In this thesis, we investigate the optical properties of m-plane InGaN/GaN multiple quantum well grown by metal organic vapor phase epitaxy. The optical spectroscopies we employed are photoluminescence (PL), polarized PL, power-dependent PL, photoluminescence excitation (PLE), polarized PLE, and Raman scattering. From the blue shift of E2 mode in Raman spectrum, we find that the epitaxial layers are under compressive stress. The PL spectrum at 10 K is dominated by the emission band peaked at 433 nm. We found the optical emission possesses the polarization anisotropy. The degree of polarization is about 80% at room temperature. It is found that the degree of polarization decreases with increasing temperature, which may be explained by carrier population effect. In addition, two major contributions to the PLE spectrum detected for the emission band have identified. Finally, the absence of quantum confined Stark effect is confirmed by power-dependent PL measurements.

Semiconductor nanowires have attracted considerable attention as possible source for lasers and optical storage media. We report the fabrication and optical characterization of ZnO and CdS nanowires. The former are produced by electrochemical deposition of Zn inside nanoporous alumina films containing regimented arrays of 10nm, 25nm and 50 nm diameter pores, followed by room temperature chemical oxidization. Fluorescence spectroscopy shows different characteristics associated with different sample diameter. The 50 nm ZnO nanowires show an exciton recombination peak and an additional peak related to the deep trap levels. 25 nm ZnO nanowires show a only the exciton recombination peak, which is red shifted, possibly due to quantum confined Stark effect associated with built in charges in the alumina. This feature can be exploited to produce light emitting devices whose frequency can be modulated with an external electric field. Such devices could be novel ultra-violet frequency modulators for optical communication and solar blind materials. In addition, we have investigated fluorescence spectra of 10-, 25- and 50-nm diameter CdS nanowires (relative dielectric constant = 5.4) self assembled in a porous alumina matrix (relative dielectric constant = 8-10). The spectra reveal peaks associated with free electron-hole recombination. The 10-nm wire spectra show an additional lower energy peak due to exciton recombination. In spite of dielectric de-confinement caused by the insulator having a higher dielectric constant than the semiconductor, the exciton binding energy increases almost 8-fold from its bulk value in the 10 nm wires. This increase is most likely due to quantum confinement accruing from the fact that the exciton Bohr radius (~5 nm) is comparable to or larger than the wire radius, especially if side depletion is taken into account. Such an increase in the binding energy could be exploited to make efficient room temperature luminescent devices in the visible range.

No description available.

Die vorliegende Arbeit beschäftigt sich mit optischen Untersuchungen von MBE-gewachsenen hexagonalen Gruppe-III-Nitridheterostrukturen. Dafür wird die Photolumineszenz von InGaN/GaN- und GaN/AlGaN-Mehrfachquantengrabenstrukturen umfangreich zeitintegriert und zeitaufgelöst studiert. Die Proben unterscheiden sich in den Dicken der Quantengräben und Barrieren (InGaN) bzw. in der kristallografischen Orientierung (AlGaN). Als Ergebnis der großen, für das Materialsystem charakteristischen, elektrostatischen Felder zeigen die konventionell [0001]-orientierten Heterostrukturen eine verringerte Übergangsenergie und längere Lebensdauern mit zunehmender Quantengrabenbreite und höherem Indiumgehalt in den Gräben. Der beobachtete Einfluss des Quantumconfined Stark-Effektes (QCSE) auf diese Größen kann auch durch Modellrechnungen quantitativ gezeigt werden. In der Arbeit wird erstmals eine umfangreiche optische Charakterisierung einer neuartigen [1-100]-orientierten GaN-Heterostruktur auf Gamma-LiAlO2 geboten. Zum Vergleich wird das Verhalten einer identisch aufgebauten, [0001]-orientierten Struktur auf SiC ebenfalls diskutiert. Die (1-100)-Probe ist in Wachstumsrichtung frei von elektrostatischen Feldern und unterscheidet sich damit deutlich von den herkömmlichen Nitridstrukturen mit [0001]-Orientierung, deren interne Felder im MV/cm-Bereich liegen. Die spektrale Lage der Photolumineszenz bei geringen Anregungsdichten bestätigt die Flachbandsituation in der Probe. Aufgrund des bei dieser Probe nicht auftretenden QCSE ist hier eine deutlich verkürzte Lebensdauer festzustellen. Entsprechend der Auswahlregeln für hexagonales GaN weist die [1-100]-orientierte Probe eine sehr starke Polarisation der Photolumineszenz bezogen auf die Lage der [0001]-Achse auf. Die geringe Abweichung des ermittelten Polarisationsgrades von der, für A-Exzitonen in Volumen-GaN zu erwartenden, totalen Polarisation kann durch das Konfinement in den Quantengräben erklärt werden. Ein Schwerpunkt der Arbeit ist die Untersuchung der Rekombinationsmechanismen der Proben in Abhängigkeit von der induzierten Ladungsträgerdichte. Diese wird in einem Bereich von sehr geringer Dichte bis über die Mottdichte variiert. Eine Abschirmung der elektrostatischen Felder mit zunehmender Ladungsträgerdichte wird festgestellt. Dabei kann bei einer InGaN/GaN-Probe mit 3.1 nm breiten Gräben gezeigt werden, dass neben den internen piezoelektrischen Feldern die in der Literatur diskutierte Lokalisation von Exzitonen an Stöchiometrieschwankungen des Quantengrabens entscheidend die Rekombinationsdynamik in der Probe beeinflusst. Dies spiegelt sich in einer Abhängigkeit der Quantengrabeneigenschaften von den Anfangsbedingungen des Abklingprozesses und damit einem nicht existierenden allgemein gültigen Zusammenhang zwischen der Lebensdauer und der Ladungsträgerdichte wider. Die zeitaufgelösten Lumineszenzspektren der InGaN/GaN-Strukturen zeigen als Folge der mit höheren Ladungsträgerdichten zunehmenden Abschirmung eine verringerte Lebensdauer durch die vergrößerte Überlappung von Elektron- und Lochwellenfunktionen. Aufgrund der wieder abnehmenden Abschirmung während des Rekombinationsprozesses verändert sich die Lebensdauer im Laufe der Zeit. Gleichzeitig kommt es zu einer Verringerung der Übergangsenergie des Lumineszenzmaximums durch den weniger abgeschirmten QCSE. Die zeitintegrierten Photolumineszenzspektren zeigen ebenfalls eine deutliche Abhängigkeit von der Anregungsdichte. Während bei der feldfreien (1-100)-Probe keine Kompensationseffekte erwartet werden, weisen die Resultate für die konventionellen Proben auf einen, die Ladungsträgerdichte wesentlich beeinflussenden Effekt hin. Die Abhängigkeit der Intensität der Photolumineszenz von der Ladungsträgerdichte deutet ab einer bestimmten Anregungsdichte auf einen zusätzlichen Prozess, welcher die Ladungsträgerdichte reduziert, sich aber nicht im Lumineszenzspektrum widerspiegelt. Als Erklärung dafür wird die Absorption von stimulierter Emission im Substrat oder in der Pufferschicht angenommen. Bei den InGaN-Proben schiebt die Übergangsenergie mit höheren Dichten zu größeren Energien und nähert sich bis 10e5 W/cm2 einem Sättigungswert an. Dieser Wert entspricht trotz Dichten oberhalb der Mottdichte noch nicht der Flachbandsituation bei vollständig kompensierten internen Feldern. Als Ursache dafür wird der genannte, bei hohen Ladungsträgerdichten einsetzende Konkurrenzprozess gesehen. Bei den GaN/AlGaN-Proben kann im untersuchten Bereich der Anregungsdichte keine spektrale Verschiebung im Photolumineszenzspektrum festgestellt werden. Zum ersten Mal werden experimentelle Untersuchungen zur stimulierten Emission einer [1-100]-orientierten GaN-Probe durchgeführt und das optische Gewinnspektrum analysiert. Die Messungen zeigen einen maximalen Nettogewinn von ca. 50 1/cm. Aus der rechnerischen Analyse der Modenausbreitung lässt sich dafür ein Materialgewinn für GaN(1-100) von 1.1x10e4 1/cm ableiten. Die Ergebnisse zeigen außerdem, dass die Rekombination eines Elektron-Loch-Plasmas der Mechanismus für die stimulierte Emission ist. Dies entspricht dem überwiegenden Teil der in der Literatur veröffentlichten Beobachtungen für [0001]-orientierte Nitridstrukturen. Ein direkter Vergleich mit der parallel untersuchten GaN/AlGaN(0001)-Probe ist aufgrund der starken Substratabsorption nicht möglich. Es zeigt sich, dass für [1-100]-orientierte GaN-Heterostrukturen gute Ausgangsbedingungen für die Realisierung von Laserdioden gegeben sind. Zu den untersuchten Heterostrukturen wird die Wellenführung in den Proben simuliert. Bei den auf SiC