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Structural Analyses of Phase Stability in Amorphous and Partially Crystallized Ge-rich GeTe Films prepared by Atomic Layer Deposition Taehong Gwon, Ahmed Mohamed, Chanyoung Yoo, Eui-sang Park, Sanggyun Kim, Sijung Yoo, Han-Koo Lee, Deok-Yong Cho, and Cheol Seong Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12946 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

Structural Analyses of Phase Stability in Amorphous and Partially Crystallized Ge-rich GeTe Films prepared by Atomic Layer Deposition Taehong Gwon1, Ahmed Yousef Mohamed2, Chanyoung Yoo1, Eui-sang Park1, Sanggyun Kim1, Sijung Yoo1, Han-Koo Lee3, Deok-Yong Cho2*, Cheol Seong Hwang1* 1

Department of Materials Science and Engineering, and Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea

2

IPIT & Department of Physics, Chonbuk National University, Jeonju 54896, Republic of Korea

3

Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea

*Electronic addresses: [email protected] and [email protected]

Abstract The local bonding structures of GexTe1-x (x=0.5, 0.6, and 0.7) films prepared through atomic layer deposition (ALD) with Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te precursors were investigated using Ge Kedge X-ray absorption spectroscopy (XAS). The results of the X-ray absorption fine structure analyses show that for all the compositions, the as-grown films were amorphous with a tetrahedral Ge coordination of a mixture of Ge-Te and Ge-Ge bonds, but without any signature of Ge-GeTe decomposition. The compositional evolution in the valence band electronic structures probed through X-ray photoelectron spectroscopy suggests a substantial chemical influence of additional Ge on the nonstoichiometric GeTe. This implies that the ALD process can stabilize Ge-abundant bonding networks like -Te-Ge-Ge-Te- in amorphous GeTe. Meanwhile, the XAS results on the Ge-rich films that had undergone post-deposition annealing at 350 oC, show that the parts of the crystalline Ge-rich GeTe became separated into Ge crystallites and rhombohedral GeTe in accordance with the bulk phase diagram, while the disordered GeTe domains still remained, consistent with the observations of transmission electron microscopy and Raman spectroscopy. Therefore, amorphousness in GeTe may be essential for the non-segregated Ge-rich phases, and the low growth temperature of the ALD enables the achievement of the structurally metastable phases. Keywords: Ge-rich GeTe, phase change memory, phase stability, atomic layer deposition, x-ray absorption spectroscopy, TEM, Raman spectroscopy, R-T

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Introduction Phase change phenomena in chalcogenide materials represented by ternary Ge-Sb-Te alloys have been widely studied for memory applications in which abrupt changes in the optoelectronic properties can be utilized.1,2 One of the phase change materials (PCMs), the GeTe-Sb2Te3 pseudo-binary compound series, has been a central issue because of its thermodynamic stability and superior phase change properties.3 Thus, examining the atomic structure and its variation upon crystallization is crucial to understand the detailed mechanism of the phase change processes in Ge-Sb-Te system.4-8 Along with the decrease of sizes in design rules, however, the stability of the amorphous phase has become more important to obtain better optoelectronic properties. It has been reported that stability in the structural and electronic properties of the amorphous phases can be obtained in GeTe-rich compositions whereas fast crystallization can be achieved in Sb2Te3-rich compositions.9,10 Therefore, binary GeTe, one of the end members in the pseudo-binary series, has been studied intensively for use as a PCM.11-13 There have been several reports on the local structures of amorphous and crystalline GeTe films prepared via physical vapor deposition methods.14-17 In the case of stoichiometric GeTe, it has been reported that Ge-centered tetrahedral coordination (or defective octahedral coordination) prevails when the material is amorphous while octahedron-like coordination prevails when the material is crystallized with a rhombohedral crystal structure.14,15,17 On the other hand, the structural properties of nonstoichiometric GexTe1-x have been rarely reported, except for its Te-rich composition,18 in spite of its superior electronic properties compared to those of stoichiometric GeTe.19,20 Thus, the examination of the structural properties of Ge-rich GeTe is of significant interest. For feasible application to the phase change random access memory (PCRAM), the PCM is contained in a peculiar structure called the “confined” structure, in which the PCM fills the trench holes to enhance the heating efficiency through the Joule heating effect, and thus decreases the operation current during the PCRAM operation.21 GeTe or Ge-Sb-Te, which is fabricated via atomic layer deposition (ALD), is known to show excellent step coverage even in a very-high-aspect-ratio (20:1) trench structure.22 The conformal filling will guarantee the uniformity in the electrical properties. The PCM, however, also bears a chemical instability issue. For instance, in the case of GeTe ALD, the reaction between the Ge(IV) and Te2precursors tends to form Ge1Te2 rather than the stoichiometric Ge1Te1.22-24 Thus, it is important to secure a method of controlling the atomic concentration ratio of Ge:Te. It is shown in the authors’ recent report20 that such controllability can indeed be achieved via ALD using Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te 2


