Type: European Regional Development Fund
Agreement No: 1.1.1.1/20/A/057
Duration: 01.01.2021 - 30.06.2023.
Project Leader: Institute of Solid State Physics University of Latvia (ISSP UL), Dr.hab.phys. Juris Purans
Project partners: SIA AGL Technologies, Dr.pys. Andis Azens, SIA BC Corporation Limited Dr.phys. Lauris Dimitrocenko.
Total budget: 537 004 EUR
ISSP UL budget: 322 000 EUR
Project description:
Gallium oxide Ga2O3 has become one of the most investigated materials of today. Nearly every issue of material-related scientific journals contains articles on growth, material properties, or device applications of gallium oxide. The reason for this large interest is the extremely promising properties for electronic and optical applications of this wide bandgap material, together with the relatively un-expensive substrate wafers. Very recently, ultrawide-bandgap spinel zinc gallate ZnGa2O4 has been demonstrated to exhibit several benefits over gallium oxide that merits to be investigated more deeply.
The aim of this industrial research project is to develop advanced high rate PVD magnetron sputtering and MOCVD technologies for deposition of functional ultrawide-bandgap gallium oxide Ga2O3 and zinc gallate ZnGa2O4 thin films for optoelectronics and electronics applications.
The main goals are:
- To develop high rate PVD magnetron sputtering technology for deposition of pure and doped (p-type dopants and RE) amorphous and crystalline gallium oxide Ga2O3 thin films and ZnGa2O4 thin films. The applications in focus are (1) deep UV TCOs/TSOs and (2) efficient inorganic luminescence devices (a-Ga2Ox:RE).
- To develop MOCVD technology of Ga2O3 and ZnGa2O4 thin films deposition and to establish epitaxial n- and p-type Ga2O3 and ZnGa2O4 thin film growth processes for deep UV optoelectronics and electronics applications.
The proposed Industrial research project will be implemented by ISSP LU, SIA AGL Technologies and SIA BC Corporation Limited. This Interdisciplinary Project consists of the research activities in Physical and Chemical sciences (1.3, 1.4) and Materials engineering (2.5).
ON THE IMPLEMENTATION OF THE PROJECT AND FINAL RESULTS (PERIOD 01.04.2023.- 30.06.2023.)
30.06.2023
Y doped Ga2O3 films (0.2–19.3 at.% Y) were deposited by reactive magnetron co-sputtering from metallic yttrium and gallium targets at room-temperature on Si substrates. Er and Eu doping of ZnGa2O4 was tried by placing small pieces of Er2O3 and metallic Eu on the Zn target erosion zone, respectively; however, sufficient sputtering from these pieces was not observed. The RE doping of Ga-based oxide films by reactive magnetron co-sputtering is difficult due to the high reactivity of REs with oxygen, resulting in low sputtering rates. To solve this, it is necessary to use a separate RE target for sputtering (instead of small pieces of RE on a solid target, such as Zn, in the case of ZnGa2O4 doping) and carefully adjust the sputtering conditions. We demonstrated that it is possible to effectively dope Ga2O3 films with Y using an additional Y target. The static deposition rate ranges from 0.3 to 77.2 nm/min. The manuscript of a scientific article on ZnGa2O4 deposition has been prepared – will be published in post-monitoring period.(Activity 1).
Process fine-tuning for MOCVD-growth of spinel ZnGa2O4 films was finalized. Technical reports were prepared and a scientific article was published: Butanovs, E., Zubkins, M., Nedzinskas, R., Zadin, V. & Polyakov, B. Comparison of two methods for one-dimensional Ga2O3-ZnGa2O4 core–shell heterostructure synthesis. J. Cryst. Growth 618, 127319 (2023). (Activity 2).
Computer modelling of the most stable (001), (101), (-201), and (010) 2D slabs of gallium oxide materials was performed using the computer code CRYSTAL. Additionally, the vibrational properties, infrared (IR) and Raman spectra of the bulk materials were calculated and compared with thin films grown via magnetron sputtering. (Activity 3).
Y doped Ga2O3 films deposited by reactive magnetron co-sputtering were studied by XRD, XPS depth profiling, UV-Vis-NIR spectroscopy, and spectroscopic ellipsometry. XPS and XRD was used as a feedback for fine-tuning MOCVD growth of ZnGa2O4 films. Technical reports were prepared. (Activity 4).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.01.2023.- 31.03.2023.)
