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The good control of the n-type doping is a key issue for the fabrication of efficient devices based on e-Ga2O3 epilayers. In this work we studied the possibility of doping the e-Ga2O3 thin films, epitaxially grown on c-oriented sapphire by metal-organic chemical vapor deposition, by means of a post-deposition treatment. For the first time, the n-type doping was achieved by depositing a tin-rich SnO2 film on top of the e-Ga2O3 layer and keeping this bi-layer system for 4 h at a temperature of 600 °C in an evacuated furnace. The diffusion of Sn atoms into the e-Ga2O3 film is evidenced by time-of-flight secondary-ion mass spectrometry depth profiles. Room-temperature resistivity of the order of 1 Ω•cm is obtained and the electrical characterization revealed a conduction mechanism based on variable range hopping, according to the Mott's model.

Low resistivity n-type epsilon-Ga2O3 epilayers were obtained for the first time either by adding silane to the gas phase during the metal organic vapour phase epitaxy deposition or by diffusing Sn in nominally undopeci, layers after the growth. The highest doping concentrations were few 10(18) cm(-3) and about 10(17) cm(-3) for Si and Sn doping, with corresponding resistivity below 1 and 10 Omega cm, respectively. Temperature dependent transport investigation in the range of 10-600 K shows a resistivity behavior consistent with the Mott law, suggesting that conduction through localized states dominates the electrical properties of Si- and Sn-doped samples. For both types of dopants, two different mechanisms of conduction through impurity band states seem to be preserit, each of them determining the transport behavior at the lower and higher temperatures of the measurement range.

The electronic structure of espilon-Ga2O3 thin films has been investigated by ab initio calculations and photoemission spectroscopy with UV, soft, and hard X-rays to probe the surface and bulk properties. The latter measurements reveal a peculiar satellite structure in the Ga 2p core level spectrum, absent at the surface, and a core-level broadening that can be attributed to photoelectron recoil. The photoemission experiments indicate that the energy separation between the valence band and the Fermi level is about 4.4 eV, a valence band maximum at the ? point and an effective mass of the highest lying bands of - 4.2 free electron masses. The value of the bandgap compares well with that obtained by optical experiments and with that obtained by calculations performed using a hybrid density-functional, which also reproduce well the dispersion and density of states

Electrical and optical properties of undoped single-phase epsilon-Ga2O3 epitaxial films prepared by MOCVD are reported. It is shown that this still unexplored polymorph of gallium oxide possesses wide bandgap and very high dark resistivity, thus allowing the design and fabrication of solar-blind UV photodetectors. Simple cost-effective photoresistors, fabricated by direct deposition of the epilayers on c-oriented sapphire substrates, exhibited good performance. The physical properties and the photoresponse of epsilon-Ga2O3 make this material very interesting in view of novel applications. (C) 2017 Elsevier B.V. All rights reserved.

The thermal stability of ?-Ga2O3 polymorph was studied by complementary methods. Epitaxial films of ?-Ga2O3 grown on c-oriented sapphire were annealed at temperatures in the range 700-1000 °C and then investigated by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). In addition, Differential Scanning Calorimetry (DSC) up to 1100 °C was carried out on fragments of pure ?-Ga2O3 taken from a very thick layer. The results clearly indicate that ?-Ga2O3 initiates modifying its crystallographic structure above 650 °C, as demonstrated by a mild endothermic bent of the DSC curves. However, the effective transition to ?-phase occurs quite suddenly at 880-920 °C, depending of the DSC heating rate. XRD and TEM results confirm this evidence. TEM investigation in particular shows that after annealing at 1000 °C and rapid cooling the film is completely made of ?-Ga2O3 grains, most of them with orientation (310) respect to the sapphire substrate. However, if the cooling rate is substantially reduced, the converted ?-Ga2O3 layer assumes the standard orientation (-201) parallel to (00.1) of the Al2O3 substrate. Based on the results of this study we conclude that ?-Ga2O3 may conveniently be used for device fabrication, exploiting its higher crystallographic symmetry and better matching to sapphire. However, all fabrication steps must be carried out at temperatures below 700 °C. © 2017 Acta Materialia Inc.