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precursors and methanol vapor. In the said method, the Ge and Te precursors react with the methanol vapor to form more reactive intermediate precursors, and the composition is strongly affected by the growth temperature. Thus, in this study, ALD was employed to prepare GeTe alloys with various Ge:Te ratios (from a stoichiometric to a Ge-rich composition). According to the bulk phase diagram,25 however, either Ge or Te excess cannot exist in GeTe in a bulk form thermodynamically, suggesting that the nonstoichiometric GeTe should be decomposed into a mixture of elemental Ge/Te and stoichiometric GeTe. Then, the additional Ge in Ge-rich GeTe cannot alter the chemistry of the stoichiometric GeTe but will merely increase the volume of the segregated Ge crystallites. On the other hand, in the nanoscale thin-film form, nonstoichiometric (Ge-rich or Te-rich) GeTe may exist because of the strong influence of the surface free energy at the nanoscale. Indeed, results of density functional theory studies on the defect formation in GeTe show that Ge vacancy or Te excess can be easily formed as to lead a p-type conductivity.26,27 Moreover, the as-grown GeTe films are prone to becoming amorphous due to the low growth temperature ( 250 nm) were measured in a 4-point probe setup with increasing temperature (from the room temperature to 350 °C). The rate of the temperature elevation was kept at 4°C/min.

Results Amorphous GeTe Figures 1a and 1b show the amplitudes and the real parts of the Fourier-transformed (FT) Ge K-edge EXAFS data of the as-grown (and amorphous) GexTe1-x films with various compositions (x=0.5, 0.6, and 0.7; hereafter abbreviated as A55, A64, and A73, respectively). The FT spectra plotted as functions of the phase-uncorrected interatomic distance R, reflect the bonding environment at various distances from the Ge atoms. The XAFS oscillation χ(k)’s (k: electron momentum), weighted by k1 are displayed in Fig. 1c. The FT for Figs. 1a and 1b was undergone for the χ(k)’s masked by a Hanning sill (shown in Fig. 1c) with ∆k = 2 Å-1 in a k-range of 3-13 Å-1. For the case of A55, the amplitude |χ(R)| (Fig. 1a) appears to have two main peaks near R=2.0 and 2.4 Å besides the small ripples caused by the limited k-range for the FT. For understanding the details of the bonding environment, the data fitting was processed based on two types of Ge- bonds that can exist in the GeTe system, namely, Ge-Ge and Ge-Te bonds. The number of bonds (N), phase-corrected bond length (R+∆) and Debye-Waller factor for bond disorders (σ2) obtained from the fittings are summarized in Table S1 in Supporting Information for the three as-grown samples (A55, A64, and A73). The simulated |χ(R)| and Re[χ(R)] of each sample are appended in Figs. 1a and 1b, respectively, and the simulated χ(k)’s are appended in Fig. 1c for comparison. The simulated spectra (dashed curves) successfully reproduced the main features in the experimental spectra, confirming the accuracy of the fit. As shown in Table S1, the bond lengths for Ge-Ge and Ge-Te bonds in (stoichiometric) A55 are ~2.48 Å and ~2.62 Å, respectively, which is very consistent with a recent report.17 Those in Ge-rich GeTe (A64 or A73) are ~2.49 Å (Ge-Ge) and ~2.65 Å (Ge-Te), being similar to the respective values in A55. Meanwhile, 5



