Browsing by Subject "Quantum dots"
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Item Adder and multiplier design and analysis in quantum-dot cellular automata(2006) Cho, Heumpil; Swartzlander, Earl E., Jr.Quantum-dot cellular automata (QCA) is an emerging nanotechnology for electronic circuits. Its advantages such as faster speed, smaller size, and lower power consumption are very attractive. The fundamental device, a quantum-dot cell, can be used to make gates, wires, and memories. As such it is the basic building block of nanotechnology circuits. While the physical nature of the nanoscale materials is complicated, the circuit designer can concentrate on the logical and structural design, so the design effort is reduced. Because of its novelty, the current literature shows only simple circuit structures. This research broadens the QCA circuit designs with larger circuits, explores the characteristics of QCA circuit designs, and shows analysis based on those design results. This dissertation proposes three kinds of adder designs in QCA from the conventional adder design approaches. Ripple carry adders, carry lookahead adders, and conditional sum adders are designed for optimization with QCA technology and simulated with several different operand sizes. Using the newly discovered knowledge of the QCA circuit characteristics, new designs for serial adders and multipliers are presented, which are the carry flow adder and the serial multiplier. The carry flow adder design is compared with the previous three adder designs. From the filter design methodology, the carry shift multiplication and the carry delay multiplication algorithms are proposed. The serial multipliers are implemented with both algorithms. The final designs are compared according to the complexity, area, and delay.Item Carrier dynamics in quantum dot and GaAs-based quantum dot cascade laser(2004-05) Cao, Chuanshun, 1972-; Deppe, Dennis G.Self-organized quantum dots provide unique atomic-like density of states and have important applications in semiconductor lasers. Energy relaxation of charge carriers in quantum dots is important for understanding the physics of devices fabricated from these artificially structured materials. Because the charge carriers relax through discrete energy levels, quantum dots provide a means to study the charge carrier interactions in the semiconductor crystal to unprecedented detail. The physics of the charge carrier relaxation is also expected to be substantially modified from that of the bulk semiconductor because of the modification of the electronic density of states. In this dissertation, we will present our work on the simulation of carrier dynamics in the quantum dot and the application of quantum dots in quantum cascade laser. Based on time-resolved InGaAs quantum dot PL measurement, a fourelectron-level quantum dot energy structure model is built up and rate equations are used to simulate the carrier distribution and relaxation in the quantum dot ensemble. By comparing simulated PL intensity and risetime vs. excitation level curves with the experiment results, we conclude that when carriers are excited into the quantum dots wetting layer, the random carrier capture process is dominated by exciton capture. And the spin blocking effect must be considered to explain relative strong first excited state emission under very low excitation level. Quantum cascade laser using quantum dot as its active region is proposed and studied experimentally. Possible advantages of quantum dot cascade laser over quantum well cascade lasers include lower threshold, high efficiency, vertical cavity surface emitting, etc. InAlAs quantum dot with ground state emission close or shorter than GaAs band edge emission has been developed and used as the active region of GaAs based quantum cascade lasers where lattice matched GaAs/AlGaAs superlattice is used for the carrier selective tunneling. Double plasmon-enhanced and Al-free waveguide is designed to form a thin, low loss and high confining waveguide for the mid-infrared emission. Crystal growth, device processing and characterization at cryogenic temperature have been performed.Item Charge control and energy level engineering in quantum-dot laser active regions(2003) Shchekin, Oleg Borisovich; Deppe, Dennis G.The research presented in this dissertation focuses on the improvement of the operating characteristics of GaAs-based InAs and InGaAs quantum dot lasers. The close spacing of discrete hole levels in quantum dots is recognized as one of the main limiting factors in laser performance. The close spacing of levels results in thermal broadening of the hole population, which lowers the optical gain and reduces the differential gain. Two methods are used to enhance laser operation. First, the carrier energy level separation is increased by engineering quantum dot shape and composition. The wide separation of the energy states reduces thermal broadening of carrier populations and is experimentally shown to reduce temperature sensitivity of laser threshold. Second, modulation p-doping of the active region is introduced to compensate for the thermal excitation of holes into higher energy levels. A quasiequilibrium model that includes multiple discrete