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Disorder-to-order induced thermal conductivity change of Ge2Sb2Te5 thin film in the cubic crystalline phaseHyung Joon KimCeNSCMR, Department of Physics and Astronomy, Seoul National University, Seoul, S. Korea2012 MRS Spring Meeting2012. 4. 12

Seoul National University, Center for Novel States of Complex Material ResearchGood morning ladies and gentleman. Im Hyung Joon Kim from Seoul National University in S. Korea. Im going to present a research topic entitled Disorder-to-order induced thermal conductivity change of Ge2Sb2Te5 thin film in the cubic crystalline phase.1CollaboratorsYisheng Chai, Jae Wook Kim, Kee Hoon Kim

Hanbeam CorporationJong Ho Lee, Seung Wook Ryu, Young Bae Ahn, Hyeong Joon KimSu Youn Lee, Byung-ki CheongThin Film Materials Research Center, KISTSeung Min LeeSchool of Materials Science and Engineering, SNUDepartment of physics and astronomy, CeNSCMR, SNU

Seoul National University, Center for Novel States of Complex Material ResearchThese are my collaborators and Prof. Kee Hoon Kim is my advisor.2IntroductionTimeTTimeT

Amorphous phase High resistivity Low reflectivity

Crystalline phase Low resistivity High reflectivityTmeltTcystTmeltTcyst

GeTeSbGeTeSb2Te3PcRAM (Phase-change RAM) : Ge2Sb2Te5Compact discDigital versatile discBlu-ray discGeTe-Sb2Te3 pseudobinarySb-Te binaryTop ElectrodeBottom ElectrodeJoule heatingTimeI~50ns~300nsLogic 1Logic 0 PcRAM (Phase-change random access memory) : Ge2Sb2Te5(GST)CrystallineAmorphousReset pulseSet pulseIt is important to know how thermal conductivity of Ge2Sb2Te5 films varies with temperature and thermal history.Seoul National University, Center for Novel States of Complex Material ResearchGe2Sb2Te5 (GST) is widely used for data storage devices in which the write/erase operations are triggered by a thermal transition between the amorphous and crystalline phases, typically through melting and annealing. If the annealing temperature is high enough, the amorphous phase becomes the crystalline phase. The amorphous state can be regained by melting and rapid cooling. Phase Change RAM is a memory device using the resistivity difference between two phases by switching the phases with Joule heating. In this thermally controlled device, it is important to know how thermal conductivity of GST films varies with temperature and thermal history.3Motivation

Ho-Ki Lyeo et al., Appl. Phys. Lett. 89, 151904 (2006)

W. P. Risk et al., Appl. Phys. Lett. 94, 101906 (2009)Applying high temperature and cooling : increment of thermal conductivity Phonon and electron thermal conductivityWiedemann-Franz law : k = kph + kel = kph + Ls T Simultaneous measurement of electrical and thermal conductivities as a function of temperatureSeoul National University, Center for Novel States of Complex Material ResearchIt is well known that the thermal conductivity of a GST film increase after phase transition from amorphous to crystalline phases. However, the electron and phonon contributions of the thermal conductivity increment are not well understood. To distinguish phonon and electron thermal conductivity values, first, we check that the thermal conductivity of GST follows the WF law. Right top figure shows the thermal conductivity vs. electrical conductivity data of various phase change materials at room temperature. Black dots are for an amorphous phase, open dots are for a crystalline phase. The dotted line is a fitting curve which is based on the WF law with considering that the phonon thermal conductivity equals the value of amorphous phase. It seems to be roughly matched. However, to more accurate analysis, we need temperature dependent thermal and electrical conductivity data. To study this, a real time thermal conductivity measurement method is a key technique. In this report, we use the 3w method to establish the real time thermal conductivity measurement.4Thermal resistance : DT/PReal Time 3w Method with a Reference StructurekSi = 1.4 W cm-1 K-1 at room T = 0.76 W cm-1 K-1 at 201 oC Reported value : 1.48 Wcm-1K-1 at room T 0.79 W cm-1 K-1 at 201 oCC. J. Glassbrenner and G. A. Slack, Phys. Rev. 134, A1058 (1964)Si(525m)Sample structure : with GSTHeater (Ti/Au=20/80nm)180nm Sputtered SiO2~ 400nm GST100nm Thermal SiO2Vw V3wI(t)W. P. Risk et al., Appl. Phys. Lett. 94, 101906(2009)

3w method + Multi-layer model Si(525m)Reference structure : without GSTHeater (Ti/Au=20/80nm)180nm Sputtered SiO2100nm Thermal SiO21D heat flow

HeaterI = I0sin(wt)T(t)TBaseThermal link

David G. Cahill, Rev. Sci. Instrum. 61, 802 (1990)I(w) P(2w) DT(2w) DR(2w) V3w

Ref.SampleHeater width : 32 mmDTSample/P- DTRef./P : frequency independentAt fixed frequency, k(t,T) can be measured successfully.