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the values of N’s change significantly according to the abundance of Ge-Ge bonds with increasing Ge concentration. The value of N for Ge-Ge bonds in A55 is nonzero (1.3 ± 0.3), evidently showing coexistence of both Ge-Ge and Ge-Te bonds in amorphous stoichiometric GeTe, in contrast to the octahedral coordination in rhombohedral GeTe. This peculiar bond structure has been known as a unique property of amorphous GeTe,17,29,30 being in a contrast to the case of crystalline GeTe (rhombohedral, R3m), in which Ge neighbors predominantly with Te.15,16 For the cases of A64 and A73, the two peaks appear to merge towards an intermediate atomic distance of R=~2.1 Å to constitute single broad peaks. This can be understood simply as a consequence of the increased number of Ge-Ge bonds due to the abundance of Ge. The surplus Ge atoms in the Ge-rich GexTe1-x films (A64 or A73) might preferentially occupy the Te sites in the crystal structure of “stoichiometric” GeTe (A55). The abundance of Ge-Ge bonds is consistent with the authors’ ALD reaction model on the formation Ge-rich GeTe, in which an intermediate precursor in the form of an oligomer with Ge-Ge bonds reacts with methanol vapor to constitute (Ge-Ge)-rich layers during the ALD cycles at a relatively high temperature.20 The details in the local structures of GexTe1-x can be further scrutinized by analyzing the features of the Xray absorption near-edge structures (XANES). XANES analysis has a strong advantage over XRD measurement in deducing structural information from the amorphous system because it reflects the local structural orders irrespective of the long-range orders. Figure 2 shows the Ge K-edge XANES of the three as-grown films. The features reflect the Ge 4p electronic structure in the GexTe1-x films through the Ge 1s → p dipole transition. The spectra were normalized by the edge jump (the difference between the absorption coefficients at the energies far below and above Ge K-edge). It is shown that the intensities of the main peaks located just above the edge energy (~11.107 keV) and the oscillatory peaks near hν=11.135 and 11.17 keV, highlighted by the dashed vertical lines, become weaker as the Ge concentration increases. In contrast, the small shoulder peak near hν=11.115 keV, highlighted by the filled triangle, becomes stronger. This feature is highlighted in the inset. For a theoretical understanding of the spectral evolution, an ab-initio full multiple scattering (FMS) approach for the XAFS simulation was employed using the FEFF8 code.31 For the simulation of the amorphous GexTe1-x, two model structures on the Ge coordination were used: one with a perfect tetrahedral Ge-Te coordination with a ~2.6 Å atomic distance (i.e., the value from the EXAFS analyses in Fig. 1), and the other with a perfect tetrahedral Ge-Ge coordination, as in a Ge crystal (diamond structure). Note that the tetrahedral GeTe4 coordination is often described in terms of defective octahedral GeTe6 6


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coordination (missing two Te; see for instance, Ref. 32). To mimic the shortage of structural orders in the amorphous system, the radius of the FMS was confined to 3 Å for Ge-Te, and to 6 Å for Ge-Ge. (See the Supporting Information for more details.) For simple and clear analyses, the Debye-Waller factors for the structural disorders were not considered in the XANES simulation. More details on the theoretical models are described in Supporting Information. The results of the FEFF simulations are appended in Fig. 2. It is clearly shown for all the experimental spectra that the main peak and the two bumps near the dashed lines are well reproduced by the Ge-Te model, although the intensities of the XAFS oscillations in the experimental data are weaker overall than the theoretical expectation because of the structural disorders in the as-grown GeTe. The bump highlighted by the filled triangle is also reproduced by the Ge-Ge model. The combination of the two models can well explain the spectral evolution upon the compositional changes. For A55, the tetrahedral Ge-Te coordination prevails, with a small contribution of the Ge-Ge coordination. As the Ge concentration increases, the Ge-Ge contribution becomes significant in that the bump (filled triangle) is prominent while the other features (the main peak and the dashed lines) are inherently weakened due to the relatively weak oscillations in the spectrum of the Ge-Ge model. The observation of the coexistence of Ge-Te and Ge-Ge in Fig. 2 most plausibly indicates the mixed coordination of Ge-Te and Ge-Ge, as is depicted in the schematic diagram shown in Fig. 2, rather than in the form of separated Ge-Te and Ge-Ge coordinations. If the two coordinations existed as separated, as in a composite of segregated GeTe (Ge-Te only) and elemental Ge (Ge-Ge only), a signature of the elemental Ge atoms should have been observed as well. The core-level XPS measurement (the inset in Fig. 3), however, showed no sign of such elemental Ge (Ge0). This supports the view that the as-grown GexTe1-x films prepared via ALD are “uniform,” without any segregated phase, even in a Ge-rich condition. Therefore, it can be concluded that the as-grown samples consisted of a tetrahedral mixture of Ge-Te and Ge-Ge and that their population ratio would change with the Ge concentration.30 The electronic structure of the Ge-rich GexTe1-x films was scrutinized to investigate the possible evolution of the electronic configurations. Figure 3 shows the valence bands (VBs) obtained through synchrotron XPS measurements of the two Ge-rich GexTe1-x films (A64 and A73). The VB spectra were taken at two photon energies: 200 and 90 eV. The VBs mainly consisted of three bump-like features, which can be referred to as Ge 4s (near binding energy (BE)=6 eV), Ge 4p (near BE=3.5 eV), and Te 5p (near the VB edge highlighted by the dashed vertical line),33 with the order of decreasing BE in accordance with the results of the density functional theory calculations.34 It is shown that the overall VB line shapes are 7



