Ferroelectric nanostructures for future high-density non-volatile memory applications - preparation methods, microstructure, and physical properties

Transcription

1 Autumn School, Berlin, Dietrich Hesse and Marin Alexe Max-Planck-Institut für Mikrostrukturphysik, Halle Ferroelectric nanostructures for future high-density non-volatile memory applications - preparation methods, microstructure, and physical properties With contributions of M.-W. Chu, C. Harnagea, S.K. Lee, Woo Lee, K. Nielsch, W. Ma, R. Scholz, I. Szafraniak, and N.D. Zakharov. Partially supported by Deutsche Forschungsgemeinschaft (via the Group of Researchers 404 at Martin-Luther-Universität Halle-Wittenberg), European Commission, and Volkswagen-Stiftung.

2 (1) Introduction Outline - Why nanostructuring ferroelectric materials? - What are ferroelectric materials? - How does nanostructuring affect the physical properties? (2) How to prepare ferroelectric nanostructures for physical studies? - Top-down methods: FIB patterning, electron-beam direct writing, nanoimprint lithography, mask-assisted pulsed laser deposition - Bottom-up methods: Physical self patterning, chemical self-patterning (3) Microstructure and properties I: The role of interfacial defects in the ferroelectric size effect - Quantitative HRTEM study of strain fields in Pb(Zr 0.52 Ti 0.48 )O 3 nanoislands - Impact of the strain fields on the polarization instability (4) Microstructure and properties II: Interfacial dislocations and the mobility of 90 domain walls (5) Microstructure and properties III: The impact of interfaces on ferroelectric imprint in arrays (6) Summary and outlook

3 (1) Introduction

4 Why nanostructuring ferroelectric materials? For example: Ferroelectric non-volatile random access memories (FRAMs) Capacitor Interconnections Electrodes Ferroelectric Gate Interval dielectric Diffusion barriers Source Drain Si PZT = Pb(Zr,Ti)O 3 For memories of Gigabit capacity, lateral condensator dimensions of about 200 nm are required. 4Mbit memory by Samsung in T/1C Cell configuration PZT used 0.6 um, 3-Metal, COB 2,5 Gbit/cm 2 = 2, Bit/cm 2 = ( ). ( ) Bit/cm 2. This corresponds to condensators/cm or cm/condensator or 200 nm x 200 nm capacitor area.

5 What are ferroelectric materials? Displacement direction of the titanium and zirconim ions = Spontaneous Polarization Oxygen, O Lead, Pb Titanium, Ti Zirconium, Zr PZT = Pb(Zr,Ti)O 3

6 Ferroelectricity - a symmetry-based phenomenon Electrostrictive 32 classes No symmetry centre 21 classes Piezoelectric 20 classes Pyroelectric 10 classes Ferroelectric symmetry centre 11 classes Non-piezoelectric 1 class Non-Pyroelectric 10 class Ion shift in the perovkite cell

7 Ferroelectric materials There are more than 500 ferroelectric compounds (without solid solutions) Landolt-Börnstein, Ferro- and Antiferroelectric Substances, Springer, 1975 For most demanding applications only oxides are seriously considered Choosing the optimum material is an application-dependent problem Main ferroelectric oxides Pb-based materials - Pb(Zr,Ti)O 3 Layered perovkites SrBi 2 Ta 2 O 9, Bi 4 Ti 3 O 12 BaTiO 3 -based materials (Ba,Sr)TiO 3 Ergo: Complex chemistry!

8 Ferroelectric materials What are they good for? Piezolectrics: Charge generation by mechanical fields Pyroelectrics: Charge generation by thermal fields Ferroelectrics: Charge generation by electrical fields And converse! Ergo: Memories, sensors, actuators...

9 Ferroelectric materials are anisotropic! For example: Relationship between piezoelectric coefficient d zz and spontaneous polarization P z tet. PZT BaTiO 3

10 Nanostructuring affects the physical properties! - Size effect: Bulk Nanoisland/Film (S. V. Kalinin and D. A. Bonnel) Bulk t: ~ µm t: < ~ 4 nm * Nanoisland/ Film - Failure mechanisms: -- Imprint: PbTiO 3 * A. Roelofs et al. (2002). -- Fatigue: size, after Many (or, maybe, all) of these effects are microstructure-determined!