The crystal structure and ferroelectric properties of epsilon-Ga2O3 deposited by low-temperature MOCVD on (0001) sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga2O3 showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga3+ sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P6(3)mc pointed out the presence of uncompensated electrical dipoles suggesting ferroelectric properties, which were finally demonstrated by independent measurements of the ferroelectric hysteresis. A clear epitaxial relation is observed with respect to the c-oriented sapphire substrate, with the Ga2O3 [10-10] direction being parallel to the Al2O3 direction [11-20], yielding a lattice mismatch of about 4.1%.

Growth of gallium oxide thin films was carried out by Metalorganic Chemical Vapor Deposition (MOCVD) at different temperatures. Pure epsilon-phase epilayers of Ga2O3, with good morphology and structural properties, were obtained, for the first time with this technique, on sapphire at the temperature of 650 degrees C. XRD analysis performed by high-resolution diffractometry confirmed the good crystallographic quality of the grown layers. At temperatures higher than 700 degrees C the usual stable beta-Ga2O3 phase was obtained. The epsilon-films were successfully deposited also on (0001)-oriented GaN and (111)- and (001)-oriented 3C-SiC templates, provided that the appropriate temperature was chosen. This indicates that the temperature, rather than substrate structure, is the growth parameter which decides what phase actually forms. The growth proceeds via coalescence of hexagonal islands and is favored when a substrate with an in-plane hexagonal arrangement of the atoms is employed. By applying Atomic Layer Deposition (ALD), epitaxial growth of the e-phase was achieved at lower temperature, while the overall uniformity resulted improved, even on large sapphire substrates. (C) 2016 Elsevier B.V. All rights reserved.

In this paper, we focus on the growth of ?- and ?/?-Ga2O3 thin films via metal-organic vapor phase epitaxy on c-plane sapphire using water and trimethyl-gallium at temperatures between 610 and 650 °C. Using these precursors, the monoclinic ?-phase is usually obtained only at temperatures higher than 700 °C. We show here, for the first time, that both ?- and ?-Ga2O3 can also be obtained by tuning the growth rate of the film, that is, by controlling the supersaturation of the vapor phase. The experimental findings are discussed in the framework of classical nucleation theory and Ostwald's step rule, showing the interplay of thermodynamic (related to different chemical potentials for the metastable ?/? phase and the stable ? phase) and kinetic effects (mainly related to different surface energy barriers for nuclei of different crystallographic phases/ planes). The experimental conditions that permit the nucleation and growth of the desired Ga2O3 polymorph are identified and thoroughly explained, giving to this work a fundamental as well as a technological relevance.

Heterostructures made of GaN and epsilon-Ga2O3 epitaxial layers may be very interesting because they could exploit the high electron mobility of GaN combined with the ferroelectric character of epsilon-Ga2O3. We have explored the possibility of using epsilon-Ga2O3 templates, deposited by metalorganic chemical vapor deposition on sapphire substrates, in order to reduce the lattice mismatch of GaN with sapphire. Considering that epsilon-Ga2O3 is metastable and undergoes a first phase transition at around 700 °C, the GaN layers were deposited at two different temperatures (690°C, 1050 °C). Preliminary electrical and SIMS investigations have enced the diffusion of oxygen from the epsilon-Ga2O3 to the GaN epitaxial layer, which results in an n-type conductivity and a sheet resistance as low as 70 Ohm/sq in a 1 ?m thick GaN layer. The rocking curve of the 2 GaN layers grown epsilon-Ga2O3/sapphire at standard high temperature (1050ºC) indicates a crystal quality worse than for GaN deposited directly on sapphire. In parallel, we studied the nucleation of epsilon-Ga2O3 on GaN templates. We evidenced that epsilon-Ga2O3 nucleates in 3D islands on the surface of GaN grown on on-axis sapphire, with coalescence taking place as the layer grows thicker. The use of off-cut sapphire substrates, instead, permits to inhibit islands formation, resulting in a smoother layer. The possibility of obtaining uniform and very thin epsilon-Ga2O3 layers on GaN layers opens interesting possibilities for the development of novel high electron mobility transistors (HEMT).

