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京都大学 / 新制・課程博士 / 博士(理学) / 甲第24074号 / 理博第4841号 / 新制||理||1692(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 寺西 利治, 教授 島川 祐一, 教授 倉田 博基 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

<p>Single junction silicon photovoltaic cells (SJSPVCs) are unable to transform all the energy in the solar spectrum into electricity, due to the broad nature of the solar spectrum and the limits imposed by a single bandgap. Furthermore, high surface recombination velocity reduces the SJSPVC external quantum efficiency response, particularly to ultraviolet photons. It is the goal of spectral engineering to optimize the light that is incident on the cell, by down-shifting high energy photons to lower energies, for example, to improve the performance of photovoltaic cells.</p> <p>This thesis represents a study into the luminescence of ion implanted films, involving silicon nanocrystals (Si-NCs) and rare-earth ions in fused silica or silicon nitride. Quantum efficiency measurements taken with an integrating sphere were used to characterize some of the samples. Other photoluminescence (PL) characterization work was carried out with a single-wavelength laser and a collection lens normal to the sample. Variable angle spectroscopic ellipsometry (VASE) was used to estimate the optical constants of the implanted films. In secondary work, Rutherford backscattering spectrometry, time-dependent PL, infrared-PL measurements, and electrical conductivity measurements were used to characterize select samples.</p> <p>It was found that the conversion efficiency of Si-NCs in fused silica was about 1% – too low to be useful according to modeled results. However, considerable variation in the peak wavelength of the Si-NC PL was obtained, depending on the peak concentration of implanted silicon. Si-NC-type PL was also produced by low-energy implantation of oxygen into a Czochralski silicon wafer.</p> <p>Oxygen was also implanted into films of cerium-doped high-purity silicon nitride, and it was shown that the photoluminescence from these films is largely dependent on the level of oxygen doping. The internal conversion efficiency of a cerium-doped fused silica sample was found to approach 20%, which indicates that this is a promising avenue for future research.</p> <p>Finally, energy transfer was demonstrated between Si-NCs and erbium ions. The lifetime of the erbium PL appears to increase with increasing implanted silicon fluence.</p> / Master of Applied Science (MASc)

<p>The aim of third-generation photovoltaics (PV) is ultimately to achieve low-cost, high-efficiency devices. This work focused on a third-generation PV concept known as down-shifting, which is the conversion of high-energy photons into low-energy photons which are more useful for a typical solar cell. Silicon nanocrystals (Si-NCs) fabricated using electron-cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) were studied as a down-shifting material for single-junction silicon cells. A calibration was done to determine optimal deposition parameters for Si-NC formation. An experiment was then done to determine the effect of film thickness on emission, optical properties, and photoluminescence quantum efficiencies.</p> <p>Photoluminescence (PL) peaks varied depending on the stoichiometry of the films, ranging from approximately 790 nm to 850 nm. Variable-angle spectroscopic ellipsometry was used to determine the optical constants of the Si-NC films. The extinction coefficients indicated strong absorption below 500 nm, ideal for a down-shifting material. Transmission Electron Microscopy (TEM) was used to determine the size, density, and distribution of Si-NCs in two of the films. Si-NCs were seen to have an average diameter of approximately 4 nm, with larger nanocrystals more common near the surface of the film. A density of approximately 10⁵ nanocrystals per cubic micron was approximated from one of the TEM samples.</p> <p>The design and implementation of a PL quantum efficiency measurement system was achieved, using an integrating sphere to measure the absolute efficiency of Si-NC emission. Internal quantum efficiencies (IQE) as high as 1.84% and external quantum efficiencies (EQE) of up to 0.19% were measured. The EQE was found to increase with thicker films due to more intense photoluminescence; however the IQE remained relatively independent of film thickness.</p> / Master of Applied Science (MASc)

<p>An ammonium polysulfide (NH₄)₂S_x solution was optimized through a series of experiments to be used for surface passivation of III-V nanowires . The effectiveness of sulfur passivation was investigated by measuring the photoluminescence from p-InP nanowires before and after passivation. The optimized parameters included solvent type, molarity and passivation time. According to the experiments, passivation of nanowires in 0.5 M solution diluted in isopropyl alcohol for 5 min produced the maximum photoluminescence improvement. It was also demonstrated that the whole surface passivation of vertical nanowires in ensemble samples caused a 40 times increase in the photoluminescence intensity while top surface passivation of individual nanowires resulted in a 20 times increase of photoluminescence intensity. A model was developed to calculate the photoluminescence from single nanowires under different surface recombination and surface potential. The model showed that the 40 times increase in the photoluminescence is mainly due to the reduction of surface state density from 10¹² cm^-2before passivation to 5×10¹⁰ cm^-2after passivation.</p> <p>The effect of sulfur passivation on core-shell p-n junction GaAs nanowire solar cells has been investigated. The relative cell efficiency increased by 19% after passivation.</p> / Doctor of Philosophy (PhD)

