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Optoelectronic Devices and Materials Group Professor Alf R Adams FRS Professor of Physics After spending two years at the University of Karlsruhe (Germany) studying the electrical andthermal transport properties of molecular crystals, Alf Adams joined Surrey in 1967. There hestarted the first work on III-V semiconductors, which now forms the core of the work of theOptoelectronic Devices and Materials Group. His main interest at the moment is in the physics ofsemiconductor lasers and light emitting diodes for use in telecommunications and informationtechnology. He was awarded the Duddell Medal and Prize in 1995 for his work on strained-layerquantum well lasers. Professor Jeremy Allam BSc, PhD (Surrey) Professor of Ultrafast Optoelectronics Jeremy Allam took up this joint appointment in the Department of Physics and ElectronicEngineering at Surrey in October 2000. He obtained his first degree in Physics from the Universityof Oxford, and his PhD from Surrey. After working for 2 years at AT&T Bell Laboratories and for 3years as a postdoc sponsored by British Telecom, in 1990 he joined the newly formed HitachiCambridge Laboratory. As Senior Researcher and Group Leader of Femtosecond Optoelectronics,he was responsible for the contribution to Hitachi’s optoelectronics effort and visited Hitachi’sCentral Research Laboratory in Japan, and the business groups and factories associated withoptoelectronic devices and systems. His present research interests are in ultrafast opticalmethods for characterisation of optoelectronics devices, and high field transport insemiconductors. Dr David A Faux BSc, MSc, PhD (Birmingham), CPhys, MInstP Senior Lecturer After studying for a PhD at Birmingham University, David Faux spent 2 years at North CarolinaState University investigating a variety of problems using Monte Carlo computer techniques. Herea theoretical research programme was established for the computer simulation of semiconductorstrained-layer growth. He was appointed as a lecturer at the University of Surrey in 1988 andpromoted to Senior Lecturer in 1995. Research interests include the calculation of thestress/strain and piezolectric fields in strained-layer, quantum-wire and quantum-dot structures.Other interests include modelling of water in zeolites. Dr Thomas J C Hosea BSc, PhD (Edinburgh) Senior Lecturer After completing his BSc and PhD degrees in his native Edinburgh, Jeff Hosea spent a further twoyears there studying crystalline phase transitions in ferroelectrics using Raman and neutronscattering techniques. He then spent 11 years in Australia and Singapore in various teaching andresearch activities including absorption spectroscopy of impurities in insulators, laser- materialsinteractions, automated laser-cutting devices, and Brillouin/Raman scattering from both crystallineand amorphous/soft materials. At Surrey a programme of research was established to investigatemodulation reflectance spectroscopy of optoelectronic semiconductor materials and devices.

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Dr Stephen Hughes BSc, PhD (Heriot Watt) Lecturer Steve obtained a PhD in Theoretical Optoelectronics in 1995 from Heriot-Watt University. He thenspent 9 months at Marburg University (Germany), 3 months at the University of Arizona in Tucson(USA), 18 months at Tokyo University (Japan) and 24 months at Washington State University(USA) before joining Surrey in October 1999. His research interests encompass the fundamentalphysics and coherent dynamics of optical interactions in solid-state materials and optoelectronicdevices with applications to semiconductor lasers, Thz-fields, ultrahigh speed optical switching andextreme non-linear optics. Dr David Lancefield MSc, PhD (Surrey) Lecturer After finishing a PhD at Surrey, David Lancefield spent a further three years as a Research Fellowfunded by DERA, Malvern studying high pressure transport properties of bulk InP, GaAs andquantum wells made from In0.53Ga0.47As/InP. He identified the importance of impurity correlationeffects on electron transport in heavily doped materials and developed detailed modellingprogrammes for bulk electron transport. He joined the staff of the department in 1988. Since thenhe has developed electrical and optical semiconductor characterisation techniques as a function oftemperature, pressure and magnetic field.

Dr Ben Murdin MSc, PhD (Edinburgh)Lecturer Ben Murdin came to the Department in August 1996. He was born in Rochester, NY, USA, wentto primary school in Sydney, Australia and secondary school in Hastings, Sussex. Aftergraduating from Cambridge in Natural Sciences, he studied for an MSc and later PhD at Heriot-Watt University in Edinburgh, followed by a 3 year post-doc in Utrecht, the Netherlands. His maininterest is the dynamics of charges and their interaction with other excitations in semiconductors,with application to optoelectronic devices. He works closely with several universities andinstitutions across Europe and the USA and is a regular user of two Free-Electron lasers in theNetherlands and France. Professor Eoin P O’Reilly BA, PhD (Cambridge), CPhys, MInstP Professor of Physics and Head of the Physics Department After studying for a PhD at the University of Cambridge on the electronic and structural propertiesof amorphous semiconductors, Eoin O’Reilly spent one year as a Research Associate in Illinoisand two years as an assistant lecturer in Dublin. He joined the University of Surrey as a lecturer in1984. His research interests are primarily in the theoretical modelling of the electronic structure ofadvanced semiconductor materials, investigating how the physical processes in these materialsenable (or limit) optoelectronic device applications. This has led to several fruitful collaborationsboth in the UK and in Europe, including leading the theoretical effort in two current Europeancontracts (GaN-based blue lasers and visible vertical cavity lasers) as well as an EPSRC fundedproject on mid infra red laser diodes.

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Long Wavelength Quantum Dot Lasers

Supervisor(s): Professor AR AdamsBack-up Supervisor(s): Professor EP O’Reilly

Major Aims:• To investigate the basic optoelectronic properties of InAs quantum dots grown on InP substrates

and to produce the first temperature insensitive lasers operating at 1.5µm wavelength for opticalfibre applications.

Techniques used and source of expertise:• The quantum dots will be grown at the Centre for Electronic Materials and Devices at Imperial

College, London. These will be characterised at Surrey, fabricated into lasers and the deviceproperties measured and compared to theory.

Despite considerable worldwide effort over the last 20 years, lasers designed to operate at 1.5µm, whereoptical fibres have their minimum loss, have fairly high threshold currents, which more importantly, are alsovery temperature sensitive. This makes packaging extremely complicated since expensive temperaturemonitoring and control circuitry must be included. Although this is not a major problem for major trunkroutes, it inhibits the use of these lasers in many smaller data networks or fibre-to-the-home applications,which are highly cost-sensitive markets. In this proposal we intend to fabricate In-plane lasers for operationat 1.55µm using InAs/InP self-assembled QDs in the active region. These form automatically during growthsince the lattice constant of InAs is so much larger than InP it cannot grow as a continuous epitaxial layer.The islands of InAs that form are so small that they have the electronic properties of giant atoms. Such QDswith narrow atomic-like densities of states are expected to provide a solution to the problems of temperaturesensitivity in long wavelength lasers.1. This may prove to be the single most important prize to be gained bymoving to QD systems: low cost, low power devices operating at 1.55µm.

