Surface Energy Driven Cubic-to-Hexagonal Grain Growth of Ge2Sb2Te5 Thin Film

Phase change memory (PCM) is a promising nonvolatile memory to reform current commercial computing system. Inhibiting face-centered cubic (f-) to hexagonal (h-) phase transition of Ge2Sb2Te5 (GST) thin film is essential for realizing high-density, high-speed, and low-power PCM. Although the atomic configurations of f- and h-lattices of GST alloy and the transition mechanisms have been extensively studied, the real transition process should be more complex than previous explanations, e.g. vacancy-ordering model for f-to-h transition. In this study, dynamic crystallization procedure of GST thin film was directly characterized by in situ heating transmission electron microscopy. We reveal that the equilibrium to h-phase is more like an abnormal grain growth process driven by surface energy anisotropy. More specifically, [0001]-oriented h-grains with the lowest surface energy grow much faster by consuming surrounding small grains, no matter what the crystallographic reconfigurations would be on the frontier grain-growth boundaries. We argue the widely accepted vacancy-ordering mechanism may not be indispensable for the large-scale f-to-h grain growth procedure. The real-time observations in this work contribute to a more comprehensive understanding of the crystallization behavior of GST thin film and can be essential for guiding its optimization to achieve high-performance PCM applications.

into {111} planes achieves and forms non-atomic layers in f-GST resembling the Van der Waals interaction gaps in h-GST, the system energy is as small as the equilibrium h-phase 17 . In view of the similar configuration and minor energy discrepancy between the 100% VOC-and h-GST lattices, a non-diffusion controlled slide of the building blocks was proposed to understand the f-to-h transformation 18 . Analogous research achievement stated that a shearing martensitic transformation from {200} planes of f-GST to {1013} planes of h-GST should be energetically favorable during f-to-h transition 19,20 . In addition, the discovery of twin crystals consisted by one f-and one h-grain led to an "epitaxial growth model" to interpret the structure evolution manner as f-GST approaching the h-phase 21,22 . One can find that previous studies inclined to utilize transformation between similarly structured f-and h-lattices (often from specific crystal orientations under static observation) to conjecture the dynamic atomic rearrangements for the whole f-to-h transition, which would inevitably be neither comprehensive nor precise enough.
In this report, in situ heating transmission electron microscopy (TEM) was utilized to characterize the dynamic crystallization procedure of GST thin films. We note previous literature 23 mainly concentrated on the phase transformation procedure and electronic structure of GST films upon in situ annealing, while no vacancy ordering process or h-grain growth mechanism was discussed. Here, we reveal that the rapid growth behavior of h-grains for GST thin film resembles an "abnormal" type 24,25 , which is driven by surface energy anisotropy. In contrast to the "normal" case in which grains get larger in a uniform manner, the abnormal growth of h-grains can be characterized by a subset of h-grains (mainly [0001]-oriented) growing bigger at a high rate and at the expense of their multifarious neighboring (small) grains. Such swift expansion of the big h-grain was usually named as the "growth-dominated crystallization" for f-to-h transition of GST 16,21 , no matter what crystallographic configurations the small grains would have. We also speculate that the vacancy-ordering process into {111} planes 11, 12, 17 may or may not occur in every f-grain, or to say, it is not an indispensable way for f-grains evolving into h-ones especially in the growth period. The adjustments by sliding the building blocks between similar f-and h-atomic configurations 17,19,21 would be more likely to involve the incubation of h-seeds from the f-matrix. The present scenarios may offer a more comprehensive perspective to understand the phase transition physics of this key material, and be essential for optimizing GST-based commercialized phase change materials to boost the performances of high density PCM device.

