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Deposition of Ga and GaN ultrathin layers on graphene substrate
Dvořák, Martin ; Nebojsa, Alois (referee) ; Mach, Jindřich (advisor)
This diploma thesis deals with preparation of graphene samples for depositions of ultrathin layers of gallium and gallium nitride. Graphene substrates were prepared by chemical vapour deposition in home-build high temperature reactor. After graphene transfer to silicon wafers, a series of chemical and thermal treatments were performed. Obtained samples were suitable for the study of growth of ultrathin layers of Ga and GaN. The growth of Ga and GaN was realized in ultra high vacuum conditions. Molecular beam epitaxy technique was used for gallium depositions together with ion source for nitridation. Obtained ultrathin layers were studied with X-ray photoelectron spectroscopy, atomic force microscopy and with scanning electron microscopy.
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The deposition of Ga and GaN nanostructures on silicon and graphene substrate
Mareš, Petr ; Hospodková,, Alice (referee) ; Mach, Jindřich (advisor)
Presented thesis is focused on the study of properties of Ga and GaN nanostructures on graphene. In the theoretical part of the thesis a problematics of graphene and GaN fabrication is discussed with a focus on the relation of Ga and GaN to graphene. The experimental part of the thesis deals with the depositions of Ga on transferred CVD-graphene on SiO2. The samples are analyzed by various methods (XPS, AFM, SEM, Raman spectroscopy, EDX). The properties of Ga on graphene are discussed with a focus on the surface enhanced Raman scattering effect. Furthermore, a deposition of Ga on exfoliated graphene and on graphene on a copper foil is described. GaN is fabricated by nitridation of the Ga structures on graphene. This process is illustrated by the XPS measurements of a distinct Ga peak and the graphene valence band during the process of nitridation.
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Diffuse x-ray scattering from GaN epitaxial layers
Barchuk, Mykhailo ; Holý, Václav (advisor) ; Caha, Ondřej (referee) ; Pietsch, Ulrich (referee)
Real structure of heteroepitaxial GaN and AlGaN layers is studied by diffuse x-ray scattering. A new developed method based on Monte Carlo simulation enabling to determine densities of threading dislocations in c-plane GaN and stacking faults in a-plane GaN is presented. The results of Monte Carlo simulations are compared with ones obtained by use of other conventional techniques. The advantages and limitations of the new method are discussed in detail. The methods accuracy is estimated as about 15%. We have shown that our method is a reliable tool for threading dislocations and stacking faults densities determination.
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Extended defects in Ga and Al nitrides
Vacek, Petr ; Holec, David (referee) ; Hospodková,, Alice (referee) ; Gröger, Roman (advisor)
III-nitridy běžně krystalizují v hexagonální (wurtzitové) struktuře, zatímco kubická (sfaleritová) struktura je metastabilní a má pouze mírně vyšší energii. Jejich fyzikální vlastnosti jsou silně ovlivněny přítomností rozsáhlých defektů, které jsou v těchto dvou strukturách od sebe odlišné. U wurtzitových nitridů se jedná primárně o vláknové dislokace. Některé vláknové dislokace tvoří hluboké energetické stavy v zakázaném pásu, kterými ovlivňují elektrické a optoelektronické vlastnosti těchto materiálů. Oproti tomu, kubické nitridy obsahují množství vrstevných chyb, které představují lokální transformace do stabilnější wurtzitové struktury. Cílem této práce je charakterizovat rozsáhlé defekty v obou krystalových strukturách pomocí elektronové mikroskopie, mikroskopie atomárních sil a rentgenové difrakce. Prokázali jsme, že vzorky GaN/AlN a AlN s orientací (0001) rostlé na substrátu Si (111) pomocí epitaxe z organokovových sloučenin obsahují velkou hustotu vláknových dislokací. Nejčastější jsou dislokace s Burgersovým vektorem s komponentou ve směru a wurtzitové struktury, následované dislokacemi s Burgersovým vektorem s komponentou ve směru a+c, zatímco dislokace s Burgersovým vektorem s c komponentou jsou relativně vzácné. Pravděpodobný původ vláknových dislokací je diskutován v souvislosti s různými mechanismy růstu těchto vrstev. Prizmatické vrstevné chyby byly nalezeny v tenkých nukleačních vrstvách AlN, ale v tlustších vrstvách již nebyly přítomny. Na rozhraní AlN / Si byla nalezena amorfní vrstva složená ze SiNx a částečně taky z AlN. Navrhujeme, že by tato amorfní vrstva mohla hrát významnou roli při relaxaci misfitového napětí. Analýza elektrické aktivity rozsáhlých defektů v AlN byla provedena pomocí měření proudu indukovaného elektronovým svazkem. Zjistili jsme, že vláknové dislokace způsobují slabý pokles indukovaného proudu. Díky jejich vysoké hustotě a rovnoměrnému rozložení však mají větší vliv na elektrické vlastnosti, než mají V-defekty a jejich shluky. Topografické a krystalografické defekty byly studovány na nežíhaných a žíhaných nukleačních vrstvách kubického GaN deponovaných na 3C-SiC (001) / Si (001) substrátu. Velikost ostrůvků na nežíhaných vzorcích se zvyšuje s tloušťkou nukleační vrstvy a po žíhání se dále zvětšuje. Po žíhání se snižuje pokrytí substrátu u nejtenčích nukleačních vrstev v důsledku difúze a desorpce (nebo leptání atmosférou reaktoru). Vrstevné chyby nalezené ve vrstvách GaN, poblíž rozhraní se SiC, byly většinou identifikovány jako intrinsické a byly ohraničené Shockleyho parciálními dislokacemi. Jejich původ byl diskutován, jako i vliv parciálních dislokací na relaxaci misfitového napětí. Díky velkému množství vrstevných chyb byly podrobněji studovány jejich interakce. Na základě našich zjištění jsme vyvinuli teoretický model popisující anihilaci vrstevných chyb v kubických vrstvách GaN. Tento model dokáže předpovědět pokles hustoty vrstevných chyb se zvyšující se tloušťkou vrstvy.
