Oxide Nanoelectronics Chang Beom Eom Department of Materials Science and Engineering University of Wisconsin Madison Supported by National Science Foundation (NIRT, FRG) Office of Naval Research Army Research Office AFOSR DOE/BES IARPA DARPA David & Lucile Packard Fellowship
Multifunctionality of Complex Oxides MRAMs FRAMs Maglev trains Ultrasound imaging Ferroelectrics Ferromagnetics Superconductors Optical waveguides Actuators Medical imaging Piezoelectrics Complex oxides Nonlinear optics LEDs Semiconductors Dielectrics Transparent conductors Metallic conductors Gate oxides Flat panel displays Fuel cells
What is unique about Multifunctional Oxides? Strong anisotropy Variable oxygen stoichiometry Controllable by cation doping Novel Hybrid devices with many multifunctional oxides Cu O Cu Cu O Silicon Cu YBa 2 Cu 3 O 7
Various forms of Multifunctional Oxides 1. Ceramics easy to prepare quickest way to search for new materials intrinsic properties get masked by grain boundaries 2. Single Crystals intrinsic properties can be studied hard to obtain large samples with uniform properties large sample dimensions 3. Thin Films well defined dimensions (thickness, lateral dimensions) device applications (multilayered heterostructures) orientation controllable artificial structures can be made
Not all films are the same! Polycrystalline Thin Film Epitaxial Thin Film
Lattice Constant (Å) Enhancement of ferroelectric transition temperature in strained BaTiO 3 thin films c 4.15 4.10 4.05 4.00 3.95 T c (50nm BaTiO 3 /DyScO 3 ) a Epitaxial BaTiO 3 GdScO 3 or DyScO 3 substrate T c (100nm BaTiO 3 /GdScO 3 ) T c (Single Crystal) a 3.90 0 100 200 300 400 500 600 700 800 Temperature ( C) K.J. Choi et al. Science, 306, 1005 (2004)
Superior J c of Co-doped Ba122 film on SrTiO 3 (BaTiO 3 )/LSAT 10 7 10 6 Critical Current density (J c at ~5K) Ba122 on BTO/LSAT Ba122 on STO/LSAT Ba122 film STO(BTO) LSAT J c (A/cm 2 ) 10 5 10 4 Ba122 bulk single crystal (Yamamoto et al., Appl. Phys. Lett., 2008) Sr122 on bare LSAT (Hiramatsu et al., Appl. Phys. Express, 2008) Sr122 on bare LAO (Choi et al., Appl. Phys. Lett.,2009) Ba122 on bare LSAT 10 3 0 2 4 6 8 10 12 14 Magnetic Field (T) Ba(Sr)122 film (La,Sr)(Al,Ta)O 3 LSAT or LaAlO 3 LAO S. Lee, Nature Materials 9, 397 (2010) 4
Why we need atomic layer control? Nanoscale Control of Interfaces and Defects Uniformity of Barrier Layers Field Effect Devices SQUID Top Electrode R I ~ nm Coated Conductors R I Bottom Electrode Substrate Magnetic Tunnel Junction Ferroelectric Memories P (µc/cm 2 ) Voltage (V)
Quasi-ideal surface of SrTiO 3 substrate AFM as received: mixed termination SrTiO 3 (001) Ti Sr O 250 nm 500 nm Etching in BHF SrO termination plane TiO 2 termination plane M. Kawasaki, Science, 266, 1540 (1994). [G. Koster et al., APL, 73, 2920, (1998).] Perfect TiO 2 -terminated SrTiO 3 substrate
Thermodynamic stability diagrams 10 800 600 400 T [ o C] 200 25 0 Log (P O 2/Torr) -10-20 -30-40 -50 V VO V 2 O 3 VO 2 V 2 O 5 Cooling Curve -60 5 10 15 20 25 30 35 10000/T [1/K] High PO 2 is needed to stabilize oxide phases
How to make atomic flux? Laser Ablation
Sputtering Target Ar + ion Atoms
RHEED Intensity and Pattern Intensity: growth kinetics Pattern: surface structures and morphologies thin reciprocal lattice rods RHEED pattern Ideal smooth Ewald sphere screen 3D islands Polycrystal RHEED intensity oscillation Real smooth broad reciprocal lattice rods
