Optical and structural properties of InGaAs self assembled quantum dots

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Capture, relaxation and recombination in quantum dots

Quantum dots (QDs) have attracted a lot of interest both from application and fundamental physics point of view. A semiconductor quantum dot features discrete atomiclike energy levels, despite the fact that it contains many atoms within its surroundings. The discrete energy levels give rise to very narrow optical emission lines at low temperature. When exciting an ensemble of quantum dots, the discrete emission lines are however broadened by the size distribution of the quantum dots. In particular, the advancement of different growth techniques like Molecular Beam Epitaxy and Metal Organic Vapour Phase Epitaxy has lead to the development of different types of ultrapure QD nanostructures with well-defined size, shape and composition. Nowadays QDs can even be grown in laterally ordered patterns. In the last decade, great progress has been obtained in the detailed physical understanding of these semiconductor nano-structures. A complete understanding of the carrier dynamics in QDs, which is essential for exploiting the utilization potential predicted for these QDs, is nevertheless lacking. In this thesis, the carrier dynamics of different types of InAs/GaAs QDs is investigated. When the carriers are photo-generated in the barrier layer much above the QD bandgap, they diffuse towards the QDs, in which they are captured and subsequently undergo relaxation towards the QD ground state. After the relaxation, each electron-hole pair within the QD ground-state with total spin J=±1, recombines radiatively and emits a single photon with an energy corresponding to the separation of the confined energy level within the QD. In this Thesis, QD-materials in which many defects are intentionally included by lowtemperature growth, is also investigated. In this case, the photo-generated carriers in the bulk GaAs-barrier and in the QDs can also be nonradiatively trapped into nearby defect states. An overview of the carrier capture, relaxation and trapping routes in both high quality QD structures and in low temperature grown QD structures is presented in Chapter 2. In this Thesis, the carrier dynamics within III-V semiconductor QDs are studied using the Time Resolved Differential Reflectivity (TRDR) and Photoluminescence (PL) measurement techniques. The TRDR experiment employs a two-color pump-probe system, in which the pump laser has a photon energy tuned above the GaAs barrier bandgap. The optically generated carriers are captured into the QDs inducing a refractive index change of the QDs. The resulting change of the reflectivity of the probe beam, which is tuned in resonance with the QD ground state, is subsequently detected as a function of the pump-probe delay to obtain the carrier dynamics. The reflectivity is interpreted under the assumption that the QD layer can be considered as a thin continuous layer with an effective dielectric constant. In this case, the observed reflectivity signal is basically the interference between the surface reflection and the small reflection from the QD-layer. The advantage of TRDR technique lies in the fact that it is non-destructive since it does not require etching of the substrate. Secondly, the TRDR technique allows the study of the QD-samples not only at low temperature but also up to room temperature. Finally, the TRDR technique allows the study of samples in which the non-radiative recombination is (intentionally) very high and in which the PL efficiency is thus too small to be measured. In Chapter 3, an array of InAs/GaAs QDs grown on a quantum wire (QWR) superlattice (SL) template is investigated to understand the capture and relaxation mechanism in these QDs. This particular QD structure is chosen since it is expected to reduce the carrier diffusion from the bottom barrier layer into the QDs, allowing a more controlled study of the actual carrier capture mechanism. The quantum wire superlattice is spectrally well separated from the QDs, thereby also allowing the study of re-distribution of carriers from the QWR template into the QDs. First, the PL spectrum from this sample is studied and observed a clear carrier re-distribution from the QWR template to the QDs at temperatures between 40 K and 100 K. Subsequently, TRDR experiments were performed, showing a distinct increase of the TRDR rise time between 40 K and 100 K. TRDR experiments as a function of the pump excitation density at 5 K showed a TRDR risetime which decreases from 28 ps at a low excitation density to 5 ps for high excitation density, also showing a plateau for excitation densities above 1 kW cm-2. At room temperature, the TRDR risetime is apparently independent of the excitation density. These observations make it clear that a simple Auger or phonon relaxation picture is not capable of explaining the carrier capture and relaxation in this sample. The data presented in this Thesis can be explained by taking into account the carrier diffusion through the barrier layers towards the capture volumes of the QDs. The diffusion through the barrier is modified by the capture and re-emission of carriers from the superlattice template. At low excitation density, the number of carriers excited directly within the QD capture volume is less than unity, implying that the carrier have to first diffuse through the barrier towards the QDs, thereby resulting in a longer risetime. As the excitation density is increased, more carriers are excited directly within the capture volume, thereby decreasing the risetime. At low temperature, the carriers generated in the bottom barrier layer are all captured by SL template, thus reducing the effective diffusion length. When the temperature increases, carrier re-emission from the SL template increases the effective carrier diffusion length, thus explaining the observed increase of the TRDR risetime. In Chapter 4, a study of an InAs/GaAs Stransky-Krastanow grown quantum rod (QR) ensemble is presented, which have been grown by the leveling and rebuilding technique. In these structures the TRDR decay dynamics was investigated, which was found to be modified by the number of resonant QDs probed within the total QR size distribution. It is found that the TRDR decay time develops a clear resonance at high excitation densities, while at low excitation density the well-known excitonic behavior is regained. At very high excitation density, and also at high temperature, the resonant enhancement of the TRDR decay time gradually decreases. These results are interpreted in terms of an electromagnetic coupling of the excited QRs. Due to this coupling, the QRs collectively decay, giving rise to a polaritonlike picture. The experimental results can be interpreted in a picture in which the TRDR lifetime of the QRs is determined by the separation between identical and excited QRs, which are in resonance with the probe. At low excitation density, the density of excited QRs is too small for such a collective decay behavior, leading to the excitonic life time. At high temperature and also at very high excitation density, the exciton dephasing rate becomes much larger than the radiative decay rate, which washes out the electromagnetic coupling. Chapter 5 presents the investigation of low temperature (250 °C) grown InAs/GaAs QDs which have been subjected to different annealing conditions. The purpose is to investigate the low temperature (LT) grown QDs as a potential device material to be used in ultra-fast optical switching. It was found that the PL-efficiency of the QDs is reasonable at low temperature, but rapidly quenches with increasing temperature and disappears at 40 K. It is also observed that the PL efficiency substantially increases when the LT-QDs are excited below the GaAs bandgap, directly into the QDs. These results indicate that LT growth indeed provides many anti-site defects within the GaAs barrier which act as fast trapping centers for the photo-excited carriers, but the QDs itself are free from these PL quenching centers. Finally, in Chapter 6, the decay dynamics of the LT-QDs is studied using the TRDR technique, since the TRDR technique is expected to be still sensitive at room temperature and for samples in which the PL is quenched by excess nonradiative recombination. From the rapid quenching of the PL-efficiency between 5 and 40 K observed in Chapter 5, a strong decrease of the TRDR decay time was expected for these LT-QDs. The TRDR decay time is however observed to be slow. Quite surprisingly, the TRDR decay time is measured to beindependent of the temperature. These data are interpreted by noting that the photo-excited electrons in the GaAs barrier are very efficiently trapped by the charged anti-site defects, while the holes are less efficiently trapped by neutral anti-site defects. As a result, the holes will be captured within the LT-QDs, while the electrons will mainly remain within the traps in the GaAs barrier. The TRDR signal is proportional to the sum of the electron and hole occupation within the LT-QDs and will thus mainly probe the hole dynamics which is not expected to be ultra-fast. The PL technique probes the product of the electron and hole densities, and thus effectively probes the very small electron occupation within the QDs. The TRDR decay time is interpreted as the trapping recombination time between holes confined within the LT-QDs and the neutral anti-site defect within the GaAs barrier, in close proximity to the LT-QD

