Vapor–Liquid–Solid and Vapor–Solid Growth of Phase-Change Sb2Te3 Nanowires and Sb2Te3/GeTe Nanowire Heterostructures

Electronic structure of Te/Sb/Ge and Sb/Te/Ge multi layer films using photoelectron spectroscopy

Te/Sb/Ge and Sb/Te/Ge multilayer films with an atomically controlled interface were synthesized using effusion cell and e-beam techniques. The layers interacted during the deposition, resulting in films composed of Sb-Te+Sb-Sb/Ge and Sb/Sb-Te/Ge-Te/Ge respectively. Atomic diffusion and chemical reactions in films during the annealing process were investigated by photoemission spectroscopy. In the case of Te/Sb/Ge, Ge diffused into the Sb-Te region released Sb in Sb-Te bonds and interacted with residual Te, resulting in a change in valence band line shape, which was similar to that of a Ge(1)Sb(2)Te(4) crystalline phase. The Ge-Sb-Te alloy underwent a stoichiometric change during the process, resulting in a 1.2:2:4 ratio, consistent with the most stable stoichiometry value calculated by ab initio density-functional theory. The experimental results strongly suggest that the most stable structure is generated through a reaction process involving the minimization of total energy. In addition, Ge in the Sb/Te/Ge film diffused into Sb-Te region by thermal energy. However, Ge was not able to diffuse to the near surface because Sb atoms of the high concentration at the surface were easily segregated and hindered the diffusion of other elements.

Phase-change Ge-Sb nanowires: synthesis, memory switching, and phase-instability

We report the synthesis and characterization of phase-change Ge-Sb nanowires with two different eutectic compositions and their memory switching characteristics. Under application of electric-fields with controlled pulse amplitude and duration times, Sb-rich (Sb > or = 86 at. %) eutectic Ge-Sb nanowires show phase-change based memory switching, while another eutectic GeSb (Ge:Sb = 1:1) nanowires do not show electronic memory switching at all. However, under repeated measurements, Sb-rich Ge-Sb nanowires display an increase of resistance of the low resistive state. The observed electrical irreversibility for Sb-rich Ge-Sb nanowires is attributed to the structural and compositional instability due to the phase-separation of Ge out of homogeneous Ge-Sb as observed from rapid thermal annealing and transmission electron microscopy experiments. Implications for design of Te-free nanoscale materials for phase change memory applications are also discussed.

Void formation induced electrical switching in phase-change nanowires

Solid-state structural transformation coupled with an electronic property change is an important mechanism for nonvolatile information storage technologies, such as phase-change memories. Here we exploit phase-change GeTe single-nanowire devices combined with ex situ and in situ transmission electron microscopy to correlate directly nanoscale structural transformations with electrical switching and discover surprising results. Instead of crystalline-amorphous transformation, the dominant switching mechanism during multiple cycling appears to be the opening and closing of voids in the nanowires due to material migration, which offers a new mechanism for memory. During switching, composition change and the formation of banded structural defects are observed in addition to the expected crystal-amorphous transformation. Our method and results are important to phase-change memories specifically, but also to any device whose operation relies on a small scale structural transformation.

Minimum voltage for threshold switching in nanoscale phase-change memory

The size scaling of the threshold voltage required for the amorphous-to-crystalline transition in phase-change memory (PCM) is investigated using planar devices incorporating individual GeTe and Sb2Te3 nanowires. We show that the scaling law governing threshold switching changes from constant field to constant voltage scaling as the amorphous domain length falls below 10 nm. This crossover is a consequence of the energetic requirement for carrier multiplication through inelastic scattering processes and indicates that the size of PCM bits can be miniaturized to the true nanometer scale.

Core-shell heterostructured phase change nanowire multistate memory

Phase-change memory, which switches reversibly between crystalline and amorphous phases, is promising for next generation data-storage devices. In this work, we present a novel, nonbinary data-storage device using core-shell nanowires to significantly enhance memory capacity by combining two phase-change materials with different electronic and thermal properties to engineer different onsets of amorphous-crystalline transitions. Electric-field induced sequential amorphous-crystalline transition in core-shell nanowires displays three distinct electronic states with high, low, and intermediate resistances, assigned as data "0", "1", and "2".