- Email: [email protected]
Insight into the role of W in amorphous GeTe for phase-change memory
Accepted Manuscript Insight into the role of W in amorphous GeTe for phase-change memory Linchuan Zhang, Naihua Miao, Jian Zhou, Jinxiao Mi, Zhimei Sun PII:
S0925-8388(17)34408-0
DOI:
10.1016/j.jallcom.2017.12.212
Reference:
JALCOM 44298
To appear in:
Journal of Alloys and Compounds
Received Date: 10 October 2017 Revised Date:
10 December 2017
Accepted Date: 19 December 2017
Please cite this article as: L. Zhang, N. Miao, J. Zhou, J. Mi, Z. Sun, Insight into the role of W in amorphous GeTe for phase-change memory, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2017.12.212. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Insight into the role of W in amorphous GeTe for phase-change memory
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Linchuan Zhanga, Naihua Miaob, Jian Zhoub, Jinxiao Mia, Zhimei Sunb,*
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a
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University, Xiamen 361005, China
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b
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Materials Engineering, International Research Institute for Multidisciplinary Science,
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Beihang University, Beijing 100191, China
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School of Materials Science and Engineering, & Center for Integrated Computational
* Corresponding Author. Email: [email protected]
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Department of Materials Science and Engineering, College of Materials, Xiamen
Abstract
GeTe is a fundamental material for phase-change memory, one of the most promising
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next-generation data storage devices. Doping GeTe with W has achieved both high writing
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(crystallization) speed at elevated temperature and long data retention (amorphous stability)
13
at room temperature, which overcame a key challenge for phase-change memory. Yet the
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effect of W on amorphous GeTe remains ambiguous at atomic and electronic scales. By
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means of ab initio calculations and molecular dynamics (AIMD) simulations, we shed light
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on this issue and reveal that W would agglomerate during the melt-quench process and the
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chemically bonded W cluster plays a key role in tuning the overall phase-change
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performances of GeTe. Furthermore, the strong W-Ge and W-Te bonds show vital impact
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on the local structure and crystallization of amorphous GeTe as well. The present work
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provides valuable clues for advancing the understanding of transition metals doping effects
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on amorphous phase-change materials, and hence promotes the development of novel
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phase-change alloys with improved information storage performance.
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Keywords: phase-change memory; data storage materials; atomic scale structure; ab initio
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molecular dynamics simulations; computer simulations.
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1. Introduction The accelerated advancement in the amount of data requires denser and faster reliable
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data storage devices. Phase-change random access memory (PCRAM) shows great promise
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as an electronic universal memory combining the high switching speed of dynamic random
5
access memory (DRAM) and the non-volatility of Flash memory, together with even higher
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recording densities [1,2]. PCRAM works in a very simple principle. The SET operation is
7
applied to crystallize the amorphous bits using a long-duration and moderate current pulse.
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While the RESET operation is conducted with an intense pulse by melting the crystalline
9
bits and quenching it into the amorphous state against a crystalline mask. The significant
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contrasts in physical properties between the amorphous and crystalline states ensure
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reliable data (“0” and “1”) readout [3]. Hence, the recording media, namely phase-change
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materials, need to possess both high crystallization (SET) speed at elevated temperature
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(~600 K) and good data retention (amorphous phase stability) at room temperature, which
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remains a key challenge for PCRAM [4].
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Germanium telluride (GeTe), an end compound in the GeTe-Sb2Te3 (GST) tie line [5],
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laid the foundation of phase change materials, whose SET process has been accelerated to
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several nanoseconds in recent years [6]. Besides, enhanced data retention and other
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valuable properties have been achieved through doping phase change materials with
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various dopants. For example, promising enhancement on the thermal stability and hence
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data retention of GeTe have been achieved through incorporating with O [7]. Li et al. [8]
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have demonstrated the same effect when doping Sb2Te3 with Y. By doping GeTe with Ag,
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Xu et al. [9] revealed that low valence elements would enhance the thermal stability of the
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amorphous phase-change alloys, consistent with our previous work [10] on Cu doped GeTe.
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Tungsten, with a unique combination of large atomic mass and moderate electronegativity,
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has been shown as a promising dopant for phase-change materials [11,12]. Ultrafast
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crystallization speed and remarkably enhanced data retention have been achieved
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simultaneously in W doped GeTe by Peng and coworkers [13]. Besides, W dopants could
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improve the adhesive strength between the recording media and the W electrodes, which is
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beneficial for enhancing device reliability [14]. Despite of such interesting findings, the
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interactions between W and Ge/Te at the atomic and electronic scales are still ambiguous.
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Here by ab initio calculations and molecular dynamics (AIMD) simulations, we have
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investigated the local structure of amorphous GeTe incorporated with two concentrations of
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W dopants and further shed light on the chemical bonding mechanisms of W-Ge-Te alloys.
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We have observed the agglomeration of W at atomic scale and the W cluster plays a key
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role in tuning the overall phase-change performances of GeTe. Furthermore, the strong
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W-Ge and W-Te bonds show vital impact on the local structure and crystallization of
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amorphous GeTe as well. The present work will contribute to advance the understanding of
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doping effect and accelerates the development of novel phase change materials.
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2. Computational methods
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The present AIMD calculations were performed within the framework of density
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functional theory (DFT) using projector augmented wave (PAW) potentials [15] as
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implemented in the Vienna ab initio simulation package (VASP) [16]. The local density
19
approximation (LDA), using the correlation energy of Ceperley and Alder [17] together
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with a plane wave kinetic energy cutoff of 167.344 eV was chosen for the AIMD
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simulations, while the generalized gradient approximation [18] of Perdew-Burke-Ernzerhof
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(GGA-PBE) [19] and a cutoff of 300 eV were applied for ab initio self-consistent
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calculations. Gaussian smearing with a smearing width of 0.1 eV was used to determine the
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partial occupancies. The convergence criteria for the self-consistent energy was 1.0×10-6
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eV. Standard PAW pseudopotentials for W (5d46s2), Ge (4s24p2) and Te (5s25p4) were used.
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The Gamma point was applied for Brillouin-zone sampling during the AIMD simulations
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and the k-mesh was set to 2×2×2 for the ab initio self-consistent calculations. A velocity
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Verlet algorithm [20] with a time step of 3 fs was employed to integrate Newton’s
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equations of motion. R.I.N.G.S. [21], VESTA [22], and home-made codes were also
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utilized for data analysis.