Work at Surrey and elsewhere has shown clearly that the high temperature sensitivity of 1.55µm quantumwell (QW) lasers grown on InP is due to the presence of non-radiative processes such as Augerrecombination1 and IVBA4. It is the removal of these by going to QD structures that is the aim of the presentproposal. Currently the drive towards QD lasers involves the deposition of InAs on GaAs for emissionaround 1µm. Success so far has been limited and a consensus is emerging that this is due to the 3D islandsize distribution which leads to inhomogeneous spontaneous emission widths of 100 meV. We haverecently demonstrated that control over the size distribution can be obtained by tight control of the growthparameters during MBE2 or by post-growth annealing techniques3. There are six main objectives to thisproposal:• To demonstrate long wavelength emission in InAs/InP and InAs/InGaAsP/InP QDs.• To establish the electronic structure of the dots and to study interband and intersubband transitions

using modulated reflectivity and fast pulse mid-IR spectroscopy.• To show that these QDs can be used as the active layers in in-plane lasers to achieve improvements in

threshold current and temperature stability and to quantify the roles of radiative and non-radiativeprocesses using temperature and pressure techniques.

• To show that the wavelength can be pre-determined and the operating wavelength of 1.55µm achievedroutinely by modifying the size/composition of the QDs.

• To optimise the conditions for growing InAs/InP and InAs/InGaAsP/InP QDs by MBE.• To establish theoretically the optimum laser structure for 1.55µm operation at room temperature and

above.

This project will give the student excellent training in the fundamental concepts and the experimentaltechniques that are presently in great demand in the booming optoelectronic communications and dataprocessing industries that feed the internet.

1 SJ Sweeney, AF Phillips, AR Adams, EP O’Reilly, PJ Thijs, Photon. Technol. Lett. 10, 1067 (Aug. 1998).2 H Yu, S Lycett, C Roberts and R Murray “Time resolved study of self assembled InAs quantum dots”,

Appl. Phys. Lett. 69 4087(1996).3 S Malik, C Roberts and R Murray “Tuning of self-assembled InAs quantum dots by rapid thermal

annealing”, Appl. Phys. Lett. 71 1987(1997).4 AR Adams, M Asada, Y Suematsu, S Arai, Jap.J Appl. Phys 19, 621,1980.

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Long Wavelength Semiconductor Lasers Using Nitrides

Supervisor(s): Professor AR AdamsBack-up Supervisor(s): Professor EP O’Reilly

Major Aims:• To develop superior semiconductor lasers for use in chemical analysis and medicine by the

introduction of nitrogen into standard III-V semiconductor materials

Techniques used and source of expertise:• The electroluminescence and photoluminescence characteristics of nitride containing III-V

optoelectronic materials and devices will be measured as a function of temperature, pressureand magnetic field. The work will be carried out in close collaboration with others in theOptelectronic Devices and Materials reseach group who are studying the theory of thesematerials

The III-V alloy InGaSbN shows quite remarkable band structure properties that raise exciting possibilities foroptoelectronic applications. Although InN has a much larger band gap than InSb, when a low concentrationof N is incorporated into InSb there is a very strong decrease in the optical band gap. This is very interestingboth theoretically and practically: theoretically, because the behavior cannot be explained by standardmodels of III-V alloys and so new approaches are clearly needed; practically, because it is believed that asthe band gap decreases due to the introduction of N, the optoelectronic properties of the materials mayactually improve instead of degrading as in all other semiconductor systems (For a brief discussion of Augerrecombination and intervalence band absorption see project description on "Infra red lasers and theirapplication to polution detection). Therefore, it may be possible to fabricate for the first time, semiconductorinterband lasers working CW at room temperature in the 3-5µm wavelength range. This is very importantbecause it is in this range that many chemical and biological molecules have strong vibrational modes. If wecan achieve a good theoretical understanding of the band structure and optical properties of InGaSbN, thepossibility exists that we may be able to propose novel and superior optoelectronic device structures. Toachieve these aims we propose to undertake measurements of the alloy composition, pressure, temperatureand magnetic field dependencies of, i) the optical and electrical properties of InGaSbN quantum wells and ii)the gain and loss processes in laser diodes. The results will be used to refine our theoretical model whichthen will be used to predict optimum laser structures.

This project will give the student a good understanding of many of the basic principles and experimentaltechniques widely used in the data processing and communications industry.

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Investigation of the Processes Limiting the Output Power ofSemiconductor Lasers (Experimental)

Supervisor(s): Professor AR Adams/Professor EP O’ReillyBack-up Supervisor(s): Professor EP O’Reilly/Professor AR Adams

Major Aims:• To discover the major physical processes that limit the output power of semiconductor lasers and

hence suggest optimum structures to increase this power.

Techniques used and source of expertise:• Measure Roll-off in Light-current characteristic as function of pulse length.• Measure temperature in the active region by studying the spontaneous emission spectrum (SES).• Measure the degree to which the carrier concentration pins by measuring (SES).

As the demands of the internet and for optical data storage grow there is a steadily increasing demand for higheroutput power from semiconductor lasers. While improvements have been achieved by introducing strainedquantum wells into the structures1, the processes that limit the power are not well understood. There are severalpossible causes, which will be studied in this project. • Heating of the active layer by Joule heating and loss processes such as Auger recombination and

intervalence band absorption (See project on"Infra-red lasers for pollution detection" for a fuller description ofthese processes). This will be evaluated by

• Measuring the threshold current, Ith, and quantum efficiency, η, at high powers as a function of the currentpulse length.

• Studying the high-energy tail of the spontaneous emission spectrum from the end facet.• Measuring the variation of the lasing wavelength with power.• Looking for spontaneous emission at involving high energy holes generated by the loss processes. eg at

Eg+∆.• Failure of the stimulated emission to pin the carrier concentration at high powers. This will be studied by

measuring the total integrated spontaneous emission from a window etched in the substrate electrode.• Spectral hole burning. This will be determined from the spectral dependence of the spontaneous emission

observed from the window close to the lasing line.• Free carrier absorption. The growth of the carrier density with current in different parts of the laser structure

will be determined again from the spectral variation of the window emission with current. This should showany increase in emission from the guiding and cladding regions with build-up in carrier concentration.

• Mode hopping. This will be determined by studying the laser wavelength and any kinking in the light-currentcharacteristic and will be related to such things as mode beating effects.

