Orlando Auciello Argonne National Laboratory
Dirk Wouters IMEC
Steven Soss Intel Corporation
Seungbum Hong Argonne National Laboratory
Applied Materials Inc
Symetrix Corp, Colorado Springs
Tuesday PM, March 25, 2008
Room 2006 (Moscone West)
9:30 AM - **F1.1
High Density Thin Film Ferroelectric Nonvolatile Memories.
Ramamoorthy Ramesh 1 Show Abstract
1 Department of Materials Science & Engineering and Department of Physics, University of California-Berkeley, Berkeley, California, United States
The field of ferroelectric nonvolatile memories has made some dramatic progress over the past 5 years. Although the field formally started in the early eighties, it was not until the late eighties and early nineties that a concerted, interdisciplinary approach was taken to solve the critical “show-stoppers” that limited their implementation as the next generation of solid state nonvolatile memories ( FRAMS). Among the key inventions, the use of conducting oxide electrodes to solve polarization fatigue and imprint in PZT based capacitors, the implementation of the SBT process, conducting barriers to create high density architectures, approaches to solve hydrogen damage, etc all have paved the way to the current status of this field. Many large companies are actively involved in designing and manufacturing low and high density IC’s that have been implemented in many applications. In this presentation, I will describe our efforts on exploring a new lead-free ferroelectric material, BiFeO3, as a possible replacement for both PZT and SBT families of ferroelectrics.
10:00 AM - F1.2
Magnetic Multilayer Ring Devices for Non-volatile Data Storage.
Caroline Ross 1 , Fernando Castano 1 , Bryan Ng 1 , Wonjoon Jung 1 , Irenee Colin 1 Show Abstract
1 Dept. Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
10:15 AM - F1.3
In-situ Studies of Surface Structural and Chemical Phenomena in Ultra-Thin Ferroelectric Films.
Junsoo Shin 1 , Peter Maksymovych 1 , Von Braun Nascimento 1 , Albina Borisevich 1 , E. Ward Plummer 1 , Vincent Meunier 1 , Sergei Kalinin 1 , Arthur Baddorf 1 Show Abstract
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
10:30 AM - F1.4
Theoretical Study of A Localized Quantum Spin Reversal by The Spin Injection in A Spin Quantum Dot: A Data Writing Method for A Single-Atom Memory.
Satoshi Kokado 1 , Kazumasa Ueda 1 , Kikuo Harigaya 2 , Akimasa Sakuma 3 Show Abstract
1 Faculty of Engineering, Shizuoka University, Hamamatsu Japan, 2 Nanotechnology Research Institute, AIST, Tsukuba Japan, 3 Graduate School of Engineering, Tohoku University, Sendai Japan
The single-atom memory is expected to be an ultimate microscopic element of the data storage device. Recently, towards development of such a memory, the anisotropy energy of a single atomic spin (S=2) of an Fe atom on CuN surface has been evaluated by using a scanning tunneling microscope . However, demonstration experiments and theories about the data writing have not yet been reported. We therefore consider the quantum spin reversal by spin injection (QSRSI) as a writing method. A related phenomenon is the magnetization reversal by spin injection , in which the magnetization of a ferromagnet (FM) is reversed by injecting a spin-polarized current into the FM. This phenomenon has been analyzed by using theoretical models, where the magnetization of the FM is described by classical spins. On the other hand, the QSRSI for small spins such as S=1 or 2 have scarcely been studied so far. In this paper, we theoretically investigated the QSRSI for a spin quantum dot that has a quantum spin S. The quantum spin has a uniaxial anisotropy energy, -|D|Sz^2, which shows a bistable potential between Sz=-S and S states, where D is an anisotropy constant. An exchange interaction acts between the localized spin and electrons. For this dot, we proposed two models; that is, (I) the single electron tunneling (SET)  and (II) the sequential tunneling. The respective details are as follows: (I) The system consists of "electrode/quantum well (QW)/dot/QW/electrode" junctions, in which the left QW has an energy level of conduction electrons with only up-spin. We consider a situation in which up-spin electrons are injected from the left electrode into the dot through the QW in the SET regime. To describe the injected electrons, we propose a simple method based on approximate solutions from the time-dependent Schrodinger equation. Using this method, it is shown that the spin reversal occurs when the right QW has energy levels of conduction electrons with only down-spin. In particular, the expression of the reversal time of a localized spin is derived and the upper and lower limits of the time are clearly expressed. (II) The system consists of FM/dot/FM junctions, where the left FM (the right FM) has energy levels of conduction electrons with up-spin (with down-spin). We calculate the time dependence of the current and the expectation value of localized spin using a combination of the Fermi golden rule and the master equation. In the master equation, we take into account the relaxation time of the localize spin from the excited states to the ground one. Whether the spin reversal is realized or not strongly depends on the relaxation time. Furthermore, the spin reversal time for S=2 (atomic spin of Fe) is evaluated for any relaxation time. Ref) C. F. Hirjibehedin et al., Science 317, 1199 (2007).  E. B. Myers et al., Science 285, 867 (1999). S. Kokado et al., Phys. Rev. B 76, 054451 (2007).
10:45 AM - F1.5
Local Piezoelectric Response and Domain Structure of PbTiO3 Nanotubes.
Yunseok Kim 1 , Changduck Bae 2 , Kyu-Hyung Lee 1 , Jeong Yong Lee 1 , Kwangsoo No 1 , Hyunjung Shin 2 Show Abstract
1 Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 School of Advanced Materials Engineering and Center for Materials and Processes of Self-Assembly, Kookmin University, Seoul Korea (the Republic of)
Ferroelectric nanostructures, mostly nanowires and nanotubes, have started to draw growing interest for the possible technical applications of the miniaturized devices such as nano-sensors, actuators, and nonvolatile memory devices. They often exhibited enhanced ferroelectric and/or piezoelectric properties compared to the bulk. Due to their superior properties and potential applications, the fabrication of ferroelectric nanowires and nanotubes has been investigated using many different synthesis methods including infiltration, molten salt technique, electrophoresis, solvothermal, and sol – gel coatings on the inside of nanotemplates. However, the preparation and analysis of ferroelectric nanotubes still remains a challenge in materials science. In this presentation, we present the fabrication and analysis of PbTiO3 (PTO) nanotubes. In addition, we discuss the local piezoelectric response and domain structures of PTO nanotubes using piezoelectric force microscopy (PFM). PTO nanotubes were fabricated based on the templating of the anodic alumina oxide (AAO) using conformal coatings of atomic layer deposition. The fabricated PTO nanotubes of ~90 nm in diameter were elliptic in cross section. Experimental results of PTO nanotubes by PFM showed the multi-domain structures. Interestingly, the different domains at left and right sides along the nanotube length direction were frequently observed due to the hollow inside. In order to analyze further the domain structures of PTO nanotubes, both lateral and vertical PFM were used and switching experiments on PTO nanotubes were also performed. Domain structures of PTO nanotubes were completely visualized based on the results of the vertical and lateral PFM as well as the switching experiments. The results showed the existence of the out-of plane, switchable ferroelectric polarization with week shear response in the PTO nanotubes. In summary, we fabricated PTO nanotubes based on the templating of the AAO and subsequent annealing at high temperature. The fabricated PTO nanotubes were elliptic of ~90 nm in diameter. They showed a substantial piezoelectricity and normal piezoresponse hysteresis behaviors. The multi-domain structure was elucidated by PFM.
11:30 AM - **F1.6
MgO-barrier-based Magnetic Tunnel Junctions for Spin Transfer Torque Random Access Memory.
Jun Hayakawa 1 2 , Shoji Ikeda 2 , Katsuya Miura 1 2 , Michihiko Yamanouchi 1 , Young Min Lee 2 , Ryutaro Sasaki 2 , Toshiyasu Meguro 2 , Masahiko Ichimura 1 , Kenchi Ito 1 , Takayuki Kawahara 3 , Riichiro Takemura 3 , Fumihiro Matsukura 2 , Hiromasa Takahashi 1 3 , Hideyuki Matsuoka 1 , Hideo Ohno 2 Show Abstract
1 , Hitachi Advanced research Laboratory, Tokyo Japan, 2 , Tohoku University, Laboratory for Nanoelectronics and Spintronics, RIEC, Sendai Japan, 3 , Hitachi Central Research Laboratory, Tokyo Japan
Spin-polarized currents exert torque on a magnetization that can switch the magnetization direction. This “current-induced magnetization switching (CIMS)” at reduced current density has been demonstrated in a number of MgO-barrier-based magnetic tunnel junctions (MTJs). In particular, CoFeB/MgO/CoFeB MTJs have been shown to exhibit high tunnel magnetoresistance (TMR) ratios  together with CIMS at low critical current density (Jc). Recently, high-capacity (2-Mb) spin transfer torque random access memory (SPRAM) utilizing the potential of CoFeB/MgO/CoFeB MTJs has been demonstrated . To attain even higher capacity SPRAM, it is necessary to further reduce Jc while maintaining a high thermal-stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature, respectively) well over 60. While Jc is proportional to the product of the magnetization and the thickness of the free layer (i.e., magnetic moment per area), the thermal-stability factor is proportional to the volume of the free layer. If the dimension of an MTJ is simply reduced to enable more bits to accommodate in a given area, Jc remains constant: however, the E/kBT degrades. We have been investigating the use of a synthetic ferrimagnetic (SyF) free layer in MgO-barrier-based MTJs to achieve low Jc and high E/kBT.  An SyF free layer consisting of two ferromagnetic layers separated by an Ru spacer layer should provide sufficiently high volume to withstand thermal fluctuations while keeping the effective magnetic moment per area low. In this study, we show the advantages of MgO-barrier-based MTJs (100 x 200nm2) with a CoFeB/Ru/CoFeB-based SyF free layer for SPRAM application. One advantage is low intrinsic J (Jc0) with maintaining the high E/kBT. When the two CoFeB layers of antiferromagnetically coupled SyF have same in thickness, the reduction of Jc0 down to 2 x 106A/cm2 is observed. High E/kBT over 60 obtained in SyF enables us not only to secure the retention time over 10 years but also to suppress the write current dispersion for SPRAM. Another remarkable advantage of SyF is to make bistable (parallel/antiparallel) magnetization configuration at zero field, it makes possible to realize CIMS without applying the external fields to compensate the offset field. This work was supported by the RR2002 IT-program from MEXT. J. Hayakawa, S. Ikeda, Y. M. Lee, R. Sasaki, T. Meguro, F. Matsukura, H. Takahashi, and H. Ohno, Jpn. J. Appl. Phys., 44, L1267 (2005). Y. M. Lee, J. Hayakawa, S. Ikeda, F. Matsukura, and H. Ohno, Appl. Phys. Lett., 90, 212507 (2007). T. Kawahara, R. Takemura, K. Miura, J. Hayakawa, S. Ikeda, Y. M. Lee, R. Sasaki, Y. Goto, K. Itoh, T. Meguro, F. Matsukura, H. Takahashi, H. Matsuoka, and H. Ohno, Int. Solid-State Circuits Conf. (ISSCC) Dig. Tech. Papers, 392-393 (2007). J. Hayakawa, S. Ikeda, Y. M. Lee, R. Sasaki, T. Meguro, F. Matsukura, H. Takahashi, and H. Ohno, Jpn. J. Appl. Phys., 45, L1057 (2006).
12:00 PM - F1.7
High-Density Ferroelectric Nanocapacitors Based on Nanowire/CNT Bottom Electrodes and Ferrelectric Nanotubes.
