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This thesis describes an experimental investigation of the photoluminescence emissions from firstly, bulk In0.53 Ga0.47 As and secondly, InGaAs\InP and AlAs\GaAs quantum well structures, as a function of hydrostatic pressure. Two high pressure systems have been developed and successfully used in the course of this work, an 8kbar piston and cylinder system and a miniature diamond anvil cell. From the high pressure measurements on the bulk InGaAs, both at room anti liquid nitrogen temperatures, it is shown that the pressure dependence of the direct band-edge luminescence is non-linear and independent of temperature. Using an empirical equation of state and making some assumptions as to the value of the bulk modulus, it is found that the band-edge luminescence is linearly dependent upon the lattice constant and may be described by a band-edge deformation potential of -8.25 +/- 0.1eV. Theory compares favourably with this value. From low temperature measurements of both quantum well systems it has been possible to deduce a quantitative description of how the conduction and valence band-edge discontinuities vary as a function of the applied pressure. It is shown that the band-offset ratio changes with pressure. This work constitutes the first observation of this phenomenon which had previously been thought either too small to be of consequence, or simply ignored. It is found that the conduction-band discontinuity in InGaAs\InP quantum wells decreases at -2.3 +/- 0.6meV/kbar while its valence-band discontinuity remains constant. The valence-band discontinuity in the AlAs\GaAs superlattice is directly measured to increase at +1.1 +/- 0. ImeV/kbar. An analysis of reported data for A1 Ga As/GaAs quantum wells shows that the pressure coefficient of the X l-x valence-band discontinuity is linearly dependent upon the alloy composition x. Theories of band-offset ratios and in particular the "model-solid" theory of Van de Walle and Martin (Van de Walle 1987), agree exceedingly well with these experimental findings. It is suggested that such agreement lends weight to the assumption that heterojunction band-edge discontinuities are intrinsic to the bulk properties of the host materials.

In der vorliegenden Arbeit werden ZnO-Mikronadeln bezüglich ihrer Anwendbarkeit als Mikroresonatoren untersucht. Dabei stehen Kavitätsmoden im Fokus der Untersuchungen, die sich nur senkrecht zur Nadelachse ausbreiten, sprich innerhalb der hexagonalen Nadelquerschnittsfläche. Folglich wird der Einfluss der Gestalt der Querschnittsfläche auf Resonatoreigenschaften wie Propagation, Form, Direktionalität und Qualität der Kavitätsmoden sowohl theoretisch simuliert als auch experimentell nachgewiesen. Die dabei beobachteten hohen Qualitätsfaktoren von Flüstergalerie-Moden ermöglichen es darüberhinaus, Wechselwirkungseffekte zwischen Kavität und Mode zu beobachten. Der erste Teil der Arbeit beschäftigt sich mit der regulären, polygonalen Resonatorform und deren Einfluss auf die Dimensionalität von Kavitätsmoden sowie deren mögliche Wechselwirkung mit dem elektronischen System des Resonators. Beispielhaft wird ein hexagonaler Resonator zur Veranschaulichung gewählt, wie er durch ZnO-Mikronadeln gegeben ist, undmittels Finite-Difference-Time-Domain (FDTD)-Simulationen sowie winkelaufgelöster Photolumineszenz (PL)-Spektroskopie untersucht. Die aufgenommenen PL-Spektren können unter Zuhilfenahme photonischer Dispersionskurven von ein- und zwei-dimensionalen Kavitätsmoden reproduziert werden. Basierend auf diesen Ergebnissen wird der Einfluß der Resonatorecken auf die Lichtauskopplung diskutiert und mittels winkelaufgelöster, anregungsabhängiger und temperaturabhängier PL-Spektroskopie nachgewiesen. Desweiteren wird auf die Wechselwirkung zwischen dem Resonator und den Kavitätsmoden eingegangen, imSpeziellen auf die starke Kopplung zwischen Flüstergalerie-Moden und freien Exzitonen imResonatormaterial. Bereits erschienende Publikationen zu diesemThema werden präsentiert und kritisch hinterfragt. Dabei wird ein Leitfaden aufgestellt, der eine Evaluierung möglicher Polaritonen-Phänomene ermöglicht. Um Wechselwirkungen dieser Art auch in den hier untersuchtenMikronadeln nachzuweisen, werden Hochanregungs-PL-Messungen durchgeführt. Dabei werden Messungen in der Mitte der Nadel sowie in der Nähe ihrer Ecken getätigt, um spezielle Polaritonen-Propagationseffekte beobachten zu können. Im zweiten Teil der Arbeit wird der Einfluß von irregulären und inhomogenen Resonatorformen auf die Bildung von Flüstergalerie-Moden diskutiert. Dafür werden elongierte Teile der Nadeln, die durch laterale Auswüchse entstehen, winkelaufgelöst bezüglich einer gerichteten Auskopplung von Kavitätsmoden vermessen und verzerrte Mikronadeln, wie sie beim Biegen entstehen, bezüglich der entstehenden Deformationseffekte und deren Einfluss auf die Kavitätsmoden mittels hochaufgelöster Mikro-PL untersucht. Die experimentellen Ergebnisse zu irregulären Resonatoren können durch FDTD-Simulationen bestätigt werden. Desweiteren wurden Mikronadel- und Nanonadel-Quantengraben-Heterostrukturen hergestellt und deren Lumineszenzeigenschaften diskutiert. Dabei wird speziell auf die Homogenität der Quantengrabenemission eingegangen und Strategien zur Realisierung einer starken Kopplung zwischen Flüstergalerie-Moden und Quantengraben-Exzitonen aufgestellt. Diese Strategien werden experimentell umgesetzt und deren Ergebnisse anhand von Kathodolumineszenzmessungen vorgestellt.

