Nanowire quantum dots tuned to atomic resonances

Quantum dots tuned to atomic resonances represent an emerging field of hybrid quantum systems where the advantages of quantum dots and natural atoms can be combined. Embedding quantum dots in nanowires boosts these systems with a set of powerful possibilities, such as precise positioning of the emitters, excellent photon extraction efficiency and direct electrical contacting of quantum dots. Notably, nanowire structures can be grown on silicon substrates, allowing for a straightforward integration with silicon-based photonic devices. In this work we show controlled growth of nanowire-quantum-dot structures on silicon, frequency tuned to atomic transitions. We grow GaAs quantum dots in AlGaAs nanowires with a nearly pure crystal structure and excellent optical properties. We precisely control the dimensions of quantum dots and their position inside nanowires, and demonstrate that the emission wavelength can be engineered over the range of at least $30\,nm$ around $765\,nm$. By applying an external magnetic field we are able to fine tune the emission frequency of our nanowire quantum dots to the $D_{2}$ transition of $^{87}$Rb. We use the Rb transitions to precisely measure the actual spectral linewidth of the photons emitted from a nanowire quantum dot to be $9.4 \pm 0.7 \mu eV$, under non-resonant excitation. Our work brings highly-desirable functionalities to quantum technologies, enabling, for instance, a realization of a quantum network, based on an arbitrary number of nanowire single-photon sources, all operating at the same frequency of an atomic transition.Comment: main text (20 pages, 3 figures) plus supplementary information, Nano Letters (2018

Room Temperature InP DFB Laser Array Directly Grown on (001) Silicon

Fully exploiting the silicon photonics platform requires a fundamentally new approach to realize high-performance laser sources that can be integrated directly using wafer-scale fabrication methods. Direct band gap III-V semiconductors allow efficient light generation but the large mismatch in lattice constant, thermal expansion and crystal polarity makes their epitaxial growth directly on silicon extremely complex. Here, using a selective area growth technique in confined regions, we surpass this fundamental limit and demonstrate an optically pumped InP-based distributed feedback (DFB) laser array grown on (001)-Silicon operating at room temperature and suitable for wavelength-division-multiplexing applications. The novel epitaxial technology suppresses threading dislocations and anti-phase boundaries to a less than 20nm thick layer not affecting the device performance. Using an in-plane laser cavity defined by standard top-down lithographic patterning together with a high yield and high uniformity provides scalability and a straightforward path towards cost-effective co-integration with photonic circuits and III-V FINFET logic

Cavity quantum electrodynamics with three-dimensional photonic bandgap crystals

This paper gives an overview of recent work on three-dimensional (3D) photonic crystals with a "full and complete" 3D photonic band gap. We review five main aspects: 1) spontaneous emission inhibition, 2) spatial localization of light within a tiny nanoscale volume (aka "a nanobox for light"), 3) the introduction of a gain medium leading to thresholdless lasers, 4) breaking of the weak-coupling approximation of cavity QED, both in the frequency and in the time-domain, 5) decoherence, in particular the shielding of vacuum fluctuations by a 3D photonic bandgap. In addition, we list and evaluate all known photonic crystal structures with a demonstrated 3D band gap.Comment: 21 pages, 6 figures, 2 tables, Chapter 8 in "Light Localisation and Lasing: Random and Pseudorandom Photonic Structures", Eds. M. Ghulinyan and L. Pavesi (Cambridge University Press, Cambridge, 2015, ISBN 978-1-107-03877-6

Resilient Pathways to Atomic Attachment of Quantum Dot Dimers and Artificial Solids from Faceted CdSe Quantum Dot Building Blocks.

