Crystalline Ge surface nanopatterns by erosion with heavy Bi-dimer and trimer ions


Crystalline Ge surface nanopatterns by erosion with heavy Bi-dimer and trimer ions

Bischoff, L.; Heinig, K.-H.; Schmidt, B.; Facsko, S.; Pilz, W.

Two features of our heavy-ion-irradiation-induced surface patterns differ drastically from patterns formed on Ge with ions so far: The surface remains crystalline as proven by Raman measurements, and the dots and ripples heights equal their wavelengths (aspect ratio ~1).
The self-organisation of these very regular, high-amplitude dot and ripple patterns on (001)Ge has been found under bombardment with heavy ions of bismuth dimers and trimers. The Bi2 +, Bi3 + and Bi3 ++ ions are formed in a Liquid Metal Ion Source, they were accelerated, focused and scanned by a Focused Ion Beam system. 30 kV acceleration voltage and up to 1017 ions/cm2 have been used.
In the ion impact angle range from normal to ~30° incidence, hexagonal patterns of dots with ~30 nm diameter and ~40 nm height are found. Using a Bi monomer ion beam having the same atomic energy and fluence like the dimer and trimer beams, an amorphous Ge nanosponge is found. In the incidence range from 30° to 60° Bi3 ++ ions smoothen the Ge surface, whereas we found for 60° to 80° and more grazing incidence ripples and shingles perpendicular to the beam, respectively.
The Bi3 ++ ions are 16 times heavier than Ar+ ions, and still 5 times heavier than Xe+ ions. This high ion mass leads to a patterning mechanism different from the Bradley-Harper model, which becomes strikingly apparent by the crystalline Ge surface. An identified threshold of this new patterning mode could help to understand the mechanism: The ion-impact-induced deposition of energy per volume (as estimated by SRIM) must exceed a value which coincides with the energy needed for melting. Thus, Bi segregation during melt pool resolidification and the 5% volume difference between molten and solid Ge can cause the observed Bi separation and Ge patterning, respectively. A consistent, qualitative model will be discussed.

Keywords: Bi- LMIS; FIB; cluster beam; surface structures

  • Invited lecture (Conferences)
    21th International Conference on the Application of Accelerators in Research and Industry CAARI 2010, 08.-13.08.2010, Fort Worth, USA

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Publ.-Id: 13870