energy levels and the energy levels of the wetting layer is used to show that active region gain can be controlled by charge carriers built into the QD discrete levels. The results of the calculations demonstrate that with moderate amounts of p-doping used to introduce excess holes in the quantum dots, the active region gain, differential gain, and consequently laser frequency response and To can be dramatically improved. The experimental evidence of the enhancement of gain and temperature performance of p-doped QD lasers is presented. Low-threshold p-doped lasers with high power output and characteristic temperature as high as 213K in CW and 232K in pulsed operation are demonstrated.Item Coherent control and decoherence of single semiconductor quantum dots in a microcavity(2008-08) Flagg, Edward Bradstreet, 1979-; Shih, Chih-kangSemiconductor quantum dots tightly confine excited electron-hole pairs, called excitons, resulting in discrete energy levels similar to those of single atoms. Transition energies in the visible or near-infrared make quantum dots suitable for many applications in quantum optics and quantum information science, but to take advantage of all the properties of quantum dot emission, it is necessary to excite them coherently which has been a great challenge due to background scattering of the excitation laser. This dissertation presents the first coherent control of a single quantum dot with observation of its resonance fluorescence and decoherence phenomena. Strong continuous-wave excitation causes the dot to undergo several Rabi oscillations before emitting. These are visible as oscillations in the first- and second-order correlation functions of the emission, and the quantum dot states are "dressed", resulting in a Mollow triplet in the emission spectrum. Some resonantly excited dots, in addition to resonance fluorescence, also emit light from excited states several meV higher in energy. Such up-conversion fits existing theories of decoherence but has never been directly observed before. The up-conversion intensity is shown to be described well by a fairly simple three-level model with single-phonon absorption. The coherent phenomena of resonance fluorescence and the decoherence due to up-conversion paint a dual picture of single quantum dots wherein they can sometimes be treated as an ideal two-level system, but their interactions with the host crystal can lead to many complex behaviors.Item Design, fabrication and characterization of quantum dot infrared photodetectors(2003-12) Ye, Zhengmao; Campbell, JoeItem Development and optimization of quantum dot-neuron interfaces(2004) Winter, Jessica O.; Schmidt, Christine E.Item Electron dynamics in nanomaterials for photovoltaic applications by time-resolved two-photon photoemission(2013-05) Tritsch, John Russell; Zhu, Xiaoyang, 1963-The impetus of unsustainable consumption coupled with major environmental concerns has renewed our society's investment in new energy production methods. Solar energy is the poster child of clean, renewable energy. Its favorable environmental attributes have greatly enhanced demand resulting in a spur of development and innovation. Photovoltaics, which convert light directly into usable electrical energy, have the potential to transform future energy production. The benefit of direct conversion is nearly maintenance free operation enabling deployment directly within urban centers. The greatest challenge for photovoltaics is competing economically with current energy production methods. Lowering the cost of photovoltaics, specifically through increasing the conversion efficiency of the active absorbing layer, may enable the invisible hand to bypass bureaucracy. To accomplish the ultimate goal of increased efficiency and lowered cost, it is essential to develop new material systems that provide enhanced output or lowered cost with respect to current technologies. However, new materials require new understanding of the physical principles governing device operation. It is my hope that elucidating the dynamics and charge transfer mechanisms in novel photovoltaic material systems will lead to enhanced design principles and improved material selection. Presented is the investigation of electron dynamics in two materials systems that show great promise as active absorbers for photovoltaic applications: inorganic semiconductor quantum dots and organic semiconductors. Common to both materials is the strong Coulomb interaction due to quantum confinement in the former and the low dielectric constant in the latter. The perceived enhancement in Coulomb interaction in quantum dots is believed to result in efficient multiexciton generation (MEG), while discretization of electronic states is proposed to slow hot carrier cooling. Time-resolved two-photon photoemission (TR2PPE) is utilized to directly map out the hot electron cooling and multiplication dynamics in PbSe quantum dots. Hot electron cooling is found to proceed on ultrafast time scales (< 2ps) and carrier multiplication proceeds through an inefficient bulk-like interband scattering. In organic semiconductors, the strong Coulomb interaction leads to bound electron-hole pairs called excitons. TR2PPE is used to monitor the separation of excitons