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similar to one another irrespective of the photon energy or composition, except for an increase in the intensity of the bumps near BE=6 eV. The maximal BE of the VB was estimated to be ~0.4 eV for both samples, as is indicated by a vertical dashed line in Fig. 3. Meanwhile, the chemistry of the films was identified with the core-level spectra. The inset in Fig. 3 shows the Ge 3d and Te 4d XPS spectra of the two films taken with photon energy hν=200 eV. The BEs of the Ge 3d peaks were larger than 30 eV, far above the BE of the elemental Ge (29.1 eV).35 This suggests that the ionic species of Ge in the GexTe1-x films were mostly Ge(II).28 Compared to the case of A64, the main peak for Ge(II) in A73 shifted to a lower BE, reflecting a slight reduction, which was consistent with the abundance of Ge in A73.20 Although the Te 4d peaks were very small due to the low photoionization cross-section, the doublet features for Te 4d5/2 and 4d3/2 were clearly shown. The small higher-BE shoulder in the spectrum of A73, indicated by a filled square in the inset of Fig. 3, can be attributed to the contribution of higher valence state such as Ge(IV). The signature of the higher valences, however, disappeared when the photon energy increased to 650 eV (not shown) for enlarged probing depth (~a few nm). This strongly suggests that the Ge(IV) existed only at the surface of the film (within ~1 nm), probably due to the surface oxidation. In principle, the surface oxidation can affect the VB spectrum because of the O 2p orbital states. Hence, to examine the possible contribution of O 2p from the surface oxidation, VB spectra were obtained twice by changing the photon energies (from 200 to 90 eV). The photoionization cross-section of the O 2p orbital at hν=90 eV was about 10 times larger than those of the other orbital states near the Fermi level (including Ge 4sp or Te 5p),36 and as such, an enhancement of the O 2p feature could be easily observed. No such strong enhancement was observed, however, throughout the VB range in the hν=90eV spectra. This suggests that the contribution of O 2p to the VB spectra is negligible, and the bump near BE=6 eV (the dashed area in Fig. 3) can be safely assigned solely to the Ge 4s orbital state. It should be noted that in the case of hard XAS (Figs. 1, 2, and 4), the surface contamination effects should be negligible even if they existed, due to the much larger probing depth (> a hundred nm). It was clearly observed that the intensity of the Ge 4s peak in the spectrum of A73 was larger than that in the spectrum of A64 (see the dashed area). Although fully occupied by electrons in nominal Ge(II), the Ge 4s orbitals in Ge-rich GeTe would be partially unfilled due to the intra-atomic mixing of the sp orbitals. Such an enhancement of the Ge 4s peak can be interpreted as a signature of an increased number of electrons or a partial reduction of Ge(II) in Ge-rich GeTe (A73). This is also consistent with the lower BE shift of the Ge 2p core levels in A73 (inset of Fig. 3). Thus, it can be inferred from the XPS measurement 8


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that the abundance of Ge in the Ge-rich as-grown films would preferentially lead to the electron filling of the Ge 4s orbitals. This, however, would not significantly alter the electrical property because the orbital states at the maximum VB are Te 5p rather than Ge 4s or Ge 4p, which should be filled almost completely (Te2-) regardless of the composition. The less significant compositional dependence of the electrical property may serve as an advantage for PCRAM cells because it will suppress a significant inhomogeneity in resistivity even under minute compositional instability along the high-aspect-ratio trench.