Hongjin Fan 1 , Susumu Kawasaki 1 , James Scott 1 , Paul Evans 2 , John Gregg 2 Show Abstract
1 Centre of Ferroics, University of Cambridge, Cambridge United Kingdom, 2 Centre for Nanostructured Media, Queens University Belfast, Belfast, Northern Ireland, United Kingdom
We present two fabrication techniques towards the high-density hard-wired FRAM capacitor arrays. The first one consists of nearly Tb/sq.in. array of BaTiO3 and PZT capacitors using Pt nanowire bottom electrodes prepared by infiltrating porous alumina thin films. As the Pt nanowires (ca. 30 nm diameter) are electrically isolated from each other, local polarization of the ferroelectric thin film (<100 nm) above each nanowire electrode defines a "bit". Hysteresis of the capacitor will be shown. In the second structure, vertical-standing multi-walled carbon nanotubes (CNTs) arrays were used as fabrication template and bottom electrode. Functional oxide layer was coated onto the CNTs for the first time using PZT metalorganic precursor and a modified Misted Chemical Deposition system. Such a room-temperature deposition avoids the burning-off problem of CNTs at high temperatures in an oxygen ambient. Present effort is to achieve good stoichiometry and functionality test.Ferroelectric nanotubes have been patented for application in inkjet printing due to their piezoelectric property. For this purpose, inner and outer electrodes are required to apply an electric field across the tube wall. We will show our result on fabrication of electrode-ferroelectric-electrode concentric nanotubes.
12:15 PM - F1.8
Chemistry, Electronic, Structural, and Transport Studies of CoFeB / MgO / CoFeB Magnetic Tunnel Junctions.
John Read 1 , Judy Cha 1 , Pinshane Huang 2 , William Egelhoff 3 , David Muller 1 , Robert Buhrman 1 Show Abstract
1 , Cornell University, Ithaca, New York, United States, 2 , Carleton College, Northfield, Minnesota, United States, 3 , National Institute of Standards & Technology, Gaithersburg, Maryland, United States
The discovery of large tunneling magnetoresistance (TMR) in MgO based Magnetic Tunnel Junctions (MTJs) and the subsequent improvements of the CoFeB / MgO /CoFeB system provide an industrially compatible structure for magnetic random access memory (MRAM) applications. Research on such systems reveals that careful engineering of the MgO tunnel barrier, the CoFeB electrodes, and the interfaces between the two, achieves significant improvements in device performance and versatility.[2,3] These advances motivate investigation of the chemical, electronic, and structural properties of working MTJ structures. We have correlated the results of scanning tunneling spectroscopy (STS), x-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) studies of MgO based MTJ structures, with current in the plane tunneling (CIPT) measurements to gain insight on the electronic structure and chemistry of the tunnel barriers and interfacial regions. We find that boron is mobile in the CoFeB alloy electrode during annealing and can either dope into the MgO barrier, diffuse away from the metal-oxide interface, or both. MgO tunnel barriers prepared using rf sputtering show modification of the oxide band gap can occur upon incorporation of boron during annealing. The CIPT, EELS and XPS measurements reveal that quite high TMR (>200%) can be obtained when there is a very substantial B component in the tunnel barrier oxide. Control of electrode oxidation plays a significant role in the quality of the tunnel barrier through the formation or suppression of interfacial iron and boron oxides. We have additionally correlated scanning tunneling microscopy (STM), x-ray diffraction (XRD), and scanning transmission electron microscopy (STEM) studies to elucidate structural modifications that occur during the annealing process. The deposition technique used to form the MgO tunnel barrier has an influence upon electrode crystallization as well as tunnel barrier crystallization. The electrodes and tunnel barriers do not crystallize uniformly during annealing and Mg seed layers enhance both barrier smoothness and crystallinity. We will discuss the impact of the various materials properties upon the transport measurements and will provide suggestions for greater control over device characteristics through specific tailoring of materials properties.  S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S-H Yang, Nature Materials 3, 862 (2004). Y. Nagamine, H. Maehara, K. Tsunekawa, D. Djayaprawira, K. Tsunekawa, N. Watanabe, S. Yuasa, and K. Ando, Appl. Phys. Lett. 89, 162507 (2006). Y. M. Lee, J. Hayakawa, S. Ikeda, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 90, 212507 (2007). J. C. Read, P. G. Mather, and R. A. Buhrman, Appl. Phys. Lett. 90, 132503 (2007). Judy J. Cha, J. C. Read, R. A. Buhrman, and David A. Muller, Appl. Phys. Lett. 91, 062516 (2007).
12:30 PM - F1.9
A Room Temperature Ferroelectric Directly on Silicon.
Maitri Warusawithana 1 , Y. Li 1 , L. Chen 1 , D. Schlom 1 , C. Cen 2 , C. Sleasman 2 , J. Levy 2 , J. Woicik 3 , L. Kourkoutis 4 , D. Muller 4 , J. Klug 5 , M. Bedzyk 5 , H. Li 6 , L. Wang 7 Show Abstract
1 Department of Materials Science and Engineering, Penn State University, University Park, Pennsylvania, United States, 2 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 5 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 6 Physical Sciences Research Laboratories, Motorola Laboratories, Tempe, Arizona, United States, 7 , Intel Corporation, Santa Clara, California, United States
With the growing need for faster and better devices, limitations with silicon based traditional electronics are becoming increasingly more apparent. This has led to the desire to interface functional materials with semiconductors and specifically with silicon to obtain new and more efficient devices. Towards this, the growth of epitaxial oxide thin films on silicon has attracted much attention. With the variety of different collective phenomena that can be achieved in complex oxides, the integration of such materials with silicon could lead to novel devices that take advantage of the properties of both the semiconductor and the functional oxide. The tendency of a pristine silicon surface to rapidly form its oxide as well as the reactivity of silicon with many elements and their oxides, however, has made this a daunting task.The epitaxial growth of SrTiO3 on (001) Si has been studied for many years. With a carefully controlled growth process1, we obtain completely strained (i.e., commensurate) SrTiO3 thin films on (001) Si. While cross sectional transmission electron microscopy shows that there is no interfacial SiO2 layer in the SrTiO3 on (001) Si films we studied, synchrotron x-ray measurements showed that the films are fully strained to a thickness of ~24Å (6 unit cells of SrTiO3). Theoretical calculations show that the substrate induced biaxial strain leads to ferroelectricity in the quantum paraelectric SrTiO3 when grown on (001) Si. Using Piezo Force Microscopy measurements, we show that these ultra thin SrTiO3 films are indeed ferroelectric. We find that a 6 unit cell thick SrTiO3 film on (001) Si is ferroelectric at temperatures above 80°C while at room temperature, the ferroelectric domains retain their polarity for periods extending over more than 3 days.1 H. Li et al., J. Appl. Phys. 93 (2003) 4521.
12:45 PM - F1.10
Spin and Symmetry Filtering Effects in Single Crystal Magnetic Tunnel Junctions.
Coriolan Tiusan 1 , Fanny Greullet 1 , Michel Hehn 1 , Francois Montaigne 1 , David Halley 2 , Olivier Bengone 2 , Martin Bowen 2 , Wolfgang Weber 2 Show Abstract
1 Laboratoire de Physique des Materiaux LPM-UMR7556, CNRS - University of Nancy, Vandoeuvre les Nancy France, 2 IPCMS UMR7504, CNRS - University of Strasbourg, Vandoeuvre les Nancy France
Orlando Auciello Argonne National Laboratory
Dirk Wouters IMEC
Steven Soss Intel Corporation
Seungbum Hong Argonne National Laboratory
F5: Poster Session: Emerging (RRAM/Crosspoint/Organic/Probe) II
Wednesday PM, March 26, 2008
Exhibit Hall (Moscone West)
1:00 AM - F5.1
Molecular Conformation-dependent Memory Effects in Non-conjugated Polymers with Pendant Carbazole Moieties.
Siew Lay Lim 1 2 , Qidan Ling 2 , Eric Yeow Hwee Teo 3 , Chun Xiang Zhu 3 , Daniel Siu Hung Chan 3 , En-Tang Kang 2 , Koon Gee Neoh 2 Show Abstract
1 NUS Graduate School of Integrative Sciences and Engineering (NGS), National University of Singapore, Singapore Singapore, 2 Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore Singapore, 3 SNDL, Department of Electrical and Computer Engineering, National University of Singapore, Singapore Singapore
Organic and polymeric materials have been used as alternatives to traditional Si, Ge, and GaAs semiconductors in the fabrication of electronic devices, such as light-emitting diodes, transistors, sensors, photovoltaic cells, switches and memory devices. Organic materials exhibiting electrically bistable behavior are also alternatives for future data storage applications. Instead of information storage and retrieval by encoding “0” and “1” as the amount of charges stored in a cell in silicon devices, polymer memory devices store data in another form, for example, based on the high and low conductivity response to an external voltage. In comparison to inorganic materials, organic materials offer the potential advantages of low cost, good processability, and properties-tuning by molecular design and appropriate syntheses. Also, polymeric memory devices exhibit good scalability, can be stacked 3-dimensionally for high-density data storage, and can be fabricated into flexible devices. In the field of organic and polymeric memory devices, molecular design and doping have been used in the fabrication of memory devices to achieve different memory properties.In our work, conformation-induced volatile and non-volatile conductance switching effects were demonstrated in non-conjugated polymers containing the same electroactive pendant groups. Single-layer devices of the structure indium-tin-oxide/polymer/aluminum were fabricated from two non-conjugated polymers with pendant carbazole groups in different spacer units. The device based on poly(2-(N-carbazolyl)ethyl methacrylate) (PMCz) exhibited non-volatile write-once-read-many-times (WORM) memory behavior with an ON/OFF current ratio up to 106, while the device based on poly(9-(2-((4-vinylbenzyl)oxy)ethyl)-9H-carbazole) (PVBCz) exhibited volatile memory behavior with an ON/OFF current ratio of approximately 103. The formation of carbazole excimers resulting from conformation-induced conductance switching under an electric field was revealed in situ by fluorescence spectroscopy. The appearance and subsequent retention of emission bands attributable to the two excimeric states of carbazole was consistent with the volatile/non-volatile nature of the device. The corresponding voltage-induced conformation ordering in the polymer film resulted in the appearance of ordered microdomains which were captured by transmission electron microscopy. In the absence of a spacer unit between the pendant carbazole group and the main chain, regioregular poly(N-vinylcarbazole) (PVK) exhibited only one conductivity state (ON state). The differences in memory behavior among the three polymers were attributed to their inherent differences in the degree of regioregularity and ease of conformational relaxation of the field-induced regioregular carbazole groups. These conformational effects were in turn dictated by the chemical structure and steric effect of the spacer unit between the pendant carbazole group and the main chain.
1:00 AM - F5.10
Electronic and Optical Data Read Out from Single Layer Organic Memory Device.
Basudev Pradhan 1 , Ghassan Jabbour 1 Show Abstract
1 School of Materials, Arizona State University, Tempe, Arizona, United States
1:00 AM - F5.11
Dependence of Organic Thickness on Electrical characteristic Behavior in Low Molecular Organic Nonvolatile Memory.