It is very interesting to probe the rotational symmetry of semiconductor quantum dots for quantum information and quantum computation applications. We studied the effects of rotational symmetry in semiconductor quantum dots using configuration interaction calculation. Moreover, to compare with the experimental data, we studied the effects of hidden symmetry. The 2D single-band model and the 3D single-band model were used to generate the single-particle states. How the spectra affected by the breaking of hidden symmetry and rotational symmetry are discussed. The breaking of hidden symmetry splits the degeneracy of electron-hole single-triplet and triplet-singlet states, which can be clearly seen from the spectra. The breaking of rotational symmetry redistributes the weight percentage, due to the splitting of px and py states, and gives a small brightness to the dark transition, giving rise to asymmetry peaks. The asymmetry peaks of 4X, 5X, and 6X were analyzed numerically. In addition, Auger-like satellites of biexciton recombination were found in the calculation. There is an asymmetry peak of the biexciton Auger-like satellite for the 2D single-band model while no such asymmetry peak occurs for the 3D single-band model. Few-particle effects are needed in order to determine the energy separation of the biexciton main peak and the Auger-like satellite. From the experiments, it was confirmed that the lower emission energy peak of X2-spectrum is split. The competed splitting of the X2- spectra were revealed when temperature dependence was implemented. However, since the splitting is small, we suggest the X2- peaks are broadened in comparison with other configurations according to single-band models. Furthermore, the calculated excitonic emission patterns were compared with experiments. The 2D single-band model fails to give the correct energy order of the peaks for the few-particle spectra; on the other hand the peaks order from 3D single-band model consistent with experimental data.

This dissertation is dedicated to investigating the thermoelectric properties of the Zn – Sb phases and particularly the Zn13Sb10 material, which was shown to achieve a high ZT (1.3 at 670K) in 1997. The Zn13Sb10 material (known as “Zn4Sb3”) was then extensively studied as a potential thermoelectric material. The Zn13Sb10 materials, however, were not widely adapted in thermoelectric applications. A new synthetic procedure was developed to synthesize phase-pure Zn13Sb10 materials in this thesis, thus allowing a robust characterization of the Zn13Sb10-based materials. This work aims to improve the thermoelectric performance of the Zn13Sb10- based materials by substituting foreign elements into the structure of the Zn13Sb10 phase, at the same time characterize and navigate the synthesis-property-composition relationship of the doped Zn13Sb10 materials. On the other hand, some relative Zn–Sb phases such as the α- and β-Zn3Sb2 were also studied in attempt to complete the characterizations of all Zn–Sb phases stable at room temperatures. The ZnSb phase, another well-studied Zn–Sb material, was investigated in coherence with the Zn13Sb10 materials to understand the effects to transport properties brought by the same atom replacement in the two systems. This was realized in the form of a comparison between the (Zn,Cd)Sb and (Zn,Cd)13Sb10 solid solution series. Materials studied in this work were mostly made using the melt and solidification method. Powder and single-crystal X-ray diffractions were employed to characterize samples’ purity and structure determination. Energy-dispersive X-ray Spectroscopy (EDS) was used to determine sample compositions, especially to confirm the presence(s) of dopants in materials in small quantities. Physical properties of materials were measured to evaluate the thermoelectric performances of materials. Computational methods, such as the linear muffin-tin orbital (LMTO) method was used to help understand the transport properties of materials and the electron localization function (ELF) method to analyze the bonding natures between atoms. / Thesis / Doctor of Philosophy (PhD) / Thermoelectric materials are incorporated into thermoelectric devices to generate electricity from heat sources. As there are environmental concerns and increasing demand of energy supplies in the society, thermoelectricity may relieve some of the pressure by waste heat recovery, from internal combustion engines for example. This work is dedicated to studying the zinc–antimony (Zn–Sb) materials and a focus on the Zn13Sb10 material for thermoelectric applications. This work aims to improve the thermoelectric efficiency of the Zn13Sb10 material, at the same time understand the changes on the physical properties brought by the structural and the compositional differences of the material. The relative Zn–Sb phases, such as ZnSb and Zn3Sb2, were also characterized to compare their structures with their physical properties.