Partially crystallized GeTe The aforementioned structural stability (tetrahedral coordination with mixed Ge-Ge and Ge-Te) in spite of the chemical influence of Ge excess should be ascribed to the amorphousness of the as-grown GeTe. The structural disorders might somehow stabilize the Ge-rich phase at the nanoscale. Thus, it is worthwhile to investigate the effects of heat treatment, by which the films could become partially crystallized, losing the structural disorders. Figure 4 comparatively shows the results of the XAFS analyses on the Ge-rich GexTe1-x films before and after the PDA at 350°C. The processing temperature is much lower than the operation temperature for crystallization in actual PRAM devices (>500°C), and it was selected to induce partial crystallization in Ge-rich GeTe films for the purpose of this research. The XANES of the Ge:Te=6:4 and 7:3 films after the PDA (hereafter abbreviated as P64 and P73, respectively) are shown in Fig. 4(a), along with their as-grown counterparts (A64 and A73). XRD confirmed that parts of GeTe in P64 and P73 crystallized and were decomposed into a mixture of rhombohedral GeTe and Ge crystal, consistent with the previous report.20 Compared to the case of the as-grown samples, the main peaks at hν=~11.117 keV in the spectra of the annealed samples became slightly more intense, as highlighted by the arrows in Fig. 4(a). The stronger main peaks for both P73 and P64 can be attributed to the enhanced structural orders due to the partial crystallization. Also, two bump-like features newly appeared at hν=~11.117 and ~11.130 keV (highlighted by the open and filled triangles, respectively) in the spectra of the annealed samples. For the case of the lower energy bump (filled-up triangle), the peak intensity significantly changed with the composition; the intensity in the spectrum of P73 was larger than that in the spectrum of P64. Meanwhile, for the case of the higher energy bump (open-down triangle), the intensities in the spectra of P73 and P64 are similar to each other. 9



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The results of the FEFF simulations were included in the figure to clarify the origins of the aforementioned bumps. Note that the higher energy bumps (open-down triangle) cannot be attributed to a certain tetrahedral Ge-Te coordination because the features do not exist in the spectra of the as-grown (amorphous) samples. Therefore, to correspond to the crystalline films, the model structures of a rhombohedral GeTe (with a complete octahedral Ge-Te coordination), a disordered GeTe (the same model as in the rhombohedral Ge-Te coordination, but with a decreased FMS radius), and a cubic Ge crystal (diamond structure) were employed. The FMS radii for the three models were set as 6, 3, and 12 Å, respectively. (See the Supporting Information for more details.) It is clearly shown in the simulated spectra that the higher energy bump is a signature of rhombohedral GeTe while the lower energy bump is a signature of elemental Ge crystallites. This suggests that in addition to the precipitation of Ge crystallites, Ge-rich GeTe will consist of an ordered rhombohedral GeTe and a disordered GeTe with octahedral coordination after PDA. Therefore, it can be stated that GeTe domains are present after PDA, and they possess octahedral coordination even when they are less ordered, in contrast to the case of the as-grown GeTe (tetrahedral coordination). The predominance of octahedral Ge-Te coordination in the partially crystalline GeTe can be closely related to the fast switching characteristics of the GeTe or GeSbTe alloys, in that the unchanged local structure upon the switching would possibly save energy cost and time for converting the local coordination to a tetrahedral one. The FT EXAFS spectra (k1-weighted) of P73 and P64 (with A73 and A64 as well) are shown in Fig. 4(b). The widths of the first-shell peaks (the main peaks at the lowest R, R=~2.1 Å) of P73 and P64 are narrower than those of A73 and A64, suggesting that the mixing of Ge-Te and Ge-Ge bonds becomes suppressed after PDA. Particularly in the case of P73, the peak intensity noticeably increased compared to the case of A73, and an additional peak was observed at the higher R (R=~3.6 Å). The former can be ascribed to the enhanced structural orders in GeTe due to crystallization while the latter can be ascribed to the order of long indirect Ge-Ge bonds in Ge crystal. All these findings suggest that parts of the Ge-Te and Ge-Ge bonds become separated into rhombohedral GeTe and Ge crystallites, consistent with the bulk phase diagram.25 The schematics of the XAFS backscattering through the direct Ge-Te bonds in the Ge-Te octahedral coordination, and through the indirect Ge-Te bonds in Ge crystallites, are also appended in Fig. 4(b). In the case of P64, those peaks are less prominent than in the case of P73, implying that the local structures are seemingly less ordered. Therefore, the results of the FT EXAFS analyses are very consistent with the observations in Fig. 4(a). It should be noted, however, that the Ge-Te bond lengths (~2.6 Å) for the first shells were much smaller 10


