In the past years, a large body of work has been dedicated to semiconductor quantum dots embedded in thin films of oxide and nitride, as the tailorable electronic and optical properties of these nanostructures make them desirable for various optoelectronic applications. The properties of these low-dimensional semiconductor systems are directly related to the atomic arrangement, distribution and size of the quantum dots, the structure of the surrounding matrices and the distance between the quantum dots. The quantum confinement of carriers in the quantum dots, their interaction, and local states in the matrices are the dominating factors determining the material properties.The present work focuses on studying the atomic structures of these materials, and how their optical and electronic properties vary as a function of size and structure. Two material systems were especially synthesized as part of this work; Si and Ge quantum dots embedded This PhD project was motivated by the potential of the third generation solar cells, which seek to increase the device efficiency above the Shockley-Queisser limit, and the recent advances in aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy that allow the investigation of the electronic structure and chemistry of materials with sub-angstrom spatial resolution. The work focused on studies of the structure and electronic properties of semiconductor quantum dots using mainly transmission electron microscopy and electron energy-loss spectroscopy. The synthesis and structural characterization of the semiconductor quantum dots were carried out at the Structure Physics group, University of Oslo. Due to the extremely small dimensions of the quantum dots and the need of high spatial and energy resolution instrument for very detail experiments, the investigation of the quantum dots' chemical bonding and electronic properties was carried out by advanced analytical scanning transmission electron microscopy at the SuperSTEM Laboratory. There is a direct continuation of, and an improvement on, the important and novel results reported in the first to the third paper, which provided one of the first direct experimental evidence for the presence of quantum confinement effects in individual Si and Ge quantum dots in a dielectric matrix. The contribution from this work very much provides an improved understanding of the condensed matter physics responsible for the quantum behaviors of the quantum dots, while from a practical point of view it would also arguably provides important result for the photovoltaic community concerned with device/materials performance.vii

Structure determination of thin CoFe films by anomalous x-ray diffraction J. Appl. Phys. 112, 074903 (2012) Surface wettability of titania thin films with increasing Nb content J. Appl. Phys. 112, 073502 (2012) Multi-scale order in amorphous transparent oxide thin films J. Appl. Phys. 112, 054907 (2012) Growth of continuous and ultrathin platinum films on tungsten adhesion layers using atomic layer deposition techniques Appl.The atomic structure and optical properties of Si-rich silicon nitride thin films have been for decades the subject of intense research, both theoretically and experimentally. It has been established in particular that modifying the chemical composition of this material (e.g., the Si excess concentration) can lead to dramatic differences in its physical, optical, and electrical properties. The present paper reports on how the incorporation of oxygen into silicon nitride networks influences their chemical bonding and photoluminescence properties. Here, by using a combination of analytical scanning transmission electron microscopy and x-ray photoelectron spectroscopy it is demonstrated that the structure of Si-rich silicon nitride with low O content can be described by the co-existence of Si nanocrystals in a Si 3 N 4 matrix, with occasional localized nano-regions of a Si 2 ON 2 phase, depending on the amount of excess Si. Furthermore, it is shown that the structure of silicon nitride with high O content can be adequately described by a so-called random bonding model, according to which the material consists in bonded networks of randomly distributed tetrahedral SiO x N 4Àx (where x ¼ 0, 1, 2, 3, and 4). Photoluminescence measurements indicate that the effect of O is to introduce a gap state in the band gap of Si 3 N 4 matrix. When a large amount of O is introduced, on the other hand, the photoluminescence measurements are in agreement with a shifted conduction band minimum in the dielectric. For both cases (high and low O content), Si dangling bonds were found to give rise to the deep level in the band gap of the nitride matrix, causing the dominant emission band in the photoluminescence of the films. V C 2012 American Institute of Physics. [http://dx.

Ion implantation induced phase transformation and the crystal structure of a series of ion implanted β-Ga2O3 samples were studied using electron diffraction, high resolution transmission electron microscopy, and scanning transmission electron microscopy. In contrast to previous reports suggesting an ion implantation induced transformation to the orthorhombic κ-phase, we show that for 28Si+, 58Ni+, and stoichiometric 69Ga+/16O+-implantations, the monoclinic β-phase transforms to the cubic γ-phase. The γ-phase was confirmed for implantations over a range of fluences from 1014 to 1016 ions/cm2, indicating that the transformation is a general phenomenon for β-Ga2O3 due to strain accumulation and/or γ-Ga2O3 being energetically preferred over highly defective β-Ga2O3.

The plasmonic properties of individual quantum-sized Ge nanocrystals (NCs) were observed and systematically analyzed by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). For this purpose, Ge NCs embedded in an SiO 2 matrix with controllable size, density, and structure were fabricated using magnetron sputtering. The size dependence of the Ge plasmon energies in the size range of 5-9 nm is shown to be well depicted by the so-called medium quantum confinement (QC) model, with an effective mass of 0.57m 0 (contrary to expectations of a stronger quantum effect). In the very low-loss region of the EEL spectra, an apparent blue shift of the E 2 interband transition peak up to 2 eV and a strong reduction in the oscillator strength were measured for the NCs in the size range of 4-6 nm. It indicates for this smaller size range a transition to a QC regime where the band structure and the density of states are modified dramatically. These trends are explained by a combination of low-loss and core-loss EELS results, which show that the Ge NCs are surrounded uniformly by nearly stoichiometric SiO 2. This local chemistry is shown to provide an infinite potential barrier and to confine electrons and holes in the spherically shaped Ge NCs. In addition to pure QC effects in the Ge NCs, the SiO 2 matrix thus plays an important role in the strength of the observed QC and interband transitions.

Defect accumulation and annealing phenomena in Si-implanted monoclinic gallium oxide (β-Ga2O3) wafers, having [Formula: see text], (010), and (001) orientations, were studied by Rutherford backscattering spectrometry in channeling mode (RBS/c), x-ray diffraction (XRD), and (scanning) transmission electron microscopy [(S)TEM]. Initially, the samples with different surface orientations were implanted with 300 keV 28Si+-ions, applying fluences in the range of 1 × 1014–2 × 1016 Si/cm2, unveiling interesting disorder accumulation kinetics. In particular, the RBS/c, XRD, and (S)TEM combined data suggested that the radiation disorder buildup in Si-implanted β-Ga2O3 is accompanied by significant strain accumulation, assisting crystalline-to-crystalline phase transitions instead of amorphization. Selected samples having [Formula: see text] orientation were subjected to isochronal (30 min) anneals in the range of 300–1300 °C in air. Systematic RBS/c and XRD characterization of these samples suggested complex structural transformations, which occurred as a function of the fluence and the temperature. Moreover, a detailed (S)TEM analysis of the sample implanted with 2 × 1016 Si/cm2 and annealed at 1100 °C was enhanced by applying dispersive x-ray and electron energy-loss spectroscopies. The analysis revealed silicon agglomerations in the form of silicon dioxide particles. Signal from silicon was also detected outside of the agglomerates, likely occurring as substitutional Si on Ga sites.