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Ultrathin films and preparation of films

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... near field optical microscopy) XPS (X-ray photoemission ... Transmission electron microscopy (TEM) With TEM one can obtain diffraction patterns and images ... – PowerPoint PPT presentation

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Title: Ultrathin films and preparation of films


1
Ultrathin films and preparation of films
  • Ultrathin films
  • Surface anisotropy, Magnetisation, M(T)
    behavior,
  • Domains
  • Film preparation
  • (a) Vacuum evaporation
  • (b) Magnetron sputtering
  • (c) Laser abrasion
  • (d) Molecular beam epitaxy
  • (e) Self-assembled monolayer
  • 3. Measurements

E.Bauer, Growth of thin films, J.Phys Condens.
Matter 11(1999)9365
2
Interesting aspects (1) For studying new phases
of materials such as, fcc cobalt and fcc
iron grown on Cu (001) (2) Two dimensional
features which not encountered in bulk
specimens. (3) Whether any dead layer appears ?
3
(1)Perpendicular anisotropy
N(P?-P?)/ (P?P?)
Spin polarization, p (in), of photoelectrons
from Fe(100) on Ag(100), versus magnetic field
along the surface normal (Stampanoni et al.,PRL
59(1987)2483).
4
Temperature dependence of the saturation
polarization of a 3.5 ML thick epitaxial bcc Fe
film on Ag(001) and a 3 ML fcc Fe on
Cu(001). Insert thickness dependence of the
Cuire temperature of the bcc Fe films.
5
Fcc Fe on (001)Cu
Polarization P(H) of a 5 ML fcc Fe film on
Cu(001), (a) for sample temperature T215, 267,
and 375 K, (b) at T30 K, H is perpendicular to
the film plan.
Polarization P(H) measured at T30 K, (a) for 3
ML fcc Fe on Cu(001), (b) for 1 ML. H is
perpendicular to the film plane.
6
Temperature dependence of the reduced
polarization P/Po for 1, 3, and 5 ML films of fcc
Fe on Cu(001). Po is the saturation
polarization at low temperature. The Curie
temperature are 230K for 1ML film, 390 K for 3
and 5 ML films.
7
Fcc Co on Cu(001) PRL 58(1987)933
Normalized polarization P/Po as a function of the
externally applied field perpendicular to the
film plane. Data are given for five film
thickness at T300 K.
8
Temperature dependence of the spin polarization
for a 1 ML film measured in saturation. Applied
field 15 KOe
9
3M 286(2005)405
10
Fe(100) on V(100)
PRB 70(2004)214406
11
Magnetisation loop of 2.2 nm thick Ni(111) on
Cu(111), coated by Cu(111), measured by TOM. The
films show a perpendicular aniso- tropy (PA)
between 1.0 2.5 nm (Gradmann, Ann.
Phys.17(1966)91).
12
J Phys Condens Matt 86(2005)s573
13
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14
  • From the above two figures, the following facts
    are evident
  • Films of 1ML or thicker of bcc Fe/Ag(001) is
    ferromagnetic, Tc 400K
  • For gt5ML bcc Fe/Ag(001) Its Tc approaches to
    bulk
  • 3.5ML and 5ML layer thickness Fe(100) on
    Ag(100) show perpendicular anisotropy.
  • (2) For fccFe/Cu(001) 1, 3 and 5 ML Tc220, 360
    and 410K, respectively, 3ML Fe/Cu appears P
    anisotropy at 30K.
  • (3) Fcc Co/Cu(001) 1ML is F and Tc 400K,
    Perpendicular anisotropy appears at room T.
  • (4) 1-2.5 ML Ni(111)/Cu(111) appears
    perpendicular anisotropy.
  • (5) Orbital Moment appears in ultra-thin Co film
    (lt2ML).
  • In short gt 1ML F appears, Perpendicular
    anisotropy appears in ultra-thin films, Orbital
    Moment appears in ultra-thin Co film.

