Phase change materials and phase change memory

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Phase change materials and phase change memory

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Phase change materials and phase change memory
Simone Raoux, Feng Xiong, Matthias Wuttig, and Eric Pop
Phase change memory (PCM) is an emerging technology that combines the unique properties of phase change materials with the potential for novel memory devices, which can help lead to new computer architectures. Phase change materials store information in their amorphous and crystalline phases, which can be reversibly switched by the application of an external voltage. This article describes the advantages and challenges of PCM. The physical properties of phase change materials that enable data storage are described, and our current knowledge of the phase change processes is summarized. Various designs of PCM devices with their respective advantages and integration challenges are presented. The scaling limits of PCM are addressed, and its performance is compared to competing existing and emerging memory technologies. Finally, potential new applications of phase change devices such as neuromorphic computing and phase change logic are outlined.

Introduction
Novel information storage concepts have been continuously developed throughout history, from cave paintings to printing, from phonographs to magnetic tape, dynamic random access memory (DRAM), compact disks (CDs), and flash memory, just to name a few. Over the last four decades, silicon technology has enabled data storage through charge retention on metal-oxide-silicon (MOS) capacitive structures. However, as silicon devices are scaled toward (sub-) 10 nm dimensions, minute capacitors become leaky by simple quantum mechanical considerations, and the memory storage density appears to plateau. Novel information storage concepts are under development that include storing data in the direction of the magnetic orientation (magnetic RAM,1 spin torque transfer RAM,2 racetrack RAM3), in the electric polarization of a ferroelectric material (ferroelectric RAM4), in the resistance of a memory device (resistive RAM,5 memristor,6 conducting bridge RAM,7 carbon nanotube memory8,9), or in the resistance of the storage media itself (phase change RAM10). Phase change materials store information in their amorphous and crystalline phases, which can be reversibly switched by the application of an external voltage. In this article, we describe the properties of phase change materials and their application to phase change memory (PCM).

Properties of phase change materials
Phase change materials exist in an amorphous and one or sometimes several crystalline phases, and they can be rapidly and repeatedly switched between these phases. The switching is typically induced by heating through optical pulses or electrical (Joule) heating. The optical and electronic properties can vary significantly between the amorphous and crystalline phases, and this combination of optical and electrical contrast and repeated switching allows data storage. This effect was initially uncovered in 1968,11 but it took the breakthrough discovery12 of fast (i.e., nanosecond time scale) switching materials along the pseudo-binary line between GeTe and Sb2Te3, notably the most studied and utilized Ge2Sb2Te5 (GST), to enable phase change storage technology.13
Many technologically useful phase change materials are chalcogenides, which owe their success in this regard to a unique combination of properties, which include strong optical and electrical contrast, fast crystallization, and high crystallization temperature (typically several hundred degrees Celsius). Figure 1 shows the ternary phase diagram of the Ge-Sb-Te system. As mentioned previously, alloys along the pseudo-binary line between Sb2Te3 and GeTe with compositions (GeTe)m(Sb2Te3)n have been intensely studied14 and are used in state-of-the-art PCM devices.15 In search of

Simone Raoux, Institute Nanospectroscopy for Energy Material Design and Optimization, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany; [email protected] Feng Xiong, Electrical Engineering, Stanford University, USA; [email protected] Matthias Wuttig, Physikalisches Institut and Jülich Aachen Research Alliance – Fundamentals of Future Information Technology, RWTH Aachen University, Germany; [email protected] Eric Pop, Electrical Engineering, Stanford University, USA; [email protected] DOI: 10.1557/mrs.2014.139

