Characterization of phase change memory materials using phase changebridge devices

Daniel Krebs,1,a Simone Raoux,2 Charles T. Rettner,2 Geoffrey W. Burr,2

Robert M. Shelby,2 Martin Salinga,1 C. Michael Jefferson,2 and Matthias Wuttig1

IBM/Macronix PCRAM Joint Project1I. Physikalisches Institut IA, RWTH Aachen University, 52056 Aachen, Germany2IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA

Received 20 March 2009; accepted 25 June 2009; published online 4 September 2009

Having become one of the most promising candidates for future nonvolatile memory applications,phase change random access memory is now driving an intensive search for phase change materialswith optimized properties. In this paper, phase change bridge devices are utilized as a uniquematerial test vehicle, allowing systematic extraction of crucial material parameters such asresistance contrast, switching speed, threshold voltage, and set and reset power. Bridge devicesfabricated from undoped Ge15Sb85 are presented that reproducibly switch between set and resetstates with one decade resistance contrast using current pulses as short as 10 ns. Since devices arefabricated in the amorphous-as-deposited phase, an interesting intermediate device state can beproduced, which we attribute to a crystalline center region that fails to completely bridge theamorphous active device volume. The subtle differences between the amorphous-as-melt-quenchedand amorphous-as-deposited phases are explored using both bridge devices and optical experiments.It is shown that the large difference in crystallization times between these two amorphous statesobserved in laser experiments is reflected also in device operation. © 2009 American Institute ofPhysics. doi:10.1063/1.3183952

I. INTRODUCTION

Although phase change materials were originally associ-ated with reversible resistance switching,1 it was the discov-ery of fast switching materials with significant optical con-trast by Yamada et al.2 that led to their first majortechnological application in optical data storage. These ma-terials based on the Sb2Te3–GeTe pseudo-binary system, arenow found in most rewritable compact disks CD, digitalversatile disks DVD, as well as the newly introduced Blu-ray™ disk.3 In addition to their high optical contrast,4 how-ever, such phase change materials also exhibit a resistancecontrast of several orders of magnitude, making them suit-able for electronic memory. Additional favorableproperties—such as fast programming speed, low powerconsumption, low voltage operation due to the thresholdswitching effect, high endurance to repeated cycling, andgood scalability to smaller dimensions—have helped phasechange random access memory PCRAM emerge as one ofthe most promising candidates for next-generation nonvola-tile memory. However, the need for further improvements inspeed, data endurance, and switching power is driving anintensive search for new and optimized phase change mate-rials.

To evaluate if a new phase change material is suitable5

for use in a PCRAM device, various critical parameters suchas crystallization temperature Tc, resistivities in the amor-phous and crystalline states, switching speed, and thresholdvoltage Vt need to be determined. Using simple thin-filmexperiments, some characteristics—resistivity, crystallization

temperature, and speed of recrystallization—can be mea-sured rapidly to evaluate if a new material shows a promise.However, much more information can be obtained from thetesting of new materials in real memory devices, includingthe effects of device fabrication on material characteristics,actual switching speed and power, achievable resistance con-trast, value of the threshold voltage, device retention, failuremechanisms, and the effects of highly inhomogeneous tem-perature and current distributions. The device structure to beused in such a material evaluation should be easy to fabricateover a wide range of device sizes, so as to help minimize theinevitable uncertainty about the precise device size in theevaluation of test results. This also enables the study of de-vice parameters as a function of device dimensions, provid-ing crucial results for evaluation of materials and devicescaling.

In this paper, phase change “bridge cell” devices6 areutilized as such a material evaluation testbed, fabricated withundoped Ge15Sb85GeSb as the active phase change mate-rial. This material shows some great advantages compared tothe most commonly applied phase change materialGe2Sb2Te5. It has a high crystallization temperature of about250 °C compared to Ge2Sb2Te5 about 150 °C.7 Laser ex-periments demonstrated that this material also offers ex-tremely fast speed.8,9 Even though it was found that the crys-tallization of amorphous-as-deposited material requires laserpulses as long as 10 s, melt quenching and recrystalliza-tion of amorphous-as-melt-quenched material can be reliablyperformed with laser pulses of only 10 ns.

