Interface Studies of GaAs/MgO-Systems for Metal-Oxide-Semiconductor Capacitor (MOSCAP) Applications

The sub-microscale miniaturisation of the metal-oxide-semiconductor field-effect transistor opens the way to small scale electronic devices. The miniaturisation forces the whole structure to downscale, including the oxide layer between the metal gate and the semiconductor substrate. This oxide functions as a capacitor allowing to induce charge in the semiconductor via the field effect. However, as the layer becomes thinner, the leakage current increases which is detrimental for the performance of the device. Oxides with higher dielectric constants can overcome this problem, however their introduction has been an enormous challenge. Furthermore, the semiconductor substrate is upgraded by using III-V semicondcutors for their high carrier mobility improving device performance. This PhD investigates the role of the interface between the semiconductor (GaAs) and the oxide layer (MgO) on the properties of the capacitor. The usage of complementary characterisation techniques yields detailed knowledge on the interplay between the structural, chemical and electrical characteristics in the developed heterostructures with an emphasis on the oxide and semiconductor interface. It was found that the structural and electrical properties are strongly correlated and this relationship plays a crucial role in this research.status: publishe

Analysis of the variation in nanohardness of pearlitic steel: Influence of the interplay between ferrite crystal orientation and cementite morphology

The influence of the relative orientation of the ferrite crystallite lattice and the cementite lamellae on the hardness of pearlitic steel has been investigated by a combination of nanoindentation and electron microscopy (electron back scatter diffraction (EBSD) and scanning electron microscopy (SEM)). Three pearlitic samples, each with a different interlamellar spacing, and one ferritic sample were nanoindented. Although the hardness of the ferritic sample is very similar at different spots on the sample, a large variation in hardness is obtained on each of the pearlitic samples. It has been found that this variation cannot be accounted for solely by the variation in interlamellar spacing and is related to differences in ferrite crystal orientaiton. As to explain the observed large variation in hardness, the ferrite crystal orientation was considered relative to the cementite lamellae orientation by calculation of the distance dislocations can glide between adjacent lamellae in the slip direction. However, no clear correlation was found for a scaling of this orientation factor with the hardness. Possible interpretations of this discrepancy are suggested

Adsorption of Te atoms on Au(111) and the emergence of an adatom-induced bound state

We report on the adsorption of Te adatoms on Au(1 1 1), which are identified and investigated relying on scanning tunnelling microscopy, Auger electron spectroscopy, and density functional theory. The Te adatoms lift the 23 × √3 surface reconstruction of the Au(111) support and their organization is similar to that of previously reported chalcogen adatoms on Au(1 1 1), which are also known to lift the herringbone reconstruction and can adopt a (√3 × √3)R30° structure. The adatoms show strong interaction with the Au(1 1 1) surface, resulting in scattering and confinement of the Au surface state (SS) electrons near the Fermi level. More remarkably, scanning tunnelling spectroscopy reveals the existence of an electronic resonance at high voltages well above the Fermi level. This resonance can be interpreted as a bound state that is split off from the bottom of the Au(1 1 1) bulk conduction band. A similar split-off state may exist for other types of adatoms on metallic surfaces that exhibit a surface band gap.journal_title: Journal of Physics: Condensed Matter article_type: paper article_title: Adsorption of Te atoms on Au(1 1 1) and the emergence of an adatom-induced bound state copyright_information: © 2017 IOP Publishing Ltd date_received: 2016-08-17 date_accepted: 2016-11-07 date_epub: 2017-02-08status: publishe

Analysis of the variation in nanohardness of pearlitic steel: Influence of the interplay between ferrite crystal orientation and cementite morphology

The influence of the relative orientation of the ferrite crystallite lattice and the cementite lamellae on the hardness of pearlitic steel has been investigated by a combination of nanoindentation and electron microscopy (electron back scatter diffraction (EBSD) and scanning electron microscopy (SEM)). Three pearlitic samples, each with a different interlamellar spacing, and one ferritic sample were nanoindented. Although the hardness of the ferritic sample is very similar at different spots on the sample, a large variation in hardness is obtained on each of the pearlitic samples. It has been found that this variation cannot be accounted for solely by the variation in interlamellar spacing and is related to differences in ferrite crystal orientation. As to explain the observed large variation in hardness, the ferrite crystal orientation was considered relative to the cementite lamellae orientation by calculation of the distance dislocations can glide between adjacent lamellae in the slip direction. However, no clear correlation was found for a scaling of this orientation factor with the hardness. Possible interpretations of this discrepancy are suggested.publisher: Elsevier articletitle: Analysis of the variation in nanohardness of pearlitic steel: Influence of the interplay between ferrite crystal orientation and cementite morphology journaltitle: Materials Science and Engineering: A articlelink: http://dx.doi.org/10.1016/j.msea.2014.08.019 content_type: article copyright: Copyright © 2014 Elsevier B.V. All rights reserved.status: publishe

Auger electron spectroscopy study of semiconductor surfaces: Effect of cleaning in inert atmosphere

