Germanium Nanostructures for Efficient Silicon-Compatible Optoelectronics

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This project is jointly funded by the Electronic and Photonic Materials Program (EPM) in the Division of Materials Research (DMR) and the Energy, Power, and Adaptive Systems Program (EPAS) in the Division of Electrical, Communications and Cyber Systems (ECCS).Technical Description: Efficient optoelectronic active materials, based on confined germanium quantum dots and germanium/silicon quantum wires, promise enhanced photoconversion mechanisms useful for chip-compatible photodetectors and solar cells with broadband absorption. This project focuses on two interrelated classes of optoelectronic materials and devices: (1) a close-packed array of Ge quantum dots for high-efficiency photodetectors and (2) Ge/Si heteronanowires for tandem solar cells. Materials properties of Ge quantum dots (such as concentration, crystallinity and surface passivation) are directly correlated to optoelectronic functionalities and optimized to improve the photodetection speed while retaining high responsivity. The response time and gain mechanisms are studied using time-resolved measurements as well as theoretical modeling of charging and inter-quantum-dot hopping. For the second class of materials and devices, the Ge/Si nanowires are grown using the vapor-liquid-solid technique to overcome lattice mismatch limitations. The transport and optical response of individual heteronanowires are correlated with nanowire diameter, length, doping and composition, and are used to predict the collective behavior of dense nanowire arrays with improved spectral coverage and reduced reflectivity. Simulations are performed to match the short-circuit currents in the Ge and Si sections in order to achieve enhanced photoconversion efficiency.Non-technical Description: This research project is on light-matter interactions in germanium-based nanostructures, including quantum dots and quantum wires. These nanostructures combine attractive physical properties enabled by more efficient light-matter interaction at the nanoscale. Specifically, the project seeks to use these nanomaterials for higher-efficiency photodetectors and broadband solar cells. The research has educational impact in and out of the classroom at Brown University. It includes long-term collaboration between faculty and graduate students at Brown University and scientists at the Los Alamos National Laboratory, on nanowire growth. Undergraduates also work on parts of the research. In addition, solar energy-based K-12 outreach activities are expanded and local high-school students participate in laboratory summer research.