Laser MBE system with in situ High Pressure RHEED RHEED works up to 1 Torr O 2 Phase transition of SrRuO 3 load lock laser-beam phosphor screen + camera substrate holder heater 0-3 o Ø 0.5 mm target holder Electron-gun Ø 1 mm 650C 100C filament SrTiO 3 850 o C 50 mtorr PO 2 I/I 0-1 <10Pa -4 <5 10Pa <100 Pa to main pump O,Ar,Ne,He 2 ` Rijnders et al. Appl Phys. Lett. 70 (1997) 1888 Time (s)
Atomic layer controlled growth of SrRuO 3 /SiTiO 3 /SrRuO 3 trilayers SrRuO 3 SrTiO 3 34 pulses SrRuO3 bottom layer SrRuO 3 SrTiO 3 substrate 400 nm 6 unit cell SrTiO3 barrier layer 22 pulses 200 nm SrRuO3 top layer RHEED intensity oscillation C.B. Eom et al. Science, 258, 1799 (1992). 400 nm
TEM images of SrRuO3/SrTiO3/SrRuO3 trilayers SrRuO3 top [110]o SrRuO3 SrRuO3 bottom [111]o [111]oSrRuO3 SrTiO3 [001] RuO2 TiO2 [110] [110]SrTiO 3 SrRuO3 SrTiO3 substrate SrTiO3 barrier SrO 6ML TiO2 RuO2 SrO Sharp interface structure Single domain structure TEM: W. Tian and.x. Pan, Univ. of Michigan
Ferroelectric tunnel junctions in nonvolatile memory and logic devices. A. Gruverman et al. Nano Letters, 9, 3539 (2009)
Ferroelectric Tunnel Junctions 100 Start Intensity (percent) 80 60 40 1 2 3 4 5 6 7 8 9 10 ML End SrRuO 3 20 0 40 80 120 160 200 Elap sed Time (seconds) BaTiO 3 SrRuO 3
I V curves for two opposite polarization directions
Mechanical Writing of Ferroelectric Polarization H. Lu et al. Science, 336, 59 (2012)
Strongly Correlated 2DEG Metallic and insulating oxide interfaces controlled by electronic correlations H. W. Jang, et al, Science, 331, 886 (2011) 22
A B Ti L 2,3 O K LaO Scan direction Intensity (arb. unit) 1 nm L 3 L 2 455 460 465 Energy loss (ev) 470 520 530 540 550 Energy loss (ev) C D Ti L 2,3 LaO SmO Intensity (arb. unit) TiO 2 /LaO TiO 2 /SmO 1 nm 1 nm 450 460 470 480 Energy loss (ev)
E.Y. Tsymbal
Giant Piezoelecticity for Hyper-Active MEMS (S.H. Baek et al. Science, 334, 958 (2011) Medical Imaging Sensing Actuation Piezotronics Energy Harvesting
(1960) Ultrasound Medical Imaging (2003) Ceramic PZT transducer array At 1 MHz GE Medical 20x20 first 2-D array (currently 256x256 = 65,536 subdiced elements) S.W. Smith, Duke Univ.
5MHz, intra-cardiac Catheter 2D Array 7 Fr, 112 Elements Side Scanning 12 Fr, 64 Elements Side Scanning Ultrasound used in virtually every medical specialty: Obstetrics, Cardiac, Abdominal Rad. (2-10 MHz) Endoscopy (5-15 MHz) Intravascular, Skin, Eye (10-50 MHz) Ultrasound Microscope, Blood cells (100-200 MHz) Better Resolution and Deeper penetration are desirable. (Giant Piezoelectric Materials!!!)
Hyper-Active NEMS MEMS spatial light modulator for maskless lithograohy (V. Aksyuk, Bell Labs) 10 million electrostatically actuated mirrors Major challenges are emerging as MEMS move to smaller size and require increased integration density with faster and larger relative motion range. A force achieved by electrostatic actuator at 100 V requires only 0.01 V in a hyper-active NEMS. force achieved by the intrinsic electrostatic
Bulk single crystal relaxor ferroelectrics (Pb(Mg 1/3 Nb 2/3 ) PbTiO 3 (PMN PT) and Pb(Mg 1/3 Zn 2/3 ) PbTiO 3 (PZN PT) ). (10 times higher piezo response than PZT ceramics, (d 33 ) = 1500 2500 pm/v ) S E. Park, T.R. Shrout J. Appl. Phys. 82, 1804 (1997). k 33 > 90%, d 33 > 2000 pc/n, strain up to 1.7%
Field Induced Phase Transition in (001) PZN 8% PT Single Crystal S E. Park, T.R. Shrout, J. Appl. Phys. 82, 1804 (1997).
