京都大学 / 新制・課程博士 / 博士(工学) / 甲第24612号 / 工博第5118号 / 新制||工||1979(附属図書館) / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 江利口 浩二, 教授 嶋田 隆広, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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Nitride-based semiconductor materials make it possible to fabricate optoelectronic devices that operate in the whole electromagnetic range, since the band gaps of these compounds can be modified by doping. Among these materials, the sp2-hybridized boron nitride has properties that make it a potential candidate for integration in devices operating in the short-wavelength limit, under harsh environment conditions, due to the strength of the B-N bond.  Nevertheless, this binary compound has been the less studied material among the nitrides, due to the lack of complete control on the growth process. This thesis is focused on the study of the optical properties of sp2-hybridized boron nitride grown by hot-wall chemical vapor deposition (CVD) method, at the Department of Physics, Chemistry and Biology, at Linköping University, Sweden. The samples received for this study were grown on c-plane aluminum nitride as the buffer layer, which in turn was grown by nitridation on c- plane oriented sapphire, as the substrate material. The first objective of the research presented in this thesis was the development of a suitable ellipsometry model in a spectral region ranging from the infrared to the ultraviolet zones of the electromagnetic spectrum, with the aim of obtaining in the process optical properties such as the index of refraction, the energy of the fundamental electronic interband transition, the frequencies for the optical vibrational modes of the crystal lattice, as well as their broadenings, and the numerical values of the dielectric constants; and on the other hand, structural parameters such as the layers thicknesses, and examine the possibility of the presence of roughness or porosity on the boron nitride layer, which may affect the optical properties, by incorporating their effects into the model. The determination of these parameters, and their relation with the growth process, is important for the future adequate design of heterostructure-based devices that incorporate this material. In particular, emphasis has been put on the modeling of the polar lattice resonance contributions, with the TO- LO model, by using infrared spectroscopic ellipsometry as the characterization technique to study the phonon behavior, in the aforementioned spectral region, of the boron nitride. On the other hand, spectroscopic ellipsometry in the visible-ultraviolet spectral range was used to study the behavior of the material, by combining a Cauchy model, including an Urbach tail for the absorption edge, and a Lorentz oscillator in order to account for the absorption in the material in the UV zone. This first step on the research project was carried out at Linköping University.  The second objective in the research project was to carry out additional studies on the samples received, in order to complement the information provided by the ellipsometry model and to improve the model itself, provided that it was possible. The characterization techniques used were X-ray diffraction, which made it possible to confirm that in fact boron nitride was present in the samples studied, and made it possible to verify the crystalline quality of the aforementioned samples, and in turn relate it to the quality of the ellipsometry spectra previously obtained; the Raman spectroscopy made it possible to further verify and compare the crystalline qualities of the samples received, as well as to obtain the frequency for the Raman active B-N stretching vibration in the basal plane, and to compare this value with that corresponding to the bulk sp2-boron nitride; scanning electron microscopy made it possible to observe the rough surface morphologies of the samples and thus relate them to some of the conclusions derived from the ellipsometry model; and finally cathodoluminescence measurements carried out at low temperature (4 K) allowed to obtain a broad band emission, on all the samples studied, which could be related to native defects inside the boron nitride layers, i.e., boron vacancies. Nevertheless, no trace of a free carrier recombination was observed. Considering that the hexagonal-boron nitride is nowadays considered to be a direct band gap semiconductor, it may be indirectly concluded, in principle, that the dominant phase present in the samples studied was the rhombohedral polytype. Moreover, it can be tentatively concluded that the lack of an observable interband recombination may be due to the indirect band gap nature of the rhombohedral phase of the boron nitride. Spectroscopic ellipsometry does not give a definite answer regarding this issue either, because the samples analyzed were crystalline by nature, thus not being possible to use mathematical expressions for the dielectric function models that incorporate the band gap value as a fitting parameter. Therefore, the nature of the band gap emission in the rhombohedral phase of the boron nitride is still an open research question. On the other hand, luminescent emissions originating from radiative excitonic recombinations were not observed in the cathodoluminescence spectra. This second step of the project was carried out at the Leroy Eyring Center for Solid State Science at Arizona State University.

<p>In vapour phase deposition, the knowledge and control of homogeneous and heterogeneous reactions in connection to precursor design may lead to the deposition of the desired material; structure or phase. This thesis is a document attempting to increase the knowledge of film growth in the B-N system.</p><p>In the present work, surface processes like adsorption, abstraction, migration and nucleation have been modelled on an atomic scale using density functional theory (DFT). The systems studied are mainly cubic and hexagonal boron nitride surfaces ((c-BN) vs. (h-BN)), but also the α-boron (001) surface.</p><p>It has been shown that DFT and a cluster approach is a reliable tool in modelling boron nitride surfaces and surface processes, provided that certain functionals, basis sets and geometrical constraints are used.</p><p>By using surface stabilisers such as H species in an electron- or radical-rich environment, it has been shown that <i>i)</i> the structure of cubic boron nitride surfaces can be sustained, and <i>ii)</i> c-BN may nucleate on the h-BN (001) basal plane. Furthermore, the nucleation of c-BN from arbitrary and experimental growth species is energetically preferable over a continuous growth of h-BN on the h-BN (001) edges.</p><p>An atomic layer deposition (ALD) process for boron nitride was developed. It resulted in turbostratic (t-BN), transparent, well-adherent and almost atomically smooth BN films. However, with the cubic phase of boron nitride absent in the ALD films, more effort needs to be put into both the theoretical and the experimental branches of this field of science.</p>

