Variations of Local Motifs around Ge Atoms in Amorphous GeTe

Subscriber access provided by - Access paid by the | UCLA Library

Article

Variations of Local Motifs Around Ge Atoms in Amorphous GeTe Ultrathin Films Ping Ma, Hao Tong, Ting Huang, Ming Xu, Niannian Yu, Xiaomin Cheng, Chengjun Sun, and Xiangshui Miao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09841 • Publication Date (Web): 24 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

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

The Journal of Physical Chemistry

Variations of Local Motifs Around Ge Atoms in Amorphous GeTe Ultrathin Films

Ping Ma1,2, Hao Tong1,2, Ting Huang1,2,3, Ming Xu1,2, Niannian Yu4, Xiaomin Cheng1,2, Chengjun Sun3, and Xiangshui Miao*,1,2

1

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan 430074, China 2

School of Optical and Electronic Information, Huazhong University of Science and Technology,

Wuhan 430074, China 3

X-ray Science Division, Argonne National Laboratory, Argonne, IL, USA

4

School of Science, Wuhan University of Technology, Wuhan, 430070, China

Correspondence and requests for materials should be addressed to X. M. ([email protected])

1

ACS Paragon Plus Environment


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

Abstract:

Page 2 of 31

Phase change materials, the highly promising candidate for nonvolatile

data recording, present a different phase change property when film thickness shrinks to very deep submicron scale. The local structure of amorphous GeTe ultrathin films, which contributes to the characteristics of phase change, is examined using X-ray absorption measurements. Ge atoms are found to be low-coordinated when the film thickness decreases. Ge atoms are linked to neighbor atoms by covalent bond, and the weaker Ge-Te bonds are easier to be broken, which suggests that Ge atoms are located in the defective Ge2Te3 local arrangement. The mixture of sp3/sp2 hybridization and 3-coordinated Ge found in Ab initio Molecular Dynamics simulations also support this local motif. The exponential rise of crystallization temperatures of ultrathin films with decreasing film thickness, which is a vital parameter for phase change process, can be well explained by the proposed defective GeTe local arrangement.

2


Page 3 of 31

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


Introduction Phase change random access memory (PCRAM), which utilizes the tremendous difference of electrical properties between amorphous and crystalline phases of phase change materials (PCMs)1, is considered to be one of the most promising next generation nonvolatile memory2. Due to the impressive retention, endurance, performance, and yield characteristics3, PCMs are believed to be excellent candidate materials for nonvolatile data recording4. The extraordinary properties5 of PCMs exhibit their noticeable capability to extend Moore’s Law, which make it possible to continuously implement the “next size smaller” device for generations6. The key challenge for PCRAM is understanding the variation of crystallization temperature7 for PCMs when memory size shrinks to very deep submicron (VDSM) scale. When the thickness decreases to a certain value, the small size effect makes crystallization temperature significantly different from bulk materials. However, this variation of crystallization temperature remain to be explained from the perspective of micromechanism8. Recent studies found that, chalcogenides are one class of materials whose amorphous phases contain unique local motifs, such as the mixture of tetrahedral and octahedral clusters9, and the ratio between these local motifs can be vastly different in various amorphous phases10. These local structures also majorly contribute to the electrical property and crystallization speed11. Thus, the study of microscopic variation may have important implications on modification of Ge-Sb-Te phase change materials, for instance, tailoring the material properties of Ge2Sb2Te5 materials by doping Si12,13, N14, O15, and Ag16 elements. Due to the lack of effective 3



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

Page 4 of 31

measurement, however, the mechanism of such size effect on amorphous phase change films is yet to be understood. The X-ray absorption (XAS) measurements have been proved to one of the most effective tools to investigate the local structure of amorphous materials17. In this letter, we took advantage of X-ray Absorption Near-Edge Structure (XANES) in fast revealing the change of local motifs of amorphous (a-GeTe), and utilized the Extended X-Ray Absorption Fine Structure (EXAFS) to investigate the chemical environment around Ge atoms18. We propose that the defective Ge2Te3 local arrangement is the major reason that accounts for the variation of chemical environment around Ge, and this proposition is supported by the structural analysis of the models calculated by Ab initio Molecular Dynamics simulations. Such variations of local motifs in a-GeTe explain the change of crystallization temperature when the film thickness is reduced below a certain value.

