1. Seoul National University, Center for Novel States of Complex Material Research Disorder-to-order induced thermal conductivity change of Ge 2 Sb 2 Te 5 thin film in the cubic crystalline phase Hyung Joon Kim CeNSCMR, Department of Physics and Astronomy, Seoul National University, Seoul, S. Korea 2012 MRS Spring Meeting 2012. 4. 12

2. Seoul National University, Center for Novel States of Complex Material Research Collaborators Yisheng Chai, Jae Wook Kim, Kee Hoon Kim Hanbeam Corporation Jong Ho Lee, Seung Wook Ryu, Young Bae Ahn, Hyeong Joon Kim Su Youn Lee, Byung-ki Cheong Thin Film Materials Research Center, KIST Seung Min Lee School of Materials Science and Engineering, SNU Department of physics and astronomy, CeNSCMR, SNU

3. Seoul National University, Center for Novel States of Complex Material Research Introduction Time T T Amorphous phase High resistivity Low reflectivity Crystalline phase Low resistivity High reflectivity T melt T cyst T melt T cyst Ge TeSb GeTe Sb 2 Te 3 PcRAM (Phase-change RAM) : Ge 2 Sb 2 Te 5 Compact disc Digital versatile disc Blu-ray disc GeTe-Sb 2 Te 3 pseudobinary Sb-Te binary Top Electrode Bottom Electrode Joule heating Time I ~50ns ~300ns Logic “1”Logic “0” PcRAM (Phase-change random access memory) : Ge 2 Sb 2 Te 5 (GST) CrystallineAmorphous “Reset pulse” “Set pulse” It is important to know how thermal conductivity of Ge 2 Sb 2 Te 5 films varies with temperature and thermal history.

4. Seoul National University, Center for Novel States of Complex Material Research Motivation 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 conductivity Wiedemann-Franz law :  ph  el =  ph  L  T  Simultaneous measurement of electrical and thermal conductivities as a function of temperature

5. Seoul National University, Center for Novel States of Complex Material Research Thermal resistance :  T/P Real Time 3  Method with a Reference Structure  Si = 1.4 W cm -1 K -1 at room T = 0.76 W cm -1 K -1 at 201 o C Reported value : 1.48 Wcm -1 K -1 at room T 0.79 W cm -1 K -1 at 201 o C C. J. Glassbrenner and G. A. Slack, Phys. Rev. 134, A1058 (1964) Si (525μm) Sample structure : with GST Heater (Ti/Au=20/80nm) 180nm Sputtered SiO 2 ~ 400nm GST 100nm Thermal SiO 2 VV3VV3 I(t) W. P. Risk et al., Appl. Phys. Lett. 94, 101906(2009) 3  method + Multi-layer model Si (525μm) Reference structure : without GST Heater (Ti/Au=20/80nm) 180nm Sputtered SiO 2 100nm Thermal SiO 2 1D heat flow Heater I = I 0 sin(  t) T(t)T(t) T Base Thermal link David G. Cahill, Rev. Sci. Instrum. 61, 802 (1990) I(  )  P(2  )   T(2  )   R(2  )  V 3  Ref. Sample Heater width : 32  m  T Sample /P-  T Ref. /P : frequency independent At fixed frequency,  (t,T) can be measured successfully. Heat sink h : thickness of the GST layer (400 nm) w : heater width

6. Seoul National University, Center for Novel States of Complex Material Research T Cryst : ~130 ℃ GST Ref.  of amorphous GST ~ 0.18 Wm -1 K -1 Phase transition T Cryst ~ 130 ℃ Time evolution of  at fixed T  (t) and  (T) of GST Film Amorphous Face-centered cubic

7. Seoul National University, Center for Novel States of Complex Material Research 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.