11 (2) How to prepare ferroelectric nanostructures

12 (2.1) Top-down methods

13 A classical micro/nanostructuring method: Soft lithography Micro Contact Printing Micro Molding G. Whitesides et al., Harvard Univ.

14 Focussed Ion Beam Patterning 1.0 µm 0.5 µm 0.25 µm 0.1 µm R. Ramesh et al., Univ.. of Maryland (1999)

15 Electron beam direct writing 1. Metalorganic layer deposition 2. E-beam E exposure 3. Developing 4. Crystallization

16 Regular arrays of polycrystalline PZT nanoislands Patterned test structure (SEM image) 1 µm x 1 µm 500 nm x 500 nm 250 nm x 250 nm 100 nm x 100 nm M. Alexe,, W. Erfurth,, D. Hesse, MPI Halle (1999)

17 Nanoimprint lithography PZT structures on SrTiO 3 :Nb C. Harnagea,, M. Alexe,, D. Hesse, MPI Halle (2002)

18 Result of nanoimprint lithography Array of 300 nm PZT structures 6 x 6 µm C. Harnagea,, M. Alexe,, D. Hesse, MPI Halle (2002)

19 deposition mask Mask-assisted pulsed laser deposition

20 Latex microsphere monolayers as deposition masks Regular arrays of epitaxial BaTiO 3 and SrBi 2 Ta 2 O 9 nanoislands Array of latex spheres (SEM image) 1. Substrate; 2. Latex sphere; 3. Interstice SEM The result AFM W. Ma, D. Hesse, MPI Halle (2003)

21 A new approach: Metal nanotube membranes as deposition masks Hexagonal lattice Back side view Back side view 7 µm 1 µm 7 µm 600 nm Square lattice Front side view 3 µm d 3 µm 3 µm 500 nm Woo Lee, K. Nielsch,, MPI Halle (2005) Example: d = 280 nm

22 Preparation of metal nanotube membranes via ordered porous alumina Metal sputter Preparation of metal nanotube membranes 1 2 Metal ECD Al 2 O 3 removal Al 2 O Plasma sputtering of metal onto nanoporous Al 2 O 3. Controlled electrochemical deposition (ECD). Removal of nanoporous Al 2 O 3. 3 Woo Lee, K. Nielsch et al., Chem. Letters, 2005 (submitted) Woo Lee, K. Nielsch,, MPI Halle (2005)

23 Metal nanotube membranes as deposition masks Large-area area ferroelectric PZT and BLT nanodot arrays S.K. Lee, W. Lee, M. Alexe, K. Nielsch, D. Hesse, and U. Gösele, Applied Physics Letters 86 (2005) (3p.) TEM SEM d 33 (pm/v) WL khz ATEC-FM PFM Applied Bias (V) S.K. Lee, M. Alexe,, D. Hesse, MPI Halle (2005)

24 (2.2) Bottom-up methods

25 Physical self-assembly techniques I Island growth mode in MOCVD PLD PZT islands on SrTiO 3 (111) (Image size 1 µm x 1 µm) Height scale: M. Shimizu, H. Fujisawa et al., Himeji Inst. Technol.. (2003) BaTiO 3 islands on SrTiO 3 A. Visinoiu and M. Alexe, MPI Halle (2003) Geral approach: Tune the growth conditions such as to achieve the island growth mode,, which allows the fabrication of nanoscale islands!

26 Physical self-assembly techniques II Making use of a microstructural instability in epitaxial ultra-thin thin CSD-deposited films T = const. a - distance between pits x - pit size; h 0 - original film thickness A. Seifert, A. Vojta, J.S. Speck, F.F. Lange (1996)

27 Physical self-assembly techniques II Self-assembled PZT structures obtained by CSD I. Szafraniak and M. Alexe,, MPI Halle (2003)

28 Physical self-assembly techniques II Nanosize ferroelectrics by self-patterning: Annealing temperature 800 o C 950 o C 1100 o C I. Szafraniak and M. Alexe,, MPI Halle (2003)

