As-deposited Versus Melt-quenched GST

Atomistic simulations on phase-change materials have focused on melt-quenched (MQ) samples, where a liquid sample is cooled rapidly. Experimental samples are often deposited on a substrate, and the structure is likely to differ. We have developed a method to generate an as deposited (AD) sample with 648 atoms at 300 K [25] and have compared the results with those for a 460-atom MQ sample, refined as described above with respect to XRD and XPS data [26]. 

The simulations were carried out at 300 K, beginning with a random template of 36 atoms in an area of 27.61 x 27.61 A (layer thickness 1.4 A). These atoms were fixed during deposition to minimize relaxation effects near the base of the system. 17 layers of 36 atoms were then deposited on the sample and allowed to relax for 5-10 ps each. The vertical box dimension was adjusted for each layer to minimize the interaction with the slab replica (vacuum region 10 A). Deposition took a total of 91 ps, and the result is shown in Fig. 17.11. There is limited mobility at 300 K, but some atoms mix with layers deposited previously, and the lower coordination of Te enhances its concentration at the surface. The final vertical dimension of the 648-atom sample is 15 % larger than in the bulk. 

The model AD system was then prepared by releasing the 32 template atoms and decreasing the vertical box size (19 steps at 300 K) to obtain a cubic supercell (size 27.61 A, density 0.0308 atoms/A ). The process lasted 67 ps in steps of 3-4ps, and the system was equilibrated for 34 ps before data collection (25 ps). The pressure was small (2.9 1.8 kbar) at the final (experimental) density, and the final structure was optimized at 0 K (Fig. 17.1 Ic-d). The structure of the MQ sample [26] does not correspond to a DF energy minimum, so we performed a simulation at 300 K using the same functional (PBEsol [27]) and atomic density as for AD. Initialization (5 ps, 300 K) was followed by data collection (35 ps). The MQ structure is 11 meV/atom more stable than AD and 58 meV/atom less stable than the rock salt structure. 

The stmcture factors (g) for the AD and MQ samples are compared with experiment in Fig. 17.12. The curves for AD and MQ are strikingly similar, and the agreement with the experiment is very good. We note only that the calculated heights for the first two peaks (2.1 and 3.3 A ) are somewhat low, and the shape of the third peak differs slightly. The calculated structures are from DF/MD simulations at 300 K, and deviations from experiment are caused in part by the approximations in the xc functional [25], 

The electronic densities of states (DQS) in AD and MQ are very similar, with band gaps of 0.2-0.3 eV at the Fermi energy. The DQS profiles are in satisfactory agreement with XPS data [19], the most significant difference being near the DOS minimum around -10 eV between two ct-bands. 

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