Guohan Hu, IBM T. J. Watson Research Center
Hyunsang Hwang, Pohang University of Science and Technology
Gabriel Molas, LETI-CEA
Eisuke Tokumitsu, Japan Advanced Institute of Science and Technology
Symposium Support Kojundo Chemical Laboratory Co. Ltd. of Japan
Tuesday PM, December 01, 2015
Hynes, Level 2, Room 202
2:30 AM - *KK2.01
Magnetization Reversal and Thermal Stability of Perpendicular Spin-Transfer-Torque Magnetic Random Access Memory Devices
Luc Thomas 1
1TDK- Headway Technologies Milpitas United StatesShow Abstract
The combination of high speed and non-volatility of perpendicular Spin-Transfer-Torque Magnetic Random Access Memories (pSTT-MRAMs) promises a significant reduction of computing power, making this technology particularly attractive for mobile and Internet-of-Things applications .
In this talk, we will present recent advances in the development of perpendicular Magnetic Tunnel Junctions (pMTJs) at TDK-Headway Technologies , which have enabled the demonstration of fully functional 8Mb pSTT-MRAM chips with sub-5ns writing . Furthermore, we will discuss the physics of current-induced switching and thermal stability in pMTJs devices, in comparison with writing and data retention properties of 8Mb test chips .
3:00 AM - KK2.02
Dictating Magnetic Easy Axis in CoFe2O4 Films with Helium Implantation
T. Zac Ward 1 Andreas Herklotz 1 Anthony T. Wong 1
1Oak Ridge National Laboratory Oak Ridge United StatesShow Abstract
Heteroepitaxial strain engineering is an essential tool in strongly correlated systems for investigating fundamental coupling effects and for more practical control of thin film properties. We demonstrate that the length of a single axis in an epitaxial CoFe2O4 (CFO) film can be controlled by strain doping helium into the lattice which in turn allows fine control over the magnetic easy axis through induced magnetostriction. Compressively strained thin films of CFO are grown coherently on MgO substrates and show pronounced out-of-plane magnetic anisotropy. Successive doping of the CFO films with He using a commercial ion gun results in an expansion of the out-of-plane lattice parameter while maintaining in-plane epitaxial lock to the substrate. We observe a continuous rotation of the magnetic easy axis towards the film plane with increasing unit cell tetragonality. A vacuum anneal above 500 °C is sufficient to evacuate the He from the lattice and return it to the pristine state. The results are in agreement with the strain-induced change of the magnetic anisotropy due to the large negative magnetostriction of CFO and demonstrates that strain doping via He implantation is an elegant path to tune desired characteristics of transition metal oxide thin films.
This work was supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.
3:15 AM - KK2.03
Tunneling Magnetoresistance and Enhanced Voltage-Controlled Magnetic Anisotropy in Magnetoelectric Tunnel Junctions with MgO/PZT/MgO Tunnel Barrier
Diana Chien 1 Xiang Li 1 Kin Wong 1 Shauna Robbennolt 1 Guoqiang Yu 1 Sarah Tolbert 1 Nick Kioussis 1 Pedram Khalili 1 Kang Wang 1 Jane Chang 1
1University of California, Los Angeles Los Angeles United StatesShow Abstract
As existing memory systems approach fundamental limitations, ultra-thin uniform and conformal PZT films are needed for next-generation ultralow-power voltage-controlled non-volatile magnetoelectric RAM devices. By utilizing the magnetoelectric effect, where an electric field or voltage can be used to control the magnetization switching (instead of current), the writing energy can be reduced, resulting in increased memory density. Previous research has shown that the voltage-controlled magnetic anisotropy (VCMA) effect increases with the capacitance of the stack. Therefore, integrating an ultra-thin PZT film (having a dielectric constant 1-2 orders of magnitudes higher than currently used MgO) into the tunneling oxide layer enhances the VCMA coefficient, allowing for a lower voltage to switch the magnetization of the free magnetic layer and thus decreasing the write energy.
In this work, the growth of MTJ stacks with an MgO/PZT/MgO tunnel barrier using a combination of sputtering and atomic layer deposition (ALD) techniques was shown to be a viable process. First, an 18 nm thick Ta layer was deposited as the MTJ bottom electrode on thermally oxidized Si substrates, and then a Co20Fe60B20 free layer (out-of-plane magnetically anisotropic) was sputtered. For the MgO MTJ, a 2.5 nm thick MgO tunnel barrier was then sputtered. For the PZT MTJ, a 1.5 nm thick PZT film was deposited via ALD with an atomic Zr:Ti ratio of 52/48 between two layers of 1.0 nm thick sputtered MgO to form the MgO/PZT/MgO tunnel barrier. The PZT thin film was deposited by ALD at a substrate temperature of 250°C with Pb(tmhd)2, Zr(tmhd)4, and Ti(O.i-Pr)2(tmhd)2 as metalorganic precursors and deionized H2O vapor as the oxidant. A Co20Fe60B20 fixed layer (in-plane magnetically anisotropic) and capping layers of Ta and Pt were then sputtered on top of the tunnel barrier. The MTJs were annealed at 200-250°C for 30 minutes under vacuum and fabricated using standard photolithography and reactive ion etching techniques.
The perpendicular magnetic anisotropy of the bottom free magnetic Co20Fe60B20 layer was verified via superconducting quantum interference device magnetometry, confirming that the ALD PZT deposition process is a viable method for synthesizing PZT MTJs. The TMR ratio and VCMA effect were measured by sweeping an in-plane magnetic field from 0shy;3000 Oe while varying the applied voltage from -300shy;300 mV. The MgO MTJs were measured to have a tunneling magnetoresistance (TMR) ratio of 61.5% and a VCMA coefficient (xi;average) of 14.32.7 fJ/V-m, whereas the PZT MTJs were measured to have a TMR ratio of 53.1% and a xi;average of 19.81.3 fJ/V-m. The VCMA coefficient of PZT MTJs was 38.5% larger than those of MgO MTJs. In conclusion, PZT MTJs were demonstrated to have tunneling magnetoresistance and an enhanced VCMA effect, making them being potential candidates for future voltage-controlled, ultralow-power, high-density memory devices.
3:30 AM - KK2.04
Etching and Smoothing Process of Materials Used in Magnetic Tunneling Junctions with Gas Cluster Ion Beams
Noriaki Toyoda 1
1Univ of Hyogo Himeji JapanShow Abstract
Etching and smoothing of materials used in magnetic tunneling junctions (MTJ) with gas cluster ion beam (GCIB) were studied. For etching of etch-resistant metals, acetic acid vapor was supply during O2-GCIB irradiation in order to enhance the chemical reactions induced by GCIB. Etching depth of various etch-resistant materials (Pt, Ru, Ta, CoFe) showed 1.8 - 10.7 times higher values than those without acetic acid. High-pressure and high-temperature effects of GCIB enhance chemical reactions between reactive gas and substrate surface at room temperature. Thus, halogen free and room-temperature etching of etch-resistant materials was achieved. In additions, surface smoothing of Ru films under MTJ with GCIB was studied. Surface roughness of Ru film decreased monotonically with increasing the GCIB fluence, and reached 0.2 nm (Ra). After smoothing of Ru films, MTJ was formed. The magnetic properties of MTJ formed on the smoothed Ru showed improvement of inter-layer coupling magnetic field (Hin). Thus, etching and smoothing with GCIB are promising process for MTJ fabrications.
