Conceptual Design With A Molecular Dynamics Simulation Of The Performance Characteristics Of A Nonvolatile Molecular Memory Cell Operating In A Room-Temperature Environment

The semiconductor industry is confronted with 2 significant challenges; the fast approaching miniaturization limits in memory technologies and the lack of a viable technology for introducing operational non-volatility in the random access memory (RAM) of a computer. In this dissertation we proposed 2 conceptual designs of a carbon nanotube (CNT)-based molecular memory cell, each of which is able to fully address both issues. The read-write operations of the memory cell are accomplished by the back-and-forth oscillations of the inner tube that is driven by a controlled electrostatic field impulse. To study its performance characteristics we employed molecular dynamics (MD) simulation that takes into account the main environmental forces such as those arising from the interactions between carbon-carbon atoms, and from the energies of the CNT-electrode binding and the external capacitive source. From the isothermal MD studies, we determined that our second design is most reliable in preserving the non-volatility at room temperatures.Our first design is a double-wall CNT-based memory cell made up of a fully-capped outer-tube and a fully-capped inner-tube or core. The molecular static calculations of the van der Waals potential indicate that the design is bistable with 2 well-defined logical states "0" and "1" for the read-write operations. The design requires an external electrostatic field of 1.5 V/nm to effect a switch between the 2 logical states. For the write operation, 2 types of external electrical field: a short-duration and a long-duration trapezoidal impulse were used. Although the former consumes less power, it is more sensitive to environmental disturbances. The latter on the other hand, uses more power but yields a better write reliability. To assess its performance at the room temperature 300K an isothermal MD simulation with the following 3 thermostat models applied in-turns: Langevin, Gunsteren-Berendsen and Guo-Berendsen was conducted. The Langevin MD study indicates that the fully-capped, double-wall CNT design behaves as a nonvolatile memory cell at room temperature with operating frequencies of up to 4 GHz. However, when replaced by either of the 2 remaining thermostat models, the fully-capped design is not only not able to function as a nonvolatile memory cell, it is also unable to perform the write operations at room temperature.The second design is an opened, center-split double-wall CNT-based memory cell with platinum electrodes placed at two extremities. The outer-tube is opened at both ends but the inner-tube remains fully-capped. To produce deeper and wider potential wells in the system energetics, we center-split the outer-tube into 2 sections. This design has the 2 structural parameters that need to be optimized: the split gap and the electrode gaps that determine the positioning of the 2 platinum electrodes. From a superposition of molecular static calculations of the van der Waals potential for each of the cell configuration, we showed that the design possesses bistable characteristics with 2 well-defined logical states "0" and "1". A switching voltage of 7.0 V is found to be sufficient to drive the core between the 2 logical states. Also, the same 2 external fields: a short-duration and a long-duration trapezoidal impulse were employed to realize the states transition. The Langevin MD results show that, the transition times of about 50ps and 40ps correspond to the short-duration and long-duration writing operations, respectively. Also, we adopted the long-duration impulse excitation in order to produce a more reliable write operation. As with the first design, an isothermal MD simulation at 300K with the same 3 thermostat models applied in-turns was conducted to assess the performance of the memory cell. The MD study showed that the opened, center-split memory cell produces stable, reliable and nonvolatile room-temperature operations for all 3 thermostat models. It is able to operate at frequencies of up to 16.6 GHz, which is at least 100 times faster than the flash memory. Therefore, the second design is clearly superior to the first proposal. We recommend the opened, center-split double-wall CNT memory cell as the more promising candidate for the further development of a novel memory device that can act both as a permanent terabit solid-state storage and a nonvolatile RAM. Additionally, it is molecular in scale based on the bottoms-up technology and hence, should not be handicapped by miniaturization limits in the foreseeable future.