© 2014 Materials Research Society

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Figure 1. Tertiary Ge-Sb-Te phase diagram with some popular phase change alloys highlighted. The red arrow indicates the trend of adding Ge to Ge2Sb1Te2 alloys.19
faster materials, undoped16 and slightly Ge-doped Sb devices with a composition of Ge15Sb8517 have been fabricated. The term “doping” is used in the phase change materials community to describe adding amounts of another element in the several percent range, but alloying would be a more accurate term. Another set of materials along the pseudobinary line between GeTe and Sb was studied as well,18 and starting from Ge2Sb1Te2 on this line and the further addition of Ge (red arrow in Figure 1) led to the design of phase change materials with very high thermal stability of the amorphous phase, suitable for high-temperature PCM applications.19
All of these materials utilize a remarkable bonding mechanism in the crystalline phase, termed resonance bonding by Linus Pauling, where a single half-filled p-band forms two bonds to its left and right neighbors.20,21 This bonding mechanism only prevails for a small subset of group V and VI compounds, which helps to identify and optimize possible phase change materials,22 as illustrated in Figure 2. Resonance bonding requires long range order, and amorphous materials only employ ordinary covalent bonding. Crystallization is hence accompanied by a change in the bonding mechanism. Understanding the microscopic mechanisms employed in crystallization is crucial to optimize the performance of PCM data storage. In recent years, it has become clear that phase change materials are bad glass formers23 that exhibit the characteristic behavior of fragile liquids,24,25 including a pronounced temperature dependence of the activation barrier for crystal growth. This ensures that the amorphous phase is stable for 10 years at about 100°C (a typical industry benchmark), while this state recrystallizes into the crystalline phase in less than 10 ns at elevated temperatures around 500°C.26,27 In fact, the fastest switching speeds reported are less than 1 ns.28

Principles of phase change memory
PCM is based on the repeated switching of a phase change material between the amorphous and the crystalline states associated with a large change in resistance. Information is stored in the phase of the material and is read by measuring the resistance of the PCM cell; the cell is programmed and read using electrical pulses. Figure 3 illustrates the principle. Switching from the high-resistance or “reset” state, where part or all of the phase change material is amorphous, occurs when a current pulse is applied that heats the amorphous material above the crystallization temperature for a sufficiently long time for the material to crystallize. This is only possible because of the threshold switching effect that leads to a drastic and sudden (within nanoseconds) reduction of the resistance of the amorphous phase when a certain threshold field is surpassed, at a given threshold voltage VT. Otherwise, it would be impossible to heat the amorphous material using Joule heating with reasonably low voltages. Switching from the low-resistance or “set” state, where the material is crystalline, is achieved by a high current pulse with a very short trailing edge. The current pulse heats the material by Joule heating, melts it, and enables very fast cooling (melt-quenching) such that the material solidifies in the amorphous state. The resistance state of the memory cell is read with a sufficiently small current pulse, which does not alter the state of the memory cell.
The previously mentioned threshold switching can be attributed to a current-voltage instability under a high electric field.29 It does not necessarily lead to phase change, and if the high voltage is removed fast enough, the material will go back to the high resistance state. Threshold switching can be described as hot-electron trap-limited transport and a three-dimensional (3D) network model assuming a random distribution of traps.30
The current pulses are provided by an access device, which also isolates the memory cell during programming and read operations. Field-effect transistors,31 bipolar junction transistors,32 and diodes33 have been used as access devices, and, in most cases, the size of the access device is larger than the PCM cell in order to provide sufficient current, ultimately limiting the storage density. Considerable development efforts have been devoted to optimizing the cell design that leads to reduced reset current, thus to reduced size of the access device and increased storage density.
Phase change materials are at the heart of PCM technology, and their properties to a large extent determine its functionality and success. Optimization of phase change materials is not only application specific but also technology node specific; for example, the threshold voltage in current typical PCM cells is on the order of 1 V, but if devices are scaled to much smaller dimensions, the threshold voltage scales with the size of the amorphous region, and for very small cells, it could become comparable to the read voltage such that every read operation could alter the cell state. In the following section, we discuss various possible PCM applications with their challenges and possible materials classes that could meet these challenges.