This large difference between laser-induced crystalliza-tion times of amorphous-as-deposited and amorphous-as-aElectronic mail: krebs@physik.rwth-aachen.de.

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melt-quenched material can be explained by the crystalliza-tion mechanisms. Ge15Sb85 is a growth-dominated phasechange material,9 where nucleation is slow but crystalgrowth is fast. The first crystallization of amorphous-as-deposited material requires the formation of critical nucleiand their subsequent growth. In the laser-induced crystalliza-tion experiments starting from the amorphous-as-melt-quenched phase, however, the amorphous-as-melt-quenchedarea is surrounded by a crystalline matrix. In such an experi-ment, nucleation is not required because crystals can growfrom the crystalline-amorphous interface. Thus the crystalli-zation speed can be very fast due to the high crystal growthvelocity. The main question, which will be answered in thispaper, is whether this important difference between theamorphous-as-melt-quenched phase and the initialamorphous-as-deposited phase, only observed optically todate, is actually relevant for device operation. Comparablebehavior in device experiments would imply that it is suffi-cient to perform straightforward laser experiments to inves-tigate the switching speed of a phase change material whileproducing results that are still predictive of memory cell per-formance. However, if the device performance is not compa-rable in terms of the observable differences betweenamorphous-as-melt-quenched and amorphous-as-deposited,then this would make laser experiments much less useful inthe search of potential materials for electronic device appli-cations.

One disadvantage of Ge–Sb phase change materials istheir tendency to phase segregate into Ge-rich and Sb-richregions upon prolonged heating.10,11 Similar segregation hasbeen observed also for Ge2Sb2Te5 “mushroom” phasechange devices after repeated cycling.9 This can be detrimen-tal to device performance because the Sb-rich regions thattypically form on top of the bottom electrode contact inmushroom cells have a lower crystallization temperature,suggesting reduced data retention. It was shown, however,6

that Ge15Sb85 bridge devices can be cycled at least 30 000times—indicating that this effect does not immediately dete-riorate device performance. This may be due to the differentgeometry bridge cell versus mushroom cell, the scaling ofthe devices the bridge cells had only 3–10 nm thickGe15Sb85 material which suppressed large-area segregation,the very short switching times, or a combination of theseeffects.

II. EXPERIMENTAL

Phase change bridge devices were fabricated by usingelectron beam lithography to pattern the narrow lines of un-doped GeSb nominally the eutectic composition with 15at. % Ge and 85 at. % Sb across two planarized 80 nm thickTiN bottom electrodes on 1 m SiO2 on a Si substrate.6

Numerous bridge devices with a uniform thickness of 30 nmwere fabricated with lengths ranging from 6 to 480 nm andwidths ranging from 15 to 500 nm. A scanning electron mi-crograph of a typical device is shown in Fig. 1. Depositionswere performed by magnetron sputtering in an argon atmo-sphere of 0.27 Pa with a flow of 40 SCCM SCCM denotescubic centimeter per minute at STP and a power of 10 W dc

on the GeSb compound sputter source. All depositions werecarried out at room temperature, resulting in as-depositedfilms in the amorphous phase. Rutherford backscatteringspectrometry measured the Ge:Sb ratio to be Ge:Sb= 14.30.5 at. % : 85.70.5 at. %. During the litho-graphic portion of the device fabrication process, the amor-phous phase change material had to be heated to 120 °C for2 min and to 90 °C for 8 min. However, since the crystalli-zation temperature of GeSb Tc=247 °C is significantlyhigher than these process temperatures, the fabrication pro-cess did not induce any phase transitions. As a result, elec-trical testing could be performed on fully fabricated bridgedevices that remained in the original amorphous-as-deposited phase.

