Hydrogen interaction with platinum and palladium metal-insulator-semiconductor devices

摘要

Hydrogen-sensitive Pd-SiO_2-Si and Pt-SiO_2-Si metal-insulator-semiconductor (MIS) devices have been studied in ultrahigh vacuum in the temperature range of 223-523 K. Adsorption/ absorption of hydrogen occurs at the metal surface, in the metal bulk, and at the metal-insulator interface. The sensor signal, caused by hydrogen adsorption at the interface, shows a logarithmic dependence on the applied hydrogen pressure. The Pt-MIS device, which is fully functional at atmospheric pressures, is sensitive to changes in hydrogen pressure down to the 10~(-12)-Torr scale. We propose that the interface adsorption follows a so-called Temkin isotherm with an interface heat of adsorption that varies with hydrogen coverage as ΔH_(i0)(1-aθ). The initial heat of adsorption ΔH_(i0) is determined to 0.78 eV/hydrogen atom. The adsorption potential at the external Pt surface is found to be 0.45 eV/hydrogen atom. These values were obtained by modeling the hydrogen interaction with the MIS devices and fitting the model to a number of experimental results. Also studies of Pd-based devices were performed and compared with Pt. The hydrogen adsorption on the metal surface, previously treated as a first-order process on Pd, is shown to follow a second-order process. Qualitatively the results from the Pd- and Pt-MIS devices agree. Quantitatively there are differences. The hydrogen sensitivity of the Pt-MIS device is only approximately one-third compared to that of the Pd-MIS structure. This agrees with the result that the concentration of available hydrogen adsorption sites at the Pt-SiO_2 interface is approximately 7 X 10~(17) m~(-2) whereas the concentrations of sites at the Pd-SiO_2 interface is roughly three times larger (2 X 10~(18) m~(-2)). An estimate of the size of the dipole moments (0.6-0.7 D) implies that the interface hydrogen atoms are strongly polarized. Differences are also observed in the microstructure of the metal films. Atomic force microscopy results show that the Pd surface reconstructs during H_2-O_2 exposures, while the Pt surface shows no such change at these temperatures.