The use of Insulated Gate Bipolar Transistors (IGBT) have enabled better switching performance than the Metal Oxide Semiconductor Field effect Transistor (MOSFET) in medium to high power applications due to their lower on-state power loss and higher current densities. The power ratings of IGBTs are slowly increasing and are envisaged to replace thyristors in medium power applications such as High Voltage Direct Current (HVDC) inverter systems and traction drive controls. Devices such as the MOS Controlled Thyristor (MCT) and Emitter Switched Thyristor (EST) were developed in an effort to further simplify drive requirements of thyristors by incorporating a voltage controlled MOS gate into the thyristor structure. However, the MCT is unable to achieve controlled current saturation which is a desirable characteristic of power switching devices while the EST has only limited control. The IGBT can achieve current saturation, however, due to the transistor based structure it exhibits a larger on-state voltage in high power applications compared with thyristor based devices. MOS Gated Thyristor (MGT) devices are a promising alternative to transistor based devices as they exhibit a lower forward voltage drop and improved current densities. This current research focuses on the Clustered Insulated Gate Bipolar Transistor (CIGBT) whilst being operated under soft-switching regimes. The CIGBT is a MOS gated thyristor device that exhibits a unique self-clamping feature that protects cathode cells from high anode voltages under all operating conditions. The self-clamping feature also enables current saturation at high gate biases and provides low switching losses. Its low on-state voltage and high voltage blocking capabilities make the CIGBT suitable as a contender to the IGBT in medium to high power switching applications. For the first time, the CIGBT has been operated under soft-switching regimes and transient over-voltages at turn-on have been witnessed which have been found to be associated with a number of factors. The internal dynamics of the CIGBT have been analysed using 2D numerical simulations and it has been shown that a major influence on the peak voltage is the P well spacing within the CIGBT structure. For example, Small adjacent P well spacings within the device results in an inability for the CIGBT to switch iv on correctly. Further to this, implant concentrations of the n well region during device fabrication can also affect the turn-on transients. Despite this, the CIGBT has been experimental analysed under soft-switching conditions and found to outperform the IGBT by 12% and 27% for on-state voltage drop and total energy losses respectively. Turn off current bumps have been seen whilst switching the device in zero voltage and zero current switching mode of operation and the internal dynamics have been analysed to show the influence upon the current at turn off. Preliminary results on the Trench CIGBT (TCIGBT) under soft switching conditions has also been analysed for the first time and was found to have a reduced peak over-voltage and better switching performance than the planer CIGBT. Through optimisation of the CIGBT structure and fabrication process, it is seen that the device will become a suitable replacement to IGBT in medium power application.

Ruthenium oxide (RuO2) nanorods are an emergent technology in nanostructure devices. As the physical size of electronics approaches a critical lower limit, alternative solutions to further device miniaturization are currently under investigation. Thin-film nanorod growth is an interesting technology, being investigated for use in wireless communications, sensor systems, and alternative energy applications. In this investigation, self-assembled RuO2 nanorods are grown on a variety of substrates via a high density plasma, reactive sputtering process. Nanorods have been found to grow on substrates that form native oxide layers when exposed to air, namely silicon, aluminum, and titanium. Samples were analyzed with Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques. Conductive Atomic Force Microscopy (C-AFM) measurements were performed on single nanorods to characterize structure and electrical conductivity. The C-AFM probe tip is placed on a single nanorod and I-V characteristics are measured, potentially exhibiting rectifying capabilities. An analysis of these results using fundamental semiconductor physics principles is presented. Experimental data for silicon substrates was most closely approximated by the Simmons model for direct electron tunneling, whereas that of aluminum substrates was well approximated by Fowler-Nordheim tunneling. The native oxide of titanium is regarded as a semiconductor rather than an insulator and its ability to function as a rectifier is not strong. An electronic model for these nanorods is described herein.

