Silicon germanium alloy layers can be grown epitaxially on a silicon substrate to provide a means of adjusting the lattice parameter of the crystal. Such a platform, known as a virtual substrate, has a number of potential applications. For instance, it allows for subsequent overgrowth of highly strained layers of silicon, or germanium, that could enable very high speed transistors, similarly it could be used as the starting point of a range a silicon-based optoelectronic devices. In this work, a novel adaptation has been made to a recently proposed reverse grading technique to create high Ge composition SiGe virtual substrates. The proposed structures consist of a relaxed, highly defected, pure Ge underlayer on a Si (001) substrate prior to reverse grading where structures have final compositions of Si0.2Ge0.8. Additionally, two grading schemes are studied, reverse linear grading and reverse terrace grading. All buffers are grown by reduced pressure chemical vapour deposition. The relaxation, defect levels and surface roughness of the fabricated buffers have been quantified whilst varying the grading rate. An ideal grading rate has been found where the quality of the buffer is very high, due to the highly defected Ge underlayer and that the buffer relaxes under tensile strain. Outside of this ideal grading rate three dimensional growth, stacking fault formation and crack generation can occur. Cracking of the buffer has been modelled and some conditions where the buffer is stable have been found. This study experimentally investigates this proposed solution and a crack-stable high quality buffer is fabricated. Comparisons have been drawn with other more popular buffer fabrication techniques and it is found that this technique has very competitive qualities.

As high-k dielectrics are introduced into commercial Si CMOS (Complimentary Metal Oxide Semiconductor) microelectronics, the 40 year channel/dielectric partnership of Si/SiO2 is ended and the door opened for silicon to be replaced as the active channel material in MOSFETs (Metal Oxide Semiconductor Field Effect Transistor). Germanium is a good candidate as it has higher bulk carrier mobilities than silicon. In addition, Si and Ge form a thermodynamically stable SiGe alloy of any composition, allowing Ge to be implemented as a thin layer on the surface of a standard Si substrate. This thesis is a practical investigation on several aspects of Ge CMOS technology. High-k dielectric Ge p-MOSFETs are electrically characterised. A large variation in interface state densities is demonstrated to be responsible for a threshold voltage shift and this is proportional to reciprocal peak mobility due to the Coulomb scattering of carriers by charged states. A theoretical mobility is fitted to that measured at 4.2 K and confirms that interface states are the main source of interface charged impurities. The model demonstrates a reduction in the interface charged impurity density in p-MOSFETs that underwent a PMA (Post Metallisation Anneal) in hydrogen atmosphere and that the anneal also reduces the RMS (Root Mean Square) dielectric/semiconductor interface roughness, from an average of 0.60 nm to 0.48 nm. High-k strained Ge p-MOSFETs are electrically characterised and have peak mobilities at 300 K (470 cm2 V-1 s-1) and 4.2 K (1780 cm2 V-1 s-1) far in excess of those measured for the unstrained Ge p-MOSFETs (285 cm2 V-1 s-1,785 cm2 V-1 s-1 respectively). Strained Ge n-MOSFETs perform significantly worse than standard Si P, - MOSFETs primarily due to a high source/drain resistance. A 10 nm thick SiGe-01 (On Insulator) layer with a Ge composition of 58% is obtained from a 55 nm Si0_88Ge1o2. initial layer on 100 nm Si-Ol substrate via the germanium condensation technique. For the first time, germanium is demonstrated to diffuse through the BOX (Buried OXide) during Ge-condensation and into the underlying Si substrate. An order of magnitude increase in the calculated ITOX (Internal Thermal OXidation) rate of the BOX in the final stages of Ge-condensation is hypothesised to be responsible for stopping this diffusion.

It is now apparent that the continued performance enhancements of silicon metal-oxide-semiconductor field effect transistors (MOSFETs) can no longer be met by scaling alone. High-mobility channel materials such as strained Si1-xGex and Ge are now being seriously considered to maintain the performance requirements specified by the semiconductor industry. In addition, alternative gate dielectric, or high-? dielectrics, will also be required to meet gate leakage requirements. This work investigates the properties of using strained Si1-xGex or Ge as alternative channel materials for pMOSFETs incorporating hafnium oxide (HfO2) high-? gate dielectric. Whilst the SiGe pMOSFETs (x = 0.25) exhibited an enhancement in hole mobility (300 K) over comparable silicon control pMOSFETs with sputtered HfO2 dielectric, high Coulomb scattering and surface roughness scattering relating to the dielectric deposition process meant that the effective hole mobilities were degraded with respect to the silicon universal curve. Germanium channel pMOSFETs with halo-doping and HfO2 gate dielectric deposited by atomic layer deposition showed high hole mobilities of 230 cm2V-1s-1 and 480 cm2V-1s-1 at room temperature and 77 K, respectively. Analysis of the off-state current for the Ge pMOSFETs over a range of temperatures indicated that band-to-band tunnelling, gate-induced drain leakage and other defect-assisted leakage mechanisms could all be important. Hole carrier velocity and impact ionisation were also studied in two batches of buried channel SiGe pMOSFET with x = 0.15 and x = 0.36, respectively. SiGe channel pMOSFETs were found to exhibit reduced impact ionisation compared to silicon control devices, which has been attributed to a strain-induced reduction of the density of states in the SiGe conduction and valence bands. Analysis of the hole carrier velocity indicated that pseudomorphic SiGe offered no performance enhancements over Si below 100 nm, possibly due to higher ion implantation damage and strain relaxation of the strained SiGe channel. The results indicate that velocity overshoot effects might not provide the performance improvements at short channel lengths that was previously hoped for.

