A self-consistent, full-band, electrothermal ensemble Monte Carlo device simulation tool has been developed. It is used to study charge transport in bulk GaN, and to design, analyze, and improve the performance of AlGaN/GaN high electron mobility transistors (HEMTs) and avalanche photodiodes (APDs). Studies of electron transport in bulk GaN show that both peak electron velocity and saturated electron velocity are higher for transport in the basal plane than along the c-axis. Study of the transient electron velocity also shows a clear transit-time advantage for electron devices exploiting charge transport perpendicular to the c-axis. The Monte Carlo simulator also enables unique studies of transport under the influence of high free carrier densities but with low doping density, which is the mode of transport in AlGaN/GaN HEMTs. Studies of isothermal charge transport in AlGaN/GaN HEMTs operating at high gate bias show a drain current droop with increasing drain-source bias. The cause of the droop is investigated and a design utilizing source- or gate-connected field plate is demonstrated to eliminate the drain current droop. Electrothermal aspects of charge transport in AlGaN/GaN HEMTs are also investigated, and the influence of non-equilibrium acoustic and optical phonons is quantified. The calculated spatial distribution of non-equilibrium phonon population reveals a hot spot in the channel that is localized at low drain-source bias, but expands towards the drain at higher bias, significantly degrading channel mobility. Next, Geiger mode operation of wurtzite GaN-based homojunction APDs is investigated. The influences of dopant profile, active region thickness, and optical absorption profile on single photon detection efficiency (SPDE) are quantified. Simulations of linear mode gain as a function of multiplication region thickness and doping profile reveal that weakly n-type active regions may be exploited to achieve higher avalanche gain, without penalty to either applied bias or active region thickness. A separate absorption and multiplication APD (SAM-APD) utilizing a AlGaN/GaN heterojunction is also investigated. The presence of strong piezo-electric and spontaneous polarization charges at the heterojunction enables favorable electric field profile in the device to reduce dark current, improve excess noise factor, improve quantum efficiency, and improve breakdown probability. To maximize SPDE, a new device structure with a buried absorber is proposed and improved SPDE is demonstrated. Lastly, a new approach for the direct generation of self-sustaining millimeter-wave oscillations is proposed. In contrast to Gunn diodes, which exploit a bulk-like active region, periodic oscillation is achieved in the proposed structures through the creation, propagation and collection of traveling dipole domains supported by fixed polarization charge and the associated two-dimensional electron gas along the plane of a polar heterojunction. Numerical simulation of induced oscillations in a simple triode structure commonly used for AlGaN/GaN HEMTs reveals two distinct modes of self-sustaining millimeter-wave oscillation.

Over the last five years, charge-coupled devices (CCD) have been improved dramatically in terms of sensitivity, manufacturability and particularly, cost. This has enabled them to be used economically in many more industrial and commercial electronic imaging processes. They are found in products ranging from video cameras to satellite-based camera systems. This has sparked my interests in these devices, and with a great deal of encouragement from Dr. Turpin, I decided to base my Master's thesis/project on a CCD. The project was mainly based on the design of the electronics and optics needed to support a CCD. The particular circuit design which I used other designs which are available. Many of the designs are microprocessor- based, which tends to limit the speed of operation of the imaging process. Other circuits employ specially coded memory chips to implement the required logic processes, but again, the speed of operation is limited by the access times of the memory chips. The circuit employed in the project uses only logic gates and flip flops, and is probably one of the fastest circuits available for the capture of single-frame images.