Electrical testing was performed by probing test padsnear the sample with tungsten tips in a customized electricaltester setup with a 500 resistor in series with the phasechange memory cell Fig. 1. A Tektronix AWG520 arbitrarywaveform generator AWG 2Ch, 1GS/s was used to applyvoltage pulses, while a Tektronix TDS7404B digital phos-phor oscilloscope 4 GHz, 20GS/s measured both the volt-age across a 50 scope input resistor parallel to the deviceand the voltage over a 50 scope input resistor in serieswith the device. From these scope traces, the voltage acrossand the current through the device could be obtained, allow-ing the dynamic resistance the resistance during the pulseand the threshold voltage of the device to be determined.Despite the absence of active probes, device current and dy-namic resistance could be accurately obtained for pulseslonger than 50 ns, below which capacitive effects complicatethe measurement. However, our measurements of thresholdvoltages remained completely unaffected by pad capacitance.In addition, highly accurate dc measurements were per-

FIG. 1. The experimental setup shown schematically at the left suppliespulses from an AWG to the device under test using two oscilloscope chan-nels to measure the device voltage and current independently. To the right, aSEM image of the fabricated bridge device shows the narrow line of phasechange material bridging two planarized TiN contacts separated by insulat-ing SiO2. The inset shows the active device volume located above the nar-row dielectric gap between the two electrodes Ref. 12.

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formed before and after each pulse at an applied voltage of50 mV to determine the resistance state of the device withouttriggering a phase change.

Optical testing was performed on 30 nm thick GeSbfilms, sputter deposited under the same conditions as the de-vices onto a 50 nm alumina heat barrier layer on a siliconwafer. A custom-built pulse/probe laser setup equipped witha 658 nm diode laser was used to apply high power laserpulses with a 1 m beam diameter to the sample in order toinduce phase transitions. A low power 632 nm cw probe lasermonitored the reflectivity of the material before, during, andafter the heating laser pulse in order to detect the phase tran-sition. To measure recrystallization times, the material wasfirst crystallized by heating it above its crystallization tem-perature in a furnace. Then, melt-quenched-amorphous areaswere produced in the crystalline matrix by short and power-ful laser pulses sufficient to melt the material. Recrystalliza-tion of these melt-quenched-amorphous areas was then per-formed with laser pulses of variable duration and power tomeasure crystallization times. The pulse laser power for theexperiments ranged from 1 to 20 mW and the laser pulsewidth from 5 ns to 30 s, while the probe laser power re-mained below 2 mW to avoid any additional probe-inducedheating.

III. DEVICE SWITCHING

Fabricated devices were found to be in a fairly high ini-tial resistance state, confirming that they remained in theas-deposited amorphous phase. For a 56 nm long bridge de-vice that was 40 nm wide and 30 nm thick, this initial resis-tance was 1 M, as shown in Fig. 2a. Since we can rea-sonably conclude that the entire bridge was in theamorphous-as-deposited state, we can estimate the resistivityof the GeSb material in the as-deposited phase to be2.1 cm. This is in good agreement with the value of2.4 cm obtained for 50 nm thick blanket films.

In order to determine the pulse parameters required toset the device into the fully crystalline state, pulses withincreasingly larger voltage 0.1 V increment were applied tothe as-fabricated device. Figure 2a shows the experimentson four different but nominally identical devices with eachcurve corresponding to pulses of different plateau durationsranging from no plateau to 4.35 s but identical leadingand trailing edges 100 ns. We observed that a first signifi-cant drop in resistance occurred at 1.2 V, largely indepen-dent of the pulse duration. This intermediate state corre-sponded to a resistance of roughly 20–30 k but was notthe final set state. Increasing the voltage further led to aneven lower resistance state of about 700–800 . Since thisis the same resistance that could be obtained by thermal an-nealing at temperatures well above the crystallization tem-perature, we conclude that this lowest resistance correspondsto full crystallization along the length of the bridge. With thisassumption, we can conclude that the crystalline phase has aresistivity of 410−4 cm. This is actually slightly lowerthan the value obtained for a blanket film 210−3 cm.