SrTiO 3 on (001) Silicon STO Si 2 nm MBE growth by D.G. Schlom, Penn State, TEM by X.Q. Pan, Michigan
Cross Sectional TEM images of PMN-PT on silicon PMN-PT PMN-PT Rocking Curve FWHM o 0.23 SrRuO 3 o 0.58 SrTiO 3 100 nm Si 2 nm SrRuO 3 TEM by X.Q. Pan, Michigan o 0.32 (bulk PMN-PT single crystal)
Effective Transverse Piezoelectric Coefficient (-e 31 ) 30 -e 31 (C/m 2 ) 25 20 15 10 Epitaxial PMN-PTon Si (This work) Best PZT Film on Si 5 PMN-PT on Si (Previous work) 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Thickness ( m) Measured by S. Trolier-McKinstry, Penn State
PMN PT Active Cantilevers PMN PT Pt SrRuO 3 Si SrTiO 3 Cantilever W5 2 7 35 µm long, 3.2 µm wide Height (µm) 0.4 0.2 0.0 0.2 0.4 0.6 0.8 Length:35µm Cantilever clamping point 0 0.02 0.04 Length (mm) 2.8V 0.375 µm / V deflection Resonant frequencies, 100s of khz. 0V Resonance frequency (khz) 400 350 300 250 200 150 100 Resonance frequency (khz) 400 350 300 250 200 150 100 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1000 / length 2 (1/µm 2 ) 30 35 40 45 50 Cantilever length (µm)
MEMS performance PMN PT piezoelectric properties unaffected by processing Simulations match experimental data using bulk PMN PT parameters Pt (60nm) PMN-PT (270nm) SrRuO 3 (100nm) SrTiO 3 (13nm) Si (substrate) 0.8 1.0 Tip displacement (µm) 0.6 0.4 0.2 Modeled PMN PT cantilever Z (μm) 0.8 0.6 0.4 0.2 0.0 0.0 1.5 3.0 Actual PMN PT cantilever Voltage (V) 0.0 Modeled electrostatic cantilever 10 0 10 1 10 2 Voltage (V)
Effective Polarization (µc/cm 2 ) 40 30 20 10 0-10 -20-30 Pt / PMN PT / SrRuO 3 Sandwich type Electrode -40-200 -100 0 100 200 Effective Electric Field (kv/cm) Effective Polarization (µc/cm 2 ) 40 30 20 10 0-10 -20-30 Inter digitated symmetric Electrode -40-200 -100 0 100 200 Effective Electric Field (kv/cm) Unipolar Operation
Wide Bandwidth Piezoelectric Micro Energy Harvester Based on Nonlinear Resonance Power density 2 W/cm 3 A. Hajati and S.G. Kim, APL, 99, 083105 (2011)
The maximum extractable power : 45μW (based on PZT: d 33 =110 pm/v). Power density: 2 W/cm 3 (PZT volume: 4 5 4 265 0.021 3 ) PMN PT: >200 W/cm 2 (d 33 = 1200 pm/v) P extractable 1 2 c e 2 45μW 2 E P PZT V PZT PZT 2 S PZT d 2 33 f ex PZT A. Hajati and S.G. Kim, APL, 99, 083105 (2011)
More transistors can be put on a chip ( Moore s Law ) BUT CMOS clock speed has not increased since 2003 (Compute performance has also slowed) Piezotronics Saturation in CMOS Computer Speed and Performance Moore s Law Reason for clock speed saturation is that Voltage has stopped scaling. Attempting to increase speed at constant V DD results in economically unacceptable power dissipation. Challenge is to find a new switch operable at low voltage. D.M. Newns, IBM From jai-on-asp.blogspot.com
Low power and fast digital switching technology A gate voltage on a piezoelectric (PE) applies pressure to a piezoresistive (PR) material which induces a insulator-metal transition, turning on the current through sense. I sense Insulator metal transition pressure Insulator SmSe Metal V gate Piezoelectronic Transistor (PET) D.M. Newns, IBM
MRAM Operating at Ultra low voltrage Ultrahigh storage capacity of up to 88 Gb/in 2 Ultralow power dissipation as low as 0.16 fj/bit Room temperature high speed operation below 10 ns. J.M. Hu et al, Nature Communications, 2, 553 (2011)
Summary 1. Oxide materials has a great potential for novel oxide electronics and discovering new solid state phenomena. 2. Strain, domain and interfacial engineering by heteroepitaxy is a general means for achieving extraordinary physical properties in thin films. 3. Oxide electronics is just beginning. There are much challenges and opportunities.
Collaborators S.H. Baek, J. Park, D.A. Felker, D.M. Kim, R. R. Das, R. Blick, M.S. Rzchowski University of Wisconsin Madison V. Aksyuk National Institute of Standards and Technology, Gaithersburg V. Vaithyanathan, J. Lettieri, N. B. Gharb, S. Trolier Mckinstry, The Pennsylvania State University D.G. Schlom Cornell University V. Nagarajan, R. Ramesh University of California Berkeley Y. B. Chen, H. P. Sun, X.Q. Pan University of Michigan S.K. Streiffer Argonne National Laboratory