We report a novel method for fabricating a highly sensitive chemical sensor based on a ZnO nanorod array that is epitaxially grown on a Pt-coated Si substrate, with a top–top electrode configuration. To practically test the device, its O2 and NO2 sensing properties were investigated. The gas sensing properties of this type of device suggest that the approach is promising for the fabrication of sensitive and reliable nanorod chemical sensors.
KeywordsZnO nanorod array Chemical sensor MOCVD
Recently, nanostructures, such as nanorods and nanowires, made of semiconducting materials have been extensively investigated for the purpose of using their unique properties in various nanoscale functional devices [1, 2]. For instance, ZnO nanostructures have received particular attention due to their many valuable properties and the ease with which ZnO can be made into various nanostructure shapes by many different methods [3–6].
Since nanorods and nanowires have much larger surface-to-volume ratios compared to their thin film and bulk material counterparts, their application to miniaturized highly sensitive chemical sensors has been predicted to be promising [7, 8]. The electrical and chemical sensing properties of single ZnO nanorods have been extensively investigated in recent years by the fabrication and testing of single nanorod field-effect transistors (FETs). According to the results, ZnO nanorods show an n-type semiconducting behavior and their electrical transport is strongly dependent on the adsorption and/or desorption nature of chemical species [9–13]. Despite significant achievements in the realization of chemical sensors based on single ZnO nanorods [14–17], there still remain many aspects that should be overcome before their actual application. Firstly, the fabrication of sensors based on individual nanorods involves a careful lithography process in which each fabrication step is expensive and tedious. Secondly, a precise system that can measure currents in the region of 10−9 A is necessary to detect the small current changes that occur in a single nanorod during the adsorption/desorption of chemical species. Finally, the slightly different sizes of each nanorod and the different natures of the electrical contacts in each sensor cause poor reproducibility.
In order to overcome the disadvantages of single nanorod chemical sensors, recently the use of vertically aligned nanorod arrays (NRAs) in chemical sensors has been attempted [18–20]. In these works, metal electrodes were simply deposited on top of nanorod arrays using sputtering [18, 20] or aerosol spray pyrolysis . However, this approach is likely to result in not distinctive but gradient interfaces between nanorods and metal electrodes, possibly deteriorating sensor efficiency. Therefore, an approach for fabricating chemical sensors based on ZnO nanorod arrays (NRAs) using more reliable electrode configurations needs to be developed.
In this work, we report a novel approach to fabricating chemical sensors based on ZnO NRAs with a top–top electrode configuration. The approach used a coating and etching process with a photoresist (PR). The results show that the proposed ZnO NRA-based chemical sensor exhibits a comparable sensitivity, a higher reproducibility and can be made in a simpler way, suggesting that the proposed approach is promising for fabricating chemical sensors based on ZnO NRAs.
ZnO NRAs were synthesized on Pt-coated Si (001) substrates using a horizontal-type metal organic chemical vapor deposition (MOCVD) system without using any metal catalyst. Pt films of ~120 nm in thickness were deposited on Si (001) substrates by a sputtering method. Before the Pt deposition, a Ti interlayer of ~5 nm in thickness was deposited on the bare Si substrates using the same sputtering method. This was done in order to enhance the adhesion of the Pt films to the Si substrates. According to the high-resolution X-ray diffraction (XRD) results (which are not presented here), the resultant 120-nm-thick Pt films possessed a (111) preferred orientation normal to the substrate plane, while showing a random alignment in the in-plane direction. ZnO NRAs were grown at 500°C for 30 min using O2 and diethylzinc as precursors with argon as a carrier gas. The pressure in the reactor was kept at 5 torr. The flow rates of the oxygen and diethylzinc were fixed to result in an O/Zn precursor ratio of 68. The microstructures and crystalline quality of the synthesized ZnO NRAs were investigated using field-emission scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM). The growth behavior, alignment nature, substrate dependency, size and shape control, fabrication of the field-effect transistors, and the temperature-dependent electrical transport of the single ZnO nanorods used in this study have been reported in detail in our previous works [21–24].
