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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.

Innovative electronic device concepts that use polarization charges to provide improved performance were validated. The strength of the electric fields created by polarization charges (PCs) was suggested to act as an additional design parameter in the creation of devices using III-nitride and other highly polar materials. Results indicated that polarization induced electric fields can replace conventional doping schemes to create the charge separation region of solar cells and would allow for a decoupling of device performance from doping requirements. Additionally, a model for calculating current through polarization induced tunnel diodes was proposed. The model was found to agree well with experimental current values. Several polarization induced tunnel junction (PTJ) designs were analyzed. A novel double-barrier PTJ was conceived that would allow for the creation of a multi-junction solar cell using strained InGaN absorption layers. Future research would include the fabrication of these devices and the inclusion of thermal effects in the model for calculating current through PTJs.

Approved for public release, distribution is unlimited / In this thesis, a series of experiments were performed to characterize the material properties of InGaAs/GaAs for use in a two-color quantum-well IR photodetector (QWIP) design. Results from room temperature studies using cathodoluminescence and photoluminescence indicated light emission at 858 nm and 1019 nm from GaAs and InGaAs, respectively. Using a direct transport imaging technique, an edge dislocation pattern was observed and shown to be confined to the InGaAs layer of the material. A dislocation density measurement was performed and was shown to be less than 2000 lines/cm. Quantitative intensity level measurements indicated fluctuation in the region of dislocations to be less than 30% of the signal to background level. Finally, a spot mode study using the direct transport imaging method was performed to evaluate the feasibility of using this technique for contact-less diffusion length measurements. / Civilian, Department of Air Force

Experimental investigations of the optical properties of GaN nanostructured light emitting diode (LED) arrays are presented. Microphotoluminescence spectroscopy with pulsed and continuous wave lasers was used to probe the carrier dynamics and emission mechanisms of nanorod LED arrays fabricated by a top down etching method. Results show a possible reduction in internal electric field as nanorod diameter decreases. Localisation effects were also observed, affecting the spectral shape of the nanorod emission. Under two-photon excitation, quantum dot-like sharp spectral peaks in the PL spectra are found to exist in abundance amongst all the nanorod samples. The optical properties of these localised states, which are shown to be associated with the nanorod free-surfaces, are characterised using non-linear and time resolved spectroscopy. An investigation into spatially resolved single nanorods was also carried out. Single nanorods were isolated, and characterised using pulsed lasers. The etching is shown to increase the carrier decay life-time at extended intervals over several hundred ns. The temporal evolution and excitation power density dependence of the quantum dot-like states are also presented for the first time. The long lived localised states are thought to arise from surface effects, in particular Fermi-surface pinning, causing localisation and spatial separation of carriers. Additional work on nano-pyramid array LEDs, with quantum wells on semi-polar surfaces is also presented. Optical properties using micro-photoluminescence are compared to cathodoluminescence studies. An uneven distribution of emission wavelengths across the pyramid facet is thought to lead to an emission mechanism involving carriers transferring between multiple spatially localised states. Finally, experimental techniques and fabrication methods for future work are documented in detail.

This dissertation is a theoretical treatment of the electric field dependence of optical properties such as Quantum Confined Stark (QCS) shifts, Photoluminescence Quenching (PLQ), and Excitonic Mixing in quantum well heterostructures. The reduced spatial dimensionality in heterostructures greatly enhances these optical properties, more than in three dimensional semiconductors. Charge presence in the quantum well from doping causes the potential to bend and deviate from the ideal square well potential. A potential bending that varies as the square of distance measured from the heterostructure interfaces is derived self-consistently. This potential is used to solve the time-independent Schrodinger equation for bound state energies and wave functions within the framework of the Wentzel, Kramers, and Brillouin (WKB) approximation. The theoretical results obtained from the WKB approximation are limited to wide gap semiconductors with large split off bands such as gallium arsenide-gallium aluminum arsenide and indium gallium arsenide—indium phosphide. Quantum wells with finite confinement heights give rise to an energy dependent WKB phase. External electric and magnetic fields are incorporated into the theory for two different geometries. For electric fields applied perpendicular to the heterostructure multilayers, QCS shifts and PLQ are found to be in excellent agreement with the WKB calculations. Orthogonality between electrons and holes gives rise to interband mixing in the presence of an external electric field. On the contrary, intraband mixing between light and heavy holes is not sufficiently accounted for in the WKB approximation.

