Sub-Ppm Gas Sensitivity and Electrical Properties of Highly (100)-Oriented SnO2 Thin Film on Sapphire Substrate
Authors: Ando Ki , Hukuda Naoki , Shinoda Hiroyuki , Mutsukura Nobuki 
Published: May 2020

Introduction

Applications of a gas sensor are not only using as environment gas sensing, but also come into use for analyzing volatile organic compound (VOC) gases in human breath. However, VOC gas concentration in human breath such as acetone and acetoin is ultra-low almost reaching 1 ppb. Therefore, further high sensitivity is required to use a metal oxide semiconductor type gas sensor which has advantages of simple and low cost in pathological diagnosis. In the field of gas sensors, crystallinity and electrical properties of metal oxide semiconductors have not been discussed deeply [1]. The metal oxide semiconductor that has less defects and less boundary such as a bulk single crystalline semiconductor provide lower electron density and higher electron mobility. It is assumed that these electrical properties caused by high crystallinity lead more significant resistance change by gas adsorption. In this study, we grew high crystallinity SnO2 (tin oxide) thin films on a sapphire substrate and investigated their crystallinity and electrical properties. And then, we selected the SnO2 thin films that had suitable characteristics for a gas sensor and demonstrated that our gas sensor was able to detect 0.5 ppm concentration gases.

Method

SnO2 thin films were grown on sapphire [α-Al2O3(0001)] substrates using our developed ultra-high vacuum RF magnetron sputtering system. The ultimate pressure of main chamber was less than 3 × 10-7 Pa. A 4-N grade SnO2 sintered compact was used as a sputtering target and the sputtering gas was 6-N grade argon gas (Ar) regulated 2 Pa. Substrate temperature while growing was ranged from room temperature (RT) to 950 °C. Electrical properties of SnO2 layers were measured by the van der Pauw method.
The structure of gas sensor prepared in the experiments is shown in fig. 1. Gold on titanium (Au/Ti) electrodes as ohmic contact were deposited on SnO2 surface in parallel with 3 mm distance. Nickel (Ni) lead wires were attached to both Au/Ti electrodes. The gas sensor was heated to working temperature in the tubular electric furnace ranged from RT to 700 °C. Control gas named “Air” that consisted of 80 % nitrogen (N2) gas and 20 % oxygen (O2) gas was regulated by the mass flow controllers and flowed to the gas sensor as shown in fig. 2. Experimental gas was hydrogen (H2), ethanol, or acetone regulated from 0.5 to 100 ppm by the mass flow controller. This experimental gas was flowed to the gas sensor after mixed with 20 % O2 gas and balanced by N2 gas. Total flow rate of the control and experimental gas was 50 sccm and each gas purity was higher than 6-N grade. 5 V was applied to the gas sensor and current was measured by the digital multi meter (DMM) to evaluate gas response of the sensor.

Results and Conclusions

Fig. 3 shows XRD spectrums of SnO2 layers by 2θ/ω scan mode. Sole SnO2 (200) peak observed with growth temperature of 400 °C or more excepting Al2O3 substrate peaks. Additionally, narrower full with half measure (FWHM) of SnO2 (200) rocking curve by ω scan mode observed with higher growth temperature. FWHMs of SnO2 (200) rocking curve were 32 and 8460 arcsec with growth temperature of 950 and 400 °C, respectively. These crystallinity was very high quality and comparable with the bulk SnO2 crystal [2][3]. Electrical conduction type of all SnO2 layers was n-type. In the range of higher growth temperature, electron mobility of SnO2 layers was also better (fig. 4) considered that SnO2 layers were highly (100)-oriented. However, at the same time, electron density was increased (fig. 5) seems to be oxygen desorption from the SnO2 layer.
In the case of using SnO2 layers that had higher electron density such as grown with 950 °C substrate temperature, the SnO2 layers had not shown sufficient gas response as a gas sensor caused by too high electron density. Therefore, we investigated gas response in detail using the SnO2 layer grown with 400 °C substrate temperature which had acceptable crystallinity and electron mobility and lowest electron density. We obtain current ratio between air and experimental gas condition as ΔI / I
air (ΔI = I
gas - I
air) to evaluate gas sensitivity. Fig. 6 shows relationships between working temperature and sensitivity of the gas sensor at 100 ppm H2 gas. The sensitivity was increased with working temperature. Additionally, the sensitivity of the gas sensor after annealed in 700 °C was significantly greater than that as grown. We considered that higher working temperature and 700 °C annealing affected to compensation for oxygen desorption of the SnO2 layer. Finally, we obtained gas response to 0.5 ppm concentration using the gas sensor with 550 °C working temperature after annealing as shown in fig. 7 and 8. To realize 1 ppb gas sensitivity, we will investigate compensating oxygen desorption by annealing for the SnO2 layer grown with 950 °C which has suitable crystallinity, high electron mobility, and too much electron density.

References

[1] Ananya Dey： “Semiconductor metal oxide gas sensors: A review”, Materials Science & Engineering B, Vol.229, pp. 206-217 (2018)
[2] Zbigniew Galazka, et al., Growth, characterization, and properties of bulk SnO2 single crystals, Phys. Status Solidi A, Vol.211, No.1, pp.66-73 (2014)
[3] Zhen Zhu, et al., Epitaxial relationships and optical properties of SnO2 films deposited on sapphire substrates, Applied Surface Science, Vol.257, No.7, pp.2516-2519 (2011)

Figure 1