The effects of self-ion bombardment (30–500 keV) on the surface topography of single-crystal germanium

High-resolution scanning electron microscopy has been used to observe the effects of self-ion bombardment on the topography of germanium surfaces. Holes (mean separation 400 Å) appeared in the surface at doses above 2×1015 ions/cm2 (ion energy 50 keV). These enlarged with increasing ion dose and developed into a complex cell structure. This structure underwent no further gross change for doses above 1×1017 ions/cm2, where the mean cell diameter was ∼1200 Å. The surface preparation was found to have no influence on the appearance of this cellular structure. Some specimens were fractured after bombardment to observe a section of the bombarded surface. A porous layer of thickness 2.5 times the projected range (Rp) was observed at doses just below those where changes in surface topography were first observed. At the highest dose (4×1017 ions/cm2) the thickness of this layer was 6Rp. The cellular surface structure was observed at all ion energies chosen so long as the energy deposited in the surface by the ion beam was kept below 0.5 W cm−2. The observed effects can be qualitatively explained by the formation of large voids. When these intersect the surface the effects of sputter etching and redeposition combine to enlarge the cell structure. At surface loadings above 0.6 W cm−2 different topographies were observed, as a consequence of the surface temperature exceeding the amorphous/crystalline transition temperature during bombardment.