In this work, the conduction mechanisms across novel contacts to epitaxial films of pure phase epsilon-Ga2O3 (ε-Ga2O3) were investigated. Different structures made by sputtered metal and oxide thin films were tested as electrical contacts. I-V characteristics show heterogeneous behaviors, revealing different conduction mechanisms according to the applied bias. The results are interesting as they offer a viable method to obtain ohmic contacts on ε-Ga2O3, which is less studied than other gallium oxide polymorphs but may find application in new electronic and optoelectronic devices. The newly developed ohmic contacts allow to fabricate simple test devices and assess the potential of this material.

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 degrees C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called epsilon phase; i.e. they exhibited hexagonal P6(3)mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga2O(3) layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5-10 nm size. Each domain has orthorhombic structure with Pna2(1) space group symmetry, referred to as kappa-Ga2O3. Further XRD analysis carried out on thicker samples (9-10 mu m) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together - if the domain size falls below the actual resolution of the probing techniques - can be misinterpreted as the disordered structure with its P6(3)mc space group symmetry usually referred to as epsilon-Ga2O3 in the current literature. The crystal structure of these Ga2O3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna2(1) space group and previous observations.

��bergangsmetalloxide zeichnen sich durch eine Vielzahl unterschiedlicher magnetischer und elektrischer Eigenschaften aus. Diese sind von gro��em Interesse da sie die Grundlage f��r Anwendungen in der Industrie, Medizin und Computertechnik liefern. Die vorliegende Arbeit untersucht zwei in dieser Hinsicht viel versprechende Materialien, TiO2 und GaFeO3. Beide zeichnen sich durch einen Grundzustand aus der nicht magnetisch und halbleitend ist. Dotiert man beide System, so ver��ndern sich Ihre magnetischen und elektrischen Eigenschaften. TiO2 ist vorwiegend f��r seine photokatalytischen Eigenschaften bekannt, w��hrend GaFeO3 als magnetoelektrisches multiferroisches Material immer mehr an Bedeutung gewinnt. Der erste Teil der Arbeit besch��ftigt sich mit der elektronischen und magnetischen Struktur von Kohlenstoff and Stickstoff dotiertem Rutil TiO2. Dabei werden substitutionelle als auch interstitielle Fehlstellen untersucht. Hierzu werden ab-initio Berechnungen an einer 48-atom gro��en Superzelle mit Hilfe des VASP Codes durchgef��hrt. Um eine realistische Beschreibung der physikalischen Eigenschaften zu erhalten, wird der Austausch-Korrelations Anteil mit dem HSE06 hybrid Funktional angen��hert. Ersetzt man nun ein Sauerstoffatom mit einen Kohlenstoff- oder Stickstoffatom so wird ein magnetisches Moment induziert. W��hrend Kohlenstoff ein Gesamtmoment von 2mueB pro Superzelle erzeugt, liefert Stickstoff ein magentisches Moment von 1mueB. Besetzt man mehrer Sauerstoffpl��tze so zeigen beide Atome die Tendenz einer langreichweitige antiferromagnetischen Kopplung. Im Fall von interstitielle Fehlstellen, verliert Kohlenstoff sein magnetisches Moment, w��hrend Stickstoff weiterhin ein Moment von 1 mueB pro Superzelle erzeugt. F��r die interstitielle Fehlstellen finden sich zwei L��sungen. Eine beschreibt einen Sattelpunkt auf dem das Fremdatom zwischen den benachbarten Titan und Sauerstoffatomen liegen bleibt. Die stabile L��sung hingegen ist jene bei der der Kohlen- und Stickstoff eine Bindung mit einem der benachbarten Sauerstoffatome eingeht. Dadurch formen sich C-O und N-O Dimere deren Bindugsl��ngen sehr ��hnlich jenen von doppel-gebundenen CO und NO Molek��len ist. Dieses Ergebnis best��tigt die schon in XPS-Experimenten nachgewiesenen N-O Komplexen. Dadurch motiviert wurden die dazugeh��rigen symmetrischen Streckschwingungsfrequenzen ausgerechnet, um eine zuk��nftig m��gliche experimentelle Verifikation zu erleichtern. F��r alle untersuchten Dotierungskonfiguration wurden Kohlenstoff und Stickstoff Zust��nde in der TiO2 Bandl��cke gefunden. Diese neu induzierte Zust��nde werden im Zusammenhang mit den photokatalytischen Eigenschaften von TiO2 diskutiert. Der zweite Teil der Arbeit untersucht die magnetischen und elektrischen Eigenschaften von GaFeO3 . Wir beginnen mit ab initio DFT Berechnungen durchgef��hrt an stoichiometrischen GaFeO3 . Im Detail wird dabei der Ursprung des antiferromagnetischen (AFM) Grundzustandes untersucht. Mit Hilfe der erhaltenen Ergebnisse l��sst sich der daf��r verantwortliche Superaustausch in einem k��nnen f��r neue technische Anwendungen attraktiv sein. Es werden vier verschiedene Sauerstoffsubstitutionen pro Dotierungsatom untersucht, wobei sich herausstellt, dass die resultierenden magnetischen Eigenschaften vom Gittergsplatz abh��ngen. Kohlenstoff kann dabei anstatt einer AFM Kopplung, wie im Falle des Fe1-O-Fe2 Komplexes, eine ferrimagnetische Wechselwirkung in der Fe1-C-Fe2 Verbindung induzieren. Abh��nging von der Dotierungsstelle f��hrt Kohlenstoff zu einer AFM oder ferrimagnetischen Kopplung zwischen den benachbarten Eisenatomen. Auch Stickstoff beeinflusst die magnetische Kopplung der Eisenatome, w��hrend Schwefel gro��e strukturelle Deformationen in der Zelle mit sich bringt. All dies hat Auswirkungen auf die Bandl��cke und die magnetischen Kopplungen in GaFeO3. Die jeweiligen Austauschmechanismen werden dabei