Self-organized surface patterns on Germanium by heavy cluster ions
Ion beam induced surface patterning of a new quality has been observed for the surface erosion of Ge by heavy Bi2+ und Bi32+ cluster ions in the HZDR in the end of 2009. The new quality concerns the excellent short range order and the large amplitude of the dot patterns, which exceeds previous results for elementary semiconductors. The implanted Bi is concentrated in the dots. Also, a qualitative step during cluster ion erosion is observed: While during perpendicular FIB irradiation with Bi+ ions the Ge surface layer obtains the well known spongy structure, for Bi32+ cluster with the same energy per atom (>10 keV) self-organized, crystalline dots, with a spacing below 50 nm und a height of 30 - 40 nm are obtained. Thus, this kind of self-structuring is dominated by a Bi cluster effect and not by single atom impacts. Contrary to the regular self-structuring of Ge with 3 -4 nm shallow holes by bombardment with 5 keV Ga single ions, models like Bradley-Harper or Kuramoto-Sivashinsky are not directly applicable in this case.
A first theoretical approach reveals that the energy density deposited by the collision cascade has to exceed a threshold to activate the novel self-structuring process. The threshold coincides with the needed energy to melt the Ge, i.e. each Bi cluster impact creates a Ge melt pool of some 100 nm3. A first model explains the Bi concentration in the dots by a Bi-Ge separation of the surface layer by repeated Bi segregation in the solidifying melt pools. The topographic swelling in den dots is caused by the 5% Ge volume increase due to melting. The Bi concentration depending melting temperature of Ge leads to a asymmetric solidification of the melt pool and, due to the volume change, to a Ge mass current towards high Bi concentration. The Bi cluster induced, extremely fast melting-solidification kinetics of smallest volumes are a unique model for GeTe based phase change memories.
Different Binm+ ion species are obtained from a pure bismuth liquid metal ion source. The source is mounted on the mass separated FIB column having a mass resolution of m/Δm ≈ 35. Both, the used beam defining and mass separation apertures had a diameter of 50 µm. Single and double charged monomer ions as well as cluster ions were used for the irradiation experiments. The acceleration voltage of 30 kV corresponds to energies of 7.5-20 keV/atom for the clusters and to 30 and 60 keV for single and double charged monomers, respectively. The irradiation was carried out in the fluence range from 5x1013 to 1x1017 ions/cm2. The focused ion beam on the Ge (001) surface was digitally scanned over square areas of 5x5 µm2 (256x256 pixel2) with a constant pixel dwell time of 20 µs and adapted repetition rates. Additionally, the angle of ion impact relative to the surface normal was adjusted between normal and nearly grazing incidence with an accuracy of 0.30°. The Bi+ monomer ion irradiation was carried out additionally on heated samples in the temperature range from RT to 600° where the samples were mounted on a Tectra BORALECTRIC® heating element.
Monomer vs. cluster beams
After irradiation with the Bi+ and Bi++ ions, at energies of 30 keV and 60 keV, respectively, the well known porous sponge-like Ge surface structures are obtained. But in contrast to that, for Bi2+, Bi3+ and Bi3++ dimer and trimer beams at normal incidence regular dot pattern with a pronounced medium-range order and an inter-dot distance in the order of 40 to 60 nm corresponding to a dot diameter of 30 to 50 nm have been observed. The formation of an ordered pattern starts alreday at fluences of 1015 ions/cm2, which is some orders of magnitude lower than the fluence for the surface pattern formation with single ions. The different behaviour are demonstrated below, where the available mass spectrum of the used ion species in the FIB and the resulting features of surface structures are shown.
The aspect ratio of dot height to dot diameter depends on the local ion energy deposition. For the Bi3++ trimer ions, having the highest energy of 20 keV/atom the highest aspect ration of nearly one has been observed. Therefor further investigation have been mostly focused to this cluster projectile.
An AFM image of a regular dot matrix after Bi3++ trimer irradiation and the corresponding surface profile are shown below confirming the high aspect ratio. The inset shows the result of a Fast Fourier Transformation (FFT) calculation, indicating the highly ordered dot ensemble. The comparison of secondary electron and backscattered electron SEM (not shown) imaging, giving topography or atomic number related contrast, respectively, indicate that the dots are enriched with Bi.
Dependence on the angle of incidence
The pattern evaluation induced by a 30 keV FIB irradiation with Bi3++ clusters during tilting the sample can be divided into four characteristic regions. In the angle range from 0 … 30° regular dot pattern were obtained). In the range from 35° to about 60° a nearly smooth surface was obtained. From 60° to 75° pronounced ripple structures with a wave length of about 150 nm were found, followed by a shingle or saw-tooth like structure at higher angles of about 85°. All ripple and shingle structures are oriented perpendicular to the beam direction. So the incoming beam looks to a projection of surface structures mostly to planes having a low incidence angle. A rotation of the pattern into parallel to the beam axis at high angles, as reported in other single ion experiments was not found. Nevertheless, the angle dependence of the structure evolution in the macroscopic scale looks like the behavior, which may be described with an adapted Bradley - Harper model.