Methods The X-ray absorption measurements for GeTe ultrathin films. A-GeTe thin ﬁlms with different thickness (100 nm, 10 nm, and 3 nm) are deposited on 2 µm thermal SiO2 coated Si (111) substrates by DC magnetron sputtering. The sputtering rate is lowered to 7 nm/min to guarantee the small surface roughness of ultrathin ﬁlms. In order to evaluate the morphology of 3 nm samples, transmission electron microscope (TEM) results of annealed ultrathin films in similar scale are shown in Fig.S2 in supplement materials. Film thickness and surface roughness measurements obtained 4


Page 5 of 31

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


with contact Atomic force microscopy (AFM) are given in Fig.S3 and Table S2 in supplement materials. X-ray absorption measurements are performed at room temperature and ambient pressure in fluorescence mode at the undulator beamline 20-ID-B of the Advanced Photon Source (APS), Argonne National Laboratory. The energy resolution is ∆E / E =1.3×10-4, or 2.2 eV. All samples are mounted at 10°to the incident X-ray beam. The polarization of the X-rays is linear. At least five sets of data are acquired for each sample, and in all instances the distinctions of spectra are negligible. The software package Athena version 0.8.056 and Artemis version 0.8.012, which include FEFF and IFEFFIT codes, were used for data analysis. K-weighting of the data is 2, and fitting was performed in the back-transformed k-space with k-range 2

from 2 to 13.85. During overall process, the amplitude reduction factor ( S0 ) was fixed to the value from the reference19. The procedure used to estimate the confidence limits is described as following: first, coordination numbers CNs was set as the value from the reference, and the interatomic distance R, the Debye-Waller factor σ2, and the edge-energy shift E0 were allowed to vary for each shell. Second, R was set to the value from the first step, and CNs, σ2, E0 were allowed to vary.

Ab initio Molecular Dynamics for a-GeTe models. Calculations have been done for a fixed volume (Canonical Ensemble) using Vienna Ab-initio Simulation Package (VASP)

20

. We use gradient corrected functional in the form of the Generalized

Gradient Approximation PBE (GGA-PBE) by Perdew, Burke, and Ernzerhof21. Periodic boundary conditions with a single point (k=0) in the Brillouin zone are 5



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

Page 6 of 31

applied, and a plane wave basis with an energy cutoff of 287 eV was used. Amorphous GeTe models are generated via melt-quenching method. The initial model is the rock salt structure of 2×2×1 supercell, and a cleaved surface is introduced in (001) direction. We build a 10 Å vacuum slab on this surface to simulate the existence of surface layers. The size of model box is 12.4×12.4×16.2 Å3 and it contains 48 atoms. The total process is as follows: First, the model is heated at 2000 K for 3ps. Then, it is slowly quenched to 298 K within 30ps, and last, the model is equilibrated at 298 K for 3ps to receive the final structure. It is worth pointing out that, Molecular Dynamics (MD) quenching method naturally generates large number of wrong bonds, and therefore the ART program22 developed by Normand Mousseau, et al. is recommended for such binary system, in the future.

The measured crystallization temperature as a function of film thickness. Films of different thickness (100 nm, 50 nm, 10 nm) are deposited on 1 µm thick SiO2 film on Si and measured using the phase change temperature tester23 which enables real time detection. Samples are heated under the background vacuum pressure of 102 Pa and by the temperature rate of 20 °C/min. The tester utilizes the reflectivity of a focused infrared laser beam to detect the phase change process. As film thickness decreases, the transmission quantity of laser becomes a major problem and the reflection quantity is reduced. When the film thickness shrinks down to 3nm, the photo detector in this tester is unable to distinguish the reflection difference for crystallization. Due to weak signals of reflection over the films, the samples of 3 nm 6


Page 7 of 31

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


in this test method failed to provide useful information. In order to explore the crystallization temperature of ultrathin films, the crystallization temperature empirical formula is introduced to extrapolate the crystallization temperature of 3nm films from the fitting curve.

Results and Discussion The X-ray absorption fine structure data for a-GeTe films of different thickness. The normalized XANES spectra at Ge K-edge for as-deposited amorphous GeTe films of 3 nm, 10 nm and 100 nm are plotted in Fig. 1, showing the following features: 1) In the vicinity of the threshold, peak A is strong in 3 nm sample and shifts to lower energies compared with that in thicker films. 2) Other than peak A, 10 nm and 100 nm samples have an extra well-resolved peak B at ca. 11110 eV. Previous work has shown the existence of a pronounced maximum at the XANES absorption edge for many solids. The phenomenon is called “white line”24. For K-edge absorption, it may be due to one of two reasons: (1) a high density of unoccupied states (DOS), with p symmetry in the neighborhood of the absorbing atom; and (2) the formation of exciton levels. Just as expected from dipole selection rules, the absorption edge should be related to p-DOS of Ge and Te atoms. Considering the arrangement of valence electrons, white line in XANES spectra corresponds to Ge 1s to Ge 4p-Te 5p anti-bonding states25. It is believed that white line (peak A in Fig.1) should be proportional to unoccupied density of states26. The strong white line, 7