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Our initial configuration is a cubic Ge108Te108 supercell with a density of ~6.03 g/cm3.
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Two W doped GeTe models were constructed as follows: (i) 6 W atoms and 2 vacancies
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occupy the Ge positions following the special quasi-random structure (SQS) method [23],
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resulting in W6Ge100Te108 (W6GT); and (ii) 12 W atoms substitute Ge atoms
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quasi-randomly resulting in W12Ge96Te108 (W12GT).
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Both cubic supercells were melted and thermalized at 1500 K for 12 ps, after which
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they were gradually quenched down to 300 K with a quenching rate of 21.1 K ps-1, the
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same as our previous work [10]. The quenched models finally re-thermalized at 300 K for
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150 ps, wherein the temperature was controlled using the algorithm of Nosé [24]. To
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eliminate the pressures, both amorphous models (a-WxGT) were relaxed to equilibrium
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volume according to the equation of states (E-V) and then re-thermalized at 300 K for
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another 27 ps to reach their equilibrium configurations. The results of amorphous structures
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presented herein are gathered during the last 3 ps at 300 K unless stated otherwise. The
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a-WxGT cells were then re-thermalized at 600 K for 180 ps to simulate the early stage of
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the SET process. Undoped Ge100Te108 (a-GeTe) and Cu6Ge100Te108 (a-Cu6GT) of our
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previous work [10] were also re-thermalized following the same protocol for comparison.
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3. Results and discussions 3.1. Local structure of amorphous W doped GeTe
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We have plotted the mean square displacements (MSD) for amorphous W6GT
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(a-W6GT) and amorphous W12GT (a-W12GT) during the last 3 ps thermostat at 300 K in
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Fig. 1. The MSD curves turn to be smoothly linear, indicating that the systems have
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reached their thermal equilibrium states.
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Figure 2 illustrates the partial pair correlation functions (PCFs) for a-W6GT and
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a-W12GT. The absence of peaks at long distances demonstrates that the models have lost
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their long range orders after the melt-quench process. As seen in Fig. 2 (a), sharp first peaks
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of Ge-Te PCFs locate at ~2.74 Å, indicating strong Ge-Te nearest coordinating
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environment with an average Ge-Te bond length of ~2.74 Å. In a-W12GT, the higher W
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concentration enhances the Ge-Ge pair correlation, while it shows very subtle impact on the
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Ge-Te and Te-Te PCFs. As illustrated in Fig. 2 (b), the W-W PCFs show very sharp first
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peaks, indicating strong correlations between W atoms. The agglomeration tendency of W
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atoms could be expected.
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Table 1. The estimated coordination numbers (CNs) for various pairs of atoms in a-W6GT
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and a-W12GT, where the cutoff radius is 3.2 Å for all atomic pairs.
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System
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a-W6GT
a-W12GT
with W with Ge with Te Total
W
0.58
4.17
3.83
8.58
Ge
0.25
1.24
2.98
4.47
Te
0.21
2.76
0.17
3.14
W
1.40
2.71
4.66
8.77
Ge
0.34
1.53
2.82
4.69
Te
0.52
2.51
0.23
3.26
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The partial and total coordination numbers (CNs) were estimated by integrating PCFs
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up to the first minimum, the results of which are listed in Table 1. While the CNs
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distributions are shown in Fig. S1 of the supplementary materials. Our results for a-GeTe in
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the previous work [10] agree well with experimental data [25,26], demonstrating good
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reliability of our computational methods. In this work, the Ge-Ge homopolar bonds are
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common where their partial CNs (1.24 in a-W6GT and 1.53 in a-W12GT) are larger than
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that in a-GeTe (1.19) [10], which is consistent with the aforementioned Ge-Ge PCF
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enhanced by W doping. It is noteworthy that W has more Ge than Te in their first
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coordination shell in a-W6GT, while CNW-Te is larger than CNW-Ge in a-W12GT. This might
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be partially attributed to the cavities induced by Ge vacancies in a-W6GT which is absent in
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a-W12GT. In addition, the CNs of W-W pair (0.58 in a-W6GT and 1.40 in a-W12GT) are
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large, which could also be an indication of the agglomeration of W dopants.
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Further insight on the amorphous structures could be obtained by analyzing the
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(first-neighbor) bond angle distributions (BAD) as shown in Fig. 3. It is obvious that the
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BAD curves are mainly composed of 2 peaks situating at ~60º and ~90º for around Ge and
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Te in both a-W6GT and a-W12GT. The main peak at ~90º is an indication of the distorted
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octahedral geometry resembling the cubic crystal structure, which should be beneficial to
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the reconstruction of crystalline structures and hence accelerate the SET process. Note that
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the BAD peaks around 60º, which have not been observed for a-GeTe [10], might be
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related to unusual three-fold ring configurations [27]. We infer that they originate from W
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dopants by analyzing the decomposed BADs (Fig. S2) and ring statistics (Fig. S3). Besides,
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large discrepancy in local coordination environment indicates that W could disturb the local
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structure of a-GeTe and tune its properties as well.
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3.2. Agglomeration of transition metal dopants
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Tungsten atoms were considered to have a tendency to form clusters as
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aforementioned, and the agglomeration initiates at 1500 K melting, the details of which are
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shown in Fig. 4. Tungsten atoms distribute quasi-randomly at the cation sites in the initial
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crystal lattice of our W6GT model with the nearest W-W distance of ~7.33 Å, as depicted
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by the red bar in Fig. 4 (a). After it was heated at 1500 K for 1 ps, the crystal lattice
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collapses and W atoms agglomerate with the shortest W-W distances of ~ 6 Å, as indicated
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by the first peak of W-W PCF in Fig. 4 (b). The first W-W PCF peak shifts to even shorter
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distances as the heat treatment continues, with the nearest W-W distances of ~4.7 Å for 4
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ps and ~4.675 Å for 7 ps. Snapshots of the structure evolutions during 1 ps, 4 ps, and 7 ps
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melting process have been depicted in Fig. 4 (c-e), respectively. We further show the MSD
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curves of W, Ge, and Te at 1500 K in Fig. 4 (f). W atoms show higher diffusivity at 1500 K
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than at 300 K (compared with results depicted in Fig. 1), explaining why W atoms could
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displace long distances to agglomerate at 1500 K while they stay nearly still and stabilize
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the amorphous structure at room temperature. Furthermore, such an atomic scale
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agglomeration of transition metal dopants would be the key to understand the mechanism
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of tuning the phase change properties during the crystallization process, which will be
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discussed in section 3.4.