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than the known value for rhombohedral GeTe (2.98 Å). The XRD analyses of P64 and P73 showed that the Ge-Te bond length in the rhombohedral GeTe crystallite should be ~2.95 Å, which is much larger than the EXAFS observation. This strongly suggests that the overall Ge-Te bond structure is different from that of the GeTe crystallites. PDA at a moderate temperature (below the crystallization temperature) will not result in the complete formation of rhombohedral GeTe; instead, most parts of the films will remain less ordered. This implies that although octahedral Ge-Te coordination may prevail, the crystalline phases are not yet fully stabilized due to the insufficient heat for crystallization. These findings were further supported by HRTEM.

Discussion Figure 5 shows the HRTEM and FFT images from a cross-section of the 500-nm-thick Ge7Te3 film after the same heat treatment with that of the thinner one, P73 (350 oC, 30 min). The local structure of the thick Ge7Te3 can be assimilated with that of P73 (50 nm thick). The low-magnification image in Fig. 5(a) shows a non-uniform contrast, suggesting that the composition or crystallinity of the film became inhomogeneous. This non-uniformity, however, was not observed in the case of the as-grown film (Supplementary Information Fig. S1). Therefore, the non-uniformity can be attributed to the thermal treatment. Figures 5(b) and (c) show the high-magnification images taken from some areas in Fig. 5(a), and some areas indicated by the boxes in Figs. 5(d)-(f) are further zoomed in. No noticeable periodic atomic arrangements can be seen in Fig. 5(d), while clear periodic contrasts can be seen in Fig. 5(e) and (f). The FFT images in Figs. 5(d)-(f) are also shown in Figs. 5(g)-(i), respectively. The diffraction patterns in Fig. 5(g) are very obscure, indicating that the atomic structures in the selected region in Fig. 5(d) are much disordered. On the other hand, the diffraction patterns in Figs. 5(h) and (i) can be analyzed in accordance with rhombohedral GeTe (PDF#00-047-1079) and cubic Ge (PDF#01-089-4164), indicating the dominance of the rhombohedral GeTe in the area in Fig. 5(e), and of the cubic Ge in the area in Fig. 5(f), Therefore, it is evident that although the low-temperature annealing induced partial rhombohedral GeTe crystallization as well as partial Ge segregation, significant parts of the films remained less-ordered GeTe. This can explain why the average Ge-Te bond length in P73 seen in Fig. 4(b) is close to that in A73. The majority of the GeTe in P73 should lack structural orders while the Ge-Te octahedral coordination must represent its local structure, as is evident in the results of the EXAFS analyses (Fig. 4(a)). The readily octahedral coordination even in the disordered phase might have been responsible for the fast PCRAM 11



