Quantum-dot Cellular Automata (QCA) is a new paradigm for computation that utilizes polarization states instead of using current switching. It is being studied because of the realization of the quickly approaching limitation of the current CMOS technology. The location of two excess electrons located within four or five quantum dots on a particular cell can transmit the binary information. These dots are located in the corner of a square cell, and if there is a fifth dot it is located in the center. The electrons are allowed to tunnel freely among the dots, but are restricted from tunneling between neighboring cells. Because of the interaction between the electrons, they will anti-align within the cell giving one of two particular configurations. This configuration can be transmitted to neighboring cells. In other words, data is flowing.We present a numerical study of the fabrication defect's influence on Quantum-dot Cellular Automata (QCA) operation. The statistical model that has been introduced simulates the random distribution of positional defects of the dots within cells and of cells within arrays. Missing dots within a QCA cell structure have also been studied.We have studied specific non-clocked QCA devices using the Inter-cellular Hartree Approximation, for different temperatures. Parameters such as success rate and breakdown displacement factor were defined and calculated numerically. Results show the thermal dependence of the breakdown displacement factor of the QCA devices. It has been shown, that the breakdown displacement factor decreases with increasing temperature. As expected, multiple defects within the same QCA array have shown a reduction in success rate greater than that of a single defect influencing the system. / Department of Physics and Astronomy

To have a useful QCA device it is first necessary to study how to control data flow in a device, then study how temperature and manufacturing defects will affect the proper output of the device. Theoretically a "quantum wire" of perfectly aligned QCA cells at zero Kelvin temperature has been examined. However, QCA processors will not be operating at a temperature of zero Kelvin and inherently the manufacturing process will introduce defects into the system. Many different types of defects could occur at the device level and the individual cell level, both kinds of defects should be examined. Device defects include but are not limited to linear and/or rotational translation, and missing or extra cell(s). The internal cell defects would include an odd sized cell, and one or more miss-sized or dislocated quantum dot(s). These defects may have little effect on the operation of the QCA device, or could cause a complete failure. In addition, the thermal effect on the QCA devices may also cause a failure of the device or system. The defect and thermal operating limit of a QCA device must be determined.In the present investigation, the thermal and defect tolerance of clocked QCA devices will be studied. In order to study tolerance of QCA devices theoretical models will be developed. In particular, some existing computer simulation programs will be studied and expanded. / Department of Physics and Astronomy

The defects and fault tolerance study is essential in the QCA devices in order to know its characteristics. Knowing the characteristics, one can understand the flow of information in a QCA system with and without manufacturing and operational defects. The manufacturing defects could be at device level or cell level. At the device level, the cell could be rotated, displaced vertically or horizontally, the cell could be missing or the size of the cell could be different. At the cell level, there could be a missing dot, dot could be displaced from its position or the size of the dots could be different. The operational defects are due to its surrounding, such as temperature or stray charge. Each of these defects and fault tolerances can be studies in detail in order to find the optimum working conditions where the information can be safely transmitted to the appropriate locations in the device.The theoretical studies have shown that at absolute temperature and without any defect, the QCA devices are operational. But it is almost impossible to manufacture a perfect or defect free device, and also it is impractical to think about operating a system at absolute zero temperature environment.Therefore, it is important to investigate the fault tolerant properties with defects and higher temperatures to see how far the QCA device can operate safely. Many studies have been done to investigate the fault tolerant properties in QCA devices. However, these studies have not completely exhausted the study of defects and temperature effects. In this study, the dot displacement and missing dots with temperature effects are investigated for the basic QCA devices and a Full Adder. In order to study fault tolerant properties, the existing theoretical model and computer simulation programs have been expanded and used. The defect characteristics have been simulated using normal distribution. / Department of Physics and Astronomy