gewachsenen Schichten werden die sich ausbreitenden Moden wegen des deutlich höheren Brechungsindexes des Substrates vornehmlich dort geführt. Die Überlappung der Moden mit dem aktiven Schichtpaket ist äußerst gering. Es ist für die Proben auf SiC kein optischer Gewinn zu erwarten. Die [1-100]-orientierte GaN/AlGaN-Probe besitzt eine deutlich bessere Wellenführung, da das LiAlO2 einen vergleichsweise kleinen Brechungsindex besitzt. Es wird ein Zusammenhang zwischen experimentell ermitteltem optischen Gewinn und dem Materialgewinn gebildet und das Ergebnis mit Rechnungen aus der Literatur verglichen. Ein Vorschlag für eine optimierte Wellenführung in allen untersuchten Proben wird gegeben. / In this thesis, the optical properties of molecular beam epitaxy grown hexagonal group-III nitride heterostructures are studied. The photoluminescence (PL) characteristics of InGaN/GaN and GaN/AlGAN multiple quantum well structures are investigated by time-integrated and time-resolved measurements. The analyzed specimens differ in the width of the quantum wells and barriers (InGaN) and in the crystallographic orientation (AlGaN), respectively. As a result of the large characteristic electrostatic fields, conventional [0001]-oriented heterostructures show a reduced transistion energy and longer lifetimes with increasing well width and higher Indium content in the wells. The observed impact of the Quantum Confined Stark Effect (QCSE) on these quantities is quantitatively shown in model calculations. In this work, a first extensive optical characterization of a novel [1-100]-oriented GaN heterostructure grown on Gamma-LiAlO2 is presented. For comparison, an identically designed [0001]-oriented structure on SiC is discussed. The (1-100)-grown specimen is free of electrostatic fields along the growth direction and shows a significant different behavior than conventional [0001]-oriented nitrides with internal fields of several MV/cm. The existing flat band conditions are confirmed by the spectral position of the PL at low excitation densities. Due to the non-existing QCSE at this specimen an significantly reduced lifetime is observed. A strong polarization of the PL is observed for the [1-100]-oriented sample, following the selection rules for hexagonal GaN. The small deviation of the degree of polarization from unity, which is expected in bulk GaN, is attributed to the quantum confinement in the heterostructures. One main topic of this thesis is the analysis of the recombination mechanisms of the specimens depending on the induced carrier density. The carrier density is varied from very low upto densities above the mott density. A screening of the electrostatic fields is observed with increasing carrier density. It is shown, that an InGaN/GaN heterostructure with a well width of 3.1 nm not only is influenced by internal piezoelectric fields but also the localization of excitons at stoichiometric inhomogenities in the quantum well is playing an important role for the recombination dynamics of the structure. This can be seen in the dependence of the decay process on the starting conditions. No general correlation is existing between lifetime and carrier density. Time-resolved PL measurements on InGaN/GaN heterostructures show a reduced lifetime due to an increased overlap of the electron and hole wave functions as a result of the increased screening at increasing carrier densities. During the recombination process the screening decreases again and the lifetime is changed with time. Simultaneously the transistion energy of the PL maximum is reduced by the less screened QCSE. A distinct dependence of the time-integrated PL spectra on the excitation density was also found. While there are no compensation effects expected at the (1-100) structure, which is free of electrostatic fields, the results for the conventional specimens point to an effect which influences the carrier density essentially. The dependence of the PL intensity on the carrier density points to an additional process, which comes into play at a special excitation density. This process reduces the carrier density but is invisible in the PL spectra. As an explanation we assume, that light of stimulated emission is absorbed either in the substrate or in the buffer layer. The transistion energy