Symposium Organizers
Arthur H. Edwards Air Force Research Laboratory
P. Craig Taylor University of Utah
Jon Maimon Ovonyx, Inc.
Alex Kolobov National Institute of Advanced Industrial
Science & Technology
H1: GeSbTe Materials Properties I
Session Chairs
Wednesday PM, April 19, 2006
Room 3012 (Moscone West)
9:30 AM - **H1.1
Parallels and Contrasts in the Properties of Ge-Sb-Te Phase Change Alloys and Chalcogenide Glasses.
Stephen Bishop 1 3 , John Abelson 2 3 , Bong-Sub Lee 2 3 , Min-Ho Kwon 4 , Stephanie Bogle 2 3 , Ying Xiao 2 3 , Simone Raoux 5 , Ki-Bum Kim 4 , Byung-ki Cheong 6 , P. Taylor 7 , Heng Li 8
1 Department of Electrical & Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 The Coordinated Sciences Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Department of Materials Science & Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 4 School of Materials Science & Engineering, Seoul National University, Seoul Korea (the Republic of), 5 , IBM Almaden Research Center, San Jose, California, United States, 6 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 7 Physics Department, Colorado School of Mines, Golden, Colorado, United States, 8 Physics Department, University of Utah, Salt Lake City, Utah, United States
Show AbstractWe are investigating the fundamental optical, electronic, and structural properties of the Ge2Sb2Te5 (GST) phase-change alloy that is used as the active medium in rewritable optical memory disks, and in current-switched phase-change random access memories now under development. Of particular interest is the micro- or nano-structural basis for the rapidly reversible, thermally-induced atomic scale rearrangements that underlie the phase change phenomenon. Our studies are informed by the history of previous investigations of more stable, bulk and thin film chalcogenide glasses. Accordingly, we will briefly review the evolution of our understanding of these materials, focusing on: (1) optically induced defects and structural modifications such as photodarkening and their relationship to possible medium range order (MRO); (2) the critical role of materials preparation conditions; and (3) parallels and contrasts in the developing understanding of the phase-change chalcogenide alloys. Optical absorption spectroscopy, spectroscopic ellipsometry (SE), temperature dependent electrical conductivity, photoconductivity, and fluctuation electron microscopy are among the techniques we are applying to the study of thin films of GST fabricated at collaborating institutions. Previously, we reported [1] the unambiguous determination of the optical band gaps of amorphous, fcc and hexagonal phases of GST by proper analyses of optical data (SE, IR absorption spectra, and photoconductivity). Our investigation of the effects of prolonged illumination by above-bandgap light on the optical properties of amorphous GST revealed a pronounced photo-oxidation effect. The optically-induced redshift in the band gap (photodarkening) that is characteristic of a broad range of chalcogenide glasses was not observed in amorphous GST. The implications of these studies for inferred structural differences between amorphous GST and other chalcogenide glasses will be discussed. Similarly, the results of our investigation of the temperature dependence of the electrical conductivity and photoconductivity of GST below room temperature will be compared with the corresponding properties of conventional chalcogenide glasses. The technique of fluctuation electron microscopy (FEM), which is carried out in the TEM, has been used to probe the existence of nanometer-scale structural order in the amorphous phase. FEM is directly sensitive to three- and four-atom position correlations within the coherent probe of 1-4 nm diameter, whereas diffraction is only a two-atom correlation. We find that all as-deposited Ge2Sb2Te5 films contain nanoscale order, in some cases crystallites of 2-4 nm diameter and in other cases finer-scale order. Using a scanning TEM to perform FEM, we statistically filter out the regions that contain nanocrystallites and extract the characteristic size of the order in the matrix. [1] Lee et al., J. Appl. Phys. 97, 093509 (2005).* Supported by NSF Grant Number DMR-0412939
10:00 AM - H1.2
Electronic and Atomic Structure of Chalcogenide Phase Change Material Ge2Sb2Te5.
John Robertson 1 , Ka Xiong 1
1 Engineering, Cambridge University, Cambridge United Kingdom
Show AbstractThe electronic structure of the phase change chalcogenide Ge2Sb2Te5 has been calculated by the local density approximation (LDA) methods for various model structures, those representing the glassy and crystallised states. Both have a band gap of order 0.4 eV in LDA, which is an under-estimate. We use various periodic model structures similar to those of Kolobov et al [1], but with smaller cells. The crystalline rock salt phase is found to be the more stable. This ordered structure maximises the resonant bonding, due to long-range order [2]. This phase possesses vacancies on the Ge/Sb sublattice [3]. However, these should not be regarded as electronically defects. The states near the band gap are not localised on the vacancies. They are purely structural features required to form an electronic closed shell. Like the O ‘vacancies’ in the bixbyite structure, they do not give rise to gap states. Nor do they increase diffusion. On the other hand, the amorphous phase has only short-range order, and no resonant bonding. A structure with 4-fold bonded Ge [1] is found to be metastable.1.A Kolobov et al, Nature Materials 3 703 (2004)2.G Lucovsky, R M White, Phys Rev B 8 660 (1973)3.N Yamada et al, J App Phys 88 7020 (1991)
10:15 AM - H1.3
Local atomic Order and Optical Properties in Amorphous and Laser-crystallized Phase-change Materials.
Wojciech Welnic 1 2 , Silvana Botti 2 , Lucia Reining 2 , Matthias Wuttig 1
1 , I. Physikalisches Institut IA, RWTH Aachen, Aachen Germany, 2 Laboratoire des Solides Irradies, Ecole Polytechnique, Palaiseau France
Show Abstract10:30 AM - H1: GSTMP
BREAK
H2: GeSbTe Materials Properties II
Session Chairs
Wednesday PM, April 19, 2006
Room 3012 (Moscone West)
11:00 AM - **H2.1
Crystallographic Studies on the Roles of the Third and Forth Elements in the Typical Phase-Change Materials, Ge-Te and Sb-Te.
Noboru Yamada 1 , Toshiyuki Matsunaga 2 , Kouichi Kifune 3 , Yoshiki Kubota 4 , Rie Kojima 1
1 AV Core Technology Development Center, Matsushita Electric Industrial Co., Ltd., Moriguchi Japan, 2 Characterization Technology Group, Matsushita Technoresearch Inc., Moriguchi Japan, 3 Faculty of Liberal Arts and Sciences, Osaka Prefecture University, Sakai Japan, 4 Graduate School of Science, Osaka Prefecture University, Sakai Japan
Show AbstractRecent phase-change amorphous alloy films for memory devices are characterized by the explosive high-speed crystallization when they are once heated up above the critical temperatures, while they are kept stable for long period at room temperature. Generally, these properties are characteristically found in Te-based multinary alloys with three or more components. Here, we will discuss the roles of the third and fourth elements through the crystallographic studies using synchrotron radiation at SPring-8. At first, we have carried out the Rietveld analyses for the laser crystallized GeTe and cubic-GeSbTe films, and found out that the cubic-GeSbTe had distinctly larger atomic vibrations as compared with GeTe. Next, we have investigated the crystal structure of Sb-Te films in the Sb-rich region of the δ-phase, and successfully identified several compounds, Sb2nTe3 (n:integer), with extremely long period stackings[1]. Matsunaga et al. have already clarified that a Sb-Te film in the δ-phase takes a pseudo-cubic structure by adding small amounts of Ag and In[2]. These results let us conclude that the third or forth elements work to increase the cystallization speed; i.e., in the former case, they work to increase the atomic vibrations in the crystal phase at high temperature, and in the latter case, they work to simplify the crystal structure and to make the atomic distribution random and highly isotropic. References: 1. K. Kifune, Y. Kubota, T. Matsunaga and N. Yamada, Acta Cryst. (2005). B61, 492.2. M. Matsunaga, T. Umetani and N. Yamada, Phys. Rev. B, 64 (2001) 184116.
11:30 AM - H2.2
Nanometer–Scale Order in Amorphous Ge2Sb2Te5 Phase Change Material.