3:45 AM - KK2.05
Dual Field Effects in Spinel Ferrite Field Effect Devices: Volatile Electrostatic Carrier Doping and Nonvolatile Redox Reactions
Hidekazu Tanaka 1 Takashi Ichimura 1 Kohei Fujiwara 1
1Osaka Univ Ibaraki JapanShow Abstract
Controlling the electronic properties of functional oxide materials via external electric fields has attracted increasing attention as a key technology for next-generation electronics. For transition-metal oxides with metallic carrier densities, the electric-field effect with ionic liquid electrolytes has been widely used because of the enormous carrier doping capabilities. The gate-induced redox reactions revealed by recent investigations have, however, highlighted the complex nature of the electric-field effect. Here, we use the gate-induced conductance modulation of spinel ZnxFe3minus;xO4 to demonstrate the dual contributions of volatile and non-volatile field effects arising from electronic carrier doping and redox reactions using field effect device structure with an ionic liquid DEME-TFSI. These two contributions are found to change in opposite senses depending on the Zn content x; virtual electronic and chemical field effects are observed at appropriate Zn compositions. In the (Fe2.5Zn0.5)O4/DEME-TFSIFET EDLT, a large hysteresis observed in the drain current vs gate voltage characteristics is not accounted for solely by electrostatic doping, strongly suggesting the presence of chemical reactions . For the heavily Zn substituted system of (Fe2.2Zn0.8)O4/DEME-TFSIFET EDLT with less carrier concentration, the characteristic hysteresis virtually disappears . These observations revealed the coexistence of two types of field effects in the Fe3minus;xZnxO4 devices, and the tuning of field-effect characteristics via composition engineering should be extremely useful for fabricating high-performance oxide field-effect devices.
References;  Adv. Mater. Interfaces 1 (2014) 1300108,  Sci. Rep. 4 (2014) 581
Present affiliation of Dr. Kohei Fujiwara: The Institute for Materials Research#12289;Tohoku Univ., Japan
4:30 AM - *KK2.06
Thermally-Assisted MRAM for Embedded Applications
Philip Trouilloud 1 Anthony Annunziata 1 Sebastien Bandiera 2 Stephen Brown 1 Erwan Gapihan 2 Eugene Orsquo;Sullivan 1 Lucien Lombard 2 Daniel Worledge 1
1IBM T.J. Watson Research Ctr Yorktown Heights United States2Crocus Technology Grenoble Cedex FranceShow Abstract
Programming of Thermally-Assisted Magnetic Random Access Memories (TA-MRAM) relies on the coincidence of a thermal pulse and a writing pulse. With the thermal pulse, crossing the energy barrier needed to write to a new state can occur with a weaker writing pulse than would be needed in regular MRAM1. The data storage element is a ferromagnetic layer pinned by an antiferromagnetic layer. Materials with stronger pinning improve data retention and make it harder to overwrite. The thermal pulse temporarily reduces the pinning strength by bringing the antiferromagnet to its blocking temperature. TA-MRAM is well suited for small devices, as they can be heated and cooled quickly and efficiently.
In addition to retention, strong pinning in the operating temperature range of the device has another advantage. The devices can be read using a self-reference readout in which the stored state is interrogated non-destructively without the need for a global reference. This makes Read more tolerant to manufacturing spreads of size and resistance inherent in small devices.
We investigated TA-MRAM for embedded applications with devices designed to survive BEOL process temperatures of 400C2. Read and write performance was characterized as a function of TA-MRAM material stack parameters. In the main part of the device population, there is a 3-sigma separation between the write voltage and the breakdown voltage observed for 10^7 pulses. Consistency of the self-reference read was tested for 10^8 write pulses.
 Prejbeanu et al., J. Physics: Condensed Matter 19 (16), 165218 (2007).
 Annunziata et al., J. Appl. Phys. 117, 17B739 (2015).
5:00 AM - KK2.07
Fabrication and Magnetoelectronic Transport of Double Stack FePt Nanodots on Ultrathin SiO2
Seiichi Miyazaki 1 Yuuki Kabeya 1 Yusuke Mitsuyuki 1 Katsunori Makihara 1
1Nagoya Univ Nagoya JapanShow Abstract
Nanodots (NDs) showing a high magnetic anisotropy have received much attention because of their potential application to magnetoelectronic devices. So far, we have demonstrated a spontaneous formation of L10-ordered FePt dots with an areal density as high as ~1011cm-2 by remote H2-plasma (H2-RP) exposure of Pt/Fe bilayers on ultrathin SiO2 without external heating and a unique magnetic-field dependent electron transport through individual FePt dots/ultrathin SiO2 by means of a CoPtCr-coated magnetic tip of atomic force microscopy (AFM) in a contact mode.
In this work, based on size dependence of magnetic properties of FePt nanodots, we designed and fabricated double stack FePt NDs with ultrathin internal SiO2 and characterized their magnetoelectronic transport by using a non-magentic Rh-coated AFM tip at room temperature (RT).
After FePt NDs with an areal density of ~4.5x1011cm-2, an average dot size of ~5.0nm and a RT coercivity as small as 0.5kOe were formed by H2-RP exposure of ultrathin Pt/Fe bilayers on 2nm-thick thermally- grown SiO2 layer/Si(100) without external heating, uniform coverage of the FePt NDs with a 2nm-thick SiO2 layer by a PVD method, a very uniform formation of Pt/Fe bilayers was followed by H2-RP exposure to form a spontaneous formation of FePt nanodots with an areal density of ~2.5x1011cm-2, an average dot size of ~8.0nm and a RT coercivity of ~2.5kOe formed was carried out in a similar way to the 1st formation of FePt NDs. After that, Al back contact was formed to measure the electron transport through the double stack FePt NDs on ultrathin SiO2.
I-V characteristics of the double stacked FePt NDs as a function of magnetic field were measured by using non-magnetic Ph-coated AFM Tip at room temperature. With an increase in magnetic field from 1.5 to 2.5kOe in the direction normal to the sample surface, the current level is increased significantly by over one order of magnitude. Then no further change in the current level was detectable by magnetic field application of 4.5kOe. In addition, such a current level occurred at 2.5kOe remains almost unchanged even in 24hr after removal of external magnetic field, which implies stable magnetization of the double stack FePt NDs. But when the magnetic field of 0.5kOe was applied in the opposite direction to the 1st magnetization, the current level was decreased markedly to the initial low current level. With further increase the magnetic field from 1.5 to 2.5kOe, the switching to a high current level occurred. The observed magnetic field dependence of the current level through the double stack FePt NDs can be interpreted in terms of alignment in the magnetization polarity between staked FePt NDs with different coercivities.
