Doctoral Theses

“Electronics and photonics”

  • Theme: Development of Ga2O3 transistors for applications in kV range
    Supervisor:Ing. Milan Ťapajna, PhD,   ( Department of III-V Semiconductors )
    Co-advisors: RNDr. Dagmar Gregušová, DrSc.
    Abstract:
    Current power electronic device market is mostly covered by Si (<1kV voltage range) and SiC and GaN (up to several kV) devices. However, there are practically no semiconductor power devices available for and above the 10-kV range. Gallium oxide (Ga2O3) is a promising ultra-wide bandgap (Eg=4.8–5.3 eV) semiconductor material, which offers technological potential for design of new electronic devices capable of handling this voltage range. Such devices can enable development of systems for transportation utilising electric drive (cars, trains, ships, aircrafts) or transformation for high-voltage DC power distribution networks. The aim of the thesis will be the design of power devices (diodes, transistors) for kV region using complex software tools as well as the fabrication of SBDs and MOSFET transistors using state-of-the-art semiconductor technologies available at IEE SAS. The mechanism of electrical transport and breakdown of the prepared devices will also be comprehensively analysed. The work will be carried out in the framework of a joint project with the Taiwanese partner ITRI (Industrial Technology Research Institute).
  • Theme: Epitaxial growth of Ga2O3 and related alloys via metalorganic chemical vapour deposition (MOCVD)
    Supervisor: Ing. Filip Gucmann, PhD.  ( Department of III-V Semiconductors )
    Abstract:
    Almost one third of all produced electricity passes through power electronic devices at some stage before consumption with the predicted increase up to 80 % for the next decade. Therefore, even small efficiency improvement of power electronics represents significant energy savings and reduction of carbon emissions. One of the ways to achieve higher efficiency of power electronic devices is the use of new (ultra) wide bandgap (Eg) semiconductor materials. Gallium oxide (Ga2O3) is a good example of such material – with Eg ~4.9 eV and high critical field which surpasses those of the mainstream semiconductors for power switching in the range up to several kVs (Si, SiC, GaN). Ga2O3-based devices can be also invaluable for faster deployment of electric means of transport thanks to development of faster charging systems and more efficient energy transfer between batteries and electric drivetrain.
    The main focus of this thesis will be a systematic study of epitaxial growth of thin film Ga2O3 or similar materials, e.g. (AlxGa1-x)2O3 or (InxGa1-x)2O3 on various substrates (e.g. Al2O3, SiC) using metalorganic chemical vapour deposition (MOCVD) and investigation of the material properties of prepared layers (structural, electrical, and optical). For this study, we will use commercial MOCVD tool Aixtron CCS and the state-of-the-art technological equipment and methods, available at the Institute of Electrical Engineering, SAS. A successful candidate will acquire a hands-on experience with the wide range of experimental techniques for material diagnostics (e.g. X-ray diffraction, atomic force microscopy, Raman spectroscopy, and various advanced electrical methods). The work will be carried out in the framework of a joint project with the Taiwanese partner ITRI (Industrial Technology Research Institute).

“Physical Engineering”

  • Theme: Influence of different substrates and buffer layers on the microstructure and electric properties of epitaxial layers of Ga2O3 and related compounds deposited via metalorganic chemical vapour deposition (MOCVD)
    Supervisor: Ing. Alica Rosová, CSc.  ( Department of III-V Semiconductors )
    Abstract:
    In recent years, Ga2O3, wide band gap semiconductor, has attracted great attention as promising material for high voltage and high-power electronic devices, short wavelength light emitting diodes, and ultraviolet semiconductor lasers. However, growth of high-quality Ga2O3 epitaxial layers on technologically-important substrates still remains a challenging task. The aim of this work will be concentrated to microstructural study of epitaxial layers of Ga2O3 and related materials deposited via metalorganic chemical vapour deposition (MOCVD) prepared at the Institute of Electric Engineering, Slovak Academy of Sciences. The microstructure of prepared Ga2O3 films will be influenced by the used substrate materials and by deposition of various buffer layers. These will result in change of the mechanical strain at the interface between Ga2O3 and substrate during the epitaxial growth. The main analytical tools used for this work will be transmission electron microscopy (TEM), scanning electron microscopy (SEM), and chemical elemental analyses with energy and wavelength dispersive X-ray spectroscopy (EDS and WDS). Also studied will be the influence of the chosen Ga2O3 growth conditions on the electrical properties of prepared films, e.g. achieved concentration of charge carriers and their mobility. The work will be carried out in the framework of a joint project with the Taiwanese partner ITRI (Industrial Technology Research Institute).