Min-Ho Kwon 1 , Bong-Sub Lee 2 4 , Stephanie N. Bogle 2 4 , John R. Abelson 2 4 , Stephen G. Bishop 3 4 , Simone Raoux 5 , Heng Li 6 , P. Craig Taylor 7 , Ki-Bum Kim 1 , Byung-Ki Cheong 8
1 School of Materials Science & Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Department of Materials Science & Engineering , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 4 The Coordinated Sciences Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 Department of Electrical & Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 5 , IBM Almaden Research Center, San Jose, California, United States, 6 Physics Department, University of Utah, Salt Lake City, Utah, United States, 7 Physics Department, Colorado School of Mines, Golden, Colorado, United States, 8 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show Abstract11:45 AM - H2.3
In-situ Growth of Ge2Sb2Te5 Nanoparticles Grown by Pulsed Laser Deposition and their Phase-change Electrical Characteristics for Memory Applications.
Hyeran Yoon 1 , William Jo 1
1 , Department of Physics and Division of Nanosciences, Ewha Womans University, Seoul Korea (the Republic of)
Show AbstractPhase change Ge-Sb-Te (GST) nanoparticles have been in-situ synthesized by a pulsed laser ablation method. Energy dispersive x-ray analysis is performed to look into chemical composition of the GST nanoparticles. Scanning and transmission electron microscopy are used to image local structure and phase formation of the nanoparticles. Fourier transformed analysis of the electron microscopic images shows a face centered cubic structure with the Ge2Sb2Te5 phase and it is confirmed that lattice parameter of GST nanoparticles is about 6Å which is in good agreement with the bulk value. The local structure of GST nanoparticles has been examined by extended x-ray absorption fine structure spectroscopy. Distance of Ge atoms in nearest and next-nearest neighbors is carefully analyzed and compared with others' results. Effect nitrogen doping into GST nanoparticles is briefly discussed. Current-voltage characteristics of the GST nanoparticles are examined in a metal-dot capacitor structure to see phase-dependent resistance effects. This measurement suggest a potential application for high density phase-change random access memories with low writing current.
12:00 PM - H2.4
Photoconductivity and Photo-oxidation in Ge2Sb2Te5 Phase Change Material.
Bong-Sub Lee 1 3 , Ying Xiao 1 3 , Stephen Bishop 2 3 , John Abelson 1 3 , Simone Raoux 4 , Min-Ho Kwon 5 , Ki-Bum Kim 5 , Byung-ki Cheong 6 , Heng Li 7 , P. Taylor 8
1 Department of Materials Science & Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 The Coordinated Sciences Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Department of Electrical & Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 4 , IBM Almaden Research Center, San Jose, California, United States, 5 School of Materials Science & Engineering, Seoul National University, Seoul Korea (the Republic of), 6 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 7 Physics Department, University of Utah, Salt Lake City, Utah, United States, 8 Physics Department, Colorado School of Mines, Golden, Colorado, United States
Show Abstract12:15 PM - H2.5
Structural and Optical Properties of Amorphous Ge2Sb2Te5
Heng Li 2 , T. Ju 2 , T. Herring 2 , P. Taylor 1 2 , D. Williamson 1 , M. Nelson 3 , C. Inglefield 3
2 Physics, University of Utah, Salt Lake City, Utah, United States, 1 Physics, Colorado School of Mines, Golden, Colorado, United States, 3 Physics, Weber State Univ, Ogden, Utah, United States
Show AbstractThe optical and structural properties of amorphous sputtered films of Ge2Sb2Te5 depend strongly on the preparation conditions. Films grown at higher growth rates exhibit greater local strains as indicated by the slope of the optical absorption in the exponential “band-tail” region, but these films also incorporate smaller densities of oxygen impurities. At slower growth rates the band-tail slopes are sharper (smaller local strains) but there is greater oxygen incorporation. We will discuss several experiments that suggest that the local strain relief in the films grown at slower growth rates is due to a greater ability of the atoms to rearrange on the growing surface and not to increased oxygen incorporation. Small angle x-ray scattering experiments show that the films exhibit small elliptical “voids” with long axes perpendicular to the growing surface. The approximate dimensions of these voids are 3 x 20 nm. These films can be switched optically with little change in surface topography as measured by atomic force microscopy. Electron spin resonance measurements indicate that paramagnetic defects exist in some films but are either absent or below the detection limit (~ 1018 cm-3) in most films. The implications of these results for the switching mechanisms will be discussed.
H3: Chalcogenide Phase Transition I
Session Chairs
Wednesday PM, April 19, 2006
Room 3012 (Moscone West)
2:30 PM - **H3.1
Local Structure of Liquid GeTe via Ab Initio Molecular Dynamics Simulation
James Chelikowsky 1
1 Institute for Computational Engineering and Sciences, University of Texas, Austin, Texas, United States
Show AbstractI will examine the local atomic order as well as some dynamic properties of the semiconducting liquid GeTe via ab initio molecular dynamics simulations. My simulations are based on interatomic forces derived from pseudopotentials constructed within density functional theory. At the melting temperature, the Peierls distortion responsible for the lower-temperature crystal phase is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder in the Ge environment determines the eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms.
3:00 PM - H3.2
Photo-induced Metastable State of S8 Clusters in Liquid Sulfur.
Yoshifumi Sakaguchi 1 , Kozaburo Tamura 1
1 Graduate School of Engineering, Kyoto University, Kyoto, Kyoto, Japan
Show AbstractRecently we have found that the long-lived products are produced for a weak illumination of a pulsed laser to liquid sulfur composed of S8 rings. Munejiri et al. performed the first-principles molecular-dynamics simulations for liquid sulfur and examined the effect of photo-excitation. They have found that a bond in one of the S8 rings is easily broken and consequently the S8 rings turns out to be the S8 chain after an electron excitation. After the electron excitation is stopped, the S8 chain does not reconstruct the S8 rings; instead a 'tadpole' structure is formed. It is considered that the S8 tadpole is the long-lived product which is experimentally observed. In the S8 tadpole, there is a pair of a threefold-coordinated atom and a singly coordinated atom, which is considered to be the intimate valence alternation pair (IVAP) proposed by Kastner et al. to explain the defects in chalcogenide glasses. The S8 tadpole changes to be the S8 chain and the chain changes to be the S8 tadpole again with time. There are rearrangements of the bonding and another type of tadpole structure is created in the process. This reminds us the sequent bond breaking and rearrangement observed in irradiated chalcogenide glasses. We discuss such dynamical change of S8 cluster in connection with the change appeared in chalcogenide glasses.
3:15 PM - H3.3
Fast Photodarkening in Amorphous and Liquid Chalcogenide.
Yoshifumi Sakaguchi 1 , Kozaburo Tamura 1
1 Graduate School of Engineering, Kyoto University, Kyoto, Kyoto, Japan
Show AbstractWe have measured the photo-induced optical absorption spectra of amorphous and liquid As2Se3 in the high absorption region using a specially designed optical cell. Photodarkening has been observed in the nanosecond and microsecond time domain for a weak laser-pulse with the intensity of 1.6mJ/cm2 in the high absorption region. The decay is quite slow so that the change is observed even at 1 ms. The decay time decreases with increasing temperature but the darkening is still observed in the microsecond time domain for the liquid at 703K. The darkening suggests the structural change since the darkening is related to the interband transitions. This photostructural change is the quick structural response for the photo-excitation of lone-pair electrons and it is regarded as the pre-photostructural change before the occurrence of the ordinary photostructural change observed for the steady-state irradiation. The slow decay observed in the liquid state shows that there is a large structural unit in amorphous state and the unit is preserved even in the liquid state. The photoinduced effect must spread all over the unit.
H4: Chalcogenide Phase Transition II
Session Chairs
Wednesday PM, April 19, 2006
Room 3012 (Moscone West)
4:00 PM - **H4.1
Understanding Structural Changes in Phase Change Memory Alloys.
Paul Fons 1 , Dale Brewe 2 , Alexander Kolobov 1 , Ed Stern 3 2 , Junji Tominaga 1
1 Center for Applied Near-Field Optics Research, Nat. Inst. of Adv. Ind. Sci. & Tech., Tsukuba, Ibaraki, Japan, 2 PNC-CAT Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States, 3 Physics Department, University of Washington, Seattle, Washington, United States
Show Abstract4:30 PM - **H4.2
Why phase-change Materials work: An EXAFS Investigation of Ge-Sb-Te Alloys.
