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Type of Document Dissertation Author Kummamuru, Ravi Author's Email Address rkummamu@nd.edu URN etd-04162004-163831 Title Experimental studies on Quantum-dot Cellular Automata Devices Degree Doctor of Philosophy Department Electrical Engineering Advisory Committee
Advisor Name Title Dr. Alan Bowling Committee Chair Dr. Gregory L. Snider Committee Co-Chair Dr. Alexei O. Orlov Committee Member Dr. Craig S. Lent Committee Member Dr. Gary H. Bernstein Committee Member Dr. Patrick J. Fay Committee Member Keywords
- Quantum-dot Cellular Automata
- QCA
- Single electronics
- Digital Logic
- Latch
- Shift-register
- Power Gain
- Leadless
Date of Defense 2004-02-13 Availability restricted Abstract Quantum-dot Cellular Automata is an exciting novel device architecture forimplementation 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.
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