STRUCTURAL-CHANGE INDUCED NEAR SURFACES OF ALPHA-AL2O3 DURING ELECTRON-IRRADIATION

A structural change produced by electron irradiation near surfaces of alpha-Al2O3 was observed through high resolution electron microscopy a nd electron diffraction. Initially smooth surfaces become rough during TEM observations at 200 and 400 kV and holes appear with a prefered o rientation of growth. Planar defects are also produced near the surfac es; they appear as dark- or bright-lines parallel to the (001) plane w hen the specimen is observed along the [110] direction, and they are a lso visible along the [001] direction as a periodic modulation of brig ht dots. The defects give rise to streaks and extra spots in electron diffraction patterns along [1101 and [001], respectively. A formation mechanism and model of the defects were proposed; the defects are indu ced by inelastic scattering on Al plane parallel to (001) through rear rangement of Al ions and Al-vacancies. Three quaters of the atomic sit es of the Al plane are occupied by Al ions and one quater is vacant, w hile 2/3 of the sites are filled with Al ions in perfect alpha-Al2O3.