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This thesis reports on the interplay between molecular structure and photophysics in light emitting conjugated polymers revealed by steady state and picosecond time-resolved fluorescence spectroscopy. The fundamental excited state relaxation of a polyfluorene derivative is compared to that of two oligofluorenes using isolated molecules in dilute solution. Their long-time time-dependent spectral dynamics are monitored by means of picosecond streak camera and single photon counting techniques. Excited state relaxation in oligofluorenes is entirely conformational and depends on solvent viscosity. The intrachain photophysics of the polyfluorene is dominated by fast excitation migration with a slow conformational component. A rigid ladder-type polymer exhibits only migrational relaxation. In analogy, the spectral dynamics of an alkoxy-substituted polyspirobifluorene are studied in dilute solution. Their qualitative dependence on solvent viscosity is elucidated by further femtosecond photobleaching measurements. Two excited states are proposed with the lower energy state involving strong spiroconjugation as confirmed by electronic structure calculations. Conversion between them occurs via conformational relaxation of the fluorene side groups. The sensitive reaction of these photophysics to the substitution pattern of the polymer suggests an easy chemical tunability of polyspirobifluorenes towards optimised charge carrier transport properties. Finally, the formation of the beta phase in amorphous polydioctylfluorene is investigated as a function of spin coating fabrication in the solid state. It is suggested that it forms by condensation at colloidal sites, which arise from incomplete solvation in the master solution. A further room temperature phase exists in the absence of these nuclei. The excited state relaxation after energy transfer from amorphous to beta phase is monitored via time-resolved spectroscopy. Within the beta phase, exciton migration is restricted as confirmed by steady state anisotropy data. This evidence for exciton confinement is an important step towards the application of the beta phase as a polymer laser.

This thesis presents the electrical characterisation of defects in novel group IV semiconducting materials: semiconducting diamond and silicon germanium (SiGe) virtual substrates. Several methods to clean diamond surfaces are introduced, which lead to the fabrication of a diamond Schottky diode with acceptable characteristics. Current-Voltage (I-V) and Capacitance-Voltage (C-V) measurements were carried out to study the electrical properties of both the diamond and SiGe Schottky diodes. Deep level transient spectroscopy (DLTS) and Laplace DLTS were then carried out to investigate the deep electronic states in these devices. Scanning Electron Microscopy (SEM) was also used to investigate defects in the diamond samples. For the diamond Schottky diodes, I-V and C-V measurements confirmed the quality of the fabricated Schottky diode; the measured phase angle between capacitance and voltage was close to 90° for temperatures greater than 300K and frequencies above 200 kHz and the device clearly exhibited rectification. DLTS and LDLTS measurements of the diamond did not show any signatures that could be attributed to isolated point defects. This could be due to the fact that it was necessary to take the samples to higher temperatures in order to fully ionize the boron in the sample. The boron acceptor is at 0.37 eV above the valence band and therefore only about 5% is ionised at room temperature. During the major part of the study at Manchester, there was no access to a high temperature cryostat. However, a clear capacitance transient was observed at lower temperatures and it is proposed that this is due to emission of holes from boron. Deep traps will be located deeper in the band gap than the boron. An additional problem was that the sample was of polycrystalline structure and is full of grain boundaries, which appear to be implicated in the leakage currents present in our devices. I-V, C-V, DLTS and LDLTS were also used to investigate the deep states in the SiGe virtual substrate. I-V and C-V measurements showed that the SiGe Schottky diode showed some leakage (reported by the suppliers) but nevertheless the diode exhibited rectification. Analysis of the DLTS data showed the presence of a defect in the SiGe samples which could be a structural defect, probably dislocation-related. However, the low background doping meant that a considerable depth below the surface was being measured in DLTS and depth profiling was not possible.