of the InGaN structures increases with increasing excitation density and reaches a saturation energy at a density of 10e5 W/cm2. Although this density is larger than the mott density, the transistion energy is not equivalent with a transition energy at flat band conditions. The origin of the observed effect is assumed to be the rival process, mentioned above, which comes into play at high carrier densities. For the GaN/AlGaN heterostructures no spectral shift of the PL was observed within the variation of excitation density. For the very first time, the stimulated emission of an [1-100]-oriented GaN structure was analyzed. A maximum netto gain of 50 1/cm was observed. From calculations of the mode propagation, a material gain of 1.1x10e4 1/cm is derived for GaN(1-100). Additionally from the results follows that the recombination of an electron-hole-plasma is the mechanism of the stimulated emission. This is in accordance with most of the published observations for [0001]-oriented GaN heterostructures. A direct comparison of both, the [1-100]-oriented specimen and the GaN/AlGaN(0001) structure, which was investigated parallel, was not possible. The reason for that is the strong absorption of the SiC substrate of the latter mentioned structure. It is generally shown, that [1-100]-oriented GaN heterostructures offers good starting conditions to realize laser diodes. The wave guiding was simulated for all of the used specimens. At structures grown on SiC the propagating modes are mainly guided in the substrate due to the larger refractive index of SiC with respect to GaN. The overlap of the amplified mode and the active layer is very small. No optical gain is expected for these structures. The [1-100]-oriented GaN/AlGaN structure shows a significantly improved wave guiding, due to the small refractive index of LiAlO2 in comparison with GaN. A correlation between the experimentally observed optical gain and the material gain is formed and the results are compared with the literature. A suggestion for an optimized wave guiding in all investigated specimens is given.

The quantum confined Stark effect (QCSE) and ultrafast absorption dynamics near the bandedge have been investigated in p-i-n waveguides comprising quantum confined heterostructures grown on GaAs substrates, for emission at 1.3um. The materials are; isolated InAs/InGaAs dot-in-a-well (DWELL) quantum dots (QD), bilayer InAs quantum dots and GaInNAs multiple quantum wells (MQW). The focus was to investigate these dynamics in a planar waveguide geometry, for the purpose of large scale integration in optical systems. Initial measurements of the QCSE using photocurrent measurements showed a small shift for isolated QDs whilst a significant shift of 40nm (at 1340nm) was demonstrated for bilayer dots, comparable to that of GaInNAs MWQ (30nm at 1300nm). These are comparable to InP based quaternary multiple quantum wells used in modulator devices. With the use of a broadband continuum source the isolated quantum dots exhibit both a small QCSE (15nm at 1280nm) and minimal broadening which is desirable for saturable absorbers used in monolithic modelocked semiconductor lasers (MMSL). A robust experimental set-up was developed for characterising waveguide modulators whilst the electroabsorption and electro-refraction was calculated (dn=1.5x10⠻³) using the Kramers-Kronig dispersion relation. Pump probe measurements were performed at room temperature using 250fs pulses from an optical parametric oscillator (OPO) on the three waveguide samples. For the isolated QDs ultrafast absorption recovery was recorded from 62ps (0V) to 700fs (-10V and the shortest times shown to be due to tunneling. Additionally we have shown good agreement of the temperature dependence of these dots and the pulse width durations from a modelocked semiconductor laser using the same material. Bilayer QDs are shown to exhibit ultrafast absorption recovery from 119ps (0V) to 5ps (-10V) offering potential for applications as modelocking elements. The GaInNAs multiple quantum wells show absorption recovery of 55ps (0V), however under applied reverse bias they exhibit long lived field screening transients. These results are explained qualitatively by the spatial separation of electrons and holes at heterobarrier interfaces.

