Electrochemical fabrication of semiconductor nanostructure arrays for photonic applications

Theoretical and experimental investigations of the properties of semiconductor nanostructures have been an active area of research due to the enhanced performance that is observed when electrons and holes are spatially confined in one, two or three dimensions. However, the development of viable photonic devices using this phenomenon requires the development of appropriate fabrication techniques that can provide control over nanostructure size, material composition, and periodicity for structures with dimensions less than 20 nm. To address these challenges, a nanostructure synthesis technique has been developed that is based on the self-organization of nanometer scale pores during the anodization of aluminum thin films. This template can then be used for direct synthesis of semiconductor material, or as a pattern transfer mask for the etching of structures in a semiconductor substrate.;In this work, alumina template technology has been transferred from the exclusive use of an aluminum substrate to a thin film technology that can be applied to an arbitrary substrate material. This thin film process has been developed and characterized to permit control and uniformity over both nanostructure length and diameter. In addition, a Al/Pt/Si structure has been developed to permit direct DC synthesis of semiconductor nanostructures. Finally, the ability of this template to serve as a mask for direct etching of nanoscale features on a semiconductor substrate has been evaluated. This technology is currently being developed to provide device applications in the area of photovoltaic devices and silicon electro-optic modulators

Control of Optically Induced Currents in Semiconductor Crystals

The generation and control of optically induced currents has the potential to become an important building block for optical computers. Here, shift and rectification currents are investigated that emerge from a divergence of the optical susceptibility. It is known that these currents react to the shape of the impinging laser pulse, and especially to the shape of the pulse envelope. The main goal is the systematic manipulation of the pulse envelope with an optical pulse shaper that is integrated into a standard THz emission setup. The initial approach, the chirping of the laser pulse only has a weak influence on the envelope and the currents. Instead, a second approach is suggested that uses the combined envelope of a phase-stable pulse-pair as a parameter. In a laser pulse, the position of the maxima of the electrical field and the pulse envelope are shifted relative to each other. This shift is known as the Carrier-Envelope Phase (CEP). It is a new degree of freedom that is usually only accessible in specially stabilized systems. It is shown, that in a phase-stable pulse-pair, at least the relative CEP is usable as a new degree of freedom. It has a great influence on the shape of the pulse envelope and thus on the current density. It is shown that this approach enables the coherent control of the current density. The experiments are corroborated by a theoretical model of the system. The potential of this approach is demonstrated in an application. A framework is presented that uses an iterative genetic algorithm to create arbitrarily shaped THz traces. The algorithm controls the optical pulse shaper, and varies the phase of the impinging laser pulses until the desired target trace is found

Fabrication And Characterization Of Electrochemically Formed Nanocrystalline Porous Si And Gaas [TA418.9.N35 B152 2008 f rb].

Bahan hablur nano berjalur tenaga terus dan tak terus (poros silicon, PS dan poros GaAs, π-GaAs) telah difabrikasi dan ciri-ciri optic mereka telah dikaji dengan mendalam. Indirect and direct band gap nanocrystalline materials (porous silicon, PS and porous GaAs, π-GaAs) have been fabricated and their optical properties were extensively studied. In this work, two approaches to manufacture these materials are employed

Electric field dependent spectroscopy of single nanocrystal systems

A suite of single molecule spectroscopic techniques and data analysis methods were implemented to explore the complex role of electric fields in single semiconductor nanocrystal photophysics. This dissertation spans the synthesis, characterization, biological applications, and photophysics of semiconductor nanocrystals. The core single molecule techniques employed in the current work include time-resolved fluorescence, time-correlated single photon counting, single molecule spectroscopy, and photon correlation spectroscopy. Various electrode devices were patterned to investigate the optical properties of single nanocrystal systems under an applied electric field. Electric field dependent spectroscopy and data analysis have revealed distributed kinetics and multiple charging of nanocrystals. In addition, interactions of nanocrystal excited states with plasmonic gold films have revealed strong enhancement of multiple exciton emission from single nanocrystals, and control by an applied electric field. The broader implications of this work can be extended to bioimaging, light harvesting, electro-optics, and lasing technologies

