Spontaneous pattern formation on Ge induced by ion irradiation

Ion irradiation of Ge surfaces leads to a variety of morphologies depending on the ion beam parameters. In the energy range of a few hundreds down to a few tens of keV swelling and the formation of sponge like structures is observed [1]. 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 [2] or hole patterns can be formed [3]. The structure size of the patterns is in the range of 10 - 100 nm and, occasionally, a remarkable 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: ion induced surface roughening due to local variation of erosion rate and smoothing via diffusional processes [4]. In addition, atomic relocations on the surface and in the bulk resulting from the collision cascade have been identified as equally important or even dominant [5]. At the atomic level the creation of surface and bulk defects, sputtering, and the influence of the ion beam on kinetic processes in the surface play a decisive role in the morphology evolution.
At high temperature, when amorphization of the Ge surface is prevented, novel crystalline surface patterns are developing during ion irradiation. In this case, regular checkerboard patterns are evolving on the Ge (001) surface with structures oriented along the <100> direction [6]. Moreover, an additional mechanism for pattern formation on Ge has been discovered recently: by very heavy ion irradiation melt pools are induced at the Ge surface by the incident ions. These melt pools can also lead to a surface instability 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 briefly the dominant formation mechanism.

References
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