Heat sinkh : thickness of the GST layer (400 nm)w : heater widthSeoul National University, Center for Novel States of Complex Material ResearchTop figure shows our sample and reference structures for the 3w method. The sample structure is composed with thermal SiO2, GST, and room temperature sputtered SiO2 thin films on Si substrate. The only difference of reference structure is the absence of GST layer. The 3w method is a way to measure a frequency dependent thermal resistance, delta T over P, between a heater and a heat sink by applying ac current through the heater. Here, P is the given power. If we apply ac current at w frequency, the temperature of heater oscillates at 2w frequency which is originated from the joule heating. This oscillating temperature induces a resistance oscillation of a heater at 2w frequency. This resistance oscillation makes 3w voltage signal. We can get the amplitude of temperature oscillation, delta T, through 3w voltage signal using this equation. Usually, the thermal conductivity is obtained by fitting the frequency dependent delta T over P. However, if we measure the delta T over P of both structures, we dont need to frequency dependent data. There is 1D heat flow through films due to the 32 um width of heater which is larger than total thickness of films. The difference of delta T over P which comes from the GST layer is almost frequency independent due to this 1D heat flow nature. Left bottom figure shows our experimental data. As you can see, the frequency independent difference of the delta T over P was observed even at high temperature. Thus, we can get the thermal conductivity of a GST film using this equation, by measuring delta T over P of both sample and reference structures at fixed frequency. Through this way, we established a real time thermal conductivity measurement.5

TCryst : ~130GSTRef. k of amorphous GST ~ 0.18 Wm-1K-1 Phase transition TCryst ~ 130 Time evolution of k at fixed Tk(t) and k(T) of GST FilmAmorphousFace-centeredcubicSeoul National University, Center for Novel States of Complex Material ResearchLeft figures show the temperature profile, dt over P, and calculated thermal conductivity as a function of time. we slowly increase the temperature and anneal the film for 10 hrs. Right figure shows the resultant thermal conductivity vs temperature plot. There exists a sharp increasing of the thermal conductivity which is corresponding to the phase transition of GST from amorphous to fcc crystalline phase at around 130 oC. As you can see, it is clearly shown that the thermal conductivity of fcc GST film is larger than the one of amorphous GST. The measured thermal conductivity of amorphous phase is around 0.18 W/m/K which is similar with reported values. Interesting point is the time dependent increment of the thermal conductivity during annealing.6

Electrical and Thermal Conductivities of GST Films The electrical and thermal conductivities are simultaneously measured. The higher annealing temperature makes the higher thermal and electrical conductivity values after cooling.

Seoul National University, Center for Novel States of Complex Material ResearchWe measure thermal conductivity of GST films with various annealing temperatures. Due to the merit of real time measurement, we succeeded to simultaneously measure the electrical and thermal conductivities. To get electrical conductivity, we measured resistivity of a GST film with conventional 4 probe method. Left figure shows the thermal conductivity and right figure shows the electrical conductivity. Both conductivities abruptly increase at around the phase transition temperature and become larger during temperature increasing and annealing. As decreasing temperature, both conductivities decrease. The higher annealing temperature makes the higher thermal and electrical conductivity values after cooling. With these thermal and electrical data, we tried to check whether GST follows Wiedemann-Franz law or not.7Wiedemann-Franz Law and Phonon Thermal Conductivity

lph~ 4 smaller than lattice constant (6)

L = 2.44 x 10-8 WW/K2 During cooling : linear behavior in k vs sT Wiedemann-Franz law The kph increases as increasing the annealing temperature.lphonon~ 4 smaller than lattice constant (6) ttotal ~ 0.7m FrequencyT/PRef.SampleTf /P TemperatureSeoul National University, Center for Novel States of Complex Material ResearchThermal Conductivity (k) Measurement of Films : 3w MethodJoule heatingI(t)V(t) = V2(t) /R = (P0 - P0cos2t)/2T(t) TBase + (bP0 + bP0cos2t)/2, [ = {T(V) -T(V=0)}/P0]R(t)R(TBase) + DT + DTcos2t, [ = dR/dT]V(t)(I0R + 3I0DT/2)sint + (I0DT/2)sin3t ~ Vwsint + V3sin3tT(t)T = bP0/2DT = DT2D + DT1DHeaterI = I0sin(wt)T(t)TBaseThermal link

David G. Cahill, Rev. Sci. Instrum. 61, 802 (1990)P0,rms: power per unit length of the heater