Effects of crossed states on photoluminescence excitation spectroscopy of InAs quantum dots

In this report, the influence of the intrinsic transitions between bound-to-delocalized states (crossed states or quasicontinuous density of electron-hole states) on photoluminescence excitation (PLE) spectra of InAs quantum dots (QDs) was investigated. The InAs QDs were different in size, shape, and number of bound states. Results from the PLE spectroscopy at low temperature and under a high magnetic field (up to 14 T) were compared. Our findings show that the profile of the PLE resonances associated with the bound transitions disintegrated and broadened. This was attributed to the coupling of the localized QD excited states to the crossed states and scattering of longitudinal acoustical (LA) phonons. The degree of spectral linewidth broadening was larger for the excited state in smaller QDs because of the higher crossed joint density of states and scattering rate

Strain Balancing of Metal-Organic Vapour Phase Epitaxy InAs/GaAs Quantum Dot Lasers

Incorporation of a GaAs0.8P0.2 layer allows strain balancing to be achieved in self-assembled InAs/GaAs quantum dots (QDs) grown by metal organic vapor phase epitaxy. Tuneable wavelength and high density are obtained through growth parameter optimization, with emission at 1.27 μm and QD layer density 3 × 10 10 cm-2. Strain balancing allows close vertical stacking (30 nm) of the QD layers, giving the potential for increased optical gain. Modeling and device characterization indicates minimal degradation in the optical and electrical characteristics unless the phosphorus percentage is increased above 20%. Laser structures are fabricated with a layer separation of 30 nm, demonstrating low temperature lasing with a threshold current density of 100 A/cm2 at 130 K without any facet coating

