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Publications in 2023

ABSTRACT: This paper reports on the deposition of amorphous and crystalline thin films of Ga2O3 by reactive pulsed direct current magnetron sputtering from a liquid gallium target onto fused (f-) quartz and c plane (c-) sapphire substrates, where the temperature of the substrate is varied from room temperature (RT) to 800 °C. The deposition rate (up to 37 nm/min at RT on f-quartz and 5 nm/min at 800 °C on c-sapphire) is two to five times higher than the data given in the literature for radio frequency sputtering. Deposited onto unheated substrates, the films are X-ray amorphous. Well-defined X-ray diffraction peaks of β-Ga2O3 start to appear at a substrate temperature of 500 °C. Films grown on c-sapphire at temperatures above 600 °C are epitaxial. However, the high rocking curve full width at half maximum values of ≈2.4–2.5° are indicative of the presence of defects. A dense and void-free microstructure is observed in electron microscopy images. Composition analysis show stoichiometry close to Ga2O3 and no traces of impurities. The optical properties of low absorptance (<1%) in the visible range and an optical band gap of approximately 5 eV are consistent with the data in the literature for Ga2O3 films produced by other deposition methods.

Vacuum, Volume 209, March 2023, 111789
DOI: 10.1016/j.vacuum.2022.111789
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ABSTRACT: Solid state ionics is one of the key research topics of the Institute of Solid State Physics, University of Latvia since its establishment. The research direction included topics ranging from electrochromic phenomena in transition metal oxides through gas sensors and electronic nose to materials for rechargeable battery electrodes and materials for hydrogen energy. By the late 1980s, the institute had become one of the biggest and most prolific solid state ionic centres in the USSR and Eastern Europe and continues to maintain its position among the regional leaders in the field. Regular regional conferences and workshops were organized and some of the published works can be ranked among the pioneering works in the world of science. This extensive historical review summarizes information on the development of solid state ionics, actual research and achievements from establishment of the institute to the present day. Currently many collaborations are ongoing with partners across Europe and beyond in research ranging from battery materials and smart windows for zero energy buildings to hydrogen production, maintaining and growing its strength as key national, regional, and international centre of research excellence in solid state ionics.

Journal of Solid State Electrochemistry, February 2023
DOI: 10.1007/s10008-023-05419-8
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ABSTRACT: An experimental investigation was conducted to explore spectroscopic and structural characterization of semiconducting yttrium oxide thin film deposited at 623 K (±5K) utilizing reactive pulsed direct current magnetron sputtering. Based on the results obtained from both x-ray diffraction and transmission electron microscope measurements, yttrium monoxide is very likely formed in the transition region between β-Y2O3 and α-Y2O3, and accompanied by the crystalline Y2O3. Resulting from either the low energy separation between 4d and 5s orbitals and/or different spin states of the corresponding orbitals’ sublevels, the stability of monoxide is most presumably self-limited by the size of the crystal in thermodynamic terms. This behavior develops a distortion in the structure of the crystal compared to the metal oxide cubic structure and it also effectuates the arrangement in nanocrystalline/amorphous phase. In addition to this, spectroscopic ellipsometry denotes that the semiconducting yttrium oxide has the dominant, mostly amorphous, formation character over crystalline Y2O3. Our purpose, by means of the current findings, is to advance the understanding of formation kinetics/conditions of yttrium with an unusual valency (2+).

Vacuum, Volume 211, May 2023, 111942
DOI: 10.1016/j.vacuum.2023.111942


ABSTRACT: In this critical review, we call attention to a widespread problem related to the vast disagreement in elastic moduli values reported by different authors for nanostructures made of the same material. As a particular example, we focus on ZnO nanowires (NWs), which are among the most intensively studied nanomaterials due to their remarkable physical properties and promising applications. Since ZnO NWs possess piezoelectric effects, many applications involve mechanical deformations. Therefore, there are plenty of works dedicated to the mechanical characterization of ZnO NWs using various experimental and computational techniques. Although the most of works consider exactly the same growth direction and wurtzite crystal structure, reported values of Young’s modulus vary drastically from author to author ranging from 20 to 800 GPa. Moreover, both – diameter dependent and independent – Young’s modulus values have been reported. In this work, we give a critical overview and perform a thorough analysis of the available experimental and theoretical works on the mechanical characterization of ZnO NWs in order to find out the most significant sources of errors and to bring out the most trustable results.