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

Page 8 of 31

usually regarded to be located at octahedral environment rather than tetrahedral one27, appears in 100 nm sample group, which indicates that in the thicker films, majority of Ge atoms are in the octahedral-like sites. On the contrary, the white line in 10 nm sample is obviously weaker than 100 nm sample, which suggests that, Ge atoms in the thinner films are more prone to be in the tetrahedral bonding environment, rather than the octahedral one. This is consistent to previous work about amorphous GeTe ultrathin films28. The peak B at ca. 11110 eV, present in 100 nm and 10 nm sample spectra, used to be the characteristic peak of Ge-oxide in the previous bulk materials research29. The spectra of 3 nm sample has a shoulder at ca. 11110 eV, and the broad peak B is also recognized by Gaussian fitting, as shown in Fig.S1 in supplement materials. Figure 2 compares the Fourier transform magnitude for amorphous GeTe samples of different thickness. The peaks above 2 Å are apparently weaker in the thinner films. It is consistent with previous opinion that, low intensity corresponds to local structure of thinner films30. Furthermore, the location is close to the sum of the corresponding covalent radii for the elements (rGe=1.22 Å, rTe=1.35 Å)31. Thus, this part of peaks are inferred to correlate with the local environment of Ge atoms in GeTe films. To get further insights, we perform simulation analysis using the region extending from 2 to 3 Å. Structural parameters obtained are summarized in Table 1. The fitting results for samples of different thickness is present in Fig. 3 (a) (b) (c). It is worth pointing out that, in order to focus on reproducing the main features of Ge atoms local environment, only two-body contributions are included in performed 8


Page 9 of 31

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


simulations. For tetrahedron structure, three-body contributions in multiple-scattering theory, though it makes no difference to the following analyses, lead to discrepancies from experimental results in our fitting results32. We can see that the main bonding environment of Ge atoms is composed of Ge-Ge and Ge-Te bonding structure33. The presence of so-called “wrong” bonds34, or Ge-Ge bonds, which is characteristic of as-deposition chalcogenide films, is observed. When the film thickness decreases, the Ge-Te and Ge-Ge bond lengths are almost constant, which suggests that Ge atoms are located at relatively stable blocks. The Ge-Te and Ge-Ge bond lengths are close to the sum of the covalent radii for the corresponding elements (rGe = 1.22 Å, rTe = 1.35 Å)35, and thus it is reasonable to consider that, inside these blocks, Ge atoms prefer to connect with neighboring atoms by covalent bonds. Debye-Waller factors σ2 is based on the following equation:

Θ h2 σ = coth( Ε ) + σ 02 2µkBΘΕ 2T 2

Here

Θ Ε is Einstein temperature, µ

Boltzmann’s constant, and

σ0

(1) is the reduced mass,

kB is

is static disorder. The Einstein temperature Θ Ε is

relatively constant for tetrahedrally bonded Ge atoms, and µ , T are as well constant

h2 Θ coth( Ε ) is a constant, marked as CT. So the for different samples. The 2µ k B ΘΕ 2T relationship between static disorder

σ0

and Debye-Waller factor σ2 should be

σ 02 = σ 2 − CT In Table 1, the σ2 increases when the film thickness decreases, and hence static disorder

σ0

becomes larger in thinner films. It demonstrates that, as neighboring 9



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

Page 10 of 31

atoms could not be able to fully occupy each sublattice position around Ge atoms, Ge atoms begin to present a soft bond relationship. The coordination numbers (CNs) of Ge-Te bonds present a declining trend when the film thickness decreases. It is worth pointing out that the surface to volume ratio increases as well with the film thickness decreasing. However, it is not difficult to rule out its influence on the observed changes in co-ordination. The CNs are almost constant from 100nm to 10nm when the surface to volume ratio increases by a factor of ten. The CNs undergo a sharp decrease from 10 nm to 3 nm but the surface to volume ratio just triples. Thus it is inferred that the surface to volume ratio has weak relation with the changes in co-ordination, and the observed results principally come from the disorder. The above results suggest that, the local structure of a-GeTe films could be described as a defective Ge2Se3 type local motif36, in which the position of Te atoms may be replaced with vacancy, as shown in Fig. 3 (d). The influence of surface becomes significant in thinner films and these Ge2Se3 blocks would be gradually broken. The Ge-Ge interatomic distance (2.50 Å) is shorter than Ge-Te (2.60 Å), and thus the homopolar Ge-Ge bond energy is slightly higher than Ge-Te bonds. Ge-Te bonds are likely to be broken firstly, and as a result a vacancy replaces Te atom. When GeTe transfers from crystalline to amorphous state, the bond length decreases concomitant with the density decrease, which is reminiscent of molecular solids (e.g. selenium)37. Similar to the case of selenium, upon the loss of the long-range order, the first nearest neighbor distances of GeTe become shorter and the second nearest 10


