solid electrolytes. Physical vapour deposition and chemical vapour deposition can be used to deposit lithiated films. However, the issue of conformality on non-planar substrates with large surface area makes them impractical for nanobatteries the capacity of which scales with surface area. Atomic layer deposition (ALD) avoids these issues and is able to deposit conformal films on 3D substrates. However, ALD is limited in the range of chemical reactions, due to the required volatility of the precursors. Moreover, relatively high temperatures are necessary (above 100 °C), which can be detrimental to electrode layers and substrates, for example to silicon into which the lithium can easily diffuse. In addition, several highly reactive precursors, such as Grignard reagents or n-butyllithium (BuLi) are only usable in solution. In theory, it is possible to use BuLi and water in solution to produce thin films of LiH. This theoretical reaction is self-saturating and, therefore, follows the principles of solution atomic layer deposition (sALD). Therefore, in this work the sALD technique and principles have been employed to experimentally prove the possibility of LiH deposition. The formation of homogeneous air-sensitive thin films, characterized by using ellipsometry, grazing incidence X-ray diffraction (GIXRD), in situ quartz crystal microbalance, and scanning electron microscopy, was observed. Lithium hydride diffraction peaks have been observed in as-deposited films by GIXRD. X-ray photoelectron spectroscopy and Auger spectroscopy analysis show the chemical identity of the decomposing air-sensitive films. Despite the air sensitivity of BuLi and LiH, making many standard measurements difficult, this work establishes the use of sALD to deposit LiH, a material inaccessible to conventional ALD, from precursors and at temperatures not suitable for conventional ALD.

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.

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

Posted on by Lubomír Vančo

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.

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.

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.

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.