Vacuum, Volume 211, May 2023, 111942
DOI: 10.1515/ntrev-2022-0505
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ABSTRACT: While gallium oxide Ga2O3 has recently shown promise as a new ultra-wide bandgap semiconductor in the form of thin films and nanowires (NWs), its widespread applicability is limited due to lack of native p-type conductivity, thus requiring fabrication of heterojunctions. A potential matching material is spinel zinc gallate ZnGa2O4. In this work we demonstrated and compared two novel approaches of one-dimensional Ga2O3-ZnGa2O4 core-shell NW heterostructure preparation: (a) a direct deposition of a ZnGa2O4 coating using a reactive magnetron co-sputtering; (b) annealing of a sacrificial few-nm-thick ZnO coating, deposited via atomic layer deposition, at high temperature to enable solid state reaction between ZnO and Ga2O3. The as-grown nanostructures were characterized via scanning and transmission electron microscopies, X-ray diffraction and X-ray photoelectron spectroscopy. Room temperature optical features were disclosed using photoluminescence and optical absorption. While both methods are viable for production of the heterostructures, smoother and more uniform ZnGa2O4 coating around Ga2O3 NWs was obtained via sacrificial layer conversion in comparison to the sputter-deposited one. These heterostructures could potentially be used for photocatalysis and nanoscale ultra-wide bandgap electronics.

Journal of Crystal Growth, Available online 12 June 2023, 127319 (In Press, Journal Pre-proof)
DOI: 10.1016/j.jcrysgro.2023.127319
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ABSTRACT: Recently gallium oxide (Ga2O3) has become one of the most actively studied materials due to its competitive electronic properties such as wide bandgap, high breakdown field, simple control of carrier concentration, and high thermal stability. These properties make gallium oxide a promising candidate for potential applications in high-power electronic devices. β-Ga2O3 crystals are commonly grown by the Czochralski method in an iridium (Ir) crucible. For this reason, Ir is often present in Ga2O3 crystals as an unintentional dopant. In this work the impact of Ir incorporation defects on potential p-type conductivity in β-Ga2O3 is studied by means of density functional theory. The metastable α-Ga2O3 phase was investigated as the model object to understand the processes caused by iridium doping in gallium oxide-based systems. Obtained results allow us to understand better the influence of Ir on Ga2O3 electronic structure, as well as provide interpretation for optical transitions reported in recent experiments.

Scientific Reports volume 13, Article number: 8522 (2023)
DOI: 10.1038/s41598-023-35112-9
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ABSTRACT: Many modern applications, including quantum computing and quantum sensing, use substrate-film interfaces. Particularly, thin films of chromium or titanium and their oxides are commonly used to bind various structures, such as resonators, masks, or microwave antennas, to a diamond surface. Due to different thermal expansions of involved materials, such films and structures could produce significant stresses, which need to be measured or predicted. In this paper, we demonstrate imaging of stresses in the top layer of diamond with deposited structures of Cr2O3 at temperatures 19°C and 37°C by using stress-sensitive optically detected magnetic resonances (ODMR) in NV centers. We also calculated stresses in the diamond-film interface by using finite-element analysis and correlated them to measured ODMR frequency shifts. As predicted by the simulation, the measured high-contrast frequency-shift patterns are only due to thermal stresses, whose spin-stress coupling constant along the NV axis is 21±1 MHz/GPa, that is in agreement with constants previously obtained from single NV centers in diamond cantilever. We demonstrate that NV microscopy is a convenient platform for optically detecting and quantifying spatial distributions of stresses in diamond-based photonic devices with micrometer precision and propose thin films as a means for local application of temperature-controlled stresses. Our results also show that thin-film structures produce significant stresses in diamond substrates, which should be accounted for in NV-based applications.