at the model CuPc/C₆₀ interface. Exciton dissociation is determined to proceed through "hot" charge transfer states that set a fundamental time limit on charge separation. TR2PPE is used to investigate charge and energy transfer from organic semiconductors undergoing singlet fission, an analog of multiple exciton generation. The dynamic competition between one and two-electron transfer is determined for the tetracene/C₆₀ and tetracene/CuPc interfaces. These findings allow for the formulation of design principles for the successful harvesting of hot or multiple carriers for solar energy conversion.Item Electron transfer in sensitized TiO₂ systems studied by time resolved surface second hermonic generation(2012-05) Williams, Kenrick John; Zhu, Xiaoyang, 1963-; Bard, Allen J.; Rossky, Peter J.; Webb, Lauren J.; Korgel, Brain A.Obtaining abundant, clean, sustainable energy has become an increasingly large need globally. To date, solar cells have had a limited impact in meeting energy demands. This is primarily due to their relatively high cost and low power conversion efficiencies. Sensitized solar cells, or Grätzel cells, have the potential for being made with low cost materials, and achieving power conversion efficiency high enough to economically compete with fossil fuels. Understanding the dynamics of charge carriers as they separate at the interface of the light absorbing donor and their semiconducting acceptor becomes an important first step in the realization of an inexpensive and efficient sensitized solar cell. Presented is the theory of treating electrons at donor-acceptor interfaces, and why time-resolved surface second harmonic generation (TR-SHG) is used to probe the dynamics of charge carriers at these interfaces. A series of experiments are described where various preparations of thin films of sensitizers on single crystal titanium dioxide, a common acceptor in Grätzel cells, are prepared and studied. TR-SHG studies of thin films of colloidal PbSe and CdSe QDs showed remarkably different electron cooling and transfer dynamics. The electron cooling in PbSe is thermally activated in PbSe QDs. By cooling samples, electron transfer from higher excited “hot” states was observed. Contrary, for CdSe QDs electron transfer rates were dependent on the energy of the excited state. When higher states were excited, charge transfer rates decreased, indicating that only low energy, electrically “cold”, states participate in charge transfer. When carbon based grapheme QDs are used, the electron dynamics mimic PbSe QDs. In this system, increasing the pump energy leads to slower recombination rates, indicating that electrons have to drift further back to the interface.Item Epitaxial regrowth based fabrication process for vertical cavity lasers(2006) Gazula, Deepa; Deppe, Dennis G.Item GaAs-based quantum dot vertical-cavity surface-emitting lasers and microcavity light emitting diodes(2002-05) Zou, Zhengzhong; Deppe, Dennis G.Due to quantum dots’ atom-like density of states, quantum dot lasers have been expected to exhibit ultralow threshold current, temperature-insensitive operation, high modulation speed due to narrow spectral linewidth, high material gain and high differential gain. Quantum dot optoelectronic devices have advanced rapidly since the advent of self-organized QD growth technique. To obtain temperature-insensitive low threshold QD lasers, ground state lasing is highly desirable, the devices should operate close to transparency and far below the saturation, and thermal population of the upper energy levels needs to be suppressed. Three stack InAs/GaAs QD edge-emitting lasers are fabricated and characterized with either 500 Å or 300 Å GaAs spacer thickness. For 500 Å spaced QD lasers, a combination of wide energy separation of 95 meV and relatively high internal efficiency of 74% leads to low threshold operation and high characteristic temperature To of 126 K beyond room temperature. In semiconductor vertical-cavity surface-emitting lasers (VCSELs), extremely high reflectivities are required for both top and bottom mirrors. Critical VCSEL design issues include alignment between the cavity resonance and the optical gain peak, lateral optical confinement, high contrast-ratio DBR mirrors. Ground state lasing of a 1.07 mm oxide-confined InGaAs/GaAs QD VCSEL is demonstrated using intracavity contacts and low loss cavity design, with the room temperature lasing threshold of ~ 700 mA and ~ 270 mA for 10 mm and 2 mm devices, respectively. The much lower experimental cavity Q is shown due to the excessive distributed loss in the upper dielectric mirror. In planar microcavity light-emitting diodes (MCLEDs) enhanced efficiency and narrower spectral linewidth are achieved through modifying the optical mode structure. To fully take advantage of the enhanced mode coupling provided by the microcavity, it is important to electronically confine the carriers to small optical mode volume, which can be achieved in apertured QD-MCLEDs. Apertured QD-MCLEDs are first demonstrated showing the efficiency enhancement with reduced mode size. The highest efficiency of 16 % for apertured QD-MCLED is achieved by designing resonance tuning at low temperature of 160 K.Item High power high efficiency