8. Seoul National University, Center for Novel States of Complex Material Research Wiedemann-Franz Law and Phonon Thermal Conductivity l ph ~ 4 Å smaller than lattice constant (6 Å ) L = 2.44 x 10 -8 W  /K 2 During cooling : linear behavior in  vs  T  Wiedemann-Franz law The  ph increases as increasing the annealing temperature. l phonon ~ 4 Å smaller than lattice constant (6 Å ) << grain size (~ 20nm)  Intra-grain mechanism : structural disorder The source of disorder :vacancies at Ge/Sb site ~ 15% in the cubic GST

9. Seoul National University, Center for Novel States of Complex Material Research T. Siegrist et al., Nature Materials 10, 202 (2011) Disorder Induced Localization

10. Seoul National University, Center for Novel States of Complex Material Research Time dependent  ph  at fixed temperatures Intrinsic origin : vacancy and small rigid building block Disordered Ordered Alexander V. Kolobov et al., Nature Materials 3, 703(2004) We believe that the formation of the ordered chain structure is a prelude for the phase transformation from the rocksalt-like to hexagonal structure. Possible origins Decreasing of film thickness Increasing of grain size Decreasing thermal boundary resistance Disordered to ordered state in the film Assumption :  D -1 ∝ n D (t) An evidence of time dependent disorder-to-order change in the fcc GST  0 -1 : scattering rate of ideal structure  D -1 : scattering rate of disorder n D : the occupation probability of the disordered state

11. Seoul National University, Center for Novel States of Complex Material Research Summary We successfully established the real time thermal conductivity measurement using 3  method with a reference structure. By simultaneously measuring  (T) and κ(T)  GST follows the Wiedemann-Franz law, once a structure is fixed.  Annealing temperature dependence of phonon thermal conductivity : a strong evidence of a structure ordering For the first time, we observed the time dependent κ ph.  Time dependent disorder relaxation during annealing The κ ph can be a good probe to understand the crystalline phase of phase change materials.

12. Seoul National University, Center for Novel States of Complex Material Research

14. Time dependent  ph  at fixed temperatures Intrinsic origin : vacancy and small rigid building block Disordered Ordered Alexander V. Kolobov et al., Nature Materials 3, 703(2004) We believe that the formation of the ordered chain structure is a prelude for the phase transformation from the rocksalt-like to hexagonal structure. Possible origins Decreasing of film thickness Increasing of grain size Decreasing thermal boundary resistance Disordered to ordered state in the film John P. Reifenberg et al., IEEE Electron Device Lett. 31, 56 (2010) Max. T : 155 o CMax. T : 175 o C Different maximum T (165 o C and 175 o C) : similar grain size and film thickness l phonon ~ 4 Å,<< grain size

15. Seoul National University, Center for Novel States of Complex Material Research 1D Differential Method with Ref. Structure Si substrate (525μm) Thermal SiO2(100nm) Sputtered SiO 2 (180nm) Heater (Ti20nm/Au80nm) GST(400nm) Sputtered SiO 2 (180nm) Si substrate (525μm) Thermal SiO2(100nm) Heater (Ti20nm/Au80nm) ΔTfΔTf ΔTfΔTf Ref. structure Sample structure Frequency ΔT/P ΔT f /P Ref. Sample GST No GST w = 32μm >> t total ~ 0.7μm Frequency ΔT/P Ref. Sample ΔT f /P Temperature

16. Seoul National University, Center for Novel States of Complex Material Research Thermal Conductivity (  ) Measurement of Films : 3  Method T(t)T(t) ∆T =  P 0 /2  T =  T 2D +  T 1D Heater I = I 0 sin(  t) T(t)T(t) T Base Thermal link David G. Cahill, Rev. Sci. Instrum. 61, 802 (1990) P 0,rms : power per unit length of the heater ΔT1ΔT1 ΔT2ΔT2 Linear region ln  TT  T 2D  T 1D : frequency independant S.-M. Lee and David G. Cahill, J. Appl. Phys. 81, 2590(1997) Heater w h w ≫ hw ≫ h Heat sink Heater I = I 0 sin(  t) T(t)T(t) T Base  thick T Base Heat sink T(t)T(t)  I = I 0 sin(  t) Heater 2D heat flow 1D heat flow

17. Seoul National University, Center for Novel States of Complex Material Research Lock-in SR830 out. AB MDAC Manganin wire : R ref Sample Computer Amplifier VRVR VSVS High T. Chamber V 3ω /V 1ω V S (ω)  V S (ω) – V R (ω) × (MDAC Value ) ~ 0 MDAC value (controlled by PC): ~ R S /R R V 3ω Measurement Scheme and Equipments MDAC :Multi digital to analog converter.

