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. 2020 Apr 26;5(17):9985–9990. doi: 10.1021/acsomega.0c00317

Highly Integrated In Situ Photoenergy Gas Sensor with Deep Ultraviolet LED

Shuang Zhang †, Huayao Li ‡, Xiaoxue Wang §, Yuan Liu †,*, Jiangnan Dai †, Changqing Chen †
PMCID: PMC7203992  PMID: 32391486

Abstract

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According to the demands of the Internet of Things (IoTs), a gas sensor is demanded to be small, portable, and easy to integrate with the environment or structure in its application. Herein, an ingenious form of in situ photoenergy gas sensor integrated with a deep ultraviolet light-emitting diode (LED) has been designed to achieve ppb level NO2 gas detection at room temperature. In this gas sensor, the deep ultraviolet LED based on AlGaN materials, which has a wider band gap and higher photoexcitation energy, acts as the substrate with emission at 280 nm. The ZnO nanorods of the gas-sensing material were directly grown on 2 inch AlGaN-based LEDs containing thousands of independent light-emitting chips, leading to photosensitive materials with uniform and controllable, as well as a sensor with low power consumption and mass manufacture with low cost. The result shows that responses of over 500% to 500 ppb of NO2 were observed by in situ irradiation of just 3 mW optical power. Meanwhile, sensitivity without real-time photoenergy is defined as the ratio of the resistance change rate in pollutant to that in air has been presented. Interestingly, the prototype gets in situ photoenergy charge for 5 min and then has responses of over 200% to 500 ppb of NO2 for 5 days in the dark. It may open a new avenue for the integrated microchip design of a gas sensor and give a novel sight into the sensitivity study without real-time assisted energy.

Introduction

Driven by ubiquitous applications such as the Internet of Things (IoTs), Microelectro Mechanical Systems (MEMS) gas sensors using micro heaters and micro hot plates have gradually become the mainstream.1−5 However, problems such as high operating temperature, poor stability and consistency, and short life of heating devices are common. In particular, the high operating temperature of MEMS gas sensors limits their detection in special environments such as flammable, explosive, and biological environments. At the same time, real-time thermal assistance will not only lead to the short lifetime of materials but also their relatively high power consumption.

In this context, light activation becomes a good choice. Many researchers have used external light-emitting diode (LED) modules to activate gas-sensitive materials.6−20 On the one hand, due to the unique optical polarization characteristics of the III–V compound semiconductor materials and the control of the angle of light by the lenses in the LED package,21,22 precise optical calibrations are required during photoexcitation. However, this method is generally more complicated and difficult to control.19 On the other hand, due to the space between the external LED modules and the gas-sensitive material, the light cannot be used sufficiently, resulting in high power of the LED module. The external light source systems make against sensor integration, miniaturization, and portability of sensors. To solve this problem, some attempts have been reported. Hsu et al. prepared ZnO films on LED lenses.18 However, its structure is an integration at the encapsulation level and not very compact. Markiewicz and Casals et al. prepared ZnO nanoparticles on p-GaN of InGaN-based LEDs, which is a novel structure with low power consumption.19,20 However, its method of micro-drop-casting cannot be mass-manufactured and has no good repeatability. Most importantly, the response or recovery time is as long as dozens of minutes, which can hardly meet the requirements of practical application. For the above-mentioned studies of light-activated gas-sensing structures, none of them has touched the deep ultraviolet (UV) as a light source. Until now, deep ultraviolet LEDs still have poor light extraction efficiency (LEE) compared to near UV or InGaN-based visible LEDs.23,24 However, many review reports have comprehensively summarized the research work of researchers to improve the LEE of deep ultraviolet LEDs.25−27 Moreover, the shorter the wavelength, the more favorable the formation of electron–hole pairs of gas-sensitive materials. The powerful photonics can also easily activate the oxygen vacancies and enhance their reactivity in sensitive processes.11,18

Herein, an in situ photoenergy gas sensor based on a deep ultraviolet LED is presented to solve the above problems. The produced prototype contains three segments: (1) the center is the sapphire substrate in 750 μm × 750 μm; (2) the backside is the flip-chip AlGaN-LED structure, which can emit the 280 nm photons to the front side; (3) the front side is the ZnO nanorod layer, which can absorb the photons and show sensitive abilities. It is worth noting that ZnO is grown directly on a 2 inch deep ultraviolet LED wafer with thousands of chips. The process is simple, and the consistency of ZnO material on the entire wafer is guaranteed. Since ZnO material is directly grown on the deep ultraviolet LED wafer, the deep ultraviolet light can be fully utilized by the ZnO layer. Therefore, the light power requirement of deep ultraviolet LEDs is relatively low. This design directly compensates for the low LEE of deep UV LEDs, so that deep ultraviolet LEDs can be applied to the field of gas sensing. In addition, there is no need to control the angle of the light, which simplifies the test equipment and steps. At the same time, the prototype does not require real-time photoenergy, which can be photoenergy charge for several minutes and then has a good sensitivity duration for several days. It can be assumed that the realized prototype of the gas sensor can be applied in any smart sensing module and has the lowest reliance on electricity.