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3.3. Density change between amorphous and crystalline WxGT
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The density change between crystalline and amorphous PCMs is an important issue
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related to the residual stress in PCRAM devices, which has vital impact on the device
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duration and RESET energy consumption as demonstrated by Liu [28]. Therefore, attention
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on the density change of transition metal doped phase-change materials should be paid. We
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estimated the equilibrium volumes of both amorphous and crystalline WxGT as well as 7
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GeTe [10] by the equation of states (as depicted in Figure S4 and Table 2). According to
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our calculations, the volume change of GeTe is ~7.72 %, comparable to typical value of
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Ge2Sb2Te5 (5-10 %) [1]. The volume change of W12GT is estimated to be ~10.76 % and
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~12.57 % for W6GT, which are larger than GeTe. It needs to be clarified that our
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amorphous models were fully randomized (as shown in Fig. 2) which resemble the
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“as-deposited” samples. As a result, their equilibrium densities might show larger
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discrepancies with their cubic counterparts than partially crystallized “melt-quenched” ones,
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whose mass density depends on their crystallinity. Nevertheless, we still argue that W
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dopants, as well as other transition metals, might enlarge the density changes between the
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amorphous and the crystalline states. So, optimizing device structure using appropriate
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electrodes and insulating substrates whose thermal expansion coefficients matching with W
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doped GeTe could be a promising way to reduce the residual stress and overcome the
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disadvantages of W doping [28]. Besides, it is worth mentioning that the structure of
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W12GT is denser than that of W6GT, which might be attributed to the tighter local
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coordination environment induced by short W-Ge/W-Te bonds. We will discuss this issue
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in section 3.5.
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Table 2．Equilibrium volumes and volume changes of our computational models, where Va
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stands for the volume of amorphous models, Vc stands for the volume of crystalline models,
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and ∆V is short for volume changes.
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System Va /Å3 Vc /Å3 ∆V /% GeTe
5722
5311
7.72
W6GT
6445
5725
12.57
W12GT
6225
5620
10.76
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3.4. Early stages of the SET process
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The early stages of the SET process of a-GeTe [10], a-Cu6GT [10], a-W6GT and
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a-W12GT at 600 K have been simulated by thermal annealing for 180 ps. The time
4
evolution of free energy was utilized as an indicator for the crystallization process. As
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shown in Fig. 5 (a), relative free energy curves of all 4 models vibrate around 0 during the
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first ~45 ps, corresponding to the incubation period as proposed by Lee and Elliott [29]
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when transient discrete planes or cubes forming and annihilating due to thermal fluctuation.
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Hereafter, free energy of a-GeTe and a-WxGT are descending rapidly with time evolution
9
until ~120 ps, during which the atoms move towards more energetic favorable (order)
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configurations to lower the free energy. While the curves for a-WxGT start to vibrate higher
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than a-GeTe during 120-180 ps. This observation might be partially attributed to large local
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structure discrepancy induced by W dopants. Moreover, cavities in a-GeTe could facilitate
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the atomic motion rendering faster growth rate compared to less/no vacancy models of
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a-WxGT. On the other hand, the relative free energy curve of a-Cu6GT only vibrates at
15
around 0 during the whole 180 ps simulation, similar to that of Ge1Cu2Te3 [30] showing
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excellent thermal stability even at elevated temperatures. It is worth mentioning that our
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models are fully amorphous rather than those with crystalline templates [7], hence, they
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crystallize rather slowly and could not fully reverse to their crystalline state within 180 ps
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simulations.
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On the other hand, the agglomerated W clusters survive the 600 K AIMD simulations
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for 180 ps as shown in Fig. 5 (b, c), indicating that the amorphous structure might be
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difficult to fully reverse to the initial SQS cubic structure. This observation might explain
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why ultrahigh concentration of W doped GeTe, such as W0.16(GeTe)0.84 [13] could hardly
24
crystallize under the PCARM SET temperature. Besides, the W cluster together with strong 9
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interactions between W and Ge/Te would play a vital role in tuning the phase change
2
properties, which will be discussed in the following section. 3.5. Chemical bonding between W and Ge/Te in a-WxGT
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We have discussed the doping effects of W on the amorphous structure of GeTe.
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Further details at the electronic level could be provided by the calculated charge density
6
difference (CDD), charge density distribution, and electron localization function (ELF) as
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illustrated in Fig. 6.
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As shown in Fig. 6 (a), W atoms are mainly surrounded by blue CDD surfaces
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(electrons depletion), while yellow CDD surfaces (electrons accumulation) locate between
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W-Ge, W-Te, and Ge-Te atomic pairs, demonstrating strong interactions between them.
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Two fractions of the model have been chosen (as labeled by red circles in Fig. 6 (a)) to
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provide deeper insight into the local bonding chemistry: the first is a W-centered W1Ge1Te6
13
cluster (Fig. 6 (b)) and the other is a cluster including 4 agglomerated W atoms (Fig. 6 (c)).
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As seen in Fig. 6 (b), W loses some electrons when forming strong W-Te or W-Ge bonds.
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Strong Ge-Te bonds and weak Ge-Ge bonds could also be expected, while there is no
16
indication of Te-Te bonds. We further investigated the bonding properties of the W clusters.
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As shown in Fig. 6 (c), electrons accumulate around the W cluster, suggesting strong W-Te,
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W-Ge, Ge-Te, and W-W chemical interactions. Local structure around the W cluster
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confirms that W dopants disturb the local order of amorphous GeTe by inducing higher
20
coordination numbers as well as bond angles around ~60º together with 3-fold rings.
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Furthermore, the 2D slice of the charge density distribution has been depicted in Fig. 6 (d).
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Although W loses some electrons, the electron densities around W nuclei are still high.