18. Seoul National University, Center for Novel States of Complex Material Research Slope GST = Slope No-GST   Si = 1.4 Wcm -1 K -1 Ref. : 1.48 Wcm -1 K -1 @300K ∆T f /P : Almost same at every freq. ∆T/P vs Frequency C. J. Glassbrenner and Glen A. Slack, Phys. Rev. 134, A1058 (1964) Thermal wavelength (λ) : (D/i2ω) 0.5 At 97Hz (1) 2D heat flow condition Si  λ = 64 μm > 16 μm (Heater width : 32μm) (2) 1D heat flow condition SiO 2  λ = 68 μm >>180nm GST  λ = 28 μm >> 400nm

19. Seoul National University, Center for Novel States of Complex Material Research  and  of GST film are measured simultaneously. (not same piece) During T up,  vs  T trend don’t follow Wiedemann-Franz law. (k increase more faster) After cool down,  vs  T shape looks linear.

20. Seoul National University, Center for Novel States of Complex Material Research Time dependent  at fixed temperatures Annealing T ( o C) C 1 (  cm) t r1 (s -1 ) C 2 (  cm) t r2 (s -1 )  f (S cm -1 ) 1550.0210530523.530.046132070.37103.51967 1650.0096115366.600.021082367.72111.85682 1750.0051913770.850.013242386.35136.42565 1830.003899007.400.005222539.10163.93443 u 2 = -112 ± 29 meV  = 10 － 4.64 ± 0.32 Hz

21. Seoul National University, Center for Novel States of Complex Material Research TEM Results

22. Seoul National University, Center for Novel States of Complex Material Research Time dependent  ph  at fixed temperatures R. C. Yu et al., Phys. Rev. Lett. 68, 2050 (1992) Single crystal C 60 Intrinsic origin Disordered Ordered Alexander V. Kolobov et al., Nature Materials 3, 703(2004) Rigid building block T. Siegrist et al., Nature Materials 10, 202 (2011)  0 -1 : scattering rate of ideal structure Assumption  D -1 ∝ n D (t)

23. Seoul National University, Center for Novel States of Complex Material Research Previous Thermal Conductivity (  ) 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  of amorphous and crystalline phase 3  method and time domain thermoreflectance 2. Estimating  by electrical measurements based on Wiedemann-Franz law  latti.  el. =  latti.  L  T  latti. : thermal conductivity of lattice vibration,  el. : thermal conductivity of electrical carriers L : Lorenz number

24. Seoul National University, Center for Novel States of Complex Material Research Previous Thermal Conductivity (  ) Studies Thermal boundary resistanceThermal conductivity of GST films Wiedemann-Franz Law W. P. Risk et al., Appl. Phys. Lett. 94, 101906 (2009) Ho-Ki Lyeo et al., Appl. Phys. Lett. 89, 151904 (2006) 3  method TDTR (time-domain thermoreflectance)

25. Seoul National University, Center for Novel States of Complex Material Research What is the origin of time dependent  and  ? Grain boundary effect l phonon ~ 4 Å, l electron ~ 10 Å << grain size ~ 200 Å  Time dependent  and  at fixed temperatures C v = 1.285 J cm -3 K -1 Lyeo et al., Appl. Phys. Lett. 89, 151904(2006) C. Peng et al., J. Appl. Phys. 82, 4183 (1997) T. Siegrist et al., Nature Materials 10, 202 (2011) phonon = 3.19 nm/ps Not dominant! R. C. Yu et al., Phys. Rev. Lett. 68, 2050 (1992) Single crystal C 60 Intrinsic origin : vacancy and small rigid building block Disordered Ordered Alexander V. Kolobov et al., Nature Materials 3, 703(2004) We believe that the formation of the ordered chain structure is a prelude for the phase transformation from the rocksalt-like to hexagonal structure.

