Phase Stability and Anisotropic Sublimation of Cubic Ge–Sb–Te Alloy

Article Cite This: J. Phys. Chem. C 2018, 122, 2968−2974

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Phase Stability and Anisotropic Sublimation of Cubic Ge−Sb−Te Alloy Observed by In Situ Transmission Electron Microscopy Katja Berlin and Achim Trampert* Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany

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S Supporting Information *

ABSTRACT: Phase stability and anisotropic sublimation dynamics of the cubic Ge−Sb−Te alloy have been investigated by in situ transmission electron microscopy (TEM). The starting point of the phase-transition study is an epitaxially aligned Ge1Sb2Te4 grain on a Si(111) substrate. Upon in situ heating, the cubic phase remains stable up to the sublimation point without a transition to the thermodynamically stable trigonal crystal structure which is attributed to Si diffusion into the Ge1Sb2Te4 grain. The sublimation process is made visible with atomic resolution. The anisotropic process leads to the formation of stable {111} facets via kink nucleation on stable steps and subsequent sublimation from those kink sites as predicated by the terracestep-kink model. Kink nucleation sites are identified and are in accordance with the broken-bond model approach.

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INTRODUCTION The ternary Ge−Sb−Te (GST) alloy has attracted great interest due toits broad application as a phase-change material in nonvolatile storage devices utilizing the fast and reversible-phase transformation between its amorphous and crystalline cubic phase.1−3 Highly intensive local heating as well as extremely fast heating and cooling rates are required to switch with great speed between the phases whereby material degradation has to be avoided at the relevant operating temperatures. Generally, Ge−Sb−Te alloys crystallize from the metastable amorphous into the cubic phase between 130 and 150 °C and undergo a second phase transition into the trigonal phase at a temperature between 300 and 350 °C depending on the actual chemical composition.4−6 While these solid−solid phase transitions as well as the transition into the molten state have been studied in detail,6−10 only very few experiments address problems related to the phase stability. For example, volume shrinkage during recrystallization can lead to void formation in encapsulated GST material after high temperature processes,6 and thus, sublimation can occur at the crystal−void interface. Besides these technological aspects, thermally induced sublimation and its associated atomistic processes are of fundamental significance. Since sublimation is a solid−vapor phase transition in which atoms dissolve away from the crystal lattice and move into the gas phase, the investigation of the underlying mechanisms on the atomic scale might shed additional light on the inverse process of crystal growth, which is not yet understood comprehensively. In situ transmission electron microscopy (TEM) offers the possibility for direct observation of sublimation at the atomic scale and in real time. So far, almost all TEM investigations on sublimation were performed on nanostructures such as © 2017 American Chemical Society

nanowires, nanorods, or nanoparticles consisting of relatively simple material systems in order to observe changes in shape, morphology, or faceting.11−22 Atomic processes during sublimation were mostly observed on two-dimensional layered materials where projection problems are excluded naturally. In case of phase change materials, the in situ TEM study of GeTe nanowires enclosed by a SiO2 shell showed a size-dependent sublimation rate. The rate is used to evaluate the vaporization coefficient of the nanowire geometry.13 On the other hand, Ge2Sb2Te5 films on SiO2 were studied by in situ high-resolution TEM and electron energy-loss spectroscopy (EELS) with emphasis on the crystallization from the amorphous phase and the cubic-hexagonal phase transition. The sublimation process is mentioned briefly in connection with electron irradiation effects but without submitting any further details.23 In the present study, high temperature stability and sublimation of Ge1Sb2Te4 thin films deposited on Si(111) substrates are investigated with atomically resolved cross-sectional TEM in real time. At 600 °C the cubic phase is still present and an anisotropic sublimation occurred via fast kink/step formation along ⟨110⟩ and ⟨112⟩ directions forming energetically favorable {111} facets. The results are discussed in the framework of a simple terrace-step-kink model.

