Hydrogen Interaction with Platinum and Palladium Metal Insulator Semiconductor devices
2005 (English)In: Journal of Applied Physics, ISSN 0021-8979, Vol. 98, no 1, 14505-14514 p.Article in journal (Refereed) Published
Hydrogen-sensitivePd–SiO2–Si and Pt–SiO2–Si metal–insulator–semiconductor (MIS) devices have been studied inultrahigh vacuum in the temperature range of 223–523 K. Adsorption/absorption ofhydrogen occurs at the metal surface, in the metal bulk,and at the metal–insulator interface. The sensor signal, caused byhydrogen adsorption at the interface, shows a logarithmic dependence onthe applied hydrogen pressure. The Pt-MIS device, which is fullyfunctional at atmospheric pressures, is sensitive to changes in hydrogenpressure down to the 10–12-Torr scale. We propose that theinterface adsorption follows a so-called Temkin isotherm with an interfaceheat of adsorption that varies with hydrogen coverage as Hi0(1–a).The initial heat of adsorption Hi0 is determined to 0.78 eV/hydrogenatom. The adsorption potential at the external Pt surface isfound to be 0.45 eV/hydrogen atom. These values were obtained bymodeling the hydrogen interaction with the MIS devices and fittingthe model to a number of experimental results. Also studiesof Pd-based devices were performed and compared with Pt. Thehydrogen adsorption on the metal surface, previously treated as afirst-order process on Pd, is shown to follow a second-orderprocess. Qualitatively the results from the Pd- and Pt-MIS devicesagree. Quantitatively there are differences. The hydrogen sensitivity of thePt-MIS device is only approximately one-third compared to that ofthe Pd-MIS structure. This agrees with the result that theconcentration of available hydrogen adsorption sites at the Pt–SiO2 interfaceis approximately 7×1017 m–2 whereas the concentrations of sites at thePd–SiO2 interface is roughly three times larger (2×1018 m–2). An estimateof the size of the dipole moments (0.6–0.7 D) implies thatthe interface hydrogen atoms are strongly polarized. Differences are alsoobserved in the microstructure of the metal films. Atomic forcemicroscopy results show that the Pd surface reconstructs during H2–O2exposures, while the Pt surface shows no such change atthese temperatures.
Place, publisher, year, edition, pages
2005. Vol. 98, no 1, 14505-14514 p.
palladium, platinum, silicon compounds, silicon, hydrogen, elemental semiconductors, MIS devices, gas sensors, atomic force microscopy, adsorption, surface reconstruction, heat of adsorption, crystal microstructure
IdentifiersURN: urn:nbn:se:liu:diva-13383DOI: 10.1063/1.1953866OAI: oai:DiVA.org:liu-13383DiVA: diva2:20563