In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal properties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling properties of PCM are illustrated with recent experimental results using special device test structures and novel material synthesis. Factors affecting the reliability of PCM are discussed.

Phase Change Memory

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.

Understanding the phase-change mechanism of rewritable optical media.

Phase change materials are materials that exist in at least two structurally distinct solid phases, an amorphous and one (or more) crystalline phases. Many materials display phase change properties in this sense and can be deposited at least as a thin film in an amorphous phase (low temperature deposition, very thin film) or crystalline phase (high temperature deposition, epitaxy). Often the amorphous and crystalline phases have very different optical and electrical properties stemming from the large differences in structure between the amorphous and the crystalline phases. These differences can be used to store information in technological applications if it is possible to switch the material repeatedly between the two phases and if both phases are stable at operating temperature. The transformation of the metastable amorphous phase to the energetically favorable, stable crystalline phase occurs by heating the material above its crystallization temperature for a time

Phase Change Materials and Their Application to Nonvolatile Memories

Phase-change materials can rapidly and reversibly be switched between an amorphous and a crystalline phase. Since both phases are characterized by very different optical and electrical properties, these materials can be employed for rewritable optical and electrical data storage. Hence, there are considerable efforts to identify suitable materials, and to optimize them with respect to specific applications. Design rules that can explain why the materials identified so far enable phase-change based devices would hence be very beneficial. This article describes materials that have been successfully employed and dicusses common features regarding both typical structures and bonding mechanisms. It is shown that typical structural motifs and electronic properties can be found in the crystalline state that are indicative for resonant bonding, from which the employed contrast originates. The occurence of resonance is linked to the composition, thus providing a design rule for phase-change materials. This understanding helps to unravel characteristic properties such as electrical and thermal conductivity which are discussed in the subsequent section. Then, turning to the transition kinetics between the phases, the current understanding and modeling of the processes of amorphization and crystallization are discussed. Finally, present approaches for improved high-capacity optical discs and fast non-volatile electrical memories, that hold the potential to succeed present-day's Flash memory, are presented.

Design Rules for Phase‐Change Materials in Data Storage Applications

Staple cartridge comprising staples with different unformed heights

The recording and retrieval of signals below 100 nm mark length were attempted with elliptical bubble-type super-resolution technology with platinum oxide (PtOx) and ductile AgInSbTe layers, using the same optical system as that of a digital versatile disk (a 635 nm wavelength red laser system). The carrier-to-noise ratio (CNR) of over 47 dB for 100 nm mark length signals (over 43 dB for 80 nm mark length signals) was obtained, which can be considered as a commercially acceptable level of CNR. The recording mechanism of the sample disk was shown through the transmission electron microscopy cross-section image observation to be by rigid elliptical bubble formation at the PtOx layer located between the AgInSbTe layers. The results of this report represent the potential for a much higher-density storage using the red laser system and a subterabyte optical storage using the blue laser system.

Super-resolution by elliptical bubble formation with PtOx and AgInSbTe layers

Germanium–antimony–tellurite (GST) is a very attractive material not only for rewritable optical media but also for realizing solid state devices. Recently, the study of the switching mechanism between the amorphous and crystal states has actively been carried out experimentally and theoretically. Now, the role of the flip-flop transition of a Ge atom in a distorted simple-cubic unit cell is the center of discussion. Turning our viewpoint towards a much wider region beyond a unit cell, we can understand that GeSbTe consists of two units: one is a Sb2Te3 layer and the other is a Ge2Te2 layer. On the based of this simple model, we fabricated the superlattice of GST alloys and estimated their thermal properties by differential scanning calorimetry (DSC). In this paper, we discuss the proof of the Ge switch on the basis of thermo-histories.

https://iopscience.iop.org/article/10.1143/JJAP.47.5763/pdf

Role of Ge Switch in Phase Transition: Approach using Atomically Controlled GeTe/Sb2Te3 Superlattice

The optical diffraction limit is rigidly determined as a simple equation of wavelength ? and lens numerical aperture NA (): ?/2/NA. In this paper, we report that Ag5.8In4.4Sb61.0Te28.8 and Ge2Sb2Te5 chalcogenide thin films, which are typical of optical recording materials used in digital versatile discs (DVDs), enable a resolution of under ?/10 due to their ferroelectric properties. In the Ag5.8In4.4Sb61.0Te28.8 film it was found that this optical super-resolution can be observed between?350 and 400??C, resulting in a second phase transition from a hexagonal (A7 belonging to ) to a rhombohedral structure of R32 or R3m. In Ge2Sb2Te5, on the other hand, the temperature range is much wider, between?250 and 450??C, which is also due to a second phase transition from a NaCl-type fcc to a hexagonal structure.

http://iopscience.iop.org/article/10.1088/0957-4484/15/5/001/pdf

Ferroelectric catastrophe: beyond nanometre-scale optical resolution

Gold nanoparticles were conjugated to an antibody (immuno-AuNP) against A/Udorn/307/1972 (H3N2) influenza virus to detect viruses on a sensing plate designed for an evanescent field-coupled waveguide-mode sensor. Experiments were conducted using human influenza A/H3N2 strains, and immuno-AuNP could detect 8×105 PFU/ml (40 pg/µl) intact A/Udorn/307/1972 and 120 pg/µl A/Brisbane/10/2007. Furthermore, increased signal magnitude was achieved in the presence of non-ionic detergent, as the virtual detection level was increased to 8×104 PFU/ml A/Udorn/307/1972. Immuno-AuNPs were then complexed with viruses to permit direct observation, and they formed a ring of confined nanodots on the membrane of both intact and detergent-treated viruses as directly visualized by scanning electron microscopy. With this complex the detection limit was improved further to 8×103 PFU/ml on anti-rabbit IgG immobilized sensing plate. These strategies introduce methods for observing trapped intact viruses on the sensing plates generated for optical systems.

/pdf/observations-of-immuno-gold-conjugates-on-influenza-viruses-3yih7v7nrm.pdf

Observations of Immuno-Gold Conjugates on Influenza Viruses Using Waveguide-Mode Sensors

We have elucidated the readout mechanism of a platinum oxide super-resolution near-field structure (PtOx super-RENS) disk. It was found that threshold readout power (Pth) depends strongly on disk rotation speed. A rough estimate of the disk temperature during readout showed Pth to be approximately 340°C, irrespective of rotation speed. Additionally, it was revealed that the phase-change recording film (AgInSbTe) has optical nonlinearity with respect to incident laser power. The results indicate that thermal effects in the AgInSbTe (AIST) film play an important role in the super-RENS readout mechanism.

Takayuki Shima

Papers

Super-resolution by elliptical bubble formation with PtOx and AgInSbTe layers

Role of Ge Switch in Phase Transition: Approach using Atomically Controlled GeTe/Sb2Te3 Superlattice

Ferroelectric catastrophe: beyond nanometre-scale optical resolution

Observations of Immuno-Gold Conjugates on Influenza Viruses Using Waveguide-Mode Sensors

Thermal Origin of Readout Mechanism of Light-Scattering Super-Resolution Near-Field Structure Disk