Bombarding a surface with energetic ions leads to the removal of surface material. This process of ion beam sputtering (IBS) is a common tool for surface modification. It is well known that optical, magnetic, tribological and other properties of thin films are strongly influenced by the roughness of surfaces and interfaces. Therefore, controlling the roughness is a main issue in a broad variety of technological applications. With IBS the roughness of solid surfaces can be modified on lateral scales of a few nanometres to micrometres or even further. Depending on the conditions, IBS can lead to surface smoothening or roughening, leading eventually to periodic patterns, i.e. ripple and hexagonal dot patterns. In 1988, over 20 years after the first observation of ripple-like patterns in 1962 by Navez et al., Bradley and Harper proposed a mechanism based on Sigmund’s sputtering theory to explain the experimentally observed features. In the current understanding, the interplay of roughening and smoothening processes leads to the formation of these self-organised patterns [1].

Here we report on in situ measurements of the surface roughness during IBS of GaSb surfaces under normal incidence of the ion beam. In particular we address the early time regime, which gives insight into the detailed mechanisms governing the dynamics at the beginning of pattern formation. In contrast to earlier investigations that were only able to probe a few intermediate steps by alternate manipulations between IBS and an analytical method e.g. AFM, we exploited an in situ setup taking advantage of contactless and non-destructive X-ray scattering techniques. During IBS, we used X-ray reflectivity to probe the surface roughness as well as the thickness of the sputtered layer, and grazing-incidence small-angle X-ray scattering (GISAXS) to investigate the lateral structure. For the first time we have observed a transition in the surface morphology: first, the GaSb surface was smoothened, followed by roughening and the formation of a dot pattern proving that the initial smoothening has the same physical origin as the relaxation mechanism for the formation of periodic structures.

Sputtering was carried out with Ar+ ions at an energy of 450 eV and an ion flux of 1x1015 cm-2s-1 in a compact high vacuum chamber equipped with a Kaufman type ion source mounted on the goniometer at beamline ID01. Reflectivity and GISAXS spectra were measured at an energy of 8 keV after sequential steps of sputtering, stopping the sputtering during the X-ray measurements.

Fig. 24: XRR at different fluences (thin lines are the simulation); inset: Evolution of the roughness (triangles) and of the transition layer (TL) thickness (experimental: dots; simulation: thick line).

Figure 24 shows reflectivity measurements of the surface in initial, untreated, and after sputtering states. Prior to IBS, the reflectivity reveals a rather rough surface with a thin surface layer that is probably due to the native oxide on top of the GaSb. After the first sputtering cycles up to a total fluence of 1.6x1017 cm-2 Ar+ ions, the reflectivity increases significantly, indicating a smoothening of the surface. Thereafter, the trend is reversed and the reflectivity decays again. The appearance of thickness fringes indicates the onset of pattern formation. The reflectivity data were analysed assuming a transition layer on top of a GaSb substrate that is characterised by a constant lower electron density, by its thickness T, and by introducing a surface roughness (inset). The GISAXS patterns in Figure 25 are in good agreement with the reflectivity: at the beginning the strong diffuse scattering is reduced, having its minimum at almost the same fluence resulting in an rms roughness of below 1 nm. At a fluence of 3x1014 cm-2, satellite peaks appear, demonstrating the existence of a periodic surface pattern. The position of these peaks is related to the mean interdot distance , whereas the number and width of the satellite peaks are related to the correlation length . While is almost constant and about 31 nm, increases by a factor of 2.

Fig. 25: GISAXS spectra (dashed lines are the simulation); inset: model used for simulation; curves are shifted for clarity.

 The experimental results have been compared with numerical calculations of the surface dynamics by a continuum Kuramoto-Sivashinsky equation that is based on the Bradley and Harper theory extended by non-linear terms (thick line in the inset of Figure 24).

In summary, we present in situ measurements of nanodot formation on an Ar-eroded GaSb surface using X-ray reflectivity and GISAXS. We show that the sputtering process leads to an initial smoothening of the surface, followed by a roughening process associated with the formation of a nanodot pattern. Both processes are the result of the same diffusion mechanism.



[1] W.L. Chan and E. Chason, J. Appl. Phys. 101, 121301 (2007).


Principal publication and authors

A. Keller (a), A. Biermanns (b), G. Carbone (c), J. Grenzer (a), S. Facsko (a), O. Plantevin (d), R. Gago (e), and T.H. Metzger (c), Applied physics letters 94, 193103 (2009).
(a) Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, Dresden (Germany)
(b) Festkörperphysik, Universität Siegen (Germany)
(c) ESRF
(d) Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Univ. Paris-Sud, Orsay (France)
(e) Instituto de Ciencia de Materiales de Madrid (Spain)