Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials

The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.

Structural transformation and phase change properties of Se substituted GeTe

GeTe_1−xSe_x (0 ≤ x ≤ 1.0) alloys have been prepared both in bulk and thin film forms to study the effect of selenium (Se) substitution for tellurium (Te) on the phase change properties. It is observed that with increasing Se substitution in GeTe, the structure transforms from rhombohdral structure to orthorhombic structure. Rietveld Refinement analysis support the phase transformation and show that the short and long bond lengths in crystalline GeTe decrease with increasing Se substitution but the rate of reduction of shorter bond length is more than the longer bond length. The GeTe_1−xSe_x thin films undergo amorphous to crystalline phase change when annealed at high temperatures. The transition temperature shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 10⁴ for GeTe, and with the Se substitution, the contrast increases considerably to 10⁶ for GeTe_0.5Se_0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistance, bandgap, transition temperature and threshold voltage of GeTe can be effectively controlled and tuned by the substitution of Te by Se, which is conducive for phase change memory applications.

Designing Multiple Crystallization in Superlattice-like Phase-Change Materials for Multilevel Phase-Change Memory

A multilevel phase-change memory device was successfully designed, which was fabricated using a Ge₄₀Te₆₀/Cr superlattice-like (SLL) structure. In the SLL films, a two-step phase change process is observed at elevated temperatures, which reveals the crystallization of Ge₄₀Te₆₀ (GT) and an interface-dominated formation of Cr₂Ge₂Te₆ (CrGT). The bonding of Cr-Te and Ge-Ge is accompanied by the breaking of a Ge-Te bond, which is mainly in the Ge-rich GeTe_4-nGe_n units. The formation of CrGT is related to the breaking apart of the edge-sharing octahedron in GT and Cr replacement at Ge sites. The crystalline GT acts as the crystallization precursors in the formation of the CrGT phase. The stable reversible two-step phase change can guarantee the reliability of the multilevel storage. The present work may shed light on the possible mechanism of the CrGT phase transition-based interfacial dynamic process. The designed multiple crystallization system demonstrates a potential for multilevel storage.

Sc-Centered Octahedron Enables High-Speed Phase Change Memory with Improved Data Retention and Reduced Power Consumption

Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. In the automotive application, intelligent system not only needs high efficiency to handle massive data processing but also good robustness to retain the existing data against high working temperature. In this work, Sc-doped GeTe is developed for PCM, which has achieved 120 °C data retention for 10 years, 6 ns operation speed, and 7 nJ low power consumption. The high data retention is attributed to the high coordination number of Sc and its strong bonds with Te atoms in the amorphous phase, which enhances the robustness of the atomic matrices. Sc-centered octahedrons in amorphous state provide a nucleation center, leading to fast crystallization. In the crystalline phase, Sc atoms occupy Ge vacancies to form a homogenous GeTe-like rhombohedral phase. The strong covalent-like Sc-Te bonds weaken the neighboring Ge-Te bonds, lowering energy for melting. Together with the increased energy efficiency originated from confined grain size, the reduced power consumption has been achieved. The improvements in data retention, speed, and power efficiency have made Sc-doped GeTe a promising candidate for high-performance automobile electronics application.

Excellent thermal stability owing to Ge and C doping in Sb2Te-based high-speed phase-change memory

The contradictory nature between transition speed and thermal stability of phase-change materials has always been the key limitation to the achievement of wide applications under harsh conditions. Ge_2.3Sb_2.0Te phase-change alloy is proposed here to feature high thermal stability (10 year data retention above 220 °C) and fast switching speed (SET programming speed up to 5 ns) for electronic storage. In mushroom-shaped device cells, the nanocomposite materials implement an endurance life of nearly 1 × 10⁵ cycles. Such operation speed among high-temperature alloys is the best ever reported. And the moderate incorporation of C offers intriguing benefits that include enhanced thermal stability and reduced RESET voltage in the above-mentioned Ge-rich Sb₂Te-based memory cells. Through microscopic analysis, the local segregation of C dopants can further refine the crystalline grains and thus induce a lower volume change and roughness upon heating. These properties are crucial with regard to the application potential in high-performance and high-density embedded memories.

Evolution of crystal structures in GeTe during phase transition

We investigated changes in the crystal structure of GeTe during its phase transition. Using density functional theory (DFT) calculations, four possible crystal structures were identified: R3m, P1, Cm, and Fm3m. Among these, P1 and Cm were examined here for the first time. By calculating the internal energy of the crystal volume change, we verified that P1, R3m, and Cm can coexist in crystalline GeTe. The X-ray diffraction spectra of annealed and laser-irradiated GeTe films revealed coexisting P1 or R3m and Cm. In addition, we confirmed that Cm transforms into P1 or R3m after laser irradiation. The presence of these new structures was revealed in the crystal Raman spectra. Many of the Raman peaks in the crystalized GeTe could be explained by the coexistence of various structures. By calculating the band gaps of these structures, we also found that a structural transformation induces a change in the crystal resistance, owing to differences in the band gaps of individual structures. The generation of new crystal structures suggests a facile phase change and instability during the structural transformation.