5:15 AM - KK2.08
Magnetoresitance of Nickel Nanoparticles Embedded in High-Quality Single-Crystal Silicon
Girish Malladi 1 Mengbing Huang 1 Hassaram Bakhru 1 Steven Novak 1 Thomas Murray 1 Vincent LaBella 1 Akitomo Matsubayashi 1
1CNSE SUNY Albany Albany United StatesShow Abstract
Integrating magnetic functionalities with Silicon, the dominant semiconductor in microelectronics, holds a promise to realize devices with multiple functionalities such as spin-valves, spinFETs etc. Earlier studies based on the incorporation of transition metal atoms into Silicon have reported ferromagnetism in Silicon but such materials have very low Curie temperature. Additionally, the observed room-temperature ferromagnetism in transition metal implanted Si has been rather associated with the implanatation-related defects. In this work, we developed a novel method based on ion implantation to fabricate a thin layer of ferromagnetic nanostructures embedded within single crystal Si. This is achieved by exploiting the effects of ion implanted hydrogen on the formation of metal nanoparticles in Si. With this approach, we demonstrate the synthesis of a layer of Ni nanoparticles (size: ~10-25 nm; density: ~ 1011/cm2) within a Si environment of a very high crystal quality. Ni nanoparticles encapsulated within such high-quality crystalline Si layer exhibit a high magnetic switching energy barrier of ~ 0.86 eV, an increase by about one order of magnitude as compared to their counterparts on a Si surface or in a highly defective Si environment. Strong ferromagnetism associated with this Ni nanostructure layer is evidenced even at room temperature. These ferromagnetic nanostructures also show enhanced positive or negative magnetoresistance depending on injected carrier types, when an external magnetic field is applied in the transverse direction. Such a thin layer of ferromagnetic nanostructures embedded in Si has a potential to act as an internal spin filter for the generation of spin-polarized carriers in Silicon.
5:30 AM - KK2.09
Thickness Dependent Multiferroic Properties of Nanoscale PZT/LSMO and PZTFT for Multiferroic Tunnel Junction Applications
Danilo G. Barrionuevo Diestra 1 2 Nora Ortega 1 2 Ram S. Katiyar 1 2 Ashok Kumar 3
1University of Puerto Rico San Juan United States2Institute of Functional Nanomaterials San Juan United States3National Physical Laboratory (CSIR), Dr K S Krishnan Road New Delhi IndiaShow Abstract
Multiferroic magnetoelectric materials combine ferromagnetism and ferroelectricity giving the possibility of controlling polarization P with a magnetic field H or magnetization M with an electric field E. Ultrathin films offer the opportunity of combining respective advantages of fast low-power electrical write operation and non-destructive magnetic read operation, resulting in the realization of four-state logic. A way to exploit these properties are to use them in multiferroic tunnel junctions (MFTJs). Pb(Zr0.53Ti0.47)0.60(Fe0.5Ta0.5)0.40O3 (PTZFT) is a single-phase multiferroic material at room temperature. In order to study thickness effect on electrical and magnetic properties from thicker to ultrathin films, we have grown films with thicknesses from 4 to 80 nm of PZTFT on (001) LSMO/(LaAlO3)0.3(Sr2AlTaO6)0.7 (LSMO/LSAT) substrates deposited by pulsed laser deposition (PLD) technique. Well saturated ferroelectric loops were observed for PZTFT films with a remanent polarization of 32, 25 and 10 mu;C/cm2 for films with thicknesses of 80, 50 and 20 nm respectively. An enhanced saturated magnetization (Ms) was observed with increased PZTFT layer thickness in PZTFT/LSMO structures. The average Ms values for PZTFT/LSMO heterostructures were 33, 25, and 15 emu/cm3 for 80, 50, and 20 nm respectively, at 300 K. Piezo force microscopy measurements for 4, 5, and 7 nm ultrathin PZTFT films showed a clear and reversible out-of-plane phase contrast above ± 4 V, which indicates the ferroelectric character of ultra-thin films. Magnetic force microscopy show magnetic stripe domains in ultrathin films. The effect of PZTFT film thickness on temperature dependent dielectric properties will be discussed. We have also studied the effect of polar capping on magnetization in nanoscale PZT(PbZr0.52Ti0.48O3)(5 and 7 nm)/LSMO(30 nm) heterostructures grown by PLD technique. PZT/LSMO heterostructures with thick polar PZT (7 nm) capping showed nearly 100% enhancement in magnetization compared with thin polar PZT (5 nm) films, probably due to excess of hole transfer from the ferroelectric to the ferromagnetic layers. Core-level X-ray photoelectron spectroscopy (XPS) studies revealed the presence of large Mn (3s) spin-orbit hybridization and high Mn3+/Mn4+-ion ratio in the LMSO with 7 nm polar capping. The transport properties for Pt/PZT(7 nm)/LSMO heterostructures shown a significant variation in tunneling electroresistance (TER) ratio when was exposed to magnetic field and its values at zero bias changed from 57 (at 0 G) to 110 under 10 kG of magnetic field. We attributed this enhancement to change in resistance near the PZT/LSMO interface under magnetic field. Ferroelectric polarization reversal and application of magnetic field changed lattice strain, chemical bonding and charge modulation near PZT/LSMO interface which in turn affects the charge carrier density and transmission probability. Our results suggest the possibility to manipulate TER by magnetic field.
5:45 AM - KK2.10
Experimental Realization of Non-Volatile Memory: A Nanodevice Based on the Interplay between Superconducting Ratchet Effect and Out of Plane Magnetization
Jose Luis Vicent 1 2 Javier del Valle 1 Alicia Gomez 1 Elvira Maria Gonzalez 1 2 Manuel R. Osorio 2 Daniel Granados 2
1Univ Complutense Madrid Madrid Spain2IMDEA-Nanociencia Madrid SpainShow Abstract
We have fabricated and tested a nanodevice that works as a non-volatile three-state memory, as well as a reading device (1). The nanodevice is a Si substrate with array of triangular-shaped nanomagnets embedded in a superconducting film. The nanomagnets are made of Co/Pd multilayers with the magnetization perpendicular to the samples plane. This magnetic configuration yields a strong stray magnetic field threading the Nb superconducting film. Consequently superconducting vortices are induced in the film without needing an applied external magnetic field. Applying an ac current in the device triggers a ratchet effect (2), so that an output dc voltage is obtained. The nanomagnet can be easily set in three states upward magnetization (+1), downward magnetization (-1) and demagnetized state (0). These three resilient states are detected by measuring the output dc voltages. We have to point out that the zero state shows a plateau where the output voltages remain constant. So the three states are well defined. In the case an external magnetic field is applied to the device the device turns into a magnetic sensor device, which more outstanding realization is that output voltage signal is zero for a precise value of the applied magnetic field. This value only depends on the fabrication characteristics of the nanodevice.
(1) J. del Valle, A. Gomez, E. M. Gonzalez, M. R. Osorio, D. Granados and J. L. Vicent, arXiv: 1505.04961
(2) J. E. Villegas, S. Savelev, F. Nori, E. M. Gonzalez, J. A. Anguita, R. Garcia, J. L. Vicent,Science 302, 1188 (2003).
