Spontaneous pattern formation on ion irradiated semiconductor surfaces


Spontaneous pattern formation on ion irradiated semiconductor surfaces

Facsko, S.; Ou, X.; Wang, X.; Hübner, R.; Grenzer, J.

Low energy ion irradiation of semiconductor surfaces induces the formation of periodic surface patterns under particular conditions. These nanostructured surfaces exhibit periodici- ties in the range of a few tens to hundreds of nanometers and are promising templates for producing nanostructured thin films [1]. During ion irradiation the surfaces are driven out of equilibrium by continuous creation of displacements in the sub-surface region. At room tem- perature (RT) the accumulation of created displacements leads to amorphization of the irradi- ated semiconductor surfaces. Under these conditions periodic ripple patterns with wave vec- tor parallel to the ion beam direction are observed frequently for ion irradiation at incidence angles between 50° and 70° to the surface normal [2]. At normal incidence dot or hole pat- terns with hexagonal symmetry are observed for specific semiconductors, i.e. GaSb [3], InSb, GaP, or for special irradiation conditions, e.g. Ga+ or Bi3+ irradiation of Ge [4, 5].
In Fig. 1 different patterns on ion irradiated Ge (001) surfaces are shown. Although the Ge (001) surface is thermodynamically stable at all temperature used in the experiments, ion irradiation induces a surface instability which is counterbalanced by surface smoothing via different relaxation mechanisms, e.g. surface diffusion, ion enhanced surface diffusion, sur- face viscous flow, etc. As a result a wavelength selection in the surface roughness manifests itself as a periodic surface pattern. For off-normal angle of incidence ripple patterns are
At higher temperatures than RT, however, point defects created by the displacements in the ion collision cascade can diffuse longer distances, thus, vacancies and interstitial recom- bine or diffuse to the surface more effectively. Eventually, at temperatures higher than the recrystallization temperature, defects in the sub-surface region are annealed or diffuse to the surface before a second ion creates new defects in the same area and the surface remains crystalline. However, the average density of surface vacancies and ad-atoms is much higher than the corresponding densities in thermal equilibrium resulting in a much higher entropy. In this regime, ion irradiation creates an excess of vacancies on the crystalline surface due to sputtering. Thus, the surfaces morphology is determined primarily by vacancy kinetics alt- hough the kinetics of ad-atoms also play an important role.
In this contribution we present investigations of the evolution of Ge surfaces with dif- ferent surface orientation irradiated at temperatures above the recrystallization temperature. The irradiations were done with 1 keV Ar+ ions at normal incidence at temperatures above 250°C which has been established to be the temperature at which the Ge surface remains crystalline even after prolonged irradiation. The samples were cut from epi-ready Ge wafers with (001) and (111) surface orientation. Irradiations were performed in a UHV chamber with a base pressure in the range of 10-8 mbar with a beam from a Kaufman ion source. During irradiation the chamber is flooded with Ar up to a pressure of 3x10-4 mbar. The flux was 1.7x1015 cm-2s-1 and the applied fluence was in the range of 1017 – 1019 cm-2.
The formation of these patterns on crystalline surfaces can be understood in analogy to the formation of 3D structures in homoepitaxy. In molecular beam epitaxy (MBE) the contin- uous deposition of atoms can lead to growth of self-organized 3D nanostructures [5]. One of the possible surface instability, which is responsible for the formation of islands or mounds is caused by the Ehrlich-Schwoebel (ES) barrier, i.e. an additional diffusion barrier for ad- atoms to cross terrace steps. Due to this effect the arriving atoms are trapped on a terraces and can again nucleate to form new terraces.
The same mechanism is also active on ion irradiated surfaces when the temperatures is above the recrystallization temperature. In this case bulk defects are dynamically annealed and amorphization is prevented. Now, ion sputtering is creating vacancies on the crystalline surface and the surfaces morphology is determined by vacancy kinetics. The diffusion of va- cancies is also biased by the ES barrier like the diffusion of ad-atoms. Consequently, the 3D growth turns into 3D erosion. The resulting structures are inverse pyramids which are grow- ing into the surface. The symmetry of these patterns is given by the crystal symmetry. In Fig. 3 zooms of AFM images and the 2D slope distributions of the surface patterns on Ge (001) and Ge (111) are shown, respectively. The detailed facet analysis of the patterns by the 2D slope distribution reveals that on Ge (001) {105} facets with a polar angle of 11° exhibiting a four-fold symmetry are formed, whereas on Ge (111) {356} facets with a polar angle of 15° are formed with a three-fold symmetry. These facets are not know to be thermodynamically stable facets in growth conditions. The {105} facets have only been observed in heteroepi- taxy, where they are stabilize by strain due to the lattice mismatch. In the case of ion erosion no strain is expected [8]. Hence, it can be concluded that these are non-equilibrium facets which are determined by the kinetics of vacancies induce by ion irradiation.
For the description of the pattern formation and evolution in reverse epitaxy a continuum equation can be used which combines the effects of ion irradiation and effective diffusion
currents due to the ES barrier on the crystalline surface. For normal incidence irradiation it is know that smoothing mechanisms dominate thus we can omit an instability term induced by the curvature dependent sputtering or ion induced mass redistribution [9]. By choosing the adequate ES barrier induced surface currents and including also a conserved Kardar-Parisi- Zhang term a remarkable qualitative agreement to the experiments is achieved for both sur- face orientations. Ge (001) and Ge (111), respectively (see Fig. 4) [7].

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Keywords: ion induced nanopatterns

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