THIN FILM DEPOSITION by yHstCIs

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									THIN LAYERS OF TRANSITION
      METAL OXIDES

                        Tjipke Hibma
Materials Science Centre, University of Groningen, The Netherlands
Contents

• Introduction to thin film deposition
• Atomic layer-by-layer growth
  -   Stoichiometry
  -   Surface “chemistry”
  -   Epitaxy
  -   Morphology
  -   Thickness
• Manipulation of properties, a few
examples
Introduction
  Atomic Layer-by-Layer Growth




Ultimate goal:
  Epitaxial growth of perfect thin layers with atomic
  precision onto (selected parts of) a single
  crystalline substrate, in order to manipulate
  materials properties (or to design ultrathin
  devices).
Introduction
Manipulation of materials properties by

• Substrate influence
     enforcement of geometric, magnetic and electronic structure
     (metastable phases, exchange bias, proximity effects, ..)
• Finite size
     thickness < characteristic length
     (quantum wells, ballistic transport,..)
• Epitaxial strain
     deformation
     (bandgap, level splittings)

 • Artificial stacking
     new layered compounds or structures
     (high-Tc, new ferromagnetic(-electric) compounds)
Introduction
 LaCrO3-LaFeO3 Atomic Superlattices
         K.Ueda, H.Tabata, T. Kawai, Science 280 (1998) 1064


                       Goodenough-Kanamori rules:
                       Cr3+-O-Fe3+ (d3-d5) 180°-superexchange
                       interaction is Ferromagnetic
Thin film deposition


        Physical Deposition          Chemical Deposition
               PVD                         CVD
 Thermal                Energetic          MOCVD
                                           LACVD
  MBE           PLD           SPUTTERING   PECVD
ALL-MBE
              UHV PLD
  ALE


most clean and precise
deposition techniques
Thin film deposition
 Molecular Beam Epitaxy (MBE)

Advantages of MBE :
• High purity elemental
  sources
• Abrupt interfaces
• RHEED growth control
• In-situ surface analysis

Disadvantages of MBE :
• Slow
• Sophisticated and
  expensive UHV
  equipment
• Multi-element rate
  control difficult
Thin film deposition
  (UHV-) Pulsed Laser Deposition (PLD)
Advantages of PLD :
• Suitable for complex
  materials
• Fast and flexible
• (RHEED growth control)
• (In-situ surface
  analysis)

Disadvantages of PLD :
• Particulates
• Loss of volatile elements
• Small area deposition
  Atomic layer-by-layer growth
     Growth processes
                             Main Growth Parameters
         Deposition


                   Arrival rates Fn           Desorption

Energies En
                                              Growth
              Diffusion        Nucleation


                 Mixing               Temperature T
 Atomic layer-by-layer growth

Control of       Growth     MBE                          PLD
                 Parameters
Stoichiometry    Relative Flux Fn   Difficult for n>2,   Loss of volatile
                                    ALL-MBE              components.
Surface          Temperature T      Tsubstrate           Tsubstrate
“Chemistry”      Energies En        thermal <0.1 eV      0.1-10eV (Pback)

Epitaxy          Substrate


Morphology        Nucleation rate   RHEED                (RHEED)
(lbl growth mode) (Fn/Dn )a
Thickness        Absolute Flux Fn RHEED,                 # Pulses,
(nr of layers)                    ALE                    (RHEED)
Stoichiometry Control
 Atomic Layer-by-layer MBE (ALL-MBE)
  (Eckstein and Bosovic, Annu. Rev. Mater. Sci., 25,679,1995)




Atomic absorption
flux control and
computer controlled
shuttering of
individual K-cell.
Stoichiometry Control
 MBE of Binary Oxides

 Stoichiometric MnOm
   excess oxygen
 Nonstoichiometric MxOy
   vary FM/FO, determine x afterwards:
 • Fe3-dO4, Moessbauer Spectroscopy
 • CrOx , XPS
 • VOx, TiOx (0.8<x<1.3), 18O-RBS
Stoichiometry Control
                                               18O-                 RBS

                                                         RBS spectra of 1.8 MeV He+ ions scattered from :

                                                                                      V       O       /A l2O                                                                                            V O       x
                                                                                                                                                                                                                      capped
                                                                                          2       3               3
I n t e n s it y ( a r b .u n it s )




                                                                                                                              I n te n s ity (a r b . u n its )
                                                                                                                                                                                                        18
                                                                                                                                                                                                             O
                                                              1 6
                                                                    O
                                                                                                        5 1
                                                                        1 8                                   V
                                                                              O
                                                                                  2 7
                                                                                        A l                                                                                                   V O            uncapped
                                                                                                                                                                                                         x


                                                                                                                                                                                    16
                                                                                                                                                                                         O         18
                                                                                                                                                                                                        O




                                       0      1 0 0   2 0 0   3 0 0           4 0 0       5 0 0        6 0 0          7 0 0   200                                   220         240          260                 280       300

                                           B a c k s c a tte r in g e n e r g y (k e V )                                                                            B a c k s c a tte r in g e n e r g y (k e V )




