1 Introduction

Phase change random access memory (PCRAM) has been hailed as the next generation nonvolatile memory to overcome scaling limitations faced by the current memory technology. Compared with memories such as flash, DRAM and SRAM, PCRAM is of faster access time, higher overwrite cycle capability, lower cost and power consumption and better compatibility with complementary metal oxide semiconductor technologies[1].

In PCRAM, the phase-change material has amorphous and crystalline phases with remarkably different properties, and each phases can reversibly transit to another on a nanosecond timescale[2]. Until now, most studies of phase-change material focus on chalcogenide pseudobinary compositions along the GeTe-Sb₂Te₃ tie line, for example, Ge₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇ [1, 3-5]. Among them, Ge₂Sb₂Te₅, known as GST, is the most widely used material. GST inherits good thermal stability of GeTe material and fast phase change ability of Sb₂Te₃ material, showing overall qualified performances [6]. As an important component, Sb₂Te₃ film has been considered as a candidate to replace GST film due to its rapid crystallization speed, but low crystallization temperature makes it inappropriate for PCRAM application [7-10]. Efforts have been made to improve the thermal stability of Sb₂Te₃ films, and a promising solution is to dope Ti, Si and N into the Sb₂Te₃ film. The doping effects on structure and phase stability of the film have been studied by experiments and simulations [7, 9-11]. However, to better understand the crystallization characteristics of the doped Sb₂Te₃ materials, atomic configuration and connectivity of Sb₂Te₃ film itself need to be studied.

In this paper, we investigate the structure of Sb₂Te₃ film by using high-energy X-ray diffraction, and reverse Monte Carlo (RMC) technique with random and crystalline configuration for comparison study. To understand the crystallization behavior, atomic configuration and connectivity of the models are discussed and compared with other work [12].

2 Experimental

Sb₂Te₃ thin film was prepared on Si wafer at room temperature by magnetron sputtering using Sb₂Te₃ alloy target. Powder sample for structural study was prepared through the following processes: (1) 1-μm Sb₂Te₃ film was deposited on a rectangle glass substrate by magnetron sputtering using Sb₂Te₃ alloy target; (2) Sb₂Te₃ film was scratched off from the glass substrate using a spatula; and (3) powders were collected in a double-side compton film-walled aluminum tube of Φ2 mm and 1 mm wall. Crystalline Sb₂Te₃ (c-Sb₂Te₃) thin film was obtained by annealing for 3 min g at 350ºC under N₂ atmosphere.

X-ray diffraction (XRD) patterns were measured by Bruker D8 Advance powder diffractometer. High-energy XRD experiment at 69.5 keV photon energy was performed on Beamline BL13W1 at the Shanghai synchrotron radiation facility (SSRF), at room temperature and atmospheric pressure[13]. Data processing was carried out using the PDFgetX3 code [14]. Total scattering structure factor S(Q) was fitted by the RMCprofile code [15, 16].

3 Results and discussion

Figure 1 shows XRD patterns of Sb₂Te₃ thin film after annealing for 3 min at 350ºC under N₂ atmosphere. The crystallized Sb₂Te₃ film shows R3m rhombohedral structure and no separate phase is observed. Each diffraction peak can be indexed according to the PDF card (JCPDS NO. 15-0874). The lattice parameters for Sb₂Te₃ film were calculated to be a = 4.26 Å and c = 30.78Å, being well consistent with previous report[17].

Fig. 1.Full-screen View

XRD pattern of Sb₂Te₃ thin film annealed at 350°C.

The RMC simulation started with a random configuration having the constraint of closest atom-atom approach to avoid physically unrealistic structure. The simulation was performed using 6000 particles with correct stoichiometry. Atomic number density is 0.03 atoms/ų. Also, we started with a crystalline configuration, performed using 4860 particles with 0.03 atoms/ų atomic number density, which led to similar atomic arrangement of the material phase. Throughout the RMC simulations, we carried out bond valence sums constraint, in which the type of neighbors was not constrained. The average BVS values in random and crystalline simulations were 2.774 and 2.672, respectively.

The measured structure factor S(Q) of Sb₂Te₃ sample is shown in Fig. 2 (circles). The pattern is of sharper Bragg reflections than that of liquid Sb₂Te₃ (l-Sb₂Te₃) in Ref. [12], indicating the existence of long-range periodicity in the atomic configuration. It means that part of the as-deposited Sb₂Te₃ film crystallized because of its low crystallization temperature. Total structure factors S(Q) of Sb₂Te₃ derived from the RMC simulations with random and crystalline models (r-model and c-model) are also shown in Fig. 2 (blue and red lines), which are consistent with the experimental data.