The fundamental bulk and surface electronic properties of a novel class of semiconductors, characterised by a significant mismatch between the size and electro-negativity of the cation and anion (SCAMS), have been investigated. The characteristic examples of CdO, In2O3, and InN were studied using high-resolution x-ray photoemission spectroscopy, infrared reflectivity, optical absorption spectroscopy, and single-field Hall effect measurements. The behaviour of not only defects, dopants and impurities, which dominate the bulk electronic properties, but also surface states was shown to depend on the position of a single energy level, the charge neutrality level (CNL), unifying bulk and surface electronic properties of semiconductors. For the materials studied, the CNL was shown to be located within the conduction band (0.39 eV, »0:65 eV, and 1.19 eV above the conduction band minimum (CBM) in CdO, In2O3, and InN, respectively; see figure) in contrast to the vast majority of semiconductors where the CNL lies within the fundamental band gap (as, for example, in the classic case of GaAs). In CdO, this was shown to lead to native defects, hydrogen impurities and surface states all being donors, even in already n-type material. The donor surface states result in electron accumulation at the CdO surface. Such an electron accumulation is also present at InN surfaces, and this was shown to exhibit a remarkable independence on surface orientation, and to lead to inversion layers at the surface of p-type InN. The changes in surface space-charge regions were investigated across the In(Ga,Al)N composition range, for both undoped and Mg-doped alloys. The influence of the CNL position on interface properties and conductivity in InN was considered. Electron accumulation was observed in In2O3, in contrast to previous reports. Muonium, and by analogy hydrogen, was also shown to be a shallow donor in this material. The location of the CNL above the CBM in SCAMS was used to explain many of their striking bulk electronic properties, such as why materials like In2O3 are able to be conducting despite being optically transparent, two normally contradictory properties. The conclusions drawn from these studies are applicable to a wide variety of other materials, in particular other SCAMS such as ZnO or SnO2. Surface electron accumulation is treated here mainly within a one-electron semi-classical approximation. The final section of this work moves beyond this, using angle-resolved photoemission spectroscopy measurements and theoretical calculations to consider both the quantized nature of an electron accumulation layer, and the influence of many-body effects.

Στην μεταπτυχιακή εργασία ειδίκευσης περιγράφεται η σύνθεση και ο χαρακτηρισμός νέων υβριδικών ανόργανων-οργανικών χαμηλοδιάστατων ημιαγωγών βασισμένων σε αλογονίδια του μολύβδου και διαφόρων χαλκογενιδίων μετάλλων για οπτικές και οπτικοηλεκτρονικές εφαρμογές. Για να γίνουν κατανοητές οι σχετικές έννοιες των χαμηλοδιάστατων ημιαγωγών αρχικά αναφερόμαστε στην κρυσταλλική δομή των στερεών, τη σχετική θεωρία των ενεργειακών ζωνών και ειδικότερα στους ημιαγωγούς. Με βάση αυτά αναπτύσσεται η θεωρία για τα χαμηλοδιάστατα ημιαγώγιμα συστήματα και ερμηνεύονται τα σχετικά κβαντικά φαινόμενα. Ειδικότερα, εξηγείται πως οι ιδιότητες που υπάρχουν στους τρισδιάστατους ημιαγωγούς αλλάζουν όταν το μέγεθος αυτών τροποποιηθεί και αποκτήσουν κάποια από τις τρεις διαστάσεις στην κλίμακα του νανομέτρου. Γίνεται αναφορά στους διάφορους τρόπους σύνθεσης νέων χαμηλοδιάστατων συστημάτων, στη μορφολογία τους καθώς και στις ιδιότητες και μεθόδους χαρακτηρισμού με έμφαση στις οπτικές και ηλεκτρονικές ιδιότητές τους. Τέλος, δίνονται πρώτα αποτελέσματα διαφόρων χαμηλοδιάστατων ημιαγώγιμων συστημάτων μαζί με τη φασματοσκοπική και ηλεκτρική ανάλυση τους. Επίσης, αναλύονται οι ιδιότητες τους σε σχέση με απλά μοντέλα και συζητείται η εφαρμογή των συγκεκριμένων σε πιθανές εφαρμογές με βάση τις οπτικές ιδιότητες τους. / In this master thesis it is described the synthesis and the characterization of hybrid inorganic - organic low dimensional semiconductors based on lead halides and various metal chalcogenides for optical and optoelectronics applications. In order to become more comprehensive, the relative notions of low dimensional semiconductors, we initially report on the theory of energy bands for crystalline solids and more specifically for semiconductors. In such a context, the theory for the low dimensional semiconductor systems is presented and is interpreted in terms of the quantum theory. More specifically, we explain that properties that exist in the three-dimensional semiconductors change when these formed in such a way that one of their dimensions are in the nanometric scale. Also, report is being given in the various methods of synthesis of new low dimensional systems, in their morphology as well as in the attributes and methods of characterization with emphasis in the optical and electronic attributes. Finally, first results of new low dimensional semiconductors systems are presented along with their optical and electric characterization. Also, their attributes are analyzed concerning simple models as well as their application is discussed in possible applications with basis their optical attributes.