31.03.2023
During the relevant time period, the deposition of CuGa2O4 and CuGaO2 films on c-sapphire substrates was performed by reactive magnetron co-sputtering from liquid/solid Ga/Cu targets. Different chemical compositions and phases were obtained by varying the sputtering power of Cu target, the O2/Ar gas flow ratio, and the sputtering pressure. The substrate temperature of 800 °C was chosen. Plasma optical emission spectroscopy was successfully used to achieve the selected process mode and ensure a stable process during deposition (Activity 1).
ZnGa2O4 films were grown on different orientation sapphire substrates by MOCVD. The dependence of their crystallographic orientation on the orientation of the sapphire surface was investigated (Activity 2).
Simulations of Ga2O3 were continued by combining the DFT with several different basis sets. Calculations on several 2D slabs of Ga2O3 with different thicknesses were continued with the focus on possible p-type conductivity in this material. (Activity 3).
The thicknesses of the films produced in Activity 1 range from 0.2 to 2.0 micrometers. X-ray diffraction of the film showed a mixture of several crystalline phases - CuGa2O4, CuGaO2, CuO. A p-type conductivity was obtained for the films with CuGaO2 phase dominance. The measured resistivity ranges from 350 to 1400 Ohm-cm. p-type conductive films absorb visible light but are partially transparent (about 60%) in the near-infrared part of the spectrum. ZnGa2O4 films produced in Activity 2 were studied in depth for their crystal structure and orientation (Activity 4).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.10.2022.- 31.12.2022.)
31.12.2022
During the relevant time period, research into the production of p-type Ga2O3 and ZnGa2O4 films by reactive magnetron sputtering continued. By varying the impurity concentration, two chemical compositions were tried: (i) Zn-doped Ga2O3 and (ii) Cu-doped ZnGa2O4. The films were grown on glass or Si substrates at an elevated substrate temperature of 400 or 700 °C. The concentration of Zn in Ga2O3 was varied by the sputtering power. Cu-doped ZnGa2O4 films were deposited by placing symmetrically small Cu pieces on the Zn target erosion zone. No measurable p-type conductivity has been obtained at the moment. Work on the deposition of these and other chemical compounds will continue, by testing different substrate materials, varying the substrate temperature and other production parameters (Activity 1).
Published article:
Zubkins M., Vibornijs V., Strods E., Butanovs E., Bikse L., Ottosson M., Hallén A., Gabrusenoks J., Purans J., Azens A., ”Deposition of Ga2O3 thin films by liquid metal target sputtering”, Vacuum (2022) 111789.
Development of the ZnGa2O4 thin film deposition process by MOCVD was continued to produce films with precise stoichiometry and spinel phase. Work has begun to grow ZnGa2O4 layers on sapphire substrates of different orientations. Pulsed laser deposition (PLD) was tested as an alternative method for obtaining ZnGa2O4 thin films, but with the available process parameters of the method, it is not possible to obtain a pure spinel phase. It would be necessary to use a ZnO/Ga2O3 sputtering target with a different composition of compounds, which is not an economically viable option for this project. (Activity 2).
Simulations of Ga2O3 were started by combining the DFT with several different basis sets. Calculations on several 2D slabs of Ga2O3 with different thicknesses have been initiated. The work on p-type conductivity in Ga2O3 was continued. (Activity 3).
Characterization of the ZnGaxO4 films produced by various methods was performed with optical spectroscopy, X-ray diffraction, ellipsometry, X-ray photoelectron spectroscopy, optical microscopy and Raman spectroscopy. Data collection was carried out, the results were used as feedback for adjusting the deposition processes. P-type Ga2O3:Zn films were studied to determine the Zn concentration by XPS, as well as their electrical properties were tested by the van der Pauw method. (Activity 4).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.07.2022.- 30.09.2022.)
30.09.2022
Activities on high rate deposition of ZnGa2O4 films by two target reactive magnetron co-sputtering were concluded by developing the technology and fine-tuning the parameters of plasma optical emission spectroscopy controlled process, allowing for stable and reproducible deposition of films with Zn:Ga ratio between 0,2 and 3. Amorphous and crystalline coatings were deposited at substrate temperatures from room to 800 °C. Activities on RF sputtering from ceramic ZnGa2O4 target were concluded by fine tuning the process parameters for deposition of stoichiometric ZnGa2O4 films. Technology development was started for deposition of doped Ga2O3 films for p-type conductivity and light emitting applications. (Activity 1).
