No description available.

Intersubband transitions (ISBTs) in quantum wells have had extensive prior research but remain interesting. This thesis continues the use of saturation experiments for the characterisation of ISBT relaxation times ie. the time taken for carriers to scatter between a quantum well's subbands. Saturation experiments were carried out using an optical parametric generator as a tunable source of mid-infrared laser light (6.4 --t 8.6 pm) and intensities of -200MWcm-2 were reached. For this thesis, two p-doped samples were saturated, each sample containing multiple GaAs/AIGaAs quantum wells. Both sample's quantum wells had two confined heavy hole levels and the ISBTs between them, had energies of 160meV and 183meV (respectively). The hhl-hh2 ISBTs were saturated for each sample, the measured carrier relaxation times were 0.3 ± 0.1 ps and 2 ± 1.4 ps respectively, indicating that relaxation occurred via LO phonon emission (for comparison, an n-type quantum well with an ISBT of similar energy would have a relaxation time between 0.3 --t 0.6 ps). The effect of circularly polarised light on the carrier dynamics ofn-doped GaAs/AIGaAs quantum wells was also investigated. Circularly polarised light can lead to spin-sensitive ISBTs; this has been shown to lead to a change in the ISBT's saturation intensity for circularly polarised light due to the addition of the spin-relaxation time to the carrier dynamics. For this thesis, saturation experiments were performed using light incident at an oblique incidence to the QWs (via a novel sample geometry) but no significant differences in saturation intensity were {ound between using circularly and linearly polarised light.

No description available.

No description available.

Quantum dots (QDs) are zero-dimensional nanostructures that confined carriers in three dimensions comparable to their de Broglie wavelengths. Therefore, carriers exhibit δ-shaped energy levels and densities of states. Due to their band structure, QD systems show significant advantages as active regions in laser cavities, both in term of lower threshold current densities and better thermal behaviour. The most studied system beingInAs/GaAs system but the antimonide-based (Sb-based) material system has been paid much attention due to their potential for optical devices in the 3-5 μm (0.25-0.40 eV) spectral regions and motivated by feasibility of active medium in high speed electronic and long wavelength photonic devices.�In most cases, QDs structures had been obtained with an intrinsic elastic strain field arising from the lattice mismatch between the matrix and QD materials. The strain field plays a very significant role in the fabrication of the self-assembled QDs (SAQDs).Strain fields inside SAQD structures strongly affect the electronic band structure, which in turn, strongly affects the performance of optoelectronic devices. Therefore,knowledge and determination of the strain field in the dots and surrounding matrix is crucial in order to obtain a well ordered SAQDs structure. While knowledge and determination of the electronic structure calculation are necessary for further device modelling to improve the performance of the devices.Numerical work based on continuum-elasticity based on Finite Element Method (FEM) and standard-deformation-potential theory has been carried out to investigate the effect of strain on the band structure for InSb-based SAQD systems with type-I and type-II band alignment. The effect of elastic anisotropy on both strain distribution and band edges profile is also performed. Next, multi-band k·p method is used to model the electronic structure of InSb-based SAQD systems.The results from the modelling show that the strain-modified band profile of the zinc-blende III-V compound semiconductor SAQDs is not very sensitive to the details of the dot shape and the major governing parameter of the geometry is the aspect ratio of the dot. The modelling results also reveal that there are no appropriate material combinations for zinc-blende III-V compound semiconductors that would applicable for the MIR 3-5 μm (0.25-0.40 eV) emission range when type-I band alignment is possible. This leads to the investigation of type-II broken gap InAsxSb(1-x)/InAs SAQDs.Finally, the optical properties of the InSb-based SAQDs are investigated by means of the photoluminescence (PL) measurement using Fourier transform infrared (FT-IR) spectroscopy. The PL results are analysed and compared to the modelling results.

No description available.

No description available.