                                       V2O3 film on Al2O3 (0001)                                                                                                  VOx film on MgO (100)
Stoichiometry Control
  18O-                             RBS of VOx

                                   1 .4

                                   1 .3
     x -O x y g e n c o n te n t




                                   1 .2

                                   1 .1

                                   1 .0

                                   0 .9

                                   0 .8

                                   0 .7

                                   0 .6

                                   0 .5

                                   0 .4
                                      3 4 3 2 3 0 2 8 2 6 2 4 2 2 2 0 1 8 1 6 1 4 1 2 1 0   8

                                                O x y g en p ressu re (m V )
Surface “Chemistry”

• Elements
  surface diffusion, nucleation
• Binary Oxides
  diffusing species: M, O, MO ??
• Complex Oxides
  ?????
Epitaxy
• Epitaxy
 = Well-defined orientation relationship between
 substrate and film lattice.
• Coherent epitaxy
                             a||film  a||substrate
  strain:
     ||  f
            2
             ||   f
           1 
       (misfit f = Da/a)
Epitaxy
    Critical Thickness




                            Energy E →
                                         Strain
Dislocation formation                    energy



                                                       Dislocation
                                                       energy

                                               tc   thicknes t →
Critical thickness:

      tc 
               b(1   cos2  )          FI
                                         tc
                                         GJ
           8 f o (1   ) sin  cos
                                      ln
                                         HK
                                          b
Epitaxy
 Critical Thickness of CoO/MgO(001)
                                                                                 k                            K                      k’

                                                                                                                                          2q

                          0.010                             Experiment
  Strain in the CoO/MgO




                                                                             2


                                                            Theory       10000

                                                                             5


                          0.008                                              2

                                                                         1000

                                                                             5


                                                                             2

                          0.006                                           100

                                                                             5


                                                                             2

                                                                           10

                          0.004                                              5


                                                                                  36.0   37.0   38.0   39.0   40.0   41.0   42.0   43.0   44.0   45.0   46.0




                          0.002
                                                                                 (002)- reflections of
                          0.000                                                  film and substrate
                                  0    200   400   600   800   1000
                                      Layer thickness (A)
Epitaxy
                                     K
Non-specular diffraction spots           k’
                                 k

  coherent growth                         2q




    Reciprocal space



                 K
Epitaxy
                                     K
Non-specular diffraction spots           k’
                                 k

  relaxed growth                          2q




    Reciprocal space
Morphology
 The three growth modes



                         “Wetting Criterion”
    Layer-by-layer          film                                    b g
                                      int erface   substrate  C ln p / p0

                         supersaturation favors lbl growth

    Layer + 3D islands



    3D-islands
Morphology
 RHEED




             MgO(001)        Fe3O4/MgO(001)


                        k’

                        k
Morphology
 RHEED
 Patterns    Reciprocal
                Lattice
                                Reciprocal
                                   Lattice
                  Rods               Rods
                       Allowed
                     Reciprocal                  Reciprocal
                        Lattice                    lattice
                       Vectors                     points




                    First             First            First
                    Order             Order            Order
                     Second           Second           Second
                     Order            Order            Order


            Perfectly flat    Surface with     Surface with
               surface         monolayer           large
             Reciprocal        roughness.       roughness.
            rods have no       Broadened       Transmission
                width             rods.          features.
Morphology
 Transmission RHEED Pattern


                 TiOx/MgAl2O4(001)
                 vacancy ordered phase
Morphology
 RHEED Oscillations

 Kinematic diffraction / Step density models

                               Do not explain
                                 - phase shifts !!!
                                 - in-/out of phase
                                   amplitude
                                 - damping
                               due to dynamic and
                               incoherent scattering
                               effects)


  only the ML period is reliable parameter
Thickness

• in-situ: quartz monitor, RHEED oscillations

• ex-situ: X-ray Reflectivity, RBS

                                      1
  Intensity (arb units, log. scale)




                                                      Reflectivity (experiment)
                                      0
                                                      Simulation
                                      -1

                                      -2                                          k   K   k’
                                      -3

                                      -4                                                  2q
                                      -5

                                      -6

                                      -7
                                           0   1      2           3           4
                                               Theta (degr.)
Manipulation of properties

 • Substrate influence
     enforcement of geometric, magnetic and electronic structure
     (metastable phases, exchange bias, proximity effects, ..)

 • Finite size
     thickness < characteristic length
     (quantum wells, ballistic transport,..)