Yool Guk Kim 1 , Sung Ho Seo 1 , Gon Sub Lee 1 , Jae Gun Park 1 , Yoon Joong Kim 2 Show Abstract
1 Department of Electrical & Computer Engineering, Hanyang University, Seoul Korea (the Republic of), 2 Electron Microscopy Team, Korea Basic Science Institute, Deajeon Korea (the Republic of)
Recently, organic nonvolatile memory of nonvolatile memories have attracted attention because application as next generation memory devices. In effort for realizing low-molecular organic nonvolatile memory, the dependence of organic thickness on electrical characteristic behavior in low-molecular organic nonvolatile memory was investigated. We developed an low-molecular organic nonvolatile memory fabricated with the device structure of Al/Alq3 (Aluminum tris(8-hydroxyquinoline))/Ni nanocrystals/Alq3/Al. Four different organic thicknesses, i.e., 30, 40, 50, and 100 nm, with fixing middle layer thickness were deposited by using a high vacuum thermal deposition method. The reason we chose Ni for middle metal layer is that Ni has smaller grain boundary which is benefit for scaling down and larger work function (~5.15 eV) that can make a deep quantum well in energy band diagram, compared with those of Al. We confirmed that, as an organic thickness increases, a current level linearly decreases by an order of magnitude in a log-scale except for the 100 nm sample case. That is, Ion of 30-nm-thick sample was about 1 mA and 50-nm-thick sample was about 10 μA. The reason why the decrease in the current with increasing an organic thickness can be attributed to that electron transfer occurs less frequently because of the decrease in the hopping frequency. In addition, a 100-nm-thick sample was not shown the electrical characteristic of low-molecular organic nonvolatile memory. Meanwhile, the switching characteristics of our device showed that Vth of 2V, Vp(program) of 4V, Ve(erase) of 7V, and Ion(after programming)/Ioff(after erasing) of ~6x10 3. In addition, the interesting behavior of those characteristics is that the switching voltages (e.g; Vth, Vp, Ve, and Ion /Ioff) were not much changed with a varying an organic thickness. Therefore, we can conclude that an organic thickness does not affect significantly to the switching characteristics but current level. In addition, it was confirmed that a 30-nm-thick organic thickness was the best process condition in realizing low-molecular organic nonvolatile memory fabrication process.
1:00 AM - F5.12
Low Molecular Organic Nonvolatile Memory Fabricated with Ni Nanocrystals Embedded in Alq3.
Oh YoungHwan 1 , Nam Woo Sik 1 , Lee Gon Sub 1 , Park Jea Gun 1 , Lee Yong Bok 2 Show Abstract
1 , hanyang university, Seoul Korea (the Republic of), 2 , Korea Basic Science Institute, Daejeon Korea (the Republic of)
Currently, an organic nonvolatile memory has attracted much interest as one of candidate devices for next generation nonvolatile memory because of its simple process, small device area, and high speed. To investigate electrical characteristics of low molecular organic nonvolatile memory using Ni as a middle metal layer, we developed low molecular organic nonvolatile memory fabricated with the device structure of Al/Alq3 (Aluminum tris (8-hydroxyquinolate))/Ni nanocrystals/Alq3/Al. A high vacuum thermal deposition method was used for the device fabrication. It is critical that the fabrication process condition for Ni nanocrystals should be optimized including ~100Å- thickness, 0.1Å/sec-evaporation rate and in-situ plasma oxidation for effective oxidation. The reasons we chose Ni for middle metal layer are that Ni has smaller grain boundary which is benefit for scale down and larger work function (~5.15 eV) that can make a deep quantum well in energy band diagram, compared with those of Al. Our device showed an electrical nonvolatile memory behavior including Vth of ~2 V, Vw (write) of ~3.5 V, NDR (negative differential region) of 3.5~7 V, Ve (erase) of 8 V, and electrical behavior at reverse bias was symmetrical. In addition, an interesting behavior of electrical properties is that, although retention and endurance characteristics were similar to middle layer-Al device, Ion/Ioff ratio was greater than 104 at Vr (read) of 1 V. This value of Ni device is higher than 102 compared to middle layer-Al device. Meanwhile, low molecular organic nonvolatile memory using Ni middle layer with α-NPD at same fabrication condition was shown much unstable characteristic compared with Alq3. We can speculate that there is a relationship in fabrication condition between middle metal material and organic material. Finally, we conclude that our device using Ni middle layer with Alq3 is capable for realizing more reliable and useful low molecular organic nonvolatile memory.*AcknowledgementThis research was supported by national research program for 0.1 Terabit Nonvolatile Memory Development sponsored by Korea Ministry of Commerce, Industry and Energy.
1:00 AM - F5.13
Current Conduction Mechanism for Low-molecular Organic Nonvolatile Memory.
Sungho Seo 1 , Woo Sik Nam 1 , Gon Sub Lee 1 , Jea Gun Park 1 Show Abstract
1 , Hanyang University, Seoul Korea (the Republic of)
Recently, it has been reported that organic devices fabricated with the structure of top metal layer/conductive organic layer/middle metal layer/conductive organic layer/bottom metal layer demonstrated nonvolatile memory behavior such as Ion (after writing)/Ioff (after erasing) of > 1x101 and the response time of ~10ns, where organic conductive layers were AIDCN (2-amino-4, 5-imidazoledicarbonitrile), Alq3 (Aluminum tris(8-hydroxyquinoline)) or α-NPD. We developed organic nonvolatile memory fabricated with the device structure of α-NPD/Al nanocrystals surrounded by Al2O3/α-NPD/Al where α-NPD is N,N’-bis(1-naphthyl)-1,1’biphenyl4-4’’diamine. One layer of Al nanocrystals was uniformly produced between α-NPD layers, confirmed by 1.2MV high voltage transmission-electron-microscope. Particularly, Al nanocrystals were produced via Al layer evaporation with 1.0 Å/sec on α-NPD followed by O2 plasma oxidation. We confirmed that the conduction bistability of ~102 and threshold voltage for a set state of 3V. Al nanocrystals surrounded by amorphous Al2O3 were included in conductive organic material α-NPD. They presented seven different reversible current paths capable for an electron charge or discharge on Al nanocrystals. The current slightly increased with the applied bias from 0V up to Vth, called high resistance state (Ioff). The current abruptly increased with the applied bias from Vth to Vw. The current decreases with increasing with the applied bias from Vw to Ve, called negative differential resistance (NDR) region. Finally, the current increased slightly increased with the applied bias above Ve. After sweeping the first applied voltage from 0 to 10V (called erase), the second applied bias was swept from 0 to Vw (called write), and then the current followed high resistance state (Ioff). After sweeping the second applied bias (write), the third applied bias was swept from 0 to Vw again, the current followed low resistance state (Ion). Surprisingly, the ratio of Ion to Ioff was ~1x102, which is enough current difference for nonvolatile memory behavior. These I-V characteristics under the positive applied bias were symmetrically repeated under the negative applied bias. All current sweeping paths were reproducible and symmetrical for applied bias polarity. In particular, our device demonstrated multi-level nonvolatile memory behavior. In addition, we observed that our device reveals the current conduction mechanism for each step of device operation regions. In other words, the high resistance regions and low resistance regions follow space-charge-limited current conduction. Vth~Vw and VNDR~Ve regions follow precisely thermionic-field-emission current conduction. Finally, above Ve regions follow F-N tunneling current conduction.Acknowledgement* This research was supported by national research program for 0.1 Terabit Non-volatile Memory Development sponsored by Korea Ministry of Commerce, Industry and Energy.
1:00 AM - F5.14
Effect of Au Nanocrystals Embedded in Conductive Polymer on Non-volatile Memory Window.
Hyun Min Seung 1 , Jong Dae Lee 1 , Byeong Il Han 1 , Gon Sub Lee 1 , Jae Gun Park 1 Show Abstract
1 , Hanyang Univ., Seoul Korea (the Republic of)
1:00 AM - F5.15
Resistive Switching Devices Based on Solid State Electrolytes.
Jiang Yin 1 , Hongxuan Guo 1 , Liang Chen 1 , Zhiguo Liu 1 Show Abstract
1 Dept. of Phys./National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, Jiangsu, China
1:00 AM - F5.16
Optical Properties of NiO Thin Films Grown by DC and RF Sputtering Deposition Studied with Spectroscopic Ellipsometry.
Seoung Ho Baek 1 , Hosun Lee 1 , Kwang Nam Choi 1 , Kwan Soo Chung 1 , Jun Woo Park 1 Show Abstract
1 Dept. of Physics, Kyung Hee University, Yong-In, Kyonggi, Korea (the Republic of)
Resistance switching phenomenon is a basic mechanism for resistance random access memory (ReRAM). In ReRAM, low and high resistance states are bistable states and are controlled by voltage without phase transformation. In order to verify the resistance switching mechanism, it is very important to understand the material properties of NiO thin films. However, the optical studies of the NiO thin films are very few. We deposited nickel oxide thin films on silicon substrates at room temperature and 500C using a nickel target by DC and RF magnetron sputtering. We annealed the NiO thin films which were deposited at room temperature. We measured the x-ray diffraction (XRD) pattern of DC-sputtering-grown NiO thin films. The thin films were RT-grown, 500C-grown, and RT-grown followed by 1000C-annealing. We found several XRD peaks such as NiO (111), (200), (220), and (311), and this suggests polycrystallinity of the thin films. The NiO(111) peak dominates in the RT-grown thin films whereas the Ni(200) peak is dominant in the RT-grown followed by 1000C annealing. According to transmission electron microscopy data, the Ni crytal grew with columnar structures vertically with a typical diameter of 20nm. Using spectroscopic eillipsometry, we obtained the refractive indexes, extinction coefficients, thicknesses, band gap energies and broadenings of NiO thin films. In order to estimate the dielectric functions of the NiO thin films, we used parametric optical semiconductor model for layer model analysis. We obtained the band gap energy and the broadening values by using standard critical point model on the second derivative spectra of the dielectric functions. We investigated the relations between the optical and structural properties of NiO thin films with the oxygen flow rate, substrate temperature and annealing. The NiO films deposited at 500C had larger refractive indexes, extinction coefficients, and crystallinity than those deposited at RT. The refractive indexes decreased and crystallinity increased when the films grown at RT were annealed. The peak of refractive index (n) spectra and the threshold of extinction coefficient (k) became sharp as the annealing temperature increased. This phenomenon suggests that the crystallinity increased with increasing annealing temperature. The crystallinity can increase when the diameter of the nanocolumn increases or the volume of the grain boundaries decrease. As O2 gas ratio increased, the refractive index n increased up to O2=50% and then n decreased as O2 ratio increased from 50 to 80%. Above 4 eV, the extinction coefficient also showed the same behavior, whereas k was constant up to 50% and then increased significantly below 4 eV. As O2 gas ratio increased, the band gap energy did not change up to O2=50% and increased from 3.8 eV to 4.0 eV above 60%. This blue shift is attributed to the band filling effect. As O2 gas increases, the excess oxygen is generated increasing free hole concentrations.
1:00 AM - F5.18
Oxide Diode Fabricated at Room Temperature as a Switch Element.