Optics Express Vol. 31, Issue 11, pp. 17950-17963 (2023)
DOI: 10.1364/OE.489901 (Open Access)


ABSTRACT: Tribovoltaic devices are attracting increasing attention as motion-based energy harvesters due to the high local current densities that can be generated. However, while these tribovoltaic devices are being developed, debate remains surrounding their fundamental mechanism. Here, we fabricate thin films from one of the world’s most common oxides, TiO2, and compare the tribovoltaic performance under contact with metals of varying work functions, contact areas, and applied pressure. The resultant current density shows little correlation with the work function of the contact metal and a strong correlation with the contact area. Considering other effects at the metal–semiconductor interface, the thermoelectric coefficients of different metals were calculated, which showed a clear correlation with the tribovoltaic current density. On the microscale, molybdenum showed the highest current density of 192 mA cm-2. This work shows the need to consider a variety of mechanisms to understand the tribovoltaic effect and design future exemplar tribovoltaic devices.

ACS Appl. Mater. Interfaces 2023, 15, 27, 33140–33147
DOI: 10.1021/acsami.3c05830
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ABSTRACT: WO3/Cu/WO3 coatings are transparent electrodes, but conductivity and transmittance have been observed to decrease with time. This paper reports the improved stability of WO3/Cu/WO3/Cu/WO3 coatings deposited by magnetron sputtering on glass and polyethylene terephthalate substrates. The stability issues due to Cu oxidation and migration can be addressed by adjusting the deposition parameters. Lowering the sputtering pressure results in denser WO3 films, confirmed by spectroscopic ellipsometry, and thus more stable coatings. The coatings retain their properties in an inert atmosphere, indicating that Cu oxidation is the main reason for the decrease in conductivity, rather than its migration observed by X-ray photoelectron spectroscopy. Optical property modeling is used to optimize the thickness of the three-layer coatings to obtain the highest figure-of-merit for a transparent electrode. A structure of glass/WO3 (70 nm)/Cu (10 nm)/WO3 (45 nm) gives a sheet resistance of 14 Ω/sq. and a light transmittance of 65% at 600 nm. In addition, the antimicrobial properties of these coatings are revealed. A decrease up to 105 of the gram-negative Escherichia coli and gram-positive Staphylococcus aureus bacterial colony formation units is found for several WO3/Cu/WO3-based coatings. In the case of the MS2 (Emesvirus zinderi) bacteriophage, a decrease in infectious particles for up to 104 plaque-forming units is obtained. The results indicate that more stable samples also had higher antimicrobial activity.

Surfaces and Interfaces, Volume 41, October 2023, 103259
DOI: 10.1016/j.surfin.2023.103259 
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ABSTRACT: Nanowires (NWs) are among the most studied nanostructures as they have numerous promising applications thanks to their various unique properties. Furthermore, the properties of NWs can be tailored during synthesis by introducing structural defects such as nano-twins, periodic polytypes, and kinks, i.e., abrupt changes in their axial direction. Here, this work reports for the first time the postsynthesis formation of such defects, achieved by exploiting a peculiar plasticity that may occur in nanosized covalent materials. Specifically, in this work the authors found that single-crystal CuO NWs can form double kinks when subjected to external mechanical loading. Both the microscopy and atomistic modeling suggest that deformation-induced twinning along the (110) plane is the mechanism behind this effect. In a single case the authors are able to unkink a NW back to its initial straight profile, indicating the possibility of reversible plasticity in CuO NWs, which is supported by the atomistic simulations. The phenomenon reported here provides novel insights into the mechanisms of plastic deformation in covalent NWs and offers potential avenues for developing techniques to customize the shape of NWs postsynthesis and introduce new functionalities.

Small 2023, 2304614
DOI: 10.1002/smll.202304614
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Institute of Solid State Physics, University of Latvia, Thin Films Laboratory