Jung Hun Seo

12-GHz thin-film transistors on transferrable silicon nanomembranes for high-performance flexible electronics.

Multigigahertz flexible electronics are attractive and have broad applications. A gate-after-source/drain fabrication process using preselectively doped single-crystal silicon nanomembranes (SiNM) is an effective approach to realizing high device speed. However, further downscaling this approach has become difficult in lithography alignment. In this full paper, a local alignment scheme in combination with more accurate SiNM transfer measures for minimizing alignment errors is reported. By realizing 1 μm channel alignment for the SiNMs on a soft plastic substrate, thin-film transistors with a record speed of 12 GHz maximum oscillation frequency are demonstrated. These results indicate the great potential of properly processed SiNMs for high-performance flexible electronics.

Materials for bioresorbable radio frequency electronics.

Materials, device designs and manufacturing approaches are presented for classes of RF electronic components that are capable of complete dissolution in water or biofluids. All individual passive/active components as well as system-level examples such as wireless RF energy harvesting circuits exploit active materials that are biocompatible. The results provide diverse building blocks for physically transient forms of electronics, of particular potential value in bioresorbable medical implants with wireless power transmission and communication capabilities.

Fast flexible electronics with strained silicon nanomembranes

Fast flexible electronics operating at radio frequencies (>1 GHz) are more attractive than traditional flexible electronics because of their versatile capabilities, dramatic power savings when operating at reduced speed and broader spectrum of applications. Transferrable single-crystalline Si nanomembranes (SiNMs) are preferred to other materials for flexible electronics owing to their unique advantages. Further improvement of Si-based device speed implies significant technical and economic advantages. While the mobility of bulk Si can be enhanced using strain techniques, implementing these techniques into transferrable single-crystalline SiNMs has been challenging and not demonstrated. The past approach presents severe challenges to achieve effective doping and desired material topology. Here we demonstrate the combination of strained- NM-compatible doping techniques with self-sustained-strain sharing by applying a strain-sharing scheme between Si and SiGe multiple epitaxial layers, to create strained print-transferrable SiNMs. We demonstrate a new speed record of Si-based flexible electronics without using aggressively scaled critical device dimensions.

Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics

Field-effect transistors (FETs) that are stretchable up to 50% without appreciable degradation in performance are demonstrated. The FETs are based on buckled thin films of polyfluorene-wrapped semiconducting single-walled carbon nanotubes (CNTs) as the channel, a flexible ion gel as the dielectric, and buckled metal films as electrodes. The buckling of the CNT film enables the high degree of stretchability while the flexible nature of the ion gel allows it to maintain a high quality interface with the CNTs during stretching. An excellent on/off ratio of >10(4), a field-effect mobility of 10 cm(2) · V(-1) · s(-1), and a low operating voltage of <2 V are achieved over repeated mechanical cycling, with further strain accommodation possible. Deformable FETs are expected to facilitate new technologies like stretchable displays, conformal devices, and electronic skins.

Interface Engineering by Piezoelectric Potential in ZnO-Based Photoelectrochemical Anode

Through a process of photoelectrochemical (PEC) water splitting, we demonstrated an effective strategy for engineering the barrier height of a heterogeneous semiconductor interface by piezoelectric polarization, known as the piezotronic effect. A consistent enhancement or reduction of photocurrent was observed when tensile or compressive strains were applied to the ZnO anode, respectively. The photocurrent variation is attributed to a changed barrier height at the ZnO/ITO interface, which is a result of the remnant piezoelectric potential across the interface due to a nonideal free charge distribution in the ITO electrode. In our system, ∼1.5 mV barrier height change per 0.1% applied strain was identified, and 0.21% tensile strain yielded a ∼10% improvement of the maximum PEC efficiency. The remnant piezopotential is dictated by the screening length of the materials in contact with piezoelectric component. The difference between this time-independent remnant piezopotential effect and time-dependent piezoelectric effect is also studied in details.