Neste trabalho, apresentamos estudos sobre o transporte eletrônico de cargas e diagramas de fase no plano ns-B em bicamadas eletrônicas ou poços quânticos duplos, formados de ligas semiconductoras de AlxGa1-xAs e GaAs, assim como também em poços quânticos triplos de GaAs. Para esta finalidade, amostras de poços duplos com diferentes concentrações de Al (x) dentro de cada poço e triplos de GaAs foram crescidas. Inicialmente, se apresenta um estudo teórico, o qual mostrou que, em poços quânticos duplos em que em cada poço a concetração de Al é diferente, a aplicação de tensões de porta permite a modulação do fator g de Landé dos elétrons confinados nesses poços. Em especial, estudou-se o caso de concentrações de Al que correspondem a valores do fator g com sinal oposto, em cada poço. Posteriormente se faz um estudo teórico da estrutura eletrônica das amostras de poços duplos e triplos, em seguida apresenta-se os fundamentos teóricos que serão de base para a interpretação de nosso resultados experimentais. Na primeira parte de nosso trabalho, medidas de magnetotransporte (Shubnikov-de Haas (SdH) e Hall), foram realizadas em todas as amostras de estudo. Na amostra de poço duplo 3242, com fator g de Landé de sinais opostos foi encontrado o colapso do gap de spin nas oscilações SdH com o incremento do campo magnético, ou seja, a soma da energia de Zeeman mais a energia de troca e correlação é igual ao potencial de desordem. Este colapso é atribuido à competição entre as energias de troca, intracamadas e intercamadas. Foi realizada uma análise das oscilações SdH através da transformada de Fourier (FFT), para mostrar que as propriedades eletrônicas tais como a concentração e mobilidade dos elétrons, nas amostras de poços duplos, decrescem à medida que aumenta a concentracão de Al. As propriedades eletrônicas nas amostras de poços triplos dependem dos parâmetros de crescimento, tal como a largura dos poços e barreira. Na segunda parte, são apresentados diagramas de fase ns-B, obtidos através da justaposição dos espectros de magnetorresistência, em amostras de poços duplos e triplos em campo magnético perpendicular e certos valores de campo inclinado. Mostra-se que, em campo magnético perpendicular, o modelo de uma partícula sem interações descreve com boa aproximação o aparecimento dos anéis no diagrama de fase para a amostra de poço duplo com g = -0,44. No entanto, na amostra com g ~ 0 o modelo não descreve em boa aproximação os diagramas de fase em campo magnético perpendicular e inclinado, precisando de um modelo que inclua termos de interação de muitos corpos para uma possível explicação. Também se prediz a existência de um estado canted antiferromagnético. O modelo também mostrará que os diagramas de fase das amostras de poços triplos têm um comportamento semelhante ao das amostras de poços duplos, quando a densidade de elétrons do poço central é baixa comparada com a densidade dos poços laterais. / In this work, we present studies about the electronic transport of charges and phase diagrams in the ns-B plane in electronic bilayers or double quantum wells formed of both AlxGa1-xAs and GaAs semiconductor alloys, also in GaAs triple quantum wells. For this purpose, double quantum wells with different aluminium compositions (Al(x)) in each well and triple quantum wells samples were growth. Firstly, a theoretical study was presented, which showed that in double quantum wells with different Al compositions, the aplication of gate voltages allow the modulation of the Landé g factor of the electrons confined within each well. In particular, the case where the quantum wells have different Al compositions was studied, which lead to the opposite signs of the electronic g-factor in each well. After this, a theoretical study of the electronic structure has been presented of both double and triple quantum wells, then, a basic theory has been presented, which will be the base for the interpretation of our experimental results. At the frst part of our work, magnetotransport measurements (Shubnikov-de Haas (SdH) and Hall) were performed in all the studied samples. In the double quantum well sample (3242), wich has Landé g-factor with opposite signs in each well, was found the spin gap collapse at the Shubnikov-de Haas oscillations with an increase in the magnetic field, that is, the sum of the bare Zeeman energy and exchange potencial energy has the same magnitude of the disorder potencial. This collapse was attributed to the competition between the interlayer and intralayer exchange energies. Fast Fourier transform (FFT) of the Shubnikov-de Haas oscillations was performed in the double and triple quantum well samples to show that the electronic properties, such as electron density and mobility decrease with the increase of the Al compositions. On the other hand, the electronic properties on the triple quantum well samples depend on growth parameters, such as width and heigh barriers of the wells. At the second part ns- B phase diagrams were determined through the superposed longitudinal magnetoresistance, in the double and triple quantum wells samples at the perpendicular magnetic field and certain values of tilted magnetic fields. It has been shown that in a perpendicular magnetic field a single particle model describes in a good aproximation the appearance of ring structures in the phase diagram of the double quantum well with g = -0:44. Meanwhile, at the sample with vanishing Landé g-factor (g ~ 0) the single particle model can not describe in a good approximation the phase diagram, being a requirement a many particle model for an possivel explanation. It has also been predicted the existence of a canted antiferromagnetic state. Finally, the model will also showed the phase diagram of triple quantum wells are similar to double quantum wells, when the electron density of the middle well is low compared to the side wells.

