NSF Award
1809284
Non-technical:<br/>Tunnel junctions are an enabling technology for future electronics. They are formed by inserting a thin insulating layer, known as a tunnel barrier, between metal or semiconductor electrodes. Devices based on tunnel junctions have the advantages of enhanced quantum coherent transport, fast speed, small size and energy efficiency. Tunnel junctions are used, for example, in sensors, flash memory and in quantum or neural computers. The performance of a tunnel junction depends critically on the quality and thickness of the tunnel barrier. Charge transport through a barrier, known as quantum tunneling, increases exponentially with decreasing layer thickness and defect concentration. Despite decades of effort, current tunnel junction technology is limited to barriers that are at least one nanometer thick and have a high defect density. Creating atomically thin (one tenth of a nanometer), defect-free tunnel barriers will enable the next-generation of tunnel junction devices. This project will advance tunnel junction technology by creating atomically thin, high-quality tunnel barriers using atomic layer deposition. Two important types of tunnel junctions will be studied. Superconducting Josephson junctions serve as quantum bits (qubits) for quantum computers. Magnetic tunnel junctions are at the heart of nonvolatile, fast magnetic random access memory and neuromorphic computers. The scientific knowledge developed through this project will broadly impact the development of future microelectronics. Atomic scale control of materials and interfaces applies to sensing, catalysis, and energy production. The project emphasizes forefront education and the cutting-edge research. This will attract students, especially those from underrepresented groups, to pursue careers in STEM.<br/><br/>Technical:<br/>This project focuses on the development of atomically thin tunnel barriers by atomic layer deposition (ALD) for use in tunnel junction devices. The goal is to understand and control the physical and chemical properties of materials used in tunnel junctions at atomic scales for high performance devices in order to achieve defect-free, atomically thin tunnel barriers. Despite exciting preliminary results on Josephson junctions and magnetic tunnel junctions with 0.1-1.0 nm thick Al2O3 tunnel barriers, many fundamental questions remain in synthesis and physical properties of these tunnel junctions. Two topics are proposed to answer these questions. Topic 1 will focus on growth of Josephson junctions and magnetic tunnel junctions using a custom-designed system for in situ ultra-high-vacuum atomic layer deposition-physical vapor deposition (ALD-PVD) for tunnel junction deposition. This system is integrated with a characterization system with scanning probe microscopy and tip-enhanced Raman spectroscopy (SPM-TERS) capabilities. The SPM-TERS system will be used to understand the growth mechanism of the ALD tunnel barriers and the metal-insulator (M/I) interface and the role of growth parameters, such as ALD growth temperature, source pulse durations and sequence will be investigated. Experimental work will be guided by simulation. Topic 2 will carry out in situ study of the tunnel barriers and the electrode/tunnel barrier interface using UHV SPM-TERS and multi-scale characterization at device and circuit levels to understand the microscopic properties of insulating barrier and electrode/tunnel barrier interface, especially the effect of defects, on the performance of superconducting qubits, Josephson junctions, and magnetic tunnel junctions. The goal is to demonstrate an innovative technological approach towards next-generation high-performance electronics based on tunnel junctions with ALD tunnel barriers.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
