Sang June Cho

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper

Todays consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. Here we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.

Sharpened VO2 Phase Transition via Controlled Release of Epitaxial Strain

Phase transitions in correlated materials can be manipulated at the nanoscale to yield emergent functional properties, promising new paradigms for nanoelectronics and nanophotonics. Vanadium dioxide (VO2), an archetypal correlated material, exhibits a metal-insulator transition (MIT) above room temperature. At the thicknesses required for heterostructure applications, such as an optical modulator discussed here, the strain state of VO2 largely determines the MIT dynamics critical to the device performance. We develop an approach to control the MIT dynamics in epitaxial VO2 films by employing an intermediate template layer with large lattice mismatch to relieve the interfacial lattice constraints, contrary to conventional thin film epitaxy that favors lattice match between the substrate and the growing film. A combination of phase-field simulation, in situ real-time nanoscale imaging, and electrical measurements reveals robust undisturbed MIT dynamics even at preexisting structural domain boundaries and significantly sharpened MIT in the templated VO2 films. Utilizing the sharp MIT, we demonstrate a fast, electrically switchable optical waveguide. This study offers unconventional design principles for heteroepitaxial correlated materials, as well as novel insight into their nanoscale phase transitions.

Transferred Flexible Three-Color Silicon Membrane Photodetector Arrays

We report the design and fabrication of transfer-printed flexible three-color multijunction 8 × 8 crystalline silicon membrane photodetector arrays. Based on the penetration-depth-dependent absorption of different wavelengths, filter-free color detection can be obtained via three junction photocurrent measurement. The optical measurements show good agreements with the optical behavior predicted by the design and simulation. No noticeable changes were observed in the device performance when it was operated in a bending state.

X-Band Compatible Flexible Microwave Inductors and Capacitors on Plastic Substrate

Microwave inductors and capacitors compatible with low temperature processes form a route to high-frequency electronics on flexible substrates in conjunction with high speed thin film transistors. We report here the process by which one can fabricate passive components with X-band compatible performance on polyethylene terephthalate substrate by changing dielectric layer material and optimized design, where the method can be applied with most of the active device technologies developed up to date on flexible substrates. High resonance frequencies were obtained, comparing with former results, confirming the effectiveness of the approach with flexible dielectric materials. Studies on bending effects for the spiral inductors and metal on insulator capacitors show miniscule difference in performance related to bending radius. Performance enhancements compared to previously reported passive elements enable higher radio-frequency electronics on plastic substrates.

Heterogeneously Integrated InGaAs and Si Membrane Four-Color Photodetector Arrays

We report the design and fabrication of heterogeneously integrated silicon and InGaAs membrane photodetector arrays. Visible and near-infrared (NIR) detection can be achieved by transfer printing a silicon membrane on InGaAs substrate. Based on the penetration-depth-dependent absorption of different wavelengths, filter-free visible color detection can be obtained via three-junction photocurrent measurement for silicon, and NIR can be detected by InGaAs. The measurements show good agreement with the optical behavior predicted by the design and simulation.

Flexible and Stretchable Microwave Microelectronic Devices and Circuits

Electronic systems built on flexible plastic films and stretchable rubber sheets have attracted new applications in many emerging fields. Integration of high-speed electronics such as microwave power amplifiers and switches can extend the applications even further with wireless capabilities. As such, flexible and stretchable microwave electronics represent opportunities for future electronics where remote capabilities are desired. Here, we review advances in numerous types of microelectronic devices used for fast, flexible, and stretchable electronic devices, as well as flexible and stretchable passive elements and circuitries. We first introduce the challenges associated with design and fabrication, and the characteristics required for high-frequency operation of the devices on foreign substrates. Second, we review the recent efforts that were made utilizing different types of high-performance semiconductors, which are ideal for high-speed flexible and stretchable electronics, such as silicon, compound semiconductors, and 1-D and 2-D materials. Third, passive electronic components fabricated on such substrates, including inductors, capacitors, and transmission lines, are reviewed. Finally, we discuss the flexible and stretchable microwave electronics at the circuit level and review the recent advances in making numerous types of flexible and stretchable microwave circuits for diverse applications.