26. Seoul National University, Center for Novel States of Complex Material Research i : different scattering mechanism  0 -1 : scattering rate of ideal structure n D (t) : occupation probability of disordered orientations : the average librational frequency u : an energy barrier between different orientations(ordered and disordered) The rate from a disordered to an ordered orientation Disorder Contribution of the Mean Free time Assumption :  OD -1 ∝ n D (t)

27. Seoul National University, Center for Novel States of Complex Material Research Time dependent  at fixed temperatures  0 -1 : scattering rate of ideal structure n D (t) : occupation probability of disordered orientations : the average librational frequency u : energy barrier DisorderOrder u Assumption :  D -1 ∝ n D (t) n D (t) : occupation probability of disordered orientations : the average librational frequency u : energy barrier DisorderOrder u

28. Seoul National University, Center for Novel States of Complex Material Research Annealing T ( o C)C (W -1 m K)t r (s -1 )  f (W m -1 K -1 ) 1550.6015730523.530.44837 1650.6520815366.600.45108 1750.5722213770.850.49051 1830.613689007.390.54764 Time dependent  at fixed temperatures u = 676 ± 148 meV = 10 3.51 ± 1.47 Hz

29. Seoul National University, Center for Novel States of Complex Material Research Motivations Time T T Amorphous phase High resistivity Low reflectivity Crystalline phase Low resistivity High reflectivity T melt T cyst T melt T cyst Ge TeSb GeTe Sb 2 Te 3 PcRAM (Phase-change RAM) : Ge 2 Sb 2 Te 5 Compact disc Digital versatile disc Blu-ray disc GeTe-Sb 2 Te 3 pseudobinary Sb-Te binary Top Electrode Bottom Electrode Joule heating Time I ~50ns ~300ns Logic “1”Logic “0” PcRAM (Phase-change random access memory) : Ge 2 Sb 2 Te 5 (GST) CrystallineAmorphous “Reset pulse” “Set pulse” It is important to know how thermal conductivity of Ge 2 Sb 2 Te 5 films varies with temperature and thermal history.

30. Seoul National University, Center for Novel States of Complex Material Research Thermal boundary resistance Thermal conductivity of GST films Wiedemann-Franz Law Motivation It is important to know how thermal conductivity of Ge 2 Sb 2 Te 5 films varies with temperature and thermal history.  ph  el =  ph  L  T  Real time  measurement using 3  method + simultaneously measuring resistivity Time dependent  variation

31. Seoul National University, Center for Novel States of Complex Material Research Wiedemann-Franz Law Motivation  ph  el =  ph  L  T  Simultaneously measuring electrical and thermal conductivities as a function of temperature and time. Annealing temperature dependence of  ph T. Siegrist et al., Nature Materials 10, 202 (2011) Metal-insulator transition : localization Localization source Disorder (Anderson localization) Disorder source in the crystalline GST  Random vacancies at Ge/Sb site ~ 15% If the disorder plays an important role, it will also affect the phonon thermal conductivity.

32. Seoul National University, Center for Novel States of Complex Material Research Time dependent  ph  at fixed temperatures Intrinsic origin Disordered Ordered Alexander V. Kolobov et al., Nature Materials 3, 703(2004) Rigid building block

33. Seoul National University, Center for Novel States of Complex Material Research

34.  Si = 1.4 W cm -1 K -1 at room T = 0.76 W cm -1 K -1 at 201 o C Reported value : 1.48 Wcm -1 K -1 at room T 0.79 W cm -1 K -1 at 201 o C C. J. Glassbrenner and G. A. Slack, Phys. Rev. 134, A1058 (1964) Si (525μm) Sample structure : with GST Heater (Ti/Au=20/80nm) 180nm Sputtered SiO 2 ~ 400nm GST 100nm Thermal SiO 2 VV3VV3 I(t) Si (525μm) Reference structure : without GST Heater (Ti/Au=20/80nm) 180nm Sputtered SiO 2 100nm Thermal SiO 2 Heater I = I 0 sin(  t) T(t)T(t) T Base Thermal link Ref. Sample Figure 1

35. Seoul National University, Center for Novel States of Complex Material Research Figure 2 GST Ref. T Cryst : ~130 ℃ Amorphous Face-centered cubic

36. Seoul National University, Center for Novel States of Complex Material Research Figure 3

37. Seoul National University, Center for Novel States of Complex Material Research Figure 4