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switching behavior of GeTe because the speed of switching towards the crystalline orders would strongly depend on the similarity in the atomic arrangements of the disordered to the ordered phase. Figure 6 shows the XRD patterns and the Raman spectra of the GeTe films before and after the PDA. Compared to XAS (Figs. 1, 2, and 4) or XPS (Fig. 3), XRD and Raman spectroscopy are strongly subject to the long-range atomic orders, and as such, the volume of the specimen is essential for obtaining appreciable features. Thus, the thicknesses of the GeTe films were increased to ~250 nm. In Fig. 6(a), the XRD patterns of the Ge:Te=5:5 and Ge:Te=7:3 films are shown. The XRD patterns of the as-grown films were very broad, suggesting the amorphous nature of the as-grown states even for the high thicknesses. After the PDA (350oC, 30 min), the signatures of rhombohedral GeTe appeared for both compositions, while in the case of Ge-rich GeTe (Ge:Te=7:3), additional peaks for Ge crystallites were also clearly observed. These findings are consistent with the XANES/EXAFS observations for the thin GeTe films (Figs. 1, 2, and 4), implying that the thickness itself will not substantially change the structural properties of GeTe. The Raman spectra in Fig. 6(b) show the signatures of Ge-Te and Ge-Ge atomic chains in the two compositions. The features at the wavenumbers below 150 cm-1 can be attributed to the Ge-Te chains, as in the octahedral Ge-Te (Ref. 37) or the corner-sharing tetrahedral Ge-Te (Ref. 38). The broad features at the higher wavenumbers (150-250 cm-1) can also be attributed to certain vibration modes in the Ge-Te chains in the as-grown (amorphous) state,17 and the intensity of those features became suppressed after the PDA. The feature highlighted by an asterisk in Fig. 6(b) can also be attributed to certain Ge-rich GeTe chains because it became pronounced at a higher Ge concentration. The sharp peak at the wavenumber near 298 cm-1 can be attributed to the Ge-Ge chains in the cubic Ge crystallites. The features are observed only in the spectra of the Ge:Te=7:3 samples, manifesting the Ge segregation in Ge-rich GeTe after the PDA. Interestingly, in the as-grown sample, a broad peak also existed at a slightly lower wavenumber (292 cm-1). This indicates the prevalence of Ge-Ge chains in Gerich GeTe even when the film is amorphous (as-grown state). The weaker Ge-Ge bonds in the amorphous state will result in a slight low energy shift of the vibration modes compared to the case of the cubic Ge crystallites. The thermal phase stability of the GeTe films was investigated by examining the temperature dependence of sheet resistance (Rs). Figure 7 shows the sheet resistance versus temperature (Rs-T) curves of the thick GeTe films, the same with the specimens measured in the HRTEM, XRD, and Raman spectroscopy (Figs. 5 and 6). The temperature of the amorphous-to-crystalline phase transition can be estimated by the sudden drops in the Rs values. They are estimated to be 170 oC, 245 oC, and 315 oC for Ge:Te=5:5, 6:4, and 7:3, 12


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respectively. The transition temperature of Ge:Te=7:3 sample (~315 oC) is close to the PDA temperature (350 oC), implying that the crystallization of P73 might be incomplete, consistent with the observations in Figs. 5 and 6.

Conclusion In conclusion, the X-ray absorption spectroscopy (XAS) study on GexTe1-x (x=0.5, 0.6, and 0.7) films together with the Fourier-transformed (FT) extended X-ray absorption fine structure (EXAFS) analyses and FEFF simulations showed that the GeTe films grown via atomic layer deposition (ALD) at 250 nm) before and after the PDA. In the case of 7:3 samples, the Raman peaks near wavenumber of 298 cm-1 are the signatures for certain Ge-Ge chains while those near 100 cm-1 are the signatures for the Ge-Te chains in GeTe. The feature highlighted by the asterisk in (b) can be attributed to a certain vibration of the Gerich Ge-Te chains in the as-grown (amorphous) state. Fig. 7: Sheet resistance versus temperature (Rs-T) curve of the thick GeTe films measured with increasing temperature. The temperature of the amorphous-to-crystalline phase transition (sudden drop in Rs) increases with Ge concentration. For Ge:Te=7:3, the transition temperature (~315 oC) is close to the PDA temperature (350 oC) implying incomplete crystallization of P73, consistent with the observations in Figs. 5 and 6.

References (1) Wuttig, M.; Yamada, N., Phase-Change Materials for Rewriteable Data Storage. Nature Materials 2007, 6, (11), 824-832.

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(18) Salicio, O.; Wiemer, C.; Fanciulli, M.; Gawelda, W.; Siegel, J.; Afonso, C. N.; Plausinaitiene, V.; Abrutis, A., Effect of Pulsed Laser Irradiation on the Structure of GeTe Films Deposited by Metal Organic Chemical Vapor Deposition: A Raman Spectroscopy Study. Journal of Applied Physics 2009, 105, 033520.

(19) Carria, E.; Mio, A. M.; Gibilisco, S.; Miritello, M.; Bongiorno, C.; Grimaldi, M. G.; Rimini, E., Amorphous-Crystal Phase Transitions in GexTe1-x Alloys. Journal of the Electrochemical Society 2012, 159, (2), H130-H139.

(20) Gwon, T.; Eom, T.; Yoo, S.; Lee, H.; Cho, D.; Kim, M.; Buchanan, I.; Xiao, M.; Ivanov, S.; Hwang, C., Atomic Layer Deposition of GeTe Films Using Ge{N[Si(CH3)3]2}2, {(CH3)3Si}2Te, and Methanol. Chemistry of Materials 2016, 28, 7158-7166.

