Associate Professor Muneaki Hase of the Faculty of Pure and Applied Sciences of the University of Tsukuba, Chief Senior Researcher Paul Fons of the Nanoelectronics Research Institute of the National Institute of Advanced Industrial Science and Technology, Lecturer Toru Shimada of Hirosaki University, Group Director Makina Yabashi of the Beam Line Research and Development Group at the RIKEN SPring-8 Center, and Researchers Tadashi Togashi and Tetsuo Katayama of the Japan Synchrotron Radiation Institute have succeeded in using the immensely powerful X-ray pulses from the X-ray free electron laser (XFEL) facility SACLA  to observe excited-state induced atomic motion on a sub-nanometer scale in a phase-change material via X-ray diffraction. Phase-change material is broadly used in the current generation of rewritable DVD media as well as serving as the basis for phase-change random access memory widely believed to be the next-generation of nonvolatile solid memory.

The XFEL SACLA produces short intense (10 fs) pulses in the x-ray region and is the first domestic X-ray laser source in Japan. The ultrashort nature of the X-ray pulses allows direct stroboscopic observation of transient changes in the atomic structure of solids.

In the research project, an ultrafast pulsed laser was used to optically excite a recording material of phase-change random access memory, a Ge₂Sb₂Te₅ single-crystalline film, and X-ray pulses from the XFEL at SACLA were used to record the subsequent atomic motion induced by the electronic excitation with sub-picosecond precision using X-ray diffraction. These observations revealed that immediately after excitation, bond breaking induced by the excited state resulted in non-thermal local structural rearrangements within a few picoseconds. The formation of the heretofore unobserved transient structural state was followed by a 2 picometer change in lattice spacing by warming of the lattice after 20 picoseconds as revealed by X-ray diffraction. The transient state was observed to persist for over 100 picoseconds, but was found to complete revert to the initial state after 1.8 nanoseconds.

The presence of this previously unknown atomic motion observed on picosecond time scales strongly suggests that the phase change of the recording material for phase-change memory may occur on picosecond scales, although the phase change has been thought to occur on nanosecond scales. In other words, the use of electronic excitation in the phase-change transition process is expected to enable picosecond time-scale memory operation.

The results of this research will be published in the British scientific journal, Scientific Reports, online on February 12, 2016.

* This research was supported by the X-ray Free Electron Laser Priority Strategy Program “Lattice dynamics of phase change materials by time-resolved X-ray diffraction” (Research representative: Muneaki Hase) of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Upon irradiation of a solid by a laser pulse, electrons are excited from the ground state to higher levels leading to the creation of an excited state in the solid. A solid in such an excited state is said to be in a metastable or unstable state and typically atoms are displaced from their equilibrium positions relative to the ground state. By intentionally creating an excited state in a solid, it is possible to induce displacements in atomic positions allowing manipulation of the crystal structure of the solid. The displacements of atomic positions in the excited state, however, are typically on the sub-nanometer (0.1 nm) scale making it impossible to assess such small changes using visible laser light with a wavelength of several hundred nanometers. To measure such small changes in position, it is necessary to use sub-nanometer wavelength light from a X-ray laser in a time-resolved X-ray diffraction measurement.

Optical recording materials such as those used for DVD-RAM are semiconducting chalcogen compounds largely composed of Te and the class of such compounds is often referred to as phase-change materials. Phase-change materials exhibit large changes in material properties such as optical reflectivity (refractive index) or electrical resistance between crystalline and amorphous states making them useful for recording information based upon the corresponding property changes. To date, the typical time for phase change between these two states has been thought to be on the order of a nanosecond (one billionth of a second). In recent years, however, first-principles calculations have predicted that by use of electronic excitation, such transitions can be made to occur on picosecond time scales (one trillionth of a second). If these predictions are realized, both low power operation and high data throughput of phase-change memory will be achieved. Switching devices of a novel operating principle are also expected.

To advance these goals, the current research team used extremely short XFEL pulses form the world’s most advanced free-electron X-ray laser facility, SACLA, to carry out real-time, time-resolved X-ray diffraction measurements of the atomic motion in a phase-change material on sub-nanometer spatial scales and picosecond time scales.

In the framework of the research project highly crystalline Ge₂Sb₂Te₅ epitaxial thin films (thickness 35 nm) were fabricated and irradiated with ultrashort duration (30 femtoseconds) laser light (wavelength 800 nm) as an excitation source at SACLA. This resulted in the optical excitation of electrons in the Ge₂Sb₂Te₅ film. In order to capture the resulting atomic motion after excitation, XFEL pulses (10 femtoseconds duration with 10 keV energy) delayed with less than one picosecond steps after the excitation were used to stroboscopically observe changes in the sample structure using a multiple port readout CCD (MPCCD) detector to record time-resolved X-ray diffraction images (Figure 1). For the current experiments, X-ray diffraction from the epitaxially grown, highly perfect single crystal sample resulted in peak-based diffraction images being recorded by the MPCCD as is schematically shown in Figure 1. The highest intensity diffraction plane (222) was chosen as the subject of the time-resolved diffraction experiments and changes in both the location and intensity of the X-ray diffraction peaks were observed with sub-picosecond time resolution (Figure 2). In this way, the changes in the positions of the atoms constituting the Ge₂Sb₂Te₅ single crystal could be followed on the ultrafast sub-picosecond time scale.

The displacement of atoms was found to reach a maximum at about 20 picoseconds (Figure 2 right) resulting in a maximum diffraction peak shift (0.45 nm^-1) with a corresponding atomic displacement of about 2 picometers. Subsequently after about 1.8 nanoseconds, the atom positions were found to revert to their original positions. Changes in the positions of the X-ray diffraction peaks reflect the increase in the lattice spacing of the crystal while reductions in the intensity of the peaks reflect changes in the magnitude of mean square vibrations of the atoms about their average positions (Debye-Waller Effect ). The changes are schematically indicated in Figure 1 where the initial lattice softening resulting from bond breaking and local atomic rearrangements induced by the electrons excitation is shown, even though the lattice remains at room temperature (the second frame (II)). Subsequent to this, the temperature of the lattice rises and leads to further expansion of the lattice plane spacing (frame III).

The current measurements as visualized in Figure 1 allowed observation of extremely small atomic motion, less than about 0.08 nm. In addition, the excitation-induced structural changes that occurred in less than 1.8 nanoseconds were thought to be intermediate states between crystalline and amorphous states, from the results of X-ray absorption spectroscopy measurements taken at the Advanced Photon Source, and will provide important information to understand the microscopic details of the transition from the crystalline to the amorphous state (Figure 3).

The current research results suggest that the phase change process in both rewritable optical recording films and nonvolatile memory phase-change materials can occur on picosecond time scales. Also recently it was found that similar sub-picosecond processes occur in thin film GeTe/Sb₂Te₃ superlattices. The application of the current method to the phase change of superlattices structures may lead to the promise of future generations of phase-change material based devices working at unprecedented speeds and lower power operation compared to the current generation of Ge₂Sb₂Te₅ polycrystalline films.

Furthermore, it was demonstrated that sub-nanometer, sub-picosecond scale processes can be observed by time-resolved X-ray diffraction measurements at SACLA. In future developments, it is anticipated that time-resolved diffraction measurements will be carried out with a time resolution better than 100 femtoseconds allowing for the measurement and understanding of transition dynamics of a wide range of materials.

Scientific Reports (doi:10.1038/srep20633)