This thesis studies p-i-n GaAs solar cells with self-assembled InAs quantum dots (QDs) inserted. The values of this work lie in three aspects. First, by comparing the cell performance with QDs in the i-region and the n-region, the photocurrent (PC) production from QDs by thermal activation and/or intermediate band (IB) absorption is proved to be much lower in efficiency than tunneling. Second, the efficiency of PC production from QDs, characterized by PC spectrum, is helpful to design QD-based photodetectors. Third, closely spaced InAs QD layers allow a strong inter-layer tunneling, leading to an effective PC production from QD deep states, potential for solar cell application. Fourth, from the temperature-dependent PC spectra the minority photohole thermal escape is found to be dominant on PC production from QDs in the n-region. The thermal activation energy reflects the potential variations formed by electron filling in QDs. Apart from InAs QDs, this thesis also explores the blinking correlation between two colloidal CdSe QDs. For QD distance of 1 µm or less, there is a bunched correlation at delay τ = 0, meaning that the two QDs blink synchronously. Such correlation disappears gradually as QD distance increases. The correlation is possibly caused by the stimulated emission between the two nearby QDs. / QC 20120507

Light manipulation is an important method to enhance the light-matter interactions in micro and nano photonic materials and structures by generating usefulelectric field components and increasing time and pathways of light propagationthrough the micro and nano materials and structures. For example, quantum wellinfrared photodetector (QWIP) cannot absorb normal incident radiation so thatthe generation of an electric field component which is parallel to the original incident direction is a necessity for the function of QWIP. Furthermore, the increaseof time and pathways of light propagation in the light-absorbing quantum wellregion will increase the chance of absorbing the photons.The thesis presents the theoretical studies of light manipulation and light-matter interaction in micro and nano photonic materials and structures, aiming atimproving the performance of optical communication devices, photonic integrateddevices and photovoltaic devices.To design efficient micro and nano photonic devices, it is essential to knowthe time evolution of the electromagnetic (EM) field. Two-dimensional and three-dimensional finite-difference time-domain (FDTD) methods have been adopted inthe thesis to numerically solve the Maxwell equations in micro and nano photonicmaterials and structures.Light manipulation in micro and nano material and structures studied in thisthesis includes: (1) light transport in the photonic crystal (PhC) waveguide, (2)light diffraction by the micro-scale dielectric PhC and metallic PhC structures(gratings); and (3) exciton-polaritons of semiconductor quantum dots, (4) surfaceplasmon polaritons at semiconductor-metallic material interface for subwavelengthlight control. All these aspects are found to be useful in optical devices of multiplebeam splitter, quantum well/dot infrared photodetectors, and solar cells. / QC 20120507

Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others). / <p>QC 20120920</p>

Solar energy harvesting demands low-cost energy conversion in the infrared from 1 – 2 μm. However, solution-processed photovoltaic devices have remained relatively inefficient in this spectral region. Herein, lead sulfide colloidal nanocrystal quantum dots are used to facilitate efficient infrared power conversion. Solution-cast nanocrystal films are employed in a simple metal/semiconductor/metal architecture to produce a photovoltaic effect. It is shown that a Schottky barrier is induced, which is responsible for the charge separating action. Through optimization of chemical processes and device fabrication, the photovoltaic response is maximized. The infrared power conversion efficiency reaches 4.2%, which sets a new precedent for solution-processed photovoltaic cells. Furthermore, the devices exhibit efficient broadband solar power conversion and show promise for multijunction cell architectures. Carrier drift through a large depletion region near the Schottky contact is determined to be the dominant transport mechanism.

Solar energy harvesting demands low-cost energy conversion in the infrared from 1 – 2 μm. However, solution-processed photovoltaic devices have remained relatively inefficient in this spectral region. Herein, lead sulfide colloidal nanocrystal quantum dots are used to facilitate efficient infrared power conversion. Solution-cast nanocrystal films are employed in a simple metal/semiconductor/metal architecture to produce a photovoltaic effect. It is shown that a Schottky barrier is induced, which is responsible for the charge separating action. Through optimization of chemical processes and device fabrication, the photovoltaic response is maximized. The infrared power conversion efficiency reaches 4.2%, which sets a new precedent for solution-processed photovoltaic cells. Furthermore, the devices exhibit efficient broadband solar power conversion and show promise for multijunction cell architectures. Carrier drift through a large depletion region near the Schottky contact is determined to be the dominant transport mechanism.