(21) Cho, S. L.; Yi, J. H.; Ha, Y. H.; Kuh, B. J.; Lee, C. M.; Park, J. H.; Nam, S. D.; Horii, H.; Cho, B. K.; Ryoo, K. C.; Park, S. O.; Kim, H. S.; Chung, U.-i.; Moon, J. T.; Ryu, B.-I. In Highly Scalable on-Axis Confined Cell Structure for High Density PRAM Beyond 256mb, Symposium on VLSI Technology 2005, 2005; IEEE.

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Buchanan, I.; Cho, D.-Y.; Hwang, C. S., Conformal Formation of (GeTe2)1-x(Sb2Te3)x Layers by Atomic Layer Deposition for Nanoscale Phase Change Memories. Chemistry of Materials 2012, 24, (11), 20992110.

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Layers Using Ge4+-Alkoxide Precursors. Chemistry of Materials 2014, 26, (4), 1583-1591.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ToC graphic

21



FT[k χ(k)]

Intensity (arb. unit)

Ge–Ge

(a)

(b) Real part

As-grown samples Ge:Te = 7:3

Ge–Te

Ge:Te = 6:4 Ge:Te = 5:5

Amplitude 0

1

2

Exp. 3

Fit

40

1

2

R (Å) Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 22 of 28

(c)

3

4

R (Å) Hanning Sill

k χ(k)

Ge:Te = 7:3 6:4 5:5 Exp.

4

6

8

Fit -1

k (Å )

10


12

Fig.1UHY

Page 23 of 28

Ge K-edge XAS As-grown samples

Absorption Coefficient (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Ge:Te = 7:3

Ge:Te = 6:4 Ge:Te = 5:5 Tetrahedral Ge–Te Tetrahedral Ge–Ge

Increasing Ge content

Te Te

Ge

Sim.

Te Ge

11.110 11.10

11.12

11.14

11.120 11.16

11.130 11.18

Photon Energy (keV)


Fig.2rev

As-grown samples Ge:Te = 7:3 Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (a. u.)


h= 200 eV

Ge

7:3 6:4 Te 4d 45

Ge 4s

2+

Ge 3d

Ge

4+

40 35 30 Binding Energy (eV)

h= 90 eV

Ge 4p Te 5p Ge 4s

h = 200 eV

Ge:Te = 6:4

h= 90 eV

Valence Band h = 200 eV 8

6

4

2

Binding Energy (eV)


0

Page 24 of 28

(a)

(b)

XANES

EXAFS Te

Te

7:3 PDA 7:3 As-grown

Ge

Ge–Te or Ge–Ge Te

Te

Ge

6:4 PDA 6:4 As-grown 7:3 PDA 7:3 As-grown

Ge

FEFF Simulation

0

Ge

Ge


Octa. Ge–Te (RFMS= 3 Å) Octa. Ge–Te (RFMS= 6 Å) Ge crystal 11.10 11.11 11.12 11.13 11.14

Ge Ge

Ge

FT Magnitude (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Absorption Coefficient (arb. unit)

Page 25 of 28

1

2

Photon Energy (keV)


3

4

5

R (Å)

Fig.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

(a)

100nm

(d)

(b)

Page 26 of 28

(g)

5 1/nm

Ge7Te3

Amorphous

(e)

SiO2 Si

Ge7Te3

(h)

20nm

(c) Ge7Te3

(f)

(i)

20nm

Fig.5 ACS Paragon Plus Environment

Page 27 of 28

(a)

Ge–Ge

> 250 nm-thick GeTe Ge (220)

Ge (111)


(b) Raman

XRD

* 7:3 PDA 7:3 As-grown

GeTe (223)

(042) Si (311)

GeTe (021)(220)

GeTe (021) GeTe (202)

Ge–Te



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


GeTe

20

30

40

50

2θ (degree)

60

100

150

200

250

300 -1

350

Raman Shift (cm )


Fig.6UHY


10

R s ( Ω/sq)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 28 of 28

10

10

8

10

6

10

4

10

2

10

0

7:3 6:4 5:5

0

100

200

300

400

o

Temperature ( C)


Fig.7QHZ

Structural Analyses of Phase Stability in ... - ACS Publications

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