Visible Light‐Driven p-Type Semiconductor Gas Sensors Based on CaFe2O4 Nanoparticles

In this work, we present conductometric gas sensors based on p-type calcium iron oxide (CaFe2O4) nanoparticles. CaFe2O4 is a metal oxide (MOx) with a bandgap around 1.9 eV making it a suitable candidate for visible light-activated gas sensors. Our gas sensors were tested under a reducing gas (i.e., ethanol) by illuminating them with different light-emitting diode (LED) wavelengths (i.e., 465-640 nm). Regardless of their inferior response compared to the thermally activated counterparts, the developed sensors have shown their ability to detect ethanol down to 100 ppm in a reversible way and solely with the energy provided by an LED. The highest response was reached using a blue LED (465 nm) activation. Despite some responses found even in dark conditions, it was demonstrated that upon illumination the recovery after the ethanol exposure was improved, showing that the energy provided by the LEDs is sufficient to activate the desorption process between the ethanol and the CaFe2O4 surface

Nanomechanical Traceable Metrology of Vertically Aligned Silicon and Germanium Nanowires by Nanoindentation

Silicon and germanium pillar structures (i.e., micro- and nanowires) were fabricated by a top-down approach including nanoimprint lithography and cryogenic dry etching. Various etching parameters were tested to ensure a reliable fabrication process. The impression of nanomechanical properties of such 3-D structures were extracted experimentally by nanoindentation showing promising and comparative results to utilize such nanostructures as small force artefacts

Nanofabrication of SOI-Based Photonic Waveguide Resonators for Gravimetric Molecule Detection

A silicon photonic microresonator comprising two curved vertical grating couplers and a single suspended Si nanowaveguide (NWG) is developed to investigate the giant enhanced Brillouin scattering in subwavelength NWGs caused by photon-phonon interaction. Finite element modelling based on COMSOL Multiphysics is conducted to optimize the critical device parameters (e.g., waveguide width, height, and length). As the smallest structures that need to be resolved are down to ~15 nm in size, electron-beam nanolithography is employed. In this case, dosage tests are carried out to minimize proximity charging effects during the nanopatterning of the silicon-on-insulator (SOI) surface, resulting in appropriate adaptive current area dosage distributions for the periodic gratings, couplers peripheral areas, and NWG, respectively. Furthermore, an enhanced inductively coupled plasma dry reactive ion etching (ICP-DRIE) process at a cryogenic temperature is used to realize smooth vertical sidewalls. Finally, buffered hydrofluoric acid (BHF)-based wet chemical etching is carried out to remove the buried oxide resulting in a suspended Si waveguide

UV-LED Photo-Activated Room Temperature NO2 Sensors Based on Nanostructured ZnO/AlN Thin Films

UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of planar and nanostructured ZnO/AlN thin films on Si substrate towards NO2 at room temperature. A significant increased sensitivity ((Rg − Ra)/Ra = 65.3 ppm NO2 in air) and a strong reduction in recovery time (Trec = 14 min) were already observed for the planar ZnO/AlN thin films under UV-B (305 nm) irradiation compared to the other UV wavelengths, while the device showed no obvious response in dark. By enlarging the surface-to-volume ratio of the sensors (i.e., creating nanostructured ZnO/AlN thin films), an increased response time is expected to be observed

Nanofabrication of Vertically Aligned 3D GaN Nanowire Arrays with Sub-50 nm Feature Sizes Using Nanosphere Lift-off Lithography

Vertically aligned 3D gallium nitride (GaN) nanowire arrays with sub-50 nm feature sizes were fabricated using a nanosphere lift-off lithography (NSLL) technique combined with hybrid top-down etching steps (i.e., inductively coupled plasma dry reactive ion etching (ICP-DRIE) and wet chemical etching). Owing to the well-controlled chemical surface treatment prior to the nanobead deposition and etching process, vertical GaN nanowire arrays with diameter of ~35 nm, pitch of ~350 nm, and aspect ratio of >10 could be realized using 500 nm polystyrene nanobead (PN) masks. This work has demonstrated a feasibility of using NSLL as an alternative for other sophisticated but expensive nanolithography methods to manufacture low-cost but highly ordered 3D GaN nanostructures

Nanoscale GaN LED arrays for chip-based optical nanoscope

Since the invention of high-power gallium nitride (GaN) light-emitting diodes (LEDs), enormous investigations have been carried out in the last decade, not only to improve the material quality and device performance but also to extend their applications. InGaN/GaN LEDs have been broadly employed in general illumination and backlight units because of their higher luminous efficacy and longer lifetime compared to conventional light sources. Furthermore, several innovative optoelectronic devices have been introduced into industrial markets (e.g., high-brightness display and optical sensors in smartphones). By integrating LEDs with CMOS control electronics, matrix-addressed and individually controlled GaN microLED arrays could be realized with a display luminance of 106 cd/m2 (12 W/cm2), which is a factor of 103 higher than normal commercial displays [1]. However, their spatial resolution was still low, which resulted from the LED dimensions with pixel and pitch sizes of several micrometers. Thus, in this work, nanoscale InGaN/GaN LED arrays with individual pixel control were designed and fabricated to be integrated as a novel illumination source in a chip-based lensless microscope (i.e., ChipScope) for real-time monitoring of biological cells. The challenging 3D processing steps of the high-aspect-ratio nano-/microLED arrays have been optimized to create tiny optoelectronic modules. To fabricate the well-ordered high-aspect-ratio nano-/microstructures, a top-down approach comprising nanophotolithography and hybrid etching was employed [2]. In this case, GaN LED fins with smooth sidewalls could be realized from the sequential processes of SF6/H2-based ICP-RIE and KOH-based wet chemical etching. As top and bottom surfaces of the structures are distantly separated by about 3.5 – 5 µm, device planarization plays a critical role for the feasibility of top metal contact deposition. Thus, different polymer filling materials have been carefully investigated (e.g., photoresist, spin-on-glass, and benzocyclobutene (BCB)). Along with the device fabrication, simulations of the light emission patterns have been conducted with different conditions of the integrated materials to optimize the nanoLED designs. References J. Herrnsdorf, J. J. D. McKendry, S. Zhang, E. Xie, R. Ferreira, D. Massoubre, A. M. Zuhdi, R. K. Henderson, I. Underwood, S. Watson, A. E. Kelly, E. Gu, M. D. Dawson, “Active-matrix GaN micro light-emitting diode display with unprecedented brightness,” IEEE Transactions on Electron Devices, 62(6), 1918-1925 (2015). DOI: 10.1109/TED.2015.2416915 F. Yu, S. Yao, F. Römer, B. Witzigmann, T. Schimpke, M. Strassburg, A. Bakin, H.W. Schumacher, E. Peiner, H.S. Wasisto, A. Waag, “GaN nanowire arrays with nonpolar sidewalls for vertically integrated field-effect transistors,” Nanotechnology, 28(9), 095206 (2017). DOI: 10.1088/1361-6528/aa57b