posted on 2019-04-24, 00:00authored byAndrea N. D. Kolb, Nicolas Bernier, Eric Robin, Anass Benayad, Jean-Luc Rouvière, Chiara Sabbione, Françoise Hippert, Pierre Noé
The
outstanding properties of chalcogenide phase-change materials (PCMs)
led to their successful use in innovative resistive memory devices
where the material is switched between its amorphous and crystalline
phases. However, PCMs are easily oxidized at interfaces. Oxidation
is detrimental to device performances. In particular, it reduces the
data retention time since oxidized PCMs crystallize at a lower temperature
than nonoxidized ones. The aim of this study is to investigate how
oxidation affects the crystallization process of germanium telluride
(GeTe), a prototypical PCM. By using advanced scanning transmission
electron microscopy (STEM) techniques, including spatially resolved
correlations between composition maps measured by energy-dispersive
X-ray (EDX) spectroscopy and structural information deduced from electron
diffraction patterns and high-resolution X-ray photoelectron spectroscopy,
we obtained a thorough description of the local chemistry and structure
of an oxidized GeTe thin film, partly crystallized by heating an initially
amorphous film at ∼180 °C. Under an oxide layer consisting
of amorphous GeOx and TeOx, the upper part (∼30–40 nm thick)
of the film consists of segregated amorphous GeOx, crystalline GeTe, and, strikingly, pure Te crystallites.
The bottom part of the film, in which no oxygen has penetrated, stayed
amorphous. This study reveals why oxidation promotes crystallization
of GeTe through segregation of Te regions and heterogeneous nucleation.
These results explain why oxidation at the surface or interfaces reduces
the crystallization temperature of GeTe (by 50 °C with respect
to a nonoxidized material) and shed light on the major impact of interface
chemistry on the crystallization mechanism of PCMs used in resistive
memory devices.