29 Chemical self-assembly techniques Hydrothermal growth Microemulsion technique Hydrothermal growth Highly ordered epitaxial PZT nanostructures obtained by hydrothermal growth on a Nb:STO (100) single crystal substrate Agglomerated BaTiO 3 nanoparticles (top) and BaTiO 3 nanoparticles deposited on a Nb:STO substrate (bottom) S. Bhattacharyya and M. Alexe,, MPI Halle (2003)

30 (3) Microstructure and properties I: Microstructure and properties I: The role of interfacial defects in the ferroelectric size effect

31 Epitaxial, tetragonal Pb(Zr 0.52 Ti 0.48 )O 3 nanoislands on SrTiO 3 made by CSD and a self-ordering process at 800 C SEM AFM 20 nm 500 nm 10 nm HREM PZT 2.5 µm Misfit Dislocations 0 nm [010] Nb:SrTiO 3 (STO) 4 nm From cross section electron microscopy (HREM): Truncated-pyramid nanoislands, average height: ~9 nm; average base length: ~50 nm.

32 Origin of misfit dislocations: misfit (δ) and c-axis orientation 4.15 a c Lattice Parameters (A) o Tetragonal Cubic 2 Cubic 1 PZT STO o T ( C) PZT STO c c a a δ = c-domain a PZT -a STO a STO = 3.4% (001) PZT [100] PZT (001) STO [100] STO 1 B. Noheda et al., Appl. Phys. Lett. 74, 2059 (1999); 2 S. Stemmer et al., Phys. Stat. Sol. (a) 147, 135 (1995).

33 HREM investigations of misfit dislocations [010] -PbO -(Zr,Ti)O 2 -SrO PZT STO T -TiO 2 -SrO Edge Dislocation: Burgers vector (b) = <100> = ~4 Å 2 nm The PZT lattice around the dislocation core is strongly distorted!

34 Strain Field and Strain Energy of Edge Dislocations Edge Dislocation * y (r, θ) r θ x dislocation z dislocation Strain Field: * sin θ r ~b interface ( ) Strain Energy: * ~ (b interface ) 2 ln( ) h r r: dislocation core radius b ~ 5b * Elementary Dislocation Theory, J. Weertman and J. R. Weertman, Macmillian (New York), 1964.

35 Quantitative HREM measurements of strain fields * [010] Image of a perfect crystal: * PZT ΣH g exp{2πigr} ; g H g = A g exp{ip g } STO A g : amplitude P g : phase r : position in the image g : reciprocal lattice vector 2 nm * M. J. Hÿtch et al., Ultramicroscopy 74, 131 (1998); Microsc. Microanal. Microstruct. 8, 41 (1997).

36 Quantitative HREM Measurements of Strain Fields * Fourier Transform STO STO Image of a particular set of lattice fringes: * 2A g (r)cos{2πg r + P g (r)} PZT PZT * M. J. Hÿtch et al., Ultramicroscopy 74, 131 (1998); Microsc. Microanal. Microstruct. 8, 41 (1997).

37 Quantitative HREM Measurements of Strain Fields * Fourier Transform Image of a particular set of lattice fringes: * 2A g (r)cos{2πg. r + P g (r)} g g' 101 STO 101 PZT g = g - g' 2A g (r)cos{ 2π g. r + 2π g. r+p g (r)} Inverse FT Geometric Phase Gradient: P g (r) = 2π g * M. J. Hÿtch et al., Ultramicroscopy 74, 131 (1998); Microsc. Microanal. Microstruct. 8, 41 (1997).

38 Quantitative HREM Measurements of Strain Fields * Fourier Transform g 2 = 001 STO g 1 =101 STO Image of a particular set of lattice fringes: * 2A g (r)cos{2πg. r + P g (r)} g 2A g (r)cos{ 2πg. r + 2π g. r+p g (r)} Inverse FT Geometric Phase Gradient: P g (r) = 2π g * M. J. Hÿtch et al., Ultramicroscopy 74, 131 (1998); Microsc. Microanal. Microstruct. 8, 41 (1997).

39 Quantitative HREM measurements of strain fields Geometric phase images (P g ) g 1 = 101 g 2 = 001 PZT PZT STO STO c a π 2 nm c a 2 nm 0 -π Phase fluctuations in the reference lattice (STO) due to: (a) noise, (b) thickness, and/or (c) slight misorientation.

