Baomei Wen

A solution-processed high-efficiency p-NiO/n-ZnO heterojunction photodetector

This paper presents a high efficiency heterojunction p-NiO/n-ZnO thin film ultraviolet (UV) photodetector (PD) fabricated on conductive glass substrates. The devices are fabricated by using a simple spin-coating layer-by-layer method from precursor solutions. Photodiodes show good photoresponse and quantum efficiency under UV illumination. With an applied reverse bias of 1 V, the devices show maximum responsivity and detectivity of 0.28 A W−1 and 6.3 × 1011 Jones, respectively, as well as high gain with external quantum efficiency (EQE) of over 90%. By employing ultrathin Ti/Au as top UV transparent metal contacts, this architecture allows the PDs to be illuminated either through glass or metal side. Laser beam induced current is used to examine the local variation of EQE providing information on the photoresponse behavior within the device. Optical properties of NiO and ZnO deposits have also been explored.

Tunable Ultraviolet Photoresponse in Solution-Processed p–n Junction Photodiodes Based on Transition-Metal Oxides

Solution-processed p-n heterojunction photodiodes have been fabricated based on transition-metal oxides in which NiO and ternary Zn(1-x)Mg(x)O (x = 0-0.1) have been employed as p-type and n-type semiconductors, respectively. Composition-related structural, electrical, and optical properties are also investigated for all the films. It has been observed that the bandgap of Zn(1-x)Mg(x)O films can be tuned between 3.24 and 3.49 eV by increasing Mg content. The fabricated highly visible-blind p-n junction photodiodes show an excellent rectification ratio along with good photoresponse and quantum efficiency under ultraviolet (UV) illumination. With an applied reverse bias of 1 V and depending on the value of x, the maximum responsivity of the devices varies between 0.22 and 0.4 A/W and the detectivity varies between 0.17 × 10(12) and 2.2 × 10(12) cm (Hz)(1/2)/W. The photodetectors show an excellent UV-to-visible rejection ratio. Compositional nonuniformity has been observed locally in the alloyed films with x = 0.1, which is manifested in photoresponse and X-ray analysis data. This paper demonstrates simple solution-processed, low cost, band tunable photodiodes with excellent figures of merit operated under low bias.

Top-down fabrication of large-area GaN micro- and nanopillars

Large-area gallium nitride (GaN) micro- and nanopillar (NP) arrays were fabricated by plasma etching of lithographically patterned GaN thin-film grown on Si substrate. Deep-ultraviolet lithography, inductively coupled plasma (ICP) etching, and subsequent chemical treatments were effectively utilized to fabricate GaN pillars with diameters ranging from 250 nm to 10 μm. The impact of various plasma etching process parameters and chemical etchants on the morphology, strain, and surface defects of these NPs were studied using scanning-electron microscopy, photoluminescence (PL), and Raman spectroscopy. It was found that the shape of the NPs can be controlled by the substrate temperature during the plasma etch and by using different gas chemistries. Room-temperature PL and Raman spectroscopy measurements revealed significant strain relaxation in 250 nm diameter pillars as compared to 10 μm diameter pillars. PL measurement also indicated that the surface damage from the plasma etch can be removed by etching in KOH-ethylene glycol solution. Post-ICP selective wet chemical etch enabled us to fabricate functional structures such as micro- and nanodisks of GaN, which potentially could be utilized in nitride-based resonators and lasers.Large-area gallium nitride (GaN) micro- and nanopillar (NP) arrays were fabricated by plasma etching of lithographically patterned GaN thin-film grown on Si substrate. Deep-ultraviolet lithography, inductively coupled plasma (ICP) etching, and subsequent chemical treatments were effectively utilized to fabricate GaN pillars with diameters ranging from 250 nm to 10 μm. The impact of various plasma etching process parameters and chemical etchants on the morphology, strain, and surface defects of these NPs were studied using scanning-electron microscopy, photoluminescence (PL), and Raman spectroscopy. It was found that the shape of the NPs can be controlled by the substrate temperature during the plasma etch and by using different gas chemistries. Room-temperature PL and Raman spectroscopy measurements revealed significant strain relaxation in 250 nm diameter pillars as compared to 10 μm diameter pillars. PL measurement also indicated that the surface damage from the plasma etch can be removed by etching in ...