  • Theme: Preparation and characterization of Ga2O3-diamond heterostructure UV photodetectors
    Supervisor:  Ing. Marian Varga, PhD.  (Department of Microelectronics and Sensors )
    Co-advisors: Ing. Filip Gucmann, PhD.
    Co-advisors: prof. Ing. Alexander Kromka, DrSc.
    Abstract:
    One of the most significant challenges of the recent years, related to the development of new electronic devices and having potential for major improvement of the current status in a vast variety of human activities can be found in the field of solar-blind (SB) ultraviolet (UV) photodetectors (PDs). Novel UV PDs, especially for deep-UV (DUV) optoelectronics (wavelengths shorter than 280 nm) are needed. These cannot typically be manufactured from conventional materials. This resulted in the development of ultrawide bandgap (UWBG) semiconductors (such as AlxGa1−xN, hBN, Ga2O3, and diamond) and their utilisation for SB UV PDs. Gallium oxide (Ga2O3), a naturally n-type semiconductor, represents a suitable UWBG material, showing a very good potential and capability to significantly improve the current state-of-the-art electronic devices. Furthermore, synthetic diamond has gained a strong reputation to be an exceptionally versatile material due to its attractive physical and chemical properties and availability of facile preparation of films with p-type conduction. On the other hand, it is very difficult to prepare p-type Ga2O3 and n-type diamond.
    In this work, we will target the development and detailed characterisation of new high-performance DUV SB PDs with p-n or p-i-n device structure, comprising n– and i-type Ga2O3 and p-type diamond. Such heterostructure devices represent a favourable solution combining the best of the two UWBG worlds. The Ga2O3 layers will be grown by liquid-injection metal-organic chemical vapour deposition (LI-MOCVD), a growth technique developed at IEE SAS. Polycrystalline diamond films will be prepared by MW plasma-enhanced CVD method in close cooperation with the Institute of Physics of the Czech Academy of Sciences in Prague. UV PDs and other electronic devices will be fabricated using modern techniques and tools available at the IEE SAS, i.e. optical or electron-beam lithography, reactive ion etching, and thin films deposition. The designed periodic structures, electric contacts, interdigitated arrays, and van der Pauw test structures will be prepared using a metallic mask prepared by the vacuum sputtering or evaporation through lithographically-prepared masks and patterned by a lift-off process.The main goal of this PhD thesis is the fabrication, characterisation, and deep understanding of the effects at the Ga2O3-diamond heterostructure interface, and to assess their influence on the properties of such a heterojunction. In addition, focus will be also on testing of the properties of prepared UV photodetectors and their radiation hardness when exposed to the ionising radiation.
  • Theme: Study and optimization of detectors of ionizing detectors based on wide bandgap semiconductors SiC and diamond
    Supervisor:  Mgr. Bohumír Zaťko, PhD.  (Department of Microelectronics and Sensors )
    Abstract:
    The aim of the thesis is the technological preparation of ionizing detectors, the study of electrical and detection properties and the influence of radiation dose on their performance. The detection materials used are high quality epitaxial layers of 4H-SiC, polycrystalline and monocrystalline diamond layers. The work will initially focus on the design and fabrication of detection structures based on Schottky contacts. This will be followed by electrical characterization (current-voltage, capacitance-voltage measurements at different temperatures). SiC and diamond are wide bandgap materials that can operate at elevated temperatures. Selected suitable detection structures will be connected to a low noise spectrometric setup and their properties for particle detection at room and elevated temperatures will be evaluated. Subsequent structures will be irradiated with high doses of electron, proton or neutron-based radiation and their properties after irradiation will be investigated. Finally, the radiation hardness will be evaluated and compared with standard silicon detectors.