Photosynthesis involves the absorption of photons by light-harvesting pigments and the subsequent transfer of excitation from the absorption centre to the reaction centre. This highly efficient phenomenon of excitation transfer has traditionally been explained by the Forster mechanism of incoherent hopping of excitation from one chromophore to another. Recently 2D electronic spectroscopic evidences were gathered by Fleming and coworkers on the photosynthetic Fenna-Matthews-Olson (FMO) complex in green sulfur bacteria [1]. Subsequent simulation studies by the same group [2] led to the proposition of a quantum-mechanical, coherent, wave-like transfer of excitation among the chromophores. However, Fleming's conclusions regarding retention of coherence appeared surprising because, the complex would interact with the numerous degrees of freedom of the protein scaffold surrounding it, leading to decoherence, which is expected to be rapid. Thus, we were interested in proposing an analytical treatment to rationalize the excitation transfer. Traditional approaches employed for studying excitation energy transfer involve the master equation techniques where the system-bath coupling is perturbative and is truncated after a few orders. It is important to note that the system-bath coupling causes both decoherence and population relaxation. Such a perturbative approximation is difficult to justify for the photosystem, as the system-bath coupling and the interchromophoric electronic coupling have comparable values. Also, these treatments are largely numerical studies and demand involved calculations. Thus, exact calculations for such a system (7-level) are very difficult. Consequently, we were interested in developing an analytical approach where the coupling is treated as non-perturbative. We devised a novel analytical treatment which employs a unitary transformation analogous to the one used for the theory of nonadiabatic effects in chemical reactions [3]. Our treatment rests on an adiabatic basis which are eigenstates calculated at each nuclear position (i.e. at each configuration of the bath) bearing a parametric dependence in Qi, where Qi denotes the shift of the exciton at site `i' due to the environment. The treatment is justified because in the case of coherent transfer, the excitation would travel mostly amongst the adiabatic states and the effects of non-adiabaticity are small. We observed that the system-bath coupling, after the unitary transformation, could be decoupled at the lowest order into two parts: a) an adiabatic contribution, which accounts solely for decoherence (this is evaluated almost exactly in our approach) and b) a non-adiabatic contribution which accounts for population relaxation from one adiabatic state to another (treated by a Markovian master equation). When we applied our technique to the FMO complex, our prediction for population evolution at the chromophores showed excellent correspondence with those obtained by Nalbach and coworkers using path-integral calculations [4], which are exact. These were calculations where the environment was modelled using a Drude spectral density. Our method allowed the calculations to be readily performed for different temperatures as well. It should be specifically emphasized that, unlike the involved and cumbersome path-integral calculations by Nalbach and coworkers [4] or the hierarchical equation calculations by Ishizaki et al. [2], our method is simple, easy to apply and computationally expedient. Further it became evident that the ultra-efficiency of energy transfer in photosynthetic complexes is not completely captured by coherence alone but is the result of an interplay of coherence and the dissipative influence of the environment (also known as ENAQT or Environment Assisted Quantum Transport [5]). An added advantage of our analytical treatment was the flexibility it offered. Thus, we could use our formalism to perform expedient analyses on the behavior of the system under various conditions. For example, we may wish to evaluate the consequences of introducing correlations among the bath degrees of freedom on the efficiency of transfer to the reaction centre. To this end, we applied our formalism by introducing correlations among the bath degrees of freedom and then by introducing anticorrelations among the bath degrees of freedom. The conclusions were interesting, for they suggested that the efficiency of transfer to the reaction centre was enhanced by the presence of anti-correlations, when compared with an uncorrelated bath. Uncorrelated baths, in turn, had a higher efficiency of energy transfer than correlated baths [6]. Thus, the population evolution is fastest for the anti-correlated bath, followed by the uncorrelated bath and is slowest for the correlated bath. Similar conclusions have been reached at by Tiwari et al. [7]. We could also extend the formalism for studying the system under different spectral densities for the environment, apart from just the Drude spectral density which is popularly used in literature associated with FMO calculations. For instance, the FMO system could be analyzed for the Adolphs-Renger spectral density [3, 8]. Once again our results showed excellent agreement with those reported by Nalbach. We also analyzed the FMO system under the spectral density proposed by Kleinekathofer and coworkers [9]. It was found that these latter spectral densities had more profound participation from the environment, therefore coherences were destroyed more effectively and population relaxation was faster. The excitation transfer to the final site (site closest to the reaction centre in the FMO complex) was found to be faster for the Adolphs and Renger spectral density and the spectral density proposed by Kleinekathofer and coworkers, when compared to the Drude spectral density. Also, the excitation transfer was fastest when we modelled the environment using the Kleinekathofer spectral density. This reinforced the previous conclusions that the dissipative effects of the environment promote a faster energy transport. Being an almost analytical approach, our technique could be applied to systems with larger number of levels as well. A good example of such a case is the MEH-PPV polymer. 