<p>This Licentiate thesis summarizes the work carried out by the author the years 2004 to 2008 at the University of Iceland and the Royal Institute of Technology (KTH) in Sweden. The aim of the project was to investigate the structure of sapphire (alpha-Al2O3) surfaces, both for pure scientific reasons and also for potential use as substrate for GaN-growth by molecular beam epitaxy.</p><p>More generally the thesis describes some surface science methods used for investigating the substrates; the general physical back ground, the experi- mental implementation and what information they can give. The described techniques are used for surface analysis on sapphire substrates which have been treated variously in order to optimize them for use as templates for epi- taxial growth of GaN or related III-V compounds.</p><p>The thesis is based on three published papers.</p><p>The first paper focuses on the formation a thin AlN layer on sapphire, which may act as a buffer layer for potential epitaxial growth of GaN or any related III-V materials. Two types of sapphire substrates (reconstructed and non- reconstructed) were exposed to ammonia resulting in the formation of AlN on the surface. The efficiency of the AlN formation (nitridation efficiency) for the two surfaces was then compared as a function of substrate temperature through photoelectron spectroscopy and low electron energy diffraction. The reconstructed surface showed a much higher nitridation efficiency than the non-reconstructed surface.</p><p>In the second paper, the affect of different annealing processes on the sapphire morphology, and thus its capability to act as a template for GaN growth, was studied. Atomic force microscopy, X-ray diffraction analysis together with ellipsometry measurements showed that annealing in H2 ambient and subse- quent annealing at 1300 °C in O2 for 11 hours resulted in high quality and atomically flat sapphire surface suitable for III-V epitaxial growth.</p><p>The third paper describes the effect of argon sputtering on cleaning GaN surfaces and the possibility of using indium as surfactant for establishing a clean and stoichiometric GaN surface, after such sputtering. Soft sputtering, followed by deposition of 2 ML of indium and subsequent annealing at around 500 °C resulted in a well ordered and clean GaN surface while hard sputtering introduced defects and incorporated both metallic gallium and indium in the surface.</p>

<p>This report is the result of a diploma work made at Linköping University from August 2006 till September 2007 by Erik Lundin, under the guidance of Doctor Mike Andersson and Professor Anita Lloyd Spetz. Its purpose was to find suitable materials for the construction of an oxygen sensor. The hope was not to construct such a sensor, but to investigate materials that may be suitable in creating one. In the preparatory time period of the diploma work, different papers and books were studied in order to get a proper understanding of the sensor mechanism. During this period of time, a design proposal was made and the theory behind it is presented in this thesis. The main objective in this thesis has been to investigate the response of field effect devices to oxygen and other gases that are compounds in exhaust or flue gases. Devices were created by employing the materials which were investigated. Special material combinations were proposed for field effect devices suitable for oxygen detection by Doctor Mike Andersson. One material combination showed promising results for selective detection of the oxygen concentration in exhaust gases.</p> / This diploma work has been confidential

This thesis is the result of investigations regarding the processes in InAs III-V semiconductor surfaces induced by additional charge incorporated by adsorbates. The aim of the project is to study the development of the accumulation layer on the metal/InAs(111)A/B surfaces and its electronic structure. InAs(111)A is indium-terminated and InAs(111)B is arsenic-terminated. In addition, InAs(100) is also studied. These three substrates are different; InAs(111)A has a (2x2)-termination, explained by an indium vacancy model, and the clean surface exhibits a two-dimensional electron gas (2DEG). InAs(111)B(1x1) is bulk-truncated and unreconstructed and does not host a 2DEG. InAs(100)(4x2)/c(8x2) exhibits a more covalent character of the surface bonds compared to InAs(111)A/B, and the surface is terminated by a complex reconstruction. Photoelectron spectroscopy and LEED (low energy electron diffraction) have been used as the main tools to study surfaces with sub-monolayer to monolayer amounts of adsorbates. A photoemission peak related to a two-dimensional electron gas appears close to the Fermi level. This 2DEG has in most cases InAs bulk properties, since it is located in the InAs conduction band. A systematic study of core levels and valence bands reveals that the appearance of the 2DEGs is a complex process connected to the surface order. Adsorption of lead, tin or bismuth on InAs(111)B(1x1) induces emission from a 2DEG, but only at monolayer coverage and when the surface is ordered. Cobalt reacts strongly with InAs forming InCo islands and no accumulation is observed. Examination of Cs/InAs(111)B does not reveal any 2DEG and the surface reaction is strongly related to the clean surface stabilization process. Examination of the In-terminated InAs(111)A(2x2) surface shows that In reacts strongly with cobalt and tin adatoms and with oxygen in cases of large exposure, which decreases the 2DEG intensity, while adatoms of cesium and small doses of oxygen enhance the emission from the 2DEG. InAs(100) is terminated with one kind of atom - the InAs(100)(4x2)/c(8x2) is indium terminated. Bismuth creates dimers on the surface and a 2DEG is observed. More generally, this thesis describes some of the general physical background applied to surface science and 2DEG. The first part is a general overview of the processes on the surface. The second part concentrates on the methods related to preparation of samples, and the third part on the measurement methods. The photoelectron spectroscopy part concerns the theory used in mapping electronic structure. The inserted figures are taken from different experiments, including results for InAs(111)A not previously published. / QC 20100910