Development of High Efficiency III/V Photovoltaic Devices

Developments of photovoltaic (PV) devices are driven by increasing needs for economically competitive renewable energy conversion. To improve the efficiency of PV devices for outdoor applications, the concept of intermediate band solar cell (IBSC) has been proposed to boost the conversation efficiency to 63% under concentrated suns illumination, which requires two-step photon absorption (TSPA) dominates among other competing processes: carrier thermal escape, tunneling and recombination. To optimize the design of III-V QD-IBSCs, first, the effect of electric field on band structure and carrier dynamics and device performances were quantitative investigated via simulation and experiments. Second, to experimentally increase TSPA at room temperature, novel QD systems related QD-IBSCs were designed, fabricated and characterized. The InAs/Al0.3GaAs QD-IBSC shows high TSPA working temperature towards 110K, promising for a room temperature IBSC under concentrated sunlight. Alternative QD systems including GaSb/GaAs and type II InP/InGaP were also investigated via band structure simulations. Meanwhile, developments of PV devices under indoor low intensity light (0.1 µW/cm2-1 mW/cm2) illumination not only enable long lifetime radio-isotope based batteries, but also, more important for the daily life, have the potential to promote an emerging market of internet of things by efficiently powering wireless sensors. Single junction InGaP PV devices were optimized for low intensity light sources using via simulations and statistical control. To reduce the dark current and increase the absorption at longer wavelengths (\u3e550 nm), several parameters including doping and thickness were evaluated. The experimental results on the devices show higher conversion efficiencies than other commercial PVs under varied indoor light sources: 29% under 1µW/cm2 phosphor spectrum and over 30% efficiency under LEDs illumination. In addition, the work includes developments of InAs nanowires epi-growth for PV applications. Several marks for selective area growth were successfully made

Three Essays on Time-Varying Risk Aversion and Investor Sentiment

This dissertation explores issues regarding the effect of investor risk aversion and sentiment on financial markets. It is widely considered that a risk averse investor requires risk premium to hold risky assets, and the required risk premium is proportional to the riskiness of the underlying assets. From the myopic loss aversion perspective, investors, who get utility by frequently evaluating their portfolios and are more sensitive to losses, will require a higher risk premium as compensation for the fear of a major drop in financial wealth. Recent literature has shown that large tail jumps are often contributed to such major drop in financial markets. Rare events, which often accompanied with tail jumps, have a more drastic impact on risk averse investor, and the compensation for rare events accounts for a large fraction of the equity risk premium. However, there is no theoretical framework that has been developed to separate the risk aversion component of rare events from daily volatility.;In this work, I continue to argue the importance of the rare events have different impact on investors decision regarding to equity risk premium. Rational investor risk aversion should spike and require higher risk premium to compensate for higher risk when rare events happen. I develop a theoretical framework to decompose the risk aversion component with rare events from the part associated with daily volatility and prove that they have varying impact on risk premium. Specifically, I first extend the jump-diffusion model incorporated with disaster models and develop a theoretical framework to decompose the risk aversion component of rare events from frequent events. Then I attempt to use monetary surprises as empirical application of the theoretical framework. Bernanke and Kuttner (2005) show that the effect of monetary surprises on expected excess returns can be related to the impact of monetary policy on investor risk aversion or the riskiness of stocks. Finally I test how market response to macroeconomic news surprises conditional on investor sentiment. The results are arranged in the following order.;Chapter 1 of the dissertation makes the argument that investors should treat rare events differently from frequent events. Intuitively, a rare event (disaster) reduces the fundamental value of a stock by a time-varying amount. A rise in disaster probability lowers the expected rate of return on equity, and it also motivates investors to shift toward the risk-free asset or buy deep out-of-the-money puts. Comparing to previous models, I extend the general jump-diffusion model by decomposing investor risk aversion towards frequent and rare events separately. I show that investor treats frequent volatility (quadratic variations) and rare events (tail variations) differently, and decompose representative agent\u27s risk aversion into volatility risk aversion and tail risk aversion. The model implicates that tail risk aversion can be considered as a tractable way to model changes in expectation of rare events and equity risk premium.;In Chapter 2 of the dissertation, I apply the model implication by using monetary surprises and decomposed risk premium from the VIX. Monetary surprises are not only likely to influence stock prices, but also to affect the degree of uncertainty and risk aversion faced by investors. This paper examines the interdependence of stock market, tail risk aversion, and monetary shocks across conventional and unconventional monetary policy periods. The empirical sample is from January 1998 to October 2014. During conventional monetary policy period, I find that there is contemporaneous response among stock market, time-varying risk aversion, and monetary policy under a VAR analysis. Generally, both volatility and tail risk aversion components respond negatively to a positive stock market and monetary policy shock. It indicates that a bullish stock market decreases investor risk aversion, and a positive monetary shock also decreases investor risk aversion as increasing interest rate is considered a good indicator of the economy. During unconventional times, we find that a surprise decline in the expected short-term rate leads to an increase in stock prices and a mixed effect on tail risk aversion. A plausible explanation is that investors believe a surprise drop in expected short-term rate reflects a fast deteriorating economic outlook during unconventional monetary policy period.;Chapter 3 of the dissertation is co-authored project. In this chapter, we consider the effect of investor sentiment from market microstructure perspective. We consider investor sentiment arises when noise traders actively trade on pseudo-signals (Shiller, 1984; Lee, 2001), and investigate the effect of investor sentiment on market reaction to macroeconomic news surprises. Noise traders are considered to respond to new information sub-optimally, and smart-money investors respond to news about fundamental value rapidly and optimally. We utilize both simultaneous news release analysis and normalized estimation proposed by Swanson and Williams (2014). (Abstract shortened by ProQuest.)