Transcript PowerPoint 簡報 - National Cheng Kung University

Origin of ferromagnetism in Co doped
ZnO diluted magnetic semiconductor
Reporter: H.S Hsu
Advisor: J. C. A. Huang
1) Department of Physics, National Cheng-Kung University, Tainan, Taiwan
2) Department of Applied Physics, National Kaohsiung University, Kaohsiung,
Taiwan
Outline
• Overview of Semiconductor Spintornics
• Our experiments and results
Part I:
Co or CoFe doped ZnO grown by multilayers technique
-clustering effect or intrinsic diluted magnetic
semiconductor (DMS)?
Part II:
Thermal treatments of Zn1-xCoxO film --Variations of
structural and magnetic properties & Possible role of
oxygen vacancies
Overview of Semiconductor Spintornics
• Spintronics exploits charge + spin degrees of freedom for
processing, storage and transfer of information.
• Semiconductor spintronics is still at early stages of research
and is motivated by two broad aims:
• Improving existing functionality
• producing new functionality (e.g. Spin Transistor & Spin LED)
What is DMS?
A
B
C
The different types of semiconductors: (A) a magnetic semiconductor; (B)
a DMS and (C) a non-magnetic semiconductor.
Why DMS?
One of the most important issues in spintronics is how to
effectively inject the spin into semiconductor materials.
 However, the difference of physical property between metal
(or half-metal) and semiconductor limits the efficiency of spin
injection. An alternative choice is the use of diluted magnetic
semiconductor (DMS).
Semiconductor Spintronic Devices
Metal Oxide Semiconductor Field Effect Transistor
Datta & Das: Appl. Phys. Lett. 56, 665 (1990).
Ohmic contact
Convention
n+
Metal Gate
Ohmic Contact
n+
Oxide
P-type Si
Electron
Inversion layer
Gate Voltage changes electron density  changes conductivity
FM Metal
Schottky Gate
Spin
Injector
Spin-MOSFET
FM Metal
Spin
Analyzer
Modulation Doped
AlGaAs
B
InGaAs
2DEG
FMDMS: enhance spin injection efficiency
Diluted magnetic Semiconductors
• III-V: Combine electronic materials with magnetism
Low solubility of magnetic elements overcome by
low-temperature molecular beam epitaxy (LTMBE)
• Ferromagnetism
(In,Mn)As Ohno et al, PRL 68 2664 (1992)
Tc<RT
(Ga,Mn)As Ohno et al, ARL 73 363 (1996)
Theoretical studies predicted RT
ferromagnetism in n-type ZnO
doping with suitable TM ions .
T. Dietl,et.al., Science 287,1019
(2000)
Therefore, numerous reports of
the magnetic properties of
transition metal-doped ZnO have
appeared.
RT-Ferromagnetism
S. J. Pearton et al. J. Appl. Phys. 93, 1 (2003)
Intrinsic FM? (DMS phase)
or Extrinsic FM(formation of secondary magnetic phase?)
TiCoO2 system
Despite RHEED patterns that
show reasonably well-ordered
surface, a nanoscale
ferromagnetic Co-rich phase is
actually nucleated during
epitaxy. This phase co-exists
with the paramagnetic epitaxial
anatase TiCoO2, resulting in a
ferromagnetic signal at high
temperatures whose origin can
be easily misinterpreted.
Chambers et al, APL 82, 1257 (2003)
Extrinsic origin of Co clusters: Superparamagnetism (M-T)
M peaks at a temperature TB
whose magnitude increases with
an increase in x.
These characteristics are typical
of a blocked nanoparticle
system that has a wide particle
size distribution whose average
size increases with an increase in
x.
A. Punnoose et al. JAP, 93, 7867 (2003)
TB=KV/25kB >400K , D>8nm (hcp-Co) and 18 nm (fcc-Co)
How to stabilize intrinsic DMS phase?
clustering effect or intrinsic DMS?
Fabrication of DMS in Co doped ZnO
BY Multilayer (ML) growth technique
nominal thickness
Random alloy
1
Digital alloy
Resistance(MΩ）
0.8
0.6
0.4
0.2
discontinuous
continuous
0
2
5
8
11
14
17
CoFe layer thickness(A)
20
23
Resistance v.s. metal layer thickness.
Our Experiment
Sample growth
ZnO
By ion-beam sputtering,
[ZnO(20)/Co(x)]25 or
[ZnO(20)/CoFe(x)]25
Co(x) or CoFe
ZnO
Co(x) or CoFe
ZnO
Al2O3 (0001)
x
20A
multilayers with nominal thickness x=1,
2 , 5 and 9 Å has been prepared on αAl2O3 (0001) substrate
By changing the nominal ferromagnetic metal thickness,
the morphology and the grain size distribution can be
controlled .
Magnetic Properties: M-H curve
6
M(emu/cm )
20
3
3
M(emu/cm )
40
0
(a) x=1
-20
3
0
-3
(c) x=5
-6
-40
-3000
-1500
0
1500
-2000
3000
-1000
0
1000
2000
H(Oe)
H(Oe)
100
3
M(emu/cm )
3
M(emu/cm )
20
0
-20
(b) x=2
50
0
M(emu/cm3)
40
40
0
-40
-300
0
300
H(Oe)
(d) x=9
-50
-40
-3000
-1500
0
H(Oe)
1500
3000
-100
-3000
-1500
0
1500
3000
H(Oe)
RT ferromagnetism has been observed for samples with x=1
(~5% doping) and x=2 (~10% doping), while
superparamagnetic behavior appears for x=5 (~25% doping).
