Ultrahigh-density Memory Research Articles

Surface potential of ferroelectric domain investigated by kelvin force microscopy

We have investigated the surface potential of poled area by varying the poled size and the sign of applied voltage on 100 nm thick Pb(Zr0.25Ti0.75)O3 films grown by chemical solution deposition using Kelvin force microscopy (KFM). In the negative poled area, as the poled size increases from 300 to 4800 nm, the domain size and the KFM contrast increased in a linear way. However, in the positive poled area, the KFM contrast increased at first and then didn’t increase because of Coulomb repulsion. In two opposite poled areas, the values of the KFM contrast differed because of the internal field near the ferroelectric/electrode interface. These results imply that the surface overcharge of poled area in ferroelectric materials should be increased and the ferroelectric/electrode interface should be improved for the ultra high-density memory device.

Investigation on Ultra-high Density and High Speed Non-volatile Phase Change Random Access Memory (PCRAM) by Material Engineering

AbstractIn this paper, ultra-high memory density and high speed non-volatile phase change random access memory (PCRAM) was investigated by material engineering. The melting point, crystallization point and activation energy of crystallization of the Bismuth (Bi) doped Germanium-Antimony-Tellurium (GeSbTe) compound was measured using differential scanning calorimetry (DSC) and compared to other GeSbTe ternary compounds. It was observed that the melting temperature of Bi-doped GeSbTe was lower than that of GeSbTe. On the other hand, its activation barrier was found to be reduced, which in turn increased the speed of crystallization of Bi-doped GeSbTe. Bi-doped GeSbTe was then used as a phase change material in the fabrication of PCRAM devices. The properties of PCRAM fabricated using this material were then compared to those using GeSbTe, with emphasis on the programming current required. The results obtained revealed that lower programming current of up to 40% has been achieved for PCRAM with Bi-doped GeSbTe compared to those with other GeSbTe compounds. Bi-doped GeSbTe also has low RESET current and fast speed of crystallization with scaling, making it a suitable material for high speed, ultra-high density PCRAM fabrication in the future.

Spiers Memorial Lecture : Molecular mechanics and molecular electronics

We describe our research into building integrated molecular electronics circuitry for a diverse set of functions, and with a focus on the fundamental scientific issues that surround this project. In particular, we discuss experiments aimed at understanding the function of bistable rotaxane molecular electronic switches by correlating the switching kinetics and ground state thermodynamic properties of those switches in various environments, ranging from the solution phase to a Langmuir monolayer of the switching molecules sandwiched between two electrodes. We discuss various devices, low bit-density memory circuits, and ultra-high density memory circuits that utilize the electrochemical switching characteristics of these molecules in conjunction with novel patterning methods. We also discuss interconnect schemes that are capable of bridging the micrometre to submicrometre length scales of conventional patterning approaches to the near-molecular length scales of the ultra-dense memory circuits. Finally, we discuss some of the challenges associated with fabricated ultra-dense molecular electronic integrated circuits.

Enhanced Chemical Ordering and Coercivity in FePt Alloy Nanoparticles by Sb‐Doping

Sb doping stabilizes the chemically ordered tetragonal L10 structure in FePt alloy nanoparticles, yielding higher coercivities (Hc) at lower annealing temperatures (see Figure) than previously reported. Upon annealing to ∼ 300 °C, nanoparticles yield coercivities more than ten times those previously reported for FePt nanoparticles of similar sizes annealed to the same temperature. These findings are of importance for ultrahigh-density memory device applications.

Thermochemical Hole Burning on DPA(TCNQ)2 and MEM(TCNQ)2 Charge Transfer Complexes Using a Scanning Tunneling Microscope

A thermochemical hole burning (THB) effect was observed on two organic charge transfer complexes, when applying a suitable voltage pulse using a scanning tunneling microscope (STM), which is closely related to a STM current-induced localized thermochemical decomposition of the charge-transfer complex. The decomposition reaction evolves the low boiling point decomposition components of the charge-transfer complex, leaving a nanometer-sized hole on the crystal surface. This effect demonstrates the possibility of creating an ultrahigh-density THB memory, in which information bit is recorded as a hole.

