John C. Bean (ECE), Avik Ghosh (ECE), Lloyd R. Harriott (ECE), Lin Pu (Chemistry), Keith Williams (Physics), University of Virginia
NIRT: "Surface State Engineering" - Charge Storage and Conduction in Organo-Silicon Heterostructures as a Basis for Nanoscale Devices
Intellectual Merit: This proposal was received in response to the Active Nanostructures and Nanosystems solicitation, NSF 06-595, category NIRT. The project combines work in molecular electronics and microelectronics to lay the scientific and technological foundations of "Surface State Engineering." Specifically, organic molecules will be attached to silicon surfaces so strongly and intimately that electrons in both materials will be able to overlap quantum mechanically. This offers entirely new mechanisms for precisely introducing charge into future nanoscale metal oxide semiconductor field effect transistors. It also opens the door to new conduction phenomenon based on quantum mechanical interference between organo-molecular and semiconductor electron wave functions. The proposal bridges disciplinary boundaries and the boundary between fundamental science and technology through its development of three tools: (1) Sophisticated modeling techniques addressing the very different physics of quantum dots and silicon layers, combining these to produce a hybrid model of organo-silicon structures; (2) New vapor phase techniques for attaching high purity self-limiting single molecular layers on silicon in a manner compatible with modern microelectronics processing; (3) A characterization and device validation platform based on technologically relevant silicon-on-insulator back-gated nanoscale field effect transistors.
Broader Impacts: In addition to opening doors between scientific disciplines, this proposal suggests ways to pass through the "Brick Wall" looming ahead of the U.S. semiconductor electronics industry. It does this in a manner that does not call for miraculous development of new stand-alone nanotechnologies, but instead uses lessons learned to propose a hybrid technology combining the strengths of molecular electronics with the strengths of modern microelectronics. Finally, the proposal builds upon prior ground-breaking work in nanoscience education on both the web and in the classroom to develop a new partnership with the Science Museum of Virginia that will place nanoscience into the hands of K-12 students and teachers across the Commonwealth of Virginia.
John C. Bean (ECE), Avik Ghosh (ECE), Lloyd R. Harriott (ECE), Lin Pu (Chemistry), Keith Williams (Physics), University of Virginia This project explored the possibility of extending semiconductor-based transistor electronics through the addition of organic (i.e., carbon-based) molecules. Semiconductor electronics spans many devices and many materials, but the technology is still completely dominated by a single device possible in only a single material: the silicon "MOS" transistor. This device acts like a switch that opens or closes to allow electrical conduction. In its center is a gap that, left alone, contains essentially no freely-moving electrons. This means that the switch is intrinsically "open" (i.e. non-conducting). But this changes when a voltage is applied to a nearby metallic plate. The electric field created by this voltage draws mobile electrons into the switch's gap (its "channel"), completing the electrical circuit and closing the switch. But to achieve the initially non-conducting gap, essentially all of the electrons in that gap must be immobilized on individual atoms or in their bonds. And after fifty years, this has been achieved in only silicon capped by a layer of its own oxide, SiO2. The first goal of this project was to breach the above constraint by developing equally effective capping layers composed of organic molecules. Layers that would bond so strongly and completely to silicon surfaces that no electrons would be left to wander. Then, if this were achieved, it might become possible to delicately alter the composition of the capping organic layers in ways that would selectively liberate or remove mobile electrons from the underlying silicon, or to develop structures in which mobile electrons spanned both layers. In pursuit of these goals, detailed computer models were first developed to study the likely electronic arrangements and attachments at such surfaces. These models were then validated using experimental structures including carbon nanotube transistors incorporating controlled concentrations of single atom bonding defects. In parallel, new chemical procedures were explored with the goal of tightly bonding carbon-based molecules to essentially every atom of a silicon surface. This was achieved via a process called "hydrosilylation." The resulting carbon molecule capping layers were, indeed, nearly as effective as the industry standard SiO2 layers in bonding all electrons and creating the switch's initial non-conducting gap. This was verified in both basic measurements and by fabrication of full MOS transistor switches. The composition of these "organic" layers was then altered in an attempt to deliberately liberate or remove mobile electrons. Strong effects were not achieved with the smaller organic molecules employed, but it is believed that more complex molecules might still achieve that goal. Project funds were also used to expand UVA educational outreach efforts in the area of nanoscience. Those efforts had been nucleated by earlier NSF CCLI and NUE grants which led to the creation of the "UVA Virtual Lab" public science education website and to the creation of a prototype "Hands on introduction to nanoscience class" with fully web-posted lecture and lab materials. By the end of this project, this online content had expanded to the point that it had drawn almost eight million hits, including visitors from over two-thousand universities and one-thousand K-12 schools (see: www.virlab.virginia.edu). Further, over two summers, for-university-credit versions of the nanoscience class were developed and taught for Virginia public school STEM teachers.
