Probing Individual and Interacting Dopants in Semiconductors and Superconductors on the Nanometer Scale

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Non-Technical Abstract:Much of the technological advances in the past 50 years have been made possible by controlling the properties of materials through doping them with impurities. Modern information processing would have not been possible without the ability to tailor the electronic properties of semiconductors through doping. This program provides a new window in understanding doping in materials through nanoscale measurement and manipulation using state-of-the-art experimental technique. The uniqueness of this program comes from the demonstrated experimental ability to control the position of individual dopants with atomic precision. The program will focus on the question of how doping transforms a semiconductor into a magnet. Magnetic semiconductors have the potential to create a new class of electronic devices that utilize the spin degree of freedom for computation. They may make it possible to combine information processing and storage on a single chip. It is conceivable that the basic science explored in this project would lead to magnetic semiconductors that can operate at room temperature -- overcoming one of the major obstacles in applications of these materials. This project will also contribute significantly to training of the next generation of physicists, who are experts in materials physics as well as advanced scanning probe microscopy techniques. Undergraduate students are actively involved in the proposed program and will benefit from the development of a new Freshman Seminar on the construction of an STM. The proposed program also reaches wider audience of high-school students and educators through regular tours and demonstrations at the Princeton Nanoscale Microscopy Laboratory.Technical Abstract:The research undertaken in this project comprises two related efforts that are connected by the main objective-- to understand single dopants and dopant-dopant interactions that make insulating materials into conducting systems with collective electronic phenomena. In particular, this project focuses on atomic scale measurements of the influence of dopants that transforms a semiconductor into a ferromagnet and a charge-density-wave system into a superconductor. Nanoscale manipulation of dopants with the scanning tunneling microscope (STM) and high-resolution spectroscopic measurements will be combined to probe single dopants and their interactions. Using a novel atom-by-atom substitution technique with the STM, individual magnetic transition metal dopants will be implanted in GaAs to determine how these dopants alter the local electronic structure of this semiconductor. Interaction between these magnetic dopants is believed to result in magnetism, such as ferromagnetism in Mn-doped GaAs. The atomic scale characteristics of the interaction between transition metal dopants will be probed using various STM spectroscopic techniques. Direct mapping of the orbital state of dopants as function of external magnetic field will be used to probe spin-orbit coupling at a single-dopant level. Overall, the program provides a microscopic view of electronic structure of single dopants and uses nano-manipulation to perform controlled measurements of the interaction between isolated dopants.