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Efforts in the current semiconductor industry are focused on the production of smaller, more efficient and inexpensive devices of higher packing density. As silicon is the dominant semiconductor implemented for the fabrication of the majority of semiconductor devices, perpetual research has focused on the improvement of its properties and the realisation of the most efficient structures. This thesis presents the electrical characterisation of two different diode structures that are important for the present and future generations of electronic devices. The first part of the thesis is focused on the electrical characterisation of Ultra-Shallow Junction (USJs) Si diodes. Both p+n and n+p USJ structures that contained different implants were examined. These were very highly doped and intended to simulate the situation where a doping well is formed after heavy doping in Si for the fabrication of transistors currently used in Complementary-Metal-Oxide-Semiconductor (CMOS) technology. The implanted USJ diodes were provided by NXP, Belgium and contact deposition was performed before their electrical characterisation as part of this project. Subsequently the p+n and n+p USJ diodes were characterised by the use of Capacitance-Voltage (CV), Current-Voltage (IV), Deep Level Transient Spectroscopy (DLTS) and high resolution Laplace DLTS (LDLTS). DLTS and LDLTS are very powerful spectroscopic techniques for the profiling of defects in the bandgap of a semiconductor as well as for the identification of the electrical signatures of these defects. Transient-Enhanced Diffusion (TED) related defects were detected in these diodes as the presence of mainly carbon-related interstitial complexes was observed. In addition, certain vacancy or vacancy-dopant related levels were also discerned. The second part of this thesis presents the electrical characterisation from Schottky p-diamond/p-Si and p-diamond/n-Si p-n diodes. These diodes were readily provided, grown by the Chemical Vapour Deposition (CVD) technique, for the electrical characterisation that was performed as part of this project. The purpose of characterising both Schottky and p-n diamond on Si diodes was to detect defects near the surface of the films and near the interface with Si and hence provide a comparison between defects present at the beginning and end of growth. More defects were found near the interface with Si and the majority of observed defects were related to extended defects while the presence of grain boundaries in polycrystalline diamond was discussed.

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In this work the electrical and electroluminescence properties CdTe nanocrystal films were analysed. The structure consisted of a multilayer of CdTe nanocrystals deposited by the layer-by-layer technique, sandwiched between an ITO anode and an aluminium cathode. The first part of this work was dedicated to structural and process improvement. Earlier devices, produced through a layer-by-layer (LbL) manual procedure, had an average thickness of 30nm per nanocrystal monolayer, with a roughness near 30% of the overall thickness. Electrical tests showed current densities over 100 mA/cm^2, with a frequent occurrence of pinholes and short-circuits that caused erratic sample behaviour and device rupture. SEM and AFM microscopic analysis showed that the nanoparticles were aggregated in 15-20nm thick clusters bound by polymer. A high porosity and non- uniformity of the multilayer was observed, explaining the formation of short-circuits. Luminescence was obtained from a very small number of samples, and with a very short duration that did not make spectral analyses possible. A robotic arm was programmed to carry out the LbL deposition, in an attempt to reduce the inhomogeneities of the multilayer. A key factor was the introduction of a special routine to remove the samples from the solutions. The sample withdrawal was designed to be in the vertical at a rate of 1.18 mm/s. The idea was to use gravity and the surface tension of the aqueous solutions to remove all the excess liquid from the surface. Additionally, poly(ethylenimine) (PEI) was eliminated from the process to improve homogeneity. These modifications produced multilayers with a thickness of 3nm per layer, average roughness below 5nm and CdTe packing density of 27%. Electrical measurements showed a stabilisation of the current-voltage (I-V) characteristics. A significant improvement in luminescence occurrence frequency and intensity was also achieved, enabling first spectral analyses. Once a reliable manufacturing procedure was developed, the electrical characterisation commenced with the analyses of samples with a different number of layers, operated in air. A field dependency of the I-V curves was found. Optimal performance was obtained from 30-layer samples, and this number of layers was adopted for subsequent analyses. Best samples showed external quantum efficiencies of 0.51%, with a photometric response of 0.8 lm/W and peak brightness of 1.42 cd/m^2. However, current and electroluminescence (EL) degradation with voltage and operation time were found in the device. Single carrier devices revealed a barrier for electron injection higher than predicted by the band diagram of the structure. The presence of an aluminium oxide layer at the multilayer/cathode interface was postulated, and confirmed through experiments in nitrogen. It was proposed that the growth of this oxide layer is the cause of device degradation during operation in air. However, it was demonstrated that the presence of the oxide favoured radiative recombination prior to degradation, with device efficiencies nearly 10 times higher than in devices without the oxide film. This was justified through three effects: charge accumulation at both sides of the oxide, field concentration across the oxide barrier and a reduction in leakage current. Unequal behaviour of samples with different electrode materials revealed that charge injection was the limiting mechanism for current flow, with a current onset field in the range of 2-3x10^7 V/m. Fowler-Nordheim plots showed that field emission was responsible for hole and electron injection into the device. Also, Fowler-Nordheim plots provided evidence of the dynamic nature of the cathode oxidation process. Dielectric breakdown of the aluminium oxide barrier at a rupture field of 3x10^7 V/m was found as a possible triggering mechanism of oxide layer growth. It was found that a critical field around 4.5x10^7 V/m caused irreversible loss of photoluminescence (PL) of the nanocrystals. This loss was attributed to an avalanche effect within the multilayer. The operational range for the devices is then found to be between 2x10^7 V/m and 4.5x10^7 V/m.