Page 11 of 31

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


neighbor distances (i.e. interchain distances) increase, which as a result leaves a large numbers of gaps for Te atoms. This explains that when the coordination number of Ge-Te decreases, CNs of Ge-Ge remain unchanged in Table 1. Fitting parameters of different thickness undergoes a slight variation from 100 nm to 10 nm sample, but changes sharply from 10 nm to 3 nm, which indicates that the local structure undergoes a major change in sub-10nm scale. The fitting results of 3 nm sample show that, in surface layers, the sublattice composed by Te atoms, usually regarded as “braced structure”38, could be heavily occupied by vacancy. Mean interatomic distances (2.56 Å) of Ge-Te in 3 nm film are slightly smaller than other groups (2.60 Å). This means a positively charged Te+ atom may be formed, which leads to a smaller radii and shorter Ge-Te bonds. In Fig.2, the spectra consists of a series of peaks below 2 Å, which is inferred to be related with Ge-oxide commonly-formed in nano phase change materials39. In order to prove our assumption, least-squares curve fitting is performed to give coordination numbers CNs, mean interatomic distances R, and Debye-Waller factors σ2 of GeTe films with different thickness. The above results are listed in Table S1 in supplement materials. The parameters were consistent with the previous XASF report about GeO229.

The statistical distributions of a-GeTe model structure calculated via Ab initio Molecular Dynamics. Bond angle distributions for Ge atoms in the surface and body layer of a-GeTe thin film model are illustrated in Fig. 4, and the distribution of 11



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

Page 12 of 31

coordination numbers of Ge atoms for each layer is plotted in the inset. The two main peaks in body layers (91.5° and 153.5°) is close to the mixture of p/sp3 hybridization. In surface layers, two main peaks (95.5° and 117.5°) could be associated with p/sp3 and sp3/sp2 hybridization of silicone40, respectively. The remarkable variations near the peak of 117.5° indicates that sp3/sp2 hybridization becomes major. The core-level binding-energy shifts41 provides the explanation why Ge sites in the

surface

layer

undergo

such

changes.

In

surface

layers,

core-level binding-energy shifts have been observed in various elements42 , which include Ge. The shifts are toward higher (lower) binding energies for the surface cations (anions)43. In amorphous chalcogenides, due to valence alternation pairs44, a positively charged Te+ is usually formed for the lowest-energy configuration. As a neutral system, GeTe must contain partial negatively charged Ge- accounting for the presence of positively charged defects36. The core-level binding-energy shifts of negatively charged Ge- are toward lower energies, which increases the energy gap of 4s and 4p level, the initial states of sp3-hybridized Ge atoms. It increases the energy consumption for the formation of tetrahedral-coordinated Ge atoms and thus affects the stability of Ge-Te bonds. More Ge-Te bonds are broken under the energy shifts, and thus more three-fold local structures are formed, which is consistent with the defective GeTe local arrangement. For body atoms, less surface-binding-energy shifts results in relatively-complete tetrahedral bonding environment, which is similar to reported local order of amorphous chalcogenides45. Coordination number distributions also provide clear proof of the local configure 12


Page 13 of 31

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


change. Ge atoms in the body layer show a mixture of 4- and 3-coordination, whereas most Ge atoms in the surface layer are 3-coordinated. It demonstrates that Ge atoms are likely to be 4-coordination when less affected by surface. In the surface layer, a large density (~1018 cm-3) of dangling bonds act as acceptors and donors46, which sufficiently lower the energy of such atomic motion44. Thus, structural transition is more likely to occur and low-coordination Ge is formed.

The exponential rise of measured crystallization temperatures. The crystallization temperature Tx as a function of film thickness for GeTe (Red) is shown in Fig.5. The dash line is calculated by the following empirical formula47

Tx = Tax + (Tm − Tax ) e− d / C

(2)

where Tax is the crystallization temperature of bulk amorphous materials, Tm is the melting temperature (725°C for GeTe)48, d is the film thickness, and C is the fitting constant. The fitting parameters C varies from 4.2±0.1 nm for GeTe to 3.3± 0.1 nm for Ge2Sb2Te5. For Si, Sb2Te, and GeSb, the variation in C is as well substantial (Si: 2.4 nm, Sb2Te: 2.3 nm, GeSb: 0.7 nm). The lattice constants are believed to have a relationship with the fitting factor C. For GeTe, Ge2Sb2Te5, and Sb2Te, the lattice constants are quite similar. GeSb has a smaller lattice constant (a=4.4 Å, c=11.89 Å) and thus its fitting factor C is the smallest. For bulk materials, the crystallization temperature is constant (Tax). As the film thickness is reduced below a certain thickness, the crystallization temperature is no longer constant. Tx undergoes an exponential rise when the film thickness decreases to sub-10 nm scale. 13