15
Magnetisation loops of Pd/Co multilayers, taken
at 300 K, with the field in the film plane
(dashed curves) or along the surface normal (full
lines) (Carcia APL 47(1985)178).
16
Total anisotropy Kt for evaporated (111)
texturized polycrystalline Co/Pd multilayers
versus thickness t of Co films.
17
Co/Pt Multilatersa
Magnetic hysteresis loops at 20 oC.
18
Keff d 2ks (kV -2pMs2)d
Effective anisotropy times Co thickness versus
cobalt thickness for Co/Pt multilayers (Engle
PRL 67(1990)1910).
19
Surface Magnetic Anisotropy ?
  • The reduced symmetry at the surface (Neel 1954)
  • The ratio of Lz2 / (Lx2 Ly2) is increased near
    the surface
  • Interface anisotropy (LS coupling)

1 J.G.Gay and Roy Richter, PRL 56(1986)2728,
2 G.H.O. Daalderop et al., PRB 41(1990)11919,
3 D.S.Wang et al., PRL 70(1993)869.
20
PRL 88(2002)217202
21
Surface Magnetisation
Ferromagnetism in fcc Fe(111) on CuAu (111).
Magnetic moment µFe in the Fe film versus the
mean lattice parameter aCuAu or Au concentration
cAu in the substrate.
22
Variation of magnetic moment calculated by layer
in an 8 ML Ni/Cu (001) film. The calculated bulk
and surface moments are 0.56 µB/Ni and 0.74
µB/Ni (bulk moment 0.6 µB/Ni) (Tersoff PRB
26(1982)6186).
23
The surface-like layers (layers 7 and 8 from Cu)
The interior, bulklike layers (layers 3-6
from Cu)
Spin-resoled density of state for 8 ML Ni(001)
film on Cu(001)
24
Ms (T) behaviour
48Ni/52Fe (111) films on Cu(111)
25
Curie temperature of 48Ni/52Fe(111) versus number
of atomic layers DM. The experiments is from
Gradmann (Phys. Status Solidi 27(1968)313.
Green-function theory from Brodkorb 16(1966)225.)
26
Domain in ultrathin films
Calculated spin distribution in a thinn sample
containing A 180O domain wall.
27
(b) Vibrating sample magnetometry M-H loop of the
sample
  • Domain pattern as measured by
  • MFM above the surface of an eiptaix
  • Cu/200nmNi/Cu(100) film.

28
Schematic drawing of the evaporation chamber.
29
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30
Thermal evaporation and the uniformity of deposits
The simplest technology, raising the
temperature of the source materials An
open boat, suspended on a wire The boat or
wire is a high temperature material, such as
W or Mo and must not react with the evaporant
The substrate hold should be rotated in order to
get uniform deposits The deposition rate is
determined by the source area temperature and
the distance between the source and substrate
as well as the evaporant itself Electron beam
deposition for high temperature materials or
materials which interact with the crucible.
31
Binary alloy evaporation
dZA / dZB (MB/MA )1/2 exp-(?HA- ?HB)/RT
(CA/CB) K (CA/CB). MA, MB are
the mass of A and B element ?HA and ?HB the
evaporation heat, CA CB is the atomic ratio.
32
(the ratio of AB)
(K)
The variation of the ratio of evaporated A and B
element in binary alloy with time
33
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35
Sputtering and ion beam assisted deposition
Sputtering (Ion beam assisted deposition
(IBAD), Ion beam sputter deposition (IBSD))
provides the better quality deposits at low
substrate temperature, thus avoiding large scale
interdiffusion, adhere well to the
substrate, to realize a reactive sputtering.
36
Schematic picture of magnetron sputtering
37
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38
The sputtering rate for the different
element (using 500 eV Ar).
39
Pulsed laser width 10-20 ms, Density 1-5 J/cm2
Schematic picture of Laser ablation
40
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41
A continuum NY81-C NdYIG laser The
wavelength, pulse frequency and pulse width are
355 nm, 10Hz and 10ns, respectively The
focused laser beam with the energy density of
3-4 J/cm2 Ceramic target The distance
between the target and substrate is 55 mm
3x10-5mTorr before introducing pure O2 O2 gass
flow of 60 sccm at a pressure of 75 mTorr After
deposition, the amorphous film is post annealed
for 2 minutes at 650oC in air
42
Bi2.0Dy1.0Fe3.5Ga1.0O12
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44
Summary
(1)The chemical composition of the film is the
same as that of target (2) The
polycrystalline films on ceramic glass substrate
have easy magnetization axis normal To the
film surface, nanometer size grain and very
smooth surface (3) The film shows high squareness
of Faraday hysteresis loop (4) Magnetization of
the film at temperature range from 240K to
340K is almost temperature independent.
45
The special points of Pulse Laser Ablation
The advantages The ablated sample with the same
composition as the target composition High
energy particles is beneficial for the film
growth and realizing a chemical reaction on
substrate Reaction deposition Multilayers
growth and thickness control precisely. The
disadvantages Forming small particle, 0.1-10 µ
m, thickness deposited is not uniform
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47
Advantages of MBE
  • Growth under controlled and monitored conditions
    with in
  • situ analysis of film structure and
    composition (RHEED,
  • LHEED, XPS, AES).
  • (2) A key advantage of MBE is that it enables
    growth of the
  • layered structure along specific crystalline
    direction
  • (3) Lattice-matching between the seed film
    (prelayer) and
  • substrate can be achieved by appropriate
    choice of materials
  • and the growth axis of the magnetic
    structure selected.