Terahertz Technology for Defense and Security-Related Applications

This thesis deals with chosen aspects of terahertz (THz) technology that have potential in defense and security-related applications. A novel method for simultaneous data acquisition in time-resolved THz spectroscopy experiments is developed. This technique is demonstrated by extracting the sheet conductivity of photoexcited charge carriers in semi-insulating gallium arsenide. Comparison with results obtained using a standard data acquisition scheme shows that the new method minimizes errors originating from fluctuations in the laser system out-put and timing errors in the THz pulse detection. Furthermore, a new organic material, BNA, is proved to be a strong and broadband THz emitter which enables spectroscopy with a bandwidth twice as large as conventional spectroscopy in the field. To access electric fields allowing exploration of THz nonlinear phenomena, field enhancement properties of tapered parallel plate waveguide

Hot-carrier dynamics and transport mechanisms in InAs/AlAsSb multiple quantum wells

Semiconductor photovoltaics convert light into electricity through the extraction of photo-excited charge carriers. Among the most important parameters for a photovoltaic cell are good optical absorption in the desired region of the electromagnetic spectrum, and sufficient excited-state lifetimes and mobilities of the photocarriers to allow for charge separation and extraction before recombination. For solar cell applications there are significant challenges to overcome to improve the efficiency of the light-to-electricity conversion. The cells are most commonly made of silicon, which has a nearly perfect bandgap for absorbing the most solar radiation, an indirect bandgap to give a long photocarrier lifetime and good carrier mobility to allow for extraction of carriers. However, these single p-n junctions suffer from a thermodynamic limit of about 33% (without solar concentration), due to the detailed-balance of photocarriers excited above the bandgap, giving their energy to heating the device. Alternative methods include expensive multijunction devices with several layers absorbing and providing photovoltage at different energies, inefficient nanoparticle configurations and unproven hot-carrier solar cells (HCSCs). The latter were proposed to take advantage of good quality semiconductor materials and find mechanisms to prolong the photocarrier lifetime and allow them to be extracted via energy-selective contacts before recombination can occur. The detailed balance estimate that some HCSC devices may reach a theoretical maximum of 85%. In this dissertation, InAs/AlAsSb type-II aligned multiple quantum well (MQW) heterostructures are explored for their potential as HCSCs. III-V semiconductor devices are grown by molecular beam epitaxy and are of high quality, possess a band alignment that generally separates electrons and holes leading to a prolonged photocarrier lifetime and also exhibit low thermal conductivity that can play additional roles in preventing carriers from cooling and thus further prolonging the carrier lifetime. These systems are of great interest because of these known properties, but it is unclear whether or not they are viable for HCSC, because mechanisms for hot-carrier lifetimes are poorly understood and photocarrier transport had not been explored when the results for the this project was started. Specifically, this work uses terahertz time-domain spectroscopy, time-resolved terahertz spectroscopy and transient absorption to addresses the observation of ground-state AC conductivity, excited-state AC photoconductivity and the charge carrier dynamics. Findings show that lattice temperature significantly influences this MQW’s ground-state carrier transport due to alloy-intermixing at the well interfaces and weak coupling between long-range optical and acoustic phonons. It is also shown that excited photocarrier undergo dynamics that are also dependent on lattice temperature, dominated almost entirely by the availability of defect states in the wells and by localization/delocalization of hole bands with increasing temperature. Further investigation showed that high photocarrier concentration and low lattice temperature revealed a metastable state at early times after photoexcitation due to an intra-subband relaxation bottleneck mediated by reabsorption of optical phonons by the carrier and fast Auger scattering of carriers deeper into their respective bands. Hot-carriers also have long lifetimes outside of this regime, which can be exploited for photovoltaic applications because they generally have high carrier mobilities due to the high quality growth. Moreover, the confinement leads to ambipolar diffusion that can withstand a higher number of scattering events at ambient temperatures where such devices might operate

Optical investigations of nanostructured oxides and semiconductors

This work is motivated by the prospect of building a quantum computer: a device that would allow physicists to explore quantum mechanics more deeply, and allow everyone else to keep their credit card numbers safe on the internet. In this thesis we explore materials that are relevant to a proposed quantum computer architecture.Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature areuseful for devices. Adjusting the ferroelectric transition temperature is traditionally accomplished by chemical substitution, as in barium strontium titanate. We investigate strained-strontium titanate, which is ferroelectric at room-temperature, and a composite material of barium strontium titanate and magnesium oxide.We present optical techniques to measure electron spin dynamics with GHz dynamical bandwidth,transform-limited spectral selectivity, and phase-sensitive detection. We demonstrate the technique with a measurement of GHz-spin precession in n-GaAs. We also describe our efforts to measure single quantum dots optically.Nanoscale devices with photonic properties have been the subject of intense research over the past decade. Potential nanophotonic applications include communications, polarization-sensitive detectors, and solar power generation. Here we show photosensitivity of a nanoscale detectorwritten at the interface between two oxides