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Inverter for electric supercharger with GaN transistors
Galia, Jan ; Pazdera, Ivo (referee) ; Martiš, Jan (advisor)
This master’s thesis deals with the design and realization of a functional sample power inverter for an electric compressor, which is used in hybrid cars. The electric compressor powered by the inverter is E-compressor by Garrett Advancing Motion. An inverter will be using modern High Electron Mobility Transistors which are based on gallium nitride (GaN). The purpose of this thesis is to find if GaN transistors can be used in E-boosting application.
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Step down DC/DC converter with high efficiency
Chudý, Andrej ; Cipín, Radoslav (referee) ; Procházka, Petr (advisor)
This bachelor's thesis deals with design and realization of several versions of DC/DC buck converter. The theoretical part of the bachelor's thesis is focused on the general analysis of DC/DC converters with a detailed description of the buck converters. Another part specializes in the analysis of the current state in this field, where the basic differences between the synchronous and diode rectifier are explained, electromagnetic interference and GaN transistors are described. Subsequently, the thesis involves the design and calculations of necessary parameters of the buck converter. Calculations are verified by simulation and subsequent production of the first prototype. The basic measurements are performed on the prototype. Detected deficiencies of the prototype are removed in version 1.0. In order to achieve higher efficiency, innovated version 2.0 and version with GaN transistors are produced. The thesis is finished by comparing created buck converters.
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Diffuse x-ray scattering from GaN epitaxial layers
Barchuk, Mykhailo ; Holý, Václav (advisor) ; Caha, Ondřej (referee) ; Pietsch, Ulrich (referee)
Real structure of heteroepitaxial GaN and AlGaN layers is studied by diffuse x-ray scattering. A new developed method based on Monte Carlo simulation enabling to determine densities of threading dislocations in c-plane GaN and stacking faults in a-plane GaN is presented. The results of Monte Carlo simulations are compared with ones obtained by use of other conventional techniques. The advantages and limitations of the new method are discussed in detail. The methods accuracy is estimated as about 15%. We have shown that our method is a reliable tool for threading dislocations and stacking faults densities determination.
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Depositon Ga and GaN nanostructures on graphen substrate
Hammerová, Veronika ; Váňa, Rostislav (referee) ; Mach, Jindřich (advisor)
This diploma thesis is focused on deposition Ga and GaN structures on graphene fabricated by method of mechanical exfoliation. For mechanical exfoliation was used new method with using DGL Gel-Film with kinetically controlled adhesion. Ga is deposited by Molecular beam epitaxy with using eusion cell in UHV conditions. GaN was obtained by post-nitridation of Ga islands. These structures were investigated with optical microscope, SEM, Raman spectroscopy and photoluminiscence.
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Experimental switching supply source with power GaN MOSFETs
Matiaško, Maroš ; Martiš, Jan (referee) ; Vorel, Pavel (advisor)
This master’s thesis deals with the design of the switching power supply on the principle of high frequency converter. The goal of this thesis is construction of converter which is using GaN MOSFET transistors and SiC diodes for switching. The converter uses two switch forward power supply topology. Unusually high switching frequency was chosen for the design with power transformer with open magnetic core. The outcome of this work is functional converter which is primarily intended for educational and demonstrational purposes. Multiple parts of this converter are divided into individual blocks, which can be further used for construction of other types of switching converters.
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The deposition of Ga and GaN nanostructures on silicon and graphene substrate
Mareš, Petr ; Hospodková,, Alice (referee) ; Mach, Jindřich (advisor)
Presented thesis is focused on the study of properties of Ga and GaN nanostructures on graphene. In the theoretical part of the thesis a problematics of graphene and GaN fabrication is discussed with a focus on the relation of Ga and GaN to graphene. The experimental part of the thesis deals with the depositions of Ga on transferred CVD-graphene on SiO2. The samples are analyzed by various methods (XPS, AFM, SEM, Raman spectroscopy, EDX). The properties of Ga on graphene are discussed with a focus on the surface enhanced Raman scattering effect. Furthermore, a deposition of Ga on exfoliated graphene and on graphene on a copper foil is described. GaN is fabricated by nitridation of the Ga structures on graphene. This process is illustrated by the XPS measurements of a distinct Ga peak and the graphene valence band during the process of nitridation.
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