Three-dimensional patterned media for ultrahigh-density optical memory

We report a recording medium in which a three-dimensional nanoscale structure can be photofabricated for multilayered optical memory using a two-photon process. By fabricating the structures in the medium, we can control the shape of recorded bits and, in effect, their spatial frequency distribution. We succeeded in recording bits with a 0.5μm interval in any particular plane and 2.0μm interval between successive layers. Thus, storage density of 2.0Tbits∕cm3 is achieved.

Single Crystal Nanowire Vertical Surround-Gate Field-Effect Transistor

Harnessing the potential of single crystal inorganic nanowires for practical advanced nanoscale applications requires not only reproducible synthesis of highly regular one-dimensional (1D) nanowire arrays directly on device platforms but also elegant device integration which retains structural integrity of the nanowires while significantly reducing or eliminating complex critical processing steps. Here we demonstrate a unique, direct, and bottom-up integration of a semiconductor 1D nanowire, using zinc oxide (ZnO) as an example, to obtain a vertical surround-gate field-effect transistor (VSG-FET). The vertical device structure and bottom-up integration reduce process complexity, compared to conventional top-down approaches. More significantly, scaling of the vertical channel length is lithographically independent and decoupled from the device packing density. A bottom electrical contact to the nanowire is uniquely provided by a heavily doped underlying lattice-match substrate. Based on the nanowire-integrated platform, both n- and p-channel VSG-FETs are fabricated. The vertical device architecture has the potential for use in tera-level ultrahigh-density nanoscale memory and logic devices.

PZT Capacitors on Top of CrTiN/TiN Double Barrier Layers

In this paper we investigated the thermal stability of a novel CrTiN/TiN double barrier, compared to those of TiN and CrTiN single layer barriers. The CrTiN/TiN double layer was stable against heat treatments at higher than 700°C in oxygen ambient for 30 min. However, the double layer showed a severe Pt and Cr interdiffusion issue after crystallization annealing for PZT. We found that pre-annealing treatments prior to deposition of Pt significantly reduced the interdiffusion. In addition, we also demonstrated ferroelectric characteristics of PZT capacitors on top of Pt/CrTiN/TiO2/Si bottom electrode system for ultra-high density memory applications.

Novel ultrahigh-density flash memory with a stacked-surrounding gate transistor (S-SGT) structured cell

In order to overcome the limitation of cell area of 4F/sup 2/ per bit in conventional NAND flash memory cells, stacked-surrounding gate transistor (S-SGT) structured cell is proposed. This newly structured cell achieves a cell area of 4F/sup 2//N per bit, where N is the number of stacked memory cells in one silicon pillar, without using multibit per memory cell technology. The S-SGT structured cell consisting of two stacked memory cells in one silicon pillar achieves a cell area per bit of less than 50% of the smallest reported NAND structured cell. The novel S-SGT structured cells are fabricated by vertical self-aligned processes using a 0.2 /spl mu/m design rule. The S-SGT structured cell can be programmed and erased by uniform injection and uniform emission of Fowler-Nordheim (F-N) tunneling electrons over the whole channel area of the memory cell, respectively, which is the same program and erase mechanism as in conventional NAND structured cell. This high performance S-SGT structured cell is applicable to high-density nonvolatile memories for 16 G/64 G bit Flash memories and beyond.

Ferroelectric molecular films for nanoscopic ultrahigh-density memories.

The formation and visualization of nanometer-scale polarized domains in ultrathin ferroelectric molecular films by scanning-probe microscopy are described. These operations to the ferroelectric domains correspond to the "writing" and the "reading" process, respectively, for the data-storage application. In addition, nanometer-scale structures and the local electrical properties of the local domains, including the interface effect, are discussed. The achieved minimum diameter of the written ferroelectric domains was 30 nm. The size of the "recording" dot corresponds to the recording density of about 230 Gbit/in.(2). The "erasing" process by switching domains was also demonstrated. Furthermore, nanometer-scale ferroelectric domains using VDF oligomer molecular films were successfully formed, which has opened the way to the control of single molecular dipoles.

