Nanomaterials for Sensor Device ApplicationsView this Special Issue
UV Enhanced Oxygen Response Resistance Ratio of ZnO Prepared by Thermally Oxidized Zn on Sapphire Substrate
ZnO thin film was fabricated by thermally oxidized Zn at 600°C for 1 h. A surface containing nanostructured dumbbell and lines was observed by scanning electron microscope (SEM). The ZnO resistor device was formed after the following Ti/Au metallization. The device resistance was characterized at different oxygen pressure environment in the dark and under ultraviolet (UV) light illumination coming from the mercury lamp with a short pass filter. The resistance increases with the increase of oxygen pressure. The resistance decreases and response increases with the increase of light intensity. Models considering the barrier height variation caused by the adsorbed oxygen related species were used to explain these results. The UV light illumination technology shows an effective method to enhance the detection response for this ZnO resistor oxygen sensor.
In the past few years, the wide band gap material zinc oxide (ZnO) shows its abilities in many applications. Devices such as field emission device , surface acoustic wave device , photo diode , light emitting diodes , solar cells , and gas sensors [6–8] were studied. In the gas sensing applications, nanostructured ZnO with high surface-volume-ratio properties shows the promotive performance [9, 10].
In studies of sensor performance, higher operation temperature may improve the sensor response performance in some certain conditions [10, 11]. However, as device operated in the high temperature, response variation and device degradation can be observed also [11, 12].
As the sensing property for some oxide-based sensors changes under light illumination  and the ZnO is a wide band gap photodetector, the conductivity varies under UV light illumination [14–18]. As the oxidation of Zn metal in atmosphere to form the ZnO thin film is a simple method to achieve ZnO thin film [19–21] it is interesting to study such ZnO thin film sensing behaviour under UV illumination. In our previous study, less response time was characterized for such ZnO oxygen (O2) gas sensing characteristics under UV light illumination. In this paper, we study the effect of sensing response resistance ratio difference for ZnO O2 sensor under UV light illumination.
2. Materials and Methods
In the fabrication of ZnO, the Zn thin film was deposited first on c-plane sapphire substrate by thermal evaporation method from Zn chunk (5N, Tanaka) in vacuum chamber. The as-deposited Zn film with film thickness 510 nm shows gray-white color. After Zn deposition, the sample was then transferred to a 600°C furnace slowly in one hour to achieve the ZnO structure. The measured film thickness becomes 716 nm. With standard photolithography lift-off process, the Ti/Au (20 nm/300 nm) was formed on the film. Followed by a 20 min, 150°C thermal process in nitrogen environment, the ohmic contact was formed and the ZnO resistor structure was fabricated. The surface morphology of the film was studied using scanning electron microscopy (SEM, Hitachi S-4300N). The crystalline structure was obtained by X-ray diffraction (XRD, Bruker D8) with Cu Kα radiation. For the resistance measurement, the ZnO resistor device was put in a vacuum chamber with a diffusion pump. Ion gauge and Pirani gauge were used for the background gas sensing and pressure control. After pumping the vacuum system with background pressure less than 10−4 Torr. O2 gas was then introduced to the chamber with different pressure values. With proper valve control, the pressure reached a stable value in less than 30 s. In the O2 extracting process, the pressure reached less than 10−4 Torr in around 120 s. The resistance of the device was taken in Keithley 2400 multimeter. The film thickness was analyzed by Tencor Alpha Step 500. In the UV light illumination procedure, the dominant wavelength is 253.7 nm and comes from one 50 W mercury lamp with a short pass filter. An intensity reductor was used to control the light intensities.
3. Results and Discussion
Figure 1 shows the surface morphology characterized by SEM for the ZnO film at (a) 10,000x and (b) 50,000x magnifications. The surface shows a smooth ball structure on bottom with diameter around 200 nm and some locally distributed nanolines can be observed.
The XRD spectrum of the ZnO film was shown in Figure 2. All the peaks correspond to the hexagonal ZnO structure (JCPDS 36-1451) and no obvious Zn correlated peak  can be observed. The intensified (002) and (101) peak can be observed also for other Zn thermally oxidized ZnO films [20, 21].
Figure 3 shows the resistance transients of ZnO resistor measured at different O2 pressures in darkness and under 54.5 μW cm−2 UV light illumination. And a symmetrical current-voltage (I-V) behavior in the dark (D) and with UV light illumination (L) was shown in the inset. From Figure 3, in the O2 introduced region (O2 on), the resistance increases slowly and reaches stable values. In the O2 evacuation process (O2 off), the resistance decreases slowly and reaches a certain value. The resistance increases as of O2 pressure increases.
