Nanoscale mechanics of thermally crystallized GST thin film by in situ x-ray diffraction

Abstract

The thermal crystallization of Ge2Sb2Te5 (GST) thin film is investigated by in situ x-ray diffraction (XRD). The combination of several x-ray diffraction techniques (a) in-plane XRD, (b) out of plane XRD, (c) high resolution XRD on the substrate, and (d) 2D high energy XRD allows the characterization of the mechanical behavior of GST upon thermal crystallization. A new method is proposed for the evaluation of experimental stress vs strain dependence in thermally crystallized GST. Nanoscale strain, macroscale stress, and nucleation/crystallization are fully described by the correlation of these techniques. Upon crystallization, a progressive tensile stress build-up is observed. Concomitant stress build-up is also observed both in the in-plane and out of plane directions of the film. The 2D high energy XRD demonstrates a homogeneous nucleation process and a progressive crystallization of the GST composed partially of amorphous and crystalline parts in the film. The GST nanomechanics is then characterized at the nanoscale (crystallites scale) and at the macroscale (film scale). By plotting the stress vs strain and assuming a Poisson ratio of 0.28, the mixture of phase results in a Young modulus between 9 GPa and 37 GPa for amorphous and crystalline matrices, respectively. Intermediate states with a partial amorphous/crystalline ratio results in intermediate values of the Young modulus. Finally, cross correlation between all XRD techniques gives EGST ≈ 34 GPa and υGST ≈ 0.34 for fcc crystalline GST.