Kwan Soo Chung 1 , Kwang Nam Choi 1 , Jun Woo Park 2 , Hosun Lee 2 Show Abstract
1 Electronic Engineering, KyungHee University, Yongin-si, Gyenggi-do, Korea (the Republic of), 2 Physics, Kyunghee University, Yongin-si, Gyenggi-do, Korea (the Republic of)
Oxide diodes have been studied as ultra violet photo detectors, ultra violet light emitting diodes, and transparent field effect transistors due to their transparency. Transparent oxide semiconductors (TOS) are suitable for the fabrication of an optoelectronic device. In addition oxide diodes have been recently been developed as a switch element for nonvolatile memory. Previously silicon based transistor have been the most popularly used switch element for memory devices. However, Silicon based transistor require high processing temperatures and are difficult to grow epitaxially over metals. We show a room temperature fabricated oxide diode, Pt/p-NiO/n-ZnO/Pt, applied as a switch element for high density, nonvolatile memories. A p-n diode, like a transistor, is a fundamental circuit element for thin film electronics. Until now, expitaxial silicon commonly used to fabricate p-n diodes in electronic devices with planar structures. However, for further increase of device density we require p-n diodes that are applicable to devices with 3D stack structures. Epitaxial silicon based p-n diodes cannot be fabricated with stack structures as it is difficult to grow on a metal layer and high processing temperatures which are detrimental to underlying device layers are required. On the other hand, although amorphous silicon allows for lower processing temperatures, it does not provide the required semiconducting performance. Therefore, new low temperature and high performance materials are needed for p-n diodes, especially for application towards high density non-volatile memory devices. By replacing transistors with a diode as a switch element, there exists the possibility of producing memory cells with cross point structures composed of bit lines and word lines perpendicular to each other, with a memory element lying between them. Oxide based p-n diodes are good candidates to provide solutions to the issues associated with Si-base diodes. Most oxides, such as TiO2, ZrO2, ZnO and indium tin oxide(ITO), are well known n-type semiconductors that are characterized by electron transport properties of oxygen vacancies. However NiOx due to the presence of Ni vacancies is a p-type semiconductor. A p-type NiOx/n-type oxide is a possible alternative to silicon based diodes. All films were deposited by using RF sputtering. For the fabrication of the oxide diode, a ZnO target was reactively sputtered at a power of 150W in a mixture of oxygen and argon gas with a pressure of 3×10-3 Torr to obtain n-ZnO layer with a thickness of 30nm. Next NiO target was reactively sputtered at RF power 100W and working pressure of 5×10-3 Torr, with oxygen content of 5%, to obtain p–NiO film 50nm thick. During the deposition the substrate temperature was kept at room temperature. We measured the hall effect in each layer: n-ZnO carrier concentration was ~1×1018cm-3, with a mobility of ~40cm2(Vs)-1 and p-NiO carrier concentration was ~3×1017cm-3 with a mobility ~10cm2(Vs)-1.
1:00 AM - F5.19
Substantial Reduction of Reset Current in CoO RRAM with Ta Bottom Electrode.
Hisashi Shima 1 , Fumiyoshi Takano 1 , Yukio Tamai 2 , Hidenobu Muramatsu 1 , Hiro Akinaga 1 , Isao Inoue 3 , Hidenori Takagi 3 4 Show Abstract
1 Nanotechnology Research Institute , National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan, 2 Advanced Technology Research Laboratories, Sharp Corporation, 1 Asahi, Daimon-cho, Fukuyama, Hiroshima, Japan, 3 Correlated Electron Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki, Japan, 4 Department of Advanced Materials, University of Tokyo, Kashiwa, Chiba, Japan
Resistance random access memory (RRAM) is attracting considerable attention as a novel non-volatile memory. In the present paper, the influence of the bottom electrode (BE) material on the resistance switching properties was investigated and we report the high-speed and low-power operation in Ta/CoO/Pt with the reset current less than 150 μA. The trilayers of Pt/CoO/Pt and Ta/CoO/Pt were synthesized by magnetron sputtering. The value of the resistance in the initial state of Ta/CoO/Pt trilayer was about 10 times larger at 1.0 V than that of the Pt/CoO/Pt. In case of Pt/CoO/Pt, the forming process was carried out in both the positive and negative voltage polarity, accompanied by the nonpolar resistance switching behavior. Namely, the reset and set occur independently of the voltage polarity. On the other hands, in the case of Ta/CoO/Pt, the forming process in the negative voltage polarity was more stable, compared to that in the positive voltage polarity. Although one time forming voltage sweep caused the switching from the initial state to the low resistance state (LRS) in the negative voltage polarity, several times voltage sweeps were required in order to switch Ta/CoO/Pt into the LRS in the positive voltage polarity. In addition, the reset by the negative voltage was unstable in Ta/CoO/Pt, compared to that by the positive voltage. Those asymmetrical electric behaviors in Ta/CoO/Pt are attributed to the difference in the oxygen potential of the electrode material, which is known to be the Gibbs free energy change in the oxidation reaction. Ta has quite lower oxygen potential than Pt, being affinitive to the oxidation. In the transmission electron microscopy and electron energy loss spectroscopy studies, the intermixing between Ta and CoO as well as the oxidation of Ta was observed at the Ta/CoO interface, yielding the intermediate layer. The stable forming process was carried out in the voltage polarity in which the oxygen ions in CoO can migrate toward the intermediate layer and Ta BE, bringing about the filamentary conduction pass. In investigating the reset and set performance, we connected the load resistor in the series to the trilayer instead of the current compliance in order to suppress the transient current in the forming process which was detected using oscilloscope. The maximum value of the reset current IR is significantly influenced by the maximum current in the forming process IF. By increasing the resistance of the load resistor, the value of IF and IR were reduced. However, the value of IR in Pt/CoO/Pt was saturated around 1 mA although IF was still decreased. On the other hands, the value of IR in Ta/CoO/Pt was almost proportional to IF down to around 100 μA. For instance, by using the load resistor of 68 kohm, the reset current became 150 μA. Furthermore, the reset and set were performed by the very short voltage pulse of 20 ns in the bipolar switching operation.
1:00 AM - F5.2
Direct Metal Transfer (DMT) at Nanoscale for the Ubiquitous Organic Devices Era.
Gun Young Jung 1 , Kyeongmi Lee 1 Show Abstract
1 Materials Science and Engineering, GIST, Gwangju Korea (the Republic of)
Organic electronic devices have been widely investigated in many groups because of its application to the flexible and portable devices. To define top electrodes, conventional optical lithography and subsequent lift-off process is normally used. However, the lift-off solvent could penetrate and degrade the organic active layer during the process, resulting in weird device behaviors and failure in device fabrications accordingly. Therefore, alternative ways with non-aqueous process have been performed such as microcontact printing, cold welding and metal transfer printing. Those processes have the similar process scheme in that firstly, the metal film is deposited on top of the previously patterned stamp and then pressed on the active organic layer while applying a certain pressure. The top electrodes are defined after transferring the metal film of the protruded area in the stamp to the surface of the organic layer either physically or chemically. As described, the process is simple and performed in non-aqueous atmosphere with low cost. Successful organic electronic devices fabricated by the contact printing method have been reported at micro scale. In this study, we used a silicon stamp which was appropriate for the metal transfer at nanoscale. Metal nanowires at 70nm half-pitch were transferred uniformly on top of a polymer film which served as a memory active layer. We found that heating treatment while applying pressure is the most important key parameter for the successful metal transfer with high fidelity.
1:00 AM - F5.20
The Interfacial States between Metal/Oxide and the RRAM – Part II.
Wei Pan 1 , David Evans 1 Show Abstract
1 Materials and Device Applications Lab, Sharp Labs of America, Camas, Washington, United States
Switching resistance values of a metal/oxide/metal structure upon the stimulation of electric pulses, Pt/PrxCa1-xMnO3/Pt for example, have triggered vast research interests and activities, hoping for possible resistive RAM applications. In an earlier paper , the authors have attributed this switching resistance mechanism to be the interfacial dipoles between metal and oxide contact. The existence of interfacial diploes, between Pt electrodes and PCMO is evident in both the DC time-bias measurement as well as the AC impedance measurement. A DC bias, above a threshold voltage, causes interfacial movement of charged species, known as electrode polarization. This chemical potential change at the interface is balanced out by the change of electro-static potential across interface, which results a change of the interfacial impedance (and resistance if measured with DC). This interfacial dipole was also characterized through the admittance spectroscopy measurements, resulting so called Cole-Cole plots. In combination with temperature responses, the extracted time constants fitted very well with experimental data.In this paper, the resistance switching endurance is discussed using admittance spectra of the Pt/PCMO/Pt stack in combination with the interfacial dipole model. Experimental data indicate that those interfacial dipoles (or states) are all meta-stable. In other words long term switching endurance is a problem. We have screened many metal/PCMO contact combinations and characterized those contacts with basic memory criteria: bit separation, data retention, switch endurance, switching speed, and readout limitations. Among these combinations, TiN/PCMO showed large bit separation, reasonable data retention, and fast switching speed, but failed long term switching endurance test. Furthermore, from the free-energy-well diagram point of view, we conclude that the meta-stable dipoles (states) are the culprit for the failure of endurance. Therefore, a material system providing bi-stable states with reasonably large free energy separations is a must for RRAM applications. Phase change memory, as an example, switches between two thermodynamically separate and stable states, amorphous and crystallized, and is expected to have much better endurance as compared with metal/oxide type of memory. We have also seen resistance change caused by interfacial trap states in some of our devices. This portion of resistance change does not have data retention against external disturbs. In general, experimental results have showed the metal/oxide interfacial dipole induced resistance change have limitations toward resistive RAM application. W. Pan and David R. Evans, "The interfacial states between Pt/PCMO and the bistable resistive states", in Science and Technology of Nonvolatile Memories, edited by O. Auciello, J. Van Houdt, R. Carter, S. Hong (Mater. Res. Soc. Symp. Proc. 933E, Warrendale, PA, 2006), 0933-G04-05
1:00 AM - F5.21
Process Optimization of Ni Nanocrystals Formation Using O2 Plasma Oxidation to Fabricate Low-molecular Organic Nonvolatile Memory.
Woo Sik Nam 1 , Gon Sub Lee 1 , Sung Ho Seo 1 , Jea Gun Park 1 Show Abstract
1 , Tera-bit Nonvolatile Memory Development Center, Hanyang University, Seoul Korea (the Republic of)
Until the present time, various kinds of organic nonvolatile memory have been reported. They can classify greatly as organic nonvolatile memories embedded with metal nanocrystals in conductive organic layer (low-molecular organic, polymer etc.), with single charge-complex layer, and with single conductive organic layer etc. We researched low-molecular organic nonvolatile memories embedded with metal nanocrystals in low-molecular organic layer among them and developed organic nonvolatile memory fabricated with the device structure of Al/Alq3(aluminum tris (8-hydroxyquinoline))/Ni nanocrystals surrounded by NiO/Alq3/Al. Ni nanocrystals surrounded by NiO was formed via in-situ plasma oxidation process after Ni evaporation with a low evaporation rate.We found that Ni nanocrystals surrounded by NiO act important role to reveal I-V characteristic and memory characteristic of organic nonvolatile memory at fabrication process. So, we confirmed an amount of oxidation and Ni nanocrystals formation in accordance with various Ni evaporation rates, i.e. 0.1, 0.5, 1.0Å/sec, with and without O2 plasma oxidation. From the result analyzed by AES(Auger Electron Spectroscopy), it was confirmed that an oxygen content in the case of processing with plasma oxidation during 300sec is much about 1.5~2 times compared to the case without plasma oxidation process. In particular, if the evaporation rate of the middle Ni layer is lower preferably less than 0.1Å/sec, oxygen ratio of Ni nanocrystals layer increased. In addition, from the result analyzed by XPS(X-ray Photoelectron Spectroscopy) and TEM(Transmission Electron Microscope), we confirmed that pure poly Ni and NiO are formed together. Therefore, we found the optimal condition of Ni nanocrystals formation that exert on the most important effect in memory characteristic of organic nonvolatile memory and obtained good bistable memory characteristic in organic nonvolatile memories embedded with Ni nanocrystals. In conclusion, we obtained the best bistable switching characteristic at the Alq3 thickness of 30nm, the evaporation rate of 0.1Å/sec and the middle Ni nanocrystals layer thickness of 10nm. Electrical behavior of bistable switching device was obtained by sweeping the voltage from 0 to 10V. Our device showed excellent bistable memory characteristics; i.e., Vth of 2.3V, Vwrite of 3.2V, Verase of 6.5V, Ion/Ioff ratio of greater than 104. We found that a region of negative differential resistance (NDR) exists in between Vwrite and Verase.Acknowledgement *This research was supported by national research program for 0.1 Terabit Non-volatile Memory Development sponsored by Korea Ministry of Commerce, Industry and Energy.
1:00 AM - F5.3
AC Characterization of Cross-linkable Polymer Memory Devices.