48
Magnetic hysteresis loops for oriented Co-Pt
super- lattice recorded by MO effect.
49
Schematic representation of the three growth
modes (a) Island (b) layer-plus-island (c) layer
by layer.
Substrate
The change of AES peak with The deposition
deposite
ML
50
Two arrangements for four deposited atoms in the
same phase epitaxy
7 AA bonds
8 AA bonds (stable state)
51
In the case of the same phase epitaxy, the stable
state is one- Layer-arrangement, namely, two
demitional growth.
52
For the different phase epitaxy
(a)
-4uAB 12uAA
-8uAB-10uAA
(b)
If uAA gt 2uAB the case of (a) is beneficial for
the reduction of energy
53
  • The condition for double layers arrangement
    (Island)
  • N8, uAAgt2uAB, (b) N18, uAAgt1.5uAB, (c) N32,
  • uAAgt1.33uAB, (d) N50, uAAgt1.25uAB, (e) N72,
    uAAgt
  • 1.24 uAB.

54
Other factors should be considered
  • The size of the epitaxy atoms
  • If the size of A atom (epitaxy) is larger
    than that of B
  • (substrate), a compressive strain appears in
    the epitaxy
  • layers, conversely, tensile force appears
  • (b) The strain increases with the increase of
    epitaxy
  • thickness and finally dislocation could
    exist
  • (c) Island appears if the size A atom is largely
    different from
  • B atom (substrate).

55
Electron-based techniques for examining surface
and thin film process
AES (Auger electron spectroscopy) LEED (Low
energy electron diffraction) RHEED (Reflection
high energy electron microscopy) TEM
(Transmission electron microscopy) REM
(Reflection electron microscopy) STM (Scanning
tunneling microscopy) AFM (Atomic force
microscopy) PEEM (Photoemission microscopy) SEM
(Scanning electron microscopy) SNOM (Scanning
near field optical microscopy) XPS (X-ray
photoemission spectroscopy) UPS (Ultra-violet
photoemission spectroscopy)
56
Auger Electron Spectoscopy (AES)
Si KL1L2,3 transition
Si KLL Auger scheme (Chang Surface Sci.,
25(1974)53).
57
High resolution AES spectrum of Ge LMM for 5KV
incident energy. The strongest peaks, within the
L2M4,5M4,5 series at 1145 and L3M4,5M4,5.
58
The surface AES of Fe
The integrating spectroscopy, N(E), of the
surface AES, and N(E)dN(E)/dE.
59
Photoelectron Spectroscopies XPS and UPS
After the electron at inner shell or valence
electron absorb photon energy, they leave atom
and become photo-electron,
Ek hv Eb, where, hv photon
energy, UPS uses ultra-violet radiation as the
probe and collects electrons directly from the
valence band, XPS excites a core hole with X-rays
and collect binding energy of the electrons at
the inner cells.
60
XPS
The electron energy spectrum on Ni obtained by
bombardment of 1.25Kev photon.
61
Scanning Tunneling Microscopy(STM)
The tunneling current is measured by W needle
The distance between the tip and sample surface
is below 1 nm resolution along vertical is
0.01nm and in transverse is 0.1nm The tip is
applied a few voltage and the tunneling current
is 0.1 to 1.0 nA The current is related not
only to the height of atom on the surface, but
also to the atomic density (density state)
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64
Atomic Force Microscopy (AFM)
STM is only applied to observe surface for
conductor or semi- conductor, while AFM is an
appropriate tool for all samples. The reflect
light place is 3-10nm after the height of tip
changes 0.01nm.
  • Three operation models of AFM
  • contact (2) non-contact (3)
  • tapping model.

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66
Transmission electron microscopy (TEM)
  • With TEM one can obtain diffraction patterns and
    images
  • of the sample, revealing microstractural
    defects such as
  • dislocation, grain-, twin- and antiphase
    boundaries
  • (2) In order for the electrons to pass through
    the specimen,
  • it has to be electron transparent (hundreds
    of nm)
  • (3) High resolution than a light microscope

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69
Atoimic resolution TEM image of a Co doped
TiO2 film. No segregation of impurity phases was
obser- ved in the film.
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