Michael Paesler 1 , S. Agarwal 3 , D. Baker 1 , G Lucovsky 1 , P. Taylor 2
1 Physics, North Carolina State University, Raleigh, North Carolina, United States, 3 , Indian Institute of Technology, Kanpur India, 2 , University of Utah, Salt Lake City, Utah, United States
Show Abstract5:00 PM - **H4.3
Unravelling the Interplay of Local Structure and Physical Properties in Phase-change Materials.
Matthias Wuttig 1 , Mengbo Luo 1 , Ralf Detemple 1 , Christoph Steimer 1 , Wojciech Welnic 1
1 , I. Physikalisches Institut IA, RWTH Aachen, Aachen Germany
Show Abstract5:30 PM - H4.4
Chemical Structure and Switching Behavior of Ultrathin GeSbTe Phase Change Films.
Frances Houle 1 2 , Simone Raoux 1 2 , Robert Shelby 1 2 , Andrew Kellock 1 2 , Vaughn Deline 1 2 , Charles Rettner 1 2
1 , IBM Almaden Research Center, San Jose, California, United States, 2 IBM/Infineon/Macronix PCRAM Joint Project, IBM Almaden Research Center, San Jose, California, United States
Show Abstract5:45 PM - H4.5
What Makes Ge-Sb-Te Alloys Materials of Choice for Phase-change Optical Data Storage.
Alexander Kolobov 1 2 , Paul Fons 1 , Junji Tominaga 1
1 CANFOR, AIST, Tsukuba Japan, 2 LPMC, CNRS UMR 5617, Universite Montpellier II, Montpellier France
Show AbstractPresent-day multimedia strongly relies on re-writable phase-change optical memory. We have found that Ge2Sb2Te5 (GST), the material of choice in DVD-RAM, does not possess the rock-salt symmetry but Ge and Sb atoms are displaced from center of the cell. Amorphisation of Ge-Sb-Te results in a significant shortening of covalent bonds and a decrease in the mean-square relative displacement concomitant with a drastic change in short-range order. The order-disorder transition is primarily due to a flip of Ge atoms from octahedral positions into tetrahedral positions without rupture of strong covalent bonds. It is this nature of the transformation that ensures large changes in reflectivity, fast disk performance and repeatable switching over millions of cycles.
Symposium Organizers
Arthur H. Edwards Air Force Research Laboratory
P. Craig Taylor University of Utah
Jon Maimon Ovonyx, Inc.
Alex Kolobov National Institute of Advanced Industrial
Science & Technology
H5/G6: Joint Session: Phase Change Memories I
Session Chairs
Stephen Hudgens
Jon Maimon
Thursday AM, April 20, 2006
Room 3010 (Moscone West)
9:00 AM - **H5.1/G6.1
OUM Nonvolatile Semiconductor Memory Technology Overview
Stephen Hudgens 1
1 , Ovonyx, Inc., Sunnyvale, California, United States
Show AbstractOUM (Ovonic Unified Memory), also called PCRAM (phase-change RAM) or CRAM (chalcogenide RAM) is a nonvolatile semiconductor memory technology being developed by Ovonyx, Inc. in a number of industrial joint development programs. OUM technology is based on an electrically initiated reversible amorphous to crystalline phase change process in multi-component chalcogenide alloy materials similar to those used in rewriteable optical disks. Fundamental processes in OUM devices, manufacturing technology, and progress towards commercialization of the technology will be reviewed.
9:30 AM - H5.2/G6.2
Thermal Analysis and Structural Design of Phase Change Random Access Memory.
Rong Zhao 1 , Ler Ming Lim 1 , Luping Shi 1 , Hock Koon Lee 1 , Hongxin Yang 1 , Tow Chong Chong 1
1 , Data Storage Institute, Singapore Singapore
Show Abstract9:45 AM - H5.3/G6.3
On the Kinetic Characteristics of the Set Process in a Non-volatile Phase-change Memory.
Dae-Hwan Kang 1 , Byung-ki Cheong 1 , Jeung-hyun Jeong 1 , Taek Lee 1 , In Kim 1 , Won Kim 1 , Ki-Bum Kim 2
1 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractPhase-change memory device characteristics concerning reset and set states/processes are liable to have causal relations with the kinetics of these phase changes and the resulting phase structures. Accordingly, characterization and understanding of the relations are supposed to be indispensable to design of materials and device structures for better memory performances. This seems particularly true with regard to the set process that involves more complex and sluggish material responses than the reset, thus representing an arena of important technical and scientific issues in research and development of materials and structures for phase change memory devices. In this study, a time-resolved analysis is carried out on the kinetic nature of the set process in a non-volatile phase-change memory device by combined analyses of set voltage waveforms and low-field resistances. As it turns out, the progress of a set process may be measured in terms of three characteristic times in sequence i.e., threshold switching time t_th, apparent incubation time t_app,inc, and complete set time t_set. These characteristic times are supposed to demarcate, in some measure, different stages of crystallization in the phase-change material during a set process. Based on a qualitative model of the set process, it is suggested that t_th is required to form cold filaments in an amorphous matrix and t_app,inc – t_th and t_set – t_app,inc correspond to true incubation time t_inc for nucleation of crystallites and time for percolation t_per of growing nuclei in the heated filaments, respectively. Each of these times has a strong dependence on input pulse voltage, representing the kinetic nature of threshold switching and crystallization processes correspondingly. t_th is found to have an exponentially decaying dependence and this might be related to the decreasing capacitance of an amorphous phase-change material with approaching threshold switching. Meanwhile, each of t_inc and t_per shows a pseudo-linear dependence but with a decreasing and an increasing tendency with input voltage respectively. These results might be explained by the enhanced tendency toward faster nucleation of a lower density of crystalline nuclei at a higher input voltage (or higher temperature). The methodology advanced in the present work is being developed further for practical use in design of high-speed phase change materials and cell structures of memories with better performances.
10:00 AM - H5.4/G6.4
Multi-level Operation in Multi-layered Structure of Ge2Sb2Te5 and TiN.
Hyun-Goo Jun 1 , Dong-Ho Ahn 1 , Tae-Yon Lee 2 , Dongbok Lee 1 , Ki-Bum Kim 1
1 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Nano Systems Institute - National Core Research Center, Seoul National University, Seoul Korea (the Republic of)
Show AbstractMulti-layered structure comprising alternately deposited films of TiN and Ge2Sb2Te5 is investigated. From the multi-layered structure, it is possible to implement the multi-level operation simply by stacking metal, TiN in this case, and Ge2Sb2Te5 in resistive region of phase change random access memory (PRAM). As the metal layer, TiN is selected because of its chemical stability at interface with Ge2Sb2Te5 and relatively high resistivity for Joule heating. PRAM devices of such structure are fabricated by direct current (DC)-sputtering of Ge2Sb2Te5 and TiN layers. DC current-voltage (I-V) curves of the device were measured with the applied current in the range of 0 to 100 mA. The I-V curve on multi-layered structure exhibits several threshold switching, in contrast to the single switching observed in single layered structure. It is found that the number of switching is equal to that of Ge2Sb2Te5 layers. Furthermore I-V characteristic after each threshold switching is preserved. The differences of resistance measured after each switching and before the switching are big enough to realize multi-level device operation. I-V curves are measured on the other asymmetric double-layered structures, whose Ge2Sb2Te5 layers have different thickness. Interestingly, device which has thicker Ge2Sb2Te5 layer toward positive electrode has higher threshold voltage. It implies that electrical conduction behavior in Ge2Sb2Te5 has directional dependence. The high resistive intermediate TiN layers may modify the temperature profile during the set operation, and its interfaces with Ge2Sb2Te5 may serve as heterogeneous nucleation sites for crystallization. Thus, the preserved I-V characteristic after each switching implies that permanent changes like partial crystallization occur sequentially in each Ge2Sb2Te5 layer. As a result, discrete resistance levels are distinguished by the crystallized portion. Detailed discussion on the proposed device structure, the conduction behavior, and relation between the observed I-V characteristics and microstructure will be given.
10:15 AM - H5.5/G6.5
Investigation on Ultra-high Density and High Speed Non-volatile Phase Change Random Access Memory (PCRAM) by Material Engineering.