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Figure 2. Empirical map for materials with about three p-electrons per atomic site and even numbers of anions and cations. The axes that span the map are the tendency toward hybridization, rπ−1, and the ionicity, rσ′ . The coordinate rπ−1 describes the degree of covalency or tendency toward hybridization. It can be understood as a measure of the energetic splitting of s- and p-states, which scales with the difference between the radii of s- and p-orbitals. The coordinate rσ′ provides a quantitative measure for the ionicity of bonds similar to Pauling’s electronegativity difference. The coordinates of a large number of materials have been calculated (see the supplement to
Reference 22 for an index of materials). Different symbols in the map characterize different stoichiometry, where the subscript describes the
stoichiometry, and the superscript identifies the formal valence of the corresponding atoms. Hence IV denotes atoms of group 14, while V
and VI describe atoms of groups 15 and 16, respectively. Phase change materials are located within a small region of the map that is prone
to the occurrence of resonant bonding. The graphics on the outside illustrate the weakening of resonance effects as one leaves this region
due to the formation of more saturated covalent bonds via distortions or due to charge localization at the ions due to increasing ionicity.
The size of the white and black circles is a measure of the electronegativity of the corresponding atoms. The different shades of blue represent
different degrees of increasing electron localization. Reproduced with permission from Reference 22. © 2008 Nature Publishing Group.

The first application, which is already available in the market, is as stand-alone data storage to replace flash memory.34 The requirements are moderate in terms of operation temperature (80°C), cycle number (105), and switching speed (ms range), and Ge2Sb2Te5-based alloys can provide the required materials properties and switch at much faster times in the tens of ns range. For storage class memory applications such as a high-speed replacement for hard drives,35 the requirements are higher, in particular in terms of the cycle numbers (108–109). New materials, such as reactively sputtered and doped Ge2Sb2Te5, have been developed to increase the cycle numbers to 109 while maintaining fast switching (20 ns).36 Due to the highly competitive nature of materials development for PCM, exact material compositions are often not published.
The requirements for data retention at higher temperatures are much more stringent for automotive applications (150°C for 10 years) or pre-coded chips that need to pass a solder bonding process (250–260°C for tens of seconds). In these

cases, phase change materials with much higher crystallization temperatures compared to Ge2Sb2Te5-based alloys are required. Highly Ge-rich Ge-Sb-Te materials, sometimes doped additionally with N and C, and GaSb have been shown to be promising candidates for these applications.19,37,38
Using PCM to replace DRAM is a formidable challenge, because very fast switching times in the nanoseconds range and extremely high cycle numbers of ∼1016 present a combination of requirements that have not been achieved by phase change materials. DRAM replacement is a special case since DRAM is a volatile memory, whereas PCM is a non-volatile memory. If PCM were to achieve DRAM-like performance, it would open up possibilities to realize completely new computer architectures. Very fast switching times have been achieved for several phase change materials, including Ge2Sb2Te527,28 and GeTe26 in actual PCM devices. The high cycle number remains an enormous challenge, but it appears that scaling to smaller dimensions of the phase change material is beneficial

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power. The main failure mechanisms of PCM

devices include the case where the cell can no

longer be switched to the low resistance state,

due to void formation over the bottom electrode

contact, and is related to the change in mass density

with the phase transformation and thus with

every switching cycle. The other main failure

mechanism is elemental segregation, in

particular Sb enrichment in the switching region

caused by electromigration. This leads to poor

data retention when the cell can no longer be

Figure 3. Principle of phase change memory. Starting from the amorphous phase with large resistance R, a current pulse is applied. At the threshold voltage VT, the resistance drops suddenly, and a large current (I) flows that heats the material above the crystallization temperature Tx for a sufficiently long time to crystallize (set operation). In the crystalline state, the resistance is low. A larger, short current pulse is applied to heat the material above the melting temperature Tm. The material is melt-quenched and returns to the amorphous, high resistance state (reset operation). In the schematic, different colors represent different atoms (such as Ge, Sb, and Te in the commonly used GeSbTe compounds) in the phase change materials.

switched to the high resistance state or does not remain in the high resistance state since Sb-rich alloys have low crystallization temperatures.
A second important aspect is the cell design, which needs to consider many aspects such as number of required process steps, parameter window for each process step, availability of the required deposition methods (e.g., atomic layer

deposition of phase change materials), and other

for cycling. Data measured on highly scaled PCM cells using aspects of manufacturability. Among the greatest challenges

an Sb-rich Ge-Sb-Te phase change material demonstrated 1011 in PCM integration remains the reduction of the reset current.

cycles under accelerated testing conditions using a switching More details can be found in Chapter 17 of Reference 13.