Epitaxial VO2 thin film-based radio-frequency switches with thermal activation

In this paper, we report on the demonstration of thermally triggered “normally ON” radio-frequency (RF) switches based on epitaxial vanadium dioxide (VO2) thin films with a SnO2 template on (001) TiO2 substrates. Fast insulator-to-metal phase transition of the epitaxial VO2 at a relatively low temperature allowed RF switches made of the VO2 to exhibit sharp changes in the RF insertion loss during cooling and heating at 60 °C and 66 °C, respectively. The change of RF insertion loss due to phase transition is greater than 15 dB. The VO2 RF switches also completed the transition of S21 within less than 3 °C and showed a low-loss operation frequency of up to 24.2 GHz with a low insertion loss of −1.36 dB and isolation of 17.56 dB at 12.03 GHz, respectively. The demonstration suggests that epitaxial VO2-based RF switches can be used in switching elements up to Ku-band RF circuits.

229 nm UV LEDs on aluminum nitride single crystal substrates using p-type silicon for increased hole injection

Ultraviolet (UV) light emission at 229 nm wavelength from diode structures based on AlN/Al0.77Ga0.23N quantum wells and using p-type Si to significantly increase hole injection was reported. Both electrical and optical characteristics were measured. Owing to the large concentration of holes from p-Si and efficient hole injection, no efficiency droop was observed up to a current density of 76 A/cm2 under continuous wave operation and without external thermal management. An optical output power of 160 uW was obtained with corresponding external quantum efficiency of 0.027%. This study demonstrates that by adopting p-type Si nanomembrane contacts as hole injector, practical levels of hole injection can be realized in UV light-emitting diodes with very high Al composition AlGaN quantum wells, enabling emission wavelengths and power levels that were previously inaccessible using traditional p-i-n structures with poor hole injection efficiency.AlGaN based 229 nm light emitting diodes (LEDs), employing p-type Si to significantly increase hole injection, were fabricated on single crystal bulk aluminum nitride (AlN) substrates. Nitride heterostructures were epitaxially deposited by organometallic vapor phase epitaxy and inherit the low dislocation density of the native substrate. Following epitaxy, a p-Si layer is bonded to the heterostructure. LEDs were characterized both electrically and optically. Owing to the low defect density films, large concentration of holes from p-Si, and efficient hole injection, no efficiency droop was observed up to a current density of 76 A/cm2 under continuous wave operation and without external thermal management. An optical output power of 160 μW was obtained with the corresponding external quantum efficiency of 0.03%. This study demonstrates that by adopting p-type Si nanomembrane contacts as a hole injector, practical levels of hole injection can be realized in UV light-emitting diodes with very high Al composit...

Radio-frequency flexible and stretchable electronics: the need, challenges and opportunities

Successful integration of ultrathin flexible or stretchable systems with new applications, such as medical devices and biodegradable electronics, have intrigued many researchers and industries around the globe to seek materials and processes to create high-performance, non-invasive and cost-effective electronics to match those of state-of-the-art devices. Nevertheless, the crucial concept of transmitting data or power wirelessly for such unconventional devices has been difficult to realize due to limitations of radio-frequency (RF) electronics in individual components that form a wireless circuitry, such as antenna, transmission line, active devices, passive devices etc. To overcome such challenges, these components must be developed in a step-by-step manner, as each component faces a number of different challenges in ultrathin formats. Here, we report on materials and design considerations for fabricating flexible and stretchable electronics systems that operate in the microwave level. High-speed flexible active devices, including cost effective Si-based strained MOSFETs, GaAs-based HBTs and GaN-based HEMTs, performing at multi-gigahertz frequencies are presented. Furthermore, flexible or stretchable passive devices, including capacitors, inductors and transmission lines that are vital parts of a microwave circuitry are also demonstrated. We also present unique applications using the presented flexible or stretchable RF components, including wearable RF electronics and biodegradable RF electronics, which were impossible to achieve using conventional rigid, wafer-based technology. Further opportunities like implantable systems exist utilizing such ultrathin RF components, which are discussed in this report as well.

Materials and design considerations for fast flexible and stretchable electronics

Flexible and stretchable high-speed devices that operate at microwave level may enable wireless functionalities for many applications. The rise of flexible or stretchable electronic systems using inorganic semiconducting materials allow integration of high-performance electronic devices on foreign substrates, such as plastic or rubber. Here, we report on materials and design considerations for fabricating flexible and stretchable electronics systems that operate in the microwave level. High-speed active devices, including cost effective Si-based transistors and high-power GaN-based transistors, performing at multi-gigahertz frequencies are fabricated on either plastic or rubber substrates. Furthermore, important passive components, such as the capacitors, inductors and transmission lines that can together complete a microwave integrated circuit, are also demonstrated. All of the components presented here have comparable high-performances to their rigid counterparts, which can be combined to form microwave integrated circuits that can be used for many applications, where wireless functionalities are desired.