 • Epitaxial strain
     deformation
     (bandgap, level splittings)

  • Artificial stacking
     new layered compounds or structures
     (high-Tc, new ferromagnetic(-electric) compounds)
Manipulation of properties
 Transition metal oxides TMO
 • Substrate influence
      - new phases, CrOx, TiOx ,Sr(N,O)
      - Anti-Phase Boundaries, Fe3O4
 • Finite size
      - Electronic structure of NiO
      - Superparamagnetism in Fe3O4
 • Epitaxial strain
      - MI-transition in VOx
      - Tetragonal distortion in CoO
 • Artificial stacking
      - OFeOFeO non-polar initial phase on Al2O3
      - new ferro-magnetic(electric) materials
Substrate influence
 Metastable Chromium Monoxide CrxO
                                    (O. Rogojanu)


 • Chromium monoxide CrO does not exist as a
 bulk material, but can be grown on cubic
 substrates as CrxO (0.67<x<1) .
 • Cr2+/Cr3+ iso-electronic with Mn3+/Mn4+ (d4/d5)
  SCOO in Cr-oxides ?
   Substrate influence
      “Rocksalt”-Cr2O3/MgO(001)
                                                 (O. Rogojanu, S.Hak)



                                             (0 0 4)   Refinement of
                                                       data collected
                                  (-1-1 3)             at ID10,ESRF:
                                                       1/3 Cr-sites
LEED pattern of                                        are vacant
CrOx/MgO(001)          (-2-2 2)              (0 0 2)

                                                                         c
                                  (-1-1 1)                       z
   Areal XRD picture                                         y   x


   of CrOx/MgO(001).   (-2-2 0)                                      a
                                                         b
Epitaxial Strain
 Coherent VOx layers on MgO and STO

       (002)MgO
                                                       VOx on MgO
                          (004)MgO
                                                       (aMgO=4.21 Å)
        (002)VOx                           VO(113)     tensile strain
                           (004)VOx
                                            MgO(113)
                                           MgO(113)




         (002)STO
                                       STO(113)        VOx on STO
                          (004)STO
                                                       (aSTO=3.90 Å)
                                      VO(113)          compressive strain
       (002)VOx
                         (004)VOx


                      (004)VOx
  2Theta-omega scan
Epitaxial Strain
 MI-transition in strained VOx layers
                                      (A.D.Rata)




       SC



                                 M

     MgO         MgAl2O4       SrTiO3
     (4.213 Å)   (4.041×2 Å)   (3.903 Å)
Epitaxial Strain
 Compressed metallic VOx shows upturn
 of  and positive MR at low T
                                      -3
                                1 0

                                                                 x =      0 .8 2
   c m )




                                               H     =   0 T
   R e s is t iv it y ( o h m




                                      -4
                                1 0            H     =   5 T




                                      -5
                                1 0




                                           0   5 0       1 0 0   1 5 0     2 0 0   2 5 0   3 0 0
                                                                 T (K )
Epitaxial Strain
  XMLD of strained CoO                                      (S. Csiszar, M. Haverkort, H. Tjeng)


Compressed CoO layer,                                                                           1.0




                                                            Total electron yield (arb.units)
                                                                                                0.9
                                                                                                      Co   L3-edge        50ML CoO on Ag
                                                                                                                              T=77K
 (CoO)50/Ag                                                                                     0.8
                                                                                                                             grazing
                                                                                                0.7
                                                                                                                             normal
                   eg                                                                           0.6
                                                                                                0.5
                                                                                                                             difference
                                                                                                0.4


                                          
                                                                                                0.3
                   t2g                                                                          0.2
                                dxy                                                             0.1
                                                                                                0.0

   L=0, S=3/2                   dxz,dyz
                                                                                               -0.1
                                                                                                   770         775         780        785

                                                                                                              Photon energy h
Stretched CoO layer,                                                                           2.5




                                              Total electron yield (arb.units)
  (MnO)10 (CoO)7(MnO)50/Ag                                                                     2.0   Co L3-edge          CoO sandw. on Ag
                                                                                                                             T=77K
                                                                                               1.5                          grazing
                   eg                                                                          1.0
                                                                                                                            normal
                                                                                                                            difference
                                                                                               0.5
                   t2g          dxz,dyz
                                                                                               0.0


   L=1, S=3/2                    dxy                                                   -0.5

                                                                                        -1.0
                                                                                            770                775          780        785

“Bulk” CoO: very small effect                                                                                  Photon energy h
Artificial Stacking
    Nonpolar [OFeOFeO] stack ?


a-Al2O3(0001)
Fe3O4 (111)
FeO type
reciprocal lattice (111)



(0,3)
          (1,1)


        b*        a*



                                    End
                                    t ~ 105s
                           Start
                           t = 0s
Final remarks

• The ideal of atomic layer-by-layer growth can be
approached using MBE and UHV-PLD techniques.
However,
• Control of stoichiometry, completeness and
structure of atomic layer during growth is still
unsatisfactory.
• Knowledge of surface “chemistry” is almost fully
lacking.
• Postgrowth characterisation of composition and
structure is a tedious and tough job.
Inorganic Thin Layers Group
 Tjipke Hibma
 Henk Bruinenberg
 Wilma Eerenstein
 Diana Rata
 Sjoerd Hak
 Szilard Csiszar




 MSC-cooperations :
  Tjeng, Sawatzky (electron spectroscopy)
  Niesen, Boerma (Moessbauer spectroscopy, RBS)
  Palstra (transport measurements)

								
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