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Besides, the charge density is relatively high between W-W atoms, indicating electrons are
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shared among the W clusters. Three-dimensional display of ELF of a-W12GT was shown in
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Fig. 6 (e). It is worth mentioning that electrons around W nuclei are delocalized, where the
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electron densities are relatively high according to Fig. 6 (d). Therefore, such delocalized
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electrons might serve as carriers, explaining the experimental measurement [13] that the
4
resistance of amorphous GeTe decreases with W incorporation. In addition, the electrons
5
between W-W atoms in the W cluster are localized as demonstrated by the 2D slice of ELF
6
in Fig. 6 (f), indicating that W cluster is tightly bonded, supporting the high thermal
7
stability of surviving the 180 ps thermal annealing as discussed in section 3.4.
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Table 3. The average values of Bader charge transfer of W, Ge, and Te in a-W6GT and a-W12GT.
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0.349
-0.368
0.303
-0.361
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a-W12GT 0.824
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Bader charge analysis [31] has been implemented to quantify the electron transfer
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between different atomic species in the a-W6GT and a-W12GT. The calculated Bader
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atomic charge transfers (Table 3) of W atoms (0.803 for a-W6GT and 0.824 for a-W12GT)
15
are much larger than that of Cu (0.00548) in our previous work [10]. The results are
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consistent with the aforementioned large charge density difference, thus strong chemical
17
bonding could be verified. Through these strong bonds, W could tune the overall phase
18
change performances of a-WxGT evidently.
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4. Conclusions In summary, by means of ab initio calculations and AIMD simulations, we have
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investigated the structure and properties changes of amorphous GeTe mediated by W,
4
which is rather evident and interesting. Firstly, the local structures of the amorphous phases
5
around Ge/Te resemble their crystalline counterparts in general, thus facilitating the SET
6
process at elevated temperature and increase device writing speed. Besides, chemically
7
bonded W clusters introduce peculiar local structures into amorphous GeTe, which plays a
8
vital role in stabilizing amorphous GeTe together with strong W-Ge and W-Te bonds at
9
room temperature, thus enhancing the data retention of PCRAM. However, the W cluster
10
could increase the electron concentration, thus decrease the resistivity of the amorphous
11
phase and narrowing the readout margin. Furthermore, the W cluster could survive
12
PCRAM SET simulation for 180 ps, affecting the crystallization of the amorphous phase,
13
and might be related to device cyclability issues. On the other hand, attentions on density
14
change related issues should be paid in order to improve the device reliability. Our present
15
work will help advance the understanding the structure and properties of transition metal
16
doped GeTe related phase-change materials, and hence boost the performance of PCRAM
17
devices and pave their way towards universal data storage media.
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Acknowledgements
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This work is financially supported by the National Key Research and Development
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Program of China (Grant No. 2017YFB0701700), the National Natural Science Foundation
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for Distinguished Young Scientists of China (Grant No. 51225205), and the National
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Natural Science Foundation of China (No. 61274005).
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Appendix A. Supplementary materials
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Supplementary materials related to this article.
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[ 15 ] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B, 59 (1999) 1758-1775. [16] J. Hafner, Ab-initio simulations of materials using VASP: density-functional theory and beyond, J. Comput. Chem., 29 (2008) 2044-2078.
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[17] D.M. Ceperley, B.J. Alder, Ground-state of the electron-gas by a stochastic method, Phys. Rev. Lett., 45 (1980) 566-569.
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of Lennard-Jones molecules, Physical Review, 159 (1967) 98-103. [21] S. Le Roux, P. Jund, Ring statistics analysis of topological networks: new approach
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[25] Y. Maeda, M. Wakagi, Ge K-edge extended X-ray absorption fine-structure study of the local-structure of amorphous GeTe and the crystallization, Jpn. J. Appl. Phys. Part 1, 30
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[26] S. Kohara, K. Kato, S. Kimura, H. Tanaka, T. Usuki, K. Suzuya, Y. Moritomo, T. Matsunaga, N. Yamada, Y. Tanaka, H. Suematsu, M. Takata, Structural basis for the fast phase change of Ge2Sb2Te5: ring statistics analogy between the crystal and amorphous
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states, Appl. Phys. Lett., 89 (2006) 201910. [27] J.M. Skelton, K. Kobayashi, Y. Sutou, S.R. Elliott, Origin of the unusual reflectance
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[28] J. Liu, A multi-scale analysis of the impact of pressure on melting of crystalline phase change material germanium telluride, Appl. Phys. Lett., 105 (2014) 173509.
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[ 29 ] T.H. Lee, S.R. Elliott, Ab initio computer simulation of the early stages of crystallization: application to Ge2Sb2Te5 phase-change materials, Phys. Rev. Lett., 107
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crystallization, Acta Mater., 90 (2015) 88-93.
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significant Cu-bonding reconfiguration modulated by Te lone-pair electrons for
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Figure Captions
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Figure 1. Mean square displacements (MSD) for (a) amorphous W6GT (a-W6GT) and (b) amorphous W12GT (a-W12GT) during the last 3 ps of the re-thermalization period at 300 K, respectively. The black solid lines represent MSDs of W atoms, while the red dash-dot lines
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are for Ge atoms and blue dash lines are for Te atoms.
Figure 2. The calculated partial pair correlation function g(r). (a) The Ge-Ge (red), Ge-Te
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(green), and Te-Te (blue) pair correlation functions in a-W6GT (solid lines) and a-W12GT (dash lines). (b) The W-W (red), W-Ge (green), and W-Te (blue) pair correlation functions in a-W6GT (solid lines) and a-W12GT (dash lines).
Figure 3. Bond angle distributions (BAD) calculated around the Ge and Te atoms in
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a-W6GT and a-W12GT. The cut-off distances were 3.2 Å for both Ge and Te. Figure 4. Details of melting process of W6GT at 1500 K. (a) The initial crystalline cell with the nearest W-W distance ~7.33 Å, as depicted by a red bar. (b) PCFs during the melting
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process. (c-e) Snapshots of liquid W6GT at 1 ps, 4 ps, and 7 ps, respectively. W-W bonds
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shorter than 6 Å have been depicted by red bars. (f) MSDs for liquid W6GT at 1500 K.
Figure 5. (a) Relative free energy evolution of the early stage of the SET process of a-GeTe, a-Cu6GT, a-W6GT and a-W12GT at 600 K. (b) a-W6GT and (c) a-W12GT snapshots after 180 ps AIMD simulations. The color assignments of the atoms in (b, c) are W in red, Ge in green, and Te in blue.