abstract: This dissertation aims to study and understand relevant issues related to the electronic, spin and valley transport in two-dimensional Dirac systems for different given physical settings. In summary, four key findings are achieved. First, studying persistent currents in confined chaotic Dirac fermion systems with a ring geometry and an applied Aharonov-Bohm flux, unusual whispering-gallery modes with edge-dependent currents and spin polarization are identified. They can survive for highly asymmetric rings that host fully developed classical chaos. By sustaining robust persistent currents, these modes can be utilized to form a robust relativistic quantum two-level system. Second, the quantized topological edge states in confined massive Dirac fermion systems exhibiting a remarkable reverse Stark effect in response to an applied electric field, and an electrically or optically controllable spin switching behavior are uncovered. Third, novel wave scattering and transport in Dirac-like pseudospin-1 systems are reported. (a), for small scatterer size, a surprising revival resonant scattering with a peculiar boundary trapping by forming unusual vortices is uncovered. Intriguingly, it can persist in arbitrarily weak scatterer strength regime, which underlies a superscattering behavior beyond the conventional scenario. (b), for larger size, a perfect caustic phenomenon arises as a manifestation of the super-Klein tunneling effect. (c), in the far-field, an unexpected isotropic transport emerges at low energies. Fourth, a geometric valley Hall effect (gVHE) originated from fractional singular Berry flux is revealed. It is shown that gVHE possesses a nonlinear dependence on the Berry flux with asymmetrical resonance features and can be considerably enhanced by electrically controllable resonant valley skew scattering. With the gVHE, efficient valley filtering can arise and these phenomena are robust against thermal fluctuations and disorder averaging. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

InGaN based light emitting devices operating in the blue and near UV spectral regions are commercialized and used in many applications. InGaN heterostructures experience compositional inhomogeneity and thus potential fluctuations, such that regions of higher indium composition are formed and correspond to lower potentials. The indium rich regions form localization centers that save carriers from non-radiative recombination at dislocations, thus despite the large defect density, their quantum efficiency are surprisingly large. However, the conventional c-plane InGaN QWs suffer from high internal piezoelectric and spontaneous fields. These fields are detrimental for the performance of such structures as they lead to the quantum confined stark effect causing red-shift of the emission as well as reducing the electrons and holes wavefunctions overlap, thereby reducing the radiative recombination rate. However, growth of InGaN QWs on semipolar and nonpolar planes greatly reduced the polarization fields. Semipolar and nonpolar QWs experience an outstanding property which is polarized luminescence, opening a new frontier for applications for InGaN emitting devices. While nonpolar QWs have larger degree of polarized emission than semipolar QWs, semipolar QWs can emit in longer wavelengths due to their higher indium uptake. In this thesis, semipolar 20¯21 and nonpolar m-plane InGaN/GaN QWs were investigated. Photoluminescence, spectral and polarization dynamics were all studied in order to form a whole picture of the carrier dynamics in the QWs. Time resolved photoluminescence measurements were conducted for following carriers distribution between extended and localized states. Both the semipolar and nonpolar samples showed efficient luminescence through short radiative recombination times, as well as carrier localization in lower potential sites after thermal activation of excitons. Carrier localization was found to be benign as it didn’t degrade the performance of the samples or decrease the polarization ratio of their emission. However, the structures showed modest potential variations with the absence of deep localization centers or quantum dots. High polarization ratios were measured for both samples, which is well-known for nonpolar QWs. The high polarization ratio for the semipolar sample is of great importance, thus semipolar 20¯21 QWs should be considered for longer wavelength emitters with highly polarized spontaneous emission.

This PhD thesis is devoted to study electro-optic properties of Gemanium/Silicon-Germanium (Ge/SiGe) multiple quantum wells (MQWs) for light modulation, detection, and emission on Si platform. It reports the first development of high speed, low energy Ge/SiGe electro-absorption modulator in a waveguide configuration based on the quantum-confined Stark effect (QCSE), demonstrates the first Ge/SiGe photodiode with high speed performance compatible with 40 Gb/s data transmission, and realizes the first Ge/SiGe light emitting diode based on Ge direct gap transition at room temperature. Extensive DC and RF measurements were performed on each tested prototype, which was realized using the same epitaxial growth and fabrication process. Simple theoretical models were employed to describe experimental properties of the Ge/SiGe MQWs. The studies show that Ge/SiGe MQWs could potentially be employed as a new photonics platform for the development of a high speed optical link fully compatible with silicon technology.

