****NON-TECHNICAL ABSTRACT**** Modern electronic devices work by controlling the location and motion of electrons through semiconductors. Past and current devices work with relatively large numbers of electrons at a time, but there is a continuing drive towards smaller and smaller devices, including those that control and manipulate one electron at a time. In such one-electron devices, the role of quantum mechanics is especially important and potentially useful. Materials properties are important in enabling such devices, and silicon has especially useful characteristics. Fabrication of silicon devices in which individual electrons can be controlled and manipulated has recently been achieved. This project will optimize, characterize, and further develop these materials and devices with the aim of enabling potentially transformative applications in which quantum mechanics is a critical factor. One such application is the quantum computer, which promises to be much more powerful than the classical computers used today. Graduate students participating in this project will obtain valuable interdisciplinary training. In addition, the project will involve the creation of a new workshop for high school teachers, to provide the necessary background for understanding the operation and functionality of modern nanodevices, including the role of quantum mechanics. This award receives support from the Divisions of Materials Research and Physics, as well as the Office of Multidisciplinary Activities.
Research has shown that gated quantum dots in semiconductors can be tuned to contain a controllable number of electrons, and that number can be monitored noninvasively by using integrated charge sensors. Quantum dots in silicon are of particular interest because the electron spin coherence times in silicon quantum dots containing only a few electrons are expected to be quite long ? a feature that may be useful for storing and manipulating quantum information. This project focuses on the fundamental role of materials properties on the coherence and control of spins in silicon/silicon-germanium quantum dots. In addition to research on the manipulation of spin and the measurement of spin coherence times, this project will focus on the design, simulation, fabrication, and measurement of silicon/silicon-germanium quantum devices to study the physics of the valley degree of freedom and its interaction with spin. Students participating in this project will obtain valuable interdisciplinary training. The project also involves the creation of a new workshop for high school teachers. The workshop is designed to provide the necessary context for understanding the basic operation and functionality of modern nanodevices, particularly the role of quantum mechanics. The workshops will be accomplished in conjunction with physics outreach specialists at UW-Madison. The project receives support from the Divisions of Materials Research and Physics, as well as the Office of Multidisciplinary Activities.
The primary scientific goal of this project was to measure and better understand the quantum energy states, including spin states, in very small electronic devices. The project participants fabricated these devices in layered semiconductors made from silicon and the alloy silicon-germanium. The project showed that such devices can be made to function with a single electron occupying the device, and it showed that the rate at which electrons tunnel between two such devices could be measured and controlled. A primary outcome of the project was the successful measurement of the quantum spin state, either up or down, of a single electron in such a device. The project further demonstrated that changes in electrical voltages applied to the device could vary the likelihood that the electron in the device was either up or down, and that this probability could be controlled in a predictable way. In addition to spin, the project also studied the special features of the quantum energy states arising because silicon is an indirect band-gap semiconductor. The project showed how theory could be used to make predictions about the quantum energy level spacings in three types of silicon devices: those formed with silicon and silicon-germanium, those formed with silicon and silicon dioxide, and those formed solely from silicon and the dopant phosphorous. Experimental measurements also were made on quantum energy levels in devices formed from silicon and silicon-germanium. The "broader impact" goal for this project included primarily the training of graduate students in both experimental and theoretical techniques. The project work in quantum devices attracted students who want to make an impact on the future of technology and who wish to understand the fundamental capabilities and limitations of quantum mechanics. Students contributing to this project obtained valuable interdisciplinary training and learned skills that will help make them productive members of the workforce. They were trained in various subsets of physics, materials science, clean-room fabrication, electronic measurement, and theoretical techniques. In addition to this primary goal project participants participated in a number of outreach events to both K-12 students and the general public.