The goal of this work is to identify favored pathways for preparation of defect-resilient attached wurtzite CdX (X = S, Se, Te) nanocrystals. We seek guidelines for oriented attachment of faceted nanocrystals that are most likely to yield pairs of nanocrystals with either few or no electronic defects or electronic defects that are in and of themselves desirable and stable. Using a combination of in situ high-resolution transmission electron microscopy (HRTEM) and electronic structure calculations, we evaluate the relative merits of atomic attachment of wurtzite CdSe nanocrystals on the {11̅00} or {112̅0} family of facets. Pairwise attachment on either facet can lead to perfect interfaces, provided the nanocrystal facets are perfectly flat and the angles between the nanocrystals can adjust during the assembly. Considering defective attachment, we observe for {11̅00} facet attachment that only one type of edge dislocation forms, creating deep hole traps. For {112̅0} facet attachment, we observe that four distinct types of extended defects form, some of which lead to deep hole traps whereas others only to shallow hole traps. HRTEM movies of the dislocation dynamics show that dislocations at {11̅00} interfaces can be removed, albeit slowly. Whereas only some extended defects at {112̅0} interfaces could be removed, others were trapped at the interface. Based on these insights, we identify the most resilient pathways to atomic attachment of pairs of wurtzite CdX nanocrystals and consider how these insights can translate to the creation of electronically useful materials from quantum dots with other crystal structures

InGaAs/GaAs Quantum Dot Solar Cells by Metal Organic Chemical Vapour Deposition

Along with the ongoing research and industry development to reduce the cost of conventional PV devices such as Si-based solar cells, significant research efforts have been focused on exploring new concepts and approaches for high efficiency III-V compound semiconductor solar cells, especially through the fast emerging nanotechnology to exploit the unique properties of nanostructures such as self-assembled quantum dots (QDs). By incorporating self-assembled QDs into the intrinsic region of a standard p-i-n solar cell structure during the epitaxial growth, photons in the solar spectrum with energy lower than the energy gap of the host material can be absorbed by the QD layers, leading to an extended photoresponse to longer wavelengths and hence larger photocurrent. In addition, the size and composition of the QDs can be varied and thereby allowing the bandgap to be tuned for absorption in different regime of the solar spectrum. However, due to the small QD absorption cross section, the increase of photocurrent in QDSCs is not significant and always accompanied with some reduction in other device characteristics such as the open circuit voltage and fill factor. In this thesis, self-assembled In0.5Ga0.5As/GaAs QDSCs have been designed, fabricated, characterized and investigated in comparison with conventional GaAs p-i-n solar cells. The properties and fundamental mechanisms behind their complicated photoelectrical behaviours were analysed and understood. Several approaches were proposed and carried out to improve the device performance of QDSCs, either during the epitaxial growth process or after the growth and fabrication of the solar cells. Stacking more QD layers is supposed to enhance the total volume of QD material and hence the light absorption. We carried out experiments to grow QDSC structures with increased number of QD layers. However, much reduced photocurrent and conversion efficiency for 15 and 20-layer samples were observed, which could be due to low carrier extraction efficiency and strain-induced defects. In order to improve the carrier extraction efficiency and consequently more enhanced photocurrent, modulation doping has been introduced into QDs layers to partially populate the confined states with electrons. The modulation doping has been found to be effective to improve carrier transport and collection efficiency, leading to an enhancement of the external quantum efficiency over the whole solar cell response range and thus the conversion efficiency. We have also taken two different post-growth approaches to improve the QDSC efficiency, namely the rapid thermal annealing and surface plasmonic light trapping. Firstly, QDSCs with different layers were annealed at various temperatures between 700 and 850 °C with the device annealed at the highest temperature of 850 °C displayed the highest efficiency increase of 41.42 % from 10.26 % to 14.51 %, compared to the as-grown sample. Secondly, it was found that a combination of 120 nm diameter hemispherical Ag nanoparticle and a 5 nm thick TiO2 dielectric film pre-deposited on the back of the GaAs substrate was the optimum light trapping configuration for our QDSC. The QDSC spectral response was improved by 35.7% over the 900 nm- 1200 nm wavelength range, leading to enhancements in both Jsc and Voc and an overall efficiency enhancement of 7.6 % compared to the reference QD solar cell

Scalable photonic sources using two-dimensional lead halide perovskite superlattices

Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling

Epitaxial Growth, Processing and Characterization of Semiconductor Nanostructures

