Control of Conductance in Micro and Nano MOSFETs via Molecular Modification.

The goal of this work is to investigate methods to enhance the semiconductor device technology. In phase I of this project, we thus initiated a technology-based approach to molecular electronics, identified methods of molecular attachment on silicon, one of which is referred to as "hydrosilylation", and fabricated silicon based molecular devices. The molecular attachment was characterized and confirmed by ellipsometry and XPS. Upon the fundamental molecular characterization, we evaluated the electrical behavior of molecules in silicon-based test structures using different metal and conductive polymer as top contacts. This investigation led us to conclude that it is very unlikely that molecular devices of this type could completely replace the silicon microelectronics in the foreseeable future.

In phase II of this project, we attempted to apply the best of molecular electronics to improve the existing weaknesses of silicon microelectronics, a process we dubbed "Surface State Engineering." Two structures were fabricated and tested: a micro SOI MOSFET and a nano SOI MOSFET. Unlike the fabrication of standard MOSFETs, the order of the steps for the fabrication of the device structures was very unconventional, which created unique challenges throughout the process. The micro SOI MOSFET structure was fabricated to validate the basic science of molecular passivation and molecular dipole. A series of molecules were deposited and electrical evaluation of the structure was performed to understand the molecular effect on the micro SOI MOSFET. Subsequently, a nano SOI MOSFET was fabricated using the Electron Beam Lithography (EBL) tool.

The electrical analysis of the SOI MOSFETs suggested that there were no significant changes due to the organic molecules. The electrical measurements and the energy band diagram analysis demonstrated that the molecules had a dipole within their length that was not strong enough to actually affect the underlying silicon layer. The Kelvin probe measurements also confirmed the electrical measurements done on the micro and nano SOI MOSFETs. In order to change the electrical behavior of the MOSFET using molecules, it is necessary to synthesize molecules with strong dipole or the ability to contribute charges to the silicon layer.