The research objective of this Faculty Early Career (CAREER) proposes to address the long-standing issues of interfacing and interconnecting one-dimensional semiconductor nanowires with devices and circuits and proposes to develop novel mass-manufacturable solutions. The broad goals of this program are to advance our device physics understanding of nanowire-bulk interfaces and to facilitate new practical technologies for design, integration and mass production of nanowire based devices and systems with innovative quantum effects. Despite significant progress in nanowire synthesis and many promising single device demonstrations, applications of nanowires have been stalled by our inability to controllably incorporate them within integrated circuits. Unlike the research-based approach of sequentially connecting electrodes to individual nanowires for device physics studies, this research will employ a novel technique of epitaxial bridging of nanowires between pre-fabricated electrodes for reproducible fabrication of ultra-dense and low-cost device and circuit arrays. Growth conditions, doping techniques and wafer processing methods will be explored to understand and optimize nanowire-bulk connections. The educational objectives of the CAREER program are focused on significantly impacting the research experience of both graduate and undergraduate students at the University of California Davis and the large numbers of minority students in Oakland public school system. The PI proposes to develop a new nanotechnology course titled Nanostructured Devices: Physics and Technology that will serve to introduce and excite undergraduate students about nano-manufacturing and the career opportunities that will be available to them. The PI will develop a program for inseminating knowledge of the advancements in nanotechnology into a high school in a predominantly underprivileged and socio-economically impacted neighborhood of downtown Oakland, California.
The proposed research plan will greatly impact the fields of Nanomanufacturing by transitioning nanowires into a reliable technology through the development of cost-effective mass-manufacturing methods with revolutionary new capabilities and performances for wide variety of applications. The outcome will lead to unprecedented device density, ultimately making the nanowire based devices a commercial reality with a major improvement in the cost/performance ratio. This will facilitate mass-manufacturing of devices for electronic, photonic, energy storage and conversion, advanced light sources, sensing, and biological systems. The research community will also benefit in the near term through the development of a universal test-bed for combining conventional microfabrication (bottom-up) and synthetic nanofabrication (top-down) where self-assembled nanostructures mate to pre-fabricated microstructures to test new concepts in quantum devices.
Ultra-Low Contact Resistance of Epitaxially Bridged Si Nanowires: A significant roadblock to wide-scale integration of functional nanowire-based devices is the difficulty in forming contacts to the nanowires. A scheme aimed at integrating nanowires in devices and circuits should be universal, compatible with current IC processing methods, and cost-effective. In addition, precise control on the nanowire length, reliable and low contact resistance, and good mechanical robustness will be highly desirable. Many reported techniques for fabricating nanowires in controlled and reproducible fashion did not meet most of the above-mentioned requirements and a dramatically new approach is needed for integrating nanowires with conventional circuitry. We demonstrated a novel bridging techniques to create epitaxial connections to a large number of highly directional semiconductor nanowires between pre-fabricated electrodes. Two electrically isolated and opposing vertical semiconductor surfaces are fabricated and lateral nanowires are then grown from one surface and be epitaxially connected to the other using a metal-catalyzed chemical vapor deposition (CVD) technique, forming electrically continuous and robust "nano-bridges" with ultra-low contact resistance as shown in Figure 1. Nano-bridges are attractive choice for low noise low power applications like biological and chemical sensors and as conduction channel for nano-transistors in electronic circuits. Transfer Printing of Si Nano/micro Structure arrays: We demonstrated a novel method to fabricating single crystal photovoltaic devices in the shape of highly oriented 1D semiconductor micro/nano-pillars and then transferring them to coat a target surface of any topology using an innovative harvest/lift-off process. This method enables highly crystalline micro- and nanopillars of different materials with diverse bandgaps and physical properties to be fabricated on appropriate mother substrates and transfer to form multilayered 3D stacks for multifunctional devices (Figure 2). This approach not only ensures the incorporation of any kind of material (with the best device characteristics) on a single substrate facilitating substrate-free device fabrications on any topology, but also allows the repeated use of a mother substrate for continual production of new devices. This capability of fabricating substrate-less devices will offer a universal platform for material integration and allow solar active devices to be coated on various surface topologies that would also be suitable for solar hydrogen generation. Nanowire Integrated Electrolyte-Insulator-Semiconductor Sensors: We demonstrated an electrolyte-insulator-semiconductor (EIS) sensor with a large capacitance, near-Nernst-limit pH sensitivity, and good reliability by integrating an ensemble of Si nanowires (NWs) for the first time. The NWs were fabricated by using the electroless wet etching technique (Figure 3). An Al2O3/SiO2 bilayer coating was employed as a sensing membrane. The EIS sensors with 3.8 um long NWs exhibited about 8 times larger capacitance than that of a planar type EIS sensors that were fabricated using the same fabrication scheme without integrating NWs. The measured pH sensitivity at room temperature was 60.2 mV/pH, which is higher than the theoretical Nernstian of 59 mV/pH. Our results and analysis clearly indicate that ultra-sensitive pH sensing can be realized with optimized NW integrated EIS sensors. Fabrication of Silicon Nanostructure Based Field Ionization Gas Sensors: We demonstrated the successful fabrication of gas sensing devices with silicon nanowires (NWs) grown on silicon substrates. By the vapor-liquid-solid method, silicon NWs were synthesized with and without thin branches. Free standing NWs well aligned to (111) direction were used to sense gases by the field ionization (FI) mechanism. Even in the atmosphere pressure (760 Torr), the FI device showed very low FI threshold voltages of around 50 V, which shows promise of developing low voltage miniaturized gas ionizing devices. Branched silicon NWs (Figure 4) were employed as chemoresistance sensors to monitor gases in a real time.
