Investigation of WO₃-based metal-insulator-semiconductor Schottky diodes for hydrogen sensing

Abstract

Hydrogen is one option for global clean energy carrier in the future, and relevant new processes and technologies have been increasing over the years. Hydrogen will be used in both industrial and non-industrial environments and thus sensor for detecting hydrogen leakage is essential in these new applications. Among various solid-state hydrogen sensors, Schottky diode based on Metal-Insulator-Semiconductor (MIS) structure shows advantages of high stability, short response and recovery times, easy fabrication and low cost. However, there is still plenty of room for its sensitivity improvement. Besides, study on its sensitivity-voltage relation is very limited. Therefore, the main purpose of this research is to investigate new insulator materials based on WO3 as well as surface-modification techniques for increasing the hydrogen sensitivity of the MIS diode. Additionally, the sensitivity-voltage characteristics of the MIS diode are examined by a new technique and a new method for extracting the device parameters of the MIS diode is also proposed. Firstly, a comparative study of Schottky-diode hydrogen sensors based on Pd/WO3/Si and Pd/WO3/ZnO/Si structures is presented. Analysis of their I-V characteristics and dynamic response under different hydrogen concentrations and temperatures indicates that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10-times higher sensitivity, and shorter response/recovery times (105 s and 25 s, respectively) for 10,000-ppm H2/air at 150 ℃. Then, a semi-analytical technique for extracting the barrier height, ideality factor, series and shunt resistances of Schottky diode with high accuracy and consistency is proposed. Its application on Schottky-diode hydrogen sensor with a structure of Pd/WO3/SiC reveals excellent agreement between the extracted voltage boundaries (for exponential current-voltage relation) and the corresponding turning points on measured current-voltage curve under different temperatures and hydrogen concentrations. The average mean-squared error of the model data vs. experimental data is 0.371, more than 5 times smaller than that of traditional methods based on least-squares linear regression. Furthermore, the voltage dependence of sensitivity for Schottky-diode gas sensor is theoretically studied and experimentally verified. The study is based on the forward current-voltage (I-V) characteristics of the device and benefits from its power exponent parameter (V) = [d(ln I)]/[d(ln V)]. This analytical method is verified by using a Pd/WO3/SiC diode under exposure to hydrogen gas with different concentrations at 150 °C and 225 °C. Based on the proposed method, the parameters (barrier-height change, maximum sensitivity and corresponding bias voltage) of the sensor can be easily extracted and show excellent consistency with those obtained by the conventional method. Moreover, an investigation on the incorporation of two different kinds of high- dielectrics (HfO2 and Ta2O5) in the insulator of Pd-WO3-SiC Schottky diode is also presented. Upon exposure to 10,000 ppm H2/air, the diodes based on WHfO and WTaO show a maximum sensitivity of 89 and 147 respectively, both higher than that (31) of the control sample with WO3. From the kinetics analysis, it is demonstrated that more hydrogen atoms are accumulated at the Pd/WHfO and Pd/WTaO interfaces than their Pd/WO3 counterpart due to larger enthalpy change after hydrogen adsorption at the Pd/insulator interface, resulting in a greater barrier-height variation.

Lastly, a high-performance Pd/WO3/SiC Schottky-diode hydrogen sensor is fabricated by using fluorine plasma treatment on the WO3 film. From electrical measurements under various hydrogen concentrations and temperatures, the plasma-treated sensor exhibits a maximum barrier-height change of 279 meV and a gas sensitivity of more than 30,000, which is 30 times higher than that of the untreated sensor.