The commercially available solar cells suffer from low efficiency and high cost. This would avoid presence of solar cells as a secure energy resource in the market. Problems stem from two facts. Firstly, band gap of materials deployed for cell fabrication do not match the solar spectrum. Secondly, harvesting all the generated electrons is imperfect due to presence of many non-radiative recombination processes and, thermalization of electrons. To transcend these deficiencies, third generation of solar has been introduced. This new generation renders a whole new concept both in design and materials of solar cells scope. One of new introduction to solar cell field is Quantum Dot (QD). QD offers a broad range of tunability. The optical and electrical properties of QDs can be altered by choice of material, size and shape; therefore; they have great potential for high efficiency cell fabrication. QDs are mainly grown via MBE or synthesized via Colloidal solutions. QDs could be integrated as a part of one of new and promising third generation cells, named Intermediate Band Solar Cells. QDs could be employed as the intermediate level. If MBE is the selected method for cell fabrication, QDs would grow in a matrix of barrier material accompanied with a wetting layer. Wetting layer would disturb the ideal condition predicted in theory for gaining the high efficiency. To study how wetting layer would affect IB performance two sets of simulations have been carried out. One part is done with COSMOL. In this part different number of QDs layers have been simulated with and without wetting layer. The result showed that parasitic effect of wetting layer could not be eliminated large stacks of QD are stacked together, to achieve the promised efficient wetting layer should be eliminated from the system. In MATLAB part QDs have been approximated with simple cuboid. The main aim in this part was to compare how the result of taking into the account the real shape differs from a simple approach which has been the most reported the most in literature. If all the restrains on achieving high efficiency of IBSC are met, still one major draw- back remains and, that is high cost of MBE process. This would hinder mass production of IB cell. One possible potential method to gradually replace MBE can be Colloidal QDs. Colloidal QDs are fairly low cost and easy to fabricate. In this work, colloidal crystal growth was examined. The best condition for monolayer deposition was obtained and, the feasibility of crystal growth was demonstrated. additionally, There was an attempt to grow more than one layer and investigate result of embedding QDs in a barrier of another material.

Quantum dot (QD) has been known as three-dimensional quantum confined structure. Thus, a delta-function type of density with three-dimensional coulomb interaction can have strong dependence on field-driven optical absorption, i.e. Quantum Confine Stark Effect (QCSE), leading to lots of advantages for applications of electroabsorption modulator (EAM). In this work, based on a GaAs substrate, a self-assembly InAs quantum dot (QD) based p-i-n heterostructure is applied for fabricating electroabsorption modulator. The quantum dot electroabsorption modulation is fabricated by wet-etching technique, where the active region is formed by undercut wet-etching technique using selective etching solution (citric acid). In the device characterization, electro luminescence (EL) is first used to examine the optical transition of QD, showing 1280-1320 nm for ground state and 1220-1240 nm for the excite state. Using the photocurrent spectrum measurement, the red shift of 20 nm in photocurrent peaks from 0 V to 7 V is observed. Also, the peaks exhibit a quadratic relation against with bias, confirming QCSE effect of Q.D.. In the optical transmission measurement, 1300 nm light excites on a 300 £gm long device, obtaining 5 dB extinction by voltage swing of 7 V. By comparing with quantum well (QW) structure, the modulation efficient is in the same order of magnitude. However, the active region of QD volume is at least two orders less than QW, indicating strong QCSE can be obtained from QD and QD can have potential for high-efficient modulation. High-speed EO response with -3 dB bandwidth of 3.34 GHz is also obtained, where the main speed limitation is on the electrical isolation on the n-type GaAs substrate. Through optimizing Q.D. structure and also parasitic capacitance, Q.D. EAM can have a great potential for the application of high-speed optical modulation in optoelectronic fields.

