back to Sections
1) OPTICAL ASSESSMENT OF REACTIVE ION ETCHING OF SEMICONDUCTORS
Key words: Material sciences, nanofabrication, excitons, phonons, dry etching
Fabrication of novel semiconductor devices involves spatial definition of ultra small pieces of material. One of the main techniques used in the fabrication of such devices is Reactive Ion Etching (RIE). During RIE, material may be removed from the unmasked regions by physical and chemical processes involving ions of some 50-1000eV. The exposure of the semiconductor to the impinging ions may cause unwanted side-effects in the crystal such as (a) destruction of the crystal symmetry in the first few 10s of nm, (b) the creation of non-radiative recombination centres, (c) the appearance of strain arising from lattice mismatch strain relaxation and/or from changes in the stoichiometry. Knowledge of these potential changes to the crystalline semiconductor is essential because an increase of non-radiative traps in the material would be detrimental to applications of RIE in the fabrication of, for example, nanoscale optical devices. Similarly, a marked degradation in the conduction properties would badly affect electronic devices.
This project is concerned with the application of advanced optical spectroscopic techniques to the understanding of the damage mechanisms in III-V (eg. GaAs, InP, GaInAs, AlInAs) and wide-gap II-VI (e.g. ZnTe, ZnSe, ZnCdSe, ZnCdTe, CdMnTe, CdTe) semiconductors subjected to reactive ion etching. The techniques include photoluminescence, photoreflectance and phonon Raman scattering. The researcher / student will be expected to become familiar with the interaction of photons and electron-hole pairs and the interaction of photons with lattice vibrations as the main mechanisms involved in photoluminescence and phonon Raman scattering. Liaison with Professor Wilkinson's group working on RIE developments, electrical and crystallographic assessment; with Professor Stanley's group working on the MBE growth of the III-V semiconductors will further enhance the potential success of this (these) project (s); and with several growth groups in Britain and abroad concerning II-VI materials.
This project has benefited tremendously from the use of tripple-axis x-ray diffractometry at Linz University, to quantify and model the strain changes in nanostructures compared to the strain in the as-grown structures.
Key words: quantum dots, quantum wires, III-V and II-VI semiconductors, excitons, phonons, zeolites
Novel quantum dots and wires of dimensions from 6 to 30 Angstroms embedded in a solid matrix hold the promise of reaching lateral sizes where truly quantum phenomena are expected. So far the structural evidence of these materials is in place confirming the existence of nanostructures. These structures are called silicates and zeolites due to the nature of the solid matrix which contains a high density of cavities regularly ordered, which are then filled with III-V and II-VI semiconductor material and form a 3-dimensional network of quantum dots and wires of unique properties. these samples are available to us in collaboration with the Ioffe Physico Technical Institute of St Petersburg, Russia, where the samples are fabricated. Dr Sergei Romanov, a pioneer in this field, visiting regularly our laboratory, is providing the background expertise, and the impetus for this research in Glasgow. So far high density, smallest sizes achieved and preliminary optical studies all suggest a bright future to these "natural" nanostructures especially in the field of photonic bandgaps and of coupled nanostructures.
3) EXCITON CONFINEMENT IN NANOSTRUCTURED FREE- STANDING III-V SEMICONDUCTOR QUANTUM DOTS AND WIRES
Key words: quantum wires, quantum dots, semiconductor physics, excitons, phonons
We have demonstrated 0- and 1-Dimensional quantisation by state-of the-art optical spectroscopy in our nanostructures down to 15nm diameter. There are several questions arising from these experiments and those around the world. In this project it is intended to address the question of how different are the physical dimensions of the quantum dot and/or wire compared to those experienced by excitons and other quantum excitations. There are several approaches and we will concentrate on the use of photoreflectance (collaboration with Prof Pollak, Brooklyn College, CUNY), photoluminescence excitation and photoreflectance to obtain information on the possible potential shape and the energy of the confined levels in GaAs-GaAlAs quantum dots and wires. The use of X-rays will be explored to complement the optical information. There is the possibility of liaison with Dr J H Davies and Dr A M Asenov concerning theoretical modelling of these potentials using Poisson's law and quantum mechanics. Liaison with researchers working with other spectroscopic techniques and those involved in fabrication of the nanostructures will ensure progress in this non-trivial task.
Luminescence studies of single nanostructures are carried out in collaboartion with Boston university using 4K Near-field scanning optical microscopy (NFOM). This technique allows to separate the emission of a single dot from that of an ensemble and thereby potentiatlly to extrcat linewidth and oscillator strength parameters in nanostructures. Experiemtns using quantum rings with view to fabricating light interference devices is in progress. An in-house project using a different NFOM approach is beng pursued.
4) OPTICAL PROPERTIES OF WIDE-GAP II-VI NANOSTRUCTURES
Key words: quantum dots, quantum wires, excitons, exciton-phonon interaction.
Wide-gap II-VI semiconductors are being considered as alternatives for opto-electronic devices operating in the green-blue region of the spectrum. Lasing action in the visible has been reported in several laboratories around the world in the last 3 years. The emphasis of this project is to study the modifications to the exciton-phonon interactions in quantum dots and wires of II-VI's.