The investigations described above involve experimental techniques widely used in industry and will give thestudent excellent experience for a career in the burgeoning field of optoelectronics in communications. 1 AR Adams and EP O’Reilly, “Semiconductor lasers take the strain”, Physics World, 43-47, October 1992.

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Dynamics of Optical Gain in Dilute-Nitride Semiconductor Lasers

Supervisor(s): Professor J AllamBack-up Supervisor(s): Professor AR Adams

Major Aims:• To measure optical gain in GaInNAs (“Guinness”) dilute-nitride semiconductor lasers.• To investigate dynamics of gain depletion and recovery.• To pursue operation at longer wavelengths in both GaInNAs and in dilute-nitride antimonides.

Techniques used and source of expertise:• Ultrafast optical methods using femtosecond light pulses from a titanium-sapphire laser.• Upconversion measurements using optical parametric oscillator.

Professor Allam has experience with the optical ultrafast methods.Professor Adams is an expert in semiconductor lasers including ‘Guinness’.

The student will require:• good hands-on skills with complex experiments• some knowledge of lasers, optics and semiconductor physics• physical insight to guide the experiments performed• willingness to talk and collaborate with theorists, and industrial collaborators and hence suggest

optimum structures to increase this power.

Recently-demonstrated semiconductor lasers incorporating small amounts of nitrogen (‘dilute nitrides’)exhibit both fascinating new physical properties and useful practical applications (e.g. a surface-emittinglaser for optical fibre communications built on GaAs substrates). Unlike other conventional alloysemiconductors, the electron effective mass increases when the energy band gap is reduced by addingnitrogen. This has a profound influence on the strength of electron-photon and electron-electron interactions,and consequently on every aspect of the performance (threshold current, etc.) of semiconductor lasers. Thework at Surrey is performed in close collaboration with Infineon and with the University of Marburg(Germany), and Surrey has been recognised as a world-leader in understanding the underlying physicswhich governs the interaction of semiconductor energy bands with resonant Nitrogen states.

This project will use optical upconversion measurements of femtosecond light pulses injected intosemiconductor lasers to investigate the gain, and the dynamics of gain depletion and recovery. Initialmeasurements will use GaInNAs lasers operating at 1.3µm. An aim of the work is to extend operatingwavelengths towards 1.55µm. Furthermore, initial calculations suggest that antimonide alloys incorporationdilute nitrides may offer the new possibility of extending the operation of conventional interband lasers tomid-infrared wavelengths. Such long-wavelength lasers may also be studied depending on the availability ofthe dilute nitride antimonide alloys.

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Femtosecond Dynamics in Mid-Infrared Semiconductor Optical Devices

Supervisor(s): Professor J AllamBack-up Supervisor(s): Dr BN Murdin

Major Aims:• To develop experimental methods for femtosecond studies of mid-infrared semiconductor

optical devices, using an optical parametric amplifier light source.• To compare the switching dynamics of mid-infrared devices employing interband, type II and

intersubband transitions.• To understand the physical processes limiting the dynamics of mid-infrared sources and

detectors, and hence facilitate their optimisation.

Techniques used and source of expertise:• Investigation of dynamics of semiconductor optical devices with ~100fs resolution.• Use of advanced femtosecond laser light sources (Optical Parametric Amplifier (OPA).• Pump-probe and upconversion experiments in the mid-IR.

Professor Allam has experience with optical ultrafast methods; Dr Murdin is an expert in mid-infraredmeasurements using Free Electron lasers. Both of these complementary sources will be used in the project.

The student will require:• good hands-on skills with complex experiments• some knowledge of lasers, optics and semiconductor physics• physical insight to guide the experiments performed• willingness to talk and collaborate with theorists, and industrial collaborators.

‘Interband’ semiconductor lasers and light detectors (where photon absorption or emission is associated withelectron transitions between conduction and valence bands) offer excellent performance at visible and near-infrared wavelengths, and are widely used in optical storage (CDs, DVDs) and communication (with opticalfibres). However, they perform less well at mid-infrared wavelengths, where there are important potentialapplications including process control and environmental monitoring. Recently, new mid-IR devicesexploiting ‘Type II’ interband transitions or intersubband transitions, have shown very promising results.Manyproperties of these lasers are determined by the electron-electron and electron-phonon scattering rates. Thisproject will study these scattering rates, and their influence on laser properties including the switching time,using a dual-OPA light source producing femtosecond pulses simultaneously in the mid-infrared(intersubband transitions) and near-infrared (interband transitions) wavelengths. This will complement theongoing investigations of Dr. Murdin using the FELIX free electron laser.

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Femtosecond Dynamics in Semiconductor Lasers, Amplifiers and Optical Switches

Supervisor(s): Professor J AllamBack-up Supervisor(s): Dr S Hughes

Major Aims:• To understand the physical processes limiting the speed of real-world optical devices• To generate a comprehensive set of experimental data for comparison with both advanced

theory and systems measurements.• To investigate the effects of different structures and materials, to produce semiconductor

lasers, amplifiers and switches with optimised switching times.• To investigate new physical effects in high-intensity, coherent, short-duration pulses

propagating in semiconductor optical devices.

Techniques used and source of expertise:

• Ultrafast optical methods using femtosecond titanium-sapphire laser systems.• Optical Parametric Oscillator (OPO) and Optical Parametric Amplifier (OPA) light sources.• Mixed temporal-spectral methods (e.g. “FROG” - frequency-resolved optical gating) for

determining amplitude and phase dynamics.• Design of new semiconductor structures with optimised dynamics.

Professor Allam has experience with most of the ultrafast methods and sources described here.Experiments will be performed using new state-of-the-art equipment. Dr Hughes is an expert in thetheoretical modelling of femtosecond optical processes in semiconductors and will provide theoreticaldirection to the project. We expect that the project will be performed in collaboration with a major industrialphotonics company who will provide advanced semiconductor structures and the capability for measuringhow the devices perform in real communications systems.

The student will require:• good hands-on skills with complex experiments• some knowledge of lasers, optics and semiconductor physics• physical insight to guide the experiments performed• willingness to talk and collaborate with theorists, and industrial collaborators• optical devices, using an optical parametric amplifier light source• compare the switching dynamics of mid-infrared devices employing interband, type II and

intersubband transitions• understand physical processes limiting the dynamics of mid-infrared sources and detectors,

and hence facilitate their optimisation

Semiconductor lasers, amplifiers, and optical switches are key photonic devices which have allowed theexplosion in internet traffic. Fundamental processes limit their switching rates to less than 200GHz. Thisproject will comprise a detailed experimental investigation of femtosecond dynamics in semiconductoroptical devices, addressing both fundamental physics and the device and systems performance. Inparticular, the data will be compared to the results of unique, predictive theoretical calculations performed byDr Hughes.