Im Mittelpunkt dieser Arbeit stehen der Transport und die Manipulation optisch angeregter Elektronen-Spins in (110) Quantenfilmen (quantum wells, QWs) mittels akustischer Oberflächenwellen (surface acoustic waves, SAWs). Der starke räumliche Einschluss der Ladungsträger im akustisch erzeugten Potenzial erlaubt spinerhaltenden Ladungsträgertransport mit der akustischen Geschwindigkeit. Auf diese Weise wird langreichweitiger Spintransport über Distanzen > 60 microns demonstriert, welche Spinlebenszeiten von mehr als 20 ns entsprechen. Erreicht werden diese extrem langen Spinlebenszeiten durch drei Effekte: (i) Der D''yakonov-Perel''-Mechanismus ist für Spins in Wachstumsrichtung von (110)-QWs in III-V-Halbleitern unterdrückt. (ii) Aufgrund des Typ-II piezoelektrischen Potenzials der akustischen Oberflächenwelle ist der Bir-Aronov-Pikus Spinrelaxations-Mechanismus sehr schwach. (iii) Der starke Einschluss der Ladungsträger in mesoskopische Bereiche stabilisiert den Spin zusätzlich. In der vorliegenden Arbeit wird erstmals eine Anisotropie des Spintransports in einem externen Magnetfeld (Bext) nachgewiesen. Hierzu wurde die elektronische Spindynamik während des akustischen Transports entlang der [001]- bzw. [1-10]-Richtung untersucht. Während des Transports entlang der [001]-Richtung führt die Präzession der Elektronenspins um das fluktuierende interne Magnetfeld (Bint), das vom Fehlen eines Inversionszentrums im GaAs-Kristallgitter herrührt, zu Spinkohärenzzeiten von etwa 2 ns. Im Gegensatz hierzu ist beim Transport entlang der [1-10]-Richtung die Spinrelaxation für Spins in Wachstumsrichtung um eine Größenordnung langsamer. Grund hierfür ist die endliche mittlere Größe des internen effektiven Magnetfeldes Bint für Transport entlang dieser Richtung. Die beobachtete Anisotropie in der Spindynamik für die beiden Transportrichtungen wird vollständig im Rahmen der Spin-Bahn-Kopplung und des D''yakonov-Perel''-Mechanismus beschrieben und quantitativ erklärt. / In this work, we employ surface acoustic waves (SAWs) to transport and manipulate optically generated spin ensembles in (110) GaAs quantum wells (QWs). The strong carrier confinement into the SAW piezoelectric potential allows for the transport of spin-polarized carrier packets along well-defined channels with the propagation velocity of the acoustic wave. In this way, spin transport over distances exceeding 60 microns is achieved, corresponding to spin lifetimes longer than 20 ns. The demonstration of such extremely long spin lifetimes is enabled by three main factors: (i) Suppression of the D''yakonov-Perel'' spin relaxation mechanism for z-oriented spins in (110) III-V QWs; (ii) Suppression of the Bir-Aronov-Pikus spin relaxation mechanism caused by the type-II SAW piezoelectric potential; (iii) Suppression of spin relaxation induced by the mesoscopic carrier confinement into narrow stripes along the SAW wave front direction. A spin transport anisotropy under external magnetic fields (Bext) is demonstrated for the first time. Employing the well-defined average carrier momentum impinged by the SAW, we analyze the spin dephasing dynamics during transport along the [001] and [1-10] in-plane directions. For transport along [001], fluctuations of the internal magnetic field (Bint), which arises from the spin-orbit interaction associated with the bulk inversion asymmetry of the crystal, lead to decoherence within 2 ns as the spins precess around Bext. In contrast, for transport along the [1-10] direction, the z-component of the spin polarization is maintained for times one order of magnitude longer due to the non-zero average value of Bint. The dephasing anisotropy between the two directions is fully understood in terms of the dependence of the spin-orbit coupling on carrier momentum direction, as predicted by the D''yakonov-Perel'' mechanism for the (110) system.