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Figure 6. Charge density difference (CDD), charge density distribution and electron localization function (ELF) of a-W12GT. (a-c) The 3D display of CDD with isosurfaces =
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0.01 e/bohr3. The yellow/blue areas represent electron accumulation/depletion, respectively. (d) The 2D slice of the charge density distribution across three W atoms with a scale from 0 to 0.135 and contour lines interval = 0.01 e/bohr3. (e) ELF of a-W12GT with isosurface =
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0.9. (f) The 2D slice of ELF across the same three W atoms as shown in (d) with a scale from ELF = 0-1 and contour lines interval = 0.1. The color assignments of the atoms in (a-c,
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and e) are W in red, Ge in green, and Te in blue.
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ACCEPTED MANUSCRIPT Hightlights Atomic structures of W doped amorphous GeTe have been investigated.
Local structures around Ge/Te facilitate fast crystallization.
Chemically bonded W clusters stabilize amorphous GeTe by strong bonds.
W would increase electron concentration, thus narrowing readout margin.
Density changes between two phases might deteriorate device cyclability.
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S0925-8388(17)34408-0
DOI:
10.1016/j.jallcom.2017.12.212
Reference:
JALCOM 44298
To appear in:
Journal of Alloys and Compounds
Received Date: 10 October 2017 Revised Date:
10 December 2017
Accepted Date: 19 December 2017
Please cite this article as: L. Zhang, N. Miao, J. Zhou, J. Mi, Z. Sun, Insight into the role of W in amorphous GeTe for phase-change memory, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2017.12.212. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Insight into the role of W in amorphous GeTe for phase-change memory
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Linchuan Zhanga, Naihua Miaob, Jian Zhoub, Jinxiao Mia, Zhimei Sunb,*
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a
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University, Xiamen 361005, China
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b
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Materials Engineering, International Research Institute for Multidisciplinary Science,
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Beihang University, Beijing 100191, China
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School of Materials Science and Engineering, & Center for Integrated Computational
* Corresponding Author. Email: [email protected]
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Department of Materials Science and Engineering, College of Materials, Xiamen
Abstract
GeTe is a fundamental material for phase-change memory, one of the most promising
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next-generation data storage devices. Doping GeTe with W has achieved both high writing
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(crystallization) speed at elevated temperature and long data retention (amorphous stability)
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at room temperature, which overcame a key challenge for phase-change memory. Yet the
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effect of W on amorphous GeTe remains ambiguous at atomic and electronic scales. By
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means of ab initio calculations and molecular dynamics (AIMD) simulations, we shed light
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on this issue and reveal that W would agglomerate during the melt-quench process and the
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chemically bonded W cluster plays a key role in tuning the overall phase-change
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performances of GeTe. Furthermore, the strong W-Ge and W-Te bonds show vital impact
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on the local structure and crystallization of amorphous GeTe as well. The present work
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provides valuable clues for advancing the understanding of transition metals doping effects
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on amorphous phase-change materials, and hence promotes the development of novel
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phase-change alloys with improved information storage performance.
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Keywords: phase-change memory; data storage materials; atomic scale structure; ab initio
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molecular dynamics simulations; computer simulations.
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1. Introduction The accelerated advancement in the amount of data requires denser and faster reliable
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data storage devices. Phase-change random access memory (PCRAM) shows great promise
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as an electronic universal memory combining the high switching speed of dynamic random
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access memory (DRAM) and the non-volatility of Flash memory, together with even higher
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recording densities [1,2]. PCRAM works in a very simple principle. The SET operation is
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applied to crystallize the amorphous bits using a long-duration and moderate current pulse.
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While the RESET operation is conducted with an intense pulse by melting the crystalline
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bits and quenching it into the amorphous state against a crystalline mask. The significant
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contrasts in physical properties between the amorphous and crystalline states ensure
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reliable data (“0” and “1”) readout [3]. Hence, the recording media, namely phase-change
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materials, need to possess both high crystallization (SET) speed at elevated temperature
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(~600 K) and good data retention (amorphous phase stability) at room temperature, which
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remains a key challenge for PCRAM [4].
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Germanium telluride (GeTe), an end compound in the GeTe-Sb2Te3 (GST) tie line [5],
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laid the foundation of phase change materials, whose SET process has been accelerated to
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several nanoseconds in recent years [6]. Besides, enhanced data retention and other
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valuable properties have been achieved through doping phase change materials with
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various dopants. For example, promising enhancement on the thermal stability and hence
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data retention of GeTe have been achieved through incorporating with O [7]. Li et al. [8]
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have demonstrated the same effect when doping Sb2Te3 with Y. By doping GeTe with Ag,
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Xu et al. [9] revealed that low valence elements would enhance the thermal stability of the
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amorphous phase-change alloys, consistent with our previous work [10] on Cu doped GeTe.
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Tungsten, with a unique combination of large atomic mass and moderate electronegativity,
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has been shown as a promising dopant for phase-change materials [11,12]. Ultrafast
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crystallization speed and remarkably enhanced data retention have been achieved
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simultaneously in W doped GeTe by Peng and coworkers [13]. Besides, W dopants could
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improve the adhesive strength between the recording media and the W electrodes, which is
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beneficial for enhancing device reliability [14]. Despite of such interesting findings, the
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interactions between W and Ge/Te at the atomic and electronic scales are still ambiguous.
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Here by ab initio calculations and molecular dynamics (AIMD) simulations, we have
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investigated the local structure of amorphous GeTe incorporated with two concentrations of
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W dopants and further shed light on the chemical bonding mechanisms of W-Ge-Te alloys.
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We have observed the agglomeration of W at atomic scale and the W cluster plays a key
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role in tuning the overall phase-change performances of GeTe. Furthermore, the strong
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W-Ge and W-Te bonds show vital impact on the local structure and crystallization of
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amorphous GeTe as well. The present work will contribute to advance the understanding of
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doping effect and accelerates the development of novel phase change materials.
