On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity
Introduction
Ever since Seiyama et al. [1] discovered in 1962 that the electrical conductivity of ZnO could be dramatically changed by the presence of reactive gases in the air, there have been tremendous reports on the applications of semiconducting-metal oxides as gas sensors due to their small dimensions, low cost, and high compatibility with microelectronic processing. It was known that unintentionally doped ZnO always displayed n-type behavior and the responsible donors were usually identified as the O vacancy VO, the Zn interstitial ZnI, or defect complex [2], [3], [4]. Thus, when a ZnO gas sensor is exposed to oxidizing ambient for example, oxygen molecules are adsorbed on the surface of ZnO and then extract electrons from the surface, resulting in the increase of the depletion layer.
Semiconducting-metal-oxide gas sensors have been usually fabricated in the form of thin films, in which ceramic powders are screen printed on prefabricated electrodes, followed by firing at high temperature [5], [6], [7]. In thin-film gas sensors, only a small fraction of the species adsorbed near the grain boundaries is active in modifying the electrical transport properties. This causes the low sensitivity because of the limited surface-to-volume ratio, which is difficult to overcome.
Recently, one-dimensional (1D) semiconductor nanostructures including nanowires, nanotubes, and nanobelts have attracted considerable interest for their potential as the building blocks for fabricating various nanodevices. Due to the high surface-to-volume ratios and high crystallinity of these 1D nanostructures, their major application were first made on the chemical/gas sensors [8], [9], [10], [11]. Nanowire gas sensors have been fabricated using a variety of metal-oxide semiconductors (e.g., SnO2, ZnO, WO3, In2O3, V2O5, TiO2, etc.) as gas-sensing materials [12], [13], [14], [15], [16], [17]. Recently, there are also many reports on the enhancement of gas response by catalyzing or doping of metal-oxide semiconductor nanowires with noble-metal nanoparticles such as Pd, Ru, or Au [18], [19], [20]. On the other hand, from the viewpoint of device structure, nanowire-based gas sensors were fabricated either by pick-and-place process of a single nanowire followed by making electrical contacts to individual nanowires using expensive and time consuming fabrication techniques such as electron-beam lithography [21] or by a series of processes involving synthesis, sonication, and dispersal of ensemble nanowires on another substrate with prefabricated electrodes [22]. Due to these complicated fabrication processes, their practical application of nanowire gas sensors is very limited.
Here we report on the fabrication of ZnO-nanowire gas sensor, whose fabrication process does not require any expensive and complicated fabrication techniques such as e-beam lithography. The gas-sensing capability of ZnO-nanowire gas sensor was demonstrated for NO2 gas and the responsible gas-sensing mechanism was suggested.
Section snippets
Experimental
Fig. 1(a) shows the schematic illustration for a network of ZnO nanowires floated above SiO2/Si substrate. For the area-selective growth of ZnO nanowires, 2 nm-thick Au catalyst film was patterned using the conventional photolithography. The typical gap between two Au layers was optimized to 5 μm, taking into account the length of ZnO nanowire (∼10 μm). Since the Au layers were used as catalysts during nanowire growth, we adopted a Pt contact electrode (∼300 nm) with Ti adhesion-promotion layer (∼20
Results and discussion
Fig. 2(a) shows X-ray diffraction (XRD) measurements on ZnO nanowires grown on the Pt electrode. The nanowires were found to be grown in c-axis orientation. Trace amounts of other diffraction peaks such as (1 0 1) and (1 0 2) do not mean that some ZnO nanowires were grown in those directions, but c-axis ZnO nanowires were so randomly orientated that these diffraction planes happen to meet Bragg's law. The high-resolution (HR) TEM image and its corresponding selected-area electron diffraction (SAED)
Conclusion
We have fabricated ZnO-nanowire gas sensors, in which nanowire bridges were self-assembled between two electrodes by a selective growth of ZnO nanowires on patterned Au catalyst layers. The device structure is very simple and efficient in that electrical contacts to nanowires are self-assembled and thus the fabrication processes do not involve any tedious and time consuming steps such as electron-beam lithography. The sensors demonstrated high gas response to NO2 down to sub-ppm level and fast
Acknowledgements
This work was supported by the KIST project (2E20680) and NSI-NCRC program (2N30690).
Myoung-Won Ahn received his MS from the Department of Materials Science and Engineering of Korea University, where he worked on the synthesis and sensor applications of semiconductor nanowires. Now he is researching thin-film solar cells in a start-up company.
References (29)
- et al.
The influence of defect drift in external electric field on green luminescence of ZnO single crystals
J. Lumin.
(2003) - et al.
Grain size effects on gas sensitivity of porous SnO2-based elements
Sens. Actuators B
(1991) - et al.
Preparation and characterization of SnO2 and WOx–SnO2 nanosized powders and thick films for gas sensing
Sens. Actuators B
(2001) - et al.
Gas-sensing properties of ordered mesoporous SnO2 and effects of coatings thereof
Sens. Actuators B
(2003) - et al.
H2S sensors based on tungsten oxide nanostructures
Sens. Actuators B
(2008) - et al.
Room-temperature H2S gas sensing at ppb level by single crystal In2O3 whiskers
Sens. Actuators B
(2008) - et al.
V2O5 nanofibres: novel gas sensors with extremely high sensitivity and selectivity to amines
Sens. Actuators B
(2005) - et al.
A room temperature nitric oxide sensor actualized from Ru-doped SnO2 nanowires
Sens. Actuators B
(2005) - et al.
Au nanoparticles enhance CO oxidation onto SnO2 nanobelt
Mater. Chem. Phys.
(2006) - et al.
Electrode effects on gas sensing properties of nanocrystalline Zinc oxide
Nanostruct. Mater.
(1998)
Sensing characteristics of tin-doped ZnO thin films as NO2 gas sensor
Sens. Actuators B
A new detector for gaseous components using semiconductive thin films
Anal. Chem.
Mechanisms behind green photoluminescence in ZnO phosphor powders
J. Appl. Phys.
Oxygen vacancies in ZnO
Appl. Phys. Lett.
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Myoung-Won Ahn received his MS from the Department of Materials Science and Engineering of Korea University, where he worked on the synthesis and sensor applications of semiconductor nanowires. Now he is researching thin-film solar cells in a start-up company.
Kyung-Soo Park is studying for his Ph.D. in the Department of Materials Science and Engineering of Korea University, where he worked on the synthesis of nanostructured materials and their applications into solar cells and gas/photo-sensors.
Jeong-Hoon Heo is studying for his MS in the School of Electrical Engineering of Korea University. His research focused on the synthesis of Si-based nanowires and their electrical characterizations.
Dong-Wan Kim received his Ph.D. from the Department of Materials Science and Engineering of Seoul National University in 2001. Now he is a senior research scientist in KIST. His research interests include synthesis of nanostructured materials and their applications into electrodes of Li-secondary batteries.
Kyoung Jin Choi received his Ph.D. from the Department of Materials Science and Engineering of Pohang University of Science and Technology (POSTECH) in 2001. Now he is a senior research scientist in KIST. His research interests include synthesis of semiconductor nanowires and their applications into gas and photo sensors.
Jae-Gwan Park received his Ph.D. from Alfred University in 1995. Now he is a principal research scientist in Korea Institute of Science and Technology (KIST) and director for Nano-Materials Research Center. His research interests include multi layer ceramic capacitors (MLCCs) and semiconductor nanowires.