Formation of ferroelectric nano-domains using scanning force microscopy for the future application of memory devices

Applying voltage between the conductive tip in atomic force microscopy (AFM) and bottom electrode through Pb(Zr, Ti)O3 (PZT) films can cause switching of ferroelectric domains. Formation and imaging of ferroelectric domains in nanometer scale could be applied to develop the future ultrahigh-density memory device. Relevant issues, i.e. bit (induced ferroelectric domains) size dependence on applied voltage and pulse width, are discussed. The bit size showed a log-linear dependence on the pulse width and a linear dependence on the pulse voltage. Using the analysis of electric field distribution, the size of the induced bits under certain pulse voltage and width was estimated.

Magnetic properties of very thin single and multilayer NiFeCo and CoFe films deposited by sputtering

As the size of the giant magneto resistive (GMR) devices continues to decrease into submicrometer regime, the demagnetizing field from the magnetic layers will increase to very high values, prohibiting switching with a reasonably small current. The most feasible way to reduce it is to make the magnetic layers thinner. Single layers of NiFeCo and CoFe films as well as multilayers consisting of these with various underlayers have been deposited by rf diode sputtering in a magnetic field. NiFeCo becomes nonmagnetic at 15, 10, and 6 Å when using Ta, Si3N4, and Cu underlayers, respectively. The magnetization of NiFeCo films sandwiched with Cu decreases as the films become thinner, indicating that about one atomic layer loses its magnetic moment at each interface with Cu. The induced magnetic anisotropy is a strong function of the NiFeCo film thickness, changing from 7 Oe for 20 Å to 17 Oe for 100 Å. CoFe films lose very little magnetic moment at the interfaces with Cu. Multilayers of [NiFeCo/Cu/CoFe/Cu] with different Cu underlayer thickness have been made and the magnetic and GMR properties show a strong dependence on the Cu buffer layer thickness due to different interface roughnesses. With proper designing of the interfaces, these very thin magnetic films have high potential for ultrahigh-density magnetic memory and other applications.

Reversible Tip-induced Structural Modifications in Scanning Tip Microscopy

Tip-induced structural modifications of the Si(100) surface in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are investigated with first-principles total energy pseudopotential calculations. The atoms of the tungsten tip and the surface are fully relaxed according to the tip-surface interactions. At tip-surface distances relevant to STM measurements, the tip is found to significantly influence the dynamics of the surface atoms leading a new understanding of the microscopic STM measurement process. At smaller tip-surface distances, the tip-surface interactions can be used to directly manipulate the geometry of the surface atoms. At very low temperature, the theory predicts the existence of micromechanical hysteresis on an atomic scale which can be measured in low temperature AFM. The theory also leads to the discovery of a specific protocol of tip movements with implications towards a novel design for an ultra-high density memory storage device.

Magic numbers in the magnetic properties of gadolinium clusters.

Gadolinium clusters (N=11--92) in a molecular beam exhibit dramatic size dependences in their magnetic properties. Two principal behaviors are observed: rigid rotor precession and nutation when the magnetic moment is tightly locked to the lattice by crystal anisotropy, and superparamagnetism when the moment is approximately free. Partially locked behavior is also seen. Magnetic moments of about 3${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per atom are observed, with Curie temperatures substantially above the bulk value. Moments that remain locked above 300 K may permit ultrahigh-density magnetic memory.

Second-Harmonic Generation Green Laser for Higher-Density Optical Disks

A laser-diode-pumped intracavity frequency-doubled Nd:YAG laser (SHG green laser) is an excellent light source for reading out higher-density optical disks because of its shot-noise-limited output and diffraction-limited beam quality. Scaling of the green laser using a bundle of fibers was successfully demonstrated to obtain more than 1 W of output power. By using the high-power green laser, generation of continuous-wave fourth-harmonic ultra-violet light for future ultrahigh-density optical memory was also demonstrated.