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(a) Crystallization temperature vs Ge content in a series of 100 nm thin films. Crystallization takes place at a threshold composition of . Just above the eutectic composition of , “sits” on a midrange composition plateau. (b) Ge precipitation temperature shows a distinct minimum near midrange of . The data in (a) and (b) were determined from scanning calorimetry (DSC) and (XRD) scans, e.g., see inset in (a). As-deposited amorphous films were annealed at rate in flowing He. (c) The threshold is clearly observed in Raman spectroscopy as a softening of the vibrational mode of Sb.
(a) Laser intensity sequence during the collection of Raman spectra for films. Raman spectra of (b) pure Sb, (c) eutectic composition , and (c) . Spectra are labeled by the sequence numbers indicated in (a), each with the duration of . The left-hand panels display data taken on increasing laser power, and the spectra on the right-hand side are taken at 10% intensity after the exposure to high intensity, as indicated in (a). The power level at which full crystallinity is recovered depends on Ge content. Insets highlight the altering of the crystalline phase occurring even at a relatively low laser power when Ge is present.
(a) Comparison of Ge–Sb Raman spectra for (a) and (b) taken at 100% power and with the power subsequently reduced to 10%. For 7% Ge, the difference in spectra is very slight. Mode softening with the increased laser intensity as shown in (c). For 51% Ge in (b), there is a prominent peak at corresponding to crystalline Ge. (d) As is reduced, the ratio of the Ge to the main Sb peak intensities decreases, while Ge mode “softens” at low (below eutectic). Redshifting of the Raman modes induced by Ge precipitation may be due in part to the residual stress in the films, (Ref. 9) which decreases upon further laser heating.
In situ scanning calorimetry of free standing 100 nm Ge–Sb films near endothermic melting. The 10% Ge concentration is at the edge of the threshold region, below which the film behaves as pure Sb. Slight shifts of the “dip” positions come from the variations in the -calibrations during different scans. All curves are shifted for clarity, with red line indicating the -axis region for magnified . For , other metastable Ge–Sb phases are a likely cause for the baseline shape change, and for the reduced signature of pure Sb melting.
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