Negative longitudinal miagnetoresistance in indium antimonide subject to a quantizing magnetic field [h o_c > kT and oc tau >> 1 where o_c is the cyclotron frequency] has been investigated under ohmic and non-ohmic conditions. Conduction band electron concentrations ranged from 1 x 10¹⁴ cm up to 2.23 x 10¹⁵ cm^-3. Under ohmic conditions the negative magnetoresistance was studied at various stabilized temperatures between 4.2°K and 130°K. Non-ohmic results were taken at 4.2°K, using pulsed electric fields up to 10 V cm^-1 ^(5A) to inducefree carrier heating. Using an "electron temperature model" to represent the energy distribution of the electron system, the ohmic and non-ohmic behaviour are compared. The comparison yields soma idea of the validity of this model in the presence of large magnetic and electric fields. The mechanism proposed for the observed negative magneto-resistance is the magnetic field reduction of the small-angla scattering from collisions with the ionized impurities, as originally proposed theoretically by Argyres and Adams (1956) and extended by Dubin skaya (1969). A computer programme is formulated for the extreme quantum limit [h o_c > kT and E_F] where only the ground state Landau level is occupied. As well as incorporating arbitrary degeneracy the calculations include the effect of drift momentum relaxation by small angle forward scattering events. These contribute to momentum relaxation because of the energy uncertainty of an. electron associated with finite collision times. The role of higher Landau energy levels is also considered. The inclusion of the forward scattering events results in a fairly good quantitative agreement with experiment. Various theoretical approximations and the effects of sample inhomogeneity are discussed as limitations on the agreement. An experimental and theoretical review of the phenomenon of negative magnetoresistance in semiconductors, associated with a variety of mechanisms, is also given.

Organic semiconductors are of increasing technological interest in applications such as light emitting diodes, field effect transistors, and photovoltaic devices In order to reveal the basic principles behind these organic semiconductors, charge transport theory in these organic materials has been introduced and has been receiving increasing attention over the last few years. Although excitons are known to interact with free charges, the effect that excited states may have on the charge transport is not generally considered in the field of organic electronics. This occurs even though organic light emitting diodes (OLEDs) are known to contain large numbers of triplet states during operation. Indeed, it is quite possible that the mobility in working devices may well be a function of drive current, as the excited state population will change with operating conditions. This work is thus motivated by both technological and fundamental scientific interest. In this thesis, the hole mobilities in both poly-(3-hexylthiophene) (P3TH) and N,N’- diphenyl-N,N’-bis(3-methylphenyl)-(1,1’-biphenyl)-4,4’-diamine (TPD) devices ( P HT cm Vs Hole m 3 » 5.0´10-5 2 / , TPD cm Vs Hole m » 5.0´10-4 2 / ) have been measured, and observed a remarkable mobility reduction (～15%) in ambipolar samples (in both P3HT and TPD) after applying a small DC offset bias. This correlated to the turn-on voltage in I-V characterization, and the luminescence in the ambipolar TPD sample. In the unipolar sample, however, there is no such behaviour. This strongly suggests that the reduction of the hole mobility is due to site blocking/interacting caused by the excited triplet states. In further experiments in the presence of a magnetic field (500 mT), results an increase in the mobility ( ～ 5%) and steady state current density in ambipolar samples only, this is consistent with magnetically mediated inter-conversion of 4 (blocking/interacting) triplet states to the singlet states. The correlation between the magnetic mobility increase and the steady state current increase offers direct evidence for a microscopic mechanism behind organic magneto resistance (OMR). Given the experimental evidence, we conclude that excitons (specifically triplet states play a critical role in charge transport in organic semiconductors.

The development of retinal implants has become an important field of study in recent years, with increasing numbers of people falling victim to legal or physical blindness as a result of retinal damage. Important weaknesses in current retinal implants include a lack of the resolution necessary to give a patient a viable level of visual acuity, question marks over the amount of power and energy required to deliver adequate stimulation, and the removal of eye movements from the analysis of the visual scene. This thesis documents investigations by the author into a new CMOS stimulation and imaging chip with the potential to overcome these difficulties. An overview is given of the testing and characterisation of the componments incorporated in the device to mimic the normal functioning of the human retina. Its application to in situ experimental studies of frog retina is also described, as well as how the data gathered from these experiments enables the optimisation of the geometry of the electrode array through which the device will interface with the retina. Such optimisation is important as the deposit of excess electrical charge and energy can lead to detrimental medical side effects. Avoidance of such side effects is crucial to the realisation of the next generation of retinal implants.