Unveiling the structural origin to control resistance drift in phase-change memory materials

A variety of silicon-based nanostructures with dimensions in the 1–5 nm range now emit tunable photoluminescence (PL) spanning the visible range. Achievement of high photoluminescence quantum efficiency (PLQY) relies critically on their surface chemistry passivation and an impressive “tool box” of options have been developed. Two distinct PL bands are dominant. The “S-Band” (red–green emission with Slow microsecond decay) has PLQY that has steadily improved from ∼3% in 1990 to 65 ± 5% by 2017. The “F-Band” (blue–yellow with Fast nanosecond decay) has reported PLQY values that have improved from ∼0.1% in 1994 to as high as ∼90% by 2016. The vast literature on both bands is surveyed and for the S-band, size-structure-PL correlations and selective photo-excitation studies are highlighted. Resonant photoexcitation and single quantum dot studies have revealed the key role of quantum confinement and the excitonic phonon-assisted nature of the radiative transitions. For the F-band, in contrast, specific phenomenological studies are highlighted that demonstrate similar emission without the presence of silicon nanostructures. Low PLQY F-band emission from pure silicon–silica core shell systems is probably associated with oxide-related defects, but ultrahigh PLQY from many lower temperature synthesis routes is likely to be from carbon-based nanostructures or chromophores, not silicon nanostructures. Potential applications for both PL bands include sensing, medical imaging, theranostics, photovoltaics, LED colour converters and nano-thermometry. Emerging “green” synthesis routes are mentioned. If scalability and cost are significantly improved then a number of other proposed uses of ultra-efficient PL from “nano-Si” could become viable in cosmetics, catalysis, security and forensics.

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Introductory lecture: origins and applications of efficient visible photoluminescence from silicon-based nanostructures

Ge-rich Ge–Sb–Te alloys are materials with potential for new non-volatile memories named Phase Change Memories offering an extended range of possible applications. However, the origin of their superior properties, notably their much higher transition temperature and increased thermal stability, is unknown. Using a variety of transmission electron microscopy based techniques, we have investigated the changes that affect the structure and composition of such alloys during thermal annealing. We show that, although Ge-rich Ge–Sb–Te materials can be grown as amorphous layers of homogeneous compositions, the primary effect of annealing is to activate phase separation between stable Ge and Ge–Sb–Te phases. This phase separation starts at 380 °C while the material is still amorphous and leads to the nucleation of the first Ge nanocrystals. Increasing the annealing temperature to 400 and then to 450 °C allows the crystalline Ge phase to grow by driving the Ge excess out of the matrix, which, finally, leads to the formation of large (30–50 nm) crystals with the face-centered-cubic Ge–Sb–Te structure. After annealing at 500 °C for 30 minutes, the layer fully crystallizes and consists of a population of large (50–100 nm) face-centered-cubic Ge–Sb–Te crystals with a stoichiometry close to 225 buried in a matrix composed of small Ge nanocrystals. This study evidences that the superior properties of Ge-rich alloys do not result from the intrinsic properties of some Ge-rich crystalline phases but from kinetic factors. The formation of a two phase Ge/Ge–Sb–Te material involves long range diffusion of atomic species, first and foremost, Ge.

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Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys

The Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability.

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Ab initio molecular dynamics and materials design for embedded phase-change memory

In this review paper, several new approaches about the 3C-SiC growth are been presented. In fact, despite the long research activity on 3C-SiC, no devices with good electrical characteristics have been obtained due to the high defect density and high level of stress. To overcome these problems, two different approaches have been used in the last years. From one side, several compliance substrates have been used to try to reduce both the defects and stress, while from another side, the first bulk growth has been performed to try to improve the quality of this material with respect to the heteroepitaxial one. From all these studies, a new understanding of the material defects has been obtained, as well as regarding all the interactions between defects and several growth parameters. This new knowledge will be the basis to solve the main issue of the 3C-SiC growth and reach the goal to obtain a material with low defects and low stress that would allow for realizing devices with extremely interesting characteristics.

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New Approaches and Understandings in the Growth of Cubic Silicon Carbide

Effects of surface oxidation on the crystallization characteristics of Ge-rich Ge-Sb-Te alloys thin films

We have studied by electron microscopy and x-ray diffraction techniques the amorphous-to-crystalline phase transition which occurs during annealing of a highly Ge-rich and N-doped amorphous GeSbTe material. The crystallization onset occurs at 380 °C with the diffusion and segregation of Ge followed by the formation of Ge nanocrystals. The GeSbTe face-centered cubic (FCC) crystalline phase only appears at 400 °C. Phase separation occurs because the Ge concentration is well above what can be accommodated by the Ge2Sb2Te5 lattice. The possible formation of a two-phase material should be considered in order to simulate device characteristics and optimize material composition for electronic memory applications.

In-situ transmission electron microscopy studies of the crystallization of N-doped Ge-rich GeSbTe materials

Ultra-thin Silicon Nanowires (SiNWs) were produced by means of an industrial inductively-coupled plasma (ICP) based process. Two families of SiNWs have been identified, namely long SiNWs (up to 2–3 micron in length) and shorter ones (~100 nm). SiNWs were found to consist of a Si core (with diameter as thin as 2 nm) and a silica shell, of which the thickness varies from 5 to 20 nm. By combining advanced transmission electron microscopy (TEM) techniques, we demonstrate that the growth of the long SiNWs occurred via the Oxide Assisted Growth (OAG) mechanism, while the Vapor Liquid Solid (VLS) mechanism is responsible for the growth of shorter ones. Energy filtered TEM analyses revealed, in some cases, the existence of chapelet-like Si nanocrystals embedded in an otherwise silica nanowire. Such nanostructures are believed to result from the exposure of some OAG SiNWs to high temperatures prevailing inside the reactor. Finally, the intense photoluminescence (PL) of these ICP-grown SiNWs in the 620–950 nm spectral range is a clear indication of the occurrence of quantum confinement. Such a PL emission is in accordance with the TEM results which revealed that the size of nanostructures are indeed below the exciton Bohr radius of silicon.

Marta Agati

Papers

Chemical phase segregation during the crystallization of Ge-rich GeSbTe alloys

Effects of surface oxidation on the crystallization characteristics of Ge-rich Ge-Sb-Te alloys thin films

In-situ transmission electron microscopy studies of the crystallization of N-doped Ge-rich GeSbTe materials

Growth Mechanisms of Inductively-Coupled Plasma Torch Synthesized Silicon Nanowires and their associated photoluminescence properties.

On Some Unique Specificities of Ge-Rich GeSbTe Phase-Change Material Alloys for Nonvolatile Embedded-Memory Applications