The field ionization characteristics of individual atomic planes

A detailed study of the field ionization characteristics of nine different planes of tungsten in the (001)-(011)-(111) standard triangle has been performed as function of tip temperature (T_T) between 11°K and 86°K, and local radius of curvature (rT). The measured helium ion current-voltage characteristic curve for each plane was found to consist of two distinct regimes. The first regime was linear on a log-log plot, and the slope varied from 27 to 41. A modified version of Gomer's model for the very low field ion current fitted the regime I data reasonably well. The second regime of each characteristic curve was quite complicated and exhibited several maxima and minima whose positions were functions of both T_T and crystallographic plane. A qualitative explanation for the behavior of the ion current in regime II was given in terms of a patch field model consisting of three dominant spatial regions on the surface of the field ion microscope specimen. In addition, the explanation also considered the role played by a lateral supply of gas atoms on the specimen's surface, and a slowly increasing field dependent radial supply function of gas atoms. It was also found that the probability of ionization was a strong function of both T_T, and crystallographic plane. An expression was derived for the temperature dependence of this effect which fitted the data for atomically smooth planes [e.g., the (011) plane]. Finally, the ion current from individual planes was proportional to a power of rT (at constant electric field) which varied between 2.3 and 2.9. This result was at variance with the existing theories of the supply function, and indicated that the shank of the specimen was a significant source of imaging gas atoms.