Cette thèse est consacrée à l’étude des propriétés optiques et optoélectroniques autour de la bande interdite directe des structures à puits quantiques Ge/SiGe pour la modulation, la photodétection et l’émission de lumière sur la plateforme silicium. Les principaux composants réalisés sont : un modulateur optique en guide d’onde, rapide et à faible puissance électrique, basé sur l’Effet Stark Confiné Quantiquement, les premières photodiodes Ge/SiGe dont le comportement fréquentiel est compatible avec les transmissions de données à 40 Gbit/s, et la première diode à électroluminescence à puits quantiques Ge/SiGe, base sur la transition directe de ces structures et fonctionnant à température ambiante. Les caractérisations statiques et fréquentielles ont été réalisées sur l’ensemble des composants, qui ont tous été fabriqués avec la même structure épitaxiée et les mêmes procédés de fabrication. Des modèles théoriques simples ont ensuite été utilisés pour décrire analyser les comportements observés. Finalement les études menées permettent de conclure que les structures à puits quantiques Ge/SiGe sont un candidat de choix pour la réalisation d’une nouvelle plateforme photonique à haut débit, totalement compatible avec les technologies silicium. / This PhD thesis is devoted to study electro-optic properties of Gemanium/Silicon-Germanium (Ge/SiGe) multiple quantum wells (MQWs) for light modulation, detection, and emission on Si platform. It reports the first development of high speed, low energy Ge/SiGe electro-absorption modulator in a waveguide configuration based on the quantum-confined Stark effect (QCSE), demonstrates the first Ge/SiGe photodiode with high speed performance compatible with 40 Gb/s data transmission, and realizes the first Ge/SiGe light emitting diode based on Ge direct gap transition at room temperature. Extensive DC and RF measurements were performed on each tested prototype, which was realized using the same epitaxial growth and fabrication process. Simple theoretical models were employed to describe experimental properties of the Ge/SiGe MQWs. The studies show that Ge/SiGe MQWs could potentially be employed as a new photonics platform for the development of a high speed optical link fully compatible with silicon technology.

Designs for deep ultraviolet (DUV) edge emitting laser diodes (LDs) based on the wurtzite III-nitride (III-N) material system are presented. A combination of proprietary and commercial advanced semiconductor LD simulation software is used to study the operation of III-N based DUV LDs theoretically. Critical factors limiting device performance are identified based on an extensive literature survey. A comprehensive design parameter space is investigated thoroughly with the help of advanced scripting capabilities. Several design strategies are proposed to eliminate the critical problems completely or partially. A DUV LD design is proposed based exclusively on AlInN active layers grown epitaxially on bulk AlN substrates because AlInN offers a promising alternative to AlGaN for the realization of LDs and LEDs operating in the DUV regime. The proposed AlInN-based design also features a tapered electron blocking layer (EBL) instead of a homogeneous one. Tapered EBLs redistribute the interfacial polarization charge volumetrically throughout the entire EBL thickness via compositional grading, and eliminate the parasitic inversion layer charge. AlGaN based DUV LD designs are explored also because at present, it may be difficult to grow AlInN epitaxially with superior crystalline quality. Polarization charge matching is proposed to improve electron and hole wavefunction overlap within the active region. Although the strategy of polarization charge matching has already been proposed in the literature to enhance performance of visible wavelength LEDs and LDs, the proposed design presents the first demonstration that polarization charge matching is also feasible for DUV LDs operating at sub-300 nm wavelengths. A lateral current injection (LCI) LD design is proposed featuring polarization-charge-matched barriers and regrown Ohmic contacts to avoid a group of issues related to the highly inefficient p-type doping of wide bandgap III-N materials in vertical injection designs. The proposed design partially decouples the problem of electrical injection from that of optical confinement. Although the idea of an LCI LD design has been proposed in the literature in the 90s to be used as longer wavelength active sources in optoelectronic integrated circuits using GaInAsP/InP and related material systems, the proposed design is the first theoretical demonstration that this concept can be applied to DUV LDs based on III-N material system. To solve the problem of hole transport in vertical injection designs, a DUV LD design based exclusively on AlGaN material system is presented, featuring an inverse-tapered p-waveguide layer instead of an EBL. Several EBL designs are investigated, and compared with conventionally-tapered EBL design. Through judicious volumetric redistribution of fixed negative polarization charge, inverse tapering may be exploited to achieve nearly flat valence band profiles free from barriers to hole injection into the active region, in contrast to conventional designs. Numerical simulations demonstrate that the inverse tapered strategy is a viable solution for efficient hole injection in vertical injection DUV LDs operating at shorter wavelengths (< 290 nm).