Article pubs.acs.org/cm
Electronic Structure of Transition-Metal Based Cu2GeTe3 Phase Change Material: Revealing the Key Role of Cu d Electrons Yuta Saito,*,† Yuji Sutou,*,‡ Paul Fons,† Satoshi Shindo,‡ Xeniya Kozina,§ Jonathan M. Skelton,∥ Alexander V. Kolobov,† and Keisuke Kobayashi⊥ †
Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan ‡ Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-11, Aoba-yama, Aoba-ku, Sendai 980-8579, Japan § Helmholtz−Zentrum Berlin für Materialien und Energie GmbH, 15 Albert - Einstein - Straße, Berlin 12489, Germany ∥ Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom ⊥ SPring-8/JASRI, Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan S Supporting Information *
ABSTRACT: The electronic structure of the as-deposited amorphous and crystalline phases of transition-metal based Cu2GeTe3 phase-change memory material has been systematically investigated using hard-X-ray photoemission spectroscopy and density-functional theory simulations. We shed light on the role of Cu d electrons and reveal that participation of d electrons in bonding plays an important role during the phasechange process. A large electrical contrast as well as fast switching is preserved even in the tetrahedrally bonded crystal structure, which does not exhibit resonant bonding. On the basis of the obtained results, we propose that transition-metal based phase change memory materials, a class of materials that have been previously overlooked, will be candidates not only for nonvolatile memory applications, but also for emerging applications.
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INTRODUCTION Phase-change materials (PCMs) have attained great success as recording layers in rewritable optical discs (e.g., DVD, Blu-Ray) and have also gathered attention for nonvolatile memory applications such as phase-change random access memory (PCRAM).1,2 Both applications utilize the contrast in physical properties of two different structural states, namely the amorphous and crystalline phases, to encode binary data, harnessing a reversible phase transition to convert between them. Even though the concept for these two applications using the change in material properties due to phase change is essentially the same, the requirement of optical reflectivity and electrical resistivity contrast for optical discs and PCRAM, respectively, mean that the guiding principles in the search for ideal materials may not be the same. A large number of papers published to date, however, have reported on new PCMs for PCRAM without taking modifications to this guideline into account, tending to propose materials with similar compositions to those used for optical discs. Resonant bonding has been one of the most important concepts in PCM design, as it links the microscopic structure to the origin of the large optical contrast between the crystalline and amorphous phases.3−5 Typical PCMs exhibit higher reflectivity in the crystalline phase due to the existence of resonant bonding, while conversely the © 2017 American Chemical Society
lack of resonant bonding results in a lower reflectivity in the amorphous state. While such a large optical contrast is necessary for optical-disc applications, it is not necessary for electrically operated PCRAM. 6 Therefore, for PCRAM applications, it seems counter-productive to judge a material solely on the presence or absence of resonant bonding. We recently uncovered a clear correlation between density differences and reflectivity change upon crystallization in a variety of PCMs, wherein materials showing large optical contrast also exhibit a large density changes, regardless of the sign of the change.7−9 These trends are consistent with predictions using the Clausius-Mossotti equation, which relates the dielectric constant of a material and its density.10 It should be noted that typical phase change materials, GST, show a dielectric function larger than that expected by the ClausiusMossotti equation due to the existence of resonance bonding in the crystalline phase.4 This implies that typical phase change materials with a high degree of resonant bonding may also show large volume changes (more than 6%) upon switching.11−16 Received: June 12, 2017 Revised: August 16, 2017 Published: August 16, 2017 7440
DOI: 10.1021/acs.chemmater.7b02436 Chem. Mater. 2017, 29, 7440−7449
Article
Chemistry of Materials
Figure 1. Dependence of the core−electron spectra of Cu2GeTe3 on sample annealing temperature. Plots (a)−(c) show the Cu2p3/2 (a), Ge2p3/2 (b), and Te3d5/2 (c) spectra as a function of annealing temperature. (d)−(f) show the temperature dependence of the binding energies of the fitted peaks in the Cu2p3/2, Ge2p3/2, and Te3d5/2 spectra, respectively. (g) Temperature dependence of the area under the two peaks in the Te3d5/2 spectra. (h) Crystal structure of Cu2GeTe3, as reported in ref 38.