electron-hole and unipolar quantum dot lasers(2007) Quadery, Sonia; Deppe, Dennis G.The goal of this research work is to develop and analyze Quantum Dot (QD) lasers aimed at improving high power performance which is crucial for numerous scientific, military and industrial applications. Fundamentally two dissimilar types of lasers are investigated: namely bipolar electron-hole laser and unipolar quantum cascade laser. Planar quantum well (QW) laser diodes are already well-established as commercially available high power semiconductor lasers. However these lasers are unable to deliver power greater few 10's of watts due to reduction in efficiency at longer cavity lengths. This limitation arises from inherent optical losses tied to the two-dimensional density of available states in QWs. A novel approach is proposed here to circumvent this limitation by introducing self-assembled QDs into the laser cavity which due to their delta-like discrete density of states promise to reduce the optical losses by at least an order of magnitude, hence allowing cavity length to increase proportionally. Detailed analysis based on harmonic oscillator model and solution at quasi-equilibrium condition reveal that total internal losses as low as 0.05 per cm⁻¹ can be achieved in a QD laser enabling it to deliver 50 watts of power from each bar while maintaining efficiency close to 90%. In order to take full advantage of the discrete atom-like behavior, it is also of utmost importance to reduce the inhomogeneous broadening of the dot distribution originating from size fluctuation. Experimental data of ultra narrow linewidth InAs quantum dots having linewidth of only 22 meV is presented. Research attempt has been taken to integrate these narrowly distributed dots into a workable structure. Preliminary data shows that these dots are extremely sensitive to the laser material which calls for careful optimization of the entire structure. As for the unipolar QCL, it is shown that internal absorption caused by phonon emission of electrons in a planar quantum cascade laser represents a possible limitation to the maximum operating efficiency. Possibility of reducing this absorption is explored and it is optimistically asserted that introducing QDs into the gain stage of a QCL can eliminate this internal loss mechanism, thus greatly improving high power operating characteristics.Item III-phosphide semiconductor self-assembled quantum dots grown by metalorganic chemical vapor deposition(2001-08) Ryou, Jae-hyun, 1968-; Dupuis, RussellItem Investigation of the Emission Properties of Quantum Dot-thermoresponsive Polymer Nanocomposite Hydrogels with Temperature(2011-08-08) Juriani, Ameet RajkumarThis thesis presents a novel method for the preparation of quantum dot-thermoresponsive polymer nanocomposite hydrogels. The quantum dots (QD?s) were synthesized in a microwave reactor using a high temperature organometallic synthesis procedure. The initial hydrophobic surface layer on the QD?s was coated with an amphiphilic polymer to enable phase transfer from non-polar solvent to water followed by physical immobilization of the QD?s in the thermoresponsive polymer hydrogel by photopolymerization. Their temperature dependent emission properties were investigated as a function of concentration of the incorporated QD?s. The resultant temperature dependent changes in the position of the peak emission wavelength of the QD-polymer nanocomposite hydrogels were found to be due to the change in the physical environment causing increased interaction between the embedded amphiphilic polymer coated QD?s and/or due to aggregation of QD?s. This change in peak emission position was found to be reversible in the temperature range from 29 to 37 ?C.Item Multicolor colloidal quantum dot based inorganic light emitting diode on silicon : design, fabrication and biomedical applications(2010-12) Gopal, Ashwini; Neikirk, Dean P., 1957-; Zhang, Xiaojing, Ph. D.; Dodabalapur, Ananth; Yu, Edward T.; Becker, Michael F.; Bank, Seth R.Controlled patterning of light emitting diodes on semiconductors enables a vast variety of applications such as structured illumination, large-area flexible displays, integrated optoelectronic systems and micro-total analysis systems for real time biomedical screening. We have demonstrated a series of techniques of creating quantum-based (QD) patterned inorganic light emitting devices at room temperature on silicon (Si) substrate. In particular: (I) A combination of QDs self-assembly and microcontact printing techniques were developed to form the light emission monolayer. We expand the self-assembly method with the traditional Langmuir-Schaeffer technique to rapidly deposit monolayers of core: shell quantum dots on flat substrates. A uniform film of QDs self-assembled on water was transferred using hydrophobic polydimethylsiloxane stamps with various nano/micro-scale patterns, and was subsequently stamped. A metal oxide electron transport layer was co-sputtered onto the QDs. The structure was completed by an e-beam evaporating thin metal cathode. Multicolor light emission was observed on application of voltage across the device. (II) We also demonstrate the photolithographic patterning capability of a metal cathode for top