Experimental studies on Quantum-dot Cellular Automata Devices

Quantum-dot Cellular Automata is an exciting novel device architecture for implementation of digital logic using bistable elements. This architecture offers a number of advantages such as logical completeness, low power dissipation and possibility of miniaturization of devices into the nanometer scale. In this dissertation, we demonstrate the operation of metal-based QCA devices such as double-dot, cell, latch and shift register, and investigate properties such as memory, power gain and errors in these devices. The devices are fabricated using the aluminum tunnel junction technology. Charge is confined on islands of aluminum connected to each other by tunnel junctions formed by a thin layer of aluminum oxide. These islands or 'dots' are arranged in the form of cells so that each cell has two degenerate ground states depending on the position of electrons in the dots. Various digital logic gates can be formed using arrangements of these cells with respect to one another. We start with the demonstration of a leadless QCA double-dot and cell. Switching is accomplished in the QCA cell by application of input voltage signals through gate capacitors. Electron transfer between the dots in a QCA cell is detected by measuring the dot potentials using SET electrometers. Control of switching in a QCA cell by an external signal can be accomplished by using an extra dot between the top and bottom dots of a half-cell and modulating its potential using a clock voltage signal. We demonstrate clocking in QCA devices using a half cell containing three dots (triple-dot), with inputs applied to the top and bottom dots and clock applied to the middle dot. Memory is demonstrated in a clocked QCA half-cell by suppressing co-tunneling between the top and bottom dots by fabricating multiple tunnel junctions between them. This device is called the QCA latch. A QCA shift register can be made by placing multiple latches next to each other and applying phase-shifted clock signals. A two stage QCA shift register is demonstrated using two latches capacitively coupled to each other. Power gain is demonstrated experimentally in a latch and a shift register by calculating the work done by each latch on the next, in a row of latches. Further, the types and properties of errors in the operation of the QCA latch and shift register are investigated by statistically measuring error rates under various conditions of input magnitude and bias. Finally a circuit for microwave frequency measurements of QCA devices using an RFSET is discussed. The experiments presented in this dissertation demonstrate leadless operation, clocking, memory and power gain in QCA devices. Error analysis performed on the latch shows that as the charging energy of these devices is increased, the error rates would fall exponentially.