  The nanometer-scale size-dependent electronic transport properties of
crystalline (c-) and amorphous (a-) germanium telluride (GeTe) ultrathin films
sandwiched by titanium nitride (TiN) electrodes are investigated using ab
initio molecular dynamics (AIMD), density functional theory (DFT), and Green's
function calculations. We find that a-GeTe ultrathin films scaled down to about
38 Angstrom (12 atomic layers) still shows a band gap and the electrical
conductance is mainly due to electron transport via intra-gap states. If the
ultrathin films are further scaled, the a-GeTe band gap disappears due to
overlap of the two metal induced gap states (MIGS) regions near the TiN
electrodes, leading to sharp increase of a-GeTe conductance and significant
decrease of c-GeTe/a-GeTe conductance ratio. The c-GeTe/a-GeTe conductance
ratio drops below one order of magnitude if the ultrathin films are scaled
below about 33 Angstrom, making it difficult to reliably perform read
operations in thin film based phase change memory devices. This overlap of the
MIGS regions sets up the ultimate scaling limit of phase change memory
technology. Our results suggest that the ultimate scaling limit can be pushed
to even smaller size, by using phase change material (PCM) with larger
amorphous phase band gap than a-GeTe.