It was also possible to switch as-fabricated devices di-rectly into the fully crystallized set state without entering the

intermediate resistance state by using a voltage approxi-mately twice as large. Alternatively, longer trailing edges atlower voltage could be used to set the as-fabricated devicesdirectly, again bypassing the intermediate state. For instance,if the trailing edge was extended to 1 s, the same thresholdvoltage that would have produced an as-fabricated-to-intermediate transition for the rapid ramp-down pulsesshown in Fig. 2 instead produced the set state directly.

We attribute this intermediate state to the presence of acrystalline region in the center of the bridge which fails tooverlap one or both of the electrodes, as illustrated in Fig. 3.Once the applied voltage exceeds the threshold field for elec-trical breakdown,13 heat is generated in the center of thebridge, leading eventually to nucleation of a crystalline re-gion Fig. 3a. Since the center of the bridge is hot, thiscrystalline region can grow rapidly to fill the center of thebridge Fig. 3b. However, the presence of the high thermalconductivity electrodes tends to keep the outer portions ofthe bridge at a cooler temperature, forcing the crystal growthto proceed more slowly. Thus either much higher voltages orlonger trailing edges are required to crystallize the materialall the way to the electrode Fig. 3c and as a result avoidthis intermediate state Fig. 3b.

0.0 0.3 0.6 0.9 1.20.0

0.5

1.0

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2.0Pulse: tLE / tP / tTE in ns

100 / 500 / 100

Current(mA)

Voltage (V)0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

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0.8Pulse: tLE / tP / tTE in ns

100 / 500 / 100

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0 1 2 3 4

1k

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Pulse: tLE/ tP/ tTEin ns

100 / 0 / 100100 / 500 / 100100 / 1000 / 100100 / 4350 / 100

Resistance(Ω)

Applied Voltage (V)

0 2 4 6 8

1k

10k

100k

1M

Resistance(Ω)

Applied Power (mW)

(a)

(b) (c)

FIG. 2. Color online a Resistance of GeSb bridge devices as a functionof applied voltage pulse starting from the as-deposited amorphous phase fordifferent plateau durations. Here tLE, tP, and tTE denote leading edge time,plateau time, and trailing edge time of the pulses. The measured resistanceafter a voltage pulse drops to an intermediate level at about 1.2 V for allpulse lengths threshold voltage. However, the fully crystalline set state isreached at different pulse voltages depending on the pulse width. The insetshows the same resistance data from part a but plotted as a function ofapplied power. b The current-voltage characteristic for a pulse that pro-duces the transition from as-deposited amorphous to the intermediate state.c Current-voltage characteristic for a pulse that transforms the intermediatestate to the set state.

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In the intermediate state, the resistance is significantlylower than in the fully amorphous as-fabricated device, sincemuch of the bridge is crystalline. However, since some cur-rent must pass through a region of material that is still in theamorphous phase, the device resistance tends to be higherthan in the set state. Because of the temperature gradientproduced by the metallic TiN electrodes, we believe that thisregion is most likely found at the intersection of the elec-trodes and the phase change bridge. If the Thomson effect ispresent to a significant extent,14 it is possible that the tem-perature contours are asymmetrically located with respect tothe geometric center of the bridge. Thus one electrode willlikely be overlapped by crystalline material well before theother electrode on the “colder” side of the bridge. Becausethe electrodes possess a high thermal conductivity, a fairlylarge electrical power is required to heat the phase changematerial in immediate contact to the electrodes sufficiently toenable rapid crystal growth. Of course, at such large electri-cal powers, the peak temperature at the center of the bridgecan exceed melting, leading to the possibility of a smallamorphous region after the device cools. However, thisamorphous region can only have a noticeable effect on thedevice resistance when it blocks a significant portion of thebridge.

A pulse at moderate power can also produce the set state,but a much longer pulse duration is required due to the slowcrystal growth at lower temperatures.8 However, once a con-ductive path is formed to both electrodes, these regions nearthe electrode remain crystalline and the overall device resis-tance drops to the final low resistance set state. An additionaldetail supporting this explanation is the presence of a slightthreshold behavior, as shown in Fig. 2c, as if subsequentpulses applied to the device in the intermediate state mustperform electrical switching of the residual regions ofamorphous material.