Single emitters coupled to bow-tie nano-antennas

Approaching a metallic tip to a single quantum emitter quenches the photolumi- nescence by opening non-radiative decay channels for the excited-state. This is one of the main problems in nano-optics research which prohibits optical studies on single emitters in contact with the tip with high resolution and high sensitivity. In this thesis I have shown that a bowtie nanoantenna can be used to overcome the quenching problem at the single chromophore level. Bowtie antenna tip in- teracts with the dipole of the single emitter. This is most probably the ¯rst study on this type of measurements which opens new pathways for many disciplines. Semiconductor nanocrystals were selected as a single emitter system due to their relatively high photo-stability. Based on °uorescence confocal studies, satura- tion behavior of single CdSefZnSg nanocrystal (NC) is studied under one- and two-photon excitation. In one-photon excitation (1PE) laser wavelength of 532 nm and in two-photon excitation (2PE) laser wavelength of 830 nm were used to excite the °uorophore. Due to the broad distribution in photoluminescence (PL) intensity of nanocrystals, power dependence studies were done based on an average over » 90 nanocrystals. Using focused ion beam, bowtie antennas are sculptured at the apices of silicon nitride AFM tips, which were fully-coated with a homogeneous layer of 40 nm of aluminum ¯lm. Details of structuring procedures used to fabricate well-de¯ned bowtie antennas with smallest possible feedgaps are described. Interaction of bowtie nanoantennas with single semicon- ductor nanocrystals, is investigated using PL intensity and excited-state lifetime of the nanocrystal as two intrinsic signatures of the single emitter. Proximity of the feedgap of a bowtie nanoantenna to a single nanocrystal under one-photon excitation leads to enhanced emission in addition to enhanced excitation. This e®ect is shown, by increasing the PL intensity of the nanocrystal and shortened lifetime, in contact with the bowtie antenna feedgap. These results were com- pared with a fully-coated tip which lead to complete quenching of the nanocrystal PL. Thus, the observed e®ects in PL intensity and lifetime of the nanocrystal in contact with the antenna are originated from the metallic nanostructure. Under two-photon excitation, PL intensity is enhanced but there is no change in the lifetime of the nanocrystal in contact with the bowtie antenna. This is caused by enhanced excitation through the antenna, induced by enlarging the absorption cross section of the nanocrystal in contact with the antenna. Since, °uorescence of single nanocrystal in contact with the bowtie antenna is "not quenched", more detailed studies on their interaction were performed. Under two-photon excita- tion absorption cross section of one nanocrystal was measured with and without the presence of antenna. Free nanocrystal showed a two-photon absorption cross section in the order of 6:3£10¡37cm4s which in contact with the bowtie antenna increased to 20:2£10¡37cm4s. This proves that enhanced excitation observed in 2PE is caused by a larger absorption cross section of the system induced by the antenna structure. Emission polarization of nanocrystals was studied under 1PE using polarization microscopy. From these studies, in- and out-of-plane angles as well as the absolute value of the projection of the transition dipole moment on the sample plane were determined. Results showed in contact with the bowtie antenna, the in-plane angle turns towards the orientation of the antenna. This is induced by the strong dipole of the antenna in contact with the nanocrystal. Moreover, modulation depth and the absolute value of the transition dipole were increased dramatically in contact with the bowtie nanostructure. The results show that antenna/NC system has a highly polarized emission, whose polar- ization direction is determined by the antenna dipole. Photon antibunching of nanocrystals under 1PE was done with and without the presence of the antenna tip. Shorter lifetime of the excited-state in contact with the bowtie antenna immediately appears in antibunching results. This shows that the "dead time" for single photon generation, caused by excitation-recombination cycles, is much shorter in contact with the antenna. Therefore, a nanocrystal in contact with the bowtie antenna is a more e±cient single photon source. Moreover, taking into account the emission polarization of the antenna/NC system, polarization of single photons generated from the nanocrystal in contact with the antenna can be tuned by antenna orientation. Thus, single photons provided by antenna/NC system can have strong potentials in quantum cryptography. As a result, coupling single quantum emitters (here nanocrystal) to bowtie nanoantennas will produce a new type of emitter with widely adjustable photophysical properties, which can be called a "tunable superemitter". Emission characteristics of the antenna/NC system is highly determined by the coupling intra-superemitter

MULTI-PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO-NANOSTRUCTURES

Ph.DNUS-IITM JOINT PH.D