KK3: Poster Session I: Advanced Flash, MRAM, Ferroelectric Memory, PCM, Others
Tuesday PM, December 01, 2015
Hynes, Level 1, Hall B
9:00 AM - KK3.02
Improving Electrical Properties of Inter-Poly Dielectric Layer using Atomic Layer Deposition Process in 2D NAND Flash Memory
Jeahoon Lee 1 Byoungjun Park 1 Minho Jeong 1 Jiyul Park 1 Sungpyo Lee 1 Younghwan Choi 1 Myoungkwan Cho 1 Kun-ok Ahn 1 Jinwoong Kim 1
1SK Hynix Cheongju-si Korea (the Republic of)Show Abstract
Recently, the NAND flash memory as a storage media for various mobile devices has been scaled down into sub-20 nm. However, reliability characteristics such as endurance and retention of NAND cells are also degraded as NAND Flash memory cells became smaller and closer. Of those ones, data retention characteristic is strongly related with inter-poly dielectrics (IPD), which consist of oxide-nitride-oxide layers. Because, those ones play an important role in NAND flash memory cell structure to prevent loss of charges in the floating gate. In this study, to improve the electrical properties of IPD layers, we change the way of deposition from conventional low pressure chemical vapor deposition to atomic layer deposition, which shows excellent step coverage and has less trap sites. Electrical properties of IPD layers are characterized with time dependent dielectric breakdown method and current versus voltage curves in the test element groups. Also, we check the data retention characteristic with threshold voltage distributions of NAND cell array.
9:00 AM - KK3.03
Laser Fabricated Nanocrystals for Memory Devices
L Kastanis 2 Jacob Leonard Spear 1 A Aggelou 3 Nikolaos Kalfagiannis 1 Ch Sargentis 2 Demosthenes Koutsogeorgis 1 E.K. Evanglou 3 D. Tsamakis 2
1Nottingham Trent University Nottingham United Kingdom2NTUA Attiki Greece3University of Ioannian Ioannina GreeceShow Abstract
Flash memory is a type of electronic memory most often used in portable electronic devices and floating gate is the primary technology to construct such memories . With downscaling of the device feature size, conventional poly-silicon floating gate flash memories are facing a severe challenge that the thinner tunneling oxide will degenerate retention characteristics due to leakage current . During the last decades nanocrystals (NCs) have been extensively studied as a replacement to the polycrystalline Si layer in the floating gate nonvolatile memories (NVMs) . Among various NC materials, it is well known that nonvolatile memories utilizing metal NCs have the advantages of higher density of states around the Fermi level, stronger coupling with the conduction channel, and smaller energy perturbation due to carrier confinement . Moreover, metal nanoparticles with high work-function, like platinum (Pt), Silver (Ag) and gold (Au) NPs, are promising for the use of nanoparticle type memory due to the deep potential wells they create. At the same time, laser fabrication has been proven to be a versatile and powerful method for fabricating nanoparticle arrays with well controlled characteristics (size and distribution) .
In this work we present the growth of Ag nanocrystals (NCs) on SiO2 surfaces via sputtering followed by post deposition laser annealing (LA) as well as conventional rapid thermal annealing (RTA). Post-annealing treatment was introduced to the fabrication process to optimize device performance. Enhanced performance nanocrystal memory characteristics are shown for the laser annealed MOS devices as compared to RTA ones.
Thermally grown SiO2 with thickness around 2-5nm was prepared on n type Si(1-10 Ohm.cm) wafers. On top of the SiO2 surfaces a thin (5nm) Ag layer was sputtered followed by a blocking Y2O3 layer (around 40 nm sputtered at room temperature). After laser annealing at various fluences with an KrF laser, MOS devices were fabricated by shadow mask evaporation of Al gate contacts.
Capacitance vs Voltage (C-V) and Current vs Voltage (I-V) measurements of n-Si\SiO2\Ag NCs\Y2O3\Al devices show an almost 20% larger memory window, improved retention properties and reduced leakage currents. Fowler-Nordheim tunnelling was found to be the dominant current conduction mechanism at applied gate voltages in the range of 5 to 10 V.
Our results indicate that laser annealing leads to MOHOS-type flash memory devices with improved electrical characteristics due to the creation of superior quality metal nanocrystals.
9:00 AM - KK3.04
A Hybrid Ferroelectric-Flash Memory with Quasi-Single Crystal Pb(Zr,Ti)O3 for Blocking Layer
Jaehyo Park 1 Hyung Yoon Kim 1 Zohreh Kiaee 1 Seung Ki Joo 1
1Seoul National University Seoul Korea (the Republic of)Show Abstract
In this work, A novel concept of charge-injection (CI) type ferroelectric-assisted flash memory FET (FAFM-FET) were fabricated. The total structure of the device was Pt/Pb(Zr,Ti)O3(PZT)/ZrTiO4(ZT)/SiO2/Si, where the PZT, ZT, and SiO2 was used for the blocking layer, charge-trapping layer, and tunneling layer, respectively. Strong polarization and high dielectric constant (1500) of PZT layer showed amplification effect, which improves the retention time and P/E switching speed. The ZT layer having a high dielectric constant (45) showed a high charge storage and effective diffusion barrier. As a result, the CI FAFM-TFT exhibited excellent memory characteristics, such as large memory window (9.1 V), fast P/E speed (500 nsec), long retention time and good endurance.
9:00 AM - KK3.05
Magnetoelastic Effect in Multilayer of Ni Nanoparticle and C Film Produced by Pulsed Laser Deposition
Alexsandro dos Santos Evangelista Cruz 1 Fernando Fabris 1 Dante Ferreira Franceschini 1 Yutao Xing 1 Wallace Castro Nunes 1
1Instituto de Fiacute;sica ( Universidade Federal Fluminense) Niteroacute;i BrazilShow Abstract
The research on magnetic nanoparticle(NPs) has increased in recent years due to several potential applications, including recording media, magnetic hyperthermia, drug delivery and others. In this work, we produced multilayer in substrate of Si alternating the deposition of Ni NPs and amorphous carbon film. The film and NPs was deposited by Pulsed Laser Deposition (PLD). Carbon film was deposited in vacuum of about 1.6x10-6 Torr and Ni NPs was grown in Argon atmosphere at pressure of 1.0 Torr. The carbon film causes a mechanical stress in Ni NPs resulting in a magnetic anisotropy of magnetoelastic origin. The results shows that the samples with lower thickness of carbon film have easy axis magnetization parallel to the film surface. On the other hand, the increase of the thickness changes the easy axis to perpendicular direction. In addition, there is a change in blocking temperature of about 15K to near room temperature, respectively. The change is due to the increase of the stress of the carbon film. The energy associated with this anisotropy can be evaluate according to the and is given by E=-(3/2)lambda;σcos2theta;, where lambda; is the magnetostriction coefficient, σ is the induced strain and theta; is the angle between magnetization and deformation axis. It has been reported that anisotropy induced by magnetoelastic effect can be larger than magnetocrystalline anisotropy in Ni NPs system, and thus can define the easy axis of magnetization of the sample. The morphology of the samples were characterized by Transmission Electron Microscopy. To study the details about anisotropy effect we measured M(H) curves and ZFC(Zero Field Cooled) and (Field Cooled) curves for applied magnetic field parallel or perpendicular to the sample surface. The magnetic and structural results of the studied samples are discussed considering the magnetoelastic effect induced by carbon film in Ni NPs and potential application in magnetic recording media.
