A ZnO/porous GaN heterojunction and its application as a humidity sensor

Recent Sensing Technologies of Imperceptible Water in Atmosphere

Accurate detection and quantitative evaluation of environmental water in vapor and liquids state expressed as humidity and precipitation play key roles in industrial and scientific applications. However, the development of supporting tools and techniques remains a challenge. Although optical methods such as IR and LASER could detect environmental water in the air, their apparatus is relatively huge. Alternatively, solid detection field systems (SDFSs) could recently lead to a revolution in device downsizing and sensing abilities via advanced research, mainly for materials technology. Herein, we present an overview of several SDFS based sensing categories and their core materials mainly used to detect water in atmosphere, either in the vapor or liquid phase. We considered the governing mechanism in the detection process, such as adsorption/desorption, condensation/evaporation for the vapor phase, and surface attach/detach for the liquid phase. Sensing categories such as optical, chilled mirror, resistive, capacitive, gravimetric sensors were reviewed together with their designated tools such as acoustic wave, quartz crystal microbalance, IDT, and many others, giving typical examples of daily based real scientific applications.

Humidity sensor based on Gallium Nitride for real time monitoring applications

Gallium Nitride (GaN) remarkably shows high electron mobility, wide energy band gap, biocompatibility, and chemical stability. Wurtzite structure makes topmost Gallium atoms electropositive, hence high ligand binding ability especially to anions, making it usable as humidity sensor due to water self-ionization phenomenon. In this work, thin-film GaN based humidity sensor is fabricated through pulse modulated DC magnetron sputtering. Interdigitated electrodes (IDEs) with 100 μm width and spacing were inkjet printed on top of GaN sensing layer to further enhance sensor sensitivity. Impedance, capacitance, and current response were recorded for humidity and bio-sensing applications. The sensor shows approximate linear impedance response between 0 and 100% humidity range, the sensitivity of 8.53 nF/RH% and 79 kΩ/RH% for capacitance and impedance, and fast response (Tres) and recovery (Trec) time of 3.5 s and 9 s, respectively. The sensor shows little hysteresis of < 3.53% with stable and wide variations for accurate measurements. Especially, it demonstrates temperature invariance for thermal stability. Experimental results demonstrate fabricated sensor effectively evaluates plant transpiration cycle through water level monitoring by direct attachment onto leaves without causing any damage as well as freshness level of meat loaf. These properties of the proposed sensor make it a suitable candidate for future electronics providing a low-cost platform for real time monitoring applications.

The effect of crystallinity on the surface modification and optical properties of ZnO thin films

We have studied the effects of crystallinity on the emergence of porous morphology and strong green emission in ZnO thin films after H₂ annealing treatment. The unique multiple-stacked porous structure is observed after performing H₂ annealing treatment on the film with low crystallinity. However, the annealed high-crystallinity film exhibits surface morphology with a shallow porous structure, as revealed by SEM images. To study the effects of these unique porous structures on the optical properties, photoluminescence (PL) spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy (XPS) are conducted. The multiple-stacked porous structure produces strong green emission as compared to the shallow porous structure centered at 2.5 eV, as detected by PL. Here, the green emission originates from the electronic transition related to the oxygen vacancy (V_O). XPS spectra show that the high density of V_O located on the multiple-stacked porous surface is much higher as compared to that for the shallow porous structure due to a high surface-to-volume ratio. The results show that the multiple-stacked porous structure has the potential to enhance the functionality of ZnO for applications in light-emitting.