Bao Lei 1 , Wei Lek Kwan 1 , Yang Yang 1 Show Abstract
1 Materials Science and Engineering, UCLA, Los Angeles, California, United States
Stackable memory array has been achieved using a photo cross-linkable copolymer. Impedance spectroscopy based on alternating current (AC) response measurement has been used to explore the mechanism of the polymer memory device, which consists of a thin film of a photo cross-linkable copolymer, sandwiched between an ITO bottom electrode and aluminum top electrode. Cole-Cole plot of impedance over the range of 20Hz to 1MHz shows the pattern of a simple parallel RC equivalent circuit model, wherein the difference of high conductivity state and low conductivity state are distinguished by the significant difference in parallel resistance. Further study based on statistical impedance measurement has been carried out under different conductivity states, bias voltages and thermal conditions. The variation of low frequency capacitance values suggested that space charge may play an important role in the conduction mechanism of the different conductivity states.
1:00 AM - F5.4
Resistance Switching of Organic Devices Based on P3HT-PCBM Bulk Heterojunction.
Hey Jin Myoung 1 , Sung-Soo Bae 1 , Hu Young Jeong 1 , Sung-Yool Choi 1 Show Abstract
1 , ETRI, Daejeon Korea (the Republic of)
1:00 AM - F5.5
Stackable Polymer Memory Devices.
Wei Lek Kwan 1 , Ricky J. Tseng 1 , Wei Wu 1 , Qibing Pei 1 , Yang Yang 1 Show Abstract
1 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States
Stackable memory devices have been achieved by using a photo cross-linkable polymer. The device structure consists of a polymer film sandwiched between two metal electrodes. The active layer is a cross-linkable conjugated polymer. When the spin-cast polymer is exposed to UV light, the polymer chains cross-link to form an insoluble film. The film is able to resist temperatures as high as 200°C and is robust enough to withstand the lithographic processes. Once the polymer film is cross-linked, multiple stacks of the device can be achieved by coating additional layers of polymer films and electrodes on top of the existing layers. A two layer memory array has been demonstrated.The I-V curve of the device shows an abrupt increase in current at about 3.5 V, indicating a change in the conductivity of the device. The device remains at this high conductivity state even when the voltage bias is removed. Only a ±9V(or higher) pulse can bring the device back to its low conductivity state. A negative differential resistive region appears in the I-V curve when a voltage bias of 6V or more is applied to the device. Our device was able to achieve more than 4000 write/erase cycles with retention time of more than a month.This device is promising for future high density memory device.
1:00 AM - F5.6
Au Nanoparticles Blended poly(N-vinylcarbazole) Films for Nonvolatile Memory Applications.
Pei-Ying Lai 1 , Jen-Sue Chen 1 Show Abstract
1 Materials Science and Engineering, National Cheng Kung University, Tainan Taiwan
Nonvolatile resistance random access memories (RRAMs) based on polymer materials are now emerging as a candidate for next generation memories because of their natural flexibility, low cost, and easy manufacturability. Au nanoparticles (Au NPs) have been used as electron acceptors in the fabrication of polymer memory devices. In this work, the electrical bistability of polymer memory devices containing poly(N-vinylcarbazole) (PVK) and Au NPs is observed. In order to correlate the switching efficiency with the quantity of Au NPs, the concentration of Au NPs in the polymer matrix is varied. Details of material characterization and switching mechanism will also be examined.
1:00 AM - F5.7
Metal Nanoparticles Free Transparent Organic Bistable Memory Devices.
Sung Hyun Kim 1 , Kyoung Soo Yook 2 , Jyongsik Jang 1 , Oh Young Kim 2 , Jun Yeob Lee 2 Show Abstract
1 , Seoul National University, Seoul Korea (the Republic of), 2 , Dankook University, YoungIn Korea (the Republic of)
Metal nanoparticles free transparent organic bistable memory device was developed by using polyphenylenevinylene(PPV) as an active layer and transparent ITO as electrode. Thin Al was evaporated on PPV by thermal deposition and ITO was deposited by e-beam evaporation and sputtering. Effect of Al thickness on memory characteristics of transparent organic bistable device(OBD) was from 10 nm. Transparent organic bistable memory device could be effectively fabricated by this method with high on/off ratio of over 10*3 and low operation voltage of around 2V. E-beam deposition of ITO was batter than sputtering of ITO and stable memory characteristics could be realized in e-beam ITO deposited organic bistable memory device, negative differential resistance region was observed in e-beam ITO memory device. It can be expected from this results that organic bistable deivices can be freely combined with other organic electronic devices such as organic solar cell and Organic light emitting diodes because it is transparent in visible region.
1:00 AM - F5.9
Effect of Ferroelectric Polarization on Tunneling Current.
Evgeny Kiriranov 1 , Andrei Sokolov 3 4 , Jody Redepenning 2 4 Show Abstract
1 , Lincoln South-West High School, Lincoln, Nebraska, United States, 3 Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 4 Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
In the recent years electron transport through organic materials drew a great deal of attention due to large variety of interesting properties relevant for design of new electronic devises. One attractive example of such properties is the observance of ferroelectric effect in P(VDF-TrFE) organic co-polymer. On the other hand, the use of a thin ferroelectric film as tunnel barrier is redicted to significantly expand the functionality of commonly used tunnel junctions . However, experimental realization of organic based tunnel junctions face a major difficulty due to extreme frajility of such thin film to pattering of metal electrodes. Thus, for experimental observation of the effect of ferroelectric P(VDF-TrFE) on tunnel transport and to clarify the actual mechanism of electroresistance a new experimental setup, different to standard ‘pillar’ geometry has to be realized. We present the results of study of the electron transport through ferroelectric co-polymer P(VDF-TrFE). First, nano-gap was formed, using controlled breaking of a Au nanowire to form a nanogap and then filled by the polymer from a solution under an orientation potential. Current –voltage character, reveals a hysteretic behavior, a signature of the effect of ferroelectric polarization on resistance of the tunnel junction. Possible interpretations of the effect will be discussed.References A. Bune, V. M. Fridkin, S. Ducharme, L. M. Blinov, S. P. Palto, A. Sorokin, S. G. Yudin, A. Zlatkin “Two-Dimensional Ferroelectric Films”, Nature 391, 874-877 (1998). E. Y. Tsymbal and H. Kohlstedt, “Tunneling across a ferroelectric”, Science 313, 181-183 (2006).
Orlando Auciello Argonne National Laboratory
Dirk Wouters IMEC
Steven Soss Intel Corporation
Seungbum Hong Argonne National Laboratory
F7: Emerging (Probe/Cross-point/Organic) III
Thursday AM, March 27, 2008
Room 2006 (Moscone West)
9:30 AM - **F7.1
Recent Progress on Organic/polymer Memory Devices and Their Stacking Structure.
Yang Yang 1 2 , Wei Lek Kwan 1 , Bao Lei 1 , Ricky Tseng 1 Show Abstract
1 Materials Sci. & Eng., UCLA, Los Angeles, California, United States, 2 California Nanosystem Institute, UCLA, Los Angeles, California, United States
Recently, following our initial work on organic tri-layer memory device in 2002, we have demonstrated several versions of non-volatile memory devices based on polymer, polymer nano-fiber, and tobacco mosaic virus, when they doped with nano-particles. These devices show interesting electrical bistability, which can be useful in memory application. In this presentation, a brief history will be provided to review these progress and also covers the mechanism(s) of these devices. Finally, we will report a novel stackable memory device achieved by using photo cross-linkable polymer.
10:00 AM - F7.2
Understanding the Bi-polar Nonvolatile Switching in Oxide Based Resistive Junctions.
J. Yang 1 , F. Miao 1 , D. Ohlberg 1 , J. Borghetti 1 , M. Pickett 1 , Z. Li 1 , W. Tong 1 , D. Stewart 1 , R. Williams 1 Show Abstract
1 , Hewlett-Packard Laboratories, Palo Alto, California, United States
Oxide based electrical-resistance switches are being agressively pursued for next generation nonvolatile random access memories (R-RAMs). Laboratory demonstrations include disruptive-quality fast (~10 ns) low energy (~10 pJ/b) electrical switching. However, the switching mechanism for these metal/oxide/metal devices is far from being understood. Different models even disagree on the physical location of the switching change, i.e. insulator, electrodes or interfaces, in large part due to the great difficulty in characterizing small changes in these buried layers.We have observed non-volatile bipolar switching in micro- and nano-scale Pt / TiO2-x / Pt crosspoint junctions at 30 ns and over 1000 on/off ratios. Complementary experiments on both thin film devices and bulk single crystal oxides combined with the new scanned probe technique of pressure-modulated conductance microscopy provide compelling evidence for interface-controlled electronic switching. We show that the nature of the switching is channeling (on) and recovering (off) the Schottky barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under electrical field. Switching on involves penetration of the Schottky barrier by a nanoscale conducting channel; switching off takes place with recovery of the channeled Schottky barrier by pushing oxygen vacancies away from the interface. Using this knowledge, we have used atomic layer deposition and reactive sputter deposition to build TiOx crosspoints with engineered oxygen vacancy profiles that predictively control the fundamental switching polarity and conductance, opening real opportunities for device optimization.
10:15 AM - F7.3
Polymeric Schottky Diodes for Non-volatile Memories.
Kamal Asadi 1 , Dago de Leeuw 1 2 , Bert de Boer 1 , Paul Blom 1 Show Abstract
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 High Tech Campus, Philips Research Laboratoires, Eindhoven Netherlands
Non-volatile memories based on organic semiconductors are ideally suited for inexpensive and low-performance logic circuits on thin, flexible plastic substrates, which allows for conceptually new applications such as intelligent food packaging and smart textiles. Organic bistable devices have been fabricated using many different approaches. However these novel approaches for memory cells based on resistive elements, lack the major requirement for integration of the individual cells into an array. In order to prevent cross-talk between the memory bits, each cell must be based on a one diode-one resistor (1D1R) element instead of one resistor element (1R). Here we present an integrated solution to this challenge. A non-volatile, yet reversible switchable Schottky diode is demonstrated with relatively fast programming time of less than 100 microseconds, long information retention time of larger than 10^7 seconds, and high programming cycle endurance of more than 1000 read, write, read and erase cycles with non-destructive read-out. The resistivity of this diode is modulated between non-volatile high- and low-resistance states (Boolean 0 and 1) by applying a switching pulse, whereas lower voltages allow for reading of the information non-destructively.
10:30 AM - F7.4
Amorphous Silicon Based Nonvolatile Resistive Switching Crossbar Memory.
Sung Hyun Jo 1 , Wei Lu 1 Show Abstract
1 , University of Michigan, Ann Arbor, Michigan, United States
Conventional amorphous-silicon based nonvolatile resistive switching devices have the advantages of high on/off resistance ratio, full CMOS compatibility, and rely on proven fabrication processes and relatively inexpensive material. However, the high voltage forming process typically required in such devices lowers the device yield, and it is not clear whether such devices can be scaled down to nanoscale. Our recent studies on nanoscale a-Si resistance switching devices have shown that forming can be well controlled by using a heavily doped p-type substrate as the bottom contact electrode. At the single cell level, the two-terminal metal/a-Si/p-Si memory devices show high on/off resistive switching ratio (>10^4), large endurance cycles (>10^6) and fast switching speed (order of a few nanoseconds). Here we report ultra-high density array level integration of the a-silicon memory devices. A crossbar memory structure was adopted to maximize the device density. Crosstalk between the cells in the crossbar array is mitigated by utilizing the tunable rectifying (diode-like) behavior at the on-state of the cells. The use of a solid state switching medium (amorphous silicon) enables precisely controllable device fabrication as well as reliable and predictable switching performance. Memory arrays with cell size of 100*100nm^2, have been reliably fabricated and tested. The device yield was found to be close to 100% and the programming current can be tuned by more than three orders of magnitude by controlling the amorphous silicon deposition parameters. The ultra-high density, non-volatile crossbar based resistive switching devices may be a suitable candidate for next-generation nonvolatile memories.
