Ferroelectric Order Control of the Dirac‐Semimetal Phase in GeTe‐Sb₂Te₃ Superlattices

Abstract

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...

The Crystal Orbital Hamilton Population (COHP) Method as a Tool to Visualize and Analyze Chemical Bonding in Intermetallic Compounds

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Photovoltaic chiral magnetic effect in Weyl semimetals

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A two-step process for growth of highly oriented Sb2Te3 using sputtering

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