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2. Computational methods
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The present AIMD calculations were performed within the framework of density
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functional theory (DFT) using projector augmented wave (PAW) potentials [15] as
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implemented in the Vienna ab initio simulation package (VASP) [16]. The local density
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approximation (LDA), using the correlation energy of Ceperley and Alder [17] together
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with a plane wave kinetic energy cutoff of 167.344 eV was chosen for the AIMD
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simulations, while the generalized gradient approximation [18] of Perdew-Burke-Ernzerhof
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(GGA-PBE) [19] and a cutoff of 300 eV were applied for ab initio self-consistent
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calculations. Gaussian smearing with a smearing width of 0.1 eV was used to determine the
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partial occupancies. The convergence criteria for the self-consistent energy was 1.0×10-6
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eV. Standard PAW pseudopotentials for W (5d46s2), Ge (4s24p2) and Te (5s25p4) were used.
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The Gamma point was applied for Brillouin-zone sampling during the AIMD simulations
3
and the k-mesh was set to 2×2×2 for the ab initio self-consistent calculations. A velocity
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Verlet algorithm [20] with a time step of 3 fs was employed to integrate Newton’s
5
equations of motion. R.I.N.G.S. [21], VESTA [22], and home-made codes were also
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utilized for data analysis.
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Our initial configuration is a cubic Ge108Te108 supercell with a density of ~6.03 g/cm3.
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Two W doped GeTe models were constructed as follows: (i) 6 W atoms and 2 vacancies
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occupy the Ge positions following the special quasi-random structure (SQS) method [23],
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resulting in W6Ge100Te108 (W6GT); and (ii) 12 W atoms substitute Ge atoms
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quasi-randomly resulting in W12Ge96Te108 (W12GT).
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Both cubic supercells were melted and thermalized at 1500 K for 12 ps, after which
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they were gradually quenched down to 300 K with a quenching rate of 21.1 K ps-1, the
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same as our previous work [10]. The quenched models finally re-thermalized at 300 K for
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150 ps, wherein the temperature was controlled using the algorithm of Nosé [24]. To
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eliminate the pressures, both amorphous models (a-WxGT) were relaxed to equilibrium
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volume according to the equation of states (E-V) and then re-thermalized at 300 K for
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another 27 ps to reach their equilibrium configurations. The results of amorphous structures
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presented herein are gathered during the last 3 ps at 300 K unless stated otherwise. The
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a-WxGT cells were then re-thermalized at 600 K for 180 ps to simulate the early stage of
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the SET process. Undoped Ge100Te108 (a-GeTe) and Cu6Ge100Te108 (a-Cu6GT) of our
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previous work [10] were also re-thermalized following the same protocol for comparison.
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3. Results and discussions 3.1. Local structure of amorphous W doped GeTe
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We have plotted the mean square displacements (MSD) for amorphous W6GT
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(a-W6GT) and amorphous W12GT (a-W12GT) during the last 3 ps thermostat at 300 K in
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Fig. 1. The MSD curves turn to be smoothly linear, indicating that the systems have
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reached their thermal equilibrium states.
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Figure 2 illustrates the partial pair correlation functions (PCFs) for a-W6GT and
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a-W12GT. The absence of peaks at long distances demonstrates that the models have lost
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their long range orders after the melt-quench process. As seen in Fig. 2 (a), sharp first peaks
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of Ge-Te PCFs locate at ~2.74 Å, indicating strong Ge-Te nearest coordinating
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environment with an average Ge-Te bond length of ~2.74 Å. In a-W12GT, the higher W
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concentration enhances the Ge-Ge pair correlation, while it shows very subtle impact on the
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Ge-Te and Te-Te PCFs. As illustrated in Fig. 2 (b), the W-W PCFs show very sharp first
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peaks, indicating strong correlations between W atoms. The agglomeration tendency of W
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atoms could be expected.
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Table 1. The estimated coordination numbers (CNs) for various pairs of atoms in a-W6GT
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and a-W12GT, where the cutoff radius is 3.2 Å for all atomic pairs.
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System
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a-W6GT
a-W12GT
with W with Ge with Te Total
W
0.58
4.17
3.83
8.58
Ge
0.25
1.24
2.98
4.47
Te
0.21
2.76
0.17
3.14
W
1.40
2.71
4.66
8.77
Ge
0.34
1.53
2.82
4.69
Te
0.52
2.51
0.23
3.26
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The partial and total coordination numbers (CNs) were estimated by integrating PCFs
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up to the first minimum, the results of which are listed in Table 1. While the CNs
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distributions are shown in Fig. S1 of the supplementary materials. Our results for a-GeTe in
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the previous work [10] agree well with experimental data [25,26], demonstrating good
5
reliability of our computational methods. In this work, the Ge-Ge homopolar bonds are
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common where their partial CNs (1.24 in a-W6GT and 1.53 in a-W12GT) are larger than
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that in a-GeTe (1.19) [10], which is consistent with the aforementioned Ge-Ge PCF
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enhanced by W doping. It is noteworthy that W has more Ge than Te in their first
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coordination shell in a-W6GT, while CNW-Te is larger than CNW-Ge in a-W12GT. This might
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be partially attributed to the cavities induced by Ge vacancies in a-W6GT which is absent in
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a-W12GT. In addition, the CNs of W-W pair (0.58 in a-W6GT and 1.40 in a-W12GT) are
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large, which could also be an indication of the agglomeration of W dopants.
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Further insight on the amorphous structures could be obtained by analyzing the
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(first-neighbor) bond angle distributions (BAD) as shown in Fig. 3. It is obvious that the
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BAD curves are mainly composed of 2 peaks situating at ~60º and ~90º for around Ge and
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Te in both a-W6GT and a-W12GT. The main peak at ~90º is an indication of the distorted
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octahedral geometry resembling the cubic crystal structure, which should be beneficial to
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the reconstruction of crystalline structures and hence accelerate the SET process. Note that
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the BAD peaks around 60º, which have not been observed for a-GeTe [10], might be
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related to unusual three-fold ring configurations [27]. We infer that they originate from W
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dopants by analyzing the decomposed BADs (Fig. S2) and ring statistics (Fig. S3). Besides,
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large discrepancy in local coordination environment indicates that W could disturb the local
23
structure of a-GeTe and tune its properties as well.
