Defenses PhD Thesis

271
  • August 23, 2023 at 10,00 a.m.

Ing. Ondrej Pohorelec
Supervisor: RNDr. Dagmar Gregušová, DrSc.
Theme: Názov: Hole channel GaN-based transistor for power electronics
Abstract:
This dissertation thesis deals with threshold voltage instabilities in normally-off transistor with unintentional hole channel and a preparation of InAlN/GaN transistor with intentional hole channel. Theoretical parts summarize properties of the III-N materials, methods to prepare normally-off transistors, hole channel transistors, Mg doping and experimental methods used in this work. First part of the results deals with threshold voltage instabilities in normally-off transistor with InGaN cap and an unintentional hole channel. Unexpected threshold voltage shift was explained by a model where the unintentional 2DHG plays an important role. In second part of the results, InAlN/GaN heterostructures were grown in order to prepare a transistor with hole channel. Mg doping was used to increase hole density in the channel. Hole type conductivity was not confirmed in any of the samples. In the third part of the results, In-rich InAlN was doped with Mg to compensate intrinsic donors and to increase resistivity. Resistivity increase of 2 orders of magnitude was caused by decrease in mobility caused by defects introduced by Mg doping. Higher Mg precursor flows during growth severely degrade structural and electrical properties of the samples.

 

  • August 24, 2023 at 9,30 a.m.

Mgr. Fridrich Egyenes
Supervisor: Ing. Milan Ťapajna, PhD.
Theme: Technology and transport properties of Ga2O3 semiconductors for new generation of power electronics
Abstract:
Gallium oxide (Ga2O3) belongs to ultra-wide-bandgap semiconductors. In this work, we discuss thin Ga2O3 films grown on non-native substrates (sapphire – Al2O3) by liquid-injection metal-organic-chemical-vapor-deposition. These films were prepared in Institute of Electrical Engineering Slovak Academy of Sciences. Ultra-wide-bandgap character of Ga2O3 semiconductor is promising for high-power and high-voltage device applications. There are several polymorphs of Ga2O3. We focus on the metastable rhombohedral Ga2O3 and the monoclinic Ga2O3, which is the most stable Ga2O3 polymorph. With an aim to enhance the electrical performance, we discuss the development of Ga2O3 processing technology, where the crucial steps include Si doping and post-deposition annealing in forming gas (90 % N2 + 10 % H2). Annealing in forming gas led to passivation of acceptor defects formed by Ga-vacancies. Strong localization of electrons was observed in the Si-doped monoclinic Ga2O3, which was attributed to strong disorder. In the case of rhombohedral Ga2O3, limited stability was observed by thickness-induced vertical recrystallization to monoclinic crystal. Such recrystallization led to local filament-like monoclinic cluster formation with significantly higher conductivity, which resulted in strong electrical anisotropy. Finally, new approach for growth of monoclinic Ga2O3 on rhombohedral Ga2O3 films with advantageous orientation is introduced.

 

  • August 24, 2023 at 11,30 a.m.

Iuliia Vetrova
Supervisor: Ing. Ján Šoltýs, PhD.
Theme: Preparation of magnetic micro- and nano-scale objects for novel magnetic devices
Abstract:

This thesis is focused on the formation and detection of non-colinear magnetic states in patterned nanoelements. We experimentally and numerically demonstrated the formation of the skyrmion state in the nanodisks composed of six repetitions of Pt/Co/Au multilayer. This combination of layers gives non-zero DMI and significant dipole interaction to achieve skyrmion-stable states. Furthermore, we numerically studied the influence of the disk diameter on the evolution of self-nucleation of magnetic states and on their energy levels at room temperature. These results are qualitatively consistent with the states observed in the experiment. In addition, it was studied the possibility of inducing a skyrmion state in the nanodisk using the stray field of the MFM tip.
The second part of this work aimed to develop novel vortex-core MFM tips, which should guarantee constant magnetic moments, high resolution and a long lifetime. First, a focused ion beam was used to form the commercial conical AFM tips into cylindrical tips. Subsequently, a ferromagnetic material was deposited on the tip, creating a magnetic submicron disk on the tip. The vortex core that spontaneously formed in the center of the disk served as a sensing element for mapping the magnetic field of samples with submicron domain structures. We further improved the tip properties by modifying the ferromagnetic disk to minimize the deflection of the vortex core during scanning. Finally, we performed an experiment in which we scanned a magnetically soft sample with micron-sized domains. It has been shown that such a ferromagnetic disk tip is less invasive than conventional low-moment scanning tips. We also explained why the tip highlights domain walls when scanning larger domains. We also proved its durability and suitability for repeated scanning of large areas without wearing out the disk tip. Simulations support the results of MFM measurements.