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EXPERIMENTAL SECTION An amorphous Ge1Sb2Te4 (GST124) film was deposited at room temperature on a cleaned single crystalline silicon (111) wafer Received: October 5, 2017 Revised: November 15, 2017 Published: December 20, 2017 2968

DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974

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The Journal of Physical Chemistry C

image of the as-deposited amorphous GST film prepared in the lamella geometry is shown in Figure 1b. In order to generate epitaxially aligned GST the amorphous film was first heated to 625 °C followed by a quick cool down within 3 s. At the high temperature, sublimation instead of melting has occurred because the film was not encapsulated.10 The BF-STEM image at 625 °C in Figure 1c shows that the GST thin film decomposes leading to an overall thinning of the sample, leaving faceted grains and holes behind. Simultaneously, reorientations of crystal grains with cubic structure proceeds resulting in an epitaxial alignment to the crystalline Si(111) substrate (cf. HRTEM image in Figure 1d). Using Si as a reference, the lattice parameter of GST was determined to be 0.61 ± 0.01 nm which is in good agreement with the reported lattice parameter of 0.60440 nm for GST124 with NaCl crystal structure.28,29 The cubic grains produced in this way serve as the initial state for the sublimation study. During our in situ TEM measurements, an impact of the high energy electron beam by local heating or knock-on processes can be neglected. On the one hand, Lotnyk et al. have shown that in GST the sputtering of Ge and Sb is much smaller than our observed sublimation rate.30 On the other hand, Kooi et al. demonstrated that the main effect of the electron beam is displacement damage by knock-on collisions compared to local sample heating.6

in an ultrahigh vacuum chamber using effusion cells for the individual elements. Details about the deposition process can be found elsewhere.24,25 The thickness of the as-grown film as determined by cross-sectional transmission electron microscopy (TEM) amounts to 55 nm and furthermore, the chemical composition was examined by electron energy-loss spectroscopy in the TEM.10 The TEM data were collected in a JEOL 2100F operating at 200 kV and equipped with an UltraScan 4000 (model 895) CCD camera. The in situ videos were recorded using the continuous mode for a (1k × 1k) image in Digital Micrograph software from Gatan giving us a temporal resolution of 1.15 s. A microelectro-mechanical system (MEMS)-based double-tilt sample holder from DENSsolutions (model D6+) is used to carry out the low-drift in situ heating measurements. The DENSsolutions system regulates the temperature by monitoring the resistance of a Pt wire using a 4-point measurement.15 The cross-sectional TEM samples were prepared in a lamellae geometry and placed directly on the MEMS microhot plate. This preparation is done with a JEOL IB4501 system that combines a focused ion beam (FIB) and a scanning electron microscope (SEM). A thin film of carbon is deposited onto the wafer with a carbon coater to protect the GST thin film from direct Ga+ ion exposure. The lamella is cut out of the wafer and transferred to a half-moon copper grid by standard FIB lift-out procedure.26 The transport inside the FIB vacuum chamber is done by a micromanipulator from Kleindiek Nanotechnik GmbH. The lamella is glued onto the grid by ion beam assisted tungsten deposition. The intermediate step of transferring the lamella to a copper grid is necessary to rotate the lamella in a controlled way by 90° in order to place it flat onto the MEMS microhot plate. The final thinning to reach electron transparency was done on the chip and achieved by sputtering with the Ga+ ion beam using accelerating voltages down to 5 kV in order to minimize the thickness of the damage layer. During FIB preparation, Ga+ ions are unintentionally deposited on or incorporated into the sample mainly in the area of the carbon protective layer. During in situ heating, Ga atoms become mobile (TM,Ga = 29.8 °C27) and cluster into Ga droplets posing a risk of interfering with the process under study. An effective way to suppress the interference by Ga droplets is the application of a sufficient thickness of the protective carbon layer, in which case the Ga droplet will be formed away from the region of interest. The temperature treatment of the GST thin film inside the TEM is displayed in a schematic temperature-versus-time plot in Figure 1a. A bright-field (BF) scanning (S)TEM overview

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RESULTS Thermal Evolution of GST Grain Structure. The initial state of the GST grain before the sublimation process started is shown in Figure 2a. The TEM micrograph is taken at room temperature along the [11̅0] zone axis of Si and displays an overview of the grain with projected size of about 130 nm × 50 nm. The high-resolution image reveals a crystallographic alignment of the GST grain with the Si substrate as verified by the perfect matching of the {111} planes between cubic GST and Si across the interface (see magnified image of the edge of the grain in Figure 2b). Based on this plane matching combined with the results of the Fourier analysis (inset in Figure 2a), the following orientation relationship is deduced: (111)GST ∥(111)Si and [11̅0]GST ∥[11̅0]Si. Such a well-oriented grain is required for the in situ heating study in order to be able to observe the time-dependent variations in shape and surface morphology with respect to crystallographic planes and directions with highest spatial resolution. Accordingly, the HRTEM image in Figure 2c shows the very same GST grain after heating to 640 °C. The micrograph is recorded a few seconds after the target temperature is reached. Compared to