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High Field Transport in Semiconductors: Is there a “universal” scaling?

Supervisor(s): Professor J AllamBack-up Supervisor(s): Professor EP O’Reilly

Major Aims:

• To develop an advanced model for electron motion in semiconductors at high electric fields.• To investigate the validity of recent scaling laws for breakdown in semiconductors.• To develop the ‘model semiconductor’ approach, and hence determine the variation with

material composition of electron-phonon interactions, etc.

Techniques used and source of expertise:Detailed numerical simulation of electron motion and scattering in semiconductors, including:

• Pseudopotential calculations of electronic bands and wavefunctions.• Adiabatic bond-charge models of vibration modes.• Semi-classical calculation of electron-electron and electron-phonon scattering.• 'Monte Carlo' calculation of electron motion.There is prior experience within the ODM group of the pseudopotential, electron-electron and Monte Carlocalculations, whilst the vibrational model is described in the literature. Although this is unquestionably acomplex numerical simulation, it is modular in nature and new physical insights can be obtained even beforeall the elements are completed and integrated.

The student will require:• some knowledge of solid-state physics and quantum mechanics• facility with numerical computation, in a suitable language (e.g. Fortran 90)• management skills to handle and document a multi-component project• willingness to talk and collaborate with experimentalists

The motion of electrons in semiconductors at high electric fields is a tough problem because of the complexelectronic and vibrational wavefunctions, and their strong interactions. It has been widely thought that only afirst-principles, quantum mechanical approach can accurately describe the details of this motion andassociated phenomena such as impact ionisation and avalanche breakdown. On the other hand, we recentlydiscovered a very simple ‘universal’ scaling rule for avalanche breakdown in semiconductors.This work will attempt to find a theoretical justification for such scaling laws, and investigate their range ofvalidity. By providing a bridge between detailed first-principle calculations and simple phenomenologicalmodels it will thereby facilitate the design and optimisation of improved semiconductor devices.

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Impact Ionisation & Bandstructures in Semiconductors: AIAs and InGaAs

Supervisor(s): Professor J AllamBack-up Supervisor(s): Professor AR Adams

Major Aims:

• To determine the effect of bandstructure on impact ionisation in indirect III-V semiconductors(e.g. AlAs) and ‘moderate gap’ semiconductors (e.g. InGaAs).

• To investigate experimentally the range of validity of recent scaling laws for avalanchebreakdown in semiconductors.

Techniques used and source of expertise:

• Measurement of optoelectronic properties of semiconductors.• Application of hydrostatic pressures.• Use of a new low-temperature, high-pressure helium gas cell.

The ODM group at Surrey, and the named supervisors, have extensive experience in these areas.

The student will require:• good hands-on skills with complex experiments• some knowledge of semiconductor physics and/or electron devices• physical insight to guide the experiments performed• willingness to talk and collaborate with theorists, and industrial collaborators• To discover the major physical processes that limit the output power of semiconductor lasers and

hence suggest optimum structures to increase this power.

This project will investigate impact ionisation and avalanche breakdown in semiconductors using hydrostaticpressure. Our previous measurements on Si, Ge, InAs and GaAs revealed the different effects of thebandstructure in each case. We now wish to extend the study to encompass other classes of materials,including indirect-gap III-V (e.g. AlAs) and moderate-gap materials (e.g. InGaAs), in order to explain recentinteresting experimental results.Recently, colleagues at the University of Sheffield showed that AlxGa1-xAs with x>0.8 behaves quitedifferently from x<0.6. In particular, the ionisation coefficients for electrons and holes differ significantly. Thisproperty, desirable in certain electronic devices as it leads to current multiplication without additional noise,has previously only been reliably observed in silicon. We speculated on the physical origin of this effect in Si,and now have the opportunity to investigate whether AlAs is similar. The possibility of low-noisemultiplication in AlAs indicates that new photodetectors with improved performance can be produced.The material InGaAs is on the border of ‘wide-gap’ and ‘narrow-gap’ materials within our classification. Usinghydrostatic pressure, we will observe the transition from narrow to wide-gap behaviour. Knowledge of impactionisation in InGaAs is important since this process limits the power output of high-speed transistors used intelecommunications. Other materials such as InP and InGaP may also be studied. In addition, a new low-temperature helium gas cell may be used to continuously vary the pressure at low temperatures.

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Terahertz Electro-Optic Sampling of Electronic Devices and Circuits

Supervisor(s): Professor J AllamBack-up Supervisor(s): Professor I Robertson

Major Aims:

• To fabricate and characterise microfabricated semiconductor electro-optic probes.• To compare against conventional 100GHz measurements and simulations.• To demonstrate spatial mapping of circuit at >100GHz.

Techniques used and source of expertise:

• Electro-optic sampling using femtosecond titanium-sapphire laser.• Microfabrication of semiconductor electro-optic probes and test circuits.• Conventional high-frequency electronic measurements and simulation.Professor Allam has extensive experience in electro-optic sampling, and microfabrication.Professor Robertson is an expert in high-frequency circuits and measurements.

The student will require:• good hands-on skills with complex experiments• some knowledge of optics, electronic devices, etc.• physical insight to guide the experiments performed• willingness to talk and collaborate with theorists, and industrial collaborators

Modern high-speed electronic devices and circuits with operating frequencies up to ~100GHz (1011Hz) willsoon find widespread applications, e.g. in telecommunications and in collision-avoidance radar in cars.Measuring the performance at these (and even higher) frequencies is challenging. However, recentdevelopments in pulsed lasers have allowed the relatively easy generation of ultrashort (<100fs) light pulses− corresponding to frequencies of ~10THz (1013Hz).

Electro-optic sampling using ultrashort laser pulses allows the measurement of electronic devices andcircuits at frequencies up to a TeraHertz, as well as access to internal nodes of a circuit, and spatialmapping of electric field distributions. However wide-spread adoption has been prevented by issues such asthe ‘invasiveness’ of the probes, the non-reproducibility of probe-sample contact and calibration of themeasured electric field.

These issues can be addressed using semiconductor electro-optic probes, in which micro-fabricateddielectric or metallic features control the probe-sample separation, the illuminated region, and the applicationof reference potentials for calibration. A number of probe structures will be compared to obtain the bestcombination of invasiveness, sensitivity, reproducible positioning and calibration accuracy. By combining themicro-EO probe with an atomic force microscope, high-resolution spatial imaging at frequencies >>100GHzwill be demonstrated.

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Strain in Semiconductor Lasers

Supervisor(s): Dr DA FauxBack-up Supervisor(s): Professor EP O’Reilly

Major Aims:• To exploit established methods for calculating strain fields in semiconductor structures by

performing stress/strain calculation for a range of quantum-wire and quantum-dot structures.• To develop calculations of anisotropy on strain distributions in quantum-wires and quantum-dots.• To calculate piezoelectric fields in nitride and other relevant quantum dot structures.• To investigate the influence of strain on the behaviour of defects in semiconductors.