emitting QDLEDs on Si substrates. Lithographic patterning technique enables site-controlled patterning and controlled feature size of the electrode with greater accuracy. The stability of inorganic silicon materials and metal oxide based diode structure offers excellent advantages to the device, with no significant damage observed during the patterning and etching steps. Efficient electrical excitation of QDs was demonstrated by both the methods described above. The technique was translated to create localized QD-based light sources for two applications: (1) Three-dimensional scanning probe tip structures for near field imaging. Combined topographic and optical images were acquired using this new class of “self-illuminating” probe in commercial NSOM. The emission wavelength can be tuned through quantum-size effect of QDs. (2) Multispectral excitation sources integrated with microfluidic channels for tumor cell analyses. We were able to detect the variation of sub-cellular features, such as the nucleus-to-cytoplasm ratio, to quantify the absorption at different wavelength upon the near-field illumination of individual tumor cells towards the determination of cancer developmental stage.Item Next generation transduction pathways for nano-bio-chip array platforms(2009-05) Jokerst, Jesse Vincent; McDevitt, John ThomasIn the following work, nanoparticle quantum dot (QD) fluorophores have been exploited to measure biologically relevant analytes via a miniaturized sensor ensemble to provide key diagnostic and prognostic information in a rapid, yet sensitive manner—data essential for effective treatment of many diseases including HIV/AIDS and cancer. At the heart of this “nano-bio-chip” (NBC) sensor is a modular chemical/cellular processing unit consisting of either a polycarbonate membrane filter for cell-based assays, or an agarose bead array for detection of biomarkers in serum or saliva. Two applications of the NBC sensor system are described herein, both exhibiting excellent correlation to reference methods ((R² above 0.94), with analysis times under 30 minutes and sample volumes below 50 [mu]L. First, the NBC sensor was employed for the sequestration and enumeration of T lymphocytes, cells specifically targeted by HIV, from whole blood samples. Several different conjugation methods linking QDs to recognition biomolecules were extensively characterized by biological and optical methods, with a thiol-linked secondary antibody labeling scheme yielding intense, specific signal. Using this technique, the photostability of QDs was exploited, as was the ability to simultaneously visualize different color QDs via a single light pathway, effectively reducing optical requirements by half. Further, T-cell counts were observed well below the 200/[mu]L discriminator between HIV and AIDS and across the common testing region, demonstrating the first reported example of cell counting via QDs in an enclosed, disposable device. Next, multiplexed bead-based detection of cancer protein biomarkers CEA, Her-2/Neu, and CA125 in serum and saliva was examined using a sandwich immunoassay with detecting antibodies covalently bound to QDs. This nano-based signal was amplified 30 times versus molecular fluorophores and cross talk in multiplexed experiments was less than 5%. In addition, molecular-level tuning of recognition elements (size, concentration) and agarose porosity resulted in NBC limits of detection two orders of magnitude lower than ELISA, values competitive with the most sensitive methods yet reported (0.021 ng/mL CEA). Taken together, these efforts serve to establish the valuable role of QDs in miniaturized diagnostic devices with potential for delivering biomedical information rapidly, reliably, and robustly.Item Optical resonators and quantum dots: and excursion into quantum optics, quantum information and photonics(2007-08) Bianucci, Pablo, 1975-; Shih, Chih-KangModern communications technology has encouraged an intimate connection between Semiconductor Physics and Optics, and this connection shows best in the combination of electron-confining structures with light-confining structures. Semiconductor quantum dots are systems engineered to trap electrons in a mesoscopic scale (the are composed of [approximately] 10000 atoms), resulting in a behavior resembling that of atoms, but much richer. Optical microrseonators are engineered to confine light, increasing its intensity and enabling a much stronger interaction with matter. Their combination opens a myriad of new directions, both in fundamental Physics and in possible applications. This dissertation explores both semiconductor quantum dots and microresonators, through experimental work done with semiconductor quantum dots and microsphere resonators spanning the fields of Quantum Optics, Quantum Information and Photonics; from quantum algorithms to polarization converters. Quantum Optics leads the way, allowing us to understand how to manipulate and measure quantum dots with light and to elucidate the interactions between them and microresonators. In the Quantum Information area, we present a detailed study of the feasibility of excitons in quantum dots to perform quantum computations, including an experimental demonstration of the single-qubit