Fig. 2.Full-screen View

Total structure factor S(Q) of Sb₂Te₃. The measured data are in circles. The blue and red lines are RMC-simulated with random and crystalline configuration, respectively. The insert is distribution of coordination numbers of Sb and Te atoms derived from the RMC models.

The bond lengths and average coordination numbers around Sb and Te atoms obtained from simulations are listed in Table 1. In order to test whether Sb-Sb and Te-Te bonds exist in the sample, the simulations were carried out with different cut-off distances. All of them showed that the existence of Sb-Sb and Te-Te bonds could lead to a good fit. The r-model data have larger fraction of homopolar Sb-Sb and Te-Te bonds than that of the c-model, which is similar to the case of amorphous Sb₂Te₃ (a-Sb₂Te₃) and l-Sb₂Te₃ [12]. It can be seen that Sb atoms form bonds preferentially with Te atoms in both r- and c-model. The total coordination number around the Sb and Te atom derived from simulations is estimated at 4.59 and 4.38 for r-model, 4.65 and 4.62 for c-model, respectively. These numbers are larger than the coordination number of a-Sb₂Te₃, in which Sb and Te atoms are mostly four-coordinated and three-coordinated [12].

For a phase-change material, phase transition accomplishes in its heating and cooling process. The crystalline state is formed by heating the material to its crystallization temperature, but the crystalline structure is destroyed above its melting point. Strong atomic bonds or strongly localized atomic bonds need higher energy excitation to rearrange from crystalline state to amorphous state. In GST, Ge atoms locate in octahedral (crystalline) position and tetrahedral position (amorphous), [18]. In the phase change process, transition from amorphous to crystalline can be triggered at a low energy excitation while it takes a higher energy excitation for the structure to reorganize [6]. As an important local structural property, average coordination number represents the degree of cross link in covalently bonded solids, which strongly affects the rigidity of atomic networks. Glassy chalcogenide materials usually have a varying covalent coordination number. It is supposed that rigid networks with more atomic linkages should be more strongly localized and more stable than less cross-linked ones. Therefore, at low temperature, the unstable a-Sb₂Te₃ crystallizes easily to form a stable phase with larger coordination numbers. The insert in Fig.2 shows the distribution of coordination numbers derived from two RMC models. It can be seen that in both models the total coordination number around Sb and Te are mostly 5 and 4. From Table 1 and Fig.2, we can know that dominant short-range structural units in the models are Sb₂Te₃ and Sb₂Te₄.

Juarez L. F. Da Silva reported an increase in the average effective coordination number of crystalline phase compared with amorphous phase both in experimental and theoretical studies [19], and the average effective coordination numbers of Sb and Te in c-Sb₂Te₃ were 5.89 and 4.11. In our RMC model, Te atom has a larger total coordination number, because in our simulations, a cubic box containing fixed number of atoms is used in which fewer Te atoms locate in the border, rather than 67% of Te atoms in the border of the building blocks having coordination number of 3.17 in Ref.[19].

To understand the local structure of the sample, Sb(Te)-Sb-Sb(Te) and Sb(Te)-Te-Sb(Te) bond-angle distributions (BAD) are analyzed in Fig. 3(a). In both r-model and c-model, the maximum broad peaks are centered at around 90°, indicating an octahedral local order. Another group of broad peaks is at around 60° in the two models. This is similar to the BAD in l-Sb₂Te₃. The peaks at around 60° are probably due to the formation of Sb-Sb and Te-Te homopolar bond (This will be further discussed later). An obvious difference between the two models is the minor peak close to 180° in the c-model. It indicates that some Sb(Te) atoms and their neighbors are in a linear arrangement. Detail of the BAD in Fig.3(a) insert shows that most near 180° structures origin from Sb-Te-Sb and Te-Sb-Te units, which means that the Sb(Te) atoms and their neighboring atoms are in the same plane. In addition, in Fig.3(a) insert the peak around 60° is rather small, suggesting that these units may possess a crystal-like bond angle order. Typical atomic configuration polyhedron of Sb with 5 coordination atoms in r-model is shown in Fig. 3(b). The shape of SbTe₅ polyhedron is not a regular octahedron, which is similar to a-Sb₂Te₃ and other disordered systems that both Sb and Te atoms are in a defective octahedral environment [12, 20].