As a practical test for ZnO NRA chemical sensor, the sensing properties for O2 and NO2 were investigated. The fabricated NRA chemical sensor was introduced into a vacuum chamber equipped with a system that can measure current and voltage by changing O2 and NO2 environments using N2 as a carrier gas. HP 4140B pA Meter/DC voltage source was used as the measurement tool, which was interfaced with a personal computer through a general purpose interface bus (GPIB) card. The chamber pressure was controlled using a gate valve and verified using an ion gauge. The sensor assembly was heated to the desired temperature by using a halogen lamp, and temperature was monitored through a thermocouple. In this study, the sensing measurement was performed at 573 K. The base pressure of the vacuum chamber, which was connected to a turbomolecular pump, was typically ~5 × 10−6 torr. Using mass flow controllers, O2 and NO2 environments were monitored.
As shown in a field-emission SEM image displayed in Fig. 1, vertically well-aligned ZnO nanorods grew over the Pt/Ti/Si (001) substrate. The nanorods are uniform in diameter and length. It is clear that a continuous ZnO interfacial layer exists. Our previous work on the early growth stages of ZnO nanoneedles on sapphire (0001) revealed that a continuous ZnO layer coherently strained to the substrate grows first . On top of the existing continuous layer, aligned nanoneedles start to form as the growth proceeds further. A similar growth behavior appears to occur during the growth of the ZnO NRAs on Pt-coated Si substrates. The Ni/Au double layer that is deposited on the tip-ends of the ZnO nanorods shows a well-defined interface and the formation of a continuous layer. To further investigate the microstructure of the ZnO NRAs, TEM studies were carried out.
Figure 4a shows the change in resistance as a function of time with different O2 concentrations ranging from 1.4 to 500 ppm. Six cycles were successively recorded. As shown, the device recovery was reproducible for all O2 concentrations. The gas sensitivity (S) was estimated using the relationship,S = ((R − R0)/R0), where R0 is the initial resistance in the absence of O2 gas and R is the resistance measured in the presence of O2 gas. Figure 4b shows the sensitivities extracted from Fig. 4a as a function of O2 concentration. The sensitivity at an O2 concentration of 1.4 ppm is 0.15, which is similar to the values previously reported for oxygen sensors based on single ZnO nanorods . A linear relationship is obtained between sensitivity and O2 concentration in the O2 concentration range, as shown in Fig. 4b. The sensitivity of a semiconducting oxide is usually depicted as S = A[C] N + B, where A and B are constants and [C] is the concentration of the target gas or vapor . In the present study, the data fitting results in S = 0.0059 [C] + 0.323 for the NRA chemical sensor. R2 in the figure represents the quality of the curve fit. Figure 4c shows the dependence of resistance by successive increase in O2 concentration. The resistance quickly responds to the change in O2 concentration. The increased resistance to O2 again increases by exposure to more O2 concentration. This behavior further confirms that the fabricated sensor in this study can be used in the environment with dynamically changing O2 concentration.
In addition to the O2 sensing properties of the NRA sensor, its NO2 sensing properties were investigated. Figure 5a shows the sensing cycles of the NRA sensor measured at 1–5 ppm NO2. As shown, the sensor well responds to the introduction and removal of NO2 as low as 1 ppm. The sensitivity of the sensor to NO2 is summarized in Fig. 5b. The linear slope gives the equation of S = 0.018 [C] + 0.047. The inset of Fig. 5b displays the response and recovery times of the NRA sensor to NO2 gas of various concentrations. The response time is about 50 s and shows no considerable difference depending on NO2 concentration. In contrast, the recovery time prolongs from about 55 to 200 s with increasing NO2 concentration from 1 to 5 ppm. The prolonged recovery time with higher gas concentrations is often observed [29–31].
In case of n -type semiconductors like ZnO, oxidizing gas such as O2 or NO2 mainly act as an electron accepter in the surface reactions, and the width of electron depletion layers is widened, leading to an increased resistance of the sensors. O2 or NO2 molecules adsorbed on the surface of ZnO layers take electrons from them, eventually leading to surface depletion in ZnO. Conversely, the release of electrons occurs in desorption of O2 or NO2. This charge transfer accounts for the resistance change observed in the NRA sensor. The sensing results in this study demonstrate that the approach proposed in this study is promising for the fabrication of highly sensitive chemical sensors.
In summary, we have described a novel approach to chemical sensors based on aligned ZnO NRAs grown on Pt-coated Si substrates with a top–top electrode configuration. The O2 and NO2 sensing properties of the fabricated sensor showed both a high sensitivity and an excellent reproducibility during the repeated test cycles. The results show that the device proposed in this study is promising for use as a highly sensitive, reliable chemical sensor.
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008-521-D00177).