Once the set state is achieved, cycling between a resetand set state can be achieved at much lower power, as illus-trated in Fig. 4. Here, two experiments are shown to explore

the set-to-reset and reset-to-set transitions. For the former,indicated by open circles, a device was first placed directlyinto the set state by a pulse with 100 ns leading edge dura-tion, 5 s plateau duration at 3 V, and 5 s trailing edgeduration, after which short square pulses of 14 ns 10 nsplateau with 2 ns leading and trailing edges were applied toreset the device. After each attempted reset pulse, the devicewas switched back into the fully set state again with a setpulse of 1.5 V and 100 ns duration leading edge, plateau, andtrailing edge before the next reset pulse was applied with a0.1 V larger pulse amplitude. We found that between 2 and 3V the amorphous resistance could be tuned to different val-ues between 1 and 30 k, which we attribute to differentportions of the bridge being melt quenched. Simulations notshown indicate that the shape of this amorphous region canbe expected to be a distorted cylinder with the widest portionalong the centerline of the bridge and the narrowest portionsat the periphery,6 as illustrated schematically in Fig. 3c. Foran applied voltage between 3 and 5 V, the amorphous resis-tance reached a stable reset value of about 30 k that wasmore or less independent of the applied voltage. The initialhigh resistance observed immediately after device fabrica-tion could not be obtained again by a current pulse, thermalannealing, or laser heating of any kind once a transition outof this initial state had occurred. The intermediate and resetstates have similar resistances mostly because both stateseach have small portions of the electrical path that are in theamorphous phase. However, the two states are quite easy todistinguish in the laboratory: only in the reset state can alow-voltage low-power long pulse produce the low-resistance set state, as shown in Figs. 4 and 5. In the inter-mediate state, even though the device resistance is similar toreset, such a pulse will have no effect, and only a high-voltage, high-power pulse leads to the set state, as shown inFig. 2a.

It is known that the amorphous-as-deposited and theamorphous-as-melt-quenched phases have different crystalli-zation speed and medium range order.6,15,16 Thus it would be

FIG. 3. Color online Sketch of a phase change bridge device illustratingthe crystalline blue and amorphous yellow portions of the device duringthe a initial Joule heating of the device, b intermediate state, in which thecentral region has finally crystallized yet has not grown to overlap the elec-trodes, the c set state, once the central crystalline region overlaps theelectrodes at which point the high peak temperature in the bridge center canpotentially induce melting of a small region, and the d reset state in whichthis central crystalline region is blocked by a plug of amorphous-as-melt-quenched material.

0 1 2 3 4

1k

10k

100kSET: t

LE= 10ns, t

P= 500ns, t

TE= 50ns

RESET: tLE= 2ns, t

P= 10ns, t

TE= 2ns

Resistance(Ω)

Max. Power (mW)

FIG. 4. Color online Resistance of a GeSb bridge device as a function ofdevice power for both the set-to-reset transition open circles, 14 ns pulsesof varying amplitude and the reset-to-set transition filled squares, 560 nspulses of varying amplitude. A trailing edge of 50 ns is sufficient to preventthe device from quenching into the reset state.

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reasonable to expect that one should be able to measure dif-ferences in electrical resistivity and/or threshold field be-tween these two variants of the amorphous phase. The low-temperature fabrication procedure described here for thephase change bridge offers a unique opportunity to performsuch measurements, since the device can be fabricated with aknown volume of amorphous-as-deposited material and thebreakdown voltage can be correlated with the device lengthfor a large number of samples.13 However, in order to quan-tify the difference between the as-deposited and melt-quenched amorphous materials, it is critical to know the ex-act size and shape of the reamorphized region in the resetstate illustrated in Fig. 3d. Any uncertainty concerning thisspatial distribution will directly result in significant uncer-tainty in the resistivity and threshold field determined for theamorphous-as-melt-quenched phase but not for theamorphous-as-deposited phase, since in this case the amor-phous device length is well known.

