Publications

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Enhancement-mode Si/SiGe electron quantum dots have been pursued extensively by many groups for their potential in quantum computing. Most of the reported dot designs utilize multiple metal-gate layers and use Si/SiGe heterostructures with Ge concentration close to 30%. Here, we report the fabrication and low-temperature characterization of quantum dots in the Si/Si0.8Ge0.2 heterostructures using only one metal-gate layer. We find that the threshold voltage of a channel narrower than 1 μm increases as the width decreases. The higher threshold can be attributed to the combination of quantum confinement and disorder. We also find that the lower Ge ratio used here leads to a narrower operational gate bias range. The higher threshold combined with the limited gate bias range constrains the device design of lithographic quantum dots. We incorporate such considerations in our device design and demonstrate a quantum dot that can be tuned from a single dot to a double dot. The device uses only a single metal-gate layer, greatly simplifying device design and fabrication.

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Silicon chips hosting a single donor can be used to store and manipulate one bit of quantum information. However, a central challenge for realizing quantum logic operations is to couple donors to one another in a controllable way. To achieve this, several proposals rely on using nearby quantum dots (QDs) to mediate an interaction. In this work, we demonstrate the coherent coupling of electron spins between a single 31 P donor and an enriched 28 Si metal-oxide-semiconductor few-electron QD. We show that the electron-nuclear spin interaction on the donor can drive coherent rotations between singlet and triplet electron spin states of the QD-donor system. Moreover, we are able to tune electrically the exchange interaction between the QD and donor electrons. Furthermore, the combination of single-nucleus-driven rotations and voltage-tunable exchange provides every key element for future all-electrical control of spin qubits, while requiring only a single QD and no additional magnetic field gradients

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We fabricated a split-gate defined point contact in a double gate enhancement mode Si-MOS device, and implanted Sb donor atoms using a self-aligned process. E-beam lithography in combination with a timed implant gives us excellent control over the placement of dopant atoms, and acts as a stepping stone to focused ion beam implantation of single donors. Our approach allows us considerable latitude in experimental design in-situ. We have identified two resonance conditions in the point contact conductance as a function of split gate voltage. Using tunneling spectroscopy, we probed their electronic structure as a function of temperature and magnetic field. We also determine the capacitive coupling between the resonant feature and several gates. Comparison between experimental values and extensive quasi-classical simulations constrain the location and energy of the resonant level. We discuss our results and how they may apply to resonant tunneling through a single donor.

This SAND report is the final report on Sandia's Grand Challenge LDRD Project 27328, 'A Revolution in Lighting -- Building the Science and Technology Base for Ultra-Efficient Solid-state Lighting.' This project, which for brevity we refer to as the SSL GCLDRD, is considered one of Sandia's most successful GCLDRDs. As a result, this report reviews not only technical highlights, but also the genesis of the idea for Solid-state Lighting (SSL), the initiation of the SSL GCLDRD, and the goals, scope, success metrics, and evolution of the SSL GCLDRD over the course of its life. One way in which the SSL GCLDRD was different from other GCLDRDs was that it coincided with a larger effort by the SSL community - primarily industrial companies investing in SSL, but also universities, trade organizations, and other Department of Energy (DOE) national laboratories - to support a national initiative in SSL R&D. Sandia was a major player in publicizing the tremendous energy savings potential of SSL, and in helping to develop, unify and support community consensus for such an initiative. Hence, our activities in this area, discussed in Chapter 6, were substantial: white papers; SSL technology workshops and roadmaps; support for the Optoelectronics Industry Development Association (OIDA), DOE and Senator Bingaman's office; extensive public relations and media activities; and a worldwide SSL community website. Many science and technology advances and breakthroughs were also enabled under this GCLDRD, resulting in: 55 publications; 124 presentations; 10 book chapters and reports; 5 U.S. patent applications including 1 already issued; and 14 patent disclosures not yet applied for. Twenty-six invited talks were given, at prestigious venues such as the American Physical Society Meeting, the Materials Research Society Meeting, the AVS International Symposium, and the Electrochemical Society Meeting. This report contains a summary of these science and technology advances and breakthroughs, with Chapters 1-5 devoted to the five technical task areas: 1 Fundamental Materials Physics; 2 111-Nitride Growth Chemistry and Substrate Physics; 3 111-Nitride MOCVD Reactor Design and In-Situ Monitoring; 4 Advanced Light-Emitting Devices; and 5 Phosphors and Encapsulants. Chapter 7 (Appendix A) contains a listing of publications, presentations, and patents. Finally, the SSL GCLDRD resulted in numerous actual and pending follow-on programs for Sandia, including multiple grants from DOE and the Defense Advanced Research Projects Agency (DARPA), and Cooperative Research and Development Agreements (CRADAs) with SSL companies. Many of these follow-on programs arose out of contacts developed through our External Advisory Committee (EAC). In h s and other ways, the EAC played a very important role. Chapter 8 (Appendix B) contains the full (unedited) text of the EAC reviews that were held periodically during the course of the project.