2D electronic-spectroscopic experiments performed on this polymer in solution speculate that the excitation energy transfer might be coherent even at physiological temperatures [10]. A prototype for studying this system might be a conjugated polymer with around 80-100 chromophores. Linewidths and Lineshapes in the vicinity of Graphene It has been reported that a vibrating dipole may de-excite by transferring energy non-radiatively to a neighboring metal surface [11]. It is also understood that due to its delocalized pi-cloud, graphene has a continuum of energy states and can behave like a metal sheet and accept energies. Thus, we proposed that if a vibrationally excited dipole de-excites in the vicinity of a graphene sheet, graphene may get electronically excited and thus serve as an effective quencher for such vibrational excitations. Depending on the distance of the dipole from the graphene sheet, the transfer might be intense enough to be spectroscopically probed. We have investigated the rate of such an energy transfer. We use the Dirac cone approximation for graphene, as this enables us to obtain analyt-ical results. The Fermi Golden rule was used to evaluate the rate of energy transfer from the excited dipole to the graphene sheet [12]. The calculations were performed for both the instances: a) energy transfer from a dipole to undoped graphene and, b) energy trans-fer from a dipole to doped graphene. For undoped graphene, the carrier (electron) charge density in the conduction band is zero and we would only have transitions from the valence band to the conduction band. As a consequence of absence of carrier charge density in CB (conduction band), the screening of Coulombic interactions in the graphene plane is ineffective. Thus, one could use the non-interacting polarizability for undoped graphene in the rate expression [13]. However, when we consider the case of doped graphene where EF is shifted upwards into CB, the conduction band electrons will contribute to screening. In this case, we have two sets of transitions: a) from ki in VB (valence band) to kf in CB and b) ki in CB to kf in CB, where ki and kf are the wavevectors which correspond to the initial and final electronic states in graphene. So we have used the polarizability propagator in the random phase approximation [14] to calculate the rate following the approach of [13]. It is also known that the imaginary part of the frequency domain dipole-dipole corre-lation function is a measure of the lineshape [15]. We were, thus, interested in evaluating the lineshape for these transitions. For evaluating the correlation function, we used the partitioning technique developed by L•owdin [16] and subsequently extracted the lineshape from its imaginary part. Using this method, we calculated lineshape for the vibrational excitation of CO molecule in the vicinity of an undoped graphene lattice. The linewidth for this system also was obtained. It could be seen that the vibrational linewidth for 1 CO in the vicinity (5 A) of undoped graphene (EF = 0:00eV ) is small (0:012 cm ) but could be observed experimentally. The lineshape calculations were also extended to cases where it is possible to have atomic transitions by placing an electronically excited atom in the vicinity of the graphene sheet. We considered the following two cases: a) 3p ! 2s transition in hydrogen atom, at a distance of 12 A from the graphene sheet and, b) 4p ! 3s transition in hydrogen atom, at a distance of 20 A from the graphene sheet. The linewidths for atomic transitions could be easily probed in these cases ( 55 cm 1 for 3p ! 2s and 56 cm 1 for 4p ! 3s). In the preceding calculations, the transi-tion dipoles were considered perpendicular to the graphene surface. It is worthwhile to note that if the transition dipoles are considered parallel to the graphene surface, the respective linewidths would be half of those obtained for the case where the transition dipoles are perpendicular. Another interesting possibility would be to consider a lanthanide metal complex placed within a few nanometers from graphene. Lanthanides are known to have sharp f-f transitions [17] and consequently, one could easily observe the effects of broadening due to energy transfer to the electronic system of graphene. Energy Eigenmodes for arrays of Metal Nanoparticles In the final part of the thesis we consider organized assemblies of metal nanoparti-cles, specifically helical and cylindrical assemblies and investigate the plasmonic excitation transfer across these assemblies. These were motivated by recent studies which reported growth of chiral asymmetric assemblies of nanoparticles on D and L- isomers of dipheny-lalanine peptide nanotubes [18]. The plasmons in the helical/cylindrical assemblies are expected to couple with each other via electromagnetic interactions. We construct the Hamiltonian for such systems and evaluate the eigenmodes and energies pertaining to these modes in the wave vector space. We also perform calculations for the group velocity for each eigenmode as this gives us an idea of which eigenmode transports excitation the fastest.