As a first step we have made a study of reactive ion etching of II-VI semiconductors. This has enabled the fabrication of wires (dots) of 60 nm width (diameter) and with high aspect ratios, using electron beam lithography and reactive ion etching in methane hydrogen of ZnSe and ZnTe, both grown onto GaAs by MOCVD in Regensburg University. Raman scattering and luminescence spectroscopy of surfaces subjected to reactive ion etching have shown no evidence of crystalline damage but provides some evidence of changes in the occupancy of donor and acceptors. More recently we have investigated the CdMnTe-CdTe system in collaboration with Hull and Montpellier II Universities. The first tests of dot fabrication have revealed 0-dimensional lateral confinement as well as strain-relief or possibly electron-beam-dosage-induced effects. These are the first observation of quantum confinement in wide-gap II-VI's and look extremely promising. The research will consist in assisting with the fabrication of the nanostructures, characterising them optically (excitonic effects) and focusing in the exciton-phonon interaction, with a long term view to assess them for potential light emitting and electro-optical devices in the visible region of the spectrum.
On the device front, preliminary studies of electrical pumping of II-VI laser structures using nanovalves are in progress in collaboration with Heriot-Watt University. Fabrication of II-VI waveguide structures for second harmonic generation is also in progress in collbaoration with Regensburg University.
5) LIGHT EMISSION FROM Si-SiGe NANOSTRUCTURES
Key words: quantum dots and quantum wires, optical properties
Work in Si-SiGe nanostructures is highly relevant to devices and systems since the technologies required for their fabrication are compatible with the main Si technologies. What these nanostructures offer, in addition to the advantages discovered in the 2D case, is enhanced optical emission, which can potentially be turned into light emitting devices. Technologically we have achieved optically active nanostructures down to 15nm of lateral dimensions. Scientifically, we have made a major breakthrough with the observation of many-body interactions in the emission from dots and wires (electron-hole droplet emission enhanced for decreasing lateral sizes). Very recently we have observed strong electroluminescence up to room temperature in Si-SiGe quantum dots. The researcher will be expected to assist in the fabrication of Si-SiGe nanostructures, to become familiar with the optical properties of the 2D system and to develop further the study of electrically driven light emission in nanostructures aiming at the observation of stimulated emission. This work is in collaboration with the Universities of Warwick, Linköping, the IRC in London, Daimler-Benz in Ulm and the Semiconductor Institute in Frankfurt-Oder. Future work is planned to contribute to the optical assessment of directly grown Si-SiGe dots in Aachen's RWTHA. The driving force of this project is Si-compatible light emitters and detectors.
6) PHONONS IN VERY SMALL STRUCTURES
Key words: confined phonons, interface phonons, quantum dots and quantum wires, light scattering
In large crystals, the atomic vibrations of the lattice (i.e. phonons) are approximately those of an infinite crystal. When crystal lattices are small, the effect of the crystal surface on the atomic vibrations must be taken into consideration. Vibrations peculiar to the crystal surface arise, known as surface phonons to distinguish them from the bulk phonons of the infinite crystal. Although these modes are present on the surfaces of all real crystals (since none are infinite in extent) their relative effect is small. As the sample size is reduced the surface to volume ratio of the crystal is increased resulting in a greater contribution from surface modes. Phonon modes can be studied by Raman scattering spectroscopy in very small cylinders of some 30-100nm diameter. The structures are both fabricated and studied in this department.
The results obtained in GaAs have clearly identified surface phonon modes in cylinders of 80nm diameter and up to 310nm height. The phonon frequencies have been successfully modelled by an elastic continuum model. More recently our study of GaAs-GaAlAs quantum dots has shown that multi-phonon resonances are enhanced as a result of exciton localisation phenomena in dots. Moreover, we have observed the contribution of interface phonons and laterally confined phonons in our dots. The relevance of this work concerns the energy relaxation (electronic cooling) processes in nanostructures. It is a hot topic at international levels since the theoretical approaches have just begun to converge and there is a lack of data. Ours is the first batch available but for proper comparisons smaller structures need to be studied. The researcher will become familiar with the assumptions and results of the key phonon models, with the considerations for fabrication of nanostructures and with phonon Raman spectroscopy. It is expected that discussion with leading theorists of Professor Ridley's group at Essex University will take place.
7) QUANTUM DOT-TO-DOT COUPLING STUDIED BY OPTICAL SPECTROSCOPY
Key words: exciton tunnelling, quantum dots quantum wires, quantum dot coupling
Some optoelectronic applications of nanostructures necessitate that they be addressed individually or in small arrays by either electrical or optical signals. Use can be made of the fact that surrounding electromagnetic fields can enhance or inhibit coupling between nanostructures as has already been shown in far-infrared magneto-transmission. The aim of this project will be to carry out an experimental study of possible coupling mechanisms in nanostructures.