9:00 AM - KK3.06
Temperature Dependence of Magnetization in Interacting Nanoparticle Systems
Fernando Fabris 2 Oana Pascu 1 Maria Fialho Vaz 1 Wallace Castro Nunes 2
1Universidade Federal Fluminense Niteroacute;i Brazil2Universidade Federal Fluminense Niteroacute;i BrazilShow Abstract
The nanostructured materials have shown large potential applications in many fields of science and technology. In particular, magnetic nanoparticles have been used as high-moment soft magnetic materials, magnetic sensors, improved nanocomposite magnets, and others. Theoretical models describing the magnetic properties of nanoparticle systems usually neglect interaction effects among nanoparticles. However, in rather concentrated systems the nanoparticles are close enough for interactions among them become noticeable, affecting their macroscopic magnetic properties. Is this work we developed a phenomenological model to describe the magnetic behavior of interacting nanoparticle systems. The model is based on a simple modification of the random anisotropy model to take into account the concentration and size of the nanoparticles as well as the field dependence of the correlation length. In addition, we considered that each group of correlated nanoparticles is subjected to the external magnetic field plus a internal mean field generated by dipolar interaction of other groups of nanoparticles. The proposed model leads to a accurate description of the zero-field cooled and field cooled magnetization curve of nanoparticle systems for a wide range of nanoparticle concentration.
9:00 AM - KK3.07
Ferroelectricity in Rare-Earth (Sm, Gd) Doped-HfO2 Thin Films Fabricated by Sequential Pulsed Laser Deposition
Yogesh Sharma 1 Danilo Barrionuevo 1 Radhe Agarwal 1 Shojan Pavunny 1 Ram S. Katiyar 1
1University of Puerto Rico San Juan United StatesShow Abstract
Recently, experimental as well as theoretical studies showed ferroelectricity in aliovalent ions (Si4+, Al3+, Y3+, and Zr4+) doped hafnia (HfO2) thin films which are compatible with existing CMOS technology. Ferroelectric HfO2 films have promising potential for the three-dimensional capacitor structure required for the future field driven and energy efficient ferroelectric random-access-memory (FeRAM) devices. With this motivation, rare-earth doped hafnium oxide (HfO2) thin films are pulsed laser deposited by sequential ablation of individual ceramic binary oxide targets. 6 mol. % of Sm2O3/Gd2O3 doped-HfO2 thin films (Sm:HfO2 and Gd:HfO2) of about 60 nm thickness and crystallized in cubic phase are found to exhibit ferroelectricity. A remnant polarization (Pr) of ~12.5 (11) mu;C/cm2 along with a coercive field (EC) of ~334 (384) kV/cm are observed in Sm:HfO2 (Gd:HfO2) thin films. Piezoresponse force microscopy measurements further confirmed the ferroelectric nature of these thin films by showing phase hysteresis and butterfly amplitude loops. It can be noticed that wake-up cycles improved the remnant polarization and found to be necessary for the forming of a well saturated hysteresis loop. Our results show potential towards realization of densely scaled next generation non-volatile FeRAM devices compatible with silicon technology.
9:00 AM - KK3.08
Probing Multi-Level Ferroelectric States in Multi-Floor P(VDF-TrFE) Nanostructures
Owoong Kwon 1 Seung Hyun 2 Jin Kon Kim 2 Yunseok Kim 1
1Sungkyunkwan University Suwon-si Korea (the Republic of)2Pohang University of Science and Technology Pohang Korea (the Republic of)Show Abstract
Ferroelectric random access memory (FeRAM) is one of next-generation memories which show fast read/write time, low power consumption, high write-erase cycles and etc. However, its actual application is technically limited due to scalability and flexibility of the device. Thus, here, to overcome these limitations on the scalability and flexibility, we demonstrate multi-floor ferroelectric copolymer polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] nanostructures for achieving both flexible devices and multi-level polarization states. Since the P(VDF-TrFE) is a well-known flexible ferroelectric material with high dielectric constant and low processing temperature, we have chosen the P(VDT-TrFE) for preparing array of multi-floor cascading of the nanostructures which allows achieving spatially multi-leveled structures. Switching spectroscopy piezoresponse force microscopy (PFM) clearly shows the different switching behavior at each floor indicating spatially varied ferroelectric states. Furthermore, multi-level ferroelectric states on the same floor were examined by first-order reversal curve-type PFM. The obtained results clearly reveal the existence of the multi-level ferroelectric states on the multi-floor P(VDF-TrFE) nanostructures. To further understand the existence of the multi-level ferroelectric states, finite element modeling was performed to visualize the electric field distribution with respect to the location of the conductive probe. These results can provide not only stepwise structural effects of the piezoresponse in the P(VDF-TrFE) but also potential device structures in the FeRAM.
9:00 AM - KK3.10
Differentiating Ferroelectric/Piezoelectric Effects from Electromechanical Response in Strain Based Atomic Force Microscopy
Seongjae Park 1 Daehee Seol 1 Olexandr Varenyk 2 Tricia Meyer 3 Ho-Nyung Lee 3 Anna Morozovska 2 Yunseok Kim 1
1Sungkyunkwan University (SKKU) Suwon Korea (the Republic of)2Institute of Physics of the National Academy of Sciences of Ukraine Kyiv Ukraine3Oak Ridge National Laboratory Oak Ridge United StatesShow Abstract
The ferroelectric materials have been of great interest for multiple applications such as ferroelectric memories and energy harvesting due to its spontaneous polarization and piezoelectricity. In order to probe the exitence of ferroelectric properties at nanoscale, hysteresis loop measurements are typically performed using piezoresponse force microscopy (PFM). However, ferroelectric-like hysteresis loops were recently reported in non-ferroelectric materials and could be resulted from different mechanisms so that it can be misinterpreted as ferroelectric materials. This indicates that an approach for probing ferroelectric effect based on the hysteresis loop measurements can be limited in the interpretation of the ferroelectricity. Here, we suggest a new and facile way to differentiate ferroelectric effects from the other contributions using frequency dependent Vac measurements. Li-ion conductive glass ceramics (LICGC), which have both ionic and piezoelectric phases, and Pb(Zr,Ti)O3, which is a ferroelectric material, have been chosen as model systems to demonstrate our approach. The electromechanical (EM) response originated from piezoresponse is independent with its frequency, whereas that induced by the other contributions shows frequency dependent behavior. In addition, theoretical calculation of the other contributions to the EM response approves its frequency dependence. These results can provide a new and facile approach for differentiating the ferroelectric effects.
9:00 AM - KK3.11
The Orientation Controlled (Pb,La)(Zr,Ti)O3 Thin Films through PLD and Annealing Conditions for Robust Ferroelectric Capacitor
Takeyasu Saito 1 Taiga Amano 1 Rika Tamano 1 Yoko Takada 1 Naoki Okamoto 1 Kazuo Kondo 1 Takeshi Yoshimura 2 Norifumi Fujimura 2 Koji Higuchi 3 Akira Kitajima 3
1Osaka Prefecture University Sakai Japan2Osaka Prefecture University Sakai Japan3Osaka University Ibaraki JapanShow Abstract
Ferroelectric random access memory (FeRAM) has tremendous potential for future non-volatile memory devices due to its excellent characteristics. Ferroelectric properties were generally influenced by crystal orientation of the films that also determines device performance. Therefore, the orientation control of the ferroelectric films was very crucial for highly integrated and reliable FeRAM devices.
In this study, we fabricated (Pb,La)(Zr,Ti)O3 (PLZT) thin films using pulsed laser deposition (PLD), then, we studied effects of PLD and annealing conditions on ferroelectric properties. Also, we measured crystallographic orientation of the films and discussed the relationship between electrical properties and orientation.