Deutsch-Jozsa algorithm performed in a single semiconductor quantum dot. Our studies in Photonics involve applications of microsphere resonators, which we have learned to fabricate and characterize. We present an elaborate description of the experimental techniques needed to study microspheres, including studies and proof of concept experiments on both ultra-sensitive microsphere sensors and whispering gallery mode polarization converters.Item Resonance fluorescence and cavity quantum electrodynamics with quantum dots(2007-05) Muller, Andreas, 1978-; Shih, Chih-KangNext-generation information technology is expected to rely on discrete two-state quantum systems that can deterministically emit single photons. Quantum dots are mesoscopic (~10,000 atoms large) semiconductor islands grown in a host crystal of larger band-gap that make well-defined two-level quantum systems and are very attractive due to stability, record coherence times, and the possibility of integrating them into larger structures, such as optical microcavities. This work presents experimental progress towards understanding the coherent optical processes that occur in single quantum dots, particularly such phenomena that might be one day utilized for quantum communication applications. High resolution low temperature optical spectroscopy is used in conjunction with first order (amplitude) and second-order (intensity) correlation measurements of the emitted field. A novel technique is introduced that is capable of harvesting the fluorescence of single dots at the same frequency as the laser, previously impossible due to insurmountable scattering. This technique enables the observation, for the first time, of single quantum dot resonance fluorescence, in both the weak and strong excitation regimes, which forms the basis for deterministic generation of single photons. Guided by the rich theoretical description available from quantum optics with atoms we obtain insight into the complex dynamics of this driven system. Quantum dots confined to novel optical microcavities were further investigated using micro photoluminescence. An optical microcavity properly coupled to a two-level system can profoundly modify its emission characteristics via quantum electrodynamical effects, which are highly attractive for single photon sources. The all-epitaxial structures we probe are distinguished by a bulk morphology that overcomes the fragility problems of existing approaches, and provides high quality factors as well as small mode volumes. Lasing is obtained with larger strucutres. Additionally, isolation of individual dots is further realized in smaller cavities and the Purcell effect observed in time-resolved photon counting experiments.Item Self-assembled quantum dots in advanced structures(2012-05) Creasey, Megan Elizabeth; Li, XiaoqinAdvances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs.Item Short-wavelength InAl(x)Ga(1-x)P quantum well lasers and InP quantum dot coupled to strained InAl(x)Ga(1-x)P quantum well lasers grown by MOCVD(2003) Heller, Richard Dean; Dupuis, RussellIII-phosphide self-assembled quantum dot (SAQD) structures offer the ability to realize injection lasers operating in the visible wavelength region from as short wavelength as yellow to deep red with superior performance characteristics, such as low threshold current density and high characteristic temperature. Previously, results of InP QD lasers grown by MOCVD that lased optically pumped pulsed at 300K and continuous-wave (CW) at 77K were described. Further, by incorporating an auxiliary InGaP quantum well (QW) coupled to the QD layers (QW + QD) by a InAlGaP barrier layer due to resonant tunneling, novel QW + QD lasers can be realized with better carrier collection, better thermalization of carriers, and lateral rearrangement of carriers in the QWs; thus giving considerably better laser performance characteristics. The effects of coupling the InP QD states to the electronic states of InGaP QWs will be studied especially with regards to improving device performance and decreasing the wavelength of operation. Further, the effect of adding strained InGaP QWs underneath InP SAQDs will be studied by performing surface morphology analysis. There are several different parameters that affect the growth and the QD areal density was optimized from about 1x1010 cm-2 to as much as 3x1010 cm-2. These InP QD coupled to InGaP QW active regions have been incorporated into various laser separate-confinement active regions. The continuous-wave 300K laser operation of an In0.49Al1-xP/In0.49(AlxGa1-x)0.51P/In0.49Ga0.51P/InP QD coupled to QW laser diode was demonstrated lasing at a visible wavelength (654nm); this is the first reported CW 300K injection laser using InP QDs. Further, by using further novel devices, an InP QD coupled to InGaP QW injection laser was grown and demonstrated at 300K with a lasing wavelength of 607nm, the shortest reported wavelength for any laser diode grown on GaAs. Based on various recombination spectra, it is obvious that the gain peaks at the quantum dot excited state, thus further indicating the coupling of the quantum well states to the quantum dot states. The growth and properties of these novel QW+QD injection lasers will be discussed.