Figure 4 shows the device resistance measured at different O2 pressures (a) in darkness (dark) and under UV light illumination with power densities (b) 10.5 μW cm−2 and (c) 54.5 μW cm−2, respectively. The resistance increases with the increase of O2 pressure. The resistance decreases with the introduction of UV light illumination. With more carriers in the structure after UV light illumination, resistance decrease for the UV light illuminated ZnO film can be expected. Although the resistance decrease may be a worse effect for the sensor operation, the extension of resistance ratio can be observed in Figures 4(b) and 4(c).
If we take the resistance at 0.1 Torr, that is, the minimum resistance as the referenced resistance in each condition, the relative response resistance ratio can be defined as where is the corresponding resistance at a certain pressure. Figure 5 shows the device resistance ratio measured at different O2 pressures in darkness (dark) and under UV light illumination with power densities 10.5 μW cm−2 and 54.4 μW/cm−2. High resistance ratio can be observed for device under UV illumination.
For the ZnO gas sensors in O2 environment without UV light illumination, a chemisorption mechanism on the sensor surface can be described as  where () and () represent the gas phase and adsorbed species, respectively. For the ZnO thin film resistor after the introduction of O2 gas, the adsorbed oxygen species may accumulate around the grain boundary and cause more electron to be trapped [23, 24]. Figure 6 shows the schematic band structure drawing for the ZnO resistor across the grain boundary. The , , , and represent the conduction band, valance band, Fermi level, and interface barrier height, respectively . For Figure 6(a), in case of low O2 pressure in the dark, the adsorbed oxygen species may cause barrier bending upward slightly by trapping electron. With higher O2 pressure, as shown in Figure 5(b), more adsorbed oxygen species cause the increasing of barrier height and the resistance increases. The behavior of resistance increases with O2 pressure increasing can be observed in Figure 6(a). The variant resistance range was controlled by the pass through ability (labeled “1”) of the electron in this case.
For device under UV illumination, as shown in Figures 4(b) and 4(c), the resistance decrease can be observed. With higher light intensity, more resistance reduction can be observed. Besides, the degree of resistance reduction is less for device operated at high O2 pressure under the same UV light intensity. As the UV photon energy (4.9 eV) is higher than the band gap of ZnO (3.3 eV), electron-hole pairs will be generated. Figures 6(c) and 6(d) show the photogenerated electron-hole pair (labeled “2”) around the grain boundary. With more carriers, resistance reduction can be expected. Besides, on the way to the electrode, hole accumulated in the grain boundary due to the band bending across the grain boundary. These holes may react with the negatively charged adsorbed oxygen species according to the following reaction : where implies that the reaction is under illumination. As a result of the above reaction, under the reaction with hole (labeled 3), the adsorbed oxygen species may desorb to oxygen molecules. Compared to the crystalline ZnO , as shown in the figure, the localization of hole in grain boundary in the polycrystalline ZnO is more efficient. Thus the reaction is much more efficient. The barrier height and depletion width will be reduced both in this case by the elimination of adsorbed oxygen species. In case of low O2 pressure, as shown in Figure 6(c), the adsorbed O2 species quantity may be small and the reduction of barrier is significant. More carriers can then go through this reduced barrier. The resistance reduction for device operated in low O2 pressure under UV light illumination is more efficient. In case of high O2 pressure, as shown in Figure 6(d), the barrier height may be reduced a little as the adsorbed oxygen species quantity is large.
For further understanding of the reproduced capability, the resistor was measured under stable 54.5 μW cm−2 UV light illumination at different O2 pressure levels. Figure 7 shows the resistance measured from low O2 pressure to high O2 pressure and back to low pressure conditions. At higher O2 pressure, with more adsorbed oxygen species, more recovery time is needed for the device to reach the original resistance value. From Figure 7, we also observed that the resistance increatment for the second low pressure (13 Torr) cycle is similar to the first cycle. This implies that this ZnO resistor can operate under stable UV light illumination. Although there may be some defect-related conduction for ZnO under light illumination, a repeated and stable operation can be observed in this resistor device. The UV-light illumination shows the ability in extending the response resistance ratio for the thermally oxidized ZnO gas sensor.
In conclusion, the ZnO resistor gas sensor device was fabricated by the oxidation of Zn metal on c-plane sapphire substrate. Device current-voltage behavior in the dark and under UV illuminations was studied. The resistance reduction and relative response increatment can be observed for device under UV illumination. Band bending and oxygen related gas species adsorption/desorption models with accumulated hole around grain boundary were used to explain the efficient resistance ratio variance. The UV light illumination shows an primitive efficient method to increase the resistance ratio of polycrystalline ZnO coming from the thermal oxidation of Zn metal.
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