The substrates were highly (111)-oriented sputtered Pt (150-nm-thick) as bottom electrode. PLZT thin films (ca. 300-nm-thick) (Pb:La:Zr:Ti = 1.13~1.27:0.03:0.3:0.7) were deposited on Pt bottom electrode by using PLD. The substrate temperature during PLD was changed from R.T. to 600°C. Then, Pt top electrode was formed on the PLZT thin films using RF sputtering with a 5minus;500-mm-diameter shadow mask. Finally, the fabricated ferroelectric capacitors were annealed using rapid thermal annealing at 650~750°C (3~30 min) in air.
We investigated the effects of substrate temperature during PLD, Pb contents of the target, annealing temperature, and annealing period to improve ferroelectric properties. Lower substrate temperature (R.T.) during PLD, higher Pb contents (Pb = 1.27) of the PLD target, higher annealing temperature (750°C) and appropriate annealing period (10 min) exhibited PLZT(100) well-oriented films. Especially, PLZT(100) originated peaks appeared with increasing annealing temperature up to 750°C, in addition to PLZT(111) peaks. The annealing temperature strongly affects the orientation of PLZT thin films. The best remnant polarization and coercive voltage at an applied voltage of 10 V was 22.0 mu;C/cm2 and 7.1 V, respectively, was obtained.
9:00 AM - KK3.12
Dielectric and Ferroelectric Properties of Lead-Free BTS-BCT Thin Films Processed by Chemical Solution Deposition Method
Berk Akbay 1 Ahmet Macit Ozenbas 1
1Middle East Technical Univ Ankara TurkeyShow Abstract
For a few decades, the piezoelectric materials have been considered as important functional materials for various applications, from the microphones to the high technology scanning electron microscopes, actuators, sonar sensors, cell phones, MEMS and etc. Lead-based phases have dominated almost all these applications. However, lead based materials have been found as toxic and hazardous materials and will be prohibited to use within a short period of time. Among various lead-free piezoelectric materials, BTS-BCT based phases are good candidates instead of Pb-based materials due to having a tricritical point compared to other lead-free alternatives. In this study, lead-free Ba(Ti0.88Sn0.12)O3-0.3(Ba0.7Ca0.3)TiO3 (BTS-BCT) thin films were successfully grown on Pt/TiO2/SiO2/Si substrates using chemical solution deposition method. The effect of sintering temperatures on microstructure, dielectric and ferroelectric properties were studied systematically. Among the various high-quality BTS-BCT thin films with uniform thickness, the optimum room temperature dielectric and ferroelectric responses were observed for the thin films sintered at 850oC for 1 h. The thickness was kept constant for all measurements as 500 nm (13 layered films). Both morphological and structural analyses showed that BaTiO3 based (Ba0.91Ca0.09)(Ti0.92Sn0.08)O3 composition had homogeneously nucleated growth mechanism throughout the film. This type of growth mechanism yields polycrystalline films as observed in structural analyses and dense-small grained morphology as observed in morphological analysis. Optimum dielectric constant and dielectric loss values were obtained as 113.4 and 5.46 % at 600 kHz frequency for the BTS-BCT thin films sintered at 850°C due to perovskite phase showing full crystallization and minimum surface porosity obtained at this temperature. Also, remnant polarization (Pr) and coercive field (Ec) values were determined as 4.1 mu;C/cm2 and 57.8 kV/cm, respectively, for the films sintered at 850°C using 10 V applied voltage. Both ferroelectric and dielectric results were evaluated in the light of morphological and structural information. Thus, process parameter is selected as 850°C which was the optimum sintering temperature for the production of BaTiO3 based BTS-BCT thin films.
9:00 AM - KK3.13
Statics and Dynamics of Ferroelectric Domains in Diisopropylammonium Bromide
Haidong Lu 1 Tao Li 1 Shashi Poddar 1 Om Goit 1 Stephen Ducharme 1 Alexei Gruverman 1
1University of Nebraska-Lincoln Lincoln United StatesShow Abstract
Recent discovery of ferroelectricity in molecular salts has spurred the studies on fundamental ferroelectric ordering and polarization control in these materials in quest for new type of functionality not available in conventional inorganic and polymer ferroelectrics. Uniaxial The uniaxial molecular ferroelectric diisopropylammonium bromide (DIPA-B) is one of the emerging materials of high fundamental and potentially technological importance. Here, we report the results of an investigation of the nanoscale static and dynamic behavior of ferroelectric domains in DIPA-B microcrystals by means of piezoresponse force microscopy (PFM) technique. Effective manipulation of in-plane polarization by electrically-biased PFM tip is demonstrated. We show that stable head-to-head or tail-to-tail domain configurations can be realized in DIPA-B crystals and study the factors determining this stability. In-plane alignment of polarization allows investigation of the forward domain growth along the polar axis - a process that is difficult to attain in conventional ferroelectric switching studies. Using this sample geometry, we have directly measured anisotropy of the domain wall velocity for the forward and lateral growth and demonstrate the effect of the forming charged domain walls on morphology of the growing domains.
9:00 AM - KK3.14
Ferroelectric Retention Free BiFeO3 Mesocrystal
Ying-Hui Hsieh 1 Fei Xue 2 Yen-Chin Huang 3 Yi-Chun Chen 3 Chun-Gang Duan 4 Long-Qing Chen 2 Qing He 5 Ying-Hao Chu 1 6
1National Chiao Tung University Hsinchu Taiwan2Pennsylvania State University University Park United States3National Cheng Kung University Tainan Taiwan4East China Normal University Shanghai China5Durham University Durham United Kingdom6Academia Sinica Taipei TaiwanShow Abstract
Nowadays, one of the focal approaches to pursue next generation low power consumption, multifunctional, and green nanoelectronics is to advance the electric field control of lattice, charge, orbital, and spin degrees of freedom. In order to control these degrees of freedom, a medium possessing the coupling between these degrees needs to be established. Multiferroics that support both strong ferroelectric and magnetic orders are typically insulators with an antiferromagnetic spin arrangement. Among numerous multiferroic systems, BiFeO3 (BFO) is currently the most studied and best understood. Large ferroelectric polarization along <111> directions and G-type antiferromagnetism at room temperature make BFO appealing for applications in non-volatile devices. The orientation of the antiferromagnetism couples to the ferroelastic strain state and is always perpendicular to the ferroelectric polarization. Once the polarization is switched by an electric field, the easy plane of magnetization changes accordingly to reach a stable state, offering an opportunity for controlling spin via the electric field.
Although BFO is an ideal template of manipulating the spin via electric field, some key issues, such as imprint, retention, and fatigue, have to be solved before the realization of new devices. For example, retention can be addressed to thermodynamic instability of the domain. Asymmetric free energy landscapes, mainly due to different out-of-plane electric boundary conditions, result in at least one unstable polarization state. In the metal-ferroelectric-metal capacitor, the problem can be solved by controlling the bottom and top electrodes to ensure the electrostatic boundaries are balanced. However, to reduce the circuit size, the structure of a transistor with ferroelectric/multiferroic is more favorable. The transistor of semiconductor-ferroelectric/multiferroic-metal will form an imbalance of electrostatic boundary conditions, causing severe retention problems. To solve the problem, additional energy term has to be induced into the system to balance the free energy landscape. Although efforts on related studies have shown their ways to reduce the energy difference of the polarization double-well by controlling chemical environment, breaking the out-of-plane compositional symmetry, or using strain gradient, ferroelectric retention is still a key issue to be dealt with. In order to shed light on the retention problem, we intend to use the elastic energy as a key parameter to improve ferroelectric retention of BFO. In this study, self-assembled BFO mesocrystal will serve as a model system. The intimate connection between the mesocrystal and matrix material provides a tunable structure coupling. This elastic energy term may be exploited to improve ferroelectric/multiferroic retention. The achievement of great improvement on the retention in this system will open a new avenue to ferroelectric retention and the possible application in nonvolatile devices.
