Growth and nanostructure of tellurides for optoelectronic, thermoelectric and phase-change
applications
Vermeulen, Paul Alexander
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Publication date: 2019
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Vermeulen, P. A. (2019). Growth and nanostructure of tellurides for optoelectronic, thermoelectric and phase-change applications.
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165 *This chapter is adapted from: Vermeulen, P. A., Calon, J., Brink, G. H. & Kooi, B. J.
Chapter 9. Combining ultrafast
calorimetry and electron microscopy:
reversible phase transformations in
SeTeAs alloys.
A novel combined investigation using ultrafast calorimetry and electron microscopy reveals a nanoscale crystalline-amorphous lamellar structure in rapidly heated SeTeAs alloys.
9.1
Abstract
Reversible amorphous-crystalline phase transitions are studied using complementary ultrafast Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM) techniques, which together allow a wealth of thermal and structural properties to be determined. The SeTe(As) system is investigated, because these chalcogenide based materials have favorable properties as a phase-change memory material and in optical systems. Using calorimetry we find that the addition of 10 at.% As to SeTe alloys strongly increases their glass forming ability, increasing both glass transition and crystallization temperatures while reducing critical quench rate. Ex-situ investigation of SexTe90-xAs10 using electron microscopy and elemental mapping
reveals a 2-phase lamellar segregation mechanism, where a trigonal SeTe-phase and an amorphous As-rich phase are formed. These findings demonstrate the
9.2
Introduction
Recently a commercial UltraFast Differential Scanning Calorimeter (UFDSC) which can reach relevant heating and cooling rates of 104 K/s was introduced.5 This has led to the discovery of new crystal phases, and extended the knowledge of thermal stability of glasses and crystallization rates within a wide range of materials.2,6–9 While already increasing our knowledge of phase transformations, the UFDSC technique lacks any structural information accompanying those transitions. Solutions to this shortcoming were mostly sought using X-ray Diffraction (XRD) or Raman spectroscopy.10–12 These can provide information on the crystal symmetry and the possible presence of secondary phases, but do not reveal the microstructure or local composition of the samples. Therefore, for relatively complex microstructures it is generally essential to have a microscopic view of the samples as well, with sufficient spatial resolution in order to sufficiently understand the samples studied. Moreover, the very small sample size often employed in UFDSC is more compatible with a high spatial resolution microscopy technique than with for instance XRD, where sample size in general should be a few millimeters. Within this context we therefore now present results from Electron Microscopy (EM) investigations of UFDSC samples, which provides atomically resolved, real-space and diffraction imaging techniques, to match the XRD- and Raman capabilities, while also allowing local microstructural and compositional analysis.
Our material system of choice are SeTeAs alloys, which are prototypical chalcogenide-based PCMs, displaying clear contrast between amorphous and crystalline phases. Much interest from this field of research is on the crystal growth rate, energy required for switching operation, long-term stability of the alloy in either phase, and reversibility of the phase change.13,14 Using Kissinger analysis, the crystallization kinetics can be characterized, while determining the glass transition temperature and critical quench rate gives a measure of stability of the amorphous phase. By reversibly heating and cooling, a switching application is replicated. The glassy state of the alloy is used for fiber-optic communications because of its favorable transmissivity at IR wavelengths and stability of the amorphous phase.15,16 Many reports in literature focus on high-As content SeTe glasses, e.g. to determine optical losses due to inhomogeneity.16–18 Furthermore, the chosen alloys are melt-quenchable within the UFDSC limitations (melting below 450 °C, cooling rate -10 000 K/s), and the binary SeTe was already studied before, offering direct comparison.19
This chapter will be presented in two parts, first the thermal analysis using ultrafast DSC is shown, then microscopy data is used in order to elucidate and extend the knowledge of the structure of thermally treated and analyzed SeTeAs alloys.