40 Quantitative HREM Measurements of Strain Fields Displacement Field Images ( u x ) - u y = 2π ( )( ) a 2y 1 a 1x a 1y a 2x P g1 P g2 PZT PZT STO c a 2 nm = - 1 2π 1-1 STO c a 2 nm 0 1 PZT 0.5 PZT π STO c a 2 nm STO c a 2 nm 0 -π

41 Quantitative HREM Measurements of Strain Fields Strain Field Images (ε) * ε = ½ (e + e T ) e = ( e xx e xy ) e yy e yx = u x x u y x u x y ( ) u y y e - local distortion; e T - transpose of e. * M. J. Hÿtch et al., Ultramicroscopy 74, 131 (1998); Microsc. Microanal. Microstruct. 8, 41 (1997).

42 Quantitative HREM measurements of strain fields Strain field images ε xx ε xy ε yy PZT T PZT T PZT T [001] STO [100] 4 nm STO 4 nm STO 4 nm [001] [001] [100] [100] In-plane (ε xx ), shear (ε xy ), and out-of-plane (ε yy ) strain fields predominantly localize on the PZT side, asymmetrically extending ~4 nm into the PZT nanoislands, because PZT is softer than STO.

43 Impact of the strain fields on the polarization instability Partial volume V p of nanoislands affected by strain fields HREM [010] PZT ~8 nm STO ~4 nm 4 nm Plan-view ~50 nm [010] [100] Dislocation networks PZT Height ~10 nm g = [220] 60 nm ~8 nm V p ~ 0.5

44 ZPFM signal (a. u.) Piezoresponse force microscopy (PFM) investigations AFM bias z P s PZT E STO Applied bias (V)

45 Piezoresponse force microscopy (PFM) investigations Partial volume V p of nanoislands affected by strain fields 150 PZT~10 nm PFM signal (a.u.) V p ~ Applied Bias (V)

46 Piezoresponse force microscopy (PFM) investigations Partial volume V p of nanoislands affected by strain fields PFM signal (a.u.) V p ~ 0.3 PZT~10 nm PZT~20 nm V p ~ Applied Bias (V)

47 Piezoresponse force microscopy (PFM) investigations Partial volume V p of nanoislands affected by strain fields PFM signal (a.u.) PZT~10 nm PZT~20 nm PTO~9 nm V p ~ 0.3 V p = 0 [010] -50 PbTiO 3 (PTO) -100 V p ~ Misfit dislocation FREE STO Applied Bias (V) 3 nm

48 Conclusions: (1) An interfacial strain field scenario for the polarization instability of ferroelectrics with confined dimensions is proposed by quantitative HREM and PFM measurements. (2) Interfacial misfit engineering is indispensable for obtaining nano-structured ferroelectrics with stable polarization. (Published in the February, 2004, issue of Nature Materials.) From a comment by Waser and Rüdiger in the same issue

49 September 2004: A thermodynamic model based on Landau-Devonshire formalism

50 (4) Microstructure and properties II: Microstructure and properties II: Interfacial dislocations and the mobility of 90 domain walls

51 90 Domains in Ferroelectrics tetragonal Pb(Zr 0.4 Ti 0.6 )O 3 T > T c T < T c Paraelectric (cubic) ~45 P s c a c a Thin-film capacitors: 90 a/c domains elastically clamped by the substrate, therefore electrically inactive. Nanoislands* (~100 nm, lateral): 90 a/c domains switchable. *V. Nagarajan et al., Nature Mater. 2, 43 (2002).

52 Interaction of 90 Domains & Dislocations in Pb(Zr0.4Ti0.6)O3 Nanoislands? [010] a-domain c-domain Domain Domain Wall Wall ~45 T STO T T Ps Ps 4 nm 2 nm

53 90 Domains in Pb(Zr 0.4 Ti 0.6 )O 3 Nanoislands nm Courtesy of D. Shilo et al.* (i) Angular separation ~1, (ii) Ferroelastic strain ~3.5%, (iii) Domain wall width ~1.5nm.* 2 nm Biaxial stress of 90 domain: ~5.4 GPa *D. Shilo et al., Nature Mater. 3, 453 (2004).