10:45 AM - F7.5
High Performance MIM and Cross-Point Non-volatile Organic Memory Devices using Novel Polyfluorene-Derivative Single Layer Film.
Tae Wook Kim 1 , Seung-Hwan Oh 1 , Hyejung Choi 1 , Gunuk Wang 1 , Dong-Yu Kim 1 , Hyunsang Hwang 1 , Takhee Lee 1 Show Abstract
1 Material science and engineering, Gwangju Institute of Science and Technology& techenlogy, Gwangju Korea (the Republic of)
Due to the various merits of organic electronic devices, including low cost, easy fabrication, and printing capability, organic materials have been intensively developed for next-generation electronic devices, such as organic light-emitting diodes, memories, photovoltaic cells, sensors, and thin-film transistors. Among these organic devices, organic memory appears highly attractive, owing to its potential application in data storage. In this study, we report on the reversible switching characteristics of four kinds of polyfluorene-drivatives (WPF-oxy-F, Ag-WPF-oxy-F, Ca-WPF-oxy-F, and Na-WPF-oxy-F) in metal-insulator-metal (MIM) type and cross-point type devices. The non-volatile organic memory devices using polyfluorene-drivatives exhibit excellent reversible resistance switching between high and low resistance states. These organic memory devices show a large ON/OFF ratio (Ion/Ioff ~10^4), a long retention time (more than 10,000 sec), a good thermal stability up to 120 degree C, and an excellent device-to-device switching uniformity of both MIMs and cross-point type devices. A detailed discussion of memory performance is presented. From analysis on the current-voltage characteristics on the log-log scale and the current images of the two different resistance states, a mechanism for the reversible switching of the organic memory devices is proposed. a) C. D. Müller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature 2003, 421, 829 b) Y. Chen, G. Jung, D. A. A. Ohlberg, X. Li, D. Stewart, J. O. Jeppesen, K. A. Nielsen, J. F. Stoddart, and R. S. Williams, Nanotechnology 2003, 14, 462 c) J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. -Q. Nguyen, M. Dante, A. J. Heeger, Science 2007, 317, 222. S.-H. Oh, S.-I. Na, Y.-C. Nah, D. Vak, S.-S. Kim, D.-Y. Kim, Org. Electron. doi:10.1016/j.orgel.2007.06.011, in press. (2007)
11:30 AM - F7.6
Resistance Switching in Ionic Nanowires.
David Schoen 1 , Stephen Conner 1 , Yi Cui 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Resistance switching in copper and silver chalcogenide ion conductors has been explored for its application to nonvolatile memory. This study investigated resistance switching in chemically synthesized Ag chalcogenide nanowires. Silver selenide and silver sulfide nanowires have been synthesized using various nanowire templates. Both monolithic and core-shell structures are reported. The materials have been structurally characterized and their electrical behavior has been investigated. A variety of resistance change behavior has been observed, and when possible correlated with the related structural changes.
11:45 AM - F7.7
Dual-frequency resonance-tracking Piezoresponse Force Microscopy: High Sensitivity Imaging, Spectroscopy, and Energy Dissipation.
Anil Ganepalli 1 , Stephen Jesse 2 , Katyayani Seal 2 , Keith Jones 1 , Brian Rodriguez 2 , Clint Callahan 1 , Sergei Kalinin 2 , Roger Proksch 1 Show Abstract
1 , Asylum Research, Santa Barbara, California, United States, 2 The Center for Nanophase Materials Sciences and Materials Sciences and Technology Division, Oak Ridge national Laboratory, Oak Ridge, Tennessee, United States
High sensitivity Piezoresponse Force Microscopy (PFM) imaging and spectroscopy requires imaging at contact resonance conditions to enhance the sometimes very weak PFM signal. Typical contact resonances in PFM are in the range of hundreds of kHz and have a full-width-half maximum of a few kHz. On the other hand, the contact resonance frequencies can easily vary by tens of kHz as the tip scans over the surface – depending on the tip-sample contact conditions. This large variation causes an unacceptable degree of crosstalk between topography and the PFM signal. One way to take advantage of the contact resonance and to avoid this crosstalk is to vary the drive frequency to follow variations in the resonance frequency. In many systems, a phase-locked loop (PLL) is used to accomplish this resonance tracking. Since PFM contrast depends on the phase dependence of the sample response to an oscillating electric field, a standard PLL is not suitable. Here we present an amplitude-based frequency-tracking mode utilizing dual ac excitation.1 Briefly, the cantilever potential is modulated at two distinct frequencies - one just below and the other just above the contact resonance. By monitoring the difference between these two amplitudes, changes in the resonant frequency can be determined and corrected for using a standard PID feedback loop. This mode allows high resolution imaging and spectroscopy of domain structures and polarization dynamics in ferroelectric and multiferroic materials. Because measurements are made at two distinct frequencies near resonance, the resonance frequency, sample dissipation and sample response amplitude are all unambiguously determined by the two amplitude and phase signals. In spectroscopy mode, the voltage dependence of dissipation and elastic stiffness is used to analyze the nano-scale mechanisms of domain switching. 1. B. Rodriguez et al. Nanotechnology 18, 475504 (2007).The research is supported (SVK, KS, BJR, SJ) by the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC for the Office of Basic Energy Sciences, US Department of Energy.
12:00 PM - F7.8
Nonvolatile Resistive Switching Characteristics of HfO2 with Cu Doping.
Weihua Guan 1 , Shibing Long 1 , Ming Liu 1 , Wei Wang 2 Show Abstract
1 , Institute of Microelectronics, Chinese Academy of Sciences, Beijing China, 2 , Indiana University - Purdue University Indianapolis, Indianapolis, Indiana, United States
Recently, reversible and reproducible resistive switching phenomena induced by external electric field have been extensively studied due to its potential applications in resistive random access memories (RRAM). This type of memory devices can be characterized by two distinct resistance states: OFF state (with high resistance) and ON state (with low resistance). The current candidate materials for this type of memories include ferromagnetic material such as PCMO, doped perovskite oxide such as SrZrO3 and SrTiO3, organic materials, and binary metal oxides such as NiO, TiO2, ZrO2, Nb2O5, CuxO, and even doped SiO2. Among all these candidates, binary transition metal oxides excel the others due to their simple structure, easy fabrication process and compatibility with CMOS technology. Although HfO2 films are currently considered to be the promising gate dielectric in advanced CMOS devices, there are few works to discuss the resistive switching behavior of HfO2, which mostly show unipolar switching, i.e., writing and erasing using the same voltage polarity. In this work, we will report the bipolar resistive switching characteristics of HfO2 with Cu doping for nonvolatile memory application. The devices are with a sandwich structure of top Au electrode/ HfO2 doped with Cu/n+ Si bottom electrode. Resistive switching from ON to OFF state is induced by increasing the voltage up to a positive while the switching from OFF to ON state can be achieved at a negative bias voltage. Though the ratio of high/low resistance is lees than an order, the switching behavior is very stable and uniform with nearly 100% device yield. Tests concerning reliability issues are also performed. More than one hundred times of write-read-erase-read cycles without serious sensing window deterioration are demonstrated. For the retention test, the device is firstly turned ON or OFF by applying proper bias and then a continuous readout voltage is applied. The resistances of both ON and OFF state are stable and show no degradation over 1e4 s. The experimental features of the I-V characteristics, interestingly, are identical to those described by the model proposed Simmons and Verderber (SV model): N-shaped I–V characteristic in positive voltage and initial low resistance state for the pristine device. Moreover, due to the homogeneous conduction of SV model, the dependence of current on electrode area is also expected. SV’s model involves the deep level Au atoms provided by the electroforming process. Since we deliberately diffused the Cu atoms into HfO2 matrix during the fabrication process, the electroforming process can be exempted, consistent with experimental observation. Considering its excellent reliability performances, Cu doped HfO2 film is promising in the application for future nonvolatile resistive switching memory devices.
12:15 PM - F7.9
Lattice Strain and Properties of Self Assembled BiFeO3-CoFe2O4 Nanocomposite Thin Films.
Murali Rajaram 1 , Nico Dix 1 , Celine Lichtensteiger 4 , Jill Guyonnet 4 , Vassil Skumryev 2 , Manuel Varela 3 , Florencio Sanchez 1 , Jean-Marc Triscone 4 , Josep Fontcuberta 1 Show Abstract
1 , ICMAB-CSIC, Barcelona Spain, 4 DPMC, Université de Genève, Genève Switzerland, 2 , ICREA, Barcelona Spain, 3 Departament de Fisica Aplicada i Optica, Universitat de Barcelona, Barcelona Spain
12:30 PM - F7.10
Diblock-copolymer Mediated Nanopatterning of Solution-Derived Electronic Oxides.
Geoff Brennecka 1 , John Ekerdt 2 , Jill Wheeler 1 , Bruce Tuttle 1 Show Abstract
1 Electronic and Nanostructured Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Deparment of Chemical Engineering, University of Texas at Austin, Austin, Texas, United States
A general nanopatterning approach based on the use of self-assembled diblock copolymers for primary pattern definition has been developed for the controlled chemical solution deposition of various electronic oxides having discrete lateral features on the order of 20nm. When surface energies are properly controlled, these diblock copolymer templates naturally self assemble into patterns with local hexagonal order. The use of appropriate materials for intermediate pattern-transfer layers enables a variety of materials to be deposited using alkoxy-based chemical solution deposition. Solution-based deposition offers significant advantages over vapor-based techniques in terms of cost and modularity. The technique therefore represents a general approach for the controlled deposition of nanopatterned electronic oxides. Results will focus on work with TiO2, including pattern formation and transfer, feature definition, deposition and patterning of subsequent materials, selective etching and liftoff techniques, and crystallization of patterned nanofeatures. Controlled nanopatterning of solution-deposited electronic oxides is essential for next-generation high-density memory and sensor devices and will enable the fabrication of metamaterials with unique behaviors arising from their nanoscale periodicity. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL8500.
12:45 PM - F7.11
Evolution of the Resistive Switching Characteristics During Electroforming of Epitaxial Iron-doped SrTiO3 Thin Films.