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

Page 14 of 31

It is argued that, the phase change between crystalline and amorphous states arises from different occupation of Ge atoms between octahedral and tetrahedral sites within Te sublattice. An ordered Te sublattice is the precondition for a fast switch of Ge atoms49. The defective Ge2Te3 local arrangement induced by scale effect indicates a gradually randomizing Te sublattice, which increases the transition barrier. As a result, the Ge atoms in the surface layer, in which the local configure is heavily defective, are hard to return to the long-range order state, or called “crystalline phase” via the low-energy consumed transformation. Instead, when the film thickness is beyond the surface layer scale, due to the similar local arrangement to bulk materials, the crystallization temperature of films stays the same as that of bulk materials. The crystallization temperature of Ge2Sb2Te5 (black in Fig.5) present the same variation trend as GeTe. It means that chalcogenides may have the similar variation of local motifs around Ge atoms. It is also worth pointing out that, the increased transition barrier induced by defective local arrangement not only affect the crystallization temperature, but is also reflected in other macroscopic characteristics, such as crystallization rates. The crystallization rate of chalcogenides falls as film thickness is below 30 nm, and an introduced thin acceleration layer is possible to increase the crystallization rate50. The acceleration layer to reduce or increase the crystallization temperature is observed as well51. The defective Ge2Te3 local arrangement is adjusted by introducing a surface acceleration layer, which may be an important rule for the structural design of PCRAM.

14


Page 15 of 31

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


Conclusions The structure of a-GeTe has been studied for several decades. Several local motif models were put forward to match experimental results but the situation of amorphous phase is still not completely resolved. Moreover, the variation of phase change characteristics in VDSM scale, which is the key problem in PCRAM development, remains to be explained in atomistic level. The proposed defective Ge2Te3 local arrangement (Fig. 3d) provides a novel view that, the local motif around Ge atoms is under change and the amorphous models of chalcogenides should be verified in both bulk and ultrathin scales. It helps to understand the Ge-atom local motif of bulk amorphous chalcogenides, but also offers a guide for industry to implement reliable smaller devices. In summary, based on X-ray absorption measurement, we further investigate the local arrangement of Ge atoms in GeTe ultrathin film. We judge that, Ge atoms tend to be low-coordinated with decreasing film thickness. The EXAFS spectra gives a clear insight about the local order of Ge atoms, and thus we propose the defective Ge2Te3 local motif model. In addition, the structural distribution in ab initio Molecular Dynamics simulation shows prominent sp3/sp2 hybridization of Ge atoms and 3-coordination in the surface layer, which supports this local configure model. At last, this variation of microstructure is used to illustrate successfully the crystallization temperature of GeTe with the decreasing of film thickness.

15



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

Page 16 of 31

Associated Content Supporting Information: (1) The XANES spectra of 3 nm sample with separated Gaussian bands; (2) TEM of ultrathin GeTe films prepared by magnetron sputtering; (3) AFM image of 3 nm GeTe films; (4) summary of the Ge-O fitting results; (5) The measured thickness and surface roughness of different sample groups. This material is available free of charge via the Internet at http://pubs.acs.org/. Conflict of Interest: The authors declare no competing financial interest.

Acknowledgements. This work was supported by the grants from National High-tech R&D Program of China (863 Program) (Grant No. 2014AA032903) and the National Natural Science Foundation of China (Grant No. 61306005, 61474052, 11504281). Sector 20 facilities at the Advanced Photon Source, and research at these facilities, are supported by the US Department of Energy - Basic Energy Sciences, the Canadian Light Source and its funding partners, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

16


Page 17 of 31

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


Reference (1) Ovshinsky, S. R. Reversible Electrical Switching Phenomena in Disordered Structures. Phys. Rev. Lett. 1968, 21, 1450-1453. (2) Sosso, G. C.; Salvalaglio, M.; Behler, J.; Bernasconi, M.; Parrinello, M. Heterogeneous Crystallization of the Phase Change Material GeTe Via Atomistic Simulations. J. Phys. Chem. C 2015, 119, 6428-6434. (3) Tomforde, J.; Bensch, W.; Kienle, L.; Duppel, V.; Merkelbach, P.; Wuttig, M. Thin Films of Ge– Sb–Te-based Phase Change Materials: Microstructure and In Situ Transformation. Chem. Mater. 2011, 23, 3871-3878. (4) Beneventi, G. B.; Perniola, L.; Sousa, V.; Gourvest, E.; Maitrejean, S.; Bastien, J.; Bastard, A.; Hyot, B.; Fargeix, A.; Jahan, C. Carbon-doped GeTe: A Promising Material for Phase-change Memories. Solid-State Electron. 2011, 65, 197-204. (5) Sosso, G. C.; Colombo, J.; Behler, J.; Del Gado, E.; Bernasconi, M. Dynamical Heterogeneity in the Supercooled Liquid State of the Phase Change Material GeTe. J. Phys. Chem. B 2014, 118, 13621-13628. (6) Xiong, F.; Liao, A. D.; Estrada, D.; Pop, E. Low-power Switching of Phase-change Materials with Carbon Nanotube Electrodes. Science 2011, 332, 568-570. (7) Sosso, G. C.; Miceli, G.; Caravati, S.; Giberti, F.; Behler, J.; Bernasconi, M. Fast Crystallization of the Phase Change Compound GeTe by Large-scale Molecular Dynamics Simulations. J. Phys. Chem. Lett. 2013, 4, 4241-4246. (8) Park, S. J.; Park, H.; Jang, M. H.; Ahn, M.; Yang, W. J.; Han, J. H.; Jeong, H.-S.; Kim, C.-W.; Kwon, Y.-K.; Cho, M.-H. Laser Irradiation-induced Modification of the Amorphous Phase in GeTe Films: the Role of Intermediate Ge–Te Bonding in the Crystallization Mechanism. J. Mater. Chem. C 2015, 3, 9393-9402. (9) Xu, M.; Cheng, Y.; Sheng, H.; Ma, E. Nature of Atomic Bonding and Atomic Structure in the Phase-change Ge2Sb2Te5 Glass. Phys. Rev. Lett. 2009, 103, 195502. (10) Krbal, M.; Kolobov, A. V.; Fons, P.; Tominaga, J.; Elliott, S. R.; Hegedus, J.; Uruga, T. Intrinsic Complexity of the Melt-quenched Amorphous Ge2Sb2Te5 Memory Alloy. Phys. Rev. B 2011, 83, 054203. 17



