Identification number: lzp-2022/1-0454
Type: Fundamental and applied research project of the Latvian Council of Science
Project duration: 04.2023 - 03.2026
Project manager: Dr.Phys. Martins Zubkins, Institute of Solid State Physics, University of Latvia
Total funding: 300 000 EUR
Project aim:
To develop advanced deposition technology of metal hydride and borohydride films, and study the influence of growth process parameters on the thin film structure and properties.
Project summary:
Reactive High Power Impulse Magnetron Sputtering (r-HiPIMS) deposition technology has a huge potential for manufacturing coatings with properties exceeding those of state of the art. The available research activities have mainly been the HiPIMS deposition of metals (Ar plasma), metal oxides by reactive process (Ar+O2), and metal nitrides (Ar+N2), but deposition of metal hydrides (Ar+H2) has not yet been demonstrated. Metal hydrides (MHx) and boro-hydrides M(BH4)x are a fascinating and important class of materials that can be used for storage and other energy applications, as far as Hydrogen storage will be a key technology for energy utilization in the 21st century. Also, lanthanide hydrides (LnHx) are unique classes of materials with a rich spectrum of physical properties. The project proposes to produce MHx and M(BH4)x (M: alkali-earth Mg, transition metals Ti and Y, lanthanides La and Eu) in the thin film form by r-HiPIMS and do in-depth characterisation by advanced in-lab techniques – XRD, XPS, SEM, TEM, Raman and UV-Vis-NIR spectroscopy, and spectroscopic ellipsometry. The project aims to study the relationship between the HiPIMS deposition parameters (impulse durations, sputtering power, pressure, substrate temperature) and chemical composition, properties, structure and density. HiPIMS sputtering has been chosen as the deposition technique since it is widely used in both lab-scale and industry with possible up-scaling.
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.07.2023. - 30.09.2023.)
During the specified time frame two series of yttrium oxyhydride thin films were deposited: in the r-HiPIMS mode and in the reactive pulsed-DC magnetron sputtering mode. The coatings were made on two types of substrates: glass and silicon. Yttrium oxyhydride thin films were obtained by sputtering YH2 thin films in a reactive atmosphere (Ar+H2) from an yttrium target and oxidized in air after the process. For both series, sputtering parameters were found which allow to obtain transparent coatings with a structure corresponding to YHO and which exhibit a photochromic effect after irradiation with ultraviolet light. HiPIMS mode requires higher pressure than pulsed-DC. Changes in optical properties over time were observed with the spectrometer, indicating the continuation of oxidation of the thin films, changes in composition and structure several days after the process. The influence of the selected sputtering mode and sputtering parameters (pressure, time, temperature) on the structure of thin films (crystallite size, lattice parameter) and optical properties (transmittance, optical band gap) was investigated. The X-ray diffractograms of the YHO films made in r-HiPIMS mode at a sputtering pressure of 8.5 mTorr and in pulsed-DC mode at a pressure of 5.1 mTorr are shown in Figure 2. The thicknesses of both coatings is about 550 nm. The photochromic properties (photochromic contrast, bleaching time) of the thin films were evaluated by periodically illuminating them with ultraviolet light and measuring the change in transmittance. To evaluate and compare the effect of the two methods on the delamination rate of the coatings, the surface changes of the thin films over time are observed with an optical microscope.
Figure 2. XRD patterns of photochromic YHO thin films deposited by reactive pulsed-DC magnetron sputtering and HiPIMS.
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.04.2023. - 30.06.2023.)
During the specified time frame, a vacuum physical vapour deposition system was successfully prepared to realize reactive high power impulse magnetron sputtering (r-HiPIMS). The system is equipped with balanced magnetrons and the necessary process gases – Ar and H2. We conducted preliminary experiments on an yttrium (Y) target by sputtering the target in both inert (Ar) and reactive (Ar+H2) atmospheres. Our system is equipped with a HiPIMS power source, alongside a high time-resolved spectrometer to detect changes in plasma emission line intensities during a pulse. Figure 1 shows an example of the change in emission intensities of excited Y atoms (Y*) and ions (Y+) during a 50 µs sputtering pulse. Both the discharge current peak value of 105 A and the intense Y+ emissions correspond to the HiPIMS mode. Emission line intensity directly depends on the concentration of relevant particles in the plasma and provides essential information for further process optimization. We successfully deposited the first thin films of yttrium hydride (YH2) and yttrium oxyhydride (YHO) on glass substrates using the r-HiPIMS process.
Figure 1. Evolution of discharge current and the intensity of excited Y atom and ion emission lines (410.2 nm and 437.5 nm, respectively) during a 50 µs-long HiPIMS pulse.