Results
In situ heating crystallization of Ge 2 Sb 2 Te 5 thin film. Figure 1 exhibits in situ heating crystallization process of GST thin film at different temperatures in TEM. The as-deposited GST thin film (Fig. 1a) shows typical a-phase at room temperature. It crystallizes into f-phase with uniformly distributed (randomly oriented) nano-crystals (<~15 nm in average grain size) at 150 °C ( Fig. 1b) with calculated lattice parameter of a = 6.01 Å 16,26 . When temperature increases to 210 °C (Fig. 1c) and 270 °C (Fig. 1d), f-grains continuously grow larger as the average grain size reaching ~20 and ~30 nm, respectively. The corresponding selected area electron diffraction (SAED) rings in Fig. 1c,d are of a little discontinuity as compared to that in Fig. 1b, denoting the gradual enlargements of the f-grains at higher temperatures. The transient moment at 320 °C (Fig. 1e) shows that GST film has small grains (mixture of evenly distributed small f-and h-grains as also proved in Supplementary Fig. 1) bordering a large h-grain. The dominant h-grain quickly swallows (with ~6.7 nm/s growing speed) adjacent small grains like flood and grows into a larger one, as clearly recorded in Supplementary Movie 1 (with the observation area ~2 × 2 μm 2 ). The large h-grain with μm size, as illustrated from the SAED pattern in Fig. 1f, is of single-crystal type showing strong [0001]-oriented texture. Since we also found tiny h-grains can be incubated from f-matrix at pretty low temperature (~210 °C) ( Supplementary Fig. 2), it is reasonable to deduce that, under the circumstance of favorable energy level and similar atomic configuration [17][18][19][20][21][22] , some of them may act as seeds for quickly growing up into the dominant [0001]-oriented h-grains.
Unnecessity of vacancy-ordering mechanism at fast growth stage of hexagonal grain. In Fig. 2, we reveal the in situ heating vacancy aggregation on {111} planes in a [011]-oriented f-grain, and finally being swallowed by an adjacent large h-grain. At 150 °C (Fig. 2a), the observed grain shows typical f-lattice structure with randomly distributed vacancies. When temperature reaches 210 °C (Fig. 2b), some intersecting (parquet-like) defect layers 26 paralleling to {111} planes emerge, causing weak scattered streaks in the inserted corresponding fast-Fourier transform (FFT) pattern. These defect layers should be ascribed to the ordering of randomly distributed vacancies in f-lattice 27,28 . In the case of 250~290 °C (Fig. 2c,d), further vacancy ordering leads to gradually clearer and longer defect layers (marked by arrows) in the f-grain. The brighter super-lattice reflection spots in corresponding FFT pattern represent a long-period ordered structure separated by the defect layers. At this period, the f-grain corresponds to an incomplete vacancy-ordering status, and there are still plenty of (Ge/Sb) atoms resided in the van der Waals-like gaps 11 . At 310 °C (Fig. 2e), the f-grain becomes a little larger, while its high resolution transmission electron microscopy (HRTEM) and FFT images become a bit blurred, which may be originated from a slight grain rotation, resulting in the deviation of [011] zone axis from the incident direction of the electron beam.
As heating temperature increases to 320 °C (Fig. 2f), the [011]-oriented f-grain is about to be eaten by an adjacent big [0001]-oriented h-grain. It is worth pointing out that the non-diffusion controlled slide of building blocks between similar f-and h-atomic configurations requires parallelism between 〈111〉 f and 〈0001〉 h directions 17,18 . Nevertheless in current case the [011]-oriented f-grain deviates ~35.3° (inter-axial angle) with 〈111〉 f direction 29 (standing for 〈0001〉 h direction). Obviously, the non-diffusion controlled slide model is invalid here for the largely misaligned structure rearrangement.

Transient growth state on hexagonal grain boundary. On the boundary of a dominant
[0001]-oriented h-grain (see Fig. 3a and zooming image of region ① in Fig. 3b), there reside some small grains corresponding to selected regions ②~④ (see Fig. 3a and zooming images in Fig. 3c to e respectively), which was captured at room temperature from an annealed sample heated up to 320 °C in TEM. Through indexing, the small grains in regions ②~④ are [011]-oriented f-grain, [001]-oriented f-grain, and [ 5503]-oriented h-grain, respectively. In contrast to the vacancy-ordered f-grain (in Fig. 2f), f-grain in region ② has not undergone obvious vacancy-ordering process, but it still cannot escape being annexed by the dominant h-grain as heating continues. The complexities are also produced by f-grain in region ③ and h-grain in region ④. For the [001]-oriented f-grain in region ③ which is unparallel to the 〈0001〉 h direction, whether or not the vacancy-ordered layers exist, it could not be transformed into h-phase via a simple building-block sliding procedure. The [ 5503]-oriented h-grain in region ④ also misaligns with the dominant [0001]-oriented h-grain, and the calculated inter-axial angle between ⟨¯⟩ 5503 and 〈0001〉 orientations is about 35 6° (ref. 29). In addition, a well-resolved video in Supplementary Movie 2 (with the observation area ~20 × 20 nm 2 ) shows the quick expansion (in a few seconds) of a [0001]-oriented h-grain from the upper part by consuming a subjacent grain (without any grain-rotation). The consumed grain obviously does not belong to the vacancy-ordered type. These results further prove that vacancy-ordering into {111} planes of the f-grain may or may not happen especially at the stage of fast h-grain growth. We thus believe the VOC-to-h rearranging model is not enough to describe the f-to-h transition image, and it would be more or less only related to the stochastic (energetically favorable) h-nucleation from f-matrix.