54 Strain Fields of 90 Domains: Geometric Phase Method* Strain Field Images (ε) ε xx ε xy ε yy PZT 2 nm PZT PZT [001] [001] [001] [100] [100] In-plane (ε xx ), shear (ε xy ), and out-of-plane (ε yy ) strain fields localized on the domain wall: (1) Extending ~2 ± 0.5 nm around the domain wall, (2) Domain wall as a compressively strained region embedded in strain free PZT. [100] *M.-W. Chu et al., Nature Mater. 3, 87 (2004).

55 90 Domains + Interfacial Dislocations [010] PZT STO P s T P s 1. Domain wall does NOT terminate on the dislocation core; 2. Edge Dislocation: Burgers vector (b) = <001> = ~4 Å. In general case, b // interface!! 2 nm

56 Strain Fields of 90 Domains + Interfacial Dislocations Strain Field Images (ε) ε xx ε xy ε yy PZT PZT PZT [001] STO [100] 4 nm [001] STO [100] 4 nm [001] STO [100] 4 nm 0.03 In-plane (ε xx ), shear (ε xy ), and out-of-plane (ε yy ) strain fields : (1) Localized on domain wall and dislocation core, (2) Domain wall (compressive) elastically coupled to dislocation core (tensile)

57 Impacts of Elastic Coupling: Electric Switching of 90 Domains Ground State PZT P s [010] P s PZT P s P s P s T STO 4 nm STO 4 nm + (1) Such in-plane climb of extra-half plane is energetically unfavorable;* (2) Interfacial dislocations (b interface) behave as pinning centers for 90 domain walls. * A. Yu. Emelyanov and N. A. Pertsev, Integ. Ferroelectrics 32, 343 (2001).

58 (5) Microstructure and properties III: Microstructure and properties III: The impact of interfaces on ferroelectric imprint in arrays

59 Regular arrays of polycrystalline PZT nanoislands As-prepared regular PZT arrays (SEM image) Patterned test structure 500 nm x 500 nm 250 nm x 250 nm 100 nm x 100 nm

60 Ferroelectric properties of polycrystalline PZT nanostructures PZT 500 nm x 500 nm 250 nm x 250 nm 100 nm x 100 nm PFM hysteresis curves of a 1 µm and 100 nm large nanostructure Switching without cross talk

61 Size-dependent imprint in regular arrays of polycrystalline PZT nanoislands Size-dependent polarization imprint 1000 nm 500 nm 250 nm 100 nm bias z P s PZT E STO PFM

62 Size-dependent imprint in regular arrays of polycrystalline PZT nanoislands Size-dependent polarization imprint - Phenomenological model Ω Offset -0.4 Ω = β β 2 1 d 2β 2 1 d δ 2δ 1 h 2δ + 1 h Cell size (nm) β = (34.5 ± 3.7) nm δ = (7.5 ± 4.5) nm

63 Regular arrays of epitaxial BaTiO 3 nanoislands Preparation of regular arrays by Pulsed laser deposition Array of latex spheres (SEM image) 1. Substrate; 2. Latex sphere; 3. Interstice SEM The result AFM

64 Thickness-dependent imprint in regular arrays of epitaxial BaTiO 3 nanoislands Epitaxial structure (TEM after FIB) Thickness-dependent imprint d 33 (a. u.) 4 2 Bias (V) a b c -4 PFM a - 45 nm thick, 230 nm wide b - 25 nm thick, 160 nm wide c - 25 nm thick, 500 nm wide Non-switchable layer, about 10 nm thick? Interface-determined imprint?

65 Thickness-dependent imprint in regular arrays of epitaxial BaTiO 3 nanoislands HRTEM: Is there any interface-near structure modification??

66 Thickness-dependent imprint in regular arrays of epitaxial BaTiO 3 nanoislands Is it due to crystallization kinetics?

67 Modification of the interface-near region by a reaction-diffusion process SrBi 2 Ta 2 O 9 nanopyramid on SrTiO 3, crystallized at 950 C B.F. 022-D.F.