Results and Discussion

Sensitive Abilities of the Integrated Microchip

The resistance behaviors of the microchip by an in situ photoactivated response and recovery are shown in Figure 1a. The AlGaN-LED structure is measured during the fabrication processes and make sure that it can emit UVC photons in 280 nm wavelength (inset of Figure 1a) and 3 mW optical powers. The stable resistance of the ZnO nanorod layer in the dark is 4.3 MΩ. Due to light excitation, a response of abrupt decrease in resistance occurred within ∼10 s, and subsequently, the resistance decreased slowly to a new stable state of 13 kΩ within 5 min. After the light termination, the resistance exhibits a significantly slow recovery rate compared to the rapid response rate. Among the whole recovery process, the resistance in the first period of ∼400 s changes relatively faster than the later period. The recovery rate becomes rather slow after the time of 1000 s in Figure 1a. The above behaviors indicate that the photogenerated electrons have a super long lifetime and can be stored in the ZnO nanorod itself.

Figure 1.

Figure 1

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Sensitive abilities of the integrated microchip. (a) Photoactivated response and recovery curve (inset shows the spectrum curve of the in situ AlGaN-LED structure). (b) Exposure to different concentration levels of NO2 with real-time in situ-assisted photoenergy at room temperature (25 °C).

The light-activated gas-sensing performances of the microchip at room temperature are shown in Figure 1b. The microchip exhibits excellent reversibility over long continuous operation with different NO2 concentration levels. The resistance responds rapidly upon exposure to NO2 and recovers fast to the almost initial value upon exposure to air again. During the short response period, the sensor was not able to reach saturation at 500 ppb. It is possible for the sensor to reach equilibrium although it takes a longer time. The NO2 concentration in an ambient environment is around 100 ppb level. As can be seen, the sensor had a rapid response and recover to 100 and 200 ppb NO2, which showed potential in real applications. The sensitivity (Rg/Ra) values for NO2 get higher with increasing concentration, and they are about 533, 305, and 218% for 500, 200, and 100 ppb, respectively. The response and recovery times for all of the concentrations are approximately 20 s. For the tested NO2 concentrations of the ppb level, the excellent sensitivities, as well as the fast response and recovery rates, are considerably better than other room-temperature operations with pure ZnO sensors reported in the literature.15−18 Since the ZnO nanorods have not been coupled or doped with other elements, good performance can be attributed to the in situ light-activated form. While the optical power of the AlGaN-LED structure is just 3 mW, the illumination as to the whole area (30 mil × 30 mil) of the microchip is equal to as high as 530 mW/cm2 in optical power density. In fact, the high optical power density is benefited to the sensing abilities. With the in situ light-activated form inside the microchip, just a very low optical power from the LED structure can create a great optical power density for the sensing layer.

To further study the sensitive ability without real-time assisted photoenergy, we have designed a measurement project close to the practical application scenario. For the 1st day, the microchip is charged with in situ photoenergy for 5 min and kept in the dark during the whole process. Note that the dry air is never stopped to pass through the test chamber from the beginning to the end. For the 2nd, 3rd, 4th, and 5th days, the resistance data of the microchip in dry air is collected for 200 s. Then, with the introduction of 500 ppb NO2, the following resistance data are also collected for 200 s. As can be seen in Figure 2a, the increase rate of resistance in NO2 is obviously faster than that in air for each day, so that the sensitivity without real-time assisted photoenergy is proposed to be the ratio of the resistance change rate in the pollutant and air (response in the dark = (ΔRgas/ΔTgas)/(ΔRAir/ΔTAir)). As shown in Figure 2b, the sensitivities are calculated to be 640, 557, 365, and 244% for the duration days. Although the sensitivity gets weak with the days going on, the response is still over 200% for 5 days without any assisted energy. In addition, there is an approximately linear relationship between the response value and duration time. The results in Figure 2a,b suggest that the microchip can keep continuously monitoring the status with in situ photoenergy charge for 5 min every 5 days. When the assisted photoenergy is not needed all of the time, the gas sensor will be definitely endowed with much more expansive applications. It can be assumed that the realized model of sensitivity without real-time assisted photoenergy can be applied in any smart sensing module and has the lowest reliance on electricity. If the gas sensor runs on a button battery, the traditional real-time photoenergy model can only last a day but the new charge and duration model can last up to a year.

