Nanostructures induced by highly charged ions R. Heller, R. Wilhelm, A. S. ElSaid, S. Facsko Institute for Ion Beam Physics and Materials Research
in cooperation with: group of F. Aumayr: W. Meissl, G. Kowarik, R. Ritter Institute of Applied Physics, Vienna Univ. of Technology Ch. Lemell Institute for Theoretical Physics, Vienna Univ. of Technology and group of G. Zschornack Institute for Applied Physics, TU Dresden Dreebit GmbH Funded by European ITS LEIF network RII3\#026015
Ion beams in material science Ion beams have been traditionally used for: Ion implantation: semiconductor industry Surface modification: sputtering, sputter deposition changing the surface properties Intel Material analysis: RBS, ERDA, with sputtering: SIMS, SNMS,... Where can highly charged ions be used with advantage? New degree of freedom: potential energy
Motivation Conventional particle irradiation HCI irradiation X 1 E kin 10 kev MeV X q E pot 0 E kin < 10 kev E pot up to 100 kev E kin One impact one feature! E kin Localized electronic excitation similar to high power laser excitation!
HCI surface interaction Interaction well understood in the frame work of the Classical Over Barrier Model (COB) Neutralization involves electron transfer from the surface to the projectile into Rydberg orbitals Æ hollow atom formation Xeq e Eim e γ e e Rapid relaxation of the HA in front of the surface leads to emission of numerous secondary electrons Electrons carry a major part of the ions initial potential energy into the surface and lead to permanent structures on a nm scale Institute for Ion Beam Physics and Materials research y René Heller y www.fzd.de y Mitglied der LeibnizGemeinschaft (soon HGF)
Experimental Irradiation experiments at Rossendorf Two Source Facility Highest charge states (q<45) at lowest kinetic energies (Ekin > 20 ev q) cafm characterization of the surface exsitu Institute for Ion Beam Physics and Materials research y René Heller y www.fzd.de y Mitglied der LeibnizGemeinschaft (soon HGF)
HCI induced surface nanostructures KBr CaF2 HOPG BaF2 10 µm Institute for Ion Beam Physics and Materials research y René Heller y www.fzd.de y Mitglied der LeibnizGemeinschaft (soon HGF)
Hillocks on CaF 2 induced by highly charged ions
Nano Hillocks on CaF 2 2.0 Xe 33, E kin = 2 kev q 1.0 Z (nm) 0.0
Nano Hillocks on CaF 2 Xe q q = 24 26 27 28 29 30 33 36 40 Hillock Volume (nm 3 ) 250 200 150 100 50 E kin = 10 q x kev E kin = 0.15 q x kev 0 5 10 15 20 25 30 35 Potential Energy (kev) Well defined threshold for hillock formation in potential energy Linear increase of hillock volume with pot. energy upon E crit. Slight shift of E crit. with increasing kinetic energy
Nano Hillocks on CaF 2 Hillock volume independent on kinetic energy over a wide range For highest E kin small shift towards lower hillock volume
Nano Hillocks on CaF 2 Electrons from HCI neutralization spread a major part of E pot into a small region close to the surface Potential energy threshold related to melting energy Local phase transition Xe q e e e e h h h h Inelastic thermal spike model (M. Toulemonde) e ~ 1 nm λ ~ 4 nm Detailed electron energy transport calculation A. S. ElSaid, R. Heller et al., PRL 100, 237601 (2008)
Pits on KBr induced by highly charged ions
Nano Pits on KBr 1.0 1.0 Xe 25, E kin = 40keV 0.5 Z (nm) 0.5 Z (nm) 0.0 0.0
Nano Pits on KBr Xe 5 (E pot = 0.16 kev) E kin = 4.0 kev potential energy Xe 10 (E pot = 0.80 kev) E kin = 4.0 kev Xe 34 (E pot = 23 kev) E kin = 4.0 kev fluence < 1 10 10 cm 2 single ion impact
Nano Pits on KBr Xe 34 (E pot = 23 kev) E kin = 1.0 kev kinetic energy Xe 34 (E pot = 23 kev) E kin = 40 kev Xe 34 (E pot = 23 kev) E kin = 153 kev fluence < 1 10 10 cm 2 single ion impact
Conditions for pit formation Formation of pits depends strongly on potential and kinetic energy A threshold for the formation of pits exists in potential and kinetic energy Formation of pits is a synergistic effect! kinetic energy (ev) 10 5 10 4 10 3 2 5 10 15 20 25 34 Xe pits no pits 10 1 10 2 10 3 10 4 10 5 potential energy (ev) S. Facsko et al., J. Phys. Cond Matt. 21 (2009)
Nano Pits on KBr E kin = 300 ev/amu Pit volume and corresponding sputter yield show a linear dependence on the ions potential energy. Pit depth is constant at one ML independent on E pot
Nano Pits on KBr The X center e K 0 K 0 K 0 e e e H F H F H F H F F H H and F center creation along the full electron trajectory Cluster of 3 Fcenters (Xcenter) created by multielectron impact in HCI neutralization Cluster of 3 Fcenters recombining with the surface by desorption of 3 K 0 atoms R. Heller, R. Wilhelm, S. Facsko, PRL 101, 096102 (2008)
High fluence irradiations Xe 3 pits no pits Φ 1 10 10 cm 2 10 5 q 2 5 10 15 20 25 34 Xe Xe 25 Kinetic energy (ev) 10 4 Φ = 2 10 13 cm 2 10 3 Φ = 1 10 11 cm 2 10 1 10 2 10 3 10 4 10 5 Potential energy (ev)
High fluence irradiations Xe 3 density (µm 2 ) 20 15 10 5 0 10 20 30 40 diameter (nm) Φ = 2 10 density (µm 2 ) 13 cm 2 Xe 3 Xe 3 100 50 0 0.2 0.4 0.6 0.8 1 depth (nm) Xe 25 Φ = 1 10 11 cm 2 density (µm 2 ) 60 40 20 0 10 20 30 40 diameter (nm) 150 Xe 25 Xe 25 density (µm 2 ) 100 3:1 50 0 0.2 0.4 0.6 0.8 1 depth (nm)
Etch pits on BaF 2 induced by highly charged ions
Etch Pits on BaF 2 Xe 24, E kin = 4.5keV q Xe 33, E kin = 4.5keV q Xe 36, E kin = 4.5keV q
Etch Pits on BaF 2 Same samples after etching for 5s with 1 vol.% solution of HNO 3 10 µm 10 µm Xe 24, E kin = 4.5keV q Xe 33, E kin = 4.5keV q Xe 36, E kin = 4.5keV q Potential energy create defects in the crystal!
Etch Pits on BaF 2 Planned experiments at HITRAP: Xe 54 and U 92 at 4 MeV/u and 500 kev/u A. S. ElSaid, R. Heller, S. Facsko and F. Aumayr, PRB (2010)
Scenario of surface modifications by HCI
Outlook The way to controlled single ion placement 1m Goal! Institute for Ion Beam Physics and Materials research y René Heller y www.fzd.de y Mitglied der LeibnizGemeinschaft (soon HGF)
Conclusion HCIs can initiate the formation of individual nano structures of different types on various insulators Structureformation depends on both the potential and (indirectly) on the kinetic energy of the ions Pit formation on KBr can be ascribed to a special kind of defect mediated desorption: X center formation The modified inelastic thermal spike model can successfully explain the hillock formation on CaF 2 Further investigation on structure formation and electron emission are in progress Thank you for your attention!