9.3
Methods and Experimental details
Se45Te45As10 alloys were prepared by mixing their pure constituents into a quartz ampoule and melting them in an oven at 450 °C for 8 hours. The resulting alloy was slowly ( 1 K/min) cooled to room temperature. The resulting ingot was shattered into smaller parts for investigation in (ultrafast-) Differential Scanning Calorimetry using A Perkin Elmer DSC and a Mettler Toledo Flash DSC 1. For regular DSC 10 ml Al pans were used, while for the ultrafast DSC UFS-1 chip-sensors were used with an active area of 500 μm diameter. The samples are molten to the heater area and are approximated as a half-sphere with diameter 100 μm and sample mass 1-2ug.19 Measurements were performed in a 20 ml/min nitrogen flow environment. Single flakes of SexTe90-xAs10 and SexTe1-x were deposited on the calorimetry sensor. These samples were heated at different rates to capture their crystallization behavior, and consecutively quenched into the amorphous phase by cooling with 4000 K/s. The heat-flow traces are recorded for all heating and cooling steps. The composition of the sample shifts rather slowly through evaporation, allowing for many measurements on a single starting sample. For each crystallization run the composition is approximated from the observed melting peak temperature, as was also described in chapter 8 and Appendix section 1. The cyclic heat treatment eliminates sample to sample variations and shows the applicability of these alloys to be reversibly switching phase-change systems.
Specimen transfer from ultrafast DSC to TEM was performed by transferring the sample using paper and a hair, and embedding the samples in an epoxy resin membrane suspended on a copper ring. The samples were polished to electron transparency by ion polishing using a Gatan PIPS II.
9.4
Results
Using UFDSC SexTe90-xAs10 flakes are heated and melted, and then cooled at various rates. During these reversible cycles the composition of flakes slowly varies due to Se evaporation as we have already demonstrated for our earlier work on SeTe alloys. This can be considered as a disadvantage, because it prevents studying a perfectly reversible system, but on the other hand it is also a clear advantage. Since the change in composition is very gradual, it provides an elegant method to determine phase diagrams in which glass transition, crystallization temperature (for various heating rates) and the melting temperature can be measured as a function of alloy composition. An important point therefore is the determination of the actual composition of the SexTe90-xAs10 flake in the UFDSC. As shown in the methods section and in the first section of the Appendix) the onset of melting can be used as an adequate estimator of the alloy composition. This is a convenient method to keep track of the composition in UFDSC, because this measurement can be readily performed for these sample types in UFDSC.
To show the strength of this method we first present the critical quench rate as a function of composition, i.e. the cooling rate needed to obtain a fully amorphous (glassy) phase from the melt. When the cooling rate becomes sufficiently high, no crystallization peak is visible in the trace of the heat flow. The critical quench rate over a range of compositions is shown in figure 1c. Compared to SeTe alloys, the addition of 10 at.% As to SexTe100-x leads to a shift of over 3 orders of magnitude, while effectively leaving the general trend of decreasing glass forming ability when exchanging Se by Te unaffected. This indicates that the crystallized phase itself is of similar nature, but its formation rate is strongly retarded by the added As content.
Employing the critical quench rate to regain a fully amorphous sample, we use a temperature program to reversibly switch the specimen between melt-quenched and crystalline phases. Representative heating traces of SexTe90-xAs10 flakes are shown in Figure 1a. The first exothermal peak, which indicates crystallization, is clearly visible. The samples also exhibit a glass transition which is visible as a step in the heat flow (inset of Figure 1a). The melting (endothermal) is visible as a clear dip around 400 °C. The crystallization peak temperature (Tc), glass transition (Tg), and melting onset temperature (Tm) are used for analysis in Figures 1b and 1d. The slope of the heat-flow traces is related to the heat conduction losses to the sample surroundings. Generally a lower heating rate leads to higher losses to the environment and thus a steeper slope. Furthermore, crystalline samples are good heat conductors, followed by the glassy and finally undercooled liquid states: this is also reflected in their respective heat-flow slope.19
The SexTe90-xAs10 phase diagram in figure 1b shows a decrease of Tc and Tg for increase of at.% Te (albeit rather modest for Tg). Increasing the heating rate dramatically increases the Tc, as well as to much lesser extent the Tg. This increase in transition temperatures cannot be ascribed to systematic thermal lag, which is estimated to be below 1 °C for heating rates below 1000 K/s.19 A remarkable feature of the SexTe90-xAs10 crystallization is the appearance in the heat flow of a broad shoulder at the higher temperature side of the (initial) crystallization peak when the Tc drops below 200 °C, as is visible for the slowest 3 curves in Figure 1a. The points corresponding to heating rates where a 2-peak transition was observed are circled in the phase diagram. This 2-peak character is not observed for the binary SeTe, and hints at a more intricate crystallization behavior for SeTeAs than for SeTe.