Techniques used and source of expertise:• FORTRAN (or C) programming.• Strain calculations.• Piezoelectric fields.

Modern semiconductor technology is at the core of the information revolution. Optical storage devices (compactdiscs), communication systems, visible laser technology all rely on the production of efficient, high-quality solidstate devices. Strain is important in a wide range of devices, whether that strain has been deliberately introducedto enhance the properties of the device or whether strain is present as an unavoidable consequence of the use ofdifferent materials. It is vital to be able to calculate the strain fields in devices so that the likely effect onmechanical and electrical properties can be established. Quantum wire and quantum dot devices are a source of considerable current research effort, both within thegroup and worldwide. We have developed a simple method for calculating strain distributions in quantum-wireand quantum-dot structures which involves calculating a volume or surface integral. We have recently extendedthis work to establish the influence of the anisotropy of elastic constants and to the calculation of piezoelectricfields in nitride dots. A PhD student would be able to build on many years of experience, establish a rapid startand quickly extend their work into new areas. The aims of the proposed programme of research are to exploit established methods for calculating strain fieldsin the new semiconductor quantum-wire and quantum-dot structures, to perform calculations of strain fields in avariety of technologically-relevant structures and to elucidate the behaviour of defects in these materials. Theseresults feed into band structure calculations which, in turn, provide predictions on the proposed structure andthereby leading to the improvement of current devices and the development of new devices which are faster,more efficient, possess longer life-times and which operate at new extremes. This PhD involves a broad-based programme of research building on work undertaken over the last few years atthe University of Surrey and results will be of value to those involves in almost all aspects of semiconductordevice technology – from growers to device processors to device users. 1 AD Andreev, JR Downes, DA Faux and EP O’Reilly, J. Appl. Phys., 86, 297-305, 1999. 2 DA Faux and GS Pearson, J. Appl. Phys., 88, 730-6, 2000.3 DA Faux and GS Pearson, Phys. Rev. B, Rapid Comm., 62, 8, R4798-801, 2000.

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Photo-Modulated Reflectance, Electro-Modulated Reflectance andLuminescence Spectroscopy of Semiconductor Materials and Devices

Supervisor(s): Dr TJC HoseaBack-up Supervisor(s): Dr D Lancefield

Major Aims:• To develop reflectance, modulated reflectance and luminescence spectroscopy to investigate the

band structure of semiconductor devices and device-like structures including VCSELs, RCLEDs,nitrides and quantum dots; to correlate the measured band structure with device characteristics; tocompare these measurements with theoretical calculations.

Techniques used and source of expertise:• Optical spectroscopy, lasers, cryogenics, vacuum techniques.• Analysis of reflectance, modulated reflectance, high pressure spectra, luminescence, least-

squares data fitting/programming.• Band structure calculations.

Characterisation of semiconductor band structure is essential in order to understand optical properties for deviceapplications. Although the most popular optical technique is probably photoluminescence, this is of use mostly atlow temperature and yields mainly ground-state interband transitions. However, these, and many higher-orderoptical transitions, can be measured at room temperature using photo-modulated reflectance and electro-modulated reflectance, which are examples of a class of modulated reflectance (MR) spectroscopic techniques.MR has become important in the study of bulk parameters, such as alloy compositions and dopingconcentrations. It can also be used for example to characterise confined excitonic energies in electron and holequantum-well devices, to study strained-layer structures, and for in-situ monitoring of alloy composition and layerthickness during growth. However, due to this versatility MR spectra are often replete with detail and cantherefore be complex and difficult to interpret. The Ph.D. programme is part of a continuing study of reflectance, MR and luminescence spectra from varioustopical devices and structures. Initially the work will involve the study of a range of structures including vertical-cavity surface-emitting lasers (VCSELs), resonant-cavity LEDs (RCLEDs), nitrides and quantum dots. Deviceswith operating wavelengths ranging from 0.4 to 1.5µm will be investigated.

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High-field THz Spectroscopy of Semiconductor Nanostructures:Theory and Modelling

Supervisor(s): Dr S HughesBack-up Supervisor(s): Dr BN Murdin

Major Aims:• To develop models and computer programs to explore microscopic wavepacket dynamics in

optically excited semiconductor nanostructures under very intense THz fields and magneticfields.

• To explore nonperturbative ponderomotive energy effects in semiconductors.

Techniques used and source of expertise:• Computational modelling, Green's function techniques (equilibrium and nonequilibrium).• Numerical solution of the time-dependent Schrodinger equation.• Microscopic semiconductor optics and semiconductor Bloch equations.• Applications of free-electron lasers.• Quantum kinetics, memory effects, high-field transport.• Drive and determination.

In the past decade the interaction of very intense laser light with atoms has been of great interest because ofthe many novel nonperturbative phenomena that have been observed, such as above threshold ionization,high harmonic generation, and stabilization. This project will explore possible related phenomena insemiconductor nanostructures subjected to intense THz radiation[1]. The intense laser-matter interactionswill be simulated using, as a starting point, the time-dependent Schrodinger equation in one dimension(quantum wires), two dimensions (quantum wells), and possibly three dimensions (bulk).

The ensuing electron-hole wavepackets will be probed, prodded, agitated, and pushed to the limits in orderto understand new and related high-field effects that can be obtained in semiconductor heterostructures.Experimental observables will be extracted. The project will go on to incorporate studies of new intense-fieldphenomena involving the interaction of high magnetic and THz fields. The student will gain anunderstanding of semiconductor electro-optics, nonperturbative quantum mechanics, and state-of-the-artcomputational programming techniques. Nonrelativistic effects will also be studied.

1 S Hughes and DS Citrin, Phys. Rev. B 59, R5288 (1999)2 S Hughes and DS Citrin, Phys. Rev. Lett. April, (2000)3 KB Nordstrom at al. Phys. Rev. Lett. 81, 457 (1998)4 J Kono et al. Phys. Rev. Lett. 79, 1758 (1997)5 AP Jauho and K Johnsen, Phys. Rev. Lett. 24, 4576 (1996).

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Spatio-Temporal Simulations of Several-Cycle--Single-Cycle Optical Pulses

Supervisor(s): Dr S HughesBack-up Supervisor(s): Professor EP O’Reilly

Major Aims:• To simulate and investigate new phenomena of nonlinear optics in the "extreme" regime.• To model sub-wavelength propagation effects and sub-optical-carrier ultrafast phenomena.