E.G. Yeo 1 , L.P. Shi 1 , R Zhao 1 , T.C. Chong 1
1 , Data Storage Institute, Singapore Singapore
Show Abstract11:00 AM - **H5.6/G6.6
Modeling Considerations for Phase Change Electronic Memory Devices.
Guy Wicker 1
1 , Ovonyx, Inc., Rochester Hills, Michigan, United States
Show AbstractThis presentation will review the status of modeling phase change electronic memory devices. Beginning with a historical look at early modeling efforts in optical and electronic phase change memories, recent modeling efforts will be reviewed based on recently published results. Then the difficulties encountered with modeling of these devices will be discussed along with a discussion of the direction modeling needs to take to be useful in improving chalcogenide alloy phase change memory devices, including mechanical properties and alloy phase segregation.
11:30 AM - H5.7/G6.7
An Analysis of the Operation Characteristics of PRAM and Development of a New Multi–bit Structure through 3-D Transient Simulation Modeling.
YoungWook Park 1 , Kyung-Woo Yi 1
1 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show Abstract A simulation model of a PRAM device was developed in order to evaluate the thermal and electric characteristics of its set and reset operations. Using this model, we investigated the effects of unit cell structure, contact area and material properties on the thermal and electric characteristics of memory switching. In addition, time-dependent variations of temperature and electric potential during phase transitions were investigated using a three dimensional transient simulation model. According to our results, the structure and contact area of the unit cell had a decisive influence on the transition volumes of the phase change material. Also, smaller phase transition volumes resulted in more efficient characteristics for the device in terms of power consumption. On the basis of our simulation model, we developed a new conceptual multi-bit structure. Unlike the on and off motions of existing structures, the processes of the multi-bit structure can be divided into three or more operations with multiple states of resistance. This multi-bit system is based on our results showing the contact area’s considerable influence on the thermal and electrical characteristics of PRAM devices.Keywords: PRAM, phase change, SET & RESET, contact area, multi-bit structure
11:45 AM - H5.8/G6.8
Thermal Conductivity of Phase Change Material Ge2Sb2Te5
Ho-Ki Lyeo 1 2 , David Cahill 1 2 , Min-Ho Kwon 5 2 , Bong-Sub Lee 1 3 , John Abelson 1 3 , Stephen Bishop 3 4 , Ki-Bum Kim 5 , Byung-ki Cheong 6
1 Materials Science and Engineering, University of Illinois, Urbana, Illinois, United States, 2 Frederick-Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois, United States, 5 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 3 The Coordinated Sciences Laboratory, University of Illinois, Urbana, Illinois, United States, 4 Electrical & Computer Engineering, University of Illinois, Urbana, Illinois, United States, 6 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractGe2Sb2Te5 (GST), a material that can change structural phases between an amorphous and two crystalline states, undergoes phase transformations by optical absorption or joule heating. Practical utilization of this material associated with the phase change at a small scale depends crucially on the thermal transport in GST and between GST and surrounding materials. We hereby explore the relationship between the phase transformations and the thermal conductivity of Ge2Sb2Te5 films (ΛGST). The thermal conductivity of GST films is measured using time-domain thermoreflectance. First, we measure the thermal conductivity ΛGST for three films that include an as-deposited amorphous (a-GST) film; the other two films are annealed for 20 min at fixed temperatures of 180 °C and 360 °C to form the cubic f.c.c. (c-GST) and hexagonal (h-GST) phases, respectively. For the three films, we obtain the values of Λa-GST ≈ 0.19 Wm-1K-1, Λc-GST ≈ 0.57 Wm-1K-1, and Λh-GST ≈ 1.58 Wm-1K-1. We then measure the thermal conductivity as a function of the film temperature when the film is heated at a rate of 3 K/min. The measured conductivity ΛGST undergoes a discontinuous increase at T ≈ 130 °C and a smooth change at ≈ 340 °C. The first abrupt change appears upon crystallization, i.e. a- to c-GST transformation, and the second change appears with c- to h-GST transformation. The values of Λc-GST encompass the range of 0.45 - 0.95 Wm-1K-1 at 130 °C < T < 310 °C. Similarly, Λh-GST includes the range of 1.4 - 1.53 Wm-1K-1 at 340 °C < T < 400 °C while the values of Λa-GST are essentially constant. The thermal transport at a- and the early c-GST phases can be explained by a random walk of vibrational energy (minimum thermal conductivity). By contrast, in the h-GST phase, the thermal conduction is largely due to electronic contribution; the contribution deduced from the Wiedemann-Franz law accounts for ~70% of the measured Λh-GST. Finally, we measure the thermal conductivity of spots crystallized by laser processing as functions of the energy density and the number of applied laser pulses. The measured thermal conductivity of the rapidly transformed spots induced by laser pulses is lower and closer to the minimum thermal conductivity than that of the thermally annealed one. This implies that the rapid crystallization leads to a more disordered crystalline structure than the annealed one does.
12:00 PM - H5.9/G6.9
Investigation on the Enhanced Switching Reliability of a Phase Change Memory Device with an Oxidized TiN Electrode.
Dae-Hwan Kang 1 , In Kim 1 , Jeung-hyun Jeong 1 , Byung-ki Cheong 1 , Dong-Ho Ahn 2 , Dongbok Lee 2 , Hyun-Mi Kim 2 , Ki-Bum Kim 2
1 Thin Film Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractFluctuations (or drifts) in switching voltages such as programming set/reset voltages and threshold voltage pose serious obstacles to the reliable operation of electrical phase change memory devices. Using a phase change memory device having a GeSb2Te4 phase change material and TiN electrode, these fluctuations are demonstrated to result from device resistances varying with programming cycles, which appear to be caused by variations in contact resistance at the interface between the phase-change material and the TiN electrode as well as by inhomogeneous phase distribution across the GeSb2Te4 layer due to lack of temperature uniformity. Oxidation of a TiN electrode surface (via thermal annealing at 350^oC under an air environment with N2 gas flow) is very effective in the reduction of fluctuations in device resistances and switching voltages hence the resulting increase in the programming cycles by two orders of magnitude. From a high resolution transmission electron microscopy, the oxidized surface was shown to consist of a TiOx (x~1.5) layer with nm-sized Ti2O3 crystallites which is presumed to yield enhanced stability of the device by the following two effects. Firstly, Ge, Sb, and Te atoms would have stronger bonds to oxygen atoms than to nitrogen atoms by about 0.5 eV, thereby producing more robust interface. Accordingly, both contact resistance and its variation with programming cycles tend to be reduced significantly so as to have little influence on the device resistances and their fluctuations (interface effect). Secondly, thermally and electrically more resistive nature of the TiOx layer would tend to yield, by enhanced generation and confinement of joule heat, more uniform temperature distribution across the phase-change material layer, rendering possibly a more homogenized material in terms of phase and composition hence steadier sheet resistances with programming cycles (bulk effect).
12:15 PM - H5.10/G6.10
Formation of the Ultra Small Programming Volume of Phase Change Random Access Memory by Phase Segregation of Ge2Sb2Te5-SiO2 Mixed Layer.
Tae-Yon Lee 1 , Dongbok Lee 2 , Dong-Ho Ahn 2 , Hyungoo Jun 2 , Ki-Bum Kim 1 2
1 Nano Systems Institute - National Core Research Center, Seoul National University, Seoul Korea (the Republic of), 2 School of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show Abstract12:30 PM - H5.11/G6.11
Oxygen Contamination in Sb2Te3.
John Boyd 1 , Arthur Edwards 1
1 Electronics Foundations Group, AF Research Lab Space Vehicles Directorate, Albuquerque, New Mexico, United States
Show AbstractH6/G7: Joint Session: Phase Change Memories II
Session Chairs
Thursday PM, April 20, 2006
Room 3010 (Moscone West)
2:30 PM - **H6.1/G7.1
Localized Light Focusing and Super Resolution by Chalcogen Thin Film.