In this paper, the authors demonstrate that Auger electron spectroscopy (AES) is an effective characterization tool in the analysis of the cleaning of semiconductor surfaces under different atmospheres. AES has several advantages for this purpose: it is nondestructive, surface specific {the analysis depth is only 4–50 Å [Childs et al., Handbook of Auger Electron Spectroscopy (Physical Electronics, Eden Prairie, MN, 1995)]}, and very sensitive to common contaminants such as carbon and oxygen. Furthermore, the authors have proven that AES allows us to describe the effectiveness of surface cleaning in a quantitative manner by comparing the peak-to-peak height of the oxygen signal for different samples. In this work, the surface cleaning of five semiconductors, namely, Si, Ge, GaAs, In0.5Ga0.5As, and In0.5 Al 0.5As, was investigated. The same standard HF cleaning procedure was applied in two different atmospheres, air or nitrogen. The latter was used to prevent reoxidation after cleaning. The authors found that for most of these semiconductors, the atmosphere in which the cleaning is performed has a significant influence on the results, reducing the oxygen peak-to-peak height with an extra 11.18% (average of all the semiconductors investigated) when comparing cleaning in N2 to cleaning in air. Complementary characterization of the effectiveness of the cleaning procedure was accomplished by in situ AES, atomic force microscopy, and reflection high-energy electron diffraction for GaAs samples.status: publishe

Direct Liquid Injection - Low Pressure Chemical Vapor Deposition of Silica Thin Films from Di-t-butoxydiacetoxysilane

In this work, an unusual silicon chemical vapor deposition precursor is used, which allows the safe deposition of thin silica films in a controlled and reproducible manner at a lower thermal budget with a newly developed direct liquid injection - low pressure chemical vapor deposition system. The deposition is controlled by parameters such as deposition temperature, partial pressure of the gases, and flow rate of the precursor solution. X-ray reflectivity and spectroscopic ellipsometry of the deposited samples show that the thickness of the layers is well controlled by deposition temperature, time, and oxygen flow. A growth rate of 4.5 angstrom min(-1) is obtained without the addition of oxygen, which can be increased to 10.2 angstrom min(-1) by the addition of oxygen. Atomic force microscopy, Rutherford backscattering spectroscopy, Fourier transform infrared spectroscopy, and drop shape analysis are used to measure roughness, composition, and hydrophobicity. Thin films of silicon dioxide are successfully grown. In addition, this newly developed system can be used for a wide range of films by varying the precursors or by co-injecting nanoparticles suspension mixed with the chemical vapor deposition precursor in the direct liquid injection vaporizer.status: publishe

A novel direct liquid injection low pressure chemical vapor deposition system (DLI-LPCVD) for the deposition of thin films

In this work, the use of a newly developed direct liquid injection low pressure chemical vapor deposition (DLI-LPCVD) system is described, which allows for the deposition of thin films in a controlled and reproducible manner. The capabilities of this system are described via silica thin films deposited using the precursor tetraethyl orthosilicate (TEOS). The deposition of thin films is controlled by parameters, such as deposition temperature, partial pressure of the gases, and flow rate of the precursor solution. The thickness of the deposited layer is varied simply by changing deposition temperature and time. X-ray reflectivity and spectroscopic ellipsometry of the deposited samples show that the thickness of the layers is well controlled by deposition temperature and time. Auger electron spectroscopy, in addition, motivates our choice to use cyclohexane as a solvent. A growth rate of 12.2 Å min 1 is obtained. Atomic force microscopy, Rutherford backscattering spectroscopy, Fourier transform infrared spectroscopy, and drop shape analysis are used to measure roughness, composition, and hydrophobicity. Thin films of silicon dioxide are successfully grown by the newly developed DLI-LPCVD system. This system can be used for a wide range of films by varying the precursors.status: publishe

Annealing-Induced Bi Bilayer on Bi₂Te₃ Investigated via Quasi-Particle-Interference Mapping

Topological insulators (TIs) are renowned for their exotic topological surface states (TSSs) that reside in the top atomic layers, and hence, detailed knowledge of the surface top atomic layers is of utmost importance. Here we present the remarkable morphology changes of Bi2Te3 surfaces, which have been freshly cleaved in air, upon subsequent systematic annealing in ultrahigh vacuum and the resulting effects on the local and area averaging electronic properties of the surface states, which are investigated by combining scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and Auger electron spectroscopy (AES) experiments with density functional theory (DFT) calculations. Our findings demonstrate that the annealing induces the formation of a Bi bilayer atop the Bi2Te3 surface. The adlayer results in n-type doping, and the atomic defects act as scattering centers of the TSS electrons. We also investigated the annealing-induced Bi bilayer surface on Bi2Te3 via voltage dependent quasi-particle-interference (QPI) mapping of the surface local density of states and via comparison with the calculated constant-energy contours and QPI patterns. We observed closed hexagonal patterns in the Fourier transform of real-space QPI maps with secondary outer spikes. DFT calculations attribute these complex QPI patterns to the appearance of a "second" cone due to the surface charge transfer between the Bi bilayer and the Bi2Te3. Annealing in ultrahigh vacuum offers a facile route for tuning of the topological properties and may yield similar results for other topological materials