Techniques used and source of expertise:• Computational electrodynamics: finite-difference time-domain method.• Quantum optics.• Microscopic theory of laser-matter interactions.• Atomic and semiconductor Bloch equations.• Femtosecond and attosecond phenomena.• Drive and determination.

The development of high-intensity ultrashort optical pulses has lead to a number of fascinating nonlinearoptical propagation studies including Rabi flopping, self-induced transparency, photon echo, and opticalshock formation, to name but a few. Common to the description of these effects, is that their propagationphenomena can be described quite adequately by employing the appropriate coupled matter-Maxwellequations within the slowly-varying-envelope approximation (SVEA), i.e., the envelopes of theelectromagnetic field and polarization are assumed to vary little over an optical period and wavelength.However, recently, a number of theoretical recent works have demonstrated the limitations of the SVEA thatadopts slowly-varying phase and amplitude components[1]. These non-slowly-varying Maxwell-Bloch studiesare both timely and necessary since on one hand device sizes and absorption lengths can now be reducedto subwavelength scales, and on the other hand several-cycle fs optical pulses are readily available.Single-cycle and several-cycle THz pulses have also been receiving a lot of interest recently.

A finite-difference time-domain (FDTD) method to solve Maxwell's equations exactly will be developed tosimulate spatiotemporal shaping of electromagnetic pulses approaching the quasi-half-cycle regime. Thehalf-cycle regime is the fundamental limit of any laser pulse and recently has become an area of immenseimportance. This project could take on a number of investigations, for example, pulse shaping with arbitraryapertures, superluminal pulse propagation in waveguides (the apparant propagation at speeds faster thanthe vacuum speed of light), and explorations of nonlinear optics in the extreme regime.

1 S Hughes, Phys. Rev. Lett. 81, 3363 (1998)2 W Forysiak et al., Phys. Rev. Lett. 76, 3695 (1996)3 RG Flesch et al., Phys. Rev. Lett. 76 2488 (1996)4 T Brabec and F Krausz, Phys. Rev. Lett. 78, 3282 (1997).

16

Theory of Very-Small-Aperture Lasers (VSAL’s):Fundamentals and Applications to Near-Field Optical Storage

Supervisor(s): Dr S HughesBack-up Supervisor(s): Professor EP O’Reilly

Major Aims:• To develop a quantitative dynamical model of VSAL’S for applications and studies exploiting near-

field optics.• Recent demonstrations of VSAL’S at Bell Labs, USA, is a major step in laser development,

potentially enabling data storage densities of over 500 Gb/in2 (up to 100 times today’smagnetic or optical storage densities). The theory and modelling will hopefully drive asuccessful exploitation of this device.

Techniques used and source of expertise:• Computer modelling.• Analytical techniques.• Microscopic semiconductor laser theory.• Computational electromagnetics.• Sound physical vision.• Drive and determination.

With more powerful computers, growing networks, and the increased processing of multimedia files, thedemand for mass storage is growing exponentially - the overall storage market approached $100 billion inrevenue in the year 2000. Even so, at that time, less than 5% of the world's data will be in digital form. The21st century will require storage capabilities that exceed the potential of currently available technology tomake effective use of information systems. Enter VSALs.

In recent years, techniques such as confocal microscopy, scanning interferomic apertureless microscope,and near-field scanning optical microscopy, have extended the spatial resolution of optical microscopybeyond the diffraction limit ~ λ/2, where λ is the wavelength of light. Many important applications inspectroscopy, photolithography, and data storage are possible[1]. Very recently VSAL's[2] havedemonstrated more than a 10,000 increase of optical powers over conventional near-field lights sources.Also, the concept of these lasers can be applied to diode lasers of any wavelength and design.

The theoretical work will have a key impact in the identification and development of new ultra-high density,optical storage techniques using near-field optics, possibly delivering dramatic improvements in the a realdensity of storage devices. Besides applications, this project has tremendous potential to explore innovativescience and technologies exploiting near-field optical fields--a science still very much in its infancy.

1 E Betzig and JJ Trautman, Science 257, 189 (1995).2 A Partovi et al, App. Phys. Lett. 75, 1515 (1999).

17

Optical and Electrical Characterisation of III-NResonant-Cavity Light Emitting Diodes

Supervisor(s): Dr D LancefieldBack-up Supervisor(s): Professor EP O’Reilly/Dr TJC Hosea

Major Aims:• To study the optical and electrical properties of resonant cavity light emitting diodes being

developed under a large European research project.

Techniques used and source of expertise:• Cryogenics.• High Pressure.• Spectroscopy.• Computer based data collection.• Analysis of data using drift-diffusion models of current transport.

There is an increasing demand for data transmission rates in excess of 100 Mbs-1 for applications inentertainment systems for commercial aircraft, and in cars to provide cell phone, navigation, collision avoidanceand audio/video entertainment. Achieving such high-speed buses using conventional copper transmission linesincurs severe cost and design penalties. Plastic Optical Fibres (POF) is competitive both in cost andperformance. The major attraction of POF is its large core diameter, which means that fibre connectors arecheap, rugged and tolerant to vibration. However, a stringent requirement for optical networks linking sensorsystems within automobiles and aircraft is that the sensors, transmitters, detectors and fibre must all operate atextreme temperatures. This project is part of a European consortium developing this technology. One aspect of the work is to develophigh temperature, high-speed emitter components. To meet this objective the project will research and developgreen and amber (510 nm and 570 nm) resonant-cavity light emitting diode (RCLED) components fabricated fromthe AlGaN/GaN/GaInN material system. These wide band-gap semiconductors are ideally suited for hightemperature applications and for matching the low loss windows of polymethylmethacrylate (PMMA) based POF.The PhD student will concentrate on the optical and electrical characterisation of devices grown by MOCVD andMBE. The electroluminescent properties of test structures will be studied as a function of temperature andpressure. Electrical characterisation will include transport studies as a funtion of temperature, pressure andmagnetic field. Optical characterisation will include photoluminescence, ellipsometry, reflectance andphotoreflectance.

18

Transport Properties of III-N and Dilute NitrideSemiconductor Materials

Supervisor(s): Dr D LancefieldBack-up Supervisor(s): Professor AR Adams

Major Aims:• To use Hall effect and resistivity measurements as a function of high pressure, low temperature

and high magnetic field to investigate the transport characteristics of bulk materials and lowdimensional structures.

Techniques used and source of expertise:• Cryogenics.• High Pressure.• High magnetic field.• Computer based data collection.• Analysis of data using Iterative Solution of the Boltzmann Equation and Relaxation Time

approximations.