Figure 1d compares Tc and Tg values as a function of Te composition for SexTe100-x and SexTe90-xAs10. The region of overlap is small; due to the extremely slow crystallization of SeTeAs, no crystallization is observed for lower Te content. Adding As increases both Tg and Tc dramatically: like for the critical quench rate, it shows As serves as a strong glass stabilizer or crystallization retardant. Nevertheless, for both alloys Tc is found to decrease as the Te content increases. Finally we note that the Tg of SeTe increases, while that of SeTeAs decreases for increasing Te concentration. When instead the reduced glass transition (Trg =
Tg/Tm) is inspected, it decreases for both alloy systems for increasing Te concentration, as is expected when the material becomes a poorer glass-former.20
Figure 5. a.) Representative heat flow traces for different heating rates. Due to non-equilibrium heat losses to the environment, more energy is needed at lower heating rates. The inset shows the region around the glass transition. The step of the glass transition (Tg), the peak of crystallization (Tc), and dip due to melting
(Tm) are clearly visible. At low heating rates (low Tc) a shoulder on the high
temperature side of the crystallization peak is visible. Errors on Tg and Tc are ±3
°C, while the broad melting onset gives a larger error estimate ±5 °C or 2 at.% b.) The extended phase diagram shows thermal transitions as a function of composition. A clear decrease of Tc is observed for increasing Te content. c.) The
cooling rate required to obtain a fully amorphous sample from the melt is plotted as a function of composition. This rate was determined by inspecting a range of cooling rate traces and finding the lowest rate which did not show any crystallization. d.) The Tc and Tg of SeTe and SeTeAsare compared for a heating
rate of 100 K/s. Both are significantly higher when As is added. While Tc
decreases for both, Tg decreases only for SeTeAs10.
Kissinger analysis was performed on SexTe90-xAs10 alloys with a wide range of compositions (Figure 2a) to elucidate the crystallization kinetics. A clear non-Arrhenius behavior was observed due to the fragility of the undercooled liquid,.
This effect becomes particularly observable at higher heating rates, where the data points clearly deviate from a straight line. This was also previously found for other phase-change materials.7,8,19 The activation energy of crystallization was calculated for the lower heating rates (<100 K/s), where the crystallization can be approximated by Arrhenius behaviour, and the result is shown in Figure 2b. The results bear a great similarity to those obtained for SeTe. Although the measurements are performed in only slightly overlapping regions, trends are similar, and seem to even match quite well to SeTe. This indicates that the crystallized (part of the) sample resembles SeTe.
Figure 2. a.) A Kissinger plot showing the Tc for all heating rates shown in Figure
1b. The color gradient indicates composition. If the material would show Arrhenius behavior, the lines would be straight. The curvature indicates this is not that case, as is true for most PCMs. The curvature is highest for the higher heating rates (top left) b.) By extracting the slope assuming Arrhenius behavior at heating rates at or below 50 K/s, the activation energy for crystallization is extracted. SeTe and SeTeAs alloys are compared. Although the region of overlap is small, the general trends seem to match well.
