MECHANISM OF HYDROGEN SENSING BY PLATINUM SILICON-OXIDE SILICON MIS TUNNELING DIODES

The mechanism of hydrogen sensing by platinum (Pt)/silicon oxide/silicon (Si) MIS tunneling diodes has been studied by measurements of the current-voltage (I-V) and conductance-voltage (G-V) characteristics. The n-Si based diodes show good rectification behavior in air but become ohmic-like after the introduction of hydrogen, whereas the p-Si based diodes are ohmic in air and become rectifiable after the introduction of hydrogen. From a detailed analysis of the voltage shifts of the I-V curves caused by hydrogen, we conclude that the shifts are caused by three factors: (i) the decrease in the effective Pt workfunction, (ii) the internal field-dependent movement of hydrogen ions formed in the silicon oxide layer from hydrogen atoms, and (iii) the formation of interface states through the reaction between hydrogen ions and the Si/silicon oxide interface. Due to factor (i), the barrier height in the n-Si is lowered, while that in the p-Si is increased. The shift caused by factor (i) is very rapid, the time constant being less than a few seconds. Factor (ii) causes the shift in the bias voltage at which the constant forward current flows with time after switching from a zero (or a reverse) bias to a forward bias. For the n-Si diodes, hydrogen ions are located near the Pt/silicon oxide interface under a reverse bias, and they move toward the Si/silicon oxide interface by switching to a forward bias. This movement of hydrogen ions causes the shift of the Si band edges toward the positive potential, and consequently the barrier height in the n-Si is lowered. This factor causes the voltage shift with a time constant of about 10 s. The formation of interface states by hydrogen (factor iii) is evident from an increase in the conductance. The density of the interface states continues to increase for more than 1 h after the introduction of hydrogen, causing a slow voltage shift of the I-V curve.