McMaster University DOCTOR OF PHILOSOPHY (2017) Hamilton, Ontario (Engineering Physics) TITLE: Luminescent Silicon Carbonitride Thin Films Grown using ECR PECVD: Fabrication and Characterization AUTHOR: Zahra Khatami , M.A.Sc. (Shahid Behehsti University) SUPERVISOR: Professor Peter Mascher NUMBER OF PAGES: xx, 268 / Silicon, the cornerstone semiconductor of microelectronics, has seen growing interest as a low-cost material in photonics. Nanoscience has employed various strategies to overcome its fundamentally inefficient visible light emission such as developing new silicon-based nanostructures and materials. Each of the proposed materials has its own advantages and disadvantages in attempting to reach commercialization. Silicon carbonitride (SiCxNy) is a less-studied and multi-functional material with tunable optical features. Despite reports on promising mechanical properties of SiCxNy thin films, they have not yet been well explored optically. This thesis presents the first in-depth analysis of the luminescent properties of SiCxNy thin films at a broad range of compositions and temperatures. To better understand this ternary structure, the reported data of the two fairly well studied binary structures was used as a reference. Therefore, three classes of silicon-based materials were produced and explored; SiCxNy, SiNx, and SiCx. Samples were fabricated using one of the common methods in the semiconductor industry; electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR PECVD). A multitude of characterization techniques were utilized including; optical methods (ultraviolet-visible spectroscopy (UVVIS), variable angle spectroscopic ellipsometry (VASE), photoluminescence (PL)) and structural techniques (elastic recoil detection (ERD), Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HR-TEM)). In view of the exploring of emission properties of SiCxNy materials, our approach was towards the enhancement of the visible emission by adjusting the film composition and subsequent thermal treatment. First, a systematic study of the influence of carbon on the optical, compositional, and structural properties of SiCxNy was carried out. This investigation was followed by an exploration of influence of growth conditions on the visible emission and its connection with the other film properties including hydrogen concentration, microstructure, and composition. In addition, hydrogen diffusion was explored and associated with two featured annealing temperatures. The key element of this thesis is the comprehensive report on the interdependency of the visible light emission and all optical, structural, and compositional features of SiCxNy structures. Unlocking the potential of this ternary and less studied material can appeal to the silicon photonics community to implement it in anti-reflection, solar cell, and sensing applications, and in particular as a substitution of SiNx used in existing microelectronic devices. / Thesis / Doctor of Philosophy (PhD)

Optical studies of heavily-doped GaAs:C grown at low temperature by molecular beam epitaxy were performed using room-temperature photoluminescence, infrared transmission, and Raman scattering measurements. The photoluminescence experiments show that in LT-GaAs:C films grown at temperatures below 400 °C, nonradiative recombination processes dominate and photoluminescence is quenched. When the growth temperature exceeds 400 °C, band-to-band photoluminescence emission appears. We conclude that the films change in character from LT-GaAs:C to normal GaAs:C once the growth temperature reaches 400 °C. Annealing, however, shows a different behavior. Once grown as LT-GaAs:C, this material retains its nonconducting nonluminescing LT characteristics even when annealed at 600 °C. The Raman-scattering measurements showed that the growth temperature and the doping concentration influence the position, broadening, and asymmetry of the longitudinal-optical phonon Raman line. We attribute these effects to changes in the concentration of interstitial carbon in the films. Also, the shift of the Raman line was used to estimate the concentration of arsenic-antisite defects in undoped LT-GaAs. The infrared transmission measurements on the carbon-doped material showed that only a fraction of the carbon atoms occupy arsenic sites, that this fraction increases as the growth temperature increases, and that it reaches about 100% once the growth temperature reaches 400 °C. The details of all these measurements are discussed. Infrared transmission and photoluminescence measurements were also carried out on heavily-doped GaAs:C films grown by molecular beam epitaxy at the standard 600 C temperature. The infrared results reveal, for dopings under 5 x 10⁹ cm⁻³, a linear relation between doping concentration and the integrated optical absorption of the carbon localized-vibrational-mode band. At higher dopings, the LVM integrated absorption saturates. Formation of C_As-C_As clusters is proposed as the mechanism of the saturation. The photoluminescence spectra were successfully analyzed with a simple model assuming thermalization of photoelectrons to the bottom of the conduction band and indirect-transition recombination with holes populating the degenerately doped valence band. The analysis yields the bandgap reduction and the Fermi-level-depth increase at high doping. / Ph. D.