    The work will be carried out at the Institute of Electrical Engineering Slovak Academy of Sciences, which has modern technological equipment necessary for the preparation and testing of various types of semiconductor structures. The PhD student will acquire knowledge in the preparation of semiconductor detector structures and will be actively involved in teamwork within national and international projects dealing with the preparation and testing of radiation detector structures.
  • Theme: Study of the magnetization and transport alternating losses in advanced
    superconductors
    Supervisor: Mgr. Enric Pardo, PhD. ( Department of Superconductor Physics )
    Co-advisors: Mgr. Ján Kováč, PhD.
    Abstract:
    Progressive superconductors are usually filamentary composite materials with high current densities and critical temperature (Tc) above 30 K allowing the generation of magnetic fields or the transfer of high electric powers without losses, which allows their effective applications in energy. In spite of large number of existing superconducting materials, there are only few of them with Tc > 30 K and produced in long lengths  ³ 1000 m. There are mostly MgB2 (Tc = 39 K), BSCCO (Tc = 80-103 K) and ReBCO (Tc = 90 K) and these materials are applicable for direct and alternating current conditions. In the case of alternating currents, additional losses are generated in superconducting filaments and also in metallic sheaths, which represents the heating and increased cooling costs. Therefore, presented PhD topic will be focussed to studies of alternating current losses in MgB2 and ReBCO superconductors by magnetization and transport measurements above 20 K and quantifications of basic loss parts (magnetic, coupling and from eddy currents). Obtained experimental results will be correlated with theoretical models and potentially also with simulated results. Present topic will be connected with domestic research projects (APVV) and also within the international, e.g. project SCARLET (Superconducting cables for sustainable energy transition), in which the minimalization of alternating losses is extremely important.
  • Theme: Electrical transport in thin layers of some TMDC materials
    Supervisor: Dr. rer. nat. Martin Hulman (Department of Physics and Technology at Nanoscale)
    Abstract:
    In the past two decades, materials characterized by significant dimensional anisotropy have become one of the most extensively studied subjects in solid-state physics and materials research. This category encompasses transition metal dichalcogenides (TMDCs), compounds composed of elements such as S, Se, and Te, in conjunction with metals like Mo, W, and Pt. Substances with the chemical formula MX2 can be synthesized in extremely thin layers.
    At the Institute of Electrical Engineering, a team is dedicated to preparing thin layers of TMDC materials and conducting their characterization. The proposed dissertation would complement the endeavors of this group. The focus of the dissertation lies in examining the electrical transport properties of (ultra)thin layers of TMDC materials, particularly those within the subgroup exhibiting semi-metallic and metallic characteristics. The work predominantly entails experimental research. While it entails some involvement in the growth of thin layers, the primary emphasis is on measuring transport characteristics. In addition to transport characterization, the dissertation will explore the impact of temperature, doping, and structure on the conductivity of 2D materials.
    Throughout the dissertation, the student will engage with vacuum and low-temperature facilities. The experimental findings will be analyzed in accordance with various models of solid-state physics. Furthermore, the student is expected to present their results at both domestic and international conferences, necessitating proficient spoken and written English skills.