We therefore suggest that searching for materials without resonance bonding can potentially serve as a criterion to discover novel PCMs that show smaller density changes upon switching. In other words, reconsidering candidate PCMs that have previously been discounted for use in optical discs may pave the way toward discovering novel materials for PCRAM applications. To this end, we propose transition-metal (TM)-based PCMs as an alternative and unexplored class of potential candidates. Typical PCMs are generally composed of elements from the IIIB, IVB, VB, and VIB groups of the periodic table, where sand p-electron bonding is dominant. However, d electrons contribute to the bonding in TM compounds, so it is also anticipated that the bonding in TM-based PCMs will differ fundamentally from that in conventional s-p-bonded materials. Although TM-doped PCMs have been reported to show improved performance in PCRAM applications, as well as displaying magnetic properties, most examples of doping involve the addition of small amounts of TMs to Ge−Sb−Te (GST) and Sb−Te compositions.17−27 Such low concentrations of TM atoms cannot be expected to significantly alter the overall bonding structure of the PCM. In this work, we have studied the ternary Cu2GeTe3 (CGT) PCM, containing more than 30 atomic % of the TM Cu. We
have previously reported on several favorable properties of this compound, including the fact that Cu−Ge−Te alloys require lower switching energy (approximately a 10% reduction for the RESET process over GST)28 and show higher thermal stability than GST (spontaneous crystallization at 125 °C, GST: 2.7 × 10−4 year, CGT: 70 years),29 with comparable switching speeds (laser-induced crystallization time at a laser power of 11.0 mW, GST: 23 ns, CGT: 26 ns, laser-induced reamorphization time at a laser power of 22.7 mW, GST: 63 ns, CGT: 27 ns)7 and a smaller density change upon crystallization (GST: + 6%, CGT: −3.4%).9 Furthermore, it was found that even though the electrical resistivity contrast between amorphous and crystalline states in a blanket GST film exceeds 106 and 104 in a CGT film, the total resistance contrast is ∼104 in CGT memory cells being larger than the ∼103 value of GST devices, when the contact resistivity between the PCM and the electrodes is taken into account in addition to the film resistivity.30 Since the real resistance contrast of the cell is determined not only by the PCM itself but also by the contact resistivity, this makes the readout process of a memory cell using CGT more reliable. A particularly unusual property of CGT is that the crystalline phase has a lower optical reflectivity than the amorphous phase, a situation opposite to that in typical PCMs undergoing resonance bonding.7,8 Such an opposite reflectivity change has 7441
DOI: 10.1021/acs.chemmater.7b02436 Chem. Mater. 2017, 29, 7440−7449
Article
Chemistry of Materials
difficult to completely avoid surface oxidation;40 the Ge−O peak intensity becomes stronger with increasing annealing temperature, indicating that oxidation continues during annealing. The crystallization temperature of the stoichiometric film was determined from the resistance drop in a two-point probe measurement, and was found to be between 220 and 235 °C as shown in Figure S1 of the Supporting Information (SI). Therefore, we suggest that the appearance of the Cu−O peak might be attributed to the oxidation of Cu. The single Te 3d5/2 peak in the amorphous-phase spectra splits into two peaks above the crystallization temperature. Since the Te−O peak usually appears at a binding energy of around 576 eV,41,42 the possibility of the oxidation of Te in the same manner as Cu and Ge can be excluded, and the presence of two peaks instead suggests two different Te chemical environments in crystalline CGT. The dependence of the binding energies of the main peaks in each spectrum on the annealing temperature are shown in Figure 1(d)−(f). The Cu 2p3/2 peak shows almost negligible change upon annealing, even after the completion of crystallization. Alternatively, the Ge 2p3/2 peak exhibits a notable shift to higher binding energies upon crystallization, indicating a change in the chemical environment around the Ge atoms through the phase change. As can be seen in Figure 1(f), the single Te 3d5/2 peak in the spectrum of the as-deposited phase splits into two on crystallization, one at a lower and one at a higher binding energy (Peaks 1 and 2, respectively). By fitting to Voigt functions, the integrated peak intensities were estimated (Figure 1(g)), and the ratio of the areas was found to be approximately 1.3:1. This observation will be discussed further, along with the origin of peak splitting, with reference to the chalcopyrite-like crystal structure of Cu2GeTe3 (Figure 1(h)). Before doing so, we first explore the dependence of the spectra on the film composition. Figure 2 shows the dependence of the binding energies measured from core−
been recently reported for other materials such as Ge−Sb, Ga− Sb, and Fe−Te alloys, where it is accompanied by a density decrease upon crystallization for the former two compounds as predicted by the Clausius-Mossotti equation, and even though the density change of Fe−Te alloy has not been reported, it is likely to exhibit a decrease of density upon crystallization as well.6,10,31,32 While this was attributed speculatively to the lack of resonant bonding in the crystalline phase, the detailed electronic structure of CGT has not been experimentally investigated, and the role of Cu, in particular its d electrons, is unclear. Therefore, an in-depth study of the electronic structure of CGT could lead to a deeper understanding of the phasechange process for nonconventional PCMs and open up new avenues for materials exploration. We have analyzed the electronic structure of amorphous and crystalline Cu−Ge−Te films, including stoichiometric Cu2GeTe3 and four off-stoichiometric compositions, using high-resolution hard X-ray photoelectron spectroscopy (HAXPES) combined with ab initio density-functional theory (DFT) simulations. HAXPES, using an excitation energy of 7.94 keV, is a powerful tool to investigate the electronic structures of materials due to its high bulk sensitivity, and has previously successfully been employed to study the electronic structure of GST.33 We shed light on Cu d electrons and reveal that participation of d electrons in the bonding plays an important role during the phase-change process in this class of materials. On the basis of our findings, we propose a promising new family of candidate PCMs for PCRAM applications.
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EXPERIMENTAL SECTION
Cu−Ge−Te films were prepared on SiO2(100 nm)/Si substrates by RF-magnetron sputtering using CuTe and GeTe alloy targets. The composition of the films was controlled by adjusting the sputtering power of each target and was measured by scanning-electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). All films were grown to a thickness of 200 nm and immediately coated with several nm of carbon after growth to prevent oxidation. The samples were subsequently cut into several pieces and annealed at varying temperatures under an Ar flow at atmospheric pressure, after the annealing chamber had been purged by evacuating it to a pressure of 100 Pa. Hard X-ray photoemission spectroscopy (HAXPES) measurements were carried out at beamline BL47XU at the SPring-834 with an X-ray excitation energy of 7.94 keV. The samples were mounted in a vacuum chamber with a pressure of