This thesis aims to study external field induced alignment of semiconductor quantum dot by utilizing single molecule spectroscopy. Wurtzite structure semiconductors, such as CdSe, exhibit strong electric dipole moment along its c-axis. It is proposed that quantum dot can be aligned along the applied field with sufficient strength. Experiments with two kind of matrix: PMMA mixing with wax, and liquid crystal thin film, were performed for that quantum dots are able to rotate freely in the matrix. Experiments with PMMA matrix were also performed as its rigid matrix for comparison. Interdigitated structure electrodes was deposited on the cover glass for the electric field experiments. The topical transition (absorption and emission) of CdSe quantum dots has a bright plane perpendicular to its c-axis, and a dark axis along the c-axis. It thus used for characterizing the field alignment. For each observing quantum dot, we record the fluorescence intensity, anti-bunching, polarization anisotropy, and fluorescence lifetime information. In addition, we also analyze the fluorescence correlation spectroscopy to probe the small modulation signal from the fluctuating fluorescence intensity. However, the results indicate that we didn¡¦t observe the field induced change with the field up to 1E7(v/m).

The bandgaps of semiconductors are decreased with increasing temperature which leads to the red-shift lasing wavelength of semiconductor lasers. Therefore, how to stabilize the lasing wavelength under different working temperatures becomes an important issue. The composition and size variation of quantum dots are additional factors which affect the lasing wavelength shift. It is well known that diffusion speeds up with increasing temperature and causes the wavelength shift to occur. To avoid the change of composition and size of quantum dots during growth, the suppression of the diffusion process is necessary to ensure the quantum dots to have a well preserved initial stage. The laser active region with InAs/GaAsN digital alloy quantum well structure was grown by molecular beam epitaxy in this experiment. The self-assembled quantum dots formed in the digital alloy quantum well under high stress. The carriers congregated in the lower energy levels with broadening distribution of composition and size of quantum dots. The peak wavelength shifted toward a longer wavelength with decreasing temperature. The behavior was contrary to the Varshni equation with shrinking bangaps under increasing temperature. Therefore, the sensitivity of the wavelength with temperature decreased. The size distribution of InAs quantum dots on the gradient quantum well broadened under higher arsenic pressure. Consequently, the wavelength sensitivity of quantum dots with temperature decreased. Finally, the InAs quantum dots were capped with the InAlAs quantum well to avoid the diffusion during high temperature growth. The capped InAs quantum dots prevented the wavelength shift from the composition and size variation of quantum dots. For the reason of stabilizing the lasing wavelength of the long wavelength semiconductor laser in optical communication system, it becomes an important topic to create new materials for the active region of the laser structure to avoid the lasing wavelength shift. The next generation temperature insensitive laser devices will be produced with the method which was created in this experiment.

The purpose of this thesis is to design and fabricate cross-loop cavity filter. We fabricated optical filter by bended waveguide and 2x2 90-degree MMI crossing model. By this design, we get the power splitter with coupling coefficient is 0, 0.15, 0.5, 0.85. By MatFhcad and BPM simulation, we showed that the device volume was decrease to 34%. In the quantum dot lasers, we fabricated the Fabry-Perot laser by optical waveguide and cleavage surface. In the material, a 1.3£gm quantum dots InGaAs epitaxial wafer is used to fabricate the lasers. Broad area lasers and ridge waveguide lasers are fabricated and their static properties (IV, LI) are analyzed experimentally. In fabrication process, first, we defined the device pattern by using photo-lithography technique. Second, we etched ridge waveguide by using dry etching method. Finally, we used the etching solution HBr:HCl:H2O2:H2O=5:4:1:70 to smooth the sidewall and reduce the scattering loss. We showed that the waveguide loss was decrease to 27.9dB/cm. In the QD lasers characteristic, we can not observe laser characteristics, partly because of the low optical power. Through the optimization of QD growth conditions, we can increase the QD sheet density and increase the number of QD layers. We can also optimize the device processing techniques and laser structure design in order to reduce the series resistance and to increase the optical confinement factor. By using the methods mentioned above, we believe the laser signal can be further increase. In the cross-loop cavity filter characteristic, we get the FSR=300GHz (simulation value FSR=50GHz) in throughput port and drop port. We attribute this appearance induced by cross couple for 2x2 90-degree MMI. The contracts for the drop port of 10.22dB have been achieved.