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

Page 18 of 31

(11) Zhang, W.; Ronneberger, I.; Zalden, P.; Xu, M.; Salinga, M.; Wuttig, M.; Mazzarello, R. How Fragility Makes Phase-change Data Storage Robust: Insights From Ab Initio Simulations. Sci. Rep. 2014, 4, 6529. (12) Kölpin, H.; Music, D.; Laptyeva, G.; Ghadimi, R.; Merget, F.; Richter, S.; Mykhaylonka, R.; Mayer, J.; Schneider, J. M. Influence of Si and N Additions on Structure and Phase Stability of Ge2Sb2Te5 Thin Films. J. Phys.: Condens. Matter 2009, 21, 435501. (13) Cho, E.; Han, S.; Kim, D.; Horii, H.; Nam, H.-S., Ab Initio Study on Influence of Dopants on Crystalline and Amorphous Ge2Sb2Te5. J. Appl. Phys. 2011, 109, 043705. (14) Shelby, R. M.; Raoux, S. Crystallization Dynamics of Nitrogen-doped Ge2Sb2Te5. J. Appl. Phys. 2009, 105, 104902. (15) Song, K.-B.; Sohn, S.-W.; Kim, J.; Kim, K.-A.; Cho, K. Chalcogenide Thin-film Transistors Using Oxygenated N-type and P-type Phase Change Materials. Appl. Phys. Lett. 2008, 93, 043514. (16) Prasai, B.; Chen, G.; Drabold, D. Direct Ab-initio Molecular Dynamic Study of Ultrafast Phase Change in Ag-alloyed Ge2Sb2 Te5. Appl. Phys. Lett. 2013, 102, 041907. (17) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer: Berlin, 1986. (18) Kolobov, A. V.; Fons, P.; Krbal, M.; Simpson, R. E.; Hosokawa, S.; Uruga, T.; Tanida, H.; Tominaga, J. Liquid Ge2Sb2Te5 Studied by Extended X-ray Absorption. Appl. Phys. Lett. 2009, 95, 241902. (19) Stern, E.; Bunker, B.; Heald, S. Many-body Effects on Extended X-ray Absorption Fine Structure Amplitudes. Phys. Rev. B 1980, 21, 5521-5539. (20) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-energy Calculations Using A Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (21) Elmér, R.; Berg, M.; Carlén, L.; Jakobsson, B.; Norén, B.; Oskarsson, A.; Ericsson, G.; Julien, J.; Thorsteinsen, T.-F.; Guttormsen, M.; et al. K+ Emission in Symmetric Heavy Ion Reactions at Subthreshold Energies. Phys. Rev. Lett. 1996, 77, 4884-4886. (22) Chopra, K.; Bahl, S. Amorphous Versus Crystalline GeTe Films. I. Growth and Structural Behavior. J. Appl. Phys. 1969, 40, 4171-4178. (23) Tong, H.; Yu, N.; Yang, Z.; Cheng, X.; Miao, X. Disorder-induced Anomalously Signed Hall Effect in Crystalline GeTe/Sb2Te3 Superlattice-like Materials. J. Appl. Phys. 2015, 118, 075704. (24) Bazin, D.; Sayers, D.; Rehr, J.; Mottet, C. Numerical Simulation of the Platinum LIII Edge White 18