Figure 1. (a) Schematic experimental procedure for in situ heating of GST thin film first from rt to 625 °C to generate epitaxial aligned cubic grains and in second to 640 °C for in situ observation of sublimation of cubic grains. (b) BF-TEM image of a FIB-lamella on a DENSsolutions chip with GST thin film in initial amorphous state featuring the Si substrate and carbon protective layer, scale bar = 1 μm. (c) Same lamella at 625 °C showing the thinned areas and voids left by first heat treatment, scale bar = 1 μm. (d) HRTEM image of cubic epitaxially aligned grain taken after cooling down to room temperature (image taken from similarly treated sample), scale bar = 2 nm. 2969

DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974

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The Journal of Physical Chemistry C

Figure 2. HRTEM image of cubic GST grain at (a) 23 °C (initial state) and (c) 640 °C (after start of sublimation) stitched together from several images. Black arrows in (c) mark some of the edges along [112] that formed during sublimation. Insets showing FFT of central region of the image containing Si and GST. Close-up views in (b) and (d) show the corresponding grain surface at Si-GST interface region. (a), (c) scale bar = 20 nm and (b), (c) scale bar = 2 nm.

Figure 3. Sample geometry and overview of sublimation process: (a) HRTEM image of cubic GST grain at 640 °C recorded 17 min after the start of sublimation and display close to the Si-[11̅0] zone axis. (b) Schematic 3D model of cubic GST with {111} facets forming an octahedron and a projection of the GST structure along [11̅0]. The Te atoms (green) make up the {111} surfaces and the Ge and Sb atoms as well as vacancies (light magenta) form the second layer. (c) HRTEM image series of the cubic GST grain on Si at 640 °C featuring visible steps (red) moving along ⟨112⟩. Scale bar = 10 nm.

performed on similar treated samples has proven that there is no change in chemical composition of the GST after heating and partial sublimation. Anisotropic Sublimation of Cubic GST. The in situ observation reported here demonstrates that sublimation of the cubic GST grain is distinctly anisotropic taken place via the predominant formation of kinks at {111} facets, which subsequently move along the ⟨110⟩ direction and thus leading to an observable step motion along the ⟨112⟩ direction. Figure 3a shows the tested grain after 17 min of overall sublimation time along the [110̅ ] zone axis of Si. The grain is significantly reduced in size compared to the situation displayed in Figure 2c (onset of sublimation) leading to a thin crystal with a rhomboidal shape in projection and crystal edges whose projections are along ⟨112⟩ directions. There are contrast modulations detectable along the cubic grain indicating a change in local thickness. The {111} facets are plausible low-index candidates to form stable surfaces featuring edges along ⟨110⟩ directions which lead to

the original status, which is described by a shapeless grain morphology, the short-time heated grain has already visibly modified its shape without changing the overall crystallographic orientation (cf. HRTEM in Figure 2d). The initial surface roughness is remarkably flattened, in particular in the thin areas of the grain on top and at the right side, accompanied by the formation of straight-lined {111} oriented steps as indicated by black arrows in Figure 2c. The change in volume and shape is a result of surface and volume diffusion and atom evaporation at this high temperature. Taking into account the melting point of the bulk GST124 of 616 °C, and the partial pressures of Te (∼103 Pa) and Sb (∼101 Pa)27 at the applied temperature, the in situ TEM heating of GST is expected to lead to sublimation, even if the partial pressure for Ge is quite low (∼10−10 Pa27) and in the range of the base pressure of the microscope. Moreover, the grain does not point to any Ge excess compared to the composition before annealing. In fact, our EELS analysis 2970

DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974

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The Journal of Physical Chemistry C edges along ⟨112⟩ in [110̅ ] projection. Figure 3b displays a 3D model of cubic GST where {111} facets form an octahedron. This {111} octahedron is the simplest shape to fulfill the cubic symmetry and the projected rhomboidal shape with edges along ⟨112⟩ (cf. crystal structure projected along [110̅ ] in Figure 3b). Furthermore, the observed contrast modulations are consistent with thickness changes due to sloping {111} facets. On the other hand, abrupt transitions along crystallographic directions are visible in the HRTEM micrograph. Because of the slightly off-zone axis imaging conditions, the surface structure contributes more strongly to the HRTEM contrast and thus steps became clearly visible,31 indicating that the real shape of the crystal in Figure 3a is more complex than a perfect equilateral {111} octahedron and might be composed of monolayer and multilayer deep steps along ⟨110⟩. Figure 3c depicts a series of HRTEM images taken from a video sequence that shows subsequent stages of the sublimation process over a time period of 100 s. (Video S1 in the Supporting Information shows the whole sequence.) It is evident that the grain shrinks in a self-similar manner retaining the rhomboidal shape. Thereby, the sublimation occurs via kink-step-terrace formation mechanism. Several exemplary steps are marked with red and blue arrows in Figure 3c. Note that “kinks” are not directly visible in the [11̅0] projection. It is assumed that after being nucleated, kinks propagate along the ⟨110⟩ direction creating a step on the {111} facet (cf. Figure 3b) whose movement along ⟨112⟩ direction is then observed. The kink formation is initiated at edge corners of the grain (red arrows) and also at step-edges (blue arrows) because of the real shape being more complex than the perfect equilateral octahedron. The anisotropic and fast kink movement results in a reduction of the surface roughness and the development of straightlined steps and finally terraces provided that the process is nucleation-limited. In the following, the sublimation process is discussed using the examples of the dynamics along [111]̅ (black arrow in Figure 3c) and [1̅1̅1] (blue arrow in Figure 3c) direction. Figure 4 extensively illustrates the dynamics of step motion during sublimation at the atomic scale. Figure 4a presents three consecutive HRTEM snapshots of the border of the crystal. The images clearly resolve the {111} planes of cubic GST, which terminate the edges of the grain forming an atomically sharp and straight facet. Figure 4b displays a model of the atomic arrangement of cubic GST in projection along our viewing direction. The measured lateral step height of 0.35 nm corresponds to the distance of the {111} lattice planes, d111 = 0.3489 nm. One set of {111} planes is composed of a pair of planes belonging to both sublattices, the Te atoms on the one hand and randomly distributed Ge and Sb atoms on the other hand.29 For this reason, a step with a lateral expansion (step height) of one {111} plane (“single-step”) involves the sublimation of all three atomic species within our spatial and temporal resolution. The kink nucleates at the edge corner where two facets build a “convex” crystal edge. From there, the kink propagates along the [110̅ ] direction and is only visible as a step when the entire atomic row along the viewing direction ([11̅0]) is removed. This kink movement is faster than our temporal resolution. The kink forms a single-step or occasionally a double-step edge along the viewing direction and is easily recognizable in the HRTEM images (cf. red arrow in the central image of Figure 4a). Most commonly, steps move across the entire length of the grain edge (about 13 nm) along the ⟨112⟩ directions within

Figure 4. Sublimation at the (1̅1̅1) facet via kink nucleation and singlestep movements along [1̅1̅2̅]. (a) Series of HRTEM images taken from a video of the cubic GST grain at 640 °C showing the nucleation and movement along [1̅1̅2̅] (red arrow) of one step in three consecutive images, scale bar = 2 nm. (b) Atomic arrangement on a (110) plane where one set of atomic sites on (111) planes is occupied by Te, while the other is occupied by Ge/Sb/Vac. (c) The cumulative sublimed length is measured as a function of time along [111̅] direction (black arrow in (a)) featuring mostly single-steps.

one consecutive frame, i.e. within 1.15 s. The kink/step velocity can be therefore estimated to be equal or faster than 11 nm/s. To follow up the time dependent process more comprehensively, the cumulative sublimed length along [111̅] direction is plotted as a function of time in Figure 4c. The curve progression resembles an irregular step-function representing the linear shrinkage of the grain over a period of 2 min. Each step in the graph corresponds to the fast kink/step motion, whereas the step height corresponds to the lateral expansion of the crystal step and the plateau area measures the time between two nucleation events ranging mostly between 1.15 and 3.45 s. Regardless of the low time resolution of our measurement, it must be concluded that the sublimation process is kink nucleation-limited as furthermore evidenced by the fact that straight facets develop, and accordingly, no step roughening is observed at this high temperature. This result is further supported by the observation of the sublimation along the [1̅1̅1] direction, i.e., at the opposite site of the grain. Figure 5a presents the corresponding cumulative sublimed distance-versus-time plot. In spite of the large time frame of 7.5 min in total, only six nucleation events are observed (indicated by arrows). The time between two nucleation events ranges from 8 to 150 s according to the plateau widths. The height of the jumps in the step-function reflects the lateral step expansion, which isin this casemostly larger than one {111} layer, indicating a multikink nucleation behavior. Indeed, the HRTEM image series in Figure 5b reveals the presence and structure of a multistep construction. The exemplary step ensemble covers four {111} planes with a staggered arrangement along the viewing direction and perpendicular to the {111} plane. The nucleation site is located at the GST/Si interface (see white short arrows), and the subsequent propagation of the step ensemble proceeds along the [112] direction until it crosses the transversely running {001} facet. The velocity of the multistep ensemble depends on the number of step units involved, as shown in Figure 5c, where the sublimed length is plotted over time. The estimated velocities in the linear sectors range from 0.5 to 1.8 nm/s where smaller 2971

DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974

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The Journal of Physical Chemistry C

Figure 5. Sublimation at the (111)̅ facet via multikink nucleation and movement along [112] (exception: one single-step motion labeled with black arrow). (a) Cumulative sublimed length is measured as a function of time along [1̅1̅1] direction (blue arrow in (b)) featuring five multisteps. (b) Series of HRTEM images taken from a video of the cubic GST grain at 640 °C showing the nucleation of the first kink after 98.9 s (white arrow) followed by mass desorption from the kink site seen as step movement along [112] (red arrow), scale bar = 5 nm. (Video S2 shows the whole sequence.) (c) Graph of step movement along [112] (red arrow in (c)) showing the cumulative sublimed length of each step starting after they nucleated at the Si-GST interface. The black line shows the faster movement of the sole single-step motion.

Figure 6. Schematic drawing to illustrate the different possible kink nucleation sites featuring two heterogeneous, concave sites at the GST−Si interface and two homogeneous sites where one is convex and the other one concave. The kink sites with the most broken bonds at each facet are marked in blue. Additionally, on the left and right-hand site of the figure, the number of broken bonds after the kink nucleation and during the subsequent step motion along [112] and [1̅1̅2̅] direction, respectively, is illustrated.

Te after heating the sample to temperatures above the sublimation point and cooling it down again to room temperature. Qiao et al.33 have found that Si doping (up to 10%) of GST is able to delay the transition from cubic to trigonal phase and thus stabilize the cubic structure at higher temperatures. They speculate that the formation of stronger covalent bonds with Si play a role. Consequently, one could conclude, that the Si atoms preferentially occupy the vacancy sites and there, avoid the ordering as the first step toward phase transition. Although similar effects were measured for N doping of GST,34 a comprehensive understanding of the stabilization mechanism is still far from being attained. Kink Nucleation Site and Propagation. According to the terrace-step-kink (TSK) model,35 the sublimation of solid surfaces starts from sites with the lowest activation energy required to remove an atom from the surface. These preferential sites are discussed in terms of nearest neighbor bonds which must be broken.35 Figure 6 shows a schematic drawing of the cubic grain to illustrate the four different possible kink nucleation start points, two for each opposing crystal edge. There are

velocities are observed for wider multistep arrangements, i.e., v4‑steps > v7‑steps > v8‑steps. This demonstrates the influence of the number of kinks involved in the correlative ensemble motion. Compared to the velocity of the single-step configuration, these values are about 1 order of magnitude smaller.

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DISCUSSION Stability of Cubic Phase at High Temperature. As stated in previous reports,9,32 annealing of cubic GST should first lead to the formation of a vacancy layer ordering along the {111} planes, followed by the transition to the trigonal phase above 300 °C.4,5 Interestingly, the aligned GST grain retained the cubic NaCl structure even up to the sublimation point in our study. A highly vacancy-ordered cubic structure could not be observed as no additional periodicity along the {111} planes were found as well as no indications of the trigonal phase. On the other hand, Figure 2a,c reveal that the GST/Si interface becomes rough at higher temperatures as a result of Si diffusion into the GST layer. In fact, EELS analysis (not shown here) demonstrates the emergence of a distinct Si signal besides Ge, Sb, and 2972

DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974

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The Journal of Physical Chemistry C

Kooi et al.6 also discussed the lowering of the surface energy by a preference of Sb atoms to be at the surface. A different approach is to consider vacancies aiding the sublimation process by lowering the energies of bonds that need to be broken. Then, the plane where most vacancies are exposed to the surface is the stable facet. This would lead to the formation of {111} facets because vacancies are allocated on the Ge/Sb sublattice and are possibly already ordered to a certain degree which has not be observed yet in HRTEM phase contrast.