Junji Tominaga 1 , Paul Fons 1 , Alexander Kolobov 2 1
1 Center for Applied Near-Field Optics Research, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan, 2 Laboratoire de Physicochimie la Matiere Condensee, University of Montpellier, Montpellier France
Show AbstractRecently, several chalcogenide thin films have been used as a digital versatile disc random access memory (DVD-RAM) and a recording layer in a rewritable digital versatile disc (DVD-RW). Among them, especially, Ge2Sb2Te5 and Ag10In4Sb58Te28 are the typical materials for rewritable use. In DVD recording and erasing system, so-called “first phase transition:” phase transition between crystal and amorphous. Since 1998, our research team has investigated the optical near-field characteristics emerged from chalcogenide thin films. Although Sb is not a chalcogen, the pure thin film and the chalcogen compounds especially with Te show optical super resolution (SR) effect when a laser beam in focused on. The SR effect was first thought as an optical contrast due to the first phase transition. However, it has gradually been elucidated that the SR is induced by second phase transition between one crystalline phase and another, where a huge optical transition probably arises. Moreover, it is found that optical near-field components: higher spatial frequencies trapped at an optical aperture edge are focused within less than a 50 nm height normal to the film. We report the up-to-date topics of the light focusing, and the application to ultrahigh density optical data storage.
3:00 PM - H6.2/G7.2
Impact of Material Crystallization Characteristics on the Switching Behavior of the Phase Change Memory Cell.
Thomas Gille 1 2 , Ludovic Goux 1 , Judit Lisoni 1 , Kristin De Meyer 1 2 , Dirk Wouters 1
1 , IMEC, Leuven Belgium, 2 ESAT, KU Leuven, Leuven Belgium
Show Abstract3:15 PM - H6.3/G7.3
Kinetics of Optically-induced Crystallization and Structure of Ag-Sb-S Chalcogenide Films and its Potential Application.
Tomas Wagner 1 , Jan Gutwirth 1 , Milos Krbal 1 , Tomas Kohoutek 1 , Miloslav Frumar 1 , Petr Bezdicka 2 , Jan Pokorny 3
1 , University of Pardubice, Pardubice Czech Republic, 2 , Institute of Inorganic Chemistry, ASCR, , Rez u Prahy, 25068 Czech Republic, 3 , Department of Dielectrics, Institute of Physics, ASCR, , Praha, 18221 Czech Republic
Show Abstract3:30 PM - H6.4/G7.4
Phase-Change Dynamics of Eutectic GeSb Alloy.
Robert Shelby 1 2 , Martin Salinga 3 1 2 , Frances Houle 1 2 , Simone Raoux 1 2
1 , IBM Almaden Research Center, San Jose, California, United States, 2 IBM/Infineon/Macronix PCRAM Joint Project, IBM Almaden Research Center, San Jose, California, United States, 3 , Physikalisches Institut 1A der RWTH Aachen, Aachen Germany
Show AbstractPhase-change materials have enabled the implementation of rewritable optical storage technologies and are leading candidates for the development of advanced solid-state nonvolatile memories. Antimony-rich binary GeSb materials have been shown to exhibit both very fast transformation of the amorphous to crystalline state when the material is heated above its glass transition temperature but below its melting point, as well as good stability of the amorphous state.1 Thus, it may be attractive for applications that require both fast writing and erasure of data as well as long term data integrity. We have characterized the optical properties and investigated the crystallization dynamics of sputter-deposited films of near-eutectic composition Ge0.15Sb0.85 using an optical tester with nanosecond time resolution. As-deposited films exhibit an induction period of several microseconds before crystallization occurs, suggesting that nucleation of sites suitable for growth of larger crystalline areas is rather slow. Atomic force microscopy (AFM) observations of laser-heated spots are consistent with crystal growth from a small number of sites within the micrometer-diameter heated region. AFM also shows indications of an as yet poorly understood “pre-crystallization” transformation during this induction period, and there is evidence for changes in composition through the depth of the films during slow annealing. In annealed crystalline Ge0.15Sb0.85 films, amorphous spots are readily written via laser-melting with nanosecond laser pulses. When heated with the laser to an appropriate temperature below the melting point, these amorphous regions are re-crystallized on the nanosecond time scale. This behavior indicates a material characterized by slow nucleation but fast crystal growth, which is a favorable combination for applications to data storage.1. J. Solis and C.N. Afonso, “Ultrashort-laser-pulse-driven rewritable phase-change optical recording in Sb-based films,” Appl.Phys.A 76,331 (2003).
3:45 PM - H6.5/G7.5
Effect of Doping on the Properties of the Phase Change Materials SbTe and GeSb.
Simone Raoux 1 , Martin Salinga 1 2 , Jean Jordan-Sweet 3 , Qing Wang 1 4
1 IBM/Infineon Technologies/Macronix PCRAM Joint Project, IBM Almaden Research Center, San Jose, California, United States, 2 1. Physikalisches Institut 1A, RWTH Aachen, Aachen Germany, 3 IBM/Infineon Technologies/Macronix PCRAM Joint Project, IBM T. J. Watson Research Center, Yorktown Heights, New York, United States, 4 , San Jose State University, San Jose, California, United States
Show AbstractSolid state memory devices based on phase change materials (PCM) are a new promising non-volatile memory concept. The devices are based on the transition from the amorphous, high-resistivity phase to the crystalline, low-resistivity phase of the PCM by heating it via an electric current above its crystallization temperature Tx for a sufficiently long time. The reverse transition occurs by melt-quenching the PCM also induced by an electric current. The device state (set and reset) is indicated by the resistance which can be measured by a low current that does not lead to phase transitions. Tailoring of Tx is desirable for optimum device performance since Tx should be sufficiently above device operation temperature for thermal stability reasons, and well below the melting point to ensure well-defined set and reset currents. To evaluate which dopant will increase or decrease Tx for a given PCM we calculated the glass transition temperature Tg as a lower limit for Tx according to the model developed by Lankhorst [1]. The model is based on an empirical linear relationship between Tg and the enthalpy of atomization which is calculated by summing all individual bond enthalpies. Tg was calculated for various elemental combinations including Al and Cu doping of SbTe and GeSb. It was found that Al doping increase Tg for SbTe and GeSb. Few doping elements were found to reduce the Tg, one example was Cu doping of GeSb while Cu doping of SbTe increases Tg. Al and Cu doped Sb2Te and Ge15Sb85 thin films were deposited on Si and SiO2 coated Si substrates by co-sputtering from the corresponding elemental targets with Al concentrations up to 10% and Cu concentrations up to 30%. Time-resolved X-ray diffraction (XRD) was performed at beamline X-20C of the National Synchrotron Light Source. Samples were heated at a rate of 1K/s in a helium atmosphere and a linear-diode-array detector was used to record the diffracted peak intensities over a 2theta range of 15 degrees (which was selected according to the position of the strongest peaks for each alloy). It was found that Al doping increases Tx for Sb2Te and Ge15Sb85 while Cu doping increases Tx for Sb2Te and reduces Tx for Ge15Sb85 thin films as predicted by the calculations of Tg for these alloys. All doped alloys crystallize in a hexagonal phase similarly to the undoped alloys, with slight changes in film texture and lattice constant caused by the doping. Measurements of the resistivity as a function of temperature were performed on films on SiO2 coated Si substrates and agree with the XRD data. We have confirmed experimentally that the model for predicting changes of Tg as a function of doping for PCM can also be applied successfully to predict changes in Tx for a given PCM. This will be very useful in optimizing PCM for future applications in solid state memory devices. [1] M.H.R. Lankhorst, J. Non-Cryst. Solids 297 (2002) 210
4:30 PM - H6.6/G7.6
A Study on the Phase Transformation Behavior and Microstructures of N-doped Ge2Sb2+xTe5 Thin Films for the Application to Phase Change Memory Devices
Kihoon Do 1 , Byung ho Lee 1 , Jaesuk Kwon 1 , Dae-hong Ko 1
1 Ceramic Engineering, Yonsei University , Seoul Korea (the Republic of)
Show Abstract The crystallization behaviors and the microstructures of Ge2Sb2Te5-based thin films with the various antimony (Sb) and nitrogen (N) compositions were studied for different annealing temperatures. 100 nm thick N-doped Ge2Sb2+xTe5 films were deposited by DC magnetron sputtering on SiO2/Si substrates and annealed at various temperatures to investigate the crystallization behavior using the furnace in N2 ambient. According to the sheet resistance measurement and XRD analysis, the crystallization temperature increased with the increase of the additional Sb and N concentration. After the phase change to the crystalline state, the N-doped Ge2Sb2+xTe5 systems showed higher sheet resistance values than Ge2Sb2Te5 system. Comparing to the Ge2Sb2Te5 system, the metastable FCC phase of the N-doped Ge2Sb2+xTe5 systems was maintained even after annealing at higher temperatures. The phase transformation temperature from the FCC structure to the HCP structure also increased in the N doped-Ge2Sb2+xTe5 systems. We investigated the effects of the additional Sb and N on the Ge2Sb2Te5 system on the microstructural changes by the TEM and XRD analysis. The phase transformation temperature from FCC to HCP increased by the addition of Sb and N elements. The excess Sb atoms and the additional N in the N-doped Ge2Sb2+xTe5 system retard the grain growth and the atomic diffusion. Accordingly, the crystallization temperature increased and the sheet resistance of crystalline state increased in N-doped Ge2Sb2+xTe5 systems. The N-doped Ge2Sb2+xTe5 films are believed to be effective to retard the crystallization and the phase transformation to HCP structure.
