Interaction of highly charged ions with ultrathin dielectric films.

摘要

The excitations occurring at a solid surface due to slow highly charged ion (HCI) impacts are interesting from the perspective of fundamental processes in atomic collisions and materials science. This thesis focuses on two questions: (1) How much HCI potential energy deposition is required to form permanent surface modifications?, (2) How does the presence of a thin dielectric surface film change the classical over-the-barrier picture for neutralization above a clean metal?;I describe a measurement of craters in thin dielectric films formed by XeQ+ (26 ≤ Q ≤ 44) projectiles. Tunnel junction devices with ion-irradiated barriers were used to amplify the effect of charge-dependent cratering through the exponential dependence of tunneling conductance on barrier thickness. Electrical conductance of a crater sigmac(Q) increased by four orders of magnitude (7.9 x 10 -4 muS to 6.1 muS) as Q increased, corresponding to crater depths ranging from 2 A to 11 A. According to a heated spike model, the energy required to produce the craters spans from 8 keV to 25 keV over the investigated charge states. Considering energy from pre-equilibrium nuclear and electronic stopping as well as neutralization, we find that at least (27 +/- 2) % of available projectile neutralization energy is deposited into the thin film during impact.;Additionally, an extension of the classical over-barrier model for HCI neutralization above dielectric covered metal surfaces is presented. The model is used to obtain the critical distance for the onset of neutralization above C60/Au(111), Al2O3/ Co, and LiF/Au(111) targets. The model predicts that for thin films with low electrical permittivity and positive electron affinity, the onset of neutralization can begin with the electrons in the metal, and at further ion-surface distances than for clean metals. The model describes three distinct over-the-barrier regimes of "vacuum limited" capture from the metal, "thin film" limited capture from the metal, and capture from the insulator. These regimes are detailed in terms of charge state, target material parameters and film thickness.