power of 45 pJ, which leads to an extrapolated

cycle number of 6.5 × 1015 cycles under

normal switching conditions using 3.6 pJ,39 see

Figure 4. Elemental segregation upon repeated

cycling is one failure mechanism for PCM

cells, and the ultra-scaled volume, which

is likely completely molten in every reset

operation, probably “remixes” the elements

at every switching and also leaves very little

room for the elements to spatially segregate,

avoiding a typical failure mechanism.40

Phase change memory devices and integration
PCM will be successful in the market when it is possible to develop a manufacturing process that can achieve low cost and reliable production of large arrays of PCM cells and is compatible and easily incorporated in existing CMOS (complementary metal oxide semiconductor) processes. Device design and integration determine the PCM functionality and production costs, which are particularly important for PCM technology, and determine the size of the access device, which in turn determines the storage density.
The functionality of the PCM cell is strongly influenced by the choice of the phase change material, and large research efforts are devoted to optimizing phase change materials for specific applications with variable requirements for memory functionality such as switching speed, data retention, endurance, and switching

Figure 4. (a) Cross-sectional view transmission electron microscope (TEM) image of 7.5 nm (width) by 17 nm (length) dash confined phase change memory (PCM) cell, where the minimum size is defined by thin-film thickness rather than photolithography: (a) Lengthwise cross-section of eight devices. (b) Enlarged view of one device showing phase change film deposited by atomic layer deposition in contact with the bottom electrode contact (BEC). (c) Top view scanning electron microscope (SEM) cross-section of the bit. (d) Endurance characteristics (switching the cell back and forth between set and reset states) of dash confined cell at 4.5 × 10–11 J reset programming energy, marked in red in (e) where cycles to failure as a function of programming energy at acceleration condition are shown. The blue arrow indicates normal (not accelerated) program energy leading to an estimated cycle number of about 1015 (yellow star). Reprinted with permission from Reference 39. © 2010 IEEE.

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Scaling properties of phase change materials

The thermal conductivity of phase change materials in both

and phase change memory

amorphous and crystalline phases is important and should be

For PCM (or any new memory technology) to be viewed as a quite small, because it determines how energy-efficient the

useful technology, it must demonstrate scalability for at least PCM device is during programming. Reifenberg et al.56 stud-

several generations to justify the heavy investment the indus- ied the thermal conductivity of Ge2Sb2Te5 thin films (60 nm

try needs to commit. Many efforts have been directed toward and 350 nm) in the amorphous, face-centered cubic, and

investigating PCM scalability at both the material and device hexagonal states. They found that the thermal conductivity

level.

decreases as the film thickness decreases in all three phases.

Table I shows a list of important parameters in phase change This suggests a favorable scaling trend since it helps reduce

materials, their influence on PCM device performance, and thermal power loss during operations.

how they behave when the material is scaled down to smaller

The threshold voltage is found to show a linear relationship

dimensions. The crystallization temperature Tx of phase change with the device dimension, suggesting an underlying threshold

materials can vary as a function of the material dimensions.41–44 electric field.16 This is beneficial for scaling down of voltage and

For instance, experiments performed on nm-thin films45,46 and power, although contact resistance effects may become domi-

nanoparticles47,48 suggest that Tx of phase change materials nant for the smallest devices.57 The resistivity ratio of phase

could increase under certain conditions with decreased dimen- change materials does not appear to decrease in experiments

sions. A higher Tx means that unintentional crystallization is on nanoscale devices,58,59 although simulations mentioned

less likely, and thus a longer data retention time prevails. Tx previously predict such effects may occur at dimensions below a

becomes a strong function of the interfaces for very thin films, dozen atomic layers.52 This means there would be a large enough

and experiments on very thin Ge-Sb phase change films with window for multi-level cell programming in PCM even at

various interface materials demonstrate that an increase or diminished dimensions.60 Overall, phase change materials

decrease of Tx can be observed, depending on the interface demonstrate a highly desirable scaling behavior, which can be

material, and the difference can be as large as 200°C for very extended to several generations of technology nodes.

thins films.49 Measurements on Ge2Sb2Te5 nanowire devices by

One of the major considerations of prototype PCM devices

Lee et al.50 found that nanowires with smaller diameters have is their programming current, which can be large during the

smaller activation energy, indicating a shorter data retention power-intensive reset step.17 One approach to decrease the

time. However, Yu et al.51 did not observe a dependence of reset current (Ireset) is to reduce the volume of the phase change

