Helium bubbles ordering along the direction of low-energy He ions in silicon

A phenomenon of putting in order of helium bubbles which occur during low-energy (17 keV) implantation of He ions into silicon has been discovered. The investigation was carried out by transmission electron microscopy (TEM) on the silicon samples (Cz-Si, (100), p-type, 10 Ohm x cm), irradiated at room temperature in fluence range of 3 x 1016 - 4.5 x 1017 cm-2 and at different falling angles of ion beam with bombarded surface. Structural modifications in silicon along He ions direction were studied with a novel 'longitudinal section' TEM technique. It has been experimentally established that: a) amorphization of silicon, implanted by He+, occurred at a damage level more than 3 dpa and began in a region of peak concentration of radiation defects; b) helium bubbles are formed exclusively in an amorphous Si (a-Si) layer; c) gas porosity development in a-Si is accompanied by bubbles ordering as along an ion-beam direction as in a cross-sectional direction; d) TEM investigation showed the presence of ion tracks with a diameter near 1 nm and a linear density equal to 107 cm-1 in an a-Si layer containing helium bubbles. (author)