T1 T2 Linear regionln wDTDT2DDT1D : frequency independant S.-M. Lee and David G. Cahill, J. Appl. Phys. 81, 2590(1997)Heaterwhw hHeat sinkHeaterI = I0sin(wt)T(t)TBasekthickTBaseHeat sinkT(t)k I = I0sin(wt)Heater2D heat flow2D heat flow1D heat flowSeoul National University, Center for Novel States of Complex Material ResearchLock-in SR830out.ABMDACManganin wire : RrefSampleComputerAmplifierVRVSHigh T. ChamberV3/V1 < 1/1000, VR() > VS() VS() VR() (MDAC Value ) ~ 0MDAC value (controlled by PC): ~ RS/RRV3 Measurement Scheme and EquipmentsMDAC :Multi digital to analog converter.Seoul National University, Center for Novel States of Complex Material Research

SlopeGST = SlopeNo-GST kSi = 1.4 Wcm-1K-1 Ref. : 1.48 Wcm-1K-1@300K

Tf/P : Almost same at every freq.T/P vs FrequencyC. J. Glassbrenner and Glen A. Slack, Phys. Rev. 134, A1058 (1964)Thermal wavelength () : (D/i2)0.5

At 97Hz(1) 2D heat flow condition Si = 64 m > 16 m (Heater width : 32m)

(2) 1D heat flow condition SiO2 = 68 m >>180nm GST = 28 m >> 400nmSeoul National University, Center for Novel States of Complex Material Research

and r of GST film are measuredsimultaneously. (not same piece)

During T up, k vs sT trend dont follow Wiedemann-Franz law.(k increase more faster)

After cool down, k vs sT shape lookslinear.Seoul National University, Center for Novel States of Complex Material Research

Time dependent s at fixed temperatures

Annealing T (oC)C 1(W cm)tr1 (s-1)C 2(W cm)

tr2 (s-1)

sf (S cm-1)1550.0210530523.530.046132070.37103.519671650.0096115366.600.021082367.72111.856821750.0051913770.850.013242386.35136.425651830.003899007.400.005222539.10163.93443

u2 = -112 29 meVn2 = 10 4.64 0.32 HzSeoul National University, Center for Novel States of Complex Material Research

TEM ResultsSeoul National University, Center for Novel States of Complex Material ResearchTime dependent k ph at fixed temperatures

R. C. Yu et al., Phys. Rev. Lett. 68, 2050 (1992)Single crystal C60 Intrinsic originDisorderedOrdered

Alexander V. Kolobov et al., Nature Materials 3, 703(2004)

Rigid building block

T. Siegrist et al., Nature Materials 10 , 202 (2011)

t0-1: scattering rate of ideal structure

Assumption tD-1 nD(t)

Seoul National University, Center for Novel States of Complex Material ResearchIt seems to be natural that annealing time is also an important control parameter of disorder state. Inverse mean free time means a scattering rate. If more than two scattering mechanism exist, total scattering rate is summation of them. In our case, we can consider two different scatterings which is originated from the disorder and a scattering of an ideal structure. In order to describe the time dependence of the thermal-conductivity data quantitatively, we adopt a phenomenological model in which the tau d is proportional to the occupation probability of the disordered state, nD(t). When disorder state becomes ordered state, it is natural to follow exponential decay based on the statistical mechanics. Based on these, we can think the inverse thermal conductivity follows the bottom exponentially decaying equation. The green lines in the left top figure is the resultant fitting curves. The fitting are well explaining the experimental results. So, based on the our fitting we can suggest that the disordered states in fcc GST are gradually change to ordered states and the higher annealing temperature induces the more ordered state. 22Previous Thermal Conductivity (k) Studies

Ho-Ki Lyeo et al., Appl. Phys. Lett. 89, 151904(2006)

W. P. Risk et al., Appl. Phys. Lett. 94, 101906(2009)1. Measuring accurate k of amorphous and crystalline phase 3w method and time domain thermoreflectance 2. Estimating k by electrical measurements based on Wiedemann-Franz law k = k latti. + kel. = k latti. + Ls T k latti. : thermal conductivity of lattice vibration, kel. : thermal conductivity of electrical carriersL : Lorenz numberSeoul National University, Center for Novel States of Complex Material ResearchTo understand thermal properties of phase change materials, there are many studies to measure accurate thermal conductivities of amorphous and crystalline phase using 3w method or time domain thermoreflectance technique. Left bottom figure shows the temperature dependent thermal conductivity of GST using TDTR method. 23Previous Thermal Conductivity (k) StudiesThermal boundary resistanceThermal conductivity of GST filmsWiedemann-Franz Law

W. P. Risk et al., Appl. Phys. Lett. 94, 101906 (2009)

Ho-Ki Lyeo et al., Appl. Phys. Lett. 89, 151904 (2006)

3w method TDTR (time-domain thermoreflectance)Seoul National University, Center for Novel States of Complex Material Research24

What is the origin of time dependent k and s? Grain boundary effect

lphonon~ 4 , lelectron ~ 10