Pattern Formation by Focused Polyatomic Ion Beam Irradiation on GaAs

Surface patterning by ion beams is a well-established technique to create self-organized regular surface patterns. The type of these patterns depends on the ion species, their energy, fluence, target temperature and angle of incidence. Emerging applications of these patterns are under discussion in particular for electronic, photonic and magnetic nano devices. The irradiation of different matter with focused monatomic ions like Ga or Bi is a well-known application. Here, a novel approach is presented, the use of heavy polyatomic Bi2+ and Bi3+ ions, promising high intensity sputtering and new kinds of surface morphology. The ultra-heavy Bi ions emitted from a liquid metal ion source (LMIS) in a mass separated FIB instrument are directed to the GaAs surface. Whereas the irradiation with monatomic Bi-ions at normal incidence creates a deep sputtering crater without any strong pattering or Ga precipitates on the crater bottom which is still known from Ga FIB irradiation. But the much heavier projectiles Bi2+ and Bi3+ lead also to a Ga droplet formation on the GaAs crater base. With increasing projectile mass, i.e. higher deposited energy in the stopping cascade of polyatomic ions more and also smaller Ga droplets are found. The phenomenon of surface evolution investigated for monatomic as well as heavy Au and Bi dimer and trimer ions is presented in dependence of the ion species, the energy, the angle of incidence as well as the target temperature in the range from RT up to 400°C.