At this point we have gained a good understanding of the thermal response of the alloy. It is clear that the material behaves as a phase-change material, possessing non-Arrhenius characteristics. We have found that the addition of As strongly stabilizes the glass phase. We will now present structural analysis using various microscopy techniques, which will add crucial information on sample inhomogeneity, chemical composition, and crystal structure. To investigate the crystalline state of the alloys samples were crystallized in various ways: (1) from a closed quartz tube ingot slowly cooled from the melt, (2) slowly heated (1 K/s) from the amorphous phase in a regular DSC, and (3) using UFDSC heated with 100 K/s. For this last sample a novel method involving simple components and lab materials was developed to transfer the sub-millimeter size samples from the DSC microschip system to an electron microscope. More details can be found in the
experimental and Appendix section 2. Microscopy images of these three samples are shown in Figure 3. As is immediately obvious, an increase in heating rate significantly reduces the size of several features of the alloy. For the ingot (Figure 3a), optical microscopy suffices to image the clearly facetted crystals (features 10 μm). SEM is used for the slowly heated samples which feature 1 μm thick lamellae (Figure 3b), and TEM is required to resolve the lamellae in the quickly heated samples (Figure 3c) where the lamellae have widths of only 0.1 μm. The slowly heated sample shows distinct grain boundaries, providing microstructuring on a 100 μm length scale, while within the grain lamellar structures without clear facets can be observed. In the quickly heated sample (100 K/s), a morphology similar (albeit on a smaller scale) to that for the slower heated (1 K/s) sample is observed. While for SEM/Optical images the lighter color indicates high conductivity and hence a crystalline phase, in TEM the darker lamellae, which indicate strong scattering contrast, indicate crystalline phase. Sharp boundaries are observed, and the Back-Scattered Electron (BSE) imaging, which is sensitive to the elemental-weight distribution, shows that both phases are relatively homogeneous. The diffraction pattern (Figure 3d) taken from the circled area shows several crystals with a common crystallographic orientation and a small tilt gradient, indicating that the lamellae originate from one grain center.
Figure 3. a.) Optical microscope image of Se45Te45As10 slowly cooled from the
melt. The white facetted areas indicate a crystalline phase. b.) Back-Scattered Electron (BSE) image of a slowly heated (1 K/s) Se45Te45As10 specimen. Grain
grain center. The lamella width is on the order of 1 μm. c.) Bright Field (BF) TEM image of a rapidly heated (100 K/s) specimen close to Se45Te45As10 (c.f. Figure 4d).
The dark lamellae have strong scattering contrast, which indicates they are crystalline. The lamella width is now on the order of 0.1 μm. d.) Using selected area diffraction of the area circled in c, diffraction spots originating from individual lamellae can be observed, color coded by the red and blue lines. The angular orientation of the spots corresponds exactly to the lamella orientation. The lamellae have a similar crystalline orientation with a small twist, pointing to a common center point.
To investigate this 2-phase lamellar structure which appears after rapid crystallization upon heating of the supercooled liquid structure more closely, small-area diffraction patterns (SADPs) were taken on a dark and a bright region within the lamellar structure (Figure 4a). By selecting a dark lamella, a sharp diffraction pattern is observed which conforms to the trigonal structure of SeTe (Figure 4b).21 When a bright area is selected, diffuse rings characteristic of an amorphous phase are visible, along with some spots due to the aperture selecting a small part of the adjacent lamellae (Fig. 4c.). The 2-phase structure consists of one crystalline and one amorphous compound. The same can be concluded from an HRTEM image shown in figure 4e, which shows a lamella edge, and a rather sharp boundary between amorphous and crystalline phases.
Electron Dispersive X-ray Spectroscopy combined with Scanning Transmission Electron Microscopy (EDS-STEM) is performed, which maps the elemental composition of the alloy. Figures 4fghi show the mapping of an area consisting of a crystalline and amorphous material. The crystalline phase, which has a diameter of
150 nm is rich in Te, while the amorphous phase is rich in Se and As.
The histogram in Figure 4d shows the average composition of several amorphous and crystalline areas in the specimen, and shows a trend similar to the mapping scans. For comparison the composition of an ingot slowly cooled from the melt¸ representing an (nearly) equilibrium distribution, is shown hatched. The ingot shows no As in the crystalline phase, which is consistent with As as a glass former. The crystalline part of the ingot contains more Se and less Te than the heated alloy, but this can also be explained by slight selective Se evaporation due to thermal cycling in the DSC (c.f. Appendix section 3). So, during the phase separation which results in alternating crystalline and amorphous lamellae the predominant diffusing species are As towards the amorphous phase and Te towards the crystalline phase. These observed elemental compositions upon phase separation are consistent with the thermal properties of the alloy: a higher Te/Se ratio reduces the glass forming ability, while the addition of As significantly increases the glass forming ability. This is also consistent with literature
descriptions of these materials’ binary alloys; where SeAs is frequently listed as a glass, whereas AsTe and SeTe are mostly reported as crystalline.22–24
Figure 4. a.) The two-phase structure of the alloy is elucidated by using selected-area diffraction on a crystalline and amorphous region, the diffraction patterns resp. b.) and c) show the trigonal symmetry of SeTe and a broad amorphous ring. d.) The composition of both phases is analyzed by averaging several EDS measurements on several representative sites like b) and c) and it clearly shows that the crystalline phase is binary SeTe while the other phase is As-rich. e.) Using HRTEM a phase boundary is imaged: a clear amorphous-crystalline edge is found. f.) Using EDS-STEM mapping, an elemental map of a 2-phase region is obtained. The central object was 150 nm diameter as calibrated using bright-field imaging. g-h-i.) Like e.) the mapping shows that Se (g) and As (h) are mostly present in the amorphous phase, while Te (i) is mostly present in the crystalline phase. The maps show a normalized intensity profile (c.f. Appendix section 3-4).