Fast flexible electronics using transferrable silicon nanomembranes

A systematic review, covering the aspects of material preparation, device fabrication and process integration, is provided for flexible electronics operating in high-frequency domain based on transferrable monocrystalline silicon (Si) nanomembranes (NM). Previously demonstrated methods of releasing Si NM from silicon-on-insulator source substrates and transferring it to flexible substrates are briefly described. Due to the processing temperature limitation of most flexible substrates, a pre-release NM selective doping scheme is used for Si NMs. With proper selections of ion implantation energy and dose, fully doped Si NMs across their entire thickness with very low sheet resistivity can be obtained, allowing flip transfer of the NMs for backside and even double side processing. A general conclusion of preferred low implantation energy for shallower depth ion implantation is identified. The evolvement of radio frequency (RF) flexible Si thin-film transistor (TFT) structures is described in detail. The continuous performance enhancement of TFTs owing to process and TFT structure innovations is analysed. Demonstrations of flexible Si RF switches and RF inductors and capacitors are also briefly reviewed as valuable components of the general flexible device family, some of which also benefit from the pre-release NM doping technique. With the proved feasibility of these basic RF elements and related processing techniques, more complicated flexible RF circuits can be expected. Future research directions are also discussed, including further enhancement of device performance, building more types of semiconductor devices on flexible substrates, and process integration for flexible circuits and systems.

An aqueous solution-based doping strategy for large-scale synthesis of Sb-doped ZnO nanowires

An aqueous solution-based doping strategy was developed for controlled doping impurity atoms into a ZnO nanowire (NW) lattice. Through this approach, antimony-doped ZnO NWs were successfully synthesized in an aqueous solution containing zinc nitrate and hexamethylenetetramine with antimony acetate as the dopant source. By introducing glycolate ions into the solution, a soluble antimony precursor (antimony glycolate) was formed and a good NW morphology with a controlled antimony doping concentration was successfully achieved. A doping concentration study suggested an antimony glycolate absorption doping mechanism. By fabricating and characterizing NW-based field effect transistors (FETs), stable p-type conductivity was observed. A field effect mobility of 1.2 cm(2) V(-1) s(-1) and a carrier concentration of 6 × 10(17) cm(-3) were achieved. Electrostatic force microscopy (EFM) characterization on doped and undoped ZnO NWs further illustrated the shift of the metal-semiconductor barrier due to Sb doping. This work provided an effective large-scale synthesis strategy for doping ZnO NWs in aqueous solution.

Stable p-type conduction from Sb-decorated head-to-head basal plane inversion domain boundaries in ZnO nanowires.

We report that Sb-decorated head-to-head (H-H) basal plane inversion domain boundaries (b-IDBs) lead to stable p-type conduction in Sb-doped ZnO nanowires (NWs) due to Sb and O codoping. Aberration-corrected Z-contrast scanning transmission electron microscopy shows that all of the Sb in the NWs is incorporated into H-H b-IDBs just under the (0001) NW growth surfaces and the (0001) bottom facets of interior voids. Density functional theory calculations show that the extra basal plane of O per H-H b-IDB makes them electron acceptors. NWs containing these defects exhibited stable p-type behavior in a single NW FET over 18 months. This new mechanism for p-type conduction in ZnO offers the potential of ZnO NW based p-n homojunction devices.

Cl-Doped ZnO Nanowires with Metallic Conductivity and Their Application for High-Performance Photoelectrochemical Electrodes

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. ZnO NWs grown by hydrothermal method are pervasively used in optoelectronic, photovoltaic, and piezoelectric energy-harvesting devices. We synthesized in situ Cl-doped ZnO NWs with metallic conductivity that would fit seamlessly with these devices and improve their performance. Possible Cl doping mechanisms were discussed. UV-visible absorption spectroscopy confirmed the visible light transparency of Cl-doped ZnO NWs. Cl-doped ZnO NW/TiO2 core/shell-structured photoelectrochemical (PEC) anode was fabricated to demonstrate the application potential of highly conductive ZnO NWs. Higher photocurrent density and overall PEC efficiency compared with the undoped ZnO NW-based device were achieved. The successful doping and low resistivity of ZnO could unlock the potential of ZnO NWs for applications in low-cost flexible transparent electrodes.

Nanopatterning by Laser Interference Lithography: Applications to Optical Devices

A systematic review, covering fabrication of nanoscale patterns by laser interference lithography (LIL) and their applications for optical devices is provided. LIL is a patterning method. It is a simple, quick process over a large area without using a mask. LIL is a powerful technique for the definition of large-area, nanometer-scale, periodically patterned structures. Patterns are recorded in a light-sensitive medium that responds nonlinearly to the intensity distribution associated with the interference of two or more coherent beams of light. The photoresist patterns produced with LIL are the platform for further fabrication of nanostructures and growth of functional materials used as the building blocks for devices. Demonstration of optical and photonic devices by LIL is reviewed such as directed nanophotonics and surface plasmon resonance (SPR) or large area membrane reflectors and anti-reflectors. Perspective on future directions for LIL and emerging applications in other fields are presented.