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3.2. Agglomeration of transition metal dopants
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Tungsten atoms were considered to have a tendency to form clusters as
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aforementioned, and the agglomeration initiates at 1500 K melting, the details of which are
4
shown in Fig. 4. Tungsten atoms distribute quasi-randomly at the cation sites in the initial
5
crystal lattice of our W6GT model with the nearest W-W distance of ~7.33 Å, as depicted
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by the red bar in Fig. 4 (a). After it was heated at 1500 K for 1 ps, the crystal lattice
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collapses and W atoms agglomerate with the shortest W-W distances of ~ 6 Å, as indicated
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by the first peak of W-W PCF in Fig. 4 (b). The first W-W PCF peak shifts to even shorter
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distances as the heat treatment continues, with the nearest W-W distances of ~4.7 Å for 4
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ps and ~4.675 Å for 7 ps. Snapshots of the structure evolutions during 1 ps, 4 ps, and 7 ps
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melting process have been depicted in Fig. 4 (c-e), respectively. We further show the MSD
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curves of W, Ge, and Te at 1500 K in Fig. 4 (f). W atoms show higher diffusivity at 1500 K
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than at 300 K (compared with results depicted in Fig. 1), explaining why W atoms could
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displace long distances to agglomerate at 1500 K while they stay nearly still and stabilize
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the amorphous structure at room temperature. Furthermore, such an atomic scale
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agglomeration of transition metal dopants would be the key to understand the mechanism
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of tuning the phase change properties during the crystallization process, which will be
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discussed in section 3.4.
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The density change between crystalline and amorphous PCMs is an important issue
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related to the residual stress in PCRAM devices, which has vital impact on the device
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duration and RESET energy consumption as demonstrated by Liu [28]. Therefore, attention
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on the density change of transition metal doped phase-change materials should be paid. We
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estimated the equilibrium volumes of both amorphous and crystalline WxGT as well as 7
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GeTe [10] by the equation of states (as depicted in Figure S4 and Table 2). According to
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our calculations, the volume change of GeTe is ~7.72 %, comparable to typical value of
3
Ge2Sb2Te5 (5-10 %) [1]. The volume change of W12GT is estimated to be ~10.76 % and
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~12.57 % for W6GT, which are larger than GeTe. It needs to be clarified that our
5
amorphous models were fully randomized (as shown in Fig. 2) which resemble the
6
“as-deposited” samples. As a result, their equilibrium densities might show larger
7
discrepancies with their cubic counterparts than partially crystallized “melt-quenched” ones,
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whose mass density depends on their crystallinity. Nevertheless, we still argue that W
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dopants, as well as other transition metals, might enlarge the density changes between the
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amorphous and the crystalline states. So, optimizing device structure using appropriate
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electrodes and insulating substrates whose thermal expansion coefficients matching with W
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doped GeTe could be a promising way to reduce the residual stress and overcome the
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disadvantages of W doping [28]. Besides, it is worth mentioning that the structure of
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W12GT is denser than that of W6GT, which might be attributed to the tighter local
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coordination environment induced by short W-Ge/W-Te bonds. We will discuss this issue
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in section 3.5.
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Table 2．Equilibrium volumes and volume changes of our computational models, where Va
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stands for the volume of amorphous models, Vc stands for the volume of crystalline models,
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and ∆V is short for volume changes.
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System Va /Å3 Vc /Å3 ∆V /% GeTe
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5311
7.72
W6GT
6445
5725
12.57
W12GT
6225
5620
10.76
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3.4. Early stages of the SET process
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The early stages of the SET process of a-GeTe [10], a-Cu6GT [10], a-W6GT and
3
a-W12GT at 600 K have been simulated by thermal annealing for 180 ps. The time
4
evolution of free energy was utilized as an indicator for the crystallization process. As
5
shown in Fig. 5 (a), relative free energy curves of all 4 models vibrate around 0 during the
6
first ~45 ps, corresponding to the incubation period as proposed by Lee and Elliott [29]
7
when transient discrete planes or cubes forming and annihilating due to thermal fluctuation.
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Hereafter, free energy of a-GeTe and a-WxGT are descending rapidly with time evolution
9
until ~120 ps, during which the atoms move towards more energetic favorable (order)
10
configurations to lower the free energy. While the curves for a-WxGT start to vibrate higher
11
than a-GeTe during 120-180 ps. This observation might be partially attributed to large local
12
structure discrepancy induced by W dopants. Moreover, cavities in a-GeTe could facilitate
13
the atomic motion rendering faster growth rate compared to less/no vacancy models of
14
a-WxGT. On the other hand, the relative free energy curve of a-Cu6GT only vibrates at
15
around 0 during the whole 180 ps simulation, similar to that of Ge1Cu2Te3 [30] showing
16
excellent thermal stability even at elevated temperatures. It is worth mentioning that our
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models are fully amorphous rather than those with crystalline templates [7], hence, they
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crystallize rather slowly and could not fully reverse to their crystalline state within 180 ps
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simulations.
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On the other hand, the agglomerated W clusters survive the 600 K AIMD simulations
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for 180 ps as shown in Fig. 5 (b, c), indicating that the amorphous structure might be
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difficult to fully reverse to the initial SQS cubic structure. This observation might explain
23
why ultrahigh concentration of W doped GeTe, such as W0.16(GeTe)0.84 [13] could hardly
24
crystallize under the PCARM SET temperature. Besides, the W cluster together with strong 9
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interactions between W and Ge/Te would play a vital role in tuning the phase change
2
properties, which will be discussed in the following section. 3.5. Chemical bonding between W and Ge/Te in a-WxGT
4
We have discussed the doping effects of W on the amorphous structure of GeTe.
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Further details at the electronic level could be provided by the calculated charge density
6
difference (CDD), charge density distribution, and electron localization function (ELF) as
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illustrated in Fig. 6.
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As shown in Fig. 6 (a), W atoms are mainly surrounded by blue CDD surfaces
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(electrons depletion), while yellow CDD surfaces (electrons accumulation) locate between
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W-Ge, W-Te, and Ge-Te atomic pairs, demonstrating strong interactions between them.
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Two fractions of the model have been chosen (as labeled by red circles in Fig. 6 (a)) to
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provide deeper insight into the local bonding chemistry: the first is a W-centered W1Ge1Te6
13
cluster (Fig. 6 (b)) and the other is a cluster including 4 agglomerated W atoms (Fig. 6 (c)).
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As seen in Fig. 6 (b), W loses some electrons when forming strong W-Te or W-Ge bonds.
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Strong Ge-Te bonds and weak Ge-Ge bonds could also be expected, while there is no
16
indication of Te-Te bonds. We further investigated the bonding properties of the W clusters.
