Imaging Si Nanoclusters in Thin SiO_2 Layers by XTEM using Contrast Enhancing Decoration with Ge

The Multidot Nano-flash Memory suggested by Tiwari [APL69(1996)1232] is a promissing candidate for succeeding the common Floating Gate Flash Memory.

Its most challenging configurational feature is a layer of insulated Si nanoclusters (NCs) within the oxide of a MOS-like structure.

Here, we present experimental evidence that the theoretical concept predicting the self-organization of delta-layers of Si NCs at ion irradiated interfaces is valid (cf. Heinig [APA77(2003)17]).

In this approach of ``bottom-up" structuring, unconventionally, a 15nm thin buried SiO_2 layer, which is enclosed by a 50nm poly-Si capping layer and the Si substrate, is irradiated with Si ions. Ion impact drives the system to a state far from thermodynamic equilibrium, i.e. the local composition of the target is modified to a degree unattainable in common processes. A region of SiO_x (x<2) -- where x is a function of depth -- is formed which is not stable.

During annealing, the system relaxes towards equilibrium, i.e. phase separation (via spinodal decomposition and nucleation) sets in. Within a certain time window of annealing, the structure of the system matches with a structure similar to the Multidot Memory device, the principal character of which is a 2D layer of Si nanoclusters of (d~3nm) which is embedded in a 3D SiO_2 matrix at a distance of ~3nm from the Si substrate.

The experimental handicap that tiny Si NCs (d<3nm) which are embedded in SiO_2 are not visible in common XTEM is resolved by a novel method which applies Ge as contrast enhancing element in TEM studies of tiny Si NCs.