9.5
Discussion
The observed microstructures show that the ternary alloy has a strong tendency to separate into two phases, which was not the case for the perfectly mixable SeTe binary alloy (c.f. Appendix section 5).25 The formation of a crystalline lamellar microstructure within a matrix of grains is a known phenomenon in many materials and has been well described by many sources.26,27 The process occurs when due to a change in temperature, the alloy segregates into two separate compounds. Due to the expulsion of one constituent from the compound, an advancing front rich in expelled constituent is created; which slows the propagation of the phase boundary. The observed lamellae are created due to this self-slowed segregation reaction.26 In such processes it is obvious that the time scale involved in crystallization, e.g. the heating rate, directly affects the attainable lamella spacing. In the present case we observed that an increase of heating rate from 1 to 100 K/s results in a 10-fold reduction in lamella spacing (from about 1 to 0.1 μm). Although our data are very limited, a lamella spacing showing a square root dependence on heating rate is a reasonable first approximation result, because the phase separation into lamellae is a diffusional process26 where the spacing depends on the square root of the effective diffusion coefficient D (mainly involving As) multiplied by time t allowed for this process: 𝜆~√𝐷𝑡.
Another interesting feature to elucidate is how the observed microstructure, in particular the decomposition into two phases relates to the double peak observed in the DSC traces. The initial sharp exothermal peak observed on heating from the undercooled liquid clearly relates to crystallization. Since we have identified the second phase as amorphous using EM, the broad shoulder is not a second crystallization effect. It must therefore be understood in the context of an exothermal relaxation or reorganization effect. This is corroborated by the fact that the transition effect is not a reversible process (c.f. Appendix section 6), unless the sample is returned to a fully molten state. At relatively low crystallization temperatures (heating rates 1-100 K/s) the driving force for crystallization is high while mobility is still extremely low: the typical behavior of a fragile liquid PCM. This leads to crystallization into a highly unfavorable phase where As is partly incorporated into the SeTe structure. We speculate several thermal reorganization effects take place after this initial crystallization. The amorphous As-rich phase, stressed by the partial crystallization of the alloy might undergo a structural relaxation. Alternatively or concurrently, a redistribution of elemental species throughout both phases may take place, removing As from the crystalline phase, and redistributing all expelled As within the glassy phase. At the lower heating rates substantially more time is available for these processes resulting in a more pronounced shoulder.
The phase segregation as observed using TEM also implies that the initial determination of the specimen composition using the melting point will
overestimate the Te content, and is in fact based on an incorrect assumption since the sample segregates into two phases. As shown in Appendix section 1 however, the Tm can still be used as a reasonable estimator of composition. Nevertheless, as shown by the microscopy data, one should be careful in interpreting the data in terms of absolute composition. The segregated phases (undercooled liquid and crystal) are expected to be fully remixed when the whole sample returns to the melt. The segregation, melting and quenching therefore represent a reversible phase change system.
From our analysis, we can extract several implications for functional application in phase-change materials. Since almost complete phase separation with respect to As occurs, the system can be treated as a pseudobinary alloy, where the amorphous As2(Se/Te)3 severely hampers the SeTe crystallization (three orders of magnitude lower quench rate). The crystallization, while slower (or at higher temperature), still proceeds in similar fashion, as was shown in Figures 1 and 2. TEM analysis confirms the SeTe phase still crystallizes into the trigonal phase. For application into optics, the alloy is to remain stable in the amorphous phase however. We have shown that even the addition of 10 at.% As is enough to stabilize the alloy into the amorphous phase with modest cooling rates of a few K/s.