9:00 AM - KK3.15
Photoelectric Switchable Diode Effect in Semiconducting Sulfur-Based Ferroelectric Materials
Yiping Wang 1 Jian Shi 1
1Rensselaer Polytechnic Institute Troy United StatesShow Abstract
We will present both experimental and theoretical exploration of how ferroelectricity would manipulate the diode behaviors in ferroelectric semiconducting materials. Via van der Waals epitaxy, we will show the growth of high-quality large scale semiconducting ferroelectric SbSI and Sb2S3 sheets. The 3d transition metal-free ferroelectrics will enable greatly increased diffusion lengths of minority carriers, making the high-efficiency light-to-electricity generation process possible. We will show that the ferroelectric polarization effectively regulates the width of depletion region and therefore boosts the quantum efficiency of the sulfur-based materials as light sensitizer. The coupling of ferroelectricity and semiconductivity suggests a new avenue to high-efficient, fast, and endurable ferroelectric photovoltaic nonvolatile memory.
9:00 AM - KK3.16
Multistate Switching in Ferroelectric Multilayer Capacitors
Alexei Grigoriev 1 Pavel Salev 1
1Univ of Tulsa Tulsa United StatesShow Abstract
Ferroelectric materials are successfully used in non-volatile memory applications. Multifunction properties of ferroelectric and multiferroic complex oxides allow researchers to develop new approaches to memory storage and switchable electronic and optical devices. We found double polarization switching and multiple dielectric states in ferroelectric bilayer capacitors of thin BaTiO3 (BTO) and PbZr0.2Ti0.8O3 (PZT) layers. Computational analysis predicts that the multistate switching can be a common feature of ferroelectric multilayer capacitors made of materials with different remnant polarizations. In this work we present experimental results of polarization stability, switching dynamics, and switchable dielectric state analysis of BTO/PZT bilayer capacitors.
9:00 AM - KK3.17
Realization of Flexible Block Copolymer-Incorporated One Diode-One Phase Change Memory Array on Plastic Substrate
Beomho Mun 1 Byoung Kuk You 1 Daniel Juhyung Joe 1 Se Ryeun Yang 1 Hyeon Gyun Yoo 1 You Yin 2 Yeon Sik Jung 1 Keon-Jae Lee 1
1KAIST Daejeon Korea (the Republic of)2Gunma University Gunma JapanShow Abstract
Recently, there has been significant research effort on realization of flexible non-volatile memory, the fundamental component for data processing, storage, and radio frequency communication in flexible electronic systems. Among several emerging non-volatile memory technologies, phase-change random-access memory (PRAM) is one of the strongest candidates for next-generation non-volatile memory devices due to its remarkable characteristics of large cycling endurance, high speed, and excellent scalability. Although there are several approaches for flexible phase-change memory (PCM) devices, high reset current is the biggest obstacle for their practical operation. In this presentation, we show the fabrication of flexible PCMs by incorporating nano-insulators derived from Si-containing block copolymer (BCP) in order to significantly lower the operating current of the memory on a plastic substrate. The reduction of thermal stress by BCP nanostructures enables the reliable operation of the memory devices spatially integrated with ultrathin, single crystal flexible Si-diodes during more than 100 switching cycles and 1000 bending cycles. These results may open up a new opportunity for realizing flexible PRAMs for practical implementation in electronic applications.
9:00 AM - KK3.18
Electrodeposition of GST phase change memory
Philip Bartlett 1 Sophie Benjamin 1 2 C. H (Kees) de Groot 1 Andrew Lee Hector 1 Ruomeng Huang 1 Andrew Jolleys 1 Gabriela Kissling 1 3 William Levason 1 Gillian Reid 1
1Univ of Southampton Southampton United Kingdom2Nottingham Trent University Nottingham United Kingdom3University of Bath Bath United KingdomShow Abstract
Chalcogenide-based phase change memory (PCM) is a promising candidate for next generation non-volatile memory. Electrodeposition offers several potentially significant advantages for growth of semiconductor alloys for PCM as a fast process with lower cost compared with vapour deposition techniques. It enables excellent control over the composition across ternary phase diagrams in p-block alloys, and is well suited to deposition into patterned substrates.1,2
We have developed a tuneable electrolyte bath that is suitable for the rapid electrodeposition of ternary GexSbyTez phase change materials. Uniform and continuous films were deposited with very low impurity levels. Thin film memory cells are fabricated from these films, displaying good switching performance, stable endurance and an on/off ratio of around 1000.
Uniform filling of nano-patterned electrodes constructed by lithographic or e-beam etching of silica films on titanium nitride provided access to vertical memory devices. TiN top contacts have been applied to these and the resulting microstructured and nanostructured GST-225 devices also displayed good switching performance.
1. P. N. Bartlett, D. Cook, C. H. de Groot, A. L. Hector, R. Huang, A. Jolleys, G.P. Kissling, W. Levason, S. J. Pearce and G. Reid, RSC Adv. 3 (2013) 15645-15654.
2. P. N. Bartlett, S. L. Benjamin, C. H. de Groot, A. L. Hector, R. Huang, A. Jolleys, G.P. Kissling, W. Levason, S. J. Pearce, G. Reid and Y. Wang, Mater. Horiz. (2015) DOI: 10.1039/c5mh00030k.
9:00 AM - KK3.19
Modeling Crystallization and Void Formation in Ge2Sb2Te5 Nanostructures
Adam Cywar 1 Zachary Woods 1 Ali Gokirmak 1
1Univ of Connecticut Storrs United StatesShow Abstract
There is considerable interest in studying and modeling phase transitions in Ge2Sb2Te5 (GST) nanostructures for the development of phase-change memory technology . Typically, the crystallization models in the literature [2-3] examine a rectangular area of material in which the material is being heated uniformly throughout. We present a finite element model for simulating the nucleation and growth of crystal grains within an amorphous domain of GST alongside other physics such as joule heating and solid mechanics, allowing for modelling of crystallization in an arbitrarily shaped structure where a thermal gradient and/or a transient may be present. This approach will allow for the modeling of dynamic crystallization during a set or reset operation, allowing for updates of the electrical, thermal and mechanical properties of the material as it crystallizes.
The current models for the crystallization of GST reported in the literature [2-3] do not account for the ~ 7% volume reduction that occurs upon the as-deposited amorphous to crystalline phase change . Experimental results (STEM) have shown that a confined volume of as-deposited amorphous GST typically exhibits several voids after crystallization during annealing. The locations and sizes of the resulting voids are critically important to the quality of the device performance. Our model captures the volume change during crystallization to predict void locations and sizes based on the local strain and stress of grain boundary locations during the first-time annealing of as-fabricated devices.