Semiconductor Seeded Fibre Amplified Sources of Ultra Short Pulses By Stephen Paul Elsmere This thesis reports upon an experimental investigation of passively mode-locked optically pumped vertical-external-cavity surface-emitting semiconductor lasers (VECSEL). Mode-locked VECSELs are a compact source of ultra-short pulses at GHz repetition rates, with pulse lengths as short as 190 fs being generated directly from the laser. The VECSEL is a power scalable device offering spectral versatility through band gap engineering of semiconductor gain material. Here, for the first time the technique of frequency resolved optical gating (FROG) has been used to record a second harmonic spectrogram of the VECSEL pulse train, from which the phase information of non-transform limited sub-picosecond pulses has been retrieved. I also report the characterisation of a single stage VECSEL seeded ytterbium-doped fibre amplifier, capable of increasing the average power of a VECSEL from 20 mW to over 1.5 W while maintaining the sub-picosecond duration of the pulse train. The amplifier is capable of operating at any repetition rate obtainable with a VECSEL, amplification is demonstrated here with 1 GHz and 6 GHz seeds. Finally, the nonlinear evolution of VECSEL pulses inside a single stage fibre amplifier has been investigated. Computer modelling of the linear gain and nonlinear pulse propagation within a single fibre has been used to design an amplifier capable of producing pulses with a parabolic profile. The modelling reveals that a parabolic amplifier would produce spectrally broader linearly chirped pulses which could be compressed to below 100 fs, with average powers > 3 W. An experimental realisation of the parabolic amplifier will require a seed with average power greater than 100 mW, this could be achieved with a re-growth of an existing sample, QT1544.

This thesis is dedicated to the study of linear and nonlinear properties of closely spaced gold nanoparticle dimers, so-called nanoantennas, and hybrid nanoantenna devices consisting of metals and semiconductors. Coupled nanoparticles are of particular interest for nanophotonics because of their ability to focus light into subwavelength volumes and the associated large field enhancement in the gap. The samples used in this thesis are gold rectangles designed by electron-beam lithography, with both symmetric and asymmetric arms, as well as symmetric closely spaced 100 nm disk dimers which were fabricated by colloidal lithography in combination with angle-dependent evaporation. We investigate the linear interplay of modes in the two arms with Spatial Modulation Microscopy, an experimental technique which results in a measure directly proportional to the extinction cross-section. We find a variety of constructive and destructive interference between different order modes, which we can better understand by comprehensive simulations of antennas, varying the parameter space of gap size (coupling strength) and length-length ratio using advanced numerical methods such as the Fourier Domain Time Difference and the Boundary Element Method. We find that the presence of nonradiative modes is made visible by Electromagnetically Induced Transparency. In order to probe the nonlinear properties of the antennas and their interaction with Indium Tin Oxide substrates, a pump-probe setup is used to get an insight into ultrafast nonlinear response with picosecond resolution. These measurements (and corresponding fits using numerical simulations) lead us to identify a new energy transfer mechanism where fast electrons are injected from the nanoparticles into the semiconductor, resulting in a refractive index change due to heating of the surroundings. In follow-up experiments, we find this mechanism to be universal (and versatile) for other types of transparent conductive oxides. These results open new avenues towards application of nanoantennas for ultrafast switching.