Process development for ZnGa2O4 thin film deposition by MOCVD was continued. The optimal temperature range for the growth of the films was established, the adjustment of Zn and Ga precursor flow rates and ratios to obtain stoichiometric films is continued. (Activity 2).
DFT calculations were started on gallium oxide bulk and surface unit cells using the CRYSTAL computer code in order to analyse the existence of p-type conductivity in non-stoichiometric and doped Ga2O3, as well as to understand whether the hole defects are localized delocalized or exist at all. (Activity 3).
The films were analysed by means of optical spectroscopy, X-ray diffraction, ellipsometry, X-ray photoelectron spectroscopy, optical microscopy and Raman spectroscopy. The results were used as a feedback for process tuning conducted in Activities 1 and 2. (Activity 4).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.04.2022.- 30.06.2022.)
30.06.2022
The development of the high-speed deposition technology of ZnGa2O4 thin films was continued, by simultaneously sputtering a liquid gallium target and a zinc target in reactive DC mode. The process parameters were varied to develop the technology for obtaining highly transparent films with different Zn:Ga ratios and different degrees of crystallinity. The obtained layer manufacturing speed significantly exceeds the speeds obtained in the literature and also in this project, when the layers are manufactured in the RF mode. Tuning of the film production process was also continued by sputtering the ZnGa2O4 target in the RF mode. Preparation of a publication on the preparation and properties of Ga2O3 films was completed. (Activity 1).
Experimental work was initiated to find out the growth parameters of ZnGa2O4 for the existing MOCVD facility. Several experiments were conducted to determine the ratio of Zn and Ga precursors, and several experiments were conducted to determine the growth temperature of the ZnGa2O4 thin film. (Activity 2).
Characterization of the ZnGaxO4 films produced under different conditions was performed using optical spectroscopy, X-ray diffraction, ellipsometry, X-ray photoelectron spectroscopy, optical microscopy and Raman spectroscopy. Data collection was carried out, the results were used as feedback for adjusting the production process in Activity 1. Atomic force microscopy (AFM) measurements of Ga2O3 thin films were performed during the mission in the equipment available at the University of Tartu. AFM measurements were necessary to understand the surface morphology of the resulting thin films and to adjust their deposition parameters to obtain the smoothest possible surface. (Activity 4).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.01.2022.- 31.03.2022.)
04.04.2022.
The production of Ga2O3 thin films was continued, sputtering the liquid gallium target in the reactive DC mode and the Ga2O3 target in the RF mode. It was found that in order to ensure the necessary stoichiometry of the films, the sputtering of the Ga2O3 target must also be performed in a reactive process. The development of a high-speed deposition technology for ZnGa2O4 thin films was initiated, simultaneously sputtering the liquid gallium target and the zinc target in the reactive DC mode. A set of process parameters for the production of highly transparent films was found. The production of films by sputtering the ZnGa2O4 target in RF mode was also started (Activity 1). A patent application with no. LVP2021000105 for A method for reactive magnetron sputter deposition of gallium oxide thin films has been submitted. (Activity 1)
The optimization of the growth of Ga2O3 thin films in the MOCVD plant was continued. The process for obtaining a high-temperature buffer layer was continued in order to obtain a higher degree of crystallization of Ga2O3 thin films. The growth of Ga2O3 and ZnGa2O4 thin films with MOCVD on sapphire substrates of different orientations was continued and the sputtering of Ga2O3 and ZnGa2O4 thin films on c-plane sapphire substrates was continued. Ga2O3 and ZnGa2O4 thin films were grown with PLD on sapphire substrates of different orientations and in-depth study of the structure and morphology of the grown thin films was performed. (Activity 2)
Ab-initio DFT calculations were performed for ZnO2 materials. DFT functionals and basis sets of Gaussian type functions within the linear combination of atomic orbitals (LCAO) approximation were examined and these results were compared with the experimental data and planewave calculations. According to the performed calculations and the obtained results, the scientific article "Zinc peroxide from the first principles" has been prepared. (Activity 3)
Characterization of Ga2O3 and ZnGa2O4 films prepared under different conditions was performed by optical spectroscopy, X-ray diffraction and Raman spectroscopy. High-resolution X-ray diffraction analysis (performed in collaboration with partners from Angström Laboratory, Uppsala, Sweden) showed that the high-temperature Ga2O3 films on the sapphire substrate are epitaxial. Analysis of the composition of Ga2O3 films by ERDA (in collaboration with KTH, Stockholm, Sweden) and XPS showed that the films are free of impurities. The morphology of the films was analysed by electron microscopy (Activity 4).