Page 19 of 31

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


Line Relative to Nanometer Scale Clusters. J. Phys. Chem. B 1997, 101, 5332-5336. (25) Park, S. J.; Jang, M. H.; Park, S. J.; Ahn, M.; Park, D. B.; Ko, D.-H.; Cho, M.-H. Effect of Amorphization on the Structural Stability and Reversibility of Ge2Sb2Te5 and Oxygen Incorporated Ge2Sb2Te5 Films. J. Mater. Chem. 2012, 22, 16527-16533. (26) Zhou, J.; Fang, H.; Hu, Y.; Sham, T.; Wu, C.; Liu, M.; Li, F. Immobilization of RuO2 on Carbon Nanotube: An X-ray Absorption Near-edge Structure Study. J. Phys. Chem. C 2009, 113, 10747-10750. (27) Mottana, A.; Robert, J.-L.; Marcelli, A.; Giuli, G.; Della Ventura, G.; Paris, E.; Wu, Z. Octahedral Versus Tetrahedral Coordination of Al in Synthetic Micas Determined by XANES. Am. Mineral. 1997, 82, 497-502. (28) Yu, N.; Tong, H.; Zhou, J.; Elbashir, A.; Miao, X. Local Order of Ge Atoms in Amorphous GeTe Nanoscale Ultrathin Films. Appl. Phys. Lett. 2013, 103, 061910. (29) Bertini, L.; Ghigna, P.; Scavini, M.; Cargnoni, F. Germanium K Edge in GeO2 Polymorphs. Correlation Between Local Coordination and Electronic Structure of Germanium. Phys. Chem. Chem. Phys. 2003, 5, 1451-1456. (30) Ikemoto, H.; Miyanaga, T. Extended X-ray Absorption Fine Structure Study of Local Structure and Atomic Correlations of Tellurium Nanoparticles. Phys. Rev. Lett. 2007, 99, 165503. (31) Kolobov, A.; Wei, S.; Yan, W.; Oyanagi, H.; Maeda, Y.; Tanaka, K. Formation of Ge Nanocrystals Embedded in A SiO2 Matrix: Transmission Electron Microscopy, X-ray Absorption, and Optical Studies. Phys. Rev. B 2003, 67, 195314. (32) Di Cicco, A.; Stizza, S.; Filipponi, A.; Boscherini, F.; Mobilio, S. X-ray Absorption Investigation of SiX4 (X= Cl, F, CH3). J. Phys. B: At., Mol. Opt. Phys. 1992, 25, 2309-2318. (33) Kolobov, A.; Fons, P.; Tominaga, J.; Ankudinov, A.; Yannopoulos, S.; Andrikopoulos, K. Crystallization-induced Short-range Order Changes in Amorphous GeTe. J. Phys.: Condens. Matter 2004, 16, S5103. (34) Jóvári, P.; Kaban, I.; Steiner, J.; Beuneu, B.; Schöps, A.; Webb, A. 'Wrong Bonds' in Sputtered Amorphous Ge2Sb2Te5. J. Phys.: Condens. Matter 2007, 19, 335212. (35) Kolobov, A. V.; Tominaga, J. Chalcogenides: Metastability and Phase Change Phenomena; Springer: New York, 2012. (36) Paesler, M.; Baker, D.; Lucovsky, G.; Edwards, A.; Taylor, P. EXAFS Study of Local Order in the Amorphous Chalcogenide Semiconductor Ge2Sb2Te5. J. Phys. Chem. Solids 2007, 68, 873-877. 19



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

Page 20 of 31

(37) Bondi, A. Van Der Waals Volumes and Radii. J. Phys. Chem. 1964, 68, 441-451. (38) Nonaka, T.; Ohbayashi, G.; Toriumi, Y.; Mori, Y.; Hashimoto, H. Crystal Structure of GeTe and Ge2Sb2Te5 Meta-stable Phase. Thin Solid Films 2000, 370, 258-261. (39) Sun, X.; Yu, B.; Ng, G.; Meyyappan, M. One-dimensional Phase-change Nanostructure: Germanium Telluride Nanowire. J. Phys. Chem. C 2007, 111, 2421-2425. (40) Vogt, P.; De Padova, P.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B.; Le Lay, G. Silicene: Compelling Experimental Evidence for Graphenelike Two-dimensional Silicon. Phys. Rev. Lett. 2012, 108, 155501. (41) Citrin, P. H.; Wertheim, G.; Baer, Y. Core-level Binding Energy and Density of States From the Surface Atoms of Gold. Phys. Rev. Lett. 1978, 41, 1425-1428. (42) Kammerer,

R.;

Barth,

J.;

Gerken,

F.;

Kunz,

C.;

Flodström,

S.;

Johansson,

L.