Gallium arsenide is piezoelectric, so it is possible to generate coupled mechanical and electrical surface acoustic waves (SAWs) by applying a high-frequency voltage to a transducer on the surface of GaAs. By combining SAWs with existing low-dimensional nanostructures one can create a series of dynamic quantum dots corresponding to the minima of the travelling electric wave, and each dot carries a single electron at the SAW velocity (~ 2800 m/s). These devices may be of use in developing future quantum information processors, and also offer an ideal environment for probing the quantum mechanical behaviour of single electrons. This thesis describes a numerical and theoretical study of the dynamics ofan electron in a range of geometries. The numerical techniques for solving thetime-dependent Schrödinger equation with an arbitrary time-dependent potential will be described in Chapter 2, and then applied in Chapter 3 to calculate the transmission of an electron through an Aharonov-Bohm (AB) ring. It will be seen that an important property of the techniques used in this thesis is that they can be easily adapted to study realistic geometries, and we will see features in the AB oscillations which do not arise in simplified analytic descriptions. In Chapter 4, we will then study a device consisting of two parallel SAW channels separated by a controllable tunnelling barrier. We will use numerical simulations to investigate the effect of electric and magnetic fields upon the electron dynamics, and develop an analytic model to explain the simulation results. From the model, it will be apparent that it is possible to use this device to rotatethe state of the electron to an arbitrary superposition of the first two eigenstates. We then introduce coherent and squeezed states in Chapter 5, which are ex-cited states of the quantum harmonic oscillator. Coherent and squeezed electronicstates may be of use in quantum information processing, and could also arise dueto unwanted perturbations in a SAW device. We will discuss how these statescan be controllably generated in a SAW device, and also discuss how they couldthen be detected. In Chapter 6 we describe how to use the motion of a SAW to create a rapidly-changing potential in the frame of the electron, leading to a nonadiabatic excita-tion. The nonadiabatically-excited state oscillates from side to side within a 1Dchannel on a few-picosecond timescale, and this motion can be probed by placing a tunnelling barrier at one side of the channel. Numerical simulations will beperformed to show how this motion can be controlled, and the simulation resultswill be seen to be in good agreement with recent experimental work performed by colleagues. Finally, we will show that this device can be used to measure the initial state of an electron which is an arbitrary superposition of the first twoeigenstates.

La maîtrise des techniques de fabrication de matériaux nanostructurés a fait émerger ces dernières années de nouvelles problématiques relatives aux transferts thermiques à très courtes échelles d'espace et de temps. L'étude thermique s'effectue alors à partir de l'équation de transport de Boltzmann (ETB) pour les phonons qui sont les principaux porteurs de chaleur dans les semi-conducteurs. Ce travail résout l’ETB par une méthode statistique de type Monte Carlo en suivant le déplacement des phonons dans une nanostructure cristalline (de type nanofilm ou nanofil). On s’intéresse en particulier aux structures poreuses homogènes et avec gradient de porosité, ainsi qu’aux nanofils modulés en diamètre qui offrent des perspectives intéressantes en terme de réduction de conductivité / Since the past decades, progresses in nanomaterials engineering raise new questions about heat transport processes at very short time and space scales. Thermal properties of nanoscaled devices are determined from the resolution of the Boltzmann Transport Equation (BTE) for phonons, which are the main heat carriers in semiconductors. In this study, BTE is solved with a numerical tool based on a statistical method (Monte Carlo) which tracks phonons’ motion in two kinds of nanostructures: nanofilms and nanowires. We focus on the effect of homogeneous and heterogeneous porous materials as well as nanowires with varying diameters. All these devices present interesting prospects regarding thermal conductivity reduction

This thesis presents experimental analysis of polariton dynamics in semiconductor umicrocavities. A microcavity is a monolithic structure composed of two distribnted Bragg reflectors which are separated by a layer of active material. Strong coupling between excitons residing in the active layer and photons confined within the cavity leads to new eigenstates of the system, called microcavity exciton-polaritons. The dynamics of these qnasi-bosons are stndied using a range of optical spectroscopy techniques. It has been shown previously tlmat resonant injection of polaritons nsing a continuous wave laser allows the rnicrocavity to operate as an optical parametric oscillator. A full study of the recovery dynanucs of a transiently destabilized nucrocavity optical parametric oscillator is made in this thesis. Destabilization was achieved by optically injecting surplus polaritonms into a systeum that had reached equilibrium. The dynamics of the scattering processes is theoretically described using a rate equation model. Bose condensation and polariton lasing have recently been demonstrated at liquid hehum temperatures. In this thesis, we use a hybrid bulk gallium nitride nuicrocavity to demonstrate the operation of the first room temperature polariton laser. Polarisation measurements also show spontamieous symmetry breaking, implying observation of the first room tenmperature Bose-Einstein condensate.