In this thesis light emission from sulphur related impurity in silicon has been reported. Although, sulphur related luminescence from silicon has been stated since the 1980's, no room temperature luminescence has been achieved and no compatible devices that can be integrated to the silicon technology have been invented. Photoluminescence and electroluminescence experiments were made on a set of samples implanted with only with sulphur at doses ranging from 1011-1014 S cm-2 at 30 keV, annealed at 1000 °C or 1100 °C for 10 s and on another set of samples implanted with sulphur as above and further implanted with boron at 1015 B cm-2 at 30 keV, further annealed at 950 °C for 1 min. The experiments revealed two major emissions at 1129.5 nm (1.0997 eV) which is due to the Si TO phonon assisted transition and at 1363 nm (0.9097 eV) which is due to sulphur related impurities. Variable temperature experiments were done at both PL and EL experiments. From the EL variable temperature measurements, it was observed that the two main lines were shifting towards longer wavelengths with the increase of temperature. Sulphur emission was present at room temperature with low intensity compared to the silicon emission which was more dominant at room temperature. Of great interest was the effect of power on silicon and sulphur emission. It has revealed a sublinear and a superlinear behaviour for the sulphur and silicon integrated intensity respectively with the increase of the injection condition, which can be attributed to the saturation of sulphur related levels responsible for the 1.33 nm emission at the high excitation levels. A model of the diffusion of sulphur concentration after the annealing treatments was presented, introducing the two cases of perfect reflection and perfect loss from the samples surfaces. Finally a model explaining our PL and EL power dependence experiments was provided which showed that there are two major radiative routes via the silicon and the sulphur that take place, which are competing at each other along with a non-radiative route coming from the sulphur related level. Our model describes the trends in our experimental data well. Finally, the energy related to the sulphur peak quenching was calculated to be 32.2 +/-1.4 meV.

Avalanche photodiodes (APDs) can provide higher sensitivity, when the noise is dominated by electronic noise, than conventional p-i-n photodiodes due to its internal gain achieved via the impact ionisation process. High speed and high sensitivity photodetectors operating at the wavelength of 1.55 m for optical communication have been intensely research due to the ever increasing internet traffic, particularly in the long-haul communication systems. In this dissertation high speed InGaAs p-i-n photodiodes, InGaAs/InAlAs separate absorption and multiplication (SAM) APDs are designed and characterised. The waveguide InGaAs photodiode exhibits a maximum -3 dB bandwidth of 26.5 GHz and external quantum efficiency of 38.4% giving a bandwidth-efficiency product of 10.2 GHz, which is higher than 7.14 GHz obtained from conventional vertically illuminated diodes fabricated from the same wafer. Building on the high speed InGaAs waveguide diodes, the InGaAs/InAlAs APDs were fabricated. We demonstrated low dark currents of ~50 nA at 0.9Vbd (Vbd is the breakdown voltage), low excess noise factor k  0.2 (k is the effective ratio of ionisation coefficients ratio in excess noise model) and wide bandwidth up to 40 GHz at low gains. Our APDs also achieve higher signal amplification than the best 40 Gb/s APD reported, confirming the suitability of our APDs for use in the 40 Gb/s optical communication systems. The signal enhancement of up to 24 dB was achieved at 35 GHz. While the InGaAs/InAlAs APDs may be suitable for 40 Gb/s operation, the avalanche gain is limited due to their limited gain bandwidth products. Hence novel wide bandgap AlAsSb avalanche regions were characterised for next generation high speed SAM APDs. The temperature dependence of dark current and avalanche gain were investigated using AlAsSb p-i-n diodes with avalanche region widths of 80 and 230 nm. Extremely low temperature coefficients of breakdown voltage of 0.95 and 1.47 mV/K were obtained in these AlAsSb diodes, which are significantly lower than all semiconductor materials, with similar avalanche region widths, in the literature. Band to band tunelling current was shown to be significantly lower than those in InP and InAlAs diodes with the same avalanche region widths. By utilising an extremely thin 40 nm AlAsSb as multiplication layer, low excess noise factor corresponding to effective k values of 0.1 to 0.15 in InGaAs/AlAsSb SAM APDs was demonstrated. This is lower than that from an InAlAs pin diode with a 100 nm avalanche region. Therefore the potential of using thin AlAsSb avalanche region for next generation high speed and high sensitivity photodetectors has been demonstrated.