This report is the result of a diploma work made at Linköping University from August 2006 till September 2007 by Erik Lundin, under the guidance of Doctor Mike Andersson and Professor Anita Lloyd Spetz. Its purpose was to find suitable materials for the construction of an oxygen sensor. The hope was not to construct such a sensor, but to investigate materials that may be suitable in creating one. In the preparatory time period of the diploma work, different papers and books were studied in order to get a proper understanding of the sensor mechanism. During this period of time, a design proposal was made and the theory behind it is presented in this thesis. The main objective in this thesis has been to investigate the response of field effect devices to oxygen and other gases that are compounds in exhaust or flue gases. Devices were created by employing the materials which were investigated. Special material combinations were proposed for field effect devices suitable for oxygen detection by Doctor Mike Andersson. One material combination showed promising results for selective detection of the oxygen concentration in exhaust gases. / This diploma work has been confidential

The magnetic and structural properties of nano magnet arrays and ferromagnetic thin films are investigated. Circular x-rays are used and extensive use is made in this Thesis of the X-ray Magnetic Circular Dichroism (XMCD) technique. By means of the XMCD magneto-optic sum rules the values of the orbital and spin moments are determined. In the case of the nano magnet arrays studied, the XMCD technique is used in a spatially resolved mode using Photo Electron Emission Microscopy (PEEM) after circular light excitation. The Extended X-ray Absorption Fine Structure (EXAFS) is studied in both the Co K- and L-edges. In situ Co L-edge X-ray XMCD spectroscopy measurements are presented, in combination with spectro-microscopy results, on Co/Pt and Co/Au based nano-dot arrays, of typical dot lateral size 250×100 nm2, on self organized Si0.5Ge0.5. The Co is only a few atomic layers thick. The dot arrays display a high degree of lateral order and the individual dots, in several cases, exhibit a stable magnetic moment at 300 K. It is found possible to characterize the spin reorientation of these dot arrays. For both systems the in- versus out-of-plane orbital moment anisotropy, is not always related with an out-of-plane magnetization and the occurrence of a spin reorientation. By performing Co K-edge EXAFS measurements the local atomic structure around the Co atoms is characterized. The feasibility of a high precision quantitative structural analysis of L-EXAFS is studied on the system Au/Co/Au/W(110). The spin reorientation transition is studied as a function of the Co thickness and Au cap thickness. The L-edge EXAFS indicates that this reorientation is correlated to a lattice expansion in the perpendicular direction. High precision angle dependent XMCD work is performed on a high temperature exchange bias system. Pinned or frozen magnetic moments are studied within an exchange biased NiFe ferromagnet at the NiFe/FeMn, ferromagnet/antiferromagnet interface by XMCD and complemented by x-ray resonant reflectivity experiments, at the Ni, Fe and Mn L-edges. The Mn L-edge XMCD MnSb and of (Ga, Mn)As layers modified by high temperature annealing is studied. For MnSb an enhanced value is obtained versus theoretical calculations. This result can be explained by means of the enhanced surface to volume ratio for the samples studied. For (Ga, Mn)As differences are found in the local environment of the Mn atoms upon annealing.

