Phase Change Memory by Andrea Redaelli
Author:Andrea Redaelli
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham
In crystallized C-doped GeTe films, the C atoms rejected from the crystalline GeTe phase must form a carbon phase, which is expected to be located at the grain boundaries of polycrystalline GeTe. This carbon segregation should limit the grain growth after crystallization. This is what is observed by X-ray diffraction studies, as shown in Fig. 6.31. The crystallite size measured at room temperature after heating to 450 °C is found to decrease as the C concentration increases (grain refinement). This result is consistent with that observed in C-doped Ge2Sb2Te5 films (see, for instance, [177] where, in addition, amorphous carbon clusters could be observed in high-resolution transmission electron microscopy images of crystallized films). The localization of the GeNx phase in crystallized N-doped GeTe films is not established. However, a reduction of the crystallite size is also observed as the N concentration increases (Fig. 6.31, see also [71, 128]). Thus, one can expect that the GeNx phase is segregated at the GeTe grain boundaries. Similar conclusions are reached in the case of N-doped Ge2Sb2Te5 [58, 19, 160, and references therein]. In particular, the simulations in [19] show that insertion of N in the atomic form is energetically costly in the crystalline Ge2Sb2Te5 phase, which explains why N atoms are expelled at crystallization. Note that a few N2 molecules could remain inserted in crystalline Ge2Sb2Te5 [19]. Finally, the effect of N on the crystallization kinetics of Ge2Sb2Te5 has been recently studied by simulation [90].
Fig. 6.31Room temperature X-ray diffraction (λ = 0.1542 nm) pattern of (a) C- and (b) N-doped crystallized GeTe films with different C and N contents. All films have been previously heated to 450 °C. The theoretical positions of the diffraction peaks corresponding to rhombohedral (rh- GeTe) and cubic (c-GeTe) phases are indicated by vertical lines. The GeTe crystalline phase evolves from rhombohedral to cubic as the C or N doping level increases. (c) and (d): Change in crystallite size as a function of C or N concentration. The crystallite size was deduced by measuring the full width at half maximum (FWHM) of the (012) X-ray diffraction Bragg peak (2θ = 30°) and by calculating the diffractive coherent domain (DCD) using Scherrer’s law (Courtesy of F. Fillot)
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