Nested ZnO Nanorod Gas Sensors Grown on ALD Seed Layers Synthesized on Si Substrates and Glass Fibers
Authors: Lin Pengtao , Chen Xin , Zhang Kai , Baumgart Helmut 
Published: February 2020

In general sensor devices are used to measure the variability of physical quantities like temperature, pressure, voltage, pH, and others. Among the many kinds of sensors in different areas, gas sensors have been widely used and investigated for gas monitoring with applications especially in hazardous atmosphere detection utilizing the specific properties of gases, like rapid evaporation, invisibility, flammability, and toxicity. Metal oxide semiconductor gas sensors have attracted extensive attention for gas detection due to their low cost, simple design and production, short response time, wide detection range, and resistance to hard working condition.
Among various suitable semiconductor materials, ZnO has been widely investigated for gas sensing applications due to its wide and direct band gape of 3.37 eV, large exciton binding energy (60 meV), high mechanical and thermal stabilities, low cost, and good electrical conductivity. Because of their high surface-to-volume ratios, ZnO nanorods have been advantageously introduced into gas detection in a large variety of areas. Specifically, ZnO gas sensors have a high sensitivity for ethanol gas detection. To further enhance the sensitivity of ZnO nanorod gas sensors, researchers have been synthesizing ZnO nanorods with gold nanoparticle coatings on the surface. In this research, additional Al doped ZnO thin film surface coatings by ALD were used to improve the sensing performance of ZnO nanorod gas sensors. ALD Al doped ZnO (AZO) thin films have been well characterized featuring non-toxicity, low material cost, good thermal stability, high optical transmittance, and good electrical conductivity. [1, 2, 3]
In this study, a Savannah 100 cross-flow thermal ALD reactor was used for the deposition of the ZnO seed layer required for hydrothermal growth. The precursors of zinc and oxygen used in the ALD process for the synthesis of ZnO thin films were diethylzinc ((C2H5)2Zn) and DI water. The growth temperature of ZnO seed layer was maintained at 200 oC. The thickness of ZnO seed layer was controlled by the deposition cycles. After the ALD synthesis of the ZnO thin films, subsequently ZnO seed layers were prepared by annealing at 350 oC for 30 mins and alternatively with no annealing. The reaction solution for ZnO nanorods growth was prepared with dissolving hexahydrate (Zn(NO3)2
·6H2O) and hexamethylenetetramine ((CH2)6N4) in 50 ml DI water at room temperature. ZnO nanorods were synthesized with the hydrothermal growth method in the prepared solution by baking in an autoclave at 80 oC for 16 hours. Then the synthesized ZnO nanorods on glass fibers and silicon wafers were annealed at 150 oC for 30 mins. Finally, Al doped ZnO thin films were deposited on the surface of the ZnO nanorods by Atomic Layer Deposition. After the fabrication of ZnO nanorods gas sensors, the sensor devices were installed into the testing system for the detection of different ethanol gas concentrations at three different temperatures (room temperature, human body temperature (37oC), and high sensing performance temperature (150oC)).
For physical characterization X-ray diffraction (XRD) was performed to analyze the crystal structure of ZnO seed layers and ZnO nanorods and the final ALD AZO coating. Field emission scanning electron microscopy (FE-SEM) was used to inspect the morphology of ZnO nanorods and coated with Al doped ZnO thin films before and after annealing. The surface roughness was analyzed by atomic force microscopy (AFM).
For final testing, the ZnO gas sensor was contacted with two copper wires and sealed into a reaction chamber as shown in Figure 1(a). A Resistance Temperature Detector (RTD) was used to measure the temperature inside the testing chamber. The sensing test was operated at room temperature (20oC), human body temperature (37oC), and high sensing performance temperature (150oC). The sensitivities of ZnO nanorods grown on glass fibers and silicon substrates for ethanol detection were investigated with the help of an electrical circuit shown in Figure 1(b). The response of the ZnO gas sensor device to ethanol concentration was measured by the change in resistance of the ZnO nanorods synthesized either on glass fibers or on Si substrates. The resistance and corresponding current and voltage were recorded with a LabView program based on the CompactRio system from National Instrument Company. The program interface records the real-time values of the circuit current, voltage on reference resistor, voltage on ZnO gas sensor, resistance of ZnO gas sensor, and the temperature of the testing chamber.
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Figure 1