Tx on nanowire diameter. It has also been demonstrated that region, thus requiring lower overall power to heat up and reset

ultrathin films (1.3 nm thickness)46 and tiny nanoparticles (2 it. This can be done by reducing the contact area between the

nm diameter)41 could still transform to the crystalline phase. phase change material and the electrodes. Figure 5 shows Ireset

Ab initio simulations have suggested that GeTe films could be as a function of the effective contact area for different cell

scaled down to ∼3.8 nm thickness (12 atomic layers), although structures.39,58,59,61–64 It is evident that Ireset scales with the effec-

in thinner films, overlapping metal-induced gap states from tive contact area, indicating an average reset current density of

the two TiN electrodes could lead to a decrease in the on-off ∼40 MA/cm2. Thus, by using sub-lithographic scale electrodes,

ratio.52 The melting temperature Tm of phase change materials such as carbon nanotubes with diameters of a few nanometers,

has also been observed to decrease with reduced dimensions the reset current could be reduced to as low as a few μA.58,59,65

in thin films,40 nanowires,53 and nanoparticles.54 A lower Tm At the same time, innovative device design by optimizing the

is generally favorable, implying that less power would be thermal control could reduce the current density even further.

needed in the power-limiting reset step. Taken together, these For example, a larger thermal boundary resistance (TBR)

experimental and theoretical results suggest good potential for between the PCM bit and its surrounding materials would act

scaling down PCMs, which is promising for future generations. as a thermal insulation layer, retaining heat within the PCM

The crystallization speed of the phase change materi- volume and thus requiring lower reset current.66 Such a demon-

als either increases (desirable) or decreases (detrimental) at stration was achieved by increasing the TBR with a thin layer of

reduced dimensions, depending on the material composition.10,55 fullerenes (C60) inserted between the phase change material and

the metal electrode.67 These

Table I. Scaling of phase change memory properties.

Material Property

Influence on Phase Change Memory Device

Crystallization temperature Tx

Set power

Scaling Behavior Good

promising results provide directions on how to overcome high reset currents, which are arguably one of the greatest

Melting temperature Tm

Reset power

Good

obstacles in PCM technology.

Crystallization speed
Thermal conductivity (amorphous and crystalline)
Threshold voltage

Data rate and set power Set and reset power
Set voltage/power

Depends Good
Good

We also briefly discuss the minimum amount of energy required to melt and reset a PCM bit from a thermodynamic perspective.

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Figure 5. Reset current scaling versus electrode contact area in phase change memory (PCM) devices. The inset shows a schematic of a typical PCM cell, where A is the contact area and d is the contact diameter. Blue circles represent devices with metal contact electrodes (typically TiN), which are lithographically defined.39,61–64 The black squares are data taken on prototype devices with carbon nanotube (CNT) electrodes.58,59 The single- and multi-wall CNTs have diameters of the order of a few nm, contacting PCM bits with volumes of a few hundred nm3. Updated and reprinted with permission from Reference 59. © 2013 American Chemical Society.
Assuming perfect thermal insulation of the bit (adiabatic heating), the energy for the reset step is the sum of the energy needed to heat up the crystalline bit from room temperature up to melting, plus the latent energy of the melting process. Taking the standard Ge2Sb2Te5 (GST) properties,68 (i.e., melting temperature Tm ≈ 600°C, specific heat Cs ≈ 1.25 J/cm3/K, and latent heat of melting H ≈ 419 J/cm3), we estimate the minimum reset energy is Emin = 1.2 aJ/nm3, which represents a fundamental lower limit for nanoscale PCM based on GST. In other words, a 5 × 5 × 5 nm3 GST bit volume with perfect thermal insulation would require a minimum reset energy of approximately 150 aJ. A similar estimate based on the scaling of the bit contact area is given in Reference 69.
New applications of phase change memory
PCM cells cannot only be programmed in the on- or off-state, it is also possible to reach intermediate resistance states. Up to 16 levels have been demonstrated using a write-and-verify scheme.60 Utilizing a continuous transition between resistance levels in PCM devices in an analog manner, this effect can be used to program them to mimic the behavior of a synapse, for example. Such an attempt could lead to the design of a neuromorphic computer with electronic hardware that resembles the functions of brain elements, such as the neurons and synapses. The phenomenon of spike-timing-dependent plasticity (a biological process where the strength of connections between neurons are adjusted during learning) could be demonstrated in PCM devices using specific programming schemes.67,70 Image recognition using a neural network of PCM devices was also demonstrated.71–73 These could potentially lead to a