im Detail beschrieben und eine m��gliche Verwendung als Photokatalystator diskutiert. Zus��tzlich wird wieder eine Stauchung und Streckung der dotierten Zelle simuliert und die magnetischen Anisotropien berechnet. Die an beiden Systemen durchgef��hrten Simulationen und Berechnungen stimmen mit den Ergebnissen zahlreicher experimenteller Studien ��berein. Neben unterst��tzender Rechnungen die oft Klarheit ��ber Experimente bringen, werden ab initio DFT Berechnungen in dieser Arbeit auch als Inovationswerkzeug verwendet. Dadurch sollen bisher im Experiment nicht untersuchte Konfigurationen simuliert werden und Ansto�� f��r zuk��nftige Verwendungen und weitere Untersuchungen liefern., Transition metal oxides show a big varitey of physical properties that are interesting for a multitude of applications, including industries, medicine and electronic devices. This thesis investigates the properties of two promising compounds, TiO2 and GaFeO3. Both are non magnetic semiconductors in their ground state, but show different magnetic and electronic properties incorporating impurity atoms. TiO2 is mostly known as a photocatalyst and GaFeO3 as a magnetoelectric multiferroic. The first part of the thesis studies the electronic and magnetic structure of carbon and nitrogen doped rutile TiO2. We investigate the effects of substitutional and interstitial anion doping. To this end we perform ab-initio calculations of a 48-atom supercell employing the VASP code. In order to obtain a realistic description of the electronic and magnetic structure, exchange and correlation are treated with the HSE06 hybrid functional. Substitutional carbon and nitrogen are found to have a magnetic moment of 2 and 1mueB, respectively, with a tendency for antiferromagnetic long range order. For C/N on interstitial sites we find that carbon is non-magnetic while nitrogen always possesses a magnetic moment of 1mueB. We find that these interstitial positions are on a saddle point of the total energy. The stable configuration is reached when both carbon and nitrogen form a C-O and N-O dimer with a bond length close to the double bond for CO and NO. This result is in agreement with earlier experimental investigations detecting such N-O entities from XPS measurements. The frequencies of the symmetric stretching mode are calculated for these dimers, which could provide a means for experimental verification. For all configurations investigated both C and N states are found inside the TiO2 gap. These new electronic states are discussed with respect to tuning doped TiO2 for the application in photocatalysis. The second part of the thesis studies studies GaFeO3. We start with \textit{ab initio} DFT calculations on stoichiometric GaFeO3. A detailed discussion of the origin of the antiferromagnetic (AFM) superexchange in stoichiometric GaFeO3 is given, including a molecular orbital description of the exchange mechanism derived from our calculations. In addition we study the properties of the Fe-O-Fe bonds for different geometries to underline the angle and distance dependence of the AFM coupling as formulated in the Goodenough-Kanamori rules. We describe the AFM ground state of GaFeO3 as a result of two intrinsic Fe-O-Fe chains that meander through the crystal along the c direction. The magnetocrystalline anisotropy energies are calculated for the stoichiometric phase with and without inner cationic site disorder. Furthermore, we show the presence of a sub-lattice dependent anisotropy. Moreover, we perform studies on cation doped Ga2-xFexO3 for varying Fe concentrations x (0.0 < x < 2.0). At a value of x=0.0 and x=2.0 GaFeO3 transforms into the isomorphic epsilon-Ga2O3 and epsilon-Fe2O3 phase, respectively. The effect of strain was also studied. Incorporating dopants and applying strain to the simulation cell changes the intrinsic geometry and thus the magnetic properties of gallium ferrite. Subsequently we examine the effect of anion doping substituting O by a C, N and S atom, respectively. Replacing the superexchange mediating O atom with p-elements of a different valence electron configuration changes the underlying magnetic exchange mechanism and influence the ground state properties. This may be used for tuning properties interesting for technical applications. Four different doping configurations were examined revealing a cell site dependent influence on the magnetic properties. Carbon, for example, changes the AFM coupling present in the Fe1-O-Fe2 configuration into a ferrimagnetic exchange for the Fe1-C-Fe2 bond. Depending on the respective cell site C substitution introduces a ferrimagnetic or AFM ground state. Nitrogen alters the ground state magnetic moment as well and Sulfur introduces large structural distortions affecting the band gap and the overall AFM coupling inside the doped GaFeO3 simulation cell. We give a detailed discussion on the respective magnetic exchange mechanisms and electronic properties with regard to applications as photocatalysis. Further, we again investigate the effect of strain and determine a possible change of the magnetocrystalline anisotropy energies. The performed calculations and results obtained for both investigates systems agree with previous experimental studies. In addition to that we use the predictive power of ab initio DFT simulations, examine doping configurations that have not been investigated yet. That, however, may trigger future experiments in the very promising field of tunable multifunctional devices and photocatalysis.

