Evidence of Self-Aligned Si Nanocluster delta-Layers in Buried SiO₂ Films Induced by Ion Irradiation

Nanoparticles embedded in a host matrix are systems with a high internal structural gradient which exhibit exciting electronic, optical, and/or magnetic properties. With regard to applications, the position of the nanoparticles, their size and their size distribution have to be well defined which has been a severe challenge for materials scientists. "Bottom-up" approaches are promising alternatives to the conventional "top-down". Usually, they are driven by self-organization mechanisms of matter.
Aiming at novel non-volatile flash memories [1], we have studied the synthesis of Si nanoclusters (NCs) in a thin SiO₂ layer which is embedded between the (100)-Si substrate and a poly-Si capping layer. For this application, Si NCs of equal size are required which are located in the buried oxide layer at a given distance from the Si-SiO₂ interface. In a previous work [2] we presented a theoretical concept which suggests that such a Si multi-dot structure can be achieved in a two-step process. At first, the layer stack poly-Si/SiO₂/(100)Si is irradiated with Si ions of medium energy (50keV) which leads to ion mixing of the Si-SiO₂ interfaces. During annealing, phase separation takes place in the course of which the Si-SiO₂ interfaces recover rapidly. Excess Si of the tails of the mixing profiles remains in the oxide, precipitates, and – in a self-organizing manner – grows up to a Si NC delta-layer which is ~3nm apart from the Si-SiO₂. Because of the very low mass contrast of Si NCs to the surrounding SiO₂ and their tininess, the Si NC delta-layer structure is outside the visibility limit of common XTEM.

In this contribution, we present for the first time experimental evidence which proves the mechanism of Si NC delta-layer formation addressed above. For this purpose, the sample structure portrayed previously was modified. In order to enhance the mass contrast of the Si NCs we "decorate" the Si NCs with Ge. A 5nm thin Ge layer was inserted in between the capping and the oxide layer. In the course of annealing, Ge diffuses through the oxide and attaches to the growing Si precipitates due to the energetically favourable Si-Ge bond. Differently from delta-layer of pure Si NCs, the Si_xGe_1-x NC delta-layer is indeed observable in XTEM because of the considerably higher mass contrast to the surrounding SiO₂. Without the presence of Si NCs, which act as seeds to which Ge monomers attach, no NC delta-layer would be visible at the position of interest. Consequently, the existence of Si NCs which align in a collective manner at a distinct distance of ~3nm from the Si channel is successfully proven.

This work was supported by the GROWTH Program of the European Community (GRD1-2000-25619).

[1] S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe, and K. Chan, "A Silicon nanocrystals based memory", Appl. Phys. Lett. 68, 1377 (1996).
[2] K.-H. Heinig, T. Müller, B. Schmidt, M. Strobel, and W. Möller, "Interfaces under ion irradiation: growth and taming of nanostructures", Appl. Phys. A 77, 17 (2003).