A publication on the preparation and properties of Ga2O3 films was started.
In-depth high-resolution X-ray diffraction measurements of crystalline Ga2O3 thin films were initiated to determine their degree of monocrystallinity and epitaxial orientation to sapphire substrates. (Activity 4)
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.10.2021. - 31.12.2021.)
11.01.2022.
The development of high-speed application technology of Ga2O3 thin films was continued, and the parameters of the sample preparation process were optimized by sputtering the liquid gallium target in the reactive direct current mode. A series of amorphous and crystalline samples were prepared on quartz and sapphire substrates in the temperature range from room temperature to 800oC. Ga2O3 thin films were created by sputtering the Ga2O3 target in RF mode. A LV patent application for Ga2O3 thin film deposition technology for sputtering a liquid gallium target in reactive direct current mode was prepared and filed. (Activity 1)
The growth of Ga2O3 thin films in the MOCVD plant is further optimized by simultaneously combining H2O and O2 precursor gases. In order to obtain a higher degree of crystallization, a process for obtaining a high-temperature buffer layer is being developed. An in-depth study of the structure and morphology of the grown thin films is being carried out. (Activity 2)
Ab-initio DFT calculations were employed to study the electronic structure of oxygen and gallium vacancies in monoclinic bulk β-Ga2O3 crystals. Hybrid exchange–correlation functional B3LYP within the density functional theory and supercell approach were successfully used to simulate isolated point defects in β-Ga2O3. The calculations predict that an oxygen vacancy in β-Ga2O3 is a deep donor defect which cannot be an effective source of electrons and, thus, is not responsible for n-type conductivity in β-Ga2O3. All types of charge states of gallium vacancies are sufficiently deep acceptors, however, due to high formation energy, they cannot be considered as a source of p-type conductivity in β-Ga2O3. As a result, a scientific publication " Vacancy Defects in Ga2O3: First-Principles Calculations of Electronic Structure" was prepared and published in Materials. (Activity 3)
Optical transmittance, reflection and absorption spectra, X-ray diffraction and Raman spectroscopy data of Ga2O3 coatings prepared under different conditions were studied and collected. (Activity 4)
Publication:
Usseinov, Abay; Koishybayeva, Zhanymgul; Platonenko, Alexander; Pankratov, Vladimir; Suchikova, Yana; Akilbekov, Abdirash; Zdorovets, Maxim; Purans, Juris; Popov, Anatoli. Vacancy Defects in Ga2O3: First-Principles Calculations of Electronic Structure. Materials. 14, 7384 (2021); DOI:10.3390/ma14237384
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.07.2021. - 30.09.2021.)
11.10.2021.
Within the framework of the project, the development of high-speed Ga2O3 thin films deposition technology has been started and the production and characterization of a series of samples has been performed. The optical transmittance, reflection and absorption spectra, X-ray diffraction data and properties of Ga2O3 coating at different temperatures were studied and collected. (Activity 1).
A price survey was conducted and the purchase of sputtering targets was made (Activity 1).
The preparation of the Aixtron (AIX-200RF) MOCVD system for the growth of Ga2O3 and ZnGa2O4 films was completed and the preparation of the technical report has been started. Work has begun on growing Ga2O3 thin films on c-plane sapphire using H2O as the oxygen source. The development and drawing of a new reactor design is continued in cooperation with SIA BC Corporation Limited - the drawing of the chamber's cooling body, substrate heater and rotation mechanism is being performed. The MOCVD is now equipped with an O2 gas precursor, which is used in parallel with H2O for Ga2O3 thin film synthesis experiments. An in-depth study of the structure and composition of the grown thin films is performed. (Activity 2).
Nanocrystalline zinc peroxide (nano-ZnO2) was synthesized through a hydrothermal process and comprehensively studied using several experimental techniques. Its crystal structure was characterized by X-ray diffraction. The temperature-dependent local environment around zinc atoms was reconstructed using reverse Monte Carlo (RMC) analysis. Lattice dynamics of nano-ZnO2 was studied by infrared and Raman spectroscopy. The obtained experimental results were supported by first-principles density functional theory (DFT) calculations. As result scientific publication "A comprehensive study of structure and properties of nanocrystalline zinc peroxide" was prepared and published in Journal of Physics and Chemistry of Solids. (Activity 3).