4:45 PM - H6.7/G7.7
Nitrogen Incorporation in Ge2Sb2Te5 Films by N2+ Ion Implantation
YoungKuk Kim 2 1 , S. A. Park 1 , E. J. Jeong 3 , M.-H. Cho 1 , D. -H. Ko 3 , K. Jeong 2
2 IPAP, Yonsei University, Seoul Korea (the Republic of), 1 Nano Surface Group, korea Research Institute of Standards and Science, Daejeon Korea (the Republic of), 3 Dep. of Ceramic Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractThe phase transition properties of Ge2Sb2Te5 (GST) films after bombardment with 40 keV N2+ ions were investigated. Comparing the nitrogen incorporated GST films with a pure GST film, crystallization was suppressed more effectively in the N2+ implanted GST film than in a nitrogen co-deposited GST film; i.e., x-ray diffraction results showed that the intensities of the crystalline diffraction peaks were decreased and the FWHMs were broader than that of a pure GST film. This suppression of crystallization owing to the incorporation of nitrogen drastically improved the change in surface morphology and decreased the electrical conductivity of the film. A near edge x-ray absorption fine structure experiment and x-ray photoemission spectroscopy data demonstrated that the suppression in crystallization was due to the formation of Ge3N4 and interstitial N2 molecules. In particular, interstitial N2 molecules played a major role in the suppression of crystallization in N2+ implanted GST films.
5:00 PM - H6.8/G7.8
InGeSbTe Compounds as a New Phase Change Material for Low-power Operation of High Density PRAM.
Jin-Seo Noh 1 , Kijoon Kim 1 , Dong-Seok Suh 1 , Woong-Chul Shin 1 , Sang Mock Lee 1 , Eunhye Lee 1 , Hyo-Jeong Kim 1 , Youn-Seon Kang 1 , Yoonho Khang 1
1 Materials Center, Samsung Advanced Institute of Technology, Yongin-Si, Gyeonggi-Do Korea (the Republic of)
Show Abstract5:15 PM - H6.9/G7.9
Operating Current Reduction in Phase Change RAM by using Ge2Sb2Te5 nanoparticles
Eunhye Lee 1 , Dong-Seok Suh 1 , Kijoon Kim 1 , Jin-Seo Noh 1 , Woong-Chul Shin 1 , Hyo-Jeong Kim 1 , Youn-Seon Kang 1 , Yoonho Khang 1
1 Materials Center, Samsung Advanced Institute of Technology, Yongin-si Korea (the Republic of)
Show AbstractThe potential applications of phase change RAM (PRAM) utilizing fast and reversible phase change between amorphous and crystalline phases have driven extensive efforts to decrease the reset current and to improve the stability of the programmable part by using various methods. Since the electrical properties of PRAM have been significantly affected by its programmable area, the characteristics of the area is particularly important. The most extensively used phase change material in PRAM is Ge2Sb2Te5 (GST). Among the various kinds of GST structure, thin film GST has been the most extensively studied. Recently, the distinctive properties of nanometer-scale particles have been reported for various materials. The variety of materials includes metals, semiconductors, and insulators. Many chalcogenide nanoparticles have also been extensively studied. However, no systematic study has been done on the phase change of chalcogenide nanoparticles so far.In this paper, we present the synthesis of GST nanoparticles and the fabrication of devices by using the nanoparticles. The GST nanoparticles were grown by pulsed laser ablation. The pressure, carrier gas flow-rate and post-annealing were controlled to obtain stoichiometric nanoparticles with narrow size dispersion. The structural properties of GST nanoparticles have been characterized by scanning and transmission electron microscopy. The GST nanoparticles used to fabricate phase change memory cell. The fabricated nanoparticles were initially crystalline phase. By applying voltage pulse, the phase change from crystalline to amorphous phase (RESET) could be observed. The RESET current (Ireset) was determined by applying voltage pulse and measuring the current level at the same time. Ireset in the device using nanoparticles was much smaller by 2 or more order of magnitude comparing to the conventional T-shape PRAM. The mechanism of the low Ireset will be intensively discussed in the paper. We also have studied the reliability improvement by passivation of nanoparticle. The passivation improved write/erase endurance.
H7/G8: Joint Poster Session
Session Chairs
Friday AM, April 21, 2006
Salons 8-15 (Marriott)
9:00 PM - H7.1/G8.2
Ag-Sb-S Thin Films Prepared by RF Magnetron Sputtering and Their Properties.
Jan Gutwirth 1 , Tomas Wagner 1 , Petr Bezdicka 2 , Cestmir Drasar 3 , Miloslav Frumar 1 , Milan Vlcek 4
1 , University of Pardubice, Pardubice Czech Republic, 2 , Institute of Inorganic Chemistry, AS CR, , 25068 Rez near Prague Czech Republic, 3 , University of Pardubice, Department of Physics, , 53210 Pardubice Czech Republic, 4 , Joint Laboratory of Solid State Chemistry of University of Pardubice and Institute of Macromolecular Chemistry AS CR, 53210 Pardubice Czech Republic
Show Abstract9:00 PM - H7.10/G8.10
Preparation of Ge2Sb2Te5 thin film for Phase Change Random Access Memory by RF magnetron Sputtering and DC Magnetron Sputtering.
Shin Kikuchi 1 , Dong Oh 1 , Isao Kimura 1 , Yutaka Nishioka 1 , Koukou Suu 1
1 Institute for Semiconductor Technologies, ULVAC,Inc., Susono, Shizuoka, Japan
Show Abstract9:00 PM - H7.11/G8.11
Stack-Structured Phase Change Memory Cell for Multi-State Storage
Sangouk Ryu 1 , Kyujeong Choi 1 , Sungmin Yoon 1 , Namyeal Lee 1 , Seungyun Lee 1 , Youngsam Park 1 , Byoung-gon Yu 1
1 , Electronics and Telecommunications Research Institute, Daejeon Korea (the Republic of)
Show AbstractIn PRAM applications, the devices can be made for both binary and multi-state storage. The ability to attain intermediate stages comes either from the fact that some chalcogenide materials can exist in configurations that range from completely amorphous to completely crystalline or from designing device structure such a way that mimics multiple phase chase phenomena in single cell. We have designed stack-structured phase change memory cell which operates as multi-state storage. Amorphous GexTe100-x chalcogenide materials were stacked and a diffusion barrier was chosen for each stack layers. The device is operated by crystallizing each chalcogenide material as sequential manner from the bottom layer to the top layer. The amplitude of current pulse and the duration of pulse width was fixed and number of pulses were controlled to change overall resistance of the phase change memory cell. To optimize operational performance the thickness of each chalcogenide was controlled based on simulation results.
9:00 PM - H7.12/G8.12
Crystallization Kinetics of Phase Change Materials
Shan Liu 1
1 Materials and Engineering Physics Program, Ames Laboratory, Ames, Iowa, United States
Show Abstract9:00 PM - H7.13/G8.13
A SiTiNx Heating/Barrier Layer for Phase-Change RAMs.