In this thesis, we focus on theoretical study of adsorbates on metal and oxide surfaces that are important for surface chemistry and catalysis. Based on first principles calculations, the adsorption ofCO, NO, NO2, C4H6S2, C22H27SH and other molecules or radicals on nobel metal surfaces (gold and silver) are investigated. Also, NO oxidation on oxygen pre-covered Au(111)surface and CO oxidation on water-oxygen covered Au(111)surface aretheoretically studied. A new mechanism of water-enhanced COoxidation is proposed. As for oxide surfaces, we first investigatethe geometric, electronic and magnetic structures of FeO ultrathin film on Pt(111) surface. The experimentally observed scanning tunneling microscopy images are well reproduced for the first timewith our model. The adsorption and dissociation of water on rutileTiO2(110) surface are investigated by quantum molecular dynamics.By theoretical X-ray photoemission spectroscopy (XPS) calculations,the surface species are properly assigned. The same strategy has applied to the study of the phase transition of water covered reconstructed anatase TiO2(001) surface, from which two different phases are theoretically identified. The structure of graphene oxideis also studied by comparing experimental and theoretical XPS spectra. Based on the novel structures identified, a new cutmechanism of graphene oxide is proposed. / QC 20100819

The dissociation of hydrogen molecules at surfaces is the first step in the absorption process. If the absorbing material is covered by an oxide, this layer will determine the effective uptake rate of an underlying absorbing material. This effect is illustrated when determining the rate of transport of hydrogen through amorphous aluminium oxide layers. The transport rate was determined to be strongly thickness dependent. Hydrogen absorbed in a transition metal causes a volume expansion generated by a strain field around the absorbed hydrogen. This strain field causes a self-trapping of the hydrogen and a temperature dependent distribution in the atomic distances. The local strain field generated by the self-trapping process is found to be crucial for understanding both the hydrogen induced volume expansion as well as the diffusion of hydrogen. Ab-initio molecular dynamics simulations were used to reveal the temperature dependence of the unbinding of the hydrogen and the local strain field and its influence on the diffusion rate. The symmetry of the local strain field is also important for phase formation in metallic films and superlattices which are clamped to a substrate. As the thicknesses reduced from 50 to 10 nm thick vanadium films, substantial finite size effects become apparent in the phase diagrams. The volume change associated with the strain field cannot be accurately measured using x-ray diffraction because of its sensitivity to local arrangements of atoms. X-ray and neutron reflectivity were found to be more reliable probes of global effects of the sumof the local strainfields. Finite size effects in extremely thin V layers were also explored in metallic superlattices composed of iron and vanadium. The co-existence region, composed of a hydrogen gas and a solid-like phase, was found to be suppressed by at least 100 K to below 300 K. The hydrogen-hydrogen interaction can also be influenced by the electronic states in the non hydrogen absorbing layers, as demonstrated when comparing hydrogen absorption in Fe/V and Cr/V superlattices. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 728

In this thesis, density functional theory (DFT) calculations and DFT based ab initio molecular dynamics simulations have been employed in order to gain insights into materials properties like diffusion, adsorption, catalysis, and structure. In transition metals, absorbed hydrogen atoms self-trap due to localization of metal d-electrons. The self-trapping state is shown to highly influence hydrogen diffusion in the classical over-barrier jump temperature region. Li diffusion in Li-N-H systems is investigated. The diffusion in Li3N is shown to be controlled by the concentration of vacancies. Exchanging one Li for H (Li2NH), gives a system where the diffusion no longer is dependent on the concentrations of vacancies, but instead on N-H rotations. Furthermore, exchanging another Li for H (LiNH2), results in a blockade of Li diffusion. For high-surface area hydrogen storage materials, metal organic frameworks and covalent organic frameworks, the hydrogen adsorption is studied. In metal organic frameworks, a Li-decoration is also suggested as a way to increase the hydrogen adsorption energy. In NaAlH4 doped with transition metals (TM), the hypothesis of TM-Al intermetallic alloys as the main catalytic species is supported. The source of the catalytic effect of carbon nanostructures on hydrogen desorption from NaAlH4 is shown to be the high electronegativity of the carbon nanostructures. A space-group optimized ab initio random structure search method is used to find a new ground state structure for BeC2 and MgC2. The fast change between the amorphous and the crystalline phase of GeSbTe phase-change materials is suggested to be due to the close resemblance between the local amorphous structure and the crystalline structure. Finally, we show that more than 80% of the voltage in the lead acid battery is due to relativistic effects. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 702

