Characterization of the Response of Magnetron Sputtered In2O3-x Sensors to NO2

Abstract

The response of resistive In₂O_3-x sensing devices was investigated as a function of the NO₂ concentration in different operative conditions. Sensing layers are 150 nm thick films manufactured by oxygen-free room temperature magnetron sputtering deposition. This technique allows for a facile and fast manufacturing process, at same time providing advantages in terms of gas sensing performances. The oxygen deficiency during growth provides high densities of oxygen vacancies, both on the surface, where they are favoring NO₂ absorption reactions, and in the bulk, where they act as donors. This n-type doping allows for conveniently lowering the thin film resistivity, thus avoiding the sophisticated electronic readout required in the case of very high resistance sensing layers. The semiconductor layer was characterized in terms of morphology, composition and electronic properties. The sensor baseline resistance is in the order of kilohms and exhibits remarkable performances with respect to gas sensitivity. The sensor response to NO₂ was studied experimentally both in oxygen-rich and oxygen-free atmospheres for different NO₂ concentrations and working temperatures. Experimental tests revealed a response of 32%/ppm at 10 ppm NO₂ and response times of approximately 2 min at an optimal working temperature of 200 °C. The obtained performance is in line with the requirements of a realistic application scenario, such as in plant condition monitoring.

Keywords: In2O3 gas sensor; MOX gas sensor; NO2 sensors; magnetron sputtering deposition.

Figure 1

Gas absorption mechanism in In₂O₃ nano-grained material.

Figure 2

Characterization of an In₂O_3−x 500 nm thick film deposited on a glass substrate: XPS spectra showing peaks for indium (a) and oxygen (b); SEM analysis at 10⁵ (c) and 3 × 10⁵ (d) magnification.

Figure 3

XRD spectrum of In₂O_3−x on a glass substrate: comparison with peaks in cubic centered form BCC c-In₂O₃ (JCPDS card number 06-0416).

Figure 4

(a)—optical transmission spectrum of the In₂O_3−x 500 nm thick sample; (b)—function ${\left(\alpha h\nu \right)}^{2}vs h\nu$ plot and linear fit used to determine the forbidden gap.

Figure 5

Measured baseline resistance of the In₂O_3−x sensor evaluated at different temperatures in air (blue line) and N₂ (red line) carrier gas.

Figure 6

Measured In₂O_3−x sensor response as a function of time when gas pulses (8 min long) consisting of mixtures of air and NO₂ (10, 5 and 2.5 ppm) are injected into the measurement chamber. Gas pulses are followed by recovery phases (8 min long) in pure dry synthetic air. Different colors represent responses obtained at the different working temperatures (see legend).

Figure 7

Measured In₂O_3−x sensor response as a function of time when gas pulses (8 min long) consisting of mixtures of N₂ and NO₂ (10, 5 and 2.5 ppm) are injected into the measurement chamber. Gas pulses are followed by recovery phases (8 min long) in pure dry N₂. Different colors represent responses obtained at the different working temperatures (see legend).

Figure 8

Responses to NO₂ as a function of temperature; air is the carrier gas.

Figure 9

Responses to NO₂ as a function of temperature; N₂ is the carrier gas.

Figure 10

Sensor response to NO₂ with air (solid line) and N₂ (dashed line) as carrier gas evaluated at the optimal working temperature of 200 °C.

Figure 11

Sensor response to 4000 ppm CO₂, 50 ppm CO and 2 ppm C₂H₆O compared with the response to 10 ppm NO₂, according to Equation (1); N₂ gas carrier, T ≈ 200 °C. Data evaluated by means of repeated transient gas measurements as for plots in Figure 3 and Figure 4.