Present-day multimedia strongly rely on rewritable phase-change optical memories. We demonstrate that, different from the current consensus, Ge(2)Sb(2)Te(5), the material of choice in DVD-RAM, does not possess the rocksalt structure but more likely consists of well-defined rigid building blocks that are randomly oriented in space consistent with cubic symmetry. Laser-induced amorphization results in drastic shortening of covalent bonds and a decrease in the mean-square relative displacement, demonstrating a substantial increase in the degree of short-range ordering, in sharp contrast to the amorphization of typical covalently bonded solids. This novel order-disorder transition is due to an umbrella-flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this unique two-state nature of the transformation that ensures fast DVD performance and repeatable switching over ten million cycles.

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.

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We report on an inelastic (Raman) light scattering study of the local structure of amorphous GeTe films. A detailed analysis of the temperature-reduced Raman spectra has shown that appreciable structural changes occur as a function of temperature. These changes involve modifications of atomic arrangements such as to facilitate the rapid amorphous-to-crystal transformation, which is the major advantage of phase-change materials used in optical data storage media. A particular structural model, supported by polarization analysis, is proposed being compatible with the experimental data as regards both the structure of a-GeTe and the crystallization transition. The remarkable difference between the Raman spectrum of the crystal and the glass can thus naturally be accounted for.

The quasibinary GeTe-Sb 2 Te 3 system (primarily the Ge 2 Sb 2 Te 5 composition, or GST225) has been long used in optical memory devices such as re-writable DVD-RAM and are also a leading candidate for the nonvolatile electronic memory known as phase-change random-access memory (PC-RAM); last year Samsung and Micron have started shipping these devices into the market. The basis of the phase-change storage is a large property contrast between the crystalline and amorphous phases; the idea dates back to the 1960s. [ 1 ] In a PC-RAM device, when a voltage exceeding a certain value (a threshold voltage) is applied to the high-resistivity amorphous phase, the material switches into the low-resistivity crystalline (SET) phase. The process can be reversed (RESET) by applying another pulse, of appropriately chosen amplitude and duration, that reverts the structure to the amorphous phase.Non-volatile memory devices are currently key elements of various electronics and portable systems (digital camera, solid state disks, smartphones, computers, e-books, tablets, etc.) and their market has been increasing exponentially over the last decade. One important aspect is to develop new memory storage concepts and devices that can integrate multiple functionalities. It was recently found by some of the present authors [ 2 ] that when GeTe and Sb 2 Te 3 components are spatially separated in forms of nm-thick layers to form a superlattice (interfacial phase-change memory, or iPCM), the energy effi ciency of devices increases by orders of magnitude, the resistivity contrast is lower in iPCM and, in stark contrast to the composite material, iPCM remains in a crystalline phase in both SET and RESET states. [ 2 ] These results demonstrate that the storage mechanism is different from conventional (composite) phase-change materials, where the GeTe and Sb 2 Te 3 phases are intermixed.It is instructive to note that both individual constituents of iPCM possess rather specifi c properties. Sb 2 Te 3 is one of the best known examples of three-dimensional (3D) tolopogical insulators (TI). [ 3 ] GeTe, on the other hand, is the simplest known ferroelectric material with just two atoms in the primitive cell. [ 4 ] This conclusion was primarily reached based on structural studies; [ 5 ] the main challenge in direct studies of ferroelectricity in GeTe by conventional electrical spectroscopic techniques, such as hysteresis loop and transient current measurements, is its high conductivity: free charge carriers screen the applied electric fi eld inhibiting polarization reversal and result in high dielectric loss. Ferroelectric order has been demonstrated for nm-sized nanocrystals. [ 6,7 ] and ferroelectric switching in GeTe has been observed experimentally in polycrystalline samples using piezoresponse force microscopy (PFM) and capacitance measurements. [ 8 ] Furthermore, stable ferroelectric switching has been recently observed in epitaxial GeTe fi lms, [ 9 ] which provides tangible reasons to expect the scaling of the ferroelectric switching functio...

An amorphous-to-crystal transition in phase-change materials like Ge-Sb-Te is widely used for data storage. The basic principle is to take advantage of the property contrast between the crystalline and amorphous states to encode information; amorphization is believed to be caused by melting the materials with an intense laser or electrical pulse and subsequently quenching the melt. Here, we demonstrate that distortions in the crystalline phase may trigger a collapse of long-range order, generating the amorphous phase without going through the liquid state. We further show that the principal change in optical properties occurs during the distortion of the still crystalline structure, upsetting yet another commonly held belief that attributes the change in properties to the loss of long-range order. Furthermore, our results suggest a way to lower energy consumption by condensing phase change inducing energy into shorter pulses or through the use of coherent phonon excitation.

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