Cette thèse explore la localisation de l’excitation dans des hétéro structures semiconductrices nanoscopiques par deux approches différentes, soit la localisation sur un ion Mn2+ et la localisation dans un anneau de CdSe. Ces structures sont préparées par voie colloïdale et caractérisées principalement par spectroscopie d’émission et d’excitation de la photoluminescence à différentes températures. Les propriétés optiques du Mn2+ ont pu être modulées par les contraintes interfaciales en modifiant la composition du coeur de nanocristaux coeur/coquille de MnxZn1−xE/ZnE (où E est du soufre ou du sélénium) pour des x entre 0,1 et 1. Pour les nanocristaux à base de séléniures, le temps de vie radiatif du Mn2+ se raccourcit avec l’effet des contraintes de 900 à 110 μs. Deux formes de couplage vibronique ont aussi été observées avec une perte de couplage des phonons optiques pour des températures inférieures à 100 K. Dans le cas des nanocristaux de MnxZn1−xS/ZnS, les paramètres du champ cristallin ont été déterminés grâce aux cinq bandes d’excitation du Mn2+. L’analyse du champ cristallin en fonction de la composition du coeur permet de mieux comprendre l’effet des contraintes et de la composition. La composition du coeur influence à la fois la force du champ cristallin et les paramètres de Racah, passant de ceux du ZnS à ceux du MnS. En faisant croître un anneau de CdSe autour de puits quantiques colloïdaux de CdS, l’excitation se localise sur l’anneau. Le maximum de l’émission peut alors être ajusté de 428 à 512nm en augmentant la largeur de l’anneau pour des puits de 3 et 4 monocouches (ML). Le déplacement de l’émission provient d’un changement de la dimensionnalité du confinement de l’exciton. Des mesures d’absorption transitoire permettent de déterminer que le temps de diffusion de l’exciton du CdS vers le CdSe est de 20 ps. Après optimisation par plan factoriel, la synthèse des anneaux quantiques de 3 ML est robuste avec un rendement quantique moyen de 8,7% pour huit expérimentateurs différents. / This thesis explores localized excitation in nanoscopic semiconductor hetero structures by two different approaches: localisation on an Mn2+ ion and localisation inside a CdSe ring. These structures are prepared using colloidal synthesis and primarily characterised by photoluminescence emission and excitation spectroscopy at different temperatures. The Mn2+ optical properties can be modulated using interfacial strain by changing the core composition of MnxZn1−xE/ZnE (where E is S or Se) core/shell nanocrystals for x between 0.1 and 1. For selenide based nanocrystals, the Mn2+ radiative lifetime shortens from 900 to 110 μs. Also, two kinds of vibronic couplings are observed with the loss of optical phonon coupling at temperatures below 100 K. For MnxZn1−xS/ZnS nanocrystals, the crystal field parameters could be determined using five Mn2+ excitation bands. The composition dependent crystal field analysis allows for a better understanding of strain and composition effects. The core composition affects both the crystal field strength and the Racah parameters from those of ZnS to those of MnS. Only the crystal field strength is dependent on the temperature and its dependence is stronger as the crystal is more strained. By growing a CdSe ring around CdS colloidal quantum wells, the excitation is located inside the ring. The emission maximum can thus be tuned from 428 to 512nm by growing a wider ring around 3 and 4 monolayers (ML) quantum wells. The emission shift comes from a change in exciton confinement dimensionality. Transient absorption measurements indicate that the exciton diffusion time from CdSe to CdSe is 20 ps. After optimisation by factorial design, the 3ML quantum ring synthesis is demonstrated to be robust with a mean quantum yield of 8.7% for 8 different experimenters.

The phenomenon luminescence rigidochromism has been reported since the 1970s in tricarbonyldiimine complexes with a general formula [R(CO)3LX] using conventional unipolar diimine ligands such as 2,2;-bipyridine or 1,10-phenanthroline as L, and halogens or simple solvents as X. As a major part of this dissertation, microwave-assisted synthesis, purification, characterization and detailed photoluminescence studies of the complex fac-[ReCl(CO)3L], 1, where L = 4-[4,6-bis(3,5-dimethyl-1H-pyrazol-1-yl]-N,N-diethylbenzenamine are reported. The employment of microwaves in the preparation of 1 decreased the reaction time from 48 to 2 hours compared to the conventional reflux method. Stoichiometry variations allows for selective preparation of either a mononuclear, 1, or binuclear, fac-[Re2Cl2(CO)6], 2, complex. The photophysical properties of 1 were analyzed finding that it possesses significant luminescence rigidochromism. The steady state photoluminescence emission spectra of 1 in solution shift from 550 nm in frozen media to 610 nm when the matrix becomes fluid. Moreover, a very sensitive emission spectral analysis of 0.1 K temperatures steps shows a smooth transition through the glass transition temperature of the solvent host. Furthermore, synthetic modifications to L have attained a family of ambipolar compounds that have tunable photophysical, thermophysical and other material properties that render them promising candidates for potential applications in organic electronics and/or sensors - either as is or for their future complexes with various transition metals and lanthanides.