In this thesis, we investigate how the optical and electronic properties of graphene may be modified in proximity to various other materials. We present several examples of how modification in this way can help make graphene better suited for specific device applications. We develop a method of up-scaling the fabrication of FeCl3-intercalated few-layer graphene from micron-sized flakes to macroscopic films so that it may be used as a transparent electrode in flexible light-emitting devices. We also find that photo-responsive junctions can be arbitrarily written into FeCl3-intercalated few-layer graphene by means of optical lithography. These junctions produce photocurrent signals that are directly proportional to incident optical power over an extended range compared to other graphene photodetectors. Through theoretical analysis of these junctions, we conclude that the enhanced cooling of hot carriers with lattice phonons is responsible for this behaviour. Finally, we trial rubrene single crystals as the light-absorbing layer in a graphene phototransistor. We find that rubrene single crystal-graphene interfaces exhibit enhanced charge transfer efficiencies under illumination with extremely weak light signals. Through a comparative study with similar devices, we conclude that the wide variation in sensitivity amongst graphene phototransistors is largely due to extraneous factors relating to device geometry and measurement conditions.

The aim of this thesis work was to fabricate a hybrid LED using organic-inorganic ZnO materials. The goal of the project was to get an efficient white light emission from zinc oxide (ZnO) nanorods active layer. Since most of the organic materials are good for hole mobility and most of the inorganic materials are good for electron mobility, it is possible to fabricate a high performance heterostructure electroluminescence device from organic-inorganic materials. This thesis work was an attempt towards fabricating such a high electroluminescence LED from hybrid materials in which polymer acts as a p-type material and ZnO acts as a n-type material. The growth mechanism of ZnO nanorods using low-temperature aqueous solution method has been studied and nanorods (NRs) growth was examined with scanning electron microscope (SEM). Optimum hole injection polymers have been studied. Finally, the fabricated device was characterized using parameter analyzer. The fabricated device worked as a diode i.e. it rectified current as expected and the desirable light emission has almost been achieved.

Vertical cavity surface emitting lasers (VCSELs) are today a commodity on the short wavelength laser market due to the ease with which they are manufactured. Much effort has in the last decade been directed towards making long wavelength VCSELs as successful in the marketplace. This has not been achieved due to the much more difficult fabrication technologies needed for realising high performance long wavelength VCSELs. At one point, GaInNAs quantum wells gain regions grown on GaAs substrates seemed to be the solution as it enabled all-epitaxial VCSELs that could make use of high contrast AlGaAs-based distributed Bragg reflectors (DBRs) as mirrors and lateral selective oxidation for optical and electrical confinement, thereby mimicking the successful design of short wavelength VCSELs. Although very good device results were achieved, reproducible and reliable epitaxial growth of GaInNAs quantum wells proved difficult and the technology has not made its way into high-volume production. Other approaches to the manufacturing and material problems have been to combine mature InP-based gain regions with high contrast AlGaAs-based DBRs by wafer fusion or with high contrast dielectric DBRs. Commonly, a patterned tunnel junction provides the electrical confinement in these VCSELs. Excellent performance has been achieved in this way but the fabrication process is difficult. In this work, we have employed high strain InGaAs quantum wells along with large detuning between the gain peak and the emission wavelength to realize GaAs-based long wavelength VCSELs. All-epitaxial VCSELs with AlGaAs-based DBRs and lateral oxidation confinement were fabricated and evaluated. The efficiency of these VCSELs was limited due to the optical absorption in the doped DBRs. To improve the efficiency and manufacturability, two novel optical and electrical confinement schemes based on epitaxial regrowth of current blocking layers were developed. The first scheme is based on a single regrowth step and requires very precise processing. This scheme was therefore not developed beyond the first generation but single mode power of 0.3 mW at low temperature, -10ºC, was achieved. The second scheme is based on two epitaxial regrowth steps and does not require as precise processing. Several generations of this design were manufactured and resulted in record high power of 8 mW at low temperature, 5ºC, and more than 3 mW at high temperature, 85ºC. Single mode power was more modest with 1.5 mW at low temperature and 0.8 mW at high temperature, comparable to the performance of the single mode lateral oxidation confined VCSELs. The reason for the modest single mode power was found to be a non-optimal cavity shape after the second regrowth that leads to poor lateral overlap between the gain in the quantum wells and the intensity of the optical field. / QC 20100825