Self-assembled ordered nanostructures on Ge by cluster irradiation

Surface modification with ion beams is a well established technique to create self-organized regular patterns like ripples and dots [1,2]. The pattern can be controlled by the kind of ion species as well as by their energy, fluence and angle of incidence. Future applications in electronic or optoelectronic nanodevices are under discussion [3]. In this contribution we present a novel approach, the irradiation with focused dimer and trimer beams of heavy ions, in particular Bi₂ ⁺ and Bi₃ ⁺⁺. These clusters from a liquid metal ion source were mass separated in a CANION 31Mplus FIB system from Orsay Physics and focused onto a Ge surface. The acceleration voltage of 30 kV corresponds to energies of 10-15 keV/atom, fluences from 10¹⁵ to 10¹⁷ cluster/cm² were applied.
For normal incidence up to an angle of ~30° dot patterns with a pronounced short-range order have been found. The dots are crystalline (as confirmed by Raman measurements), enriched with Bi and have a diameter of 30 nm. The inter-dot distance is about 50 nm. A new quality of the dots is their large aspect ratio of ~1. Using the same fluence and energy/atom, irradiation with single Bi+ ions resulted in the well-known porous Ge surface. Therefore, this new kind of pattern should be caused by cluster effects, not by single ion impacts. The Bradley-Harper model is obviously not valid, in contrast to the fabrication of regular 3-4 nm deep holes in Ge by a 5 keV FIB irradiation with monomer Ga-ions [4]. According to a first analysis, the energy density deposited per volume by the cluster impact cascade must exceed a threshold value to form this new kind of surface pattern. The threshold energy deposited per Ge atom coincides with the heat per atom required for Ge melting. Thus, each cluster impact yields to a small melting pool of <1000nm³ volume. A model based on such pools explains the segregation of Bi into the dots. The Ge surface undulation is caused by a decrease of the Ge volume of 5% during melting. A Ge flux into the Bi rich region occurs due to the Bi concentration dependent Ge melt temperature.
In the range from 30° to 60° no structures occur. The surface becomes very smooth by the heavy cluster beam. Increasing the angle further leads to the formation of ripples perpendicular to the beam direction with a wavelength of about 100 nm and a height of 30 nm. Measurements with back scattered electrons reveal that the top of the ripples is Bi enriched. At still higher angles a transition from ripples to a shingle structure has been found, which are also perpendicular to the beam direction. A rotation into ripple pattern parallel to the beam has been not observed.
References: [1] R.M. Bradley and J.M.E. Harper, J. Vac. Sci Technol. A 6 (1988) 2390-2395. [2] S. Facsko, T. Dekorsy, C. Koerndt, C. Trappe, H. Kurz, A. Vogt, H.L. Hartnagel, Science 285 (1999) 1551-1553. [3] R. Gago et al. Phys. Rev. B 73 (2006) 155414-1-9. [4] Q. Wei et al., Adv. Mat. 21 (2009) 2865-2869.