Tobias Menke 1 2 , Regina Dittmann 1 2 , Krzysztof Szot 1 2 , Paul Meuffels 1 2 , Rainer Waser 1 2 Show Abstract
1 Institute of Solid State Research, Research Center Jülich, Jülich Germany, 2 Center of Nanoelectronic Systems for Information Technology, Research Center Jülich, Jülich Germany
In view of the elucidation of the resistive switching mechanism in ternary oxides, SrTiO3 plays an important role as a prototyping material. It is clearly demonstrated that the resistive switching in SrTiO3 single crystals and thin films  originates from a redox-process within extended defects which results in a local metal-to-insulator transition. Usually, an electroforming procedure has to be performed to observe reversible switching in oxide materials. We investigated epitaxial grown iron-doped SrTiO3 thin film MIM structures with the focus on the forming procedure needed. Because of the involved charge driven redox process  we performed our measurements in a current-driven mode and investigated two different methods of electroforming, namely forming with a constant current and forming by applying a triangular current signal (forming during the first sweep). The investigated sandwich-like structure consists of a SrRuO3-film bottom electrode, a 45nm thick SrTiO3(1at%Fe)-film as the switching layer and platinum dots as top electrodes. The heterostructures were grown epitaxially on (100)-oriented SrTiO3-substrates by pulsed laser deposition at a temperature of 700°C and an oxygen pressure of 0.25 mbar. Afterwards Platinum dots were deposited on the SrTiO3(1at%Fe) thin film using a shadow mask technique. We will show that the shape of the I-V-curves strongly depends on the current load which is chosen by the forming procedure of the sample. For a low current load applied to the sample, a continuous hysteresis is observed. This hysteresis becomes more pronounced when the slope dI/dt is decreased in the measurement procedure. Furthermore the shift of the saturation voltage is observed which also depends on dI/dt. Pulse measurements indicate the resistive state as volatile. We suppose that this is related to an internal counter field originating from the accumulation of oxygen ions at the interface. We attribute this kind of curves to the redistribution of oxygen ions and not to a metal-insulator transition. With increased current load, steps in the I-V-curve emerged which is an indication of resistive switching. Pulse measurements displayed the states as non-volatile. Additional two different polarities of switching were observed which we attribute to a superposition of the two involved interfaces. We suppose that the two different kind of I-V-characteristic corresponds to different stages on a DC-forming curve. K. Szot, W. Speier, G. Bihlmayer, and R. Waser, Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3, Nat. Mater. 5, 312-20 (2006) K. Szot, R. Dittmann, W. Speier and R. Waser, Nanoscale resistive switching in SrTiO3 thin films, phys. stat. sol. 2(1), R86-R88 (2007)
F8: Future Nonvolatile Memories I
Thursday PM, March 27, 2008
Room 2006 (Moscone West)
2:30 PM - **F8.1
Optimization of Advanced FeRAM Materials and Processes.
Carlos Paz de Araujo 1 2 Show Abstract
1 , Symetrix Corp., Colorado Springs, Colorado, United States, 2 Electrical Engineering, University of Colorado, Colorado Springs, Colorado, United States
Ferroelectric Random Access Memories (FeRAMs) have now achieved nearly 900 million units in sales (including both materials families of PZT and SBT), surpassing every emerging memory technology that have been under research in the last ten years. When high K ferroelectric materials are considered (eg. GaAs MMICs with BST), more than 600 million units are added, bringing Integrated Ferroelectrics to over 1.5 Billion devices in the hands of consumers. In spite of this commercial success and resolution of CMOS compatibility issues that delayed their introduction, the scientific community is still looking for memory technologies such as MRAMs, PCMS, and various forms of molecular devices that seem incapable at the very start to meet the results already enjoyed by FeRAMs. This paper shows that further improvements in FeRAMs for the 65-32 nm era seem to be more plausible as a strategy than to improve materials for devices that cannot even compete with existing FeRAMs. The paper also includes the evolution of capacitive FeRAM into the resistive FeRAMs (ReRAMs based on Ferroelectric materials and d-block oxides in general), a topic worth pursuing as ReRAMs of various types seem to be of resent interest.
3:00 PM - F8.2
Aluminum Oxide Thin Films Using Sputtering Technique as Blocking Oxide for Flash Memory Applications.
Pawan Singh 1 2 , Kaushal Singh 1 , Ralf Hofmann 1 , Karl Armstrong 1 , Souvik Mahapatra 2 , Nety Krishna 1 Show Abstract
1 , Applied Materials, Santa Clara, California, United States, 2 Electrical Engineering, Indian Institite of Technology Bombay, Mumbai India
In this work we for the first time report the excellent physical and electrical characteristics; low leakage and large breakdown, of reactive pulsed-DC sputter deposited Al2O3 films using industry standard tools for NVM application. Impact of deposition conditions and post deposition anneal (PDA) on the physical and electrical properties are also discussed. Most of prior work done on Al2O3 has concentrated only on using CVD and ALD deposition techniques [1, 2] to obtain high quality films. A replacement high-k material like Al2O3 as blocking oxide is needed to reduce the EOT of the gate stack allowing for lower voltage operation, while still keeping large physical thickness to improve leakage and reliability. Al2O3 films were reactively sputtered with a pure Al target in an Applied Materials 200mm Endura tool using Ar/O2 gas. The base pressure of the PVD chamber was maintained at ~3E-8Torr. O2 flow rate was controlled to ‘poison’ the target surface during deposition. XPS depth profiling was performed for analyzing the composition, XRR for thickness and density measurements and XRD grazing angle scan to analyze the microstructure of the films. Finally, electrical testing is performed to evaluate the leakage and breakdown of the films. All of the analyzed films were found to be stoichiometric when deposited in ‘poison’ mode. Deposition rate was found to decrease with increasing O2 flow. Deposition at a higher substrate temperature increased the density and correspondingly decreased the thickness. Density close to 80% of Al2O3 bulk (3.99g/cm3) can be achieved without the need of any post deposition processing. XRD analysis shows that the deposited films were amorphous till atleast 250°C substrate temperature. Films deposited at higher temperatures showed lower leakage suggesting some form of in-situ annealing. PDA was performed on these films in O2 and N2 ambient in a Radiance RTP chamber at temperatures ranging from 700°C to 1100°C for different times. It was observed that anneal temperature had the maximum impact on the morphology of the deposited films. An abrupt change in the density was observed for anneal temperatures greater than 800°C irrespective of time in the experiment. Density increases to ~95% of the Al2O3 bulk after PDA for these films. XRD shows minor crystallization with the nanocrystallites (size of few nm, by x-section TEM) embedded in an amorphous matrix. PDA in O2 ambient was found to be slightly better in improving the electrical characteristics, although N2 anneal gave almost similar characteristics. Al2O3 films with BD field >7MV/cm and leakage ~1E-9A/cm2 @1MV/cm can be achieved. More experimental work is in progress and details of electrical and physical work will be presented. Results of PVD Al2O3 as blocking oxide compared with CVD and ALD deposited films will be discussed. References:  S. Skordas et al, MRS Proc. 2003,  J. Buckley et. al. ESSDERC 2006.
3:15 PM - F8.3
Materials Aspects of Si-rich Silicon Nitrides for Use in Antifuse Nonvolatile Memories with Low Programming Voltages.
Scott Habermehl 1 , Roger Apodaca 1 , Robert Kaplar 1 , Liz Roherty-Osmum 1 , David Stein 1 Show Abstract
1 Microelectronics Development Lab, Sandia National Laboratories, Albuquerque, New Mexico, United States
Antifuses are electronic devices that employ a material, or set of materials, that can be converted irreversibly, via the application of a threshold electric field, from a high resistance state to a low resistance state. Antifuses are hence one-time-programmable, with the programmed state perpetual. This renders antifuses ideal for non-volatile memory (NVM) applications that require repeatable read capability, but not repeatable write capability. In this work silicon-rich silicon nitride thin films are investigated for use as low voltage, high reliability materials for incorporation into NVM antifuse devices. Observations indicate that the thin film materials Si3N4, silicon oxynitride (SiOxNy), and Si-rich silicon nitride exhibit charge transport behavior governed by Poole-Frenkel emission. Furthermore, regardless of film composition there is a direct linear correlation between the barrier lowering energy at the breakdown field, βEBD1/2, and the bulk trap ionization potential, qΦB. The implication of this is that the breakdown field of these materials is determined by the Poole-Frenkel field saturation condition, that is, qΦB – βEBD1/2= 2kT. As the electric field increases and βE1/2 approaches to within 2kT of qΦB, the bulk traps no longer effectively regulate charge transport; this gives rise to electron heating in the quasi-conduction band, impact ionization, and subsequent dielectric breakdown. This view is further supported by the observation of reduced values for βEBD1/2 at elevated temperatures.For Si3N4 and SiOxNy, qΦB is sufficiently large (1.0-1.5 eV) and EBD is sufficiently high (11-13 MV/cm), that for practical use as a low voltage antifuse material, the film thickness would need to be impractically thin (~ 3 nm). This is not the case for Si-rich silicon nitrides (SiNx; x < 1.33), which can be prepared with qΦB well below 1.0 eV and with EBD approaching 2.5 MV/cm. The consequence of this capability is that relatively thick SiNx films (~ 20 nm) can be composed that exhibit breakdown voltages of 5 V, or less. Data collected from a series of samples of varying composition and thickness show that EBD is exponentially dependent on the silicon volume fraction of the film; this indicates multiple pathways for achieving low breakdown voltage antifuses from SiNx thin films.
3:30 PM - F8.4
Molecular Memory Devices using Ruthenium-terpyridine, MLCT, Complexes.
Hyoyoung Lee 1 , Junghyun Lee 1 , Kyoungja Seo 1 , Gyeong Sook Bang 1 , Hojong Chang 1 , Sangkwan Kim 1 Show Abstract
1 NCRI, Center of Smart Molecular Memory , ETRI, Daejeon Korea (the Republic of)
For the realization of nano-scaled molecular electronics devices, it is essential to develop functionalized molecules for molecular switches that have hysteretic I-V characteristics and thermal stability in the solid state, and can eventually replace the gate of transistor by using a self-assembling process of organic materials. In this presentation, we focused on using Ruthenium-terpyridine, metal-to-ligand charge transfer (MLCT) complexes for molecular switches and memory devices. Bis(terpyridine)-transition metal complexes exhibit superior chemical and electronic stability toward redox reactions due to their octahedral configuration in coordination. kinetics of the heterogeneous electron transfer have been shown to be very fast in Ru MLCT complexes compared to ferrocene that is often used as a redox center. The Ru(II) is readily oxidized to Ru(III) at a low energy, and terpyridine (tpy) ligands are less able to shield the charge of the metal ion compared to bipyridine ligands. The metal atom can be used as charge storage vehicle and the tpy ligands can be used as insulating barriers in the molecular junctions. In this presentation, we like to report the synthesis and characterization of a novel thiol-terminated terpyridinyl ruthenium(II) hexafluorophosphate derivatives and their application for the molecular memory devices. The hysteretic properties of these SAMs were identified with ultra-high vacuum scanning tunnelling microscopy (UHV-STM). The molecular memory device of metallic molecular junction was fabricated and writing-reading-erasing sequences were implemented.
3:45 PM - F8.5
Direct Time Resolved Observation of Magnetization Switching induced by Spin-Transfer in Magnetic Tunnel Junctions for MRAM.