compact and low power neuromorphic computing system that is capable of processing information through learning, adaptation, and probabilistic association like the brain.
The thermoelectric (TE) effect, which corresponds to the conversion of thermal energy into electrical energy, has also been actively studied in the context of PCM materials and devices.74,75 TE efficiency is characterized by the figure of merit ZT = σS2T/k, where σ is the electrical conductivity, k is the thermal conductivity, T is the temperature, and S is the Seebeck coefficient. Phase change materials have good properties as p-type TEs because they have similar chemical compositions compared to traditional telluride-based TE materials, along with low thermal conductivity and relatively high electrical conductivity, which improves ZT. Chalcogenidebased phase change materials have been shown to exhibit S ∼ 350–380 μVK–1 76,77 and ZT ∼ 0.7 at elevated temperatures78 (commercial TE materials have ZT ∼ 1). This opens up opportunities for applications such as Peltier coolers (heat Q = ITΔS is removed at a junction of materials with different S as electric current I flows through) or power generators based on phase change materials. Studies have also suggested that both the Thomson effect (heating or cooling in a homogeneous material with a temperature gradient and T-dependent Seebeck coefficient) within PCM cells and the Peltier effect at the PCM electrode contacts could be used to control device heating, and thus reduce the reset current.77,79 This effect is more prominent at smaller dimensions due to better thermal insulation. Thus, understanding and optimizing such TE phenomena could have a significant impact on future PCM device performance.
Summary
Phase change memory (PCM) is a promising novel data storage concept because of its unique combination of features such as fast access time, large electrical contrast, non-volatility, and high scalability. This article summarizes the properties and working principles of phase change materials, their applications to PCM devices, and how scaling affects these properties. The breakthrough discovery of fast switching materials along the pseudo-binary line between GeTe and Sb2Te3 has motivated researchers to further optimize material properties to improve the performance of PCM devices. Depending on the specific application, the requirements for important properties such as crystallization speed, thermal stability, and endurance are different. Material composition and preparation methods must be carefully optimized to meet the prerequisites of the specific application.
We also examined the scaling properties of phase change materials. Crystallization and melting temperature, crystallization speed, and thermal and electrical resistivity tend to demonstrate favorable scaling behavior when device dimensions are reduced to the nanometer range. PCM endurance could improve to 6.5 × 1015 cycles at ultra-scaled dimensions since complete melting of the bit would eliminate elemental segregation, which is a typical failure mechanism for PCM cells.

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Recent results have also shown that the reset current and
operating power of PCM devices scale down with the electrode
contact area, also leading to a reduction of the access device
size (which needs to carry less current) and enhancing the
overall storage density.
Acknowledgments
M.W. gratefully acknowledges funding within SFB 917
(“Nanoswitches”) and by the ERC (“Disorder Control”). F.X.
and E.P. acknowledge support from the US National Science
Foundation (NSF) grant ECCS 1002026 and from the US
Office of Naval Research (ONR) grant N00014–10–1–0853.
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2015

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CALL FOR PAPERS Abstract Submission Opens September 23, 2014 | Abstract Submission Deadline October 23, 2014

Energy
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Meeting Chairs
Artur Braun 4XJTT'FEFSBM-BCPSBUPSJFT GPS.BUFSJBMT4DJFODFBOE5FDIOPMPHZ Hongyou Fan 4BOEJB/BUJPOBM-BCPSBUPSJFT Ken Haenen )BTTFMU6OJWFSTJUZBOE*.&$W[X Lia Stanciu1VSEVF6OJWFSTJUZ Jeremy A. Theil2VBOUVNTDBQF *OD
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710 MRS BULLETIN • VOLUME 39 • AUGUST 2014 • www.mrs.org/bulletin
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