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Ion Irradiation of Ge: From Sponge-Like Structures to Periodic Patterns

Böttger, R.; Ou, X.; Fritzsche, M.; Keller, A.; Heinig, K. H.; Bischoff, L.; Liedke, B.; Facsko, S.


Ion irradiation of Ge surfaces leads to a variety of different morphologies depending on the irradiation conditions. In the energy range of few MeV down to a few tens of keV swelling and the formation of sponge like structures are observed. When lowering the energy these porous structures turn into self-organized periodic surface patterns: at off-normal incidence angles well-known ripple patterns with wave vector parallel or perpendicular to the ion beam direction appear, whereas at normal incidence angles hexagonally ordered dot or hole patterns can be formed. The structure size of the patterns is in the range of 10 to 100 nm and, occa-sionally, a high degree of ordering is achieved.
On materials which turn amorphous during ion irradiation the formation of periodic patterns relies on at least two inter-playing processes: surface roughening due to local variation of ero-sion rate and smoothing via diffusional processes. In addition, atomic relocations on the sur-face and in the bulk resulting from the collision cascade have been identified as equally im-portant or even dominating. At the atomic level the creation of surface and bulk defects, sput-tering, and the influence of the ion beam on surface diffusion processes play a decisive role in the morphology evolution.
At high temperature, when amorphization of the Ge surface is prevented by recrystallization, novel surface patterns are developing during ion irradiation. Similar to the case of ion irradia-tions of crystalline metal surfaces a new instability appears on the crystalline Ge surface due to a Ehrlich-Schwoebel barrier for ascending ion induced vacancies. In this case, regular checkerboard patterns are evolving on the Ge (001) surface with structures oriented along the <100> direction.
Moreover, a new mechanism for pattern formation on Ge has been discovered recently: by polyatomic Bi ion irradiation or monoatomic Bi ion irradiation of hot Ge melt pools are in-duced at the Ge surface by the incident ions. These melt pools can also lead to a surface insta-bility and thus to the formation of periodic dot patterns at normal incidence.
We will present an overview of the different morphologies induced by ion irradiation on Ge surfaces and analyze the dominant formation mechanism.

Keywords: ion irradiation; surface patterning; vacancy diffusion

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    Workshop on "Particle - surface interactions: from surface analysis to materials processing" (PASI 2013), 03.-5.05.2013, Luxembourg, Luxembourg


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