Structural and optical nanoscale analysis of GaN core–shell microrod arrays fabricated by combined top-down and bottom-up process on Si(111)

Large arrays of GaN core–shell microrods were fabricated on Si(111) substrates applying a combined bottom-up and top-down approach which includes inductively coupled plasma (ICP) etching of patterned GaN films grown by metal–organic vapor phase epitaxy (MOVPE) and selective overgrowth of obtained GaN/Si pillars using hydride vapor phase epitaxy (HVPE). The structural and optical properties of individual core–shell microrods have been studied with a nanometer scale spatial resolution using low-temperature cathodoluminescence spectroscopy (CL) directly performed in a scanning electron microscope (SEM) and in a scanning transmission electron microscope (STEM). SEM, TEM, and CL measurements reveal the formation of distinct growth domains during the HVPE overgrowth. A high free-carrier concentration observed in the non-polar HVPE shells is assigned to in-diffusion of silicon atoms from the substrate. In contrast, the HVPE shells directly grown on top of the c-plane of the GaN pillars reveal a lower free-carrier concentration.

Faceting control in core-shell GaN micropillars using selective epitaxy

We report on the fabrication of large-area, vertically aligned GaN epitaxial core-shell micropillar arrays. The two-step process consists of inductively coupled plasma (ICP) etching of lithographically patterned GaN-on-Si substrate to produce an array of micropillars followed by selective growth of GaN shells over these pillars using Hydride Vapor Phase Epitaxy (HVPE). The most significant aspect of the study is the demonstration of the sidewall facet control in the shells, ranging from {1101} semi-polar to {1100} non-polar planes, by employing a post-ICP chemical etch and by tuning the HVPE growth temperature. Room-temperature photoluminescence, cathodoluminescence, and Raman scattering measurements reveal substantial reduction of parasitic yellow luminescence as well as strain-relaxation in the core-shell structures. In addition, X-ray diffraction indicates improved crystal quality after the shell formation. This study demonstrates the feasibility of selective epitaxy on micro-/nano- engineered templa...

Live demonstration: Chip-scale, nano-engineered, environmental gas sensors

N5 Sensors will demonstrate its state-of-the-art chip-scale nanoengineered gas sensors. The sensors can detect environmental gasses including Volatile Organic Compounds (VOCs), Carbon Dioxide, Methane, Hydrogen, etc. with high selectivity in a low power interface suitable for wearable applications.

Morphological improvements in nanostructured nitride photodiodes

Detection technologies for the UV and visible range are well established, but many are limited by either performance or size, weight, and power. The ubiquitous silicon-based photodiodes are cheap and reliable but suffer in UV applications, while photomultiplier tubes offer high performance but can be unreliable and power hungry. To solve these issues we developed a novel morphology and device architecture using gallium nitride as the functional material. Here, we outline our efforts to develop high-efficiency nitride optoelectronic devices, the properties of which are particularly dependent on morphology. To develop these systems, we considered recent improvements in commercial LED structures, which enable high efficiencies in light extraction by controlling internal photon reflection and scattering dynamics. For nitride-based LEDs, if the active area is formed on a cystallographic plane other than the conventional polar c-plane, we can achieve improved carrier (electron and hole) dynamics associated with reduced internal electric fields. By extension, PIN diode-based detectors that exploit such morphologically tuned properties can have much higher detection efficiencies than planar devices. In conventional planar diodes, the active area (the depletion region) is coincident with the substrate surface, while in radial diodes the depletion region is orthogonal to the surface. With respect to photon absorption, the active regions of planar and radial diodes are, respectively, perpendicular and parallel to incoming photons (see Figure 1). Full realization of such 3D PIN diode detectors could enable a viable solid-state replacement for the costly, bulky, and power-hungry photomultiplier tubes used in scintillator-based radiation detection applications. Figure 1. Comparison of the characteristics of conventional planar and radial PIN junction gallium nitride (GaN)-based detectors. hv: Incoming light.