Huai-Yu Cheng 1 , Tsung-Shune Chin 1 , Chain-Ming Lee 2 , Yi-Chen Chen 2
1 Department of Materials Science and Engineering, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan, 2 Electronic Research and Service Organization, Industrial Technology Research Institute, Chu-Tung Taiwan
Show AbstractBy inserting a highly resistive heating layer (~10 nm) between the bottom electrode and phase-change material was reported to successfully decrease operation current of the phase-change random-access-memory (PCRAM) as simulated using a one- dimensional heat conduction model. Highly resistive SiTiNx films were investigated as candidates of heating layer in PCRAM devices. They were deposited by RF sputtering with TiSi2 single target at an atmosphere with different ratios of N2 to Ar. The measured resistivity between 0.039 ~0.69 Ω-cm fulfills the simulated require- ment (0.1~1 Ω-cm) with increasing nitrogen content (up to 38.68 at%) and Si/(Si+Ti) ratio (up to 0.6750) in films, respectively. All the as-deposited films were amorphous and the RMS roughness is 0.639 ~ 0.44 nm with increasing nitrogen content. Such Si-rich films showed excellent thermal stability with amorphous structures sustaining until at least 800 oC which is high enough for the operation conditions of PCRAM, while N-free Si2Ti films showed nanocrystalline structure after annealing at 700 oC. The thermal stability of SiTiNx thin films was also determined by microstructure studies through the annealing tests at different temperatures. The temperature dependence of electrical resistivity for SiTiNx films was also investigated. The as-deposited samples of N-free and of high N content showed positive temperature coefficient of resistivity (TCR) values at a temperature range 400~500 oC; while the others with the medium N content present negative TCR values. The binding energy of SiTiNx films measured by XPS changes with nitrogen content and Si/(Si+Ti) ratio, conforming with the performance of electrical resistivity. Besides the heating capability, the layer was also found to perform excellent diffusion-barrier capability between W bottom-electrode and Ge2Sb2Te5 phase-change films as examined by Auger analysis after the evaluation annealing at 400 ~ 600 oC in Ar for 30 minutes. The highly resistive SiTiNx heating layer successfully obstructed the diffusion of tungsten atoms from the W bottom-electrodes into Ge2Sb2Te5 phase-change films even if only quite a thin SiTiNx film of 10 nm was inserted between them. As increasing N content, the barrier capability became more significant. Such a barrier performance facilitates avoidance of deterioration in the memory devices after millions of set-reset repetition cycles. PCRAM cells with a W/SiTiNx/Ge2Sb2Te5/TiN structure were testified. The performance of such cells was greatly enhanced over those structures without a heating/barrier layer. It is suggested that these highly resistive SiTiNx films are good candidates of heating layer as well as excellent diffusion barrier in PCRAM structures. Optimal compositions of SiTiNx films as the heating/diffusion barrier layer for PCRAM were proposed.
9:00 PM - H7.14/G8.14
Effect of Thermal and Intrinsic Stresses on the Mechanical Failure of Ge2Sb2Te5 Films.
Il-Mok Park 1 , Young-Chang Joo 1 , Jung-Kyu Jung 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - H7.15/G8.15
Characterization of Ge2Sb2Te5 Grown by the Method of an Ion Beam Sputtering Deposition.
YoungKuk Kim 2 1 , S. A. Park 1 , E. J. Jeong 3 , M. -H. Cho 1 , D. -H. Ko 3 , K. Jeong 2
2 IPAP, Yonsei University, Seoul Korea (the Republic of), 1 Nano Surface Group, korea Research Institute of Standards and Science, Daejeon Korea (the Republic of), 3 Dep. of Ceramic Engineering, Yonsei University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - H7.2/G8.2
Phase Transition Characteristics of Sb-based Phase Change Materials for PRAM Application.
Tae Jin Park 1 , M. J. Kang 1 , S. Y. Choi 1 , S. M. Yoon 2 , B. G. Yu 2
1 Department of Material Science and Engineering, Yonsei University, Seoul Korea (the Republic of), 2 , ETRI, Deajeon Korea (the Republic of)
Show Abstract9:00 PM - H7.3/G8.3
Electrochemically-Deposited Bi2S3 and Pb3xBi2(1-x)S3 Nanowires.
Jana Sommerlatte 1 2 , Woo Lee 1 , Roland Scholz 1 , Ulrich Goesele 1 , Klaus Bente 2 , Kornelius Nielsch 1
1 , Max Planck Institute of Microstructure Physics, Halle Germany, 2 Institute of Mineralogy, University of Leipzig, Leipzig Germany
Show AbstractThermoelectric nanowires have been predicted to have superior thermoelectric properties over bulk materials. Sulfosalts in general and especially bismuth sulfide and the Bi2S3-PbS compounds are semiconductors with a direct band gap transition. The band gap of the bulk Bi2S3-PbS alloys can be adjusted over a wide range from 0.25 eV up to 1.6 eV and exhibit a large variety of crystal phases. Due to the modular crystal structure Pb3xBi2(1-x)S3 tends to grow in a needle-like shape. Therefore these compound semiconductors are very suitable for the synthesis of crystallographically-oriented 1D nanostructures. In this paper we report on the synthesis of Bi2S3 and Pb3xBi2(1-x)S3 nanowires by electropdeposition in highly ordered alumina membranes with a monodisperse pore diameter of 50 nm. According to Baranski et al. [J. Electrochem. Soc. 127, 766 (1980)] we have chosen DMSO as a nonaqueous solvent for the electrolyte. The deposition of Bi2S3 and Pb3xBi2(1-x)S3 took place under inert gas atmosphere at elevated temperatures (110°C). For the Bi2S3 deposition we have obtained nearly single crystalline nanowires and a homogenous pore filling up to membrane thicknesses of 30 μm. The Bi2S3 nanowires exhibit the bismuthumite phase with a preferential orientation of the c axis parallel to the nanowire axis. In case of Bi2S3-PbS-compounds we obtained polycrystalline phase mixtures with an extended amount of lead dissolved in the crystal structure in comparison to the corresponding crystal phase of the bulk material. By varying the bismuth concentration from x= 0.2 to 0.5 we have obtained Pb3xBi2(1-x)S3 nanowires which exhibit the cosalite Pb2Bi2S5 or lillianite Pb2Bi3S6 phase as the dominating crystal structure. We thank the German Ministry for Education and Research (BMBF, project number 03N8701) for financial support.
9:00 PM - H7.4/G8.4
Effect of Heavy Ion Irradiation on the Optical Properties of a-Se85Te15 Thin Film.
Vineet Sharma 1 , Virendra Singh 2 , Anup Thakur 3 , Jeewan Sharma 3 , S Tripathi 3
1 Physics, Jaypee University of Information Technology , Waknaghat, HP, India, 2 Chemistry, Panjab University Chandigarh, Chandigarh India, 3 Physics, Panjab University , Chandigarh, Chandigarh, India
Show Abstract9:00 PM - H7.5/G8.5
In Situ Transmission Electron Microscopy Study of the Structural Transformation in Ge2Sb2Te5 Thin Films.
Yu Jin Park 1 , Jeong Yong Lee 1 , Yong Tae Kim 2
1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 Semiconductor Materials and Devices Laboratory, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractThe nucleation, grain growth, and phase change mechanism of face centered cubic (FCC)-to-hexagonal structure in Ge2Sb2Te5 (GST) thin films were studied using high voltage electron microscope operated at 1,250 kV. As a result, we have found that 2 nm-sized nucleus forms as a cluster which atoms are arranged regularly at the stage of nucleation prior to the formation of grains having crystal structure. The high-resolution transmission electron microscopy study and fast-Fourier transformations revealed that coexistence of FCC and hexagonal structure occur, and formation of twin defect is found in the hexagonal structure during the grain growth as the annealing temperature is increased. The similarity in the atomic arrangement between FCC and hexagonal structure of GST suggests a strong possibility of an epitaxial growth during the phase transition from FCC to hexagonal structure. In other words, the structural transformation of FCC-to-hexagonal structure can be explained by the “epitaxial growth model” due to the similarity in the atomic arrangement between FCC and hexagonal structure and this will be well presented. A unique phenomenon exist that according to the crystallographic relationship between the hexagonal and its adjacent FCC structured grain, twin boundaries are formed as the hexagonal structure grows along the [0001] direction.Unlike the isotropic crystallization in the FCC structure, the preferably oriented grain growth of the anisotropic hexagonal structure was observed due to the growth velocity diversions according to growth directions. Analyzing the transmission electron microscopy images and selected area electron diffraction patterns, it was found that the large grain having the hexagonal structure grow from the surface, and the growth proceeded perpendicular to the [0001], namely, the path parallel to the (0001) plane. Consequently, grain growth to a large-scale results in a lengthened shape.