Fig. 3.Full-screen View

BAD in Sb₂Te₃ (a) and snapshot of typical SbTe₅ defective octahedron in r-model (b). Circles line and full line represent c-model and r-model, respectively. The insert is BAD of Sb-Te-Sb and Te-Sb-Te structure in c-model.

Ring distribution is often used to analyze the medium range order in amorphous model. The ring sizes of the Sb₂Te₃ models of up to 20-fold ring are showed in Fig.4. In both r- and c-models, long chains are not a dominating structure. Of all the ring structures, a large percentage is small size rings, which is different from that of a-Sb₂Te₃ [12]. In fact, 60% of the rings in the two models are three-, four- and five-membered. The c-model has more four- and less three-membered rings than the r-model. In amorphous material, maximum diameter for amorphous formation is considered as a real parameter indicating glass formation ability (GFA) of alloy. Typically, glass material with lower GFA has narrower ring distribution and is more topologically ordered [21, 22]. Therefore, it is a reasonable estimation that during amorphous-crystal process, crystallization is favored by a regular arrangement with the long chains separating to a certain size rings that gives the structure a high degree of symmetry, just as the crystallization process of a-GST and a-GeTe reported in Ref. [23].

Fig. 4.Full-screen View

Ring size distribution in Sb₂Te₃. The ring size was counted according to the method in Ref. [12]. The ring distribution was counted from an atom in the starting point and returning to the atom through the shortest path length, so as to avoid counting a large-number fold ring that could be divided into smaller-number fold rings.

To obtain information on partial correlations in real space, partial pair distribution functions g_ij(r) derived from the two RMC models are showed in Fig. 5. The covalent networks obtained in the two models are similar to each other. The g_ij(r) distribution for Sb-Sb and Te-Te pairs from 2.6 to 3.4 Å might attribute to the formation of Sb-Sb and Te-Te bonds in the structural units. Differences of the nearest coordination distance for Sb-Sb, Sb-Te and Te-Te pairs between the two models are not obvious. The coordination distance main peak for Sb-Sb and Te-Te pairs in c-model is longer than that of the r-model, suggesting the reduction of Sb-Sb and Te-Te coordination number in simulation.

Fig. 5.Full-screen View

Partial pair distribution functions, g_ij(r), for r-model (solid line) and c-model (dotted line) obtained from the RMC simulations.

To further explain the structure and atomic configurations, typical three- to six-membered rings in the r-model are shown schematically in Fig. 6(a). In the ring distribution, about 23% is three-membered rings. Most three- membered rings are connected with four and five-membered rings. It is clear that the main framework of these rings consist of Sb-Te-Sb-Te bonds. Sb-Sb and Te-Te homopolar bonds mostly exist in three-membered rings. Sb₂Te₃ exhibits both odd- and even- numbered rings due to the formation of Sb-Sb and Te-Te homopolar bonds. According to the structure of c-Sb₂Te₃, rearrangement of the different size rings in a-Sb₂Te₃ is required in phase change process. Figs. 6(b) and 6(c) show the probable process of the reorganization of the rings during crystallization. It may be inferred that a similar crystallization process in GeTe also occurs in Sb₂Te₃ that the odd-numbered rings transform into even-numbered rings with the forming of Sb-Te bonds accompanied with the breaking of Sb-Sb and Te-Te bonds.

Fig. 6.Full-screen View

Typical three-, four-, five- and six-membered rings in the r-model (a), and possible rearrangement of different rings during Sb₂Te₃ crystallization process (b, c).

4 Summary

Sb₂Te₃ film samples have been studied using synchrotron based X-ray diffraction, and RMC simulations with random and crystalline configuration for comparison. The structure of Sb₂Te₃ obtained by RMC modeling is presented and compared with a-Sb₂Te₃ and l-Sb₂Te₃ reported in references to understand the crystallization behavior of Sb₂Te₃ thin film. Two models show similar atomic arrangement of the material phase. Based on BAD, partial pair distribution functions g_ij(r) and ring distribution analysis, it is found that the c-model exhibits a little bit more crystal-like structure than the r-model. The fitting results show that Sb and Te atoms are mostly five-fold and four-fold coordinated, respectively, in defective octahedral sites. Both homopolar Sb-Sb and Te-Te bonds are present in the sample. In crystallization, long chains separate to small size rings giving the structure a high degree of symmetry. It is supposed that in the amorphous-crystal process, the atomic configuration might rearrange gradually with the forming of Sb-Te bonds and breaking of Sb-Sb and Te-Te bonds.