Once the device has been placed in the reset state byapplying a voltage in the range where the resistance increasesaturates, a subsequent pulse can be used to investigate theset operation. As shown in Fig. 4, the transition from reset-to-set requires much less power than any of the other threetransitions as-fabricated-to-intermediate, intermediate-to-set, or set-to-reset, which is consistent with the picture thatonly the center of the bridge region needs to be heated andthat the required temperatures are significantly lower thanthe melting temperature. Figure 5 shows the results of vary-ing the pulse duration by changes in leading edge, plateau,and trailing edge times. For pulses longer than 260 ns, crys-tallization takes place from the melt-quenched phase for anapplied voltage of 0.6 V threshold voltage. For shorterpulses, higher voltage and power are required. Pulses asshort as 4 ns can be used to set the device, although signifi-cant power and voltage are then required in order to achieveacceptable resistance contrast. As shown in Fig. 6, stable andreproducible switching between the set and reset states with10 resistance contrast was achieved with 10 ns pulses of

2 and 4 V for set and reset, respectively. While the exactelectrical response of the device for such short pulses is af-fected strongly by reflections and capacitive effects, it isclear that both set and reset are performed in 10 ns.

IV. CRYSTALLIZATION BEHAVIOR

Figure 7 summarizes the differences in the crystalliza-tion behavior of the amorphous-as-deposited andamorphous-as-melt-quenched phases as a function of pulsepower and pulse duration. The left-hand portion of the plot

0.0 0.5 1.0 1.5 2.0 2.5 3.0

1k

10k

100k

10 / 300 / 5010 / 400 / 5010 / 500 / 502 / 1000 / 50

tLE/ tP/ tTEin ns:

Resistance(Ω)

Applied Voltage (V)

2 / 0 / 22 / 0 / 1010 / 0 / 1010 / 0 / 2010 / 0 / 5010 / 100 / 5010 / 200 / 50

FIG. 5. Color online Resistance of GeSb bridge devices during the reset-to-set transition as a function of applied voltage for various pulse durationsand trailing edge times. For a pulse longer than 260 ns, the device can be setwith an applied voltage of 0.6 V. For shorter pulses, higher applied volt-ages and thus higher switching power are required.

(c)

(b)

(a)

0 20 40 60 80

1k

10k

Resistance(Ω)

Pulse Number

0 10 20 30 40 50

0.0

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Current(mA)

Time (ns)

0 10 20 30 40 50-1

0

1

2

3

4

RESETSET

Voltage(V)

Time (ns)

(a)

FIG. 6. Color online a Resistance of a GeSb bridge device during theset-reset cycling with 10 ns pulses. Device b voltage and c current for theset black solid curve and reset red dashed curve pulses.

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combines the data from Fig. 5 together with the results ofadditional electrical testing measurements, while the right-hand portion summarizes the optical measurements of an un-patterned thin-film GeSb sample.

In Fig. 7a, the resistance after electrical pulses of in-creasing power is plotted for devices starting in theamorphous-as-deposited phase, while in Fig. 7c the mea-sured resistance is plotted starting from the amorphous-as-melt-quenched state. The applied power was calculated fromthe measured dynamic resistance and the applied voltage.Note that while Fig. 7a spans at 4.5 s in time and 3 mWin power, Fig. 7b spans only at 80 ns and 800 W. Be-cause the amorphous-as-deposited phase cannot be reattainedonce testing has begun, the data shown in Fig. 7a are ob-tained in sequence incrementing applied voltage and thusthe pulse power, leading to the two steps observed in this

plot due to the intermediate state. Despite this complication,it is clear from the electrical data in the left-hand portion thatthe amorphous-as-melt-quenched phase crystallizes muchfaster and requires much lower power than the amorphous-as-deposited phase.

This observation is confirmed by the right-hand portionof Fig. 7, which shows the results of laser experiments for a30 nm thin film of GeSb deposited on 50 nm alumina on a Siwafer. Figures 7b and 7d plot the change in reflectivityafter a laser pulse of variable power and duration. Forsamples in the amorphous-as-deposited phase, laser pulsesmust be longer than 6 s in order to crystallize the film Fig.7b, indicative of the difficulty of nucleation in this growth-dominated phase change material.

