Nanoscale Observation and Analysis of Damage Formation and Annealing Processes in Ion Beam Interactions with Surfaces

Abstract

For controlling ion beam processes at an atomic level, it is necessary to study the interaction between energetic ions and solid surfaces. This thesis aims to reveal the ion bombardment effects on surfaces and the process of annealing the damage caused by ion impacts. The formation, structure and extinction of surface defects created at high temperature were observed with a Variable Temperature Scanning Tunneling Microscope (VT-STM). A Si(111) 7x7 surface was irradiated with Xe ions and single ion impact traces of about 20 Å diameter were clearly observed with atomic resolution. In the range from 1 to 5 keV, the average size of the trace did not depend on the impact energy. When the sample was annealed at 400℃, the vacancies created in the subsurface by the impact started to diffuse toward the surface and appear on the surface, but the interstitial atoms generated together with the vacancies remained in the bulk. At 600℃, vacancy clusters were formed, whose size corresponded to the number of vacancies created near the surfaces. At 650℃, the interstitial atoms diffused and recombined with surface vacancies and the size of the vacancy cluster decreased with annealing time. When a Si(111) surface was irradiated with Ar cluster ions at 8 keV, the traces of a cluster ion impact showed a crater shape, with a diameter of about 80 Å. This indicates that cluster impact processes are quite different from a summation of separate monomer impacts. When the surface was annealed at 600℃ after irradiation, the outer rim of the crater has disappeared and the hemispherical damage in the target was removed, but the hole at the center of crater remained. On Highly Oriented Pyrolitic Graphite (HOPG) surfaces irradiated with carbon cluster of up to 70 atoms, large hills were observed. The impact site diameters were found to be proportional to cluster size for clusters of up to 10 atoms and increase discontinuously for cluster sizes above 10 atoms. This can be explained by considering that small affected areas overlap. This indicates that non-linear multiple collision effects occur only when a local area is instantaneously bombarded by more than 10 atoms. In order to reveal the role of ion bombardment during film formation, ion bombardment effects at each stage of film formation were investigated. After annealing of a Si(111) sample at 400℃, with Ge atoms deposited to a few A, the formation of many islands of Ge on that surface was observed. In order to physically change the shape of Ge islands by ion impacts, it is necessary to irradiate the surface so that the ratio of ions to deposited atoms is above 1/10. At this ion dose, the total number of defects is higher than that of deposited Ge atoms. When Ge islands on the Si(111) surface after Xe ion irradiation were annealed at 400℃, many small islands, vacancy clusters, and complicated steps were created. After annealing at 600℃, the islands either combined with the steps or became larger. The complex surface structure observed after Xe ion irradiation and thermal annealing can be caused by the appearance and surface migration of vacancies that were formed by Xe ion impacts. The Xe ion impacts can have a large effect on the film formation if samples are annealed. When the Si(111) surface was irradiated with Ar cluster ion after Ge islands formation, the large craters were found. This result indicates that Ar cluster ion impacts can physically change the shape of Ge islands with low ion dose. The cluster ion impacts can also have a large effect on the film formation even if samples are not annealed. Thus, the damage formation and annealing processes in ion beam interactions with surfaces can be analyzed with a VT-STM. The ion bombardment effects on surfaces and annealing process of surface defects are revealed at an atomic level.