Discussion
Given this, we summarize the crystallization process of GST thin film in Fig. 4. The a-GST (Fig. 4a) firstly crystallizes into small f-grains with randomly distributed vacancies at ~150 °C (Fig. 4b). Then, some randomly oriented VOC-grains and small h-grains are formed from the f-matrix at ~210 °C (Fig. 4c). By further heating up to 320 °C (Fig. 4d), a dominant (large) [0001]-oriented h-grain appears and grows up quickly to accomplish the f-to-h transformation. The enlarged sketch (Fig. 4e) illustrates the transient growth state on the boundary of the dominant h-grain. Since the concentration of adatoms on the grain boundary is a function of its curvature 30 , usually atoms from the convex side tend to increase their coordination number via migrating to the concave side so as to lower down the system free energy. This is also valid here when low-coordinated atoms in f-GST lattice with large portion of vacancies confront the compact h-lattice. They diffuse to the boundary and reconstruct into the high-coordinated configuration. The arrows marked in Fig. 4e denote the concave-to-convex expansion (growth) directions for the dominant h-grain.
In theory, the grain growth rate V can be expressed as: where M is the average grain boundary mobility and F is the combined driving force 25,31 . The latter can be qualitatively divided into three categories: surface energy, grain boundary energy, and free energy from f-to-h phase transformation.
To model the abnormal h-grain growth in f-GST thin film of thickness D (Fig. 5), we examine the case that an initial h-GST grain of radius r h is incubated from the f-GST matrix with uniform f-grain radius r f (Fig. 5a), and grows into a bigger one of radius r (Fig. 5b). The starting h-grain can be characterized by a surface energy per unit area Γ s , h and an interphase boundary energy between the h-and f-phases per unit area Γ f/h . The f-grains are  For the h-grain of radius r h to annex its surrounding f-grains contained in an area of π π − r r h 2 2 , the energy per unit volume of this region before the subsequent grain growth is: π π π π π π π π π π = Γ + − Γ where is the total number of f-grains per unit volume, π = A rD gb f is the grain boundary area associated with an average f-grain, and ΔG h and ΔG f are the free energies of formation per unit volume of h-and f-GST respectively. Apparently, ΔG h and ΔG f are both negative, and ΔG f > ΔG h . After transformation, the fast growth of the dominant h-grain achieves significant reduction of the total energy, and the energy at this time of this region can be described as: The driving force for the transformation from the higher energy state E B to the lower one E A can therefore be expressed as: SCIEntIFIC REPORTS | 7: 5915 | DOI:10.1038/s41598-017-06426-2

Conclusion
In summary, with the aid of in situ TEM heating technique, we describe a more comprehensive characterization of the crystallization process of GST thin film. We believe that vacancy ordering phenomenon in f-phase is just a specific fragment of the whole f-to-h transition process. It would be more or less related to the stochastic h-nucleation from f-phase occurred at energetically favorable areas. The abnormal growth of large h-grain at high temperature is mainly driven by surface energy anisotropy. The closest-packed (0001) atomic plane of h-GST grain has the smallest surface energy than those of diversely-oriented f-or h-grains. This driving force would be particularly prominent as the film thickness being greatly diminished and smaller than the average grain size. Thus no matter how complicated the grain boundary would be, the [0001]-oriented h-grain can achieve overall growth. The present results mainly concentrate on the h-grain growth stage, and further study on initially stochastic h-nucleation process will be a great help to more comprehensively understanding the crystallization mechanism of the GST alloy.

Methods
GST films (~15 nm in thickness) were directly deposited on TEM grids coated with ultra-thin carbon film at room temperature by sputtering GST alloy target. Amorphous SiO 2 film (~5 nm in thickness), as the anti-oxidation layer, was successively deposited on top of GST films. By using energy-dispersive spectroscopy equipped on the TEM and X-ray fluorescence spectroscopy, the average concentrations of Ge, Sb, and Te elements of the GST films were determined to be 21.4 at.%, 22.6 at.%, and 56.0 at.%. The microstructures of the GST films were characterized by bright-field TEM, SAED, and HRTEM via using JEOL 2100 F TEM under 200 kV. The in situ heating crystallization of a-GST film was carried out in a heating holder (Gatan 628) at heating rate of 10 °C/min. During the whole heating process, electron beam was shut off to avoid irradiation effects and only turned on for capturing images 35 .