The preparation and testing of structural and morphological methods (XRD, XPS, SEM, TEM) for the characterization of Ga2O3 and ZnGa2O4 thin films were completed and the preparation of a technical report has been started. The characterization of Ga2O3 thin films obtained by magnetron sputtering and MOCVD was started in order to understand and optimize deposition processes. (Activity 4).
The project results were presented at the 2021 Fall Meeting conference of the European Materials Research Society (E-MRS) with a poster presentation "Growth of gallium oxide based core-shell nanowire heterostructures". (Activity 4).
Publication:
A comprehensive study of structure and properties of nanocrystalline zinc peroxide
Bocharov D., Chesnokov A., Chikvaidze G., Gabrusenoks J., Ignatans R., Kalendarev R., Krack M., Kundzins K., Kuzmin A., Mironova-Ulmane N., Pudza I., Puust L., Sildos I., Vasil'chenko E., Zubkins M., Purans J.
Journal of Physics and Chemistry of Solids, Volume 160, January 2022, 110318
DOI: https://doi.org/10.1016/j.jpcs.2021.110318
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.04.2021.-30.06.2021.)
23.07.2021.
Preparation of the sputtering system for use in two-magnetron DC, RF and HiPIMS sputtering configuration was continued. The design and installation of target containers for metallic Ga targets suitable for reactive sputtering from a liquid metal target and arrangement of the chamber interior geometry for sputtering in upward direction was completed. Testing of the installed target has been started. Practical knowledge related to the growth of Ga2O3 and ZnGa2O4 thin films on their structure, electrical and optical physical properties has been acquired. As a result of the research, a scientific article “A comprehensive study of the structure and properties of nanocrystalline zinc peroxide” was prepared and submitted (activity 1).
Plasma Optical Emission spectroscopy line has been installed and tuned, Gallium spectral lines have been identified for process control (activity 1).
Aixtron (AIX-200RF) MOCVD system preparation for Ga2O3 and ZnGa2O4 thin film deposition has been performed: fabrication and testing of a new silicon carbide heating element in an oxidative atmosphere; growing Ga2O3 and ZnGa2O4 thin films with MOCVD on sapphire substrates of different orientations; engineering and drawing of a new reactor design in collaboration with SIA BC Corporation Limited (activity 2).
Ab-initio calculations of doped Ga2O3 was conducted. Analysis of material atoms, electronic and oscillation properties, analysis of formation energies of various configurations were performed. The article “Ab-initio calculations of oxygen vacancy in Ga2O3 crystals” was published in an internationally cited journal: LATVIAN JOURNAL OF PHYSICS AND TECHNICAL SCIENCES, Vol. 58, N 2 (2021), 3-11 (activity 3).
An open procurement was announced for the purchase of substrates, chemicals and other laboratory equipment necessary for the implementation of the project. Preparation and testing of structural and morphology characterization methods (XRD, XPS, SEM, TEM) for Ga2O3 and ZnGa2O4 thin films has been continued (activity 4).
ON THE IMPLEMENTATION OF THE PROJECT (PERIOD 01.01.2021. - 31.03.2021.)
02.04.2021.
As part of Project No 1.1.1.1/20/A/057 preparation and testing of the vacuum system, gas inlet system, substrate heating system, target cooling and heating system and connection of the necessary (DC, RF, HiPIMS) power supplies for use in two-magnetron sputtering configuration has been started. Development of design of target containers for metallic Ga targets suitable for reactive sputtering from a liquid metal target and arrangement of the chamber interior geometry for sputtering in upward direction also is started (activity 1).
Magnetron sputtering targets purchase order placed (activity 1).
Aixtron (AIX-200RF) MOCVD system preparation for Ga2O3 and ZnGa2O4 thin film deposition has been done: testing of reactor stability at high temperature while using oxidative precursor; engineering and drawing of a new reactor design in collaboration with SIA BC Corporation Limited; replacement of malfunctioning parts (activity 2).
Study was conducted and the formation energy and transition levels of oxygen vacancies in β-Ga2O3 crystal using the B3LYP hybrid exchange-correlation functional within the LCAO-DFT approach was calculated. As result information for article “Ab-initio calculations of oxygen vacancy in Ga2O3 crystals” was prepared (activity 3).
Preparation and testing of structural and morphology characterization methods (XRD, XPS, SEM, TEM) for Ga2O3 and ZnGa2O4 thin films has been done (activity 4).