Field evaporation of silicon and field desorption of hydrogen from silicon surfaces

The field evaporation of silicon in ultrahigh vacuum and in hydrogen has been studied with the use of the pulsed-laser atom probe. Measurements of the ion yields of various field-evaporated and field-desorbed species were made as a function of applied voltage, laser power, hydrogen background pressure, and laser pulse rate. The results indicate that, in ultrahigh vacuum and above cryogenic temperatures, field evaporation of silicon is qualitatively the same as for metals. In hydrogen, however, the field-evaporation process is quite different from that of metals, with the rate-limiting step being the field-enhanced formation of surface hydrides. Field-desorbed H⁺ and H₂^_⁺ ions are shown to arise from a field-adsorbed binding state, and the voltage range where H₂^_⁺ dissociates to H⁺ is used to calibrate the electric field strength. The low-temperature evaporation field of silicon is estimated from this calibration to be 3.3-3.6 V/{}, which is considerably higher than the currently accepted value of 2.0 V/{}. Field-desorbed H₃^_⁺ ions are detected only when oxide contamination is present on the silicon surface. If we assume that the mechanism of H₃^_⁺ formation is the same as that proposed for metals, this observation suggests that weakly bound, chemisorbed hydrogen atoms exist on silicon only in the presence of surface contamination.