Inverse Ostwald Ripening and Self-Organization of Nanoclusters due to Ion Irradiation

Inverse Ostwald Ripening and Self-Organization of Nanoclusters due to Ion Irradiation

Heinig, K.-H.; Schmidt, B.; Strobel, M.; Bernas, H.

Under ion irradiation collisional mixing competes with phase separation if the irradiated
solid consists of immiscible components. If a component is a chemical compound,
there is another competition between collisional forced chemical dissociation of the
compound and its thermally activated re-formation. Especially at interfaces between
immiscible components, irradiation processes far from thermodynamical equilibrium
may lead to unexpected phenomena. If the formation of nanoclusters (NCs) occurs
during ion implantation, the phase separation caused by ion implantation induced
supersaturation can be superimposed by phenomena caused by collisional mixing.
In this contribution it will be studied how collisional mixing during high-fluence ion
implantation affects NC synthesis and how ion irradiation through a layer of NCs
modifies their size and size distribution. Inverse Ostwald ripening of NCs will be
predicted theoretically and by kinetic lattice Monte-Carlo simulations. The
mathematical treatment of the competition between irradiation-induced detachment
of atoms from clusters and their thermally activated diffusion leads to a Gibbs-
Thomson relation with modified parameters. The predictions have been confirmed by
experimental studies of the evolution of Au NCs in SiO2 irradiated by MeV ions. The
unusual behavior results from an effective negative capillary length, which will be
shown to be the reason for inverse Ostwald ripening. Another unexpected
phenomenon to be addressed is self-organization of NCs in a delta-layer parallel to
the Si/SiO2 interface. Such delta-layers were found when the damage level at the
interface was of the order of 1...3 dpa. It will be discussed that the origin of the delta-
layer of NCs can be assigned to two different mechanisms: (i) The negative interface
energy due to collisional mixing gives rise to the formation of tiny clusters of substrate
material in front of the interface, which promotes heteronucleation of the implanted
impurities. (ii) Collisional mixing in the SiO2 produces diffusing oxygen, which may be
consumed by the Si/SiO2 interface. A thin layer parallel to the interface becomes
denuded of diffusing oxygen, which results in a strong pile up of Si excess. This Si
excess promotes heteronucleation too. Independent of the dominating mechanism of
self-organization of a delta-layer of NCs, its location in SiO2 close to the
SiO2/Si interface makes it interesting for non-volatile memory application.

Keywords: Nanostructures; ion implantation; Ostwald ripening; nucleation; ion beam mixing; theory; kinetic lattice Monte-Carlo simulation; nonvolatile memory device

  • Contribution to external collection
    Mat. Res. Soc. Proc. vol. 647(2001) O14.6

Publ.-Id: 3660