Ion Bombardment of Single Crystals of Aluminum

Abstract

This paper presents the results of bombarding the (100), (110), and (111) surfaces of aluminum single crystals with a narrow beam of 8-kV argon ions. The crystals were tilted at the angles required to bring [110], [100], [112], and [111] directions parallel to the beam. The experiments were designed to study the effect of incident ion direction on ejection directions and etching rates. Ejection directions, as determined from the positions of deposits on hemispherical collectors, were found to be independent of incident ion direction for a given surface orientation. The quantity of material ejected along equivalent ejection directions was observed to decrease as the angle of deviation between the incident ion beam and the ejection direction increased. The principal ejection directions observed were [110]. Apparent [116] ejection directions were observed when (110) and (111) surfaces were bombarded. These were caused by an abrupt deflection of momentum from [110] directions of the crystal to directions close to [110] of elementary twins. These are formed by surface atoms in twin positions on (111) surfaces developed, in the case of the (110) surface, during bombardment. The [110] directions of an elementary twin are parallel to certain [114] directions of the crystal, and, had multilayer twins been formed, the resulting ejection would have been along apparent [114] directions as a result of Silsbee focusing. Ejection along apparent [116] directions was not observed from (100) surfaces presumably because predominant [100] surface grooves produced by bombardment could not contain (111) facets. Under these conditions [100] ejection was detected. Etching rates, as judged by the occurrence of matte spots, were least when the beam was parallel to the close-packed [110] directions and greatest when parallel to high index directions making large angles with the [110] directions. Low etching rates were also observed when the beam became parallel to [100] and [112] directions. Any anisotropy in surface migration rates was shown to be negligible in controlling etching rates under the conditions of these experiments.