In this work, reduced graphene oxide (rGO)/GaN nanorods (NRs) hybrid structure based sensors for hydrogen (H2) and hydrogen sulfide (H2S) gases has been demonstrated at room temperature. The morphological, elemental, and structural analyses were carried out by using scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The electrical characterization of rGO/GaN NRs hybrid structures showed good rectifying behavior compared to pristine GaN NRs. The H2 and H2S gas sensing measurements at different gas concentrations revealed that the rGO/GaN NRs exhibit superior sensing properties compared to pristine GaN NRs. In order to find the gas sensing mechanism of rGO/GaN NRs, hybrid structure sensor is also tested with NOx gas. Our experimental results revealed that the rGO/GaN NRs are good candidates for selective detection of H2S gas. The rGO/GaN NRs showed remarkably improved response under ultra-violet (λ = 365 nm) illumination, the photogenerated carriers could be responsible for increasing response of the gas sensor at 30 °C under UV illumination. In addition, humidity test of the rGO/GaN NRs sensor was also conducted in this work. Our experimental results suggested that decorating GaN NRs with solution-processable rGO is one of the effective ways to enhance the response of GaN nanostructures.

H2, H2S gas sensing properties of rGO/GaN nanorods at room temperature: Effect of UV illumination

DNA-CTMA functionalized GaN surfaces for NO2 gas sensor at room temperature under UV illumination

One of the biggest issues with GaN-based hydrogen sensors is their sensitivity to humidity in the ambient. We demonstrate that encapsulation of Pt-AlGaN/GaN Schottky diode with poly(methyl methacrylate) (PMMA) provides effective mitigation of the effects of water. Without PMMA encapsulation, the absolute current signal for detection of 500 ppm of H2 was decreased by a factor of 8 in the presence of water. By sharp contrast, encapsulated diodes show no decrease in response in the presence of water. The relative current changes are of the order $2.8 \times 10^{5}$ % when 500 ppm H2 is introduced to the surface of bare or PMMA encapsulated diodes in the absence of water or to encapsulated diode in the presence of water. Detection limits of ~ 100 ppm H2 (0.01% by volume) were obtained with standard forward bias detection mode at 1.3 V.

Pt-AlGaN/GaN Hydrogen Sensor With Water-Blocking PMMA Layer

In gas sensors, metal oxide semiconductors have been considered as favorable resistive-type toxic gas sensing materials. However, the higher temperature operation of metal oxides becomes a barrier for their wide range of applications in explosive and flammable gas environments. In this regard, great efforts have been devoted to reducing the operating temperature of the sensor. We demonstrated a chemical resistor-type NO gas sensor based on p-i-n GaN nanorods (NRs) consisting of InGaN/GaN multi-quantum wells (MQW). The sensor exhibited superior NO gas sensing performance to p-type GaN NRs. Furthermore, it also showed a remarkably improved response and fast recovery under UV irradiation (λ = 367 nm) of different UV intensities (7 to 20 mw cm-2) under reverse bias. The sensing performance of MQW-embedded p-i-n GaN NRs was enhanced with the boosted response by 4-fold at 35 °C under UV irradiation. The significant decrease in the resistance of the sensor under UV irradiation was mainly due to the extraction of photo-generated carriers under reverse bias, which can enhance the ionization of oxygen molecules. In addition, the effect of relative humidity (30%-60%) on the gas sensing performance was also manifested in this study. The selectivity of the sensor was determined by using other gases (NO, NO2, O2, NH3, H2S, CO, and H2), which exhibited a low response towards all tested gases other than NO. The experimental results demonstrated that p-i-n GaN NRs with InGaN/GaN MQW is a promising material for the detection of NO gas. Specific emphasis was laid on the enhanced response of p-i-n GaN NRs in reverse bias under UV irradiation.

A novel low-temperature resistive NO gas sensor based on InGaN/GaN multi-quantum well-embedded p-i-n GaN nanorods.

Hydrogen sensing performance of a Pd/HfO2/GaOx/GaN based metal-oxide-semiconductor type Schottky diode

In this work, enhanced hydrogen sensing characteristics of a GaN-based Schottky diode-type sensor with a GaOx layer are studied and demonstrated. A thin GaOx layer inserted in Pd/GaN interface is oxidized by the immersion in an H2O2 solution at room temperature. Experimentally, a significantly high hydrogen sensing response of 1.8 × 105 and a large Schottky barrier height variation ratio of 33.1% are found upon exposure to a 1% H2/air gas at 300 K. In addition, a very low detection limit of 0.1 ppm H2/air at 300 K is obtained. These improved properties could be attributed to the effective dissociation of hydrogen molecules and rougher Pd surface caused by the presence of the GaOx layer. The response (recovery) time constant of 13.3 (23.6) s is obtained upon exposure to a 1% H2/air gas at 300 K. The related hydrogen adsorption analysis of the proposed device is also studied and demonstrated.

Hao-Yeh Liu

Papers

Enhancement of hydrogen sensing performance of a GaN-based Schottky diode with a hydrogen peroxide surface treatment