Project coordinator: Dr.Phys. Mārtiņš Zubkins
Total cost: 100 389.00 EUR
Duration (years): 2020-2021
LZP FLPP Nr. lzp-2020/2-0291
Thin films of rare-earth metal oxy-hydrides (REHO) are a new class of inorganic mixed-anion materials, which exhibit a photochromic effect and a light-induced resistivity change at room temperature and ambient pressure. This switchable optical and electrical property enables their utilization in a multitude of technological applications, such as energy-saving smart windows, sensors, ophthalmic lenses, and medical devices. In order to tune and fully exploit REHOs in these applications, complete knowledge of the dependence of physical properties on the composition and the structure is crucial. The proposed project is based on the very recent discovery of the photochromism in REHOs and aims to study the relationship between the thin film deposition parameters, chemical composition, and structure of existing REHOs with the focus on the photochromic effect and discover new photochromic REHOs. The project proposes to produce REHOs in the thin film form and do in-depth characterisation by advanced in-lab techniques both ex-situ and in-operando. Magnetron sputtering has been chosen as the deposition technique since it is among the most widely used types of deposition by the glazing industry because it can be scaled up to large-area substrates together with a high growth rate, which is highly important for the large-scale production.
Jaunumi par projektu
AR PROJEKTA ĪSTENOŠANU (LAIKA POSMS 01.09.2021. - 31.12.2021.)
Work package 1. Deposition of REHO thin films.
Vibration spectroscopy techniques (Raman and FTIR) were exploited to study the structure of YHxOy in more detail, as it is still not fully understood. The YHxOy films were deposited on silicon and metal substrates to be able to measure high quality FTIR and Raman spectra, respectively, without a disturbing signal from the substrate. To supress the interaction between YHxOy and air, the part of samples was covered by a thick metal film in the same deposition chamber before breaking the vacuum. In this case, for the Raman measurements, the metal substrates were replaced with substrates that have high visible light transmittance but low Raman signal. Isotopically exchanged films were synthesised to identify hydrogen related vibrations in the measured spectra.
In the search for a new photochromic REHO material, Eu of the rare-earth elements was selected. Since the surface oxide layer does not stop further oxidation of Eu, as in the case of Y, the work with Eu is more complicated. Process parameters for a stable discharge in the reactive pulsed-DC magnetron sputtering from metallic Eu target were found. Total sputtering pressure was the main parameter that was varied during the development of Eu hydride and oxy-hydride films deposition. Pressure is an effective parameter to control the hydrogen concentration in the Eu films, see Fig. 1(a). Oxy-hydride phase was produced by the introduction of oxygen in the deposition chamber after the deposition. The samples of EuHx, EuH2-x, and EuHxOy were directly transferred (without breaking the vacuum) and stored in an Ar atmosphere glove-box connected to the vacuum system to protect the films from the interaction with air. A thin (15 nm) capping layer of aluminium (Al) deposited by thermal evaporation was tested to protect the films from the oxidation in the ambient conditions. A photochromic effect in EuHxOy was not discovered after irradiation with UV/blue light.
Work package 2. Thin film characterisation.
Several measurement techniques/geometries and samples/structures were used to obtain the vibration spectra of YHxOy samples. In the beginning, the samples were irradiated from the film surface side. The FTIR spectra were measured in transmittance mode, and the Raman signal was collected from the scattered light. The films, which were covered by a metal layer, were irradiated through the substrate during the measurements. In this case, the FTIR spectra were measured in reflectance mode.
The detected vibration bands are relatively wide due to the small crystallite size of approximately 10 nm according to XRD data. Vibration data of YHO are almost non-existent to the best of our knowledge, so no reference can be used to identify the structure. Both experiment and theory were used to interpret the spectra. Based on the theoretically stable YHxOy structures found in literature and our XRD data, YHxOy was modelled using the crystallographic structure belonging to different space groups. YHxOy was modelled using linear combination of atomic orbitals (LCAO) method within the framework of the hybrid density functional approach. LCAO calculations, including analyses of phonon frequencies and vibration intensities, were performed using algorithms as implemented in CRYSTAL17 code. The activities on the vibration properties of YHxOy and interpretation will continue after the project to complete the results for publication in a scientific journal.
EuHxOy and alloyed Y-Eu hydride films were produced in accordance with the project objectives. The films were characterised by XRD, SEM (Fig. 1(b)), ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy, and spectroscopic ellipsometry. Due to the lack of information on the deposition of this type of materials, the results obtained appear to be suitable for a short communication paper
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Figure 1. EuHx thin films on glass substrates synthesised at different H2 pressures (a), and surface morphology of the EuHxOy film imaged by an electron scanning microscope at high magnification (b)
An article has been submitted in international scientific journal within the framework of this project:
“Oxidation dynamics and optical properties of oxygen-containing yttrium hydride thin films”, M. Zubkins, I. Aulika, E. Strods, V. Vibornijs, L. Bikse, A. Sarakovskis, H. Arslan, J. Purans.