 H. Wong et al., "Phase Change Memory," Proc IEEE, vol. 98, pp. 2201-2227, 2010.
 P. Ashwin, et al., "Fast simulation of phase-change processes in chalcogenide alloys using a Gillespie-type cellular automata approach," J. Appl. Phys., vol. 104, pp. 084901, 2008.
 G. W. Burr et al., "Observation and modeling of polycrystalline grain formation in Ge2Sb2Te5," J. Appl. Phys., vol. 111, pp. 104308-104308-12, 2012.
 W. K. Njoroge et al., "Density changes upon crystallization of Ge2Sb2.04Te4.74 films," Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, vol. 20, pp. 230-233, 2002.
9:00 AM - KK3.20
Growth of Crystalline Hexagonal GexSbyTez by Metal Organic Vapour Phase Epitaxy
Martin Schuck 1 2 Sally Riess 1 2 Kristof Keller 1 2 Daniel Wilson 4 5 3 Christoph Schmitz 5 2 Denis Rudolf 1 3 2 Manuel Bornhoefft 6 7 2 Joachim Mayer 6 7 2 Astrid Besmehn 8 Gregor Mussler 1 2 Martina von der Ahe 1 2 Hilde Hardtdegen 1 2 Detlev Gruetzmacher 1 2
1Forschungszentrum Juelich GmbH Juuml;lich Germany2Juelich-Aachen Research Alliance (JARA) Juelich Germany3RWTH Aachen University Juelich Germany4Forschungszentrum Juelich Juelich Germany5Forschungszentrum Juelich Juelich Germany6RWTH Aachen University Aachen Germany7Forschungszentrum Juelich Juelich Germany8Forschungszentrum Juelich Juelich GermanyShow Abstract
Phase change memory (PCM) based on chalcogenides such as the Ge-Sb-Te compounds along the Sb2Te3 - GeTe pseudo-binary line have been widely used for optical data storage and in recent years also as non-volatile resistive memory devices. In these applications, the ultra-fast and reversible phase change between the amorphous and the metastable cubic crystalline phase, associated with a high contrast in reflectivity and resistivity is used for data storage. They are deposited in the amorphous state by atomic layer deposition or physical vapour deposition (sputtering). Due to the lack of applications, the thermodynamically stable crystalline hexagonal phase was not in the centre of attention up to now. However, recently superlattices of highly textured hexagonal Sb2Te3 - GeTe layers have received increasing interest due to an altered switching mechanism with reduced switching energy. Switching is field induced and occurs at the interfaces of the materials between two crystalline states circumventing the melting needed for the phase change. The layered structure of monocrystalline hexagonal Ge-Sb-Te compounds inherently resembles the superlattice structure with respect to atomic stacking and crystal orientation to the substrate. For this reason, the preparation and intense study of epitaxial, hexagonal Ge-Sb-Te can be of fundamental interest for future applications.
In this contribution, we present the growth and characterization of crystalline Ge-Sb-Te films on Si (111) deposited by MOVPE. At a reactor pressure of 50 hPa and growth temperatures around 450°C epitaxial films are grown using nitrogen as the carrier gas to transport the precursors DETe, TESb and digermane to the reactor. Different partial pressures of the precursors were employed to vary the film composition. The morphology of the deposited material was investigated using AFM and SEM, while the structure of the as-grown samples was studied by XPS, XRD and TEM. The chemical composition was determined using EDS.
The two compositions Ge1Sb2Te4 and Ge2Sb2Te5 were controllably achieved. XRD studies indicate, that the 100nm thick Ge-Sb-Te is crystallized in the stable hexagonal structure (P-3m1 or R-3m). TEM investigations reveal that the Ge, Sb and Te atoms form building blocks, consisting of 7 (Ge1Sb2Te4) or 9 (Ge2Sb2Te5) alternating cation and anion layers parallel to the Si (111) substrate surface, stacked along the  axis. These building blocks are separated by van der Waals gaps originating from hexagonal Sb2Te3, where they are naturally present. The samples are monocrystalline and exhibit a low amount of defects. XPS reveals oxidation mainly of Ge and Sb at the surface of the films. Additionally the occupation of the cation sites by Ge and Sb atoms in the hexagonal lattice was investigated by TEM and XPS.
9:00 AM - KK3.21
In-Situ XRD Measurements and Simulations to Determine Grain Sizes in GeSbTe at Various Annealing Temperatures
Kadir Cil 1 Zachary Woods 1 Lhacene Adnane 1 Adam Cywar 1 Faruk Dirisaglik 1 Yu Zhu 2 Chung Lam 2 Ali Gokirmak 1 Helena Silva 1
1University of Connecticut Storrs United States2IBM T. J. Watson Research Center Yorktown Heights United StatesShow Abstract
PCM utilizes the large electrical resistivity contrast between the amorphous (high resistance) and the crystalline (low resistance) phases of chalcogenides -Ge2Sb2Te5 (GST) being the most common - that can be reversibly and rapidly switched between the two phases by self-heating via electric pulses . Understanding the crystallization dynamics during set operation is critically important.
We have performed XRD measurements to characterize grain size distribution in GeSbTe as a function of temperature and test the crystallization numerical models we have constructed using nucleation and growth rates from the literature . The material crystallizes over time as the chuck temperature is increased with 2oC/min heating rate and is monitored by the peaks in the XRD measurements. Peaks start appearing at T ~ 175 oC. The small widening of intensity peaks is attributed to an increase grain size due to temperature dependent nucleation and growth dynamics . When the chuck temperature reaches 300 oC, the material transitions from fcc phase to hcp phase. The X-ray peak width and maximum intensity increase with temperature, showing increasingly larger crystalline grain sizes up to melting temperature. A step-wise change is visible on the average grain size of GST at maximum nucleation rate (~325oC) and the grain sizes tend to saturate as the peak growth rate temperature is approached. However, in these slow measurements, the material goes through an fcc-hcp mixed phase that is not expected to take place during normal device operation as the phase transition to hcp is a very slow process.
Our crystallization model captures transitions between fcc and amorphous phases but does not capture movement of grain-boundaries if the material is not molten. At this point, we are able to compare the simulated grain-size distributions with the experimental data assuming that the simulation is starting from amorphous material. The average grain sizes obtained from these simulations are in good agreement with the experimental ones even though there are important differences such as the temperature ramp rate which is significantly slower in the experiments.
 H. -. P. Wong, S. Raoux, S. B. Kim, J. Liang, J. P. Reifenberg, B. Rajendran, M. Asheghi and K. E. Goodson, "Phase Change Memory," Proc. IEEE, vol. 98, pp. 2201-2227, 2010.
 G. W. Burr, P. Tchoulfian, T. Topuria, C. Nyffeler, K. Virwani, A. Padilla, R. M. Shelby, M. Eskandari, B. Jackson and B. Lee, "Observation and modeling of polycrystalline grain formation in Ge2Sb2Te5," J. Appl. Phys., vol. 111, pp. 104308, 2012.
9:00 AM - KK3.22
Fusing Behavior of Flexible Silver-Fuse Memories Fabricated by Reverse Offset Printing
Nobuko Fukuda 1 Jaakko Leppaeniemi 2 Hirobumi Ushijima 1 Ari Alastalo 2
1AIST Tsukuba Japan2VTT Technical Research Centre of Finland, Ltd. Espoo Finland