Structural characterization is essential for the understanding of the magnetic properties of thin films and multilayers. In this thesis, both crystalline and amorphous thin films and multilayers were analyzed utilizing transmission electron microscopy (TEM). High resolution TEM and electron diffraction studies emphasize on the growth of amorphous Fe91Zr9 and Co68Fe24Zr8 on both Al2O3 and Al70Zr30 in multilayer structures by magnetron sputtering. The properties of the growth surfaces were found to strongly influence the formation of nano-crystallites of the magnetic material at interfaces. Field induced uniaxial magnetic anisotropy was found to be possible to imprint into both fully amorphous and partially crystallized Co68Fe24Zr8 layers, yielding similar magnetic characteristics regardless of the structure. These findings are important for the understanding of both growth and magnetic properties of these amorphous thin films. As magnetic systems become smaller, new analysis techniques need to be developed. One such important step was the realization of electron energy-loss magnetic circular dichroism (EMCD) in the TEM, where information about the ratio of the orbital to spin magnetic moment (mL/mS) of a sample can be obtained. EMCD makes use of angular dependent inelastic scattering, which is characterized using electron energy-loss spectroscopy. The work of this thesis contributes to the development of EMCD by performing quantitative measurements of the mL/mS ratio. Especially, methods for obtaining energy filtered diffraction patterns in the TEM together with analysis tools of the data were developed. It was found that plural inelastic scattering events modify the determination of the mL/mS ratio, wherefore a procedure to compensate for it was derived. Additionally, utilizing special settings of the electron gun it was shown that EMCD measurements becomes feasible on the nanometer level through real space maps of the EMCD signal.

The work presented in this thesis deals with experimental and theoretical studies related to alumina thin films. Alumina, Al2O3, is a polymorphic material utilized in a variety of applications, e.g., in the form of thin films. However, controlling thin film growth of this material, in particular at low substrate temperatures, is not straightforward. The aim of this work is to increase the understanding of the basic mechanisms governing alumina growth and to investigate novel ways of synthesizing alumina coatings. The thesis can be divided into two main parts, where the first part deals with fundamental studies of mechanisms affecting alumina growth and the second part with more application-oriented studies of high power impulse magnetron sputter (HiPIMS) deposition of the material. In the first part, it was shown that the thermodynamically stable α phase, which normally is synthesized at substrate temperatures of around 1000 °C, can be grown using reactive sputtering at a substrate temperature of merely 500 °C by controlling the nucleation surface. This was done by predepositing a Cr2O3 nucleation layer. Moreover, it was found that an additional requirement for the formation of the α phase is that the depositions are carried out at low enough total pressure and high enough oxygen partial pressure. Based on these observations, it was concluded that energetic bombardment, plausibly originating from energetic oxygen, is necessary for the formation of α-alumina (in addition to the effect of the chromia nucleation layer). Moreover, the effects of residual water on the growth of crystalline films were investigated by varying the partial pressure of water in the ultra high vacuum (UHV) chamber. Films deposited onto chromia nucleation layers exhibited a columnar structure and consisted of crystalline α-alumina if deposited under UHV conditions. However, as water to a partial pressure of 1*10-5 Torr was introduced, the columnar α-alumina growth was disrupted. Instead, a microstructure consisting of small, equiaxed grains was formed, and the γ-alumina content was found to increase with increasing film thickness. To gain a better understanding of the atomistic processes occurring on the surface, density functional theory based computational studies of adsorption and diffusion of Al, O, AlO, and O2 on different α-alumina (0001) surfaces were also performed. The results give possible reasons for the difficulties in growing the α phase at low temperatures through the identification of several metastable adsorption sites and also show how adsorbed hydrogen might inhibit further growth of α-alumina crystallites. In addition, it was shown that the Al surface diffusion activation energies are unexpectedly low, suggesting that limited surface diffusivity is not the main obstacle for low-temperature α-alumina growth. Instead, it is suggested to be more important to find ways of reducing the amount of impurities, especially hydrogen, in the process and to facilitate α-alumina nucleation when designing new processes for low-temperature deposition of α-alumina. In the second part of the thesis, reactive HiPIMS deposition of alumina was studied. In HiPIMS, a high-density plasma is created by applying very high power to the sputtering magnetron at a low duty cycle. It was found, both from experiments and modeling, that the use of HiPIMS drastically influences the characteristics of the reactive sputtering process, causing reduced target poisoning and thereby reduced or eliminated hysteresis effects and relatively high deposition rates of stoichiometric alumina films. This is not only of importance for alumina growth, but for reactive sputter deposition in general, where hysteresis effects and loss of deposition rate pose a substantial problem. Moreover, it was found that the energetic and ionized deposition flux in the HiPIMS discharge can be used to lower the deposition temperature of α-alumina. Coatings predominantly consisting of the α phase were grown at temperatures as low as 650 °C directly onto cemented carbide substrates without the use of nucleation layers. Such coatings were also deposited onto cutting inserts and were tested in a steel turning application. The coatings were found to increase the crater wear resistance compared to a benchmark TiAlN coating, and the process consequently shows great potential for further development towards industrial applications.