In vapour phase deposition, the knowledge and control of homogeneous and heterogeneous reactions in connection to precursor design may lead to the deposition of the desired material; structure or phase. This thesis is a document attempting to increase the knowledge of film growth in the B-N system. In the present work, surface processes like adsorption, abstraction, migration and nucleation have been modelled on an atomic scale using density functional theory (DFT). The systems studied are mainly cubic and hexagonal boron nitride surfaces ((c-BN) vs. (h-BN)), but also the α-boron (001) surface. It has been shown that DFT and a cluster approach is a reliable tool in modelling boron nitride surfaces and surface processes, provided that certain functionals, basis sets and geometrical constraints are used. By using surface stabilisers such as H species in an electron- or radical-rich environment, it has been shown that i) the structure of cubic boron nitride surfaces can be sustained, and ii) c-BN may nucleate on the h-BN (001) basal plane. Furthermore, the nucleation of c-BN from arbitrary and experimental growth species is energetically preferable over a continuous growth of h-BN on the h-BN (001) edges. An atomic layer deposition (ALD) process for boron nitride was developed. It resulted in turbostratic (t-BN), transparent, well-adherent and almost atomically smooth BN films. However, with the cubic phase of boron nitride absent in the ALD films, more effort needs to be put into both the theoretical and the experimental branches of this field of science.

<p>The electronic structures of the nucleobases, 5-fluorouracil compounds, DNA, metallic DNA, and samples of boron nitride are investigated. Soft X-ray absorption (XAS) and emission (XES) spectroscopy using synchrotron radiation are used to probe the unoccupied and occupied partial densities of electronic states, respectively. Hartree-Fock and density functional theory calculations have been included to compare with experimental results.</p> <p>A systematic approach to understanding the complicated electronic structure of DNA and metallic DNA systems is to initially examine smaller components. Detailed experiment and theory for both absorption and emission spectroscopy was. performed for the nucleobases and 5-fluorouracil compounds. Main transitions in the XAS and XES spectra are identified. X-ray spectroscopy has proven to be extremely sensitive to changes in the environment of various DNA samples. The local chemical environment plays an important role in determining the electronic structure of DNA. In agreement with previous results indicating metallic DNA is more efficient at the transfer of electrons than DNA, XES measurements reveal that there are a higher number of charge carriers in the metallic system. Both liquid and powder samples of (Ni)·M-DNA are found to have a high spin Ni(II) configuration. The drying process significantly alters the electronic structure of the metallic DNA sample. A comparison of high quality single crystals and thin films of boron nitride found that differences between the electronic structures of the nanocrystalline films and the single crystal samples exist, and the surface roughness of the substrate plays an important role in determining the structure of the resulting deposited film.</p>

<p>The electronic structures of the nucleobases, 5-fluorouracil compounds, DNA, metallic DNA, and samples of boron nitride are investigated. Soft X-ray absorption (XAS) and emission (XES) spectroscopy using synchrotron radiation are used to probe the unoccupied and occupied partial densities of electronic states, respectively. Hartree-Fock and density functional theory calculations have been included to compare with experimental results.</p> <p>A systematic approach to understanding the complicated electronic structure of DNA and metallic DNA systems is to initially examine smaller components. Detailed experiment and theory for both absorption and emission spectroscopy was. performed for the nucleobases and 5-fluorouracil compounds. Main transitions in the XAS and XES spectra are identified. X-ray spectroscopy has proven to be extremely sensitive to changes in the environment of various DNA samples. The local chemical environment plays an important role in determining the electronic structure of DNA. In agreement with previous results indicating metallic DNA is more efficient at the transfer of electrons than DNA, XES measurements reveal that there are a higher number of charge carriers in the metallic system. Both liquid and powder samples of (Ni)·M-DNA are found to have a high spin Ni(II) configuration. The drying process significantly alters the electronic structure of the metallic DNA sample. A comparison of high quality single crystals and thin films of boron nitride found that differences between the electronic structures of the nanocrystalline films and the single crystal samples exist, and the surface roughness of the substrate plays an important role in determining the structure of the resulting deposited film.</p>

Formation of hexagonal boron nitride (h-BN) by carbothermic reduction of B2O3 under nitrogen atmosphere at 1500oC was investigated. Reaction products were subjected to powder X-ray diffraction analysis, chemical analysis and were examined by SEM. B4C was found to exist in the reaction products of the experiments in which h-BN formation was not complete. One of the aims of this study was to investigate the role of B4C in the carbothermic production of h-BN. For this purpose, conversion reaction of B4C into h-BN was studied. B4C used in these experiments was produced in the same conditions that h-BN was formed, but under argon atmosphere. It was found that formation of h-BN from B4C&ndash / B2O3 mixtures was slower than activated C&ndash / B2O3 mixtures. It was concluded that B4C is not a necessary intermediate product in the carbothermic production of h-BN. Some additives are known to catalytically affect the h-BN formation. The second aim of this study was to examine the catalytic effect of some alkaline earth metal oxides and carbonates, some transition metal oxides and cupric nitrate. It was found that addition of 10wt% CaCO3 into the B2O3+C mixture was optimum for increasing the rate and yield of h-BN formation and decreasing the B4C amount in the products and that the reaction was complete in 2 hours. CaCO3 was observed to be effective in increasing the rate and grain size of the formed h-BN. Addition of cupric nitrate together with CaCO3 provided a further increase in the size of the h-BN grains.