Sharp transition from ripple patterns to a flat surface for ion beam erosion of Si with simultaneous co-deposition of iron

K Zhang,H Hofsäss,M Brötzmann

doi:10.1063/1.4739843

Abstract

We investigate pattern formation on Si by sputter erosion under simultaneous co-deposition of Fe atoms, both at off-normal incidence, as function of the Fe surface coverage. The patterns obtained for 5 keV Xe ion irradiation at 30° incidence angle are analyzed with atomic force microscopy. Rutherford backscattering spectroscopy of the local steady state Fe content of the Fe-Si surface layer allows a quantitative correlation between pattern type and Fe coverage. With increasing Fe coverage the patterns change, starting from a flat surface at low coverage (&lt; 2×1015 Fe/cm2) over dot patterns (2-8×1015 Fe/cm2), ripples patterns (8-17×1015 Fe/cm2), pill bug structures (1.8×1016 Fe/cm2) and a rather flat surface with randomly distributed weak pits at high Fe coverage (&gt;1.8×1016 Fe/cm2). Our results confirm the observations by Macko et al. for 2 keV Kr ion irradiation of Si with Fe co-deposition. In particular, we also find a sharp transition from pronounced ripple patterns with large amplitude (rms roughness ∼ 18 nm) to a rather flat surface (rms roughness ∼ 0.5 nm). Within this transition regime, we also observe the formation of pill bug structures, i.e. individual small hillocks with a rippled structure on an otherwise rather flat surface. The transition occurs within a very narrow regime of the steady state Fe surface coverage between 1.7 and 1.8×1016 Fe/cm2, where the composition of the mixed Fe-Si surface layer of about 10 nm thickness reaches the stoichiometry of FeSi2. Phase separation towards amorphous iron silicide is assumed as the major contribution for the pattern formation at lower Fe coverage and the sharp transition from ripple patterns to a flat surface.

Highlights

The simultaneous co-deposition of atoms during ion beam erosion of surfaces maintaining a net erosion rate of the irradiated substrate has a tremendous influence on self-organized nanopattern formation
Macko et al have shown that pronounced dot and ripple pattern formation on Si under ion beam erosion with near normal ion incidence is governed by small amounts of co-deposited iron atoms incident at oblique incidence.[11]
transmission electron microscopy (TEM) analyses of Fe-Si surface layers prepared by surfactant sputtering using slightly different irradiation conditions compared to the present study reveal an amorphous several nm thick Fe-Si layer with Fe concentration up to about 20 at.% as determined with Rutherford backscattering spectroscopy (RBS).[2]

Summary

Introduction

The simultaneous co-deposition of atoms during ion beam erosion of surfaces maintaining a net erosion rate of the irradiated substrate has a tremendous influence on self-organized nanopattern formation. Macko et al have shown that pronounced dot and ripple pattern formation on Si under ion beam erosion with near normal ion incidence is governed by small amounts of co-deposited iron atoms incident at oblique incidence.[11] The manifold of patterns (holes, dots, chains of dots or ripples) and the pattern height and shape depend on the local co-deposition flux and the co-deposition direction of Fe atoms It was observed, that the direction of incidence of co-deposited surfactant atoms determines the pattern orientation and the shape of the patterns.[2,11,12] In accordance with

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