The transport properties of bulk materials and quantum-well (QW) structures are of great importance in materialsassessment and in developing and optimising new device structures. Transport studies typically give newinformation about free carrier densities and mobilities. From this data information about defect levels andconcentrations, scattering times and scattering mechanisms can be inferred. In addition material parameterssuch as deformation potentials, optical phonon energies and effective masses may be determined. The PhD programme is part of a continuing study of the transport properties of materials and substructuresrequired for a range of electronic and optoelectronic devices1,2. Work will involve the study of electron and holetransport in nitride based structures of particular importance for light emitting structures in the visible and IR partof the spectrum. In GaN and related materials high dislocation densities associated with the lattice mismatch withthe substrate mean that dislocation scattering and transport in an interfacial layer have been proposed to limit thetransport properties. We have investigated the role of dislocation scattering and impurity band transport3 and willextend this work to look at two-dimensional electron gas structures where polarization effects can have a markedeffect on the carrier density and mobility. In dilute nitride semiconductors such as GaAsN and GaInAsN theincorporation of N at the level of 1% to 2% has a dramatic effect on the band gap. This is expected to have amarked effect on the electron effective mass and consequently on electron transport. 1 M Kasap and D Lancefield, “The temperature and pressure dependence of electron transport in plastically

relaxed InxGa1-xAs”, Phys. Stat. Sol. (b) 199, 481-493, 1997.2 MA di Forte-Poisson, F Huet, A Romann, M Tordjman, D Lancefield, E Pereira, J Di Persio and B Pecz,

“Relationship between physical properties and gas purification in GaN grown by metalorganic vapourdeposition”, J. Cryst. Growth, 195, 314-318, 1998.

3 H. Eshghi and D. Lancefield B. Beaumont and P. Gibart, “Electron transport in MOVPE GaN grown on siliconnitride treated sapphire”, phys. Stat. Sol. (b) 216, 733 (1999).

19

High Pressure Spectroscopy of Quantum Cascade Lasers

Supervisor(s): Dr BN MurdinBack-up Supervisor(s): Professor AR Adams

Major Aims:• To determine the inhibiting factors in Quantum Cascade lasers and design better services.

Techniques used and source of expertise:• A combination of low temperatures (2 K), high hydrostatic gas pressure (10,000 atm), ultrafast

Ti:sapphire laser spectroscopy (9 fs pulses at 800nm wavelength) and Free electron laserspectroscopy (200 fs pulses at 10 microns wavelength.

Quantum Cascade (QC) lasers are a new type of semiconductor optoelectronic device. Traditionalsemiconductor lasers utilise transitions between the conduction and valence band and the wavelength isthus determined by the bandgap of the material used. Some freedom to choose the wavelength is given byuse of alloys, which have varying bandgap as a function of composition. The QC laser uses transitionsbetween quantum confined states within the conduction band. These are created by making very thin layers(quantum wells) of one type of semiconductor sandwiched between layers of another, and the transitionenergy depends also on the width of the well, which gives easier tuning. They also have many unusualproperties, such as quantum efficiency > 1 (more than one photon out per electron injected). However, theysuffer from many problems such as high threshold. You will use a combination of low temperature, highpressure and ultrafast laser spectroscopy to determine the inhibiting factors, and design better devices.

The project will be a collaboration with Profesor C Sirtori of Thomson-CSF in Paris and Professor J Faist ofthe University of Neuchatel, two of the original members from the Bell Labs group that developed the firstQC lasers. You will need to visit these labs for training and experiments, and work will also be carried out atthe FELIX Free Electron Laser facility in the Netherlands.

20

Mid Infrared Near Field Microscopy

Supervisor(s): Dr BN MurdinBack-up Supervisor(s): Professor AR Adams

Major Aims:• To develop a near field microscope working in the mid-infrared region of the spectrum, for use

in semiconductor physics and bio/medical/chemical applications.

Techniques used and source of expertise:• Mid-infrared spectroscopy will be used in conjunction with an atomic force microscope, to

develop false colour images on a sub-micron scale while using light of wavelength severalmicrons.

Mid-infrared spectroscopy is very useful in the fields of semiconductor physics and elsewhere, in particulardue to the different vibrational frequencies which different chemical bonds produce. It enables detection ofmolecular species and this is important for a broad range of applications. At present, this has only beenpossible on “macroscopic” samples (such as gas samples in pollution monitoring), because the smallestthing it is possible to detect with a given wavelength is normally several times the size of the wavelength ofthe light used. However, a new technique called near field optical microscopy has been developed for thevisible region of the spectrum, in which photons “tunnel” out of the end of an optical fibre which has beendrawn to a point much smaller than the wavelength. This has enabled images to be taken with resolutiondown to 1/100th the wavelength. Such microscopes for the mid-infrared region of the spectrum have not sofar been developed due to the lack of suitable laser sources in that region. Using recently availablesemiconductor lasers, we shall develop an instrument based on an idea recently published in Nature,involving an Atomic Force Microscope, which will give surface relief information as well as the identificationof the molecules.The microscope will be used specifically to investigate new quantum dot semiconductor structures, and theordering of polymers. New samples and applications, e.g. in biochemistry, are likely to arise in the course ofthe project, which is open ended and depends on the interest of the student.

21

Physics of “spintronic” Materials

Supervisor(s): Dr BN MurdinBack-up Supervisor(s): Professor AR Adams

Major Aims:• To investigate the physics of “spintronic” materials, using ultra-fast lasers, very high magnetic

fields, and very high pressures as diagnostic techniques.

Techniques used and source of expertise:• Infrared spectroscopy of novel low-dimensional semiconductor structures, measurement of

excited state lifetimes using a picosecond laser at FELIX in the Netherlands, and non-linearabsorption techniques. High pressure and high magnetic field spectroscopy of such materials.

Very recently a new concept for semiconductor memory has been developed based on electron spin. Inspecial materials the energy states of electrons with different spin are non-degenerate (i.e. they are atdifferent energy) even without a magnetic field. This opens up the possibility to design circuits in whichelectrons remember their spin even as they flow through the circuit. However, relatively little is understoodabout the magnitude of this effect, and how to improve the length of time before the memory is lost (the spin-flip time).

There will be two aspects to this project. One will involve measurement of the spin-flip lifetimes in variousquantum confined semiconductor spintronic materials with a direct time-resolved technique, using apicosecond pulsed laser in the Netherlands. The other will be to measure the electron spin transport of thematerials while under extreme high pressures and/or magnetic fields. The effect of magnetic fields on spinstates is given, broadly speaking, by the well-known Zeeman effect. It would be interesting to know howmuch the spin-flip times can be improved by such fields, and this will also help the design of better zero-fieldmemory. High pressure is another useful diagnostic tool since it changes the electronic states of thesemiconductor in a way very similar to changing alloy compositions. It allows testing of ideas about howproperties such as the spin-flip time should depend on the alloy composition and configuration of electronicstates. This information can be used to improve the design of the quantum confinement etc.