The results presented above clearly indicate the merit of combining powerful thermal techniques with well-established electron-microscopy methods to obtain a full understanding of the material.
9.6
Conclusions
Ultrafast Differential Scanning Calorimetry and Transmission Electron Microscopy were performed to form a comprehensive understanding of the reversible crystallization behaviour of SeTeAs alloys. Using ultrafast DSC, extended phase diagrams showing glass transition and crystallization as a function of elemental composition were presented, and we showed that the addition of 10 at.% As reduced the critical quench rate by 3 orders of magnitude. Kissinger analysis was performed and the alloys were found to show non-Arrhenius fragile behavior, which is common for phase-change-materials. Using a novel technique to transfer ultrafast-DSC specimens to TEM, we investigated the 2-phase lamellar microstructure which was formed upon crystallization. It was found that feature size reduces by orders of magnitude when rapid heating was used. Using electron diffraction and elemental mapping, we found coexisting crystalline Te-rich and amorphous As/Se-rich lamellae. The observed microstructure allows explanation of an exothermic shoulder after the initial crystallization in the DSC traces at lower heating rates. These phenomena make compelling arguments to combine ultrafast thermal analysis with in-depth structural analysis using TEM.
9.7
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9.8
Appendix
Sample CompositionThe composition of the alloy is determined analogously to that of the binary SeTe alloys as described in.19 The melting point (Tm) of the alloys is recorded and is transformed into the composition under the assumption that the compound undergoing the melt is SexTe1-x, and melts according to the liquidus line of the binary SeTe system.25 To verify these assumptions, EDS (Energy Dispersive X-ray Spectroscopy) is performed on samples still attached to the UFS-1 sensor, as well as several local measurements using TEM EDS. The melting point obtained from DSC agrees well with that obtained through EDS and phase diagram. We may conclude that Tm is still a good indicator of the average specimen stoichiometry. The second half of the manuscript reveals however, that the sample is in fact distinctly inhomogeneous (c.f. figure 4d), and the crystallized component of the sample has a relatively low As content, and higher Te/(Se+Te) ratio than the average specimen stoichiometry. Since this crystalline phase is the origin of the melting peak in fig. 1a of the main text, we conclude that the Te content in the specimen would be overestimated when based on the melting point as described above. As presented in Fig. S1 however, the actual mismatch between Tm-determined and EDS-determined composition is negligible. Compared to the composition determined using TEM-EDS, the UFDSC-determined composition slightly overestimates the Te-content in the sample. This is consistent with a Te-rich crystallized phase.
Figure S1. The melting point obtained from UFDSC is compared to the theoretical melting point inferred from the elemental composition as determined using EDS. The analyzed samples are sufficiently close to the black line which represents perfect agreement. We therefore use the onset of melting as an indicator for composition.
Specimen preparation: Flash DSC to Electron Microscope
We developed a simple procedure to transfer a specimen that has been thermally analyzed using Ultrafast DSC to an SEM or TEM for structural analysis. The Mettler-Toledo Flash DSC 1 makes use of MEMS-chips (UFS-1), where the specimen is placed directly on a ‘hot-plate’ area. To prevent the sample from sticking to the surface, and to allow re-use of the MEMS-chip, a thin layer of silicone oil is commonly used. The sample can be manipulated and picked up from the chip surface using either a thin hair or a piece of rolled up tissue paper.
The sample needs to be embedded in a support which allows insertion in TEM. A standard copper ring (3 mm outer diameter and 1 mm inner diameter) was used. Using a tooth pick, a thin membrane of Power Epoxy glue is applied to cover the hole in the ring. It is essential that this glue membrane is kept as thin as possible, since most of it will need to be removed later. After the glue is applied, the sample needs to be placed in the glue membrane, as much towards the center as possible. Multiple specimen may be placed within one sample support to optimize the chance of success of the next steps. The glue is allowed to harden at low temperature. (Fig. S2 left)
To obtain a sample which is electron transparent, and therefore useful for TEM analysis, the glue membrane and the samples within have to be polished down to a thickness of less than 100 nm. This is achieved using an Ion polisher (Gatan PIPS II), where consecutively both sides of the ring are milled using argon ion guns set to grazing angles. The milling on the first side was stopped when the sample emerged from the glue: this was clear from the shiny appearance of their surface (Fig. S2 right). The bottom side was then milled until a hole appeared next to the specimen. This usually yielded a thin, electron-transparent wedge within the specimen.
