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A Simulator for the Clocking of Molecular Quantum-Dot Cellular Automata G

Supervisor: Dr Gerard Edwards

BEng Second Year Project ERASMUS Student: Mr Linus Minkevicius

Molecular Quantum-Dot Cellular Automata (QCA) is a revolutionary nano-computing paradigm which is scalable to the molecular level, where a single QCA molecular cell encodes a single bit (Blair, et al., 2010) (Hennessy & Lent, 2001) (Lent, et al., 2006). Current Complementary Metal Oxide Semiconductor (CMOS) integrated circuit technology is coming to the end of its life, in terms of further transistor integration, with severe problems due to heat dissipation that can melt the chip and challenges to define the transistor off state, due to quantum mechanical tunnelling leakage (Lent, et al., 2006). Clocked control of the QCA is essential to avoid the trapping of the system into metastable states, the QCA working with ground state computing (Lent, et al., 2006).

A half - cell for a QCA is shown in Figure 1 with a single mobile electron charge and the dots being molecular redox centres, with bridging ligands (Hennessy & Lent, 2001) (Lent, et al., 2006). For a strong positive field applied to the cell the electrons are drawn into the lower dot and the cell is in the “null” state. However, a negative field pushes the electrons into either the upper left of upper right dots representing binary “1” or “0” respectively and these states are degenerate. An E field in the x direction, due to the presence of the cell’s neighbours, can split the degeneracy (Hennessy & Lent, 2001) (Lent, et al., 2006). This field due to the neighbours, determines the logical state of the cell (Hennessy & Lent, 2001) (Blair, et al., 2010).

Figure 1 A cross section for a half - cell for a molecular QCA showing the three dots of a cell with bridging ligands. For sufficiently strong positive attractive clocking field strength along y, the mobile electron is pulled into the lower dot and the half - cell is in the “null” state. For a negative repulsive clocking field along y, the electrons are pushed upwards and occupy either the upper left or right dot, the states encoding “1” and “0” respectively, on the cell. The central line is along the x direction (Hennessy & Lent, 2001).

 

A four phase clock with voltages supplied to the clocking wires placed beneath the QCA molecular layer (see Figure 2) will induce the flow of data down a QCA line of cells. This system acts as a molecular shift register (Hennessy & Lent, 2001) (Blair, et al., 2010).

Figure 2 The geometry for the clocking of a line of molecular cells. A line of molecules are placed along x while the clocking is provided by wires, buried beneath the molecular layer, running along z, out of the page. A four phase voltage clocking scheme is applied to these clocking wires, in a repeating pattern of groups of four. In addition there is a grounded conducting layer along x, above the QCA layer.

The aim of this project was to develop a MATLAB based simulator for the clocking of Molecular QCA, containing a Graphical User Interface (GUI), to facilitate the setting of input parameters, to display the E field produced by the clocking wires, fed by the four phase clock, investigate the feasibility of the QCA molecular shift register and general clocking control. The Object Oriented Programming method was employed in the software development (see Figure 3).

Figure 3 A Class Diagram for the MATLAB Based QCA Clocking Simulator including a GUI to enable parameter input and display output information. The Conductor class is an aggregation of the individual wires that are clocked by applying a voltage.

A screenshot of the GUI for the final product shows how easily the input simulation parameters can be set and the display of the insightful calculated output information, for the spatial variation of the E field, for a specified four phase clock signal.

Figure 4 The GUI for the QCA Clocking Simulator. There are panels to input geometric parameters and control the Simulation {Wire and Conductor Settings; Clocking Wire Signal Parameters; Time Settings; QCA Settings; Additional Setting Controlling the Graphical Display}. There are Graphical Outputs for {The geometry of molecular array/clocking with the E field distribution; The E field at the QCA surface as a function of both time and position; The E field at the QCA surface as a function of position; The Four Phase Clock Waveforms with voltage against time applied to adjacent wires, in groups of four.}

References

Blair, E. P., Yost, E. & Lent, C. S., 2010. Power dissipation in clocking wires for clocked molecular quantum-dot cellular automata. Journal of Computational Electronics, Volume 9, p. 49.

Hennessy, K. & Lent, C. S., 2001. Clocking of molecular quantum-dot cellular automata. Journal of Vacuum Science and Technology B, Volume 19, p. 1752.

Lent, C. S., Liu, M. & Lu, Y., 2006. Bennett clocking of quantum-dot cellular automata and the limits to binary logic. Nanotechnology, Volume 17, p. 4240.