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As shown in Fig. 6 (c), electrons accumulate around the W cluster, suggesting strong W-Te,
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W-Ge, Ge-Te, and W-W chemical interactions. Local structure around the W cluster
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confirms that W dopants disturb the local order of amorphous GeTe by inducing higher
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coordination numbers as well as bond angles around ~60º together with 3-fold rings.
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Furthermore, the 2D slice of the charge density distribution has been depicted in Fig. 6 (d).
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Although W loses some electrons, the electron densities around W nuclei are still high.
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Besides, the charge density is relatively high between W-W atoms, indicating electrons are
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shared among the W clusters. Three-dimensional display of ELF of a-W12GT was shown in
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Fig. 6 (e). It is worth mentioning that electrons around W nuclei are delocalized, where the
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electron densities are relatively high according to Fig. 6 (d). Therefore, such delocalized
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electrons might serve as carriers, explaining the experimental measurement [13] that the
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resistance of amorphous GeTe decreases with W incorporation. In addition, the electrons
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between W-W atoms in the W cluster are localized as demonstrated by the 2D slice of ELF
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in Fig. 6 (f), indicating that W cluster is tightly bonded, supporting the high thermal
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stability of surviving the 180 ps thermal annealing as discussed in section 3.4.
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Table 3. The average values of Bader charge transfer of W, Ge, and Te in a-W6GT and a-W12GT.
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Bader charge transfer/e
System
Ge
Te
0.803
0.349
-0.368
0.303
-0.361
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a-W6GT
W
a-W12GT 0.824
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Bader charge analysis [31] has been implemented to quantify the electron transfer
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between different atomic species in the a-W6GT and a-W12GT. The calculated Bader
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atomic charge transfers (Table 3) of W atoms (0.803 for a-W6GT and 0.824 for a-W12GT)
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are much larger than that of Cu (0.00548) in our previous work [10]. The results are
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consistent with the aforementioned large charge density difference, thus strong chemical
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bonding could be verified. Through these strong bonds, W could tune the overall phase
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change performances of a-WxGT evidently.
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4. Conclusions In summary, by means of ab initio calculations and AIMD simulations, we have
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investigated the structure and properties changes of amorphous GeTe mediated by W,
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which is rather evident and interesting. Firstly, the local structures of the amorphous phases
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around Ge/Te resemble their crystalline counterparts in general, thus facilitating the SET
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process at elevated temperature and increase device writing speed. Besides, chemically
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bonded W clusters introduce peculiar local structures into amorphous GeTe, which plays a
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vital role in stabilizing amorphous GeTe together with strong W-Ge and W-Te bonds at
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room temperature, thus enhancing the data retention of PCRAM. However, the W cluster
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could increase the electron concentration, thus decrease the resistivity of the amorphous
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phase and narrowing the readout margin. Furthermore, the W cluster could survive
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PCRAM SET simulation for 180 ps, affecting the crystallization of the amorphous phase,
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and might be related to device cyclability issues. On the other hand, attentions on density
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change related issues should be paid in order to improve the device reliability. Our present
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work will help advance the understanding the structure and properties of transition metal
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doped GeTe related phase-change materials, and hence boost the performance of PCRAM
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devices and pave their way towards universal data storage media.
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Acknowledgements
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This work is financially supported by the National Key Research and Development
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Program of China (Grant No. 2017YFB0701700), the National Natural Science Foundation
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for Distinguished Young Scientists of China (Grant No. 51225205), and the National
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Natural Science Foundation of China (No. 61274005).
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Appendix A. Supplementary materials
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Supplementary materials related to this article.
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Figure Captions
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Figure 1. Mean square displacements (MSD) for (a) amorphous W6GT (a-W6GT) and (b) amorphous W12GT (a-W12GT) during the last 3 ps of the re-thermalization period at 300 K, respectively. The black solid lines represent MSDs of W atoms, while the red dash-dot lines
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are for Ge atoms and blue dash lines are for Te atoms.
Figure 2. The calculated partial pair correlation function g(r). (a) The Ge-Ge (red), Ge-Te
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(green), and Te-Te (blue) pair correlation functions in a-W6GT (solid lines) and a-W12GT (dash lines). (b) The W-W (red), W-Ge (green), and W-Te (blue) pair correlation functions in a-W6GT (solid lines) and a-W12GT (dash lines).
Figure 3. Bond angle distributions (BAD) calculated around the Ge and Te atoms in
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a-W6GT and a-W12GT. The cut-off distances were 3.2 Å for both Ge and Te. Figure 4. Details of melting process of W6GT at 1500 K. (a) The initial crystalline cell with the nearest W-W distance ~7.33 Å, as depicted by a red bar. (b) PCFs during the melting
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process. (c-e) Snapshots of liquid W6GT at 1 ps, 4 ps, and 7 ps, respectively. W-W bonds
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shorter than 6 Å have been depicted by red bars. (f) MSDs for liquid W6GT at 1500 K.
Figure 5. (a) Relative free energy evolution of the early stage of the SET process of a-GeTe, a-Cu6GT, a-W6GT and a-W12GT at 600 K. (b) a-W6GT and (c) a-W12GT snapshots after 180 ps AIMD simulations. The color assignments of the atoms in (b, c) are W in red, Ge in green, and Te in blue.
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Figure 6. Charge density difference (CDD), charge density distribution and electron localization function (ELF) of a-W12GT. (a-c) The 3D display of CDD with isosurfaces =
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0.01 e/bohr3. The yellow/blue areas represent electron accumulation/depletion, respectively. (d) The 2D slice of the charge density distribution across three W atoms with a scale from 0 to 0.135 and contour lines interval = 0.01 e/bohr3. (e) ELF of a-W12GT with isosurface =
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0.9. (f) The 2D slice of ELF across the same three W atoms as shown in (d) with a scale from ELF = 0-1 and contour lines interval = 0.1. The color assignments of the atoms in (a-c,
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and e) are W in red, Ge in green, and Te in blue.
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ACCEPTED MANUSCRIPT Hightlights Atomic structures of W doped amorphous GeTe have been investigated.
Local structures around Ge/Te facilitate fast crystallization.
Chemically bonded W clusters stabilize amorphous GeTe by strong bonds.
W would increase electron concentration, thus narrowing readout margin.
Density changes between two phases might deteriorate device cyclability.
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