This thesis deals with the growth, processing and characterization of nano-sized structures, eg., self-assembled quantum dots and nano-wires. Such structures are promising candidates for the realization of nano-scale electronic and optical devices, like for instance single electron transistors, resonant tunneling devices, and single photon emitters. For such purposes, the main focus of this work has been on the controlled growth of self-assembled quantum dots. For epitaxy, which is the fundament of this work, low-pressure metal organic vapor phase epitaxy (MOVPE) and ultra high vacuum chemical vapor deposition (UHV-CVD) were used. The structures grown were composed of III/V materials, and SiGe/Si was used for some experiments. For the first group of structures, fundamental investigations on quantum dot growth enabled in-situ growth of InAs/InP self-assembled quantum dot samples in MOVPE. These studies were carried out on freestanding as well as epitaxially overgrown dots. Topography and photo-luminescence were measured with atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FTIR) respectively. InAs/InP low-density quantum dot samples were grown in single or multiple layers, suitable for electrical measurements. These structures were studied by electrical characterization (IV), transmission electron microscopy (TEM), and cross sectional scanning tunneling microscopy (STM). Resonant tunneling through these quantum dots was observed, with peak-to-valley ratios as high as 1300 and negative differential resistance up to a point above the temperature of liquid nitrogen. For the second, more complex, group of structures, patterns on semiconductor surfaces were created, either by electron beam lithography and wet chemical etching, or by the partial overgrowth of electron beam induced carbonaceous material. Spatially ordered growth of III/V and SiGe/Si quantum dots on such patterns was studied by AFM. For the InAs/InP system, conditions were found for which dots could be grown selectively in the patterns by the use of As-P exchange reactions. For the SiGe/Si system, commonly quadruples of islands were observed around each pit. The third group of structures was grown from size selected gold particles, deposited in-house in an aerosol machine, or from Au colloids that were dispersed on the semiconductor surface. These gold particles enabled vapor-liquid-solid (VLS) growth of highly anisotropic one-dimensional structures that were characterized by scanning electron microscopy

III-Nitride Self-assembled Nanowire Light Emitting Diodes and Lasers on (001) Silicon.

Substantial research is being devoted to the development of III-nitride light emitting diodes (LEDs) and lasers, which have numerous applications in solid state lighting. In particular, white LEDs play an increasingly important role in our daily lives. Current commercially available white LEDs are nearly all phosphor-converted, but these have some serious disadvantages. Planar quantum well (QW) devices on foreign substrates exhibit large threading dislocation densities, strong strain induced polarization field, and In-rich nanoclusters resulting in poor electron-hole wavefunction overlap, large emission peak shift with injection, and large efficiency drop at high injection currents in LEDs and large threshold current densities in lasers. The objective of this doctoral research is to investigate the prospects of self-assembled InGaN/GaN disks-in-nanowire (DNW) LEDs and lasers for solid state lighting. The research described here embodies a detailed study of the optical and structural characteristics of the nanowire heterostructures by varying the growth conditions and by surface passivation, and using the disks as the active region in high performance nanowire LEDs and gain medium in nanowire lasers on (001) silicon. Self-assembled InGaN/GaN DNWs are grown in a plasma-assisted molecular beam epitaxy (PA-MBE) system. Due to their large surface to volume ratio, the growth optimized and surface passivated DNWs on (001) silicon are relatively free of extended defects and have smaller polarization field resulting in higher radiative efficiencies. Blue-, green- and red-emitting DNW LEDs, with optimized nanowire densities, are demonstrated with reduced efficiency droop and smaller peak shift with injection. Phosphor-free white nanowire LEDs are realized by incorporating InGaN/GaN disks with different color emissions in the active region. The first ever monolithic edge-emitting electrically pumped green and red nanowire lasers on (001) silicon are demonstrated using DNWs as the gain media and are characterized by low threshold current densities of 1.76-2.88 kA/cm2, small peak shifts of 11-14.8 nm, large T0 of 234 K and large differential gain of 3x10-17 cm-2. Dynamic measurements performed on these lasers yield a maximum small signal modulation bandwidth of 5.8 GHz, extremely low value of chirp (0.8 Å) and a near-zero linewidth enhancement factor at the peak emission wavelength.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111490/1/shafat_1.pd