Light extraction efficiency (LEE) in AlGaN deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated using finite-difference time-domain simulations. For flip-chip and vertical LED structures, LEE is obtained to be <10% due to strong DUV light absorption in the p-GaN layer. In flipchip LEDs, LEE of transverse-magnetic (TM) modes is found to be more than ten times smaller than that of transverse-electric (TE) modes, which explains the decreasing behavior of external quantum efficiency of DUV LEDs with decreasing wavelength. It is also found that vertical LED structures can have advantages over flip-chip structures for increasing LEE in the TM mode. #

Ionic tactile sensors (ITS) represent a new class of deformable sensory platforms that mimic not only the tactile functions and topological structures but also the mechanotransduction mechanism across the biological ion channels in human skin, which can demonstrate a more advanced biological interface for targeting emerging human-interactive technologies compared to conventional e-skin devices. Recently, flexible and even stretchable ITS have been developed using novel structural designs and strategies in materials and devices. These skin-like tactile sensors can effectively sense pressure, strain, shear, torsion, and other external stimuli with high sensitivity, high reliability, and rapid response beyond those of human perception. In this review, the recent developments of the ITS based on the novel concepts, structural designs, and strategies in materials innovation are entirely highlighted. In particular, biomimetic approaches have led to the development of the ITS that extend beyond the tactile sensory capabilities of human skin such as sensitivity, pressure detection range, and multimodality. Furthermore, the recent progress in self-powered and self-healable ITS, which should be strongly required to allow human-interactive artificial sensory platforms is reviewed. The applications of ITS in human-interactive technologies including artificial skin, wearable medical devices, and user-interactive interfaces are highlighted. Last, perspectives on the current challenges and the future directions of this field are presented.

Biological cellular structures have inspired many scientific disciplines to design synthetic structures that can mimic their functions. Here, we closely emulate biological cellular structures in a rationally designed synthetic multicellular hybrid ion pump, composed of hydrogen-bonded [EMIM + ][TFSI − ] ion pairs on the surface of silica microstructures (artificial mechanoreceptor cells) embedded into thermoplastic polyurethane elastomeric matrix (artificial extracellular matrix), to fabricate ionic mechanoreceptor skins. Ionic mechanoreceptors engage in hydrogen bond-triggered reversible pumping of ions under external stimulus. Our ionic mechanoreceptor skin is ultrasensitive (48.1–5.77 kPa −1 ) over a wide spectrum of pressures (0–135 kPa) at an ultra-low voltage (1 mV) and demonstrates the ability to surpass pressure-sensing capabilities of various natural skin mechanoreceptors (i.e., Merkel cells, Meissner’s corpuscles, Pacinian corpuscles). We demonstrate a wearable drone microcontroller by integrating our ionic skin sensor array and flexible printed circuit board, which can control directions and speed simultaneously and selectively in aerial drone flight.

The electrical and optical improvements of AlGaInP micro-light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) sidewall passivation were demonstrated. Due to the high surface recombination velocity and minority carrier diffusion length of the AlGaInP material system, devices without sidewall passivation suffered from high leakage and severe drop in external quantum efficiency (EQE). By employing ALD sidewall treatments, the 20×20 µm2 µLEDs resulted in greater light output power, size-independent leakage current density, and lower ideality factor. The forward current-voltage characteristic was enhanced by using surface pretreatment. Furthermore, ALD sidewall treatments recovered the EQE of the 20×20 µm2 devices more than 150%. This indicated that AlGaInP µLEDs with ALD sidewall treatments can be used as the red emitter for full-color µLED display applications.

Artificial smart designs inspired by structural and functional features of biological organisms have opened new avenues to develop high-performance flexible tactile sensors and advanced artificial sensory systems.

We developed a method to control threshold voltage and on/off ratio of ZnO thin-film transistor (TFT) grown by metalorganic chemical vapor deposition (MOCVD). ZnO usually shows oxygen deficiency, which shows up as n-type defects of zinc interstitial or oxygen vacancy. In order to reduce these defects, we allowed sufficient oxidation time during growth. Instead of one long oxidation step, we repeated thin-layer growth and oxidation, until desired thickness is achieved. By using this method, we could obtain high quality ZnO TFT by MOCVD. Our ZnO TFT grown at 450 °C showed 15 cm2/(V s) mobility and 107 on/off ratio, with +5 V threshold voltage, which enables enhancement mode TFT operation.

Representative tin sulfide compounds, tin monosulfide (SnS) and tin disulfide (SnS) are strong candidates for future nanoelectronic devices, based on non-toxicity, low cost, unique structures and optoelectronic properties. However, it is insufficient for synthesizing of tin sulfide thin films using vapor phase deposition method which is capable of fabricating reproducible device and securing high quality films, and their device characteristics. In this study, we obtained highly crystalline SnS thin films by atomic layer deposition and obtained highly crystalline SnS thin films by phase transition of the SnS thin films. The SnS thin film was transformed into SnS thin film by annealing at 450 °C for 1 h in HS atmosphere. This phase transition was confirmed by x-ray diffractometer and x-ray photoelectron spectroscopy, and we studied the cause of the phase transition. We then compared the film characteristics of these two tin sulfide thin films and their switching device characteristics. SnS and SnS thin films had optical bandgaps of 1.35 and 2.70 eV, and absorption coefficients of about 10 and 10 cm in the visible region, respectively. In addition, SnS and SnS thin films exhibited p-type and n-type semiconductor characteristics. In the images of high resolution-transmission electron microscopy, SnS and SnS directly showed a highly crystalline orthorhombic and hexagonal layered structure. The field effect transistors of SnS and SnS thin films exhibited on-off drain current ratios of 8.8 and 2.1 × 10 and mobilities of 0.21 and 0.014 cm V s, respectively. This difference in switching device characteristics mainly depends on the carrier concentration because it contributes to off-state conductance and mobility. The major carrier concentrations of the SnS and SnS thin films were 6.0 × 10 and 8.7 × 10 cm, respectively, in this experiment.

We report here that SnS2 films deposited at 150 °C and annealed at below 350 °C have good potential for using 2D SnS2 in flexible electronic devices.