Figure S2 Left: Two samples taken from ultrafast DSC sensor surface have been placed in a glue membrane within the hole of a copper ring. Right: The same sample after ion milling. The ring is clamped within the Ion Miller. The glue has been polished away from the bottom right sample, which looks shiny. The center sample is still covered in glue.
Elemental composition of the two-phase mixture
The histogram in figure 4d contains a comparison of the composition of two specimen, one slowly cooled from the melt, and one heated with 100 K/s. Both contain a 2-phase mixture, as shown in figure 3. Since the 100 K/s sample has been thermally cycled, its overall composition was slightly different due to the selective evaporation of Se. By normalizing the elemental composition to the total amount available in the alloy, we can eliminate this effect from the comparison. The resulting histogram is shown in figure S3. Be aware that the normalized atomic concentration is now a fraction of the total available in the sample, and not a true concentration intercomparable between samples. We can however, clearly observe that the 100 K/s sample and the ingot show highly similar phase separation: except for the small fraction of As present in the quickly heated sample.
Figure S6. The elemental fraction per specific phase normalized to the total amount of that element in the sample is shown. We see both samples have a nearly identical distribution, and the large majority of the As and to a lesser extent the Se is present in the amorphous phase, while the Te is mostly present in the crystalline phase.
(STEM)-EDS operation and quantitative analysis Calibration and errors
Electron-Dispersive X-ray diffraction (EDS) spectra are be obtained by capturing the X-ray emissions from an electron-beam irradiated surface. The spectra contain emission peaks corresponding to electronic transitions within the chemical elements of the sample. While EDS software will generally provide a fitting error and compositional accuracy of 1-2 at.% when analyzing a spectrum, the
actual measurement error depends on many factors such as sample roughness, thickness, and the potential overlap of spectral lines. The measurements have a high accuracy, but a low precision: it is therefore recommended to only compare results between similar samples or locations on one sample. Therefore no error bars are displayed since they give an overly optimistic view of the error. We do not analyze the absolute values obtained from EDS, but merely analyze trends and compare differently treated samples amongst themselves, such as the compositional change due to evaporation, or the different heat-treatments in figure 4d.
STEM mapping accuracy
When one wants to obtain an EDS map, many emission spectra each corresponding to a small part of the sample are recorded. While for accurate composition determination a long measurement time is taken to acquire a spectrum, for mapping one is more interested in local composition differences of just a few elements. In this case short dwell times per measured point can be chosen at the cost of compositional accuracy. Figure 4d has been obtained using long-time averaged scans, while f-I were obtained in a mapping mode. These figures therefore give an indication of the normalized elemental density, but should not be interpreted quantitatively.
Microscopy on binary SeTe
To compare SeTeAs and SeTe alloys, some microscopy analysis was duplicated on pure SeTe, which shows a much greater phase and compositional homogeneity.
Figure S4. Left: Bright-field TEM image Right: selected diffraction pattern of a Se75Te25 alloy heated with 100 K/s in UFDSC. The crystalline phase is
The second transition in Ultrafast DSC
An attempt was made to elucidate the origin of the shoulder on the high temperature side of the crystallization peak, which appears below certain heating rates and temperatures (c.f. figure 1b of the main text). The sample was quenched to room temperature at various points during the transition. It is clearly observed that only when the sample is cooled and reheated after the second transition has fully taken place, no exothermal effect is visible in the reheating trace. This indicates the effect is irreversible (unless we melt the sample), and therefore not a glass transition.
Figure S5. Left: Heating traces where the peak-shoulder transition is clearly visible and has been interrupted at various stages by a quick cooling. The reheating curves are shown on the right. It is clear that the shoulder will still appear on reheating, unless it has been full traced on the first heating. This indicates the transition is irreversible.