68 (6) Summary and outlook Ferroelectric nanostructures can be grown by a variety of physical and chemical methods. Nanoparticles in powders, and arrays of polycrystalline or epitaxial nanostructures on various substrates can be grown. Which method to choose depends on the envisaged properties. The real structure of the nanostructures considerably influences the physical properties. Interfaces, in particular, may have a strong impact: - Interfacial misfit dislocations were clearly shown to contribute to the size effect of epitaxial PZT nanoislands. Most probably they also hinder switching by pinning of 90 domains. - Structurally modified, non-switchable regions near interfaces can lead to size- or thickness-dependent imprint effects. From a combined application of high-resolution transmission electron microscopy and piezoresponse scanning force microscopy one can expect further progress in clarifying the role of the microstructure in nanostructured ferroelectrics.

69 Some publications C. Harnagea, M. Alexe, J. Schilling, J. Choi, R.B. Wehrspohn, D. Hesse, and U. Gösele, Mesoscopic ferroelectric cell arrays prepared by imprint lithography, Appl. Phys. Lett. 83 (2003) I. Szafraniak, C. Harnagea, R. Scholz, S. Bhattacharyya, D. Hesse, and M. Alexe, Ferroelectric epitaxial nanocrystals obtained by a self-patterning method Appl. Phys. Lett. 83 (2003) W. Ma, C. Harnagea, D. Hesse, and U. Gösele, Well-ordered arrays of pyramid-shaped ferroelectric BaTiO 3 nanostructures. Appl. Phys. Lett. 83 (2003) M.-W. Chu, I. Szafraniak, R. Scholz, D. Hesse, M. Alexe, and U. Gösele, Impact of misfit dislocations on the polarization instability of epitaxial nanostructured ferroelectric perovskites Nature Materials 3 (2004) W. Ma and D. Hesse, Polarization imprint in ordered arrays of epitaxial ferroelectric nanostructures Appl. Phys. Lett. 84 (2004) W. Ma and D. Hesse, Microstructure and piezoelectric properties of sub-80 nm high polycrystalline SrBi 2 Ta 2 O 9 nanostructures within well-ordered arrays Appl. Phys. Lett. 85 (2004), October issue. S.K. Lee, W. Lee, M. Alexe, K. Nielsch, D. Hesse, and U. Gösele, Well-ordered large-area arrays of epitaxial ferroelectric (Bi,La) 4 Ti 3 O 12 nanostructures fabricated by gold nanotube-membrane lithography Applied Physics Letters 86 (2005) (3p.)

70 Thickness-dependent imprint in regular arrays of epitaxial BaTiO 3 nanoislands Spacing of misfit dislocations varies extensively. Example for narrow spacing: BaTiO 3 SrTiO 3 3nm 12.1 nm 11.7 nm d(110) = 2.76 Å The equilibrium distance of misfit dislocations is P = b / f. f Obviously, b 3.95 Å. The misfit is f = 2 (a( f - a s ) / (a( f + a s ). a BTO = Å; c BTO = Å; a STO = Å. (a) If BaTiO 3 is c-axis-up oriented: f = %. Hence P = b / f = 3.95 / = 180 Å. (b) If BaTiO 3 is a-axis-up oriented: f = %. Hence P = b / f = 3.95 / = 120 Å.

71 Thickness-dependent imprint in regular arrays of epitaxial BaTiO 3 nanoislands HRTEM: What is the interface-near structure modification like? P s P s A 10 nm thin layer having partly in-plane, partly out-of-plane polarization?

72 The polarization instability: Size effect in nano-ferroelectrics? Bulk Nanoisland/Film Bulk t: ~ µm t: < ~4 nm * Nanoisland/ Film PbTiO 3 (S. V. Kalinin and D. A. Bonnel) Depolarization Field (E dp ) Perovskite a p = ~3.8 Å P s E dp P s + + * A. Roelofs et al., Appl. Phys. Lett. 81, 5231 (2002).

73 A new approach: Metal nanotube membranes as deposition masks Hexagonal lattice Back side view Back side view 7 µm 1 µm 7 µm 600 nm Square lattice Front side view 3 µm d 3 µm 3 µm 500 nm Example: d = 280 nm

74 A new approach: Metal nanotube membranes as deposition masks Large-area area ferroelectric PZT and BLT nanodot arrays S.K. Lee, W. Lee, M. Alexe, K. Nielsch, D. Hesse, and U. Gösele, Applied Physics Letters 86 (2005) (3p.) TEM SEM d 33 (pm/v) WL khz ATEC-FM PFM Applied Bias (V)