Figure 2.

Figure 2

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Energy storage of the integrated microchip. (a) Exposure to 500 ppb NO2 without real-time in situ-assisted photoenergy at room temperature (25 °C) for different days. (b) Calculated responses in the dark for different days. (c) Resistance variation curve of the microchip at 0–80% relative humidity (RH).

The resistance variation curve of the microchip at 0–80% RH is illustrated in Figure 2c. With RH increasing from 0 to 80%, the change of the resistance is approximately 10%. It is confirmed that the microchip shows a very small response to a large change in relative humidity.

Discussion for Sensitivity Without Real-Time-Assisted Photoenergy

It has been widely reported that ZnO nanorods are able to store the photogenerated electrons for a long time. The oxygen vacancy photoionization mechanism is mainly responsible for electron storage that can last for hundreds of hours.28−31 The neutral oxygen vacancy (VO0), which is a type of insulating atomic configuration (α-type) in ZnO, captures two electrons and forms a deep defect-localized-state (DLS) in the band gap. The VO can be photoionized to be the VO··, which is a metastable conducting atomic configuration (β-type). The two photoexcited electrons change to be the perturbed-host state (PHS), which has good electrical conduction properties. After photoexcitation, the reverse configuration transition from the VO photoexcited state (with a large Zn–Zn distance of 4.0 Å) to the VO0 ground state (with a small Zn–Zn distance of 3.0 Å) involves a large lattice relaxation and need to overcome an energy barrier of ∼0.26 eV. Accordingly, the metastable β-type configuration, as well as the photoexcited electrons, can be retained for a long time.

Accordingly, the whole reaction processes can be illustrated in Figure 3 and described as follows

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Figure 3.

Figure 3

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Illustration of the whole reaction processes in sensitivity without real-time-assisted photoenergy. The red arrows indicate the reactions with light excitation. The green arrows indicate the reactions in air after light excitation. The blue arrows indicate the reactions of NO2 in the dark.

The O2(ads) is the adsorbed O2. During the in situ photoenergy charge for 5 min, eqs 1 and 2 are dominant in the initial 10 s, and then eq 3 is dominant in the latter several minutes. After photoenergy is absent, eqs 4 and 5 are dominant in the initial hundreds of seconds. When the atmosphere is air, eqs 6 and 7 are dominant in the latter several days.

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While the microchip is exposed to the NO2 gas, the eqs 8 and 9 are dominant. The NO2(ads) is the adsorbed NO2. Since the reaction rates of eqs 8 and 9 are faster than those of eqs 6 and 7, the electron loss rate in NO2 is significantly faster than that in air, so that the increase rate of resistance in NO2 is obviously faster than that in air.

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In conclusion, the long lifetime electrons in PHS contribute to the long sensitive duration after photoenergy charge. Meanwhile, the sensitivity value without real-time-assisted photoenergy is determined by the difference in reaction rates between the PHS electrons to the external pollutant and that to the internal VO··.

Conclusions

We have combined deep ultraviolet LED device preparation technology with gas sensor technology to design an ingenious in situ photoenergy gas sensor, which has realized (1) ppb level gas detection at room temperature and excellent gas-sensitive performance; (2) a microchip gas sensor that is integrated, miniaturized, and portable; (3) mass production and low cost. This is the first time that deep ultraviolet LEDs have been applied to the field of gas sensing using the integrated method and the concept of energy storage for gas sensors has been proposed. The transduction of many materials’ physical and chemical properties requires photoenergy excitation. Designs similar to this structure can also be applied to other optoelectronic fields, such as photocatalysis and ultraviolet fluorescence detection. Therefore, this development may lay the foundation for new-type optoelectronic devices.

Experimental Section

The design and fabrication processes are illustrated in Figure 4. The fabrication of the integrated microchip starts from a 2 inch sapphire substrate. Firstly, the AlGaN epitaxial layers were orderly grown on one side of the sapphire substrate by metal–organic chemical vapor deposition (MOCVD): (1) A 2.5 um thick AlN buffer layer was grown on a (0001) sapphire substrate; (2) a 2.5 μm thick Si-doped n-Al0.55Ga0.45N layer was grown as n-contact; (3) the active region included seven periods of Al0.45Ga0.55N (3 nm)/Al0.55Ga0.45N (11 nm) multiple quantum wells (MQWs); (4) an ∼25 nm thick Mg-doping p-Al0.65Ga0.35N electron blocking layer (EBL) was grown, followed by a ∼250 nm thick Mg-doped p-Al0.5Ga0.5N layer; and (5) an ∼50 nm thick Mg-doped p-GaN layer served as the p-contact layer.