The proposed work will entail a training period of 6-9 months in fabrication of nanostructures (shorter for MSc projects with adequate technical support) in a uniquely equipped fabrication laboratory. In parallel the researcher will become familiar with elementary optical spectroscopy techniques and with semiconductor nanostructure physics. Proximity effects will be studied by using varying inter-dot separation in the presence of an electromagnetic field by monitoring electromagnetic modes in phonon Raman scattering. The dots will be fabricated in undoped material grown in Glasgow in the first instance. Work on doped nanostructures will proceed to study Coulomb interaction and exciton tunnelling controlled by Coulomb effects, as recently proposed by G Bryant of the USA.
The materials to be used are GaAs-GaAlAs and GaInAs-AlInAs-GaAs grown by MBE in the department. The project is currently at the stage when the photoluminescence of coupled quantum dots can be alterd by an applied electric field.
8) EXCITONS AND PHONONS IN CORRUGATED SUPERLATTICES AND CLUSTERS
Key words: excitons, phonons, localisation, energy relaxation
This project is based upon the collaboration with the A F Ioffe Physico-Technical Institute of St Petersburg (Dr N N Ledentsov and Prof S A Permogorov). It relies on the growth of corrugated superlattices on (100) and (311) GaAs substrates leading to the formation of regular arrays of exciton binding centres buried in GaAs. These are 3D excitons in nature subjected to a strong localisation field and therefore emitting with high efficiency and linewidth. This property makes a good candidate for optical devices, too. The presence of several buried layers grown on top of each other separated by a layer of GaAs can be interpreted as a superlattice in which phonon folding effects are expected and already predicted. The optical techniques are state of the art optical spectroscopy, with the possibility of carrying out time-resolved emission experiments in collaborating laboratories and exchanges on aspects of theoretical phonon models with Dr Gaspar Armelles in Madrid's National Centre for Microelectronics. The underlying philosophy is to correlate the optical signature with the geometrical arrangement of exciton binding centres and its impact on energy relaxation.
9) EXCITONS BOUND TO A BURIED 2-D LATTICE OF SELF-ORGANISED NANOSTRUCTURES
Key words: excitons, phonons, localisation, energy relaxation, nanofabrication
This project is based upon the collaboration with the A F Ioffe Physico-Technical Institute of St Petersburg (Dr N N Ledentsov and Prof S A Permogorov). It relies on the growth of fractional monolayers of InAs on (100) and (311) GaAs substrates leading to the formation of regular arrays of exciton binding centres buried in GaAs. So far the emission strength and linewidth of the emission from excitons bound to these InAs quasi-wires is the bright and sharp, holding the promise of highly efficient lasers. In this project we will concentrate in optimising exciton emission strength in dots and wires fabricated in fractional monolayer samples, bringing together the advantages of the built-in exciton localisation with the device-flexibility of nanostructure fabrication. Of especial interest are energy and momentum relaxation process, multiphonon processes, determination of the quantized energy levels and associated information to allow the initial design of device structures to take place.
Collaboration with the group at VTT Electronics in Finland is planned for the InP-based system.
10) INFRARED STUDIES OF SEMICONDUCTOR NANOSTRUCTURES
Key words: intersubband transitions in 0-D, absorption saturation, cyclotron resonance, shallow impurities.
This project is a collaboration with Prof Bruce MaCombe of the State University of New York at Buffalo and Prof Carl Pidgeon of Heriot Watt University. It aims at the study of the infrared properties of quantum dot and wire arrays, with view to assess their properties as potential IR photodetectors in the first instance. While the fabrication and preliminary assessment takes place in Glasgow, all the IR work will take place in the laboratories of our collaborators. The basis of this study are the predictions of the "phonon bottleneck" model for energy and momentum relaxation in nanostructures, which suggests a much inhibited across-the-gap emission, which can be expected to enhance within the band (intersubband) transitions occurring in the infrared.
11) II-VI SEMICONDUCTOR LASERS PUMPED BY NANOVALVES
Key words: II-VI quantum well lasers, nanotechnology, electrical pumping
This project addresses the need to pump electrically II-VI quantum well lasers. The approach taken is to use nanovalves fabricated by electron beam lihtography to produce nanometer sized metallic tips lined up with the optical gain area. It is expected that using planar technology these tips or nanovalves can be fabricated in parallel and be powered by a single source. The project will concentrate in improving the fabrication of microtips, testing II-VI laser structures for electroluminescence first at low temperature and then at higher temperatures. The next step will be to increase the current aiming to achive lasing action. The laser characteristics will then be analized. This work is in collaboration with Heriot-Watt University in Edinburgh.
12) OPTICAL CHARACTERISATION OF PHOTONIC BANDGAP MATERIALS
Key words: two-dimensional photonic bandgaps, light emission, waveguides
This project is based on the need to characterize photonic bandgap structures fabricated in the department in collaboration with Prof R De La Rue and Dr T Krauss. The structures are hexagonal two dimensional lattices fabricated by electron beam lithography and dry etching in MBE-grown quantum wells of GaAs-GaAlAs. Initial studies of the angular dependednce of the light emission for comparing structures with different lattice parameters are promising and this project intends to build on these first results.
to E & E Eng. Home page
Nanospectroscopy home page