Ion Surface Charge Research Articles

Studies of the near-surface ionic space charge in silver halide crystals†

Abstract Two ongoing experiments dealing with ionic surface charge effects in AgCl and AgBr are described. The first consists of a mapping of the depth profile of the electrostatic potential in macroscopic single crystals, using as a probe the distribution of trace levels of radioactive, divalent 54Mn. The crystals are sectioned chemically after aging, with a depth resolution of 10 A, using the radiotracer 36C1 as a depth gauge. The resulting potential profiles agree with theoretical expectation. The effects of temperature, dopants, and surface orientation are now being investigated. The second experiment is a study of the effects of adsorbents (such as photographic dyes) and surface treatments (e.g., transient exposure to water or methanol) on the sub-surface space charge, as manifested through changes in the longitudinal conductance of evaporated thin films (about 4000 A) of AgBr. Rather large effects have been observed, sometimes corresponding to a change in the space charge contribution to the conduct...

Metal-oxide-metal tunneling junctions on Ta and Nb: Background conductivity resulting from different oxide barriers

Two different ways of preparing the oxide barrier in tunneling junctions on Ta and Nb yield different conductance-voltage characteristics. While a ’’dirty’’ preparation method with atmospheric air as an oxidation medium leads to strongly polarity-dependent but temperature-independent characteristics, we find rather symmetric but strongly voltage-dependent and temperature-dependent (hence ’’low-quality’’) characteristics when oxidation is performed in a pure oxygen dc plasma discharge. High or low quality of the junctions in the former and latter case, respectively, are also reflected in the superconducting characteristics. The results are tentatively interpreted in terms of band bending in the oxide on account of ionic surface charges in the dirty case.

Ion Induced Surface Alterations Due to Electronic Charge Exchange and Chemical Reactions

AbstractA general discussion of the phenomena that can occur during ion—surface encounters is presented and specific examples of charge exchange processes on LiF surfaces and chemical reactions of N+2 on metals are shown. During rare gas ion bombardment of LiF, ejection of F+ ions and excited Li atoms and the energy distributions of secondary F+ ions can be correlated with Auger primary ion—surface charge exchange transitions determined by the potential energy of the primary ion vacancy and electron promotions resulting from close atomic encounters. Nitridation of Al, Cu, Mo, and Ni to produce very thin nitride surface films is induced by 0–50 eV mass selected N+2 beams. The efficiency of nitridation, monitored by the KL2L2 nitrogen Auger electron signal, exhibits a large kinetic energy Ek dependence below ca. 30 eV. The threshold impact energy for nitridation varies for the different metals; it is nominally zero for Al and Ni, ∼ 4 eV for Mo, probably > 0 for Cu, and does not occur at any energy below 200 eV for Ag. That the metal electronic structure is a critical parameter is evidenced by the facts that different ion doses are required to produce similar nitride signal levels on various metals and that plots of nitride intensity versus Ek exhibit structure or at least different shapes in the low Ek region. A simple quantum mechanical model is used to simulate the nitridation reaction by delineation into four elementary steps: (1) neutralization of incoming N+2, (2) decomposition of N2 into 2N, (3) deexcitation and thermalization of N, and (4) chemical reaction proper.

The absolute positions of calculated energy bands

The absolute positions of calculated crystalline energy bands depend upon the choice of charge cluster used to generate the total crystalline charge for the coulomb potential calculation. The coulomb potential (and hence the bands) will be shifted from the Ewald limit by 2π times the trace of the second moment tensor of the charge cluster divided by three times the volume of the unit cell. Energy bands obtained from different calculational models can thus all be corrected to the same Ewald limit. Energy bands can also be aligned more realistically with those in a finite crystal by calculating the shift in the bands from the Ewald limit due to a free atom or ion surface charge density.