9:00 PM - H7.6/G8.6
Dielectric Constants and Endurance of Chalcogenide Phase-Change Non-Volatile Memory.
Semyon Savransky 1 , Eugenio Prokhorov 2
1 , The TRIZ Experts, Newark, California, United States, 2 , CINVESTAV del IPN, Queretaro Mexico
Show AbstractInitial attempts to create memory from chalcogenide glasses (g-Ch) had limited success particularly because the first generation of these materials (labeled as CG1) has inferior endurance (about 106±1 SET-RESET cycles). Recent progress in phase-change non-volatile memory (PC-RAM) related to superior properties of Ge2Sb2Te5 (GST) alloy [1,2]. The paper answers the vital for PC-RAM development question: “Why is endurance of GST memory cells (about 1011±2 cycles) much higher than of CG1 cells?” We show that superior endurance is related to features of –U centers [3] creation in GST during RESET of PC-RAM cells. The native –U centers exist in g-Ch due to the softness of atomic potentials [3]. They play a significant role in SET process of CG1 and GST [2,4]. The –U centers behavior in GST [5] is different compared with CG1 while other properties (threshold voltage, resistivity, etc.) are basically the same [1-4]. We noticed that known dielectric permittivities ε of CG1 and GST are also different.The ε values in Ge-Sb-Te alloys films have been determined from impedance measurements in sandwich samples using method described in [6] and reported in the paper. Amorphous GST has relatively high and distinct static and optical dielectric permittivities εo=16.5 and ε∞=15.3 to compare with CG1 where ε practically independent on frequency. Hence the second term in the effective polarization potential Ep = q2[(1-1/ε∞)/r + (1/ε∞ - 1/εo)/L] is strong (here r and L are the average atomic radius and interatomic bond distance, q is the electron charge). It allows to screen the Coulomb repulsion at an –U center quite effectively in GST. Therefore polarization helps –U centers creation in amorphous GST and impedes these centers formation in crystalline hexagonal GST films where eo=38 and e∞=61 [6]. In contrast to CG1 [3], the creation and destruction of –U centers during RESET and SET processes in GST [4] are not accompanied by strong plastic mechanical stresses in a memory cell. This feature (higher barrier between elastic and plastic deformations) predetermines possibility of numerous SET-RESET cycles in this alloy. Therefore, the deformation mechanism of –U centers formation in CG1 leads to inferior endurance while the polarization mechanism of their creation in GST ensures decent endurance. Obtained experimental ε values in non-stochiometric films allow to conclude about expected endurance in the framework of the proposed model.References[1] S. Lai, IEDM '03 Tech. Digest, p. 10.1.1-10.1.4, 2003.[2] A. Pirovano, et. al., IEEE Trans. Electron Dev., 51 (2004) 452.[3a] N.F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline Materials, Oxford: Clarendon, 1979, 590 pp.[3b] Yu.M. Galperin, V.G. Karpov and V.I. Kozub, Adv.Phys., 38 (1989) 669. [4] S.D. Savransky, Journal of Ovonic Research, 1 (2005) 25.[5] J.K. Olson, et. al., Phys. Rev. B, 2005, in press.[6] E. Morales-Sánchez, et. al., J. Appl. Phys., 91 (2002) 697.
9:00 PM - H7.7/G8.7
The Phase Change of the Sb1Tex(x=0.3, 1.0, 1.5, 2.0, 2.7) Thin Films Grown by Thermal Evaporation on Si(001): Studies on Critical Te Layer Thickness and Corresponding Changes of the Structural Properties.
SangYub Ie 1 , ByungTack Bea 1 , Kyumin Lee 1 , Jinmoon Choi 1 , Kwangho Jeong 1
1 Physics, IPAP Yonsei Univ, Seoul Korea (the Republic of)
Show AbstractThe crystallinity of Sb1Tex dichalcogenide compound which is known as showing the phase change properties was studied by measuring the X-ray diffraction at different temperatures. The dichalcogenide Sb1Tex (x=0.3, 1.0, 1.5, 2.0, 2.7) compound thin films were deposited by layer-by-layer manner with controlled thickness for each element of Antimony and Tellurium on Si(001), and post annealed during the X-ray diffraction measurement. The thickness of repeating units consist of Tellurium and Antimony element layers was not less than 3.5nm, in which Antimony layer thickness was fixed with 0.884nm and the Tellurium layer thickness was varied from 0.348nm to 2.16nm and the total thickness of samples were 120nm. By modulating the thickness of each element, stoichiometry of Sb1Tex was controlled, and measured by Particle Induced X-ray Emission (PIXE). The PIXE spectrum showed the variations of Tellurium atomic concentration in the Sb1Tex thin films. X-ray diffraction was measured from 15° to 65° at different temperatures of the room temperature, 100C, 120C, 150C, 180C, 200C, 250C, 300C, 350C, 400C with 45min iso-thermal time and temperature raising rate of 10C/min. The enhancement of crystalline quality not forming the different crystalline phase was found in the case of x=0.3, which showed the enlargement of lattice size in the crystal structure. The other samples with different Tellurium concentration showed the crystalline phase change with the function of time and the annealing temperatures. In the case of x=2.7, abrupt phase change behavior started from the temperature of 150C was found and this sample also showed the second phase change started at 300C. From these results, we suggested that quantity of Tellurium element in the Sb1Tex dichalcogenide thin films can control the structural phase change properties and that there exists the critical Tellurium layer thickness. In this study we found new way of synthesizing the Sb-Te dichalcogenide compound showing phase changing behavior.
9:00 PM - H7.8/G8.8
A Study of Crystallization Characteristics on N-doped Ge2Sb2Te5 Thin Films for Phase Change Memory Devices
Byung Ho Lee 1 , Jaesuk Kwon 1 , Kihoon Do 1 , Dae-Hong Ko 1
1 Dept. of Ceramic Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractPhase change memory device has many advantages such as fast access time, good scalability, simple cell structure and good endurance. However, the additional requirements are needed for the better performance of PRAM. The higher resistivity is recommended in the crystalline state, for reducing the power consumption during the RESET operation by the joul-heating, and the higher crystallization temperature is also required for thermal stability.We investigated the electrical properties and thermal stability in the nitrogen doped Ge2Sb2Te5 thin films. 100 nm thick Ge2Sb2Te5 thin film was deposited by D.C. Magnetron sputter on SiO2/Si substrate using the Ge2Sb2Te5 single target. The concentration of the nitrogen was controlled by adjusting N2/Ar gas ratio during the sputtering. Samples were annealed at the various temperature ranges by vertical furnace in the N2 ambient. The sheet resistance measurement of Ge2Sb2Te5 and N-doped Ge2Sb2Te5 thin films showed that the crystallization temperature increased by the addition of nitrogen element. With the increase of the nitrogen contents, the sheet resistance at crystalline state also increased. The XRD analysis also showed the crystallization temperature increased with the addition of nitrogen element, and the phase transformation from FCC to HCP structure was retarded. TEM analysis showed that grain growth was retarded by the addition of the nitrogen contents upon annealing.
9:00 PM - H7.9/G8.9
Precursor Study of MOCVD GST Thin Films for the Integration of High-Density PRAM Device
Sun Heu Kim 1 , Sang Jun Lee 1 , Hyo Jung Kim 1 , June Key Lee 1
1 School of Materials Science and Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show AbstractPhase-change Random Access Memory(PRAM) is one of the promising candidates for next generation memory because of the many advantages such as high speed, low power, non-volatility, high density and low cost. For the integration of the PRAM, metal-organic chemical vapor deposition(MOCVD) method is essential. In this work, we study the decomposition behavior of GeH4(g), Sb(i-pr)3 Te(i-pr)2 precursors in terms of chamber pressure, deposition temperatures by MOCVD on SiO2 substrates. Based on the experimental results, we can design the GST MOCVD process for PRAM applications.