  • Theme: Optimization of the TMD- graphene heterostructures fabrication
    Supervisor: Mgr. M. Sojková, PhD. (Department of Microelectronics and Sensors )
    Abstract:
    Unlike metallic graphene, some two-dimensional transition metal dichalcogenides (TMD) exhibit semiconductor properties, offering interesting applications in many areas (electronics, sensing). However, for the application of TMD materials, the limiting factor is the small crystallinity of the layers and rapid chemical degradation due to the external environment. One solution to the degradation problem is the use of a protective layer formed by a material that is perfectly insulating (e.g., graphene, h-BN). In the dissertation, we propose a solution based on the simple idea of producing TMD material under graphene. Such a two-dimensional heterostructure will be prepared in one step, where a thin metal layer is covered with a layer of graphene oxide and subsequently annealed in the presence of a chalcogen. During annealing, a TMD layer is formed, while graphene oxide is reduced to graphene. Graphene will serve as a protective layer and prevent oxidation/degradation of the TMD material. This may allow the production of otherwise unstable TMD materials such as NbSe2 or TaS2. We will study how the graphene layer on the metal surface affects the crystallinity and spatial orientation of the final TMD layers. Moreover, these heterostructures in combination with a suitable insulating bottom layer (SiO2, h-BN) may be suitable for multiple applications. The work will focus on optimizing the preparation of TMD-graphene heterostructures using ultrathin layers of various types of 2D TMD materials (PtSe2, MoS2, NbSe2, TaS2). We will investigate the influence of heterostructure preparation parameters on their properties. Prepared heterostructures will be examined using X-ray diffraction analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, optical measurements, electrical property measurements, and other analyses.
    The work will be carried out at the Institute of Electrical Engineering SAS, which has the necessary technological and characterization equipment. The PhD student will acquire universal skills with a variety of experimental methods and will be actively involved in several projects.

“Physics of Condensed Matter and Acoustics”

  • Theme: Electrical transport in thin layers of some TMDC materials
    Supervisor: Dr. rer. nat. Martin Hulman (Department of Physics and Technology at Nanoscale)
    Abstract:
    In the past two decades, materials characterized by significant dimensional anisotropy have become one of the most extensively studied subjects in solid-state physics and materials research. This category encompasses transition metal dichalcogenides (TMDCs), compounds composed of elements such as S, Se, and Te, in conjunction with metals like Mo, W, and Pt. Substances with the chemical formula MX2 can be synthesized in extremely thin layers.
    At the Institute of Electrical Engineering, a team is dedicated to preparing thin layers of TMDC materials and conducting their characterization. The proposed dissertation would complement the endeavors of this group. The focus of the dissertation lies in examining the electrical transport properties of (ultra)thin layers of TMDC materials, particularly those within the subgroup exhibiting semi-metallic and metallic characteristics. The work predominantly entails experimental research. While it entails some involvement in the growth of thin layers, the primary emphasis is on measuring transport characteristics. In addition to transport characterization, the dissertation will explore the impact of temperature, doping, and structure on the conductivity of 2D materials.
    Throughout the dissertation, the student will engage with vacuum and low-temperature facilities. The experimental findings will be analyzed in accordance with various models of solid-state physics. Furthermore, the student is expected to present their results at both domestic and international conferences, necessitating proficient spoken and written English skills.
  • Theme: Optimization the preparation of magnetic nanostructures for magnonic applications
    Supervisor: Ing. Ján Šoltýs, PhD. (Department of Physics and Technology at Nanoscale)
    Co-advisors : Mgr. Juraj Feilhauer, PhD.
    Abstract:
    Magnonics is an emerging field of science in nanomagnetism that deals with spin waves to transmit, store and information processing. Magnons are the quanta associated with spin waves. They carry information about the collective spin behavior, while they can be modulated through a suitable arrangement of magnetic structures in combination with an external electric and magnetic field. Potential use includes a wide range of applications such as RF components (reconfigurable filters, delay lines), multiplexers, interference-based logic gates, unconventional spin-wave computing, and neuromorphic and quantum computing.
    The work will be focused on designing a suitable shape and arrangement of magnetic nanoelements and also on optimizing their preparation. Electron-beam lithography in combination with other microfabrication technologies such as deposition and etching of metallic layers will be used for their preparation. Process optimization also includes characterization of structures using scanning techniques such as SEM and AFM. The prepared structures will be then studied using various techniques, mainly using a magnetic force microscope.