In der vorliegenden Arbeit werden verschiedene Aspekte des Einflusses von Lokalisation, Unordnung und Polarisationsfeldern auf Elektron-Loch Zustände in Quantengräben (QWs von engl. quantum wells) aus Halbleitern mit großer Bandlücke theoretisch untersucht. Unter Verwendung eines Schwerpunktseparationsansatzes wird das Verhalten von QW Exzitonen und Biexzitonen im Grenzfall schwacher Lokalisation beschrieben. Es zeigt sich, daß die Lokalisationsenergie des Biexzitons mehr als doppelt so groß ist wie die des Exzitons. Dies wird verursacht durch ein universelles Gesetz der Lokalisation in schwachen zwei-dimensionalen Potentialen, welches lediglich durch das "Potentialvolumen" und die Masse des lokalisierten Teilchens bestimmt wird. Ein einfaches Modell des QW Biexzitons wird entwickelt, dessen Ergebnisse gut mit jenen übereinstimmen, die man mit Hilfe eines aufwendigeren numerischen Modells erhält. Der Grenzfall starker Lokalisation von QW Exzitonen und höheren Exzitonenkomplexen wird mittels einer Dichtefunktionalrechnung untersucht. Es wird gezeigt, daß Zustände bis mindestens zum X4 in den nm-großen Potentialminima lokalisieren können, die durch Phasenseparation in (In,Ga)N/GaN QWs enstehen. Es wird das Übergangsspektrum des sukzessiven Zerfalls eines lokalisierten X4 berechnet. Auf Grundlage der selbstkonsistenten Lösung von Poisson- und Schrödinger-Gleichung wird der Einfluß des Probendesigns von (In,Ga)N/GaN QW-Strukturen auf den makroskopischen Verlauf des Polarisationsfeldes in Wachstumsrichtung und somit auf optische Übergangsenergie und Oszillatorstärke systematisch untersucht. Besondere Bedeutung kommt dabei der Abschirmung der Felder durch Raumladungszonen zu. Es wird gezeigt, daß die Position des QW bezüglich einer ausgedehnten Oberflächen-Verarmungszone - die in n-dotierten, Ga-polarisierten Proben auftritt - erheblichen Einfluß auf Übergangsenergie und Oszillatorstärke hat. Durch die räumliche Variation der Polarisationsfeldstärke in dieser Verarmungszone kann das optische Übergangsspektrum eines Mehrfach-QW Schultern oder mehrere Maxima aufweisen. Indium Oberflächen-Segregation ruft eine Blauverschiebung der Übergangsenergie hervor, die bis zu einem Drittel der vom Polarisationsfeld verursachten quantum confined Stark-Verschiebung kompensiert. Diese Blauverschiebung wird von einer Verringerung des Elektron-Loch Überlapps begleitet. Die Polarisationsfelder in (In,Ga)N/GaN Mehrfach-QWs verschmieren das stufenförmige Einteilchen-Absorptionsspektrum. Durch die Aufhebung der näherungsweisen Diagonalität von Inter-Subband Übergängen und durch die Miniband-Dispersion in höheren, gekoppelten Zuständen haben diese Felder, neben dem Beitrag von Potentialfluktuationen, einen entscheidenden Einfluß auf die Form des Absorptionsspektrums. Ein in der Literatur diskutierter Mechanismus, der allein durch Polarisationsfelder eine Verbreiterung optischer Spektren hervorruft, kann nicht bestätigt werden. Unter Annahme einer unkorrelierten Zusammensetzung von (In,Ga)N und einer lateral korrelierten Grenzflächenrauhigkeit von einer Monolage in jeder Grenzfläche zeigt die spektrale Breite des Exzitonen-Schwerpunktpotentials eine Verschmälerung mit zunehmendem Feld. Diese wird verursacht durch das Eindringen der Teilchen in die binären Barrieren und durch ein vergrößertes Exzitonenvolumen. Im Fall einer langreichweitigen Grenzfächenrauhigkeit findet man eine Aufspaltung des Spektrums in einzelne Linien. / In this thesis, various aspects of the influence of localization, disorder, and polarization fields on electron-hole states in wide-gap semiconductor quantum wells (QWs) are investigated theoretically. A theoretical treatment of quantum well exciton and biexciton states in the limit of weak localization is presented, using a center-of-mass separation ansatz. It shows that the localization energy of the biexciton is more than twice as large as that of the exciton due to the universal behaviour of localization in weak two-dimensional potentials which is ruled only by the potential "volume" and the mass of the localized particle. A useful simple model of the QW biexciton wavefunction is developed which provides good agreement with the results obtained with an extensive numerical solution. The limit of strong localization of QW excitons and higher exciton complexes is investigated with a density functional calculation. It is demonstrated that states at least up to X4 may localize in nm-scale potential boxes caused by indium phase separation in (In,Ga)N/GaN QWs. The transition spectrum of the successive recombination of a localized X4 is calculated. A