Thibaut Devolder 1 , Jun Hayakawa 2 3 , Kenchi Ito 2 , Hiromasa Takahashi 2 , Shoji Ikeda 3 , Paul Crozat 1 , Joo-von Kim 1 , Claude Chappert 1 , Hideo Ohno 3 Show Abstract
1 IEF-UMR CNRS 8622, University Paris Sud, Orsay France, 2 Advanced Research Laboratory, Hitachi, Ltd, Tokyo Japan, 3 Laboratory for Nanoelectronics and Spintronics, RIEC-Tohoku University , Sendai Japan
The Spin-RAM [1, 2] promises a high density, non volatile, scalable random access magnetic memory. The core device is a nanopillar of magnetic tunnel junction (MTJ) of basic structure F(free)/I/F(fixed). F(fixed) is a ferromagnetic layer of fixed magnetization, I the tunnel barrier and F(free) the ferromagnetic layer which stores the binary information on the direction of its magnetization. The transfer of spin angular momentum from conduction electrons to the magnetization allows to write the orientation of the magnetization of F(free) by sending a bipolar current density through the nanopillar , and the tunnel magnetoresistance DR/R of the MTJ, up to nearly 1000% with a MgO tunnel barrier , allows reading the stored information. But one major advantage of the Spin-RAM could be the intrinsic switching speed of the magnetization of a nano-element, if the natural precessional dynamics of magnetization (in the 1-10 GHz range) can be preserved. This could open the way to magnetic logic circuits .Spin transfer switching is however a complex process. Assuming a uniform rotation of the magnetization in the nano-element, beyond a critical current density the spin transfer effect excites a precession of increasing angle up to full switching to the reverse direction . A stochastic character is expected from thermally activated fluctuations of the trajectory starting point. To explore the switching dynamics under spin transfer, we have measured the distribution of switching times induced by fast current pulses in 100x300nm2 pillars of MgO-based magnetic tunnel junctions. The high DR/R values also allowed to follow individual switching events with 18 GHz bandwidth. The switching proceeds through a ns-scale random incubation delay during which the resistance is quiet, followed by a sudden (400ps duration) transition terminated by a pronounced ringing that is damped within 1.5 ns. While the incubation delay is probabilistic, the following time-dependence of the resistance is reproducible. The ringing after switching indicates a high coherence of the sample's micromagnetic state, recalling uniform switching, while the absence of resistance fluctuations before the main switching trajectory rather signals non uniform excitations. Such modes may have a strong impact on speed and reliability of the switching. Possible solutions will be discussed [7, 8]. Hosomi, M. et al., IEDM Technical Digest. IEEE International, 2005 Kawahara, T. et al., International Solid-State Circuts Conference ISSCC Technical Digest, 2007 Hayakawa, J. et al., Jpn. J. Appl. Phys. 44, L1267 (2005). Hayakawa, J. et al., Appl. Phys. Lett. 89 (23), 232510 (2006). Zhao, W. et al., 8th International Conference on Solid-State and Integrated Circuit Technology ICSICT 2006 Sun, J, Phys. Rev. B 62, 570 (2000) Ito, K. et al., Appl. Phys. Lett. 89 (25), 252509 (2006). Devolder, T., Chappert, C., and Ito, K., Phys. Rev. B 75 (22), 224430 (2007)
4:30 PM - F8.6
Electron Energy-loss Spectroscopy of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions.
Judy Cha 1 , J. Read 2 , William Egelhoff 3 , R. Buhrman 1 , David Muller 1 Show Abstract
1 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 2 Department of Physics, Cornell University, Ithaca, New York, United States, 3 Metallurgy Division, Materials Science and Engineering Laboratory, National Institute of Standards & Technology, Gaithersburg, Maryland, United States
4:45 PM - F8.7
Highly Robust Thermal Stability of Metal-Rich Gd Nanocrystal Memory for High Density Flash Application.
Chao-Sung Lai 1 , Jian-Yi Wong 1 , Jer-Chyi Wang 3 , Chin Ting Lin 1 , Hsing-Kan Peng 1 , Yu-Ching Fang 2 , Li Hsu 2 , Hui-Chun Wang 2 Show Abstract
1 Electronic Engineering, Chang Gung University, Tao-Yuan Taiwan, 3 , Nanya, Tao-Yuan Taiwan, 2 Science & Technology, Materials & Electro-Optics Research Division, Chung-Shan, Tao-Yuan Taiwan
Recently, Poly-Si-Oxide–Nitride–Oxide–Silicon (SONOS)-type structure memories, which include nitride and nanocrystal (NC) memories, have attracted much attention for their application in the next-generation nonvolatile memories. Unfortunately, for conventional SONOS memory, erase saturation and vertical stored charge migration are two major drawbacks, whereas for NC memories, the most challenging tasks are how to maintain acceptable charge capability of the discrete storage nodes and fabricate NC with constant size, high density, and uniform distributions. In this work for the first time, we propose a novel metal-rich Gadolinium (Gd) NC memory that is fully compatible with the current CMOS technologies. The memory exhibits excellent thermal stability and can be applied into future high density Flash storage.The metal-oxide-semiconductor capacitors (MOSCAP) were fabricated by n-type (100) wafers. First, a 3 nm tunnel oxide was thermally grown in a horizontal furnace system after standard RCA clean. Next, a 10 nm amorphous Gd2O3 wetting layer was deposited by sputtering with pure Gadolinium (99.9% pure) target in O2 and Ar mixture. The sputtering process was also performed with 7.6E-3 torr at room temperature and with gas source of O2(3 sccm) and Ar (21 sccm); in which RF sputter power was set at 100 W. These thin films were then subjected to RTA treatment in N2 ambient at 800oC, 900oC, 1000oC for 30s and O2 ambient at 800oC, respectively. A 10 nm blocking oxide were then deposited through plasma enhance chemical vapor deposition (PECVD). Subsequently, Al deposition, gate patterning, and backside Al deposition were completed to fabricate the metal-rich Gd NC memory capacitor. Flat-band voltages were extracted from the capacitance-voltage curves measured by HP 4285 LCR meter.The C-V hysteresis curves sweep from -15V to 15V, then 15V to -15V. The hystersis width of thin films are almost the same as the temperature increased until 1000oC. It means the excellent thermal stability of Gd NC. The AFM images of Gd NC annealed at (a) O2 800oC (b) N2 800oC (c) N2 1000oC explain the outstanding thermal characteristic. The programming and erase characteristics of Gd NC storage film with various program voltages are also performed. In this paper, a novel high-k Gd NC memory is proposed and analyzed for the first time. The conventional Hf NC memory presents severe thermal stability problem, which is greatly improved by novel Gd NC memory. The charge retention and endurance are also superior for the Gd NC memory and this memory can be the candidate for next generation Flash memory applications.
5:00 PM - F8.8
Temperature Dependence of Electrical Properties of NiO Thin Films for Resistive Random Access Memory.
Ryota Suzuki 1 , Jun Suda 1 , Tsunenobu Kimoto 1 2 Show Abstract
1 Department of Electronic Science and Engineering, Kyoto University, Kyoto Japan, 2 Photonics and Electronics Science and Engineering Center (PESEC), Kyoto University, Kyoto Japan
In order to design the characteristics of resistive random access memory (ReRAM), it is essential to elucidate the resistive switching mechanism. Although various models have been suggested, the details have not been clarified yet. It is useful to investigate temperature dependence of current-voltage (I-V) characteristics of resistive switching materials for understanding the electrical conduction mechanism.Samples with Pt/NiO/Pt stack structure were fabricated on p-Si substrates. The 200 nm-thick NiO films were deposited by a reactive RF sputtering method at substrate temperature and working pressure of 300°C and 1.5 Pa, respectively. The ratio of O2 flow rate in the Ar + O2 gas mixture was 5%. Pt top electrodes with 300 µm diameter were deposited at room temperature by electron beam evaporation through a metal mask. The chemical composition of NiO films was determined to be NiO1.07 by using Rutherford backscattering (RBS). Two-terminal I-V measurements were carried out by sweeping voltage, and reproducible resistive switching characteristics were observed after so-called “forming” process at 6.5 V. The compliance current during the measurements was set to 10 mA. Typical values of the reset voltage (Vreset) where switching from low resistance state (LRS) to high resistance state (HRS) occurs and the set voltage (Vset) where HRS switches to LRS were 0.8 V and 2.5 V, respectively. The RHRS /RLRS ratio was about 104. I-V measurements were carried out in HRS and LRS in the wide temperature range from 100K to 573K. In HRS, while the resistance was almost independent of temperature below 250K, the resistance decreased with an activation energy of 0.30 eV above 250K. Hopping conduction and band conduction may be dominant in the low and high temperature range, respectively, in HRS. On the other hand, the LRS resistance was almost independent of temperature or slightly increased in the whole temperature range. This temperature dependence of LRS resistance is very similar to that of an Ni thin film deposited by RF sputtering. The Pt/NiO/Pt structure exhibited stable resistance switching characteristics at temperature as high as 250°C or even higher. Although RHRS /RLRS ratio decreased with temperature, the ratio is still 70 at 250°C. Since other competitive nonvolatile memories will face severe difficulty in high-temperature operation, the present ReRAM shows promise for high-temperature applications.
5:15 PM - F8.9
Simulation of Resistance Switching Behavior in Oxide Thin Film Heterostructures.
Sukwon Choi 1 , Joanna Meador 1 , Kevin Jiang 1 , Rozana Hussin 2 , Mohammad Noman 2 , Paul Salvador 1 , Marek Skowronski 1 , James Bain 2 Show Abstract
1 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Many oxides are known to show resistance switching behavior when they are made in metal-insulator-metal heterostructures. They have gained much attention as candidates for data storage. However, the physical theory that explains this behavior is still unclear. A common feature of many models is oxygen vacancy migration, although no quantitative description has been presented. In this work, we have developed a quantitative model that includes mobile oxygen vacancies.First, experiments were done with metal/Cr-doped SrZrO3/SrRuO3 structures. The effects of the Cr-content and the thickness of the SrZrO3 film were examined by their current-voltage characteristics (IVCs). The results provided guidance to develop a switching model. From the IVCs, space-charge-limited current (SCLC) was identified as the dominating electrical transport mechanism for both resistance states. The two coefficients in the SCLC equation transitioned between two values upon the switching event. Also, a steep increase of resistance with respect of thickness was observed. For high Cr-doping levels, R∝t3 was observed, while for low Cr-doping levels an even stronger relationship was observed.Next, IVCs of SCLC were simulated using the commercial TCAD software. A one-dimensional model was developed with the two contacts designed to behave as carrier-injecting electrodes, providing free electrons (or holes) into the SrZrO3. Simulated IVCs showed good agreement with SCLC. Simulations made with a series of doping levels and thicknesses provided a plausible explanation for experimental observations made earlier; however, no switching was observed.Finally, mobile donors (oxygen vacancies) were introduced into the switching model and simulations were performed. Donor motion was allowed only when the electric field inside the functional layer reached a threshold value. The two electrodes were modeled as blocking interfaces for donors, such that the total donor concentration was conserved in the functional layer. Numerical calculations of donor motion were made by iterating between matlab, to calculate new donor profiles, and TCAD, to calculate electrical potentials. Iterations were performed until a converged distribution was obtained at each applied voltage. Although donor motion occurs in these simulations, no switching event was observed in the IVC when the only driving force for donor motion was the electric field. The simulations were then modified to include an effective chemical driving force (chemical field) that would emulate the potential difference between the metal electrode and the functional layer. When donor motion was driven by the combined fields, switching in the IVC was observed, as will be discussed. The simulation results suggest that a model combining chemical and electric fields may ultimately yield a quantitative description of the resistance switching phenomenon.
5:45 PM - F8.11
Nanostructured, Non-volatile Charge Trap Flash Memory Devices Based on Layer-by-layer Assembled Functional Nanoparticle/polyelectrolyte Multilayers.
Inpyo Kim 1 , Jinhan Cho 1 , Jang-sik Lee 1 , Jeongju Park 1 , Chiyoung Lee 1 , Yong-mu Kim 1 , Hyunjung Shin 1 , Jaegab Lee 1 , Frank Caruso 2 Show Abstract
1 School of Advanced Material Engineering, Kookmin University, Seoul Korea (the Republic of), 2 Centre for Nanoscience and Nanotechnology, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Melbourne, Victoria, Australia
We present a novel and versatile approach for preparing a flash memory device composed of nanostructured functional nanoparticle (NP) / polyelectrolyte (PE) multilayer films. Anionic NPs were used as the charge storage elements, and polyelectrolyte multilayers deposited onto hafnium oxide (HfO2))-coated silicon substrates, formed the insulating layers. A blocking oxide layer (HfO2) and platinum were sputter coated onto the PE/NP multilayers to complete device fabrication. The tunneling oxide layer thickness was optimized by varying it between 0.9 to 1.9 nm, as followed by recording capacitance-gate voltage (C-V) curves of the multilayered devices. We also investigated the effect of increasing the number of PE and NP layers on memory performance. The (PE/NP) device improved memory window according to increasing the number of layer. The reported approach offers new opportunities to prepare nanostructured PE/ NP -based memory devices with tailored performance.