In order to investigate the recrystallization ofamorphous-as-melt-quenched material, a GeSb sample was

Ω

FIG. 7. Color online Comparison of the pulse power-duration parameter space for a and c electrically- and b and d optically-induced crystalliza-tions of GeSb starting from the a and b amorphous-as-deposited and c and d amorphous-as-melt-quenched phases Ref. 12. The color scale signifiesa the resistance Ra after an electrical pulse, c the resistance change R after an electrical pulse normalized to the initial resistance Ri, or b and d thereflectivity change R measured after a laser pulse normalized to the initial reflectivity Ri. Please note the different time axes in the upper two microsecondand the lower two nanosecond plots. Power comparisons between the electrical and optical experiments are complicated by the differences in the heating andcooling environments.

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crystallized thermally by annealing it in a furnace in nitrogenflow at a temperature of 300 °C for 10 min. Just prior toeach attempted recrystallization pulse of variable power andpulse width, the sample was first illuminated with an amor-phization pulse 40 mW for 50 ns in order to produce amelt-quenched mark suitable for recrystallization. During theexperiment, the same amorphization pulse was used, so thateach recrystallization attempt begins with the same sizemelt-quenched mark. The resulting data Fig. 7d reveal arecrystallization time of about 10 ns, 600 times shorterthan for the initial crystallization of the as-deposited amor-phous state.

As discussed earlier, this pronounced difference in speedis due to the difference between the difficulty of nucleatingnew crystalline material within the amorphous material andthe ease of propagating an existing crystalline region into aneighboring amorphous region. Both in devices and in thinfilms, the phase change material must first form crystallinenuclei within the as-deposited state during an incubation pe-riod before these nuclei can grow Fig. 3a. However, sincethe melt-quenched phase can only be formed over a limitedregion of crystalline material, a boundary between theamorphous-as-melt-quenched and crystalline phases is un-avoidable Fig. 3d. Therefore nucleation is no longer re-quired and rapid growth can occur as the crystalline interfacepropagates into the melt-quenched amorphous material.

The much faster crystallization times we observe for themelt-quenched amorphous phase confirm the growth domi-nated nature of GeSb.6,15 Crystal growth velocity is a strongfunction of temperature, peaking for GeSb at a temperaturearound 400 °C.8 An upper limit for the maximal growth ve-locity can roughly be estimated to be about 50 m/s assumingfull crystallization of the laser spot with a radius of about500 nm in 10 ns. It has been observed using atomic forcemicroscopy AFM that the crystallization of GeSb typicallybegins near the edge of the laser spot and then propagatesthrough the spot.6,15 Thus we can assume that the crystalli-zation front travels through nearly the full spot diameter.

V. CONCLUSIONS

The phase change bridge device is a unique test deviceto evaluate new phase change memory materials, since itrequires only thin film deposition of the material and onesubsequent lithographic patterning step. This makes itstraightforward to fabricate devices with a short turn-aroundtime over a wide range of bridge sizes. In addition, the low-temperature fabrication opens up the opportunity to test de-vices that remain in the original amorphous-as-depositedphase, which many characterization experiments study, yetfew devices actually exhibit. Using bridge devices as such atest vehicle, we have studied the operation of undoped GeSbas a phase change material. GeSb with recrystallization times

as low as 4 ns shows 600 times faster recrystallization once acrystalline-amorphous boundary is formed by melt quench-ing. These results were confirmed with both optical and elec-trical switching data. Reproducible electrical cycling be-tween set and reset resistance states with more than onedecade resistance contrast was shown for pulses as short as10 ns. Finally, an interesting intermediate device state wasdescribed, in which a crystalline region is formed in the cen-ter of the bridge device that does not completely reach theelectrodes. Given the straightforward, low-temperature fabri-cation procedure, the wide range of achievable device sizes,the well-defined geometry, and the accessibility of the activevolume to scanning electron microscope SEM and AFMscanning, the phase change bridge device offers unique op-portunities to study the operation of phase change memorydevices.

ACKNOWLEDGMENTS

The authors would like to thank Andrew Kellock forperforming RBS and PIXE measurements.

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