Kinetics of hydrogen adsorption and absorption

Silicon diodes employing Pd as a contact (gate) on top of a thin intervening SiO2 layer on Si make sensitive detectors for molecular hydrogen. The response to hydrogen results from the population of Pd/SiO2 interfacial traps for atomic H. Response time is of the order of minutes of 30 degrees C for 100 ppm of H2, through the speed of response can be increased by orders of magnitude by sputter cleaning the Pd in ultrahigh vacuum (UHV). Two aspects of the problem of contamination by exposure to the atmosphere are reported: the time dependence of the loss of activity of a cleaned diode after removal from UHV, and the response characteristics of a contaminated diode. Comparison is made between the rates of filling and emptying of the interfacial traps, as measured by the diode signal, with the rate of filling and emptying of the bulk, as measured by the resistance of the Pd. A model based on quasi-equilibrium between H in the Pd bulk and at the Pd/SiO2 interface is proposed, and it is concluded that, for diodes with a contaminated Pd surface, the bulk and interface are in equilibrium throughout the filling and emptying processes, with adsorption and desorption being the rate-limiting steps