We report the first formation of arrays of GaN nanorods inside the nanoscale channels of mesoporous silica SBA-15. GaCl3 dissolved in toluene was incorporated into the methyl group-functionalized SBA-15 powder. The pore surfaces functionalized with methyl groups should facilitate the impregnation with GaCl3. Formation of GaN nanorod arrays within SBA-15 was carried out by heating the powder to 700 degrees C for 3 h under nitrogen atmosphere, followed by ammonolysis at 900 degrees C for 5 h. epsilon-Ga2O3, an unusual phase for Ga2O3, formed after the first thermal process and was converted into wurtzite GaN during ammonolysis. The final products have been characterized by FT-IR spectra, powder XRD patterns, TEM images and SAED patterns, EDS analysis, and nitrogen adsorption-desorption isotherm measurements to confirm the presence of GaN nanostructures. The nanorods are 6-7.5 nm in diameter, and can be a few hundreds of a nanometer in length to exhibit nanowire structure. Free-standing GaN nanorod arrays were revealed upon removal of the silica framework with HF solution. Optical characterization of the isolated GaN nanorod arrays shows a strong and sharp near band-edge emission at 375 nm, and two phonon-assisted donor-acceptor peaks at 395 and 415 nm. A broad but weak emission in the region of 335-360 nm due to the quantum confinement effect of short nanorods was observed.