Figure 4.

Figure 4

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Schematic illustration for the fabrication processes of the integrated microchip.

Secondly, the array of the flip-chip structures was fabricated based on the AlGaN epitaxial layers. The size of each unit in the array is 750 μm × 750 μm. The key processes of the microfabrication technique in each unit can be briefly concluded as follows: (1) a depth of ∼670 nm was etched to form mesa geometry and the n-AlGaN exposed surface; (2) a depth of ∼5 um was etched to form a deep isolation trench and sidewall of n-AlGaN/AlN; (3) the contact electrode (Ti/Al/Ti/Au) for n-AlGaN layer was deposited; (4) the contact electrode (Ni/Au) for the p-GaN layer was deposited; (5) the reflecting layer of SiO2/Al was fabricated to be covered on the whole flip-chip structure to ensure that the total photons can be reflected the opposite side of the sapphire substrate that was designed to cover with the sensing material; and (6) the SiO2 passivation layer and the NP-Pad (Cr/Pt/Au) layer were successively deposited on the whole unit. The etch step was operated by inductively coupled plasma (ICP). The metal deposition was operated by electron beam evaporation (EBE). The SiO2 deposition was operated by plasma-enhanced chemical vapor deposition (PECVD).

Thirdly, the sapphire substrate was fabricated with the second side. The array of an Au interdigital electrode was deposited by photolithography and EBE. The location of each interdigital electrode corresponds to that of each flip-chip structure. Then, the ZnO seed-layer with a thickness of 40 nm was prepared on the whole of interdigital electrodes by magnetron sputtering. After that, the sapphire substrate was put in a Teflon-lined autoclave for 6 h at 80 °C to grow nanorods. The solution in the autoclave was prepared with 0.03 M Zn(NO3)2·6H2O and 0.03 M C6H12N4. The sapphire substrate was rinsed with deionized water and dried after the growth process.

Finally, the sapphire substrate was divided by a laser cutting machine and a splitter machine to get each unit separately. One unit was an integrated microchip as a gas sensor. Through all of the above processes, a 2 inch sapphire substrate can be fabricated to obtain about 2600 pcs integrated microchips. The cost of each microchip in mass manufacture is very cheap and only about 0.01$. This cost is very low for practical use. The schematic for the entire structure of the prepared integrated microchip and the morphological representation diagram of ZnO (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) are shown in Figure S1. To measure the sensitivity in this study, the integrated microchip was welded on the planar transfer substrate and then bonded to the TO bracket (Figure S2). The sensor was placed in a quartz chamber. A Keithley instrument is used to power the AlGaN-LED structure and collect sensing signals. During all of the gas-sensing measurements, the dry synthetic air was continuously passed through the chamber to maintain a dry ambience. The NO2 concentrations were controlled by changing the mixing ratio of the NO2 gases (the concentration of the gas was maintained at 5 ppm using a dry synthetic air balance) and dry synthetic air. The temperature of the laboratory was controlled at 25 ± 1 °C by an air conditioner. The response time was determined according to the time required for the resistance to change from Ra to Ra – 90% (Rg – Ra). The recovery time was determined according to the time required for the resistance to change from Rg to Rg + 90% (Ra – Rg), where the Ra and Rg are the resistances of the sensor in air and in the measured gas, respectively.

Acknowledgments

This work was supported by the Key Project of Chinese National Development Programs (Grant No. 2018YFB0406602), the Key Laboratory of infrared imaging materials and detectors, the Shanghai Institute of Technical Physics, the Chinese Academy of Sciences (Grant No. IIMDKFJJ-17-09), the National Natural Science Foundation of China (Grant No. 61774065), and the Postdoctoral Science Foundation of China (Grant No. 2019M652632).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00317.

  • Schematic for the entire structure of the prepared integrated microchip, the morphological representation diagram of ZnO (SEM and TEM), and the photo of the real microchip (PDF)

Author Contributions

Y.L., H.L., and S.Z. discussed and defined the research project. S.Z. performed experimental fabrication and measurements. X.W. assisted with the ZnO growth and sensitivity tests. H.L. built the test platform for sensing measurement. S.Z. and Y. L. wrote the paper with significant input from J.D. and C.C. All the authors commented on the paper.

The authors declare no competing financial interest.

Supplementary Material

ao0c00317_si_001.pdf (416.2KB, pdf)

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Supplementary Materials

ao0c00317_si_001.pdf (416.2KB, pdf)

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