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.06.2021. - 31.08.2021.)
Work package 1. Deposition of REHO thin films.
During this period, a set of YHO samples for X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy measurements at different film deposition pressures were prepared on titanium substrates. Samples were stored under an inert gas (argon) atmosphere to evaluate their stability to samples stored at ambient conditions.
Work package 2. Thin film characterisation.
The XPS results confirm that the previously observed gradient of optical properties perpendicular to the film substrate is related to changes in chemical composition (Fig. 1). The ratio of oxygen to yttrium concentration decreases from the surface of the films to the substrate.
The results of this project are presented in several reports at the international conference E-MRS Fall meeting 2021:
- Magnetron sputtered YHO thin film oxydation dynamics and optical properties;
- Optical band gap determination issues for amorphous and crystalline metal-oxide thin films;
- Nanocrystalline/Amorphous semiconducting yttrium monoxide.
An article has been published within this project:
Figure 1. YHO film on Ti substrate XPS depth profile measurement. Y signal - blue line, O signal - green line, Ti signal - red line.
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Figure 2. Schematic illustration of the sensory device based on reflectivity measurement setup with the capability to measure both the ratio of the amplitude and the phase shift (a). Main ellipsometric angles (Δ, Ψ) as a function of photon energy with 0.02 eV step for the structure Si/Au(29 nm)/YHO(12 nm)/SiO2(35 nm)/YHO(15 nm)/Au(6 nm) (b).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.03.2021. - 31.05.2021.)
Work package 1. Deposition of REHO thin films.
During the appropriate time period, a vacuum device was prepared for film deposition in plasma at elevated substrate temperature – 300 ° C, and a short set of YHO samples were prepared to evaluate the effect of substrate temperature on film oxidation both during the process and after in the oxygen-containing atmosphere. The vacum coater is being equipped with an additional gas supply to vary the composition of the films and to decipher the vibration spectra of the films in detail.
Figure 1. Surface morphology of an YHO film imaged by an electron scanning microscope at high magnification
Work package 2. Thin film characterisation.
Surface morphology studies of the prepared YHO films were performed by scanning electron microscopy (Fig. 1) and its change depending on the deposition pressure was observed. SE studies of new sample series show an increase in the refractive index n and the fundamental optical bandwidth Eg and a decrease in the extinction coefficient k, increasing the pressure from 3 to ~6.5 mTorr. As the pressure increases above 6.5 mTorr, the (n, k) decrease. Semiconductor films have a pronounced optical gradient: n decreases from the bottom to the top of the film. With an increase of sputtering pressure, the gradient decreases.
A scientific paper is currently being prepared that includes all the results obtained from the deposition of the YHO thin films and their characterization. The paper describes in detail the oxidation dynamics of the films depending on the deposition parameters. In addition, a comprehensive description of the structure, surface morphology, and optical properties of the films is provided. At the international conference CSW2021, a poster presentation entitled “Multilayer Structures With Spectral Darkness For Biomedical Sensor Applications” was presented, which included the results of the respective project. In addition, two abstracts have been submitted for the international conference E-MRS Fall meeting 2021.
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.12.2020. - 28.02.2021.)
Work package 1. Deposition of REHO thin films.
During the appropriate time period, a reactive magnetron sputtering vacuum system for the deposition of YHO thin films was successfully prepared and several sets of YHO samples were prepared on different types of substrates – glass, silicon and titanium. In a narrow range of process parameters (sputtering pressure), photosensitive films were obtained (see Figure 1). For the first time in the study of this material, the oxidation dynamics was determined both during the deposition process and during the introduction of oxygen gas after the process by measuring the visible light transmittance spectra while the sample is still in the vacuum chamber.
Figure 1. Photochromic effect of YHO thin film using UVA and blue light.
Work package 2. Thin film characterisation.
The following measurements were performed on the prepared samples – X-ray diffraction (XRD), light (UV-Vis-NIR) transmittance and reflectance, spectral ellipsometry (SE) and analysis, optical microscopy, and profilometry measurements. An experimental system was prepared and the first measurments were performed to determine the change in light absorption during illumination, and a photochromic effect was observed. The films are polycrystalline and the transmitance of visible light is controlled by the deposition pressure. SE studies show a significant decrease in the refractive index n and the fundamental optical bandwidth Eg and a decrease in the extinction coefficient k, increasing the pressure from 3 to ~8.0 mTorr. As the pressure increases above 8 mTorr, the optical constants decrease. Semiconductor films have a pronounced optical gradient: n decreases from the bottom to the top of the film. During the photochromic effect, the k increase and the Eg decreases.
As a result of the experiments, the milestone 1 provided in the project plan has been achieved - definition of optimal process parameters for the stable/reproducible synthesis of YHO films.