Depending on the time-scale we may be able to take advantage of the new ultra-fast laser and highpressure facilities of the new Advanced Technology Institute.

22

Spectroscopy of Self-Organised Photonic Materials

Supervisor(s): Dr BN MurdinBack-up Supervisor(s): Professor AR Adams

Major Aims:• To investigate completely new 3D photonic materials.

Techniques used and source of expertise:• Infrared spectroscopy of novel low-dimensional semiconductor structures.• High pressure and high magnetic field spectroscopy of such materials.

Quantum dots are very small islands of semiconductor with very interesting and useful properties, and may beformed spontaneously when depositing epitaxially a very thin layer of the dot material on a substrate of verydifferent lattice constant. In most semiconductor systems they site themselves randomly with large variations insize from dot to dot.

It has recently been shown that quantum dots based on the lead-salt semiconductor system (PbSe onPbEuTe) exhibit a three-dimensional ordering so that the dots arrange themselves in a face-centred cubicarray, by Prof. G. Bauer’s group at Linz [G Springholtz et al, Science vol 282, p 734, 1998]. The ordered dotarrays display very high homogeneity in size. So far, there have been no systematic optical studies of thesedots.

The subject of "photonic" materials is very similar to X-ray scattering by crystals. It is possible to show thatarrays of blobs of material with differing refractive index can set up Bragg scattering in such a way thatcertain wavelengths of light cannot propagate through the array in any direction, just like the bandgap of asemiconductor. Special defects can be introduced to mimic semiconductor dopants so light can propagateonly at a single frequency and in a specific direction. These effects have been demonstrated in themicrowave region of the spectrum using mm-scale balls of polystyrene etc. This hasenormous numbers of technological applications for optoelectronics, if the devices can actually be scaleddown in size to optical wavelengths. So far most optical photonic structures are only 2-dimensional. Wewould like to investigate the possibility of using the self-organised dot material, whose nearest neighbourdot-dot distance is in the optical wavelength regime, as a photonic material.

23

The Dynamic Franz-Keldysh Effect

Supervisor(s): Dr BN MurdinBack-up Supervisor(s): Dr S Hughes

Major Aims:• To investigate what happens to an electron and hole pair are excited by an ultra-short pulse of

ultra-high-intensity light. This project is of fundamental physics interest, but has applications tohigh-speed terahertz telecommunications.

Techniques used and source of expertise:• Infrared spectroscopy of semiconductors structures. Use of ultrafast lasers including the Free-

Electron Laser FELIX in the Netherlands.

In a semiconductor, when a photon of light is absorbed an electron is excited from the valence band to theconduction band leaving behind a hole. The electron orbits around the hole in exactly the same way anelectron orbits around a proton in the hydrogen atom, except that the hole is much "lighter" than a proton.The orbiting electron/hole pair is called an "exciton". A growing field of interest in atomic physics is thebehaviour of Rydberg (hydrogen-like) atoms under intense short pulses of an a.c. electric field (i.e. light),and it would be interesting to do the same in the semiconductor. The electric field will drive the electron andhole to and fro away from and then towards each other, and it has been predicted that strange excited statesof the exciton will appear. Effectively the attraction of the charges (i.e. the Coulomb field) will be overcomeby the external a.c. field, and rather than hydrogenic states the system will have "Floquet" states. We wouldlike to show that such states can exist in semiconductors using a Free-Electron Laser FELIX as the sourceof the teraherz driving field.

24

Gain Mechanisms in Semiconductor Lasers with Localised States

Supervisor(s): Professor EP O’ReillyBack-up Supervisor(s): Dr A Andreev

Major Aims:• To understand the role of disorder in determining the gain characteristics of InGaN-based blue

lasers.• To establish the requirements for stable laser emission in disordered laser structures such as

InGaN quantum well lasers, and also InGaAs-based quantum dot lasers.

Techniques used and source of expertise:• Computational modelling using FORTRAN (or C) programming.• Analytical techniques.

There have been two exciting developments recently in the field of semiconductor lasers, namely thedemonstration of: • InGaN-based lasers, emitting at the blue end of the spectrum and• InGaAs-based quantum dot lasers, with low threshold current densities, and for which the threshold current is

only weakly temperature dependent, in agreement with theoretical predictions. While these two developments appear largely unrelated, they share at least one feature not found in conventionallasers: in both cases the optical recombination and lasing takes place between localised energy levels. This canlead to considerably stronger nonlinear gain effects than observed in conventional bulk and quantum welldevices, including in particular bleaching of the gain at the lasing transition energy and laser mode hopping, asthe transitions at one energy are quenched and gain increases at a neighbouring energy. The first aim of this project is to establish the requirements for stable laser emission from localised states, and theinfluence of the localised states in the noise characteristics of semiconductor lasers. In addition, there is evidencethat the presence of localised states actually enables lasing to occur in InGaN, lasing has proved much moredifficult to demonstrate in GaN, where there are no alloy disorder effects to give localised states. The second aim is to undertake a theoretical analysis of the different gain characteristics associated withrecombination between localised states and between extended states in wide band gap semiconductors such asGaInN: Why should the gain characteristics be improved in disordered compared to ordered structures?

25

Novel Gain Mechanisms in Semiconductor Multi-Layer Structures

Supervisor(s): Professor EP O’ReillyBack-up Supervisor(s): Dr A Andreev

Major Aims:• Basic properties of type-II transitions in superlattices.• Survey of semiconductor materials systems viable for type-II structures.• Modelling of performance of specific structures.

Techniques used and source of expertise:• Computational modelling using FORTRAN (or C) programming.• Analytical techniques.

There are considerable difficulties in developing semiconductor laser structures for operation in the near tomid infra red wavelength range (2-10µm). The development of robust room temperature lasers in this wavelength range would open manyapplications, including pollution and environmental monitoring, and medical diagnosis and treatment.However, Auger recombination, a strongly temperature sensitive intrinsic loss mechanism, limits themaximum operating temperature of conventional long-wavelength lasers to temperatures well below roomtemperature. Several strategies have been proposed to reduce or eliminate Auger recombination in suchlasers and thereby enable room temperature operation. These include the development of type II multi-layerstructures, where the conduction and valence band line-ups are such that electrons and holes are confinedin separate layers of the structure. The aims of this project are three-fold: • Carrying out a survey of the different semiconductor material systems which could be viable for type-II

structures.• Investigating the basic characteristics of radiative and non-radiative recombination processes in type-II

superlattices and• Modelling the performance of specific structures, for comparison with experiment and to predict

optimised designs for future devices.