homogeneous sites where only the GST crystal is involved in the sublimation process and heterogeneous sites where the GST and Si crystal form an interface. Additionally, the potential sites can be differentiated in “convex” and “concave” depending on the crystal facets that meet at the specific site. Invoking the TSK model, the most preferable kink nucleation site is located at the homogeneous, “convex” site of the grain (bonds to be broken: 1) compared to the other possible sites marked in Figure 6. There is no “convex” site at the [111̅] facet due to a stable facet in [001̅] direction crossing it. The second most favorable kink nucleation site considering the number of bonds is the heterogeneous, “concave” site on the [111̅] facet (bonds to be broken: 4). The discussed preferred nucleation sites at each facet (marked in blue in Figure 6) are those which are observed during sublimation. The rate with which the kinks are formed is 30 times faster for the [1̅1̅1] facet compared to the opposing facet. As expected, the kink nucleation rate is much faster for the most preferable kink nucleation site. Besides the preferential starting points for kink formation, the subsequent kink propagation along the [11̅0] direction should be identical on both crystal sites because of the same number of bonds which must be broken. Indeed, step velocities along the [112] and [1̅1̅2̅] direction of about 11 and 5 nm/s, respectively, were estimated for the single step motion. The apparent discrepancy is attributed to the fact that the step width along e-beam direction (local thickness) is different at both sides of the grain and therefore the kink propagation along [11̅0] takes longer as more material has to sublime for the same visible step motion along [1̅1̅2̅]. Surface Morphology and Crystal Shape During Sublimation. Based on our observation of the grain morphology, shape, and crystallographic orientation, we have concluded that {111} facets were predominantly generated during the anisotropic sublimation process. Therefore, it is implied that these facets must be energetically favorable for cubic GST crystals at high temperature. From a general point of view, faceting of surfaces is driven by the anisotropy in surface energy γhkl as a function of the crystallographic orientation {hkl}. Due to the lack of data on specific surface energies for GST, a first approximation can be made by applying the simplified broken-bond model on a rock-salt crystal structure and including the surface density of atoms.36 This model yields that the {100} surface should have the lowest energy followed by the {110} and {111} surface, respectively. Consequently, a small particle in equilibrium shape consists primarily of those facet types. And indeed, we have observed cubic grains that have been formed after crystallization from melt droplet on a Si(111) substrate at 600 °C exhibiting only {100}, {110}, and {111} facets (shown in Supporting Information). Furthermore, an estimation of the corresponding surface energies by applying the Wulff−Kaishev construction demonstrate the following sequence γ111 < γ110 ≈ γ100, which means that {111} surface has the lowest energy in accordance with the sublimation result. Nonetheless, during sublimation {111} facets are detected mainly pointing out that the shape might not be in equilibrium. The discrepancy can be ascribed on the influence of the particular GST structure and the different partial pressure for the different atomic species: According to the crystal structure, the {111} surfaces consists of atoms from only one sublattice, either Te or Ge/Sb.29 Due to the low vapor pressure of Ge, one can imagine that there is a preference for the Ge/Sb sublattice to form stable surfaces during sublimation. For the formation of {111} surfaces during the growth of trigonal GST grain,

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CONCLUSIONS The phase stability and sublimation behavior of cubic GST are investigated in this study. The cubic phase remains stable even up to the sublimation point, and no ordering phenomena or transition to the trigonal phase is observed. This behavior could be related to Si diffusion into GST at high temperatures. The sublimation of this ternary structure on a silicon substrate is observed with atomic resolution. The sublimation is anisotropic leading to the formation of {111} facets as plausible low-index candidates, which is attributed to vacancies playing a role in the sublimation process. The observed dynamics are in accordance with the TSK model for nucleation-limited dynamics: The sublimation occurs via kink nucleation on stable steps and subsequent sublimation from those kink sites. Slow and fast kink nucleation in accordance with the broken-bond model and subsequent step movement with similar velocities is identified on opposing facets. Open questions include the formation process of the observed “multi” steps. A possible approach is to study the influence of interfacial defects which form due to a high lattice mismatch between Si substrate and GST thin film on the sublimation mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09855. Surface energies of GST (PDF) In situ HRTEM overview sequence of the sublimation process of a cubic GST grain on a silicon substrate which is discussed in Figure 3c (AVI) In situ HRTEM sequence of the multistep sublimation process discussed in Figure 5 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 (0)30 20377280. ORCID

Katja Berlin: 0000-0001-8182-5773 Achim Trampert: 0000-0001-7949-643X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank DENSsolutions for giving us the opportunity to perform the presented experiments by providing us with their heating sample holder (model D6+) and Dr. Michael Hanke for carefully reading the manuscript. Part of this work was financially supported by the European Union and the state of Berlin via the EFRE project 2016011843. 2973

DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974

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DOI: 10.1021/acs.jpcc.7b09855 J. Phys. Chem. C 2018, 122, 2968−2974