Thesis (Ph. D.)--School of Electrical and Computer Engineering, Georgia Institute of Technology, 2004. Directed by Douglas M. Blough. / Includes bibliographical references (leaves 62-63).

An advancement of the two stage growth recipe for the fabrication of CIGS solar cells was developed. The developed advancement was inconsistent in producing samples of similar stoichiometry. This was a huge barrier for up scaling the process as the behavior of devices would be different due to variation in stoichiometry. Samples with reproducible stoichiometry were obtained once the heating rate of elements, selenium in particular was better understood. This is mainly attributed to the exponential increase of selenium flux after its evaporation temperature. Monitoring the selenium flux was vital in getting constant selenium fluxes. Few changes to the growth recipe were induced to optimize the amount of selenium being used. Depositions were done using constant selenium to metal flux ratio of 5. Elemental tradeoffs were observed as a result of the growth recipe change. These tradeoffs are in favor of the two stage growth recipe. The solar cells were fabricated on a soda lime glass substrate with a molybdenum back contact. Improper sample cleaning and storage were found to affect the deposition outcome of the molybdenum back contact. This also had a cascading effect on the absorber layer. Residual precipitates during deposition of CdS were avoided by increasing the spinner speed which increased the reaction rate. This is attributed to the growth of CdS either by cluster-by-cluster growth or by ion-by-ion growth. SEM, EDS were some important tools used to characterize the devices. EDS in particular, was used extensively at different stages throughout the growth process to ensure that we were heading in the right direction. Current-voltage (I-V) measurements were done to study the solar cell performance under light and dark.

Understand the behavior of queueing networks in heavy tra c is very important due to its importance in evaluating the network performance in related applications. However, in many cases, the stationary distributions of such networks are intractable. Based on di usion limits of queueing networks, we can use Re ected Brownian Motion (RBM) processes as reasonable approximations. As such, we are interested in obtaining the stationary distribution of RBM. Unfortunately, these distributions are also in most cases intractable. However, the tail behavior (large deviations) of RBM may give insight into the stationary distribution. Assuming that a large deviations principle holds, we need only solve the corresponding variational problem to obtain the rate function. Our research is mainly focused on how to solve variational problems in the case of rotationally symmetric (RS) data. The contribution of this dissertation primarily consists of three parts. In the rst part we give out the speci c stability condition for the RBM in the octant in the RS vi case. Although the general stability conditions for RBM in the octant has been derived previously, we simplify these conditions for the case we consider. In the second part we prove that there are only two types of possible solutions for the variational problem. In the last part, we provide a simple computational method. Also we give an example under which a spiral path is the optimal solution. / text

This dissertation presents two efficient circuit simulation techniques for very large scale integrated (VLSI) circuits. Model order reduction (MOR) plays a significant role in VLSI circuit simulation as nowadays the system model may contain millions of equations or variables. MOR is needed to reduce the order of the original system to allow the simulation to be performed with an acceptable amount of time, reasonable storage and reliable accuracy. Multi-point moment matching is one of the state-of-the-art methods for MOR. However, the moment order and expansion points are usually selected in a heuristic way, which cannot guarantee the global accuracy of the reduced-order model (ROM). Therefore, it is important to utilize an adaptive algorithm in exercising multi-point moment matching. In this regard, we propose a novel automatic adaptive multi-point moment matching algorithm for MOR of linear descriptor systems. The algorithm implements both adaptive frequency expansion point selection and automatic moment order control guided by a transfer function-based error metric. Without a priori information of the system response, the proposed algorithm leads to a much higher global accuracy compared with standard multipoint moment matching without adaptation. The moments are computed via a generalized Sylvester equation which is subsequently solved by a newly proposed generalized alternating direction implicit (GADI) method. Another technique for circuit simulation proposed in this thesis is the matrix exponential (MEXP) method. MEXP method has been demonstrated to be a competitive candidate for transient simulation of VLSI circuits. Nevertheless, the performance of MEXP based on ordinary Krylov subspace is unsatisfactory for stiff circuits, because the underlying Arnoldi process tends to oversample the high magnitude part of the system spectrum while under-sampling the low magnitude part that is important to the final accuracy. In this thesis, we explore the use of extended Krylov subspace to generate more accurate and efficient approximation for MEXP.We also develop a formulation, called generalized extended Krylov subspace, that allows unequal positive and negative dimensions in the subspace for better performance, and propose an adaptive scheme based on the generalized extended Krylov subspace to select the ratio between the positive and negative dimensions. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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