Surface-binding-energy Shifts for Sodium, Magnesium, and Aluminum Metals. Phys. Rev. B 1982, 26, 3491-3494. (43) Eastman, D.; Chiang, T.-C.; Heimann, P.; Himpsel, F. Surface Core-level Binding-energy Shifts for GaAs (110) and GaSb (110). Phys. Rev. Lett. 1980, 45, 656-659. (44) Kastner, M.; Adler, D.; Fritzsche, H. Valence-alternation Model for Localized Gap States in Lone-pair Semiconductors. Phys. Rev. Lett. 1976, 37, 1504-1507. (45) Jóvári, P.; Kaban, I.; Steiner, J.; Beuneu, B.; Schöps, A.; Webb, M. Local Order in Amorphous Ge2Sb2Te5 and GeSb2Te4. Phys. Rev. B 2008, 77, 035202. (46) Street, R.; Mott, N. States in the Gap in Glassy Semiconductors. Phys. Rev. Lett. 1975, 35, 1293-1296. (47) Zacharias, M.; Bläsing, J.; Veit, P.; Tsybeskov, L.; Hirschman, K.; Fauchet, P. Thermal Crystallization of Amorphous Si/SiO2 Superlattices. Appl. Phys. Lett. 1999, 74, 2614-2616. (48) Senkader, S.; Wright, C. Models for Phase-change of Ge2Sb2Te5 in Optical and Electrical Memory Devices. J. Appl. Phys. 2004, 95, 504-511. (49) Kolobov, A. V.; Fons, P.; Tominaga, J.; Frenkel, A. I.; Ankudinov, A. L.; Yannopoulos, S. N.; Andrikopoulos, K. S.; Uruga, T. Why Phase-change Media Are Fast and Stable: A New Approach to An Old Problem. Jpn. J. Appl. Phys. 2005, 44, 3345-3349. (50) Zhou, G.-F.; Jacobs, B. A. High Performance Media for Phase Change Optical Recording. Jpn. J. Appl. Phys. 1999, 38, 1625-1628. 20


Page 21 of 31

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


(51) Ohshima, N. Crystallization of Germanium–antimony–tellurium Amorphous Thin Film Sandwiched Between Various Dielectric Protective Films. J. Appl. Phys. 1996, 79, 8357-8363.

21



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

Page 22 of 31

Figure Legends

Figure 1. Normalized XANES spectra at Ge K-edge for a-GeTe films of 3 nm, 10 nm, and 100 nm. Black, 3 nm; Red, 10 nm; Blue, 100 nm. The weaker white line in 10 nm sample indicates that in thinner films, Ge atoms are more prone to be in the tetrahedral bonding environment rather than the octahedral one.

22


Page 23 of 31

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


Figure 2. Fourier-transform magnitude of Ge K-edge EXAFS of 3 nm, 10 nm and 100 nm samples. The peak above 2 Å is considered to be the contribution of Ge atoms in the tetrahedral bonding environment because the location is close to the sum of the corresponding covalent radii for the elements (rGe = 1.22 Å, rTe = 1.35 Å).

Figure 3. The Fourier-filtered EXAFS data for GeTe films of (a) 100 nm, (b) 10 nm, (c) 3 nm. The simulated region extends from 2 to 3 Å. Solid line, experiment; scatterplot, fit. Three-body contributions excluded from our simulations, though it has no effect on the following discussions, lead to discrepancies from experimental results in fitting results. (d) Defective Ge2Te3 local arrangement, in which vacancy may replace Te atoms position.

23



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

Page 24 of 31

Figure 4. Angle distribution function resolved for Ge atoms in the surface and body layer of a-GeTe thin film model. Insets are coordination number distribution of Ge atoms for each layer.

Figure 5. Measured temperature Tx as a function of film thickness for GeTe and Ge2Sb2Te5. The dash line is calculated by empirical formula Equation (2).

24


Page 25 of 31

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


Tables Table 1. Summary of the Ge bonds fitting results. Thickness(nm)

Bond type

R(Å)

CN

σ2 (10-3Å2)

100

Ge-Te

2.60±0.01

2.5±0.8

3.65±1.91

Ge-Ge

2.50±0.01

1.0±0.6

9.98±4.56

Ge-Te

2.60±0.01

2.3±0.8

5.46±3.99

Ge-Ge

2.50±0.01

1.1±0.3

5.49±1.48

Ge-Te

2.56±0.02

1.2±0.4

7.73±2.67

Ge-O

3.12±0.01

1.2±0.4

5.20±1.56

10

3

25



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

Page 26 of 31

TOC Graphic

26


Page 27 of 31

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


Fig1 120x86mm (300 x 300 DPI)



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

Fig2 116x86mm (300 x 300 DPI)


Page 28 of 31

Page 29 of 31

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


Fig3 86x64mm (300 x 300 DPI)



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

Fig4 118x86mm (300 x 300 DPI)


Page 30 of 31

Page 31 of 31

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


Fig5 76x53mm (300 x 300 DPI)


Variations of Local Motifs around Ge Atoms in Amorphous GeTe

Recommend Documents