systematic investigation of the influence of the sample design of (In,Ga)N/GaN QW structures on optical transition energy and oscillator strength reveals the importance of space charge layers with regard to screening of polarization fields along the QW-axis. Based on a self-consistent solution of the Schrödinger-Poisson equations, the overall situation of the macroscopic spontaneous and piezoelectric polarization fields is discussed in dependence on various substantial sample parameters. It is found that the position of a QW in the sample with respect to an extended surface depletion layer - which is shown to exist in n-type Ga-face grown material - severely affects transition energy and electron-hole overlap. Due to the spatial variation of the field strength in this surface depletion layer, the optical transition spectrum of a Ga-face grown multiple-QW can display shoulders or even a multiple-peak structure. Indium surface segregation results in a blueshift of the transition energy compensating up to one third of the quantum confined Stark shift produced by the polarization field. This blueshift is accompanied by a decrease of the electron-hole overlap. Polarization fields in (In,Ga)N/GaN multiple-QWs result in a smoothing of the step-like single-particle absorption spectrum. Apart from the contribution of compositional fluctuations, the fields have significant influence on the shape of the spectrum via the abrogation of the nearly diagonality of inter-subband transitions and via the mini-band dispersion of higher coupled states in case of a periodic structure. A line broadening-mechanism due to polarization fields in (In,Ga)N/GaN QWs, as sometimes discussed in literature, could not be confirmed. Assuming uncorrelated (In,Ga)N alloy and in-plane-correlated interface roughness of one monolayer in each interface, the calculation of the spectral width of the QW exciton center-of-mass potential yields a narrowing with increasing average field. This is a result of the penetration of the carriers into the barriers and of an increasing exciton volume. In case of a long-range interface roughness, a splitting of the spectrum into individual lines can be predicted.

Diese Arbeit stellt eine gründliche Analyse der Schwerpunktsbewegung von Exzitonen in Halbleiter-Quantengräben und -Quantendrähten dar. Dabei wurde die k.p-Kopplung der schweren und leichten Löcher im Valenzband sowie das Coulomb-Potential voll berücksichtigt. Die Optimierung der Schwerpunktstransformation auf der Basis eines Ansatzes für die Abhängigkeit des Grundzustands des Exzitons vom Schwerpunktsimpuls Q ermöglichte numerische Ergebnisse hoher Qualität. Es zeigt sich nämlich, daß in einer Subbandentwicklung die Enveloppe des Grundzustands des Exzitons in guter Näherung unabhängig vom Schwerpunktsimpuls ist. So konnten erstmalig Multiband-Exziton-Berechnungen in Quantendrähten mit voller Berücksichtigung der Coulomb-Wechselwirkung durchgeführt werden. Die in dieser Arbeit dargestellten Untersuchungen zeigen interessante physikalische Effekte auf, wie beispielsweise eine nichtmonotone Zunahme der Bindungsenergie des exzitonischen Grundzustands mit wachsendem Q und einen zur entsprechenden Kontinuumskante weitestgehend parallelen Verlauf der Dispersion des exzitonischen Grundzustands. Die Optimierung der Schwerpunktstransformation führt außerdem zu einem analytischen Ausdruck für eine mittlere Masse, die relevant für den exzitonischen Grundzustand ist. / This thesis presents a thorough investigation of the center-of-mass dispersion properties of excitons in semiconductor quantum wells and quantum wires. The k.p coupling of heavy and light holes as well as the Coulomb coupling are taken fully into account. High-quality numerical calculations of the exciton center-of-mass dispersion are achieved by optimizing the center-of-mass transformation, making use of an Ansatz for the dependence of the groundstate exciton upon the center-of-mass momentum Q. Indeed, the envelope in the subband expansion of the groundstate exciton is to a good approximation independent of Q. This technique made possible for the first time multiband calculations in quantum wires that take the Coulomb coupling fully into account. Various physically interesting effects are found and investigated, like, e.g., the non-monotonous increase of the exciton groundstate binding energy with Q or the fact that the exciton groundstate energy follows the exciton continuum edge rather closely. The center-of-mass optimization leads also to an analytical expression for an estimate of the exciton groundstate center-of-mass mass.