Magnetic Properties:ZFC-FC M-T curves
Moment(emu/cm3)
14
Both x=1 and x=2 samples do not show
any sign of blocking temperature (TB) in
ZFC data and their FC magnetizations do
not go to zero up to 350 K.
FC
ZFC
x=1
12
10
This is in marked contrast to x=5 an x=9
samples. (shows a blocking temperature)
8
Moment(emu/cm3)
10 0
100
200
FC
ZFC
Temperature(K)
300
x=5: TB=18K and the magnetization goes
to zero by 300 K (the estimated cluster
size ~1 nm).
x=2
8
6
4
x=9: TB=80 K Ferromagnetism should be
caused by the magnetic clusters (the
2
0
FC100
200
ZFC Temperature(K)
300
estimated cluster size ~8 nm).
60
x=5
40
20
3
Moment(emu/cm )
Moment(emu/cm3)
80
FC
ZFC
160
80
x=9
0
0
100
200
300
Temperature(K)
0
0
100
200
Temperature(K)
300
400
Nevertheless, the M(T) behaviors are
quite different between x=1 and x=2
samples in low temperature region.
Fitting of the M-T curves
x=1 ML
13
x=2 ML
FerroM
x=1ML
x=5 ML
Mixed State
ParaM
x=5ML
70
11
60
50
M(emu/C.C.)
M-T {20 :1}
3-D SP Model
10
10
9
40
M-T 20:5
Curie-Weiss Law
30
20
10
0
100
200
300
9
400
0
Temperature(K)
0
8
3-D spin wave model
3
2
 k T 
M (T)  M 0  0.117 B  B 2 
2SJd 
M(emu/C.C.)
M(emu/C.C.)
12
100
200
300
400
Temperature(K)
7
M-T 20:2
3-D SP Model+Curie-Weiss Law
3-D SP Model
6
5
Curie-Weiss Law
4
M(T)=ρH{C/(T-θ)}
3
x=2ML
0
100
200
Temperature(K)
300
400
Local structure: X-ray absorption Spectroscopy
ln(I/I0)=u
absorption coefficient
6.0
XANES
Absorption Coefficient
5.5
0~40eV
5.0
4.5
Cr reference
pre-edge
EXAFS
4.0
40~1000eV
3.5
Z dependence 
3.0
atomic selectivity
2.5
2.0
1.5
1.0
5600
5800
6000
6200
6400
Energy(eV)
6600
6800
7000
XAS Analysis
u(E)=u0(1- (E))
NR 2
2 2k 2 2 R /   k 
     2 S0   f i k ,   sin 2kR  2 c   e
e
kR
R
EXAFS data is background
subtracted and normalized (1),
transformed into k space and
weighted (2) and Fourier
transformed into R space (3).
XAS studies: Electronic and local structures
•XANES
The spectrum of x=5 sample
resemble the Co reference,
particularly in the pre-edge.
(a)Co K edge
Normalized Absorption
1.2
1.0
0.8
0.6
x=1
x=2
x=5
Co metal
Co oxide
0.4
0.2
0.0
7700 7710 7720 7730 7740 7750
Energy(eV)
On the other hand, the edge
position of x=1 sample is shifted
by 1 eV and is closed to that of
CoO reference. It implies that Co
may be in +2 oxidation state
throughout the film .
The spectrum of x=2 sample
shows a mixture of Co(0) and
Co(Ⅱ) and is more closer to x=5
sample
•EXAFS
Fourier transform amplitude of
EXAFS at the Co K-edge
ZnO
Co Metal
FT(k2)
x=1
x=2
x=5
25%(x=1)+75%(x=5)
0
2
4 6
R(Å)
8
10
These EXAFS results
indicated the formation of
magnetic clusters for the x=2
and 5 samples in comparison
to the Co reference .
For x=1 sample, the
interatomic distances for the
two major peaks of the film are
essentially the same as undoped ZnO. It implies that the
origin of RT ferromagnetism
for x=1 sample is intrinsic.
Summary of Part I
Multilayer growth technique is employed to stabilize
the formation of a good DMS phase for
[ZnO(20Å )/Co0.7Fe0.3(1Å)]25 .
The magnetic phase of the ZnO/CoFe multilayers
can be clearly determined by detailed M(T) studies
and fitted by the models.
The magnetic origins of the ZnO/CoFe multilayers
can be further revealed by the assistance of
electronic state and local structure analyses by xray absorption spectroscopy. These approaches can
be useful to study diluted magnetic semiconductors.
Outline
• Overview of Semiconductor Spintornics
• Our experiments and results
Part I:
Fabrication and characterization of diluted magnetic
semiconductor (DMS) -- Co or CoFe doped ZnO grown
by multilayers technique
Part II:
Thermal treatments of Zn1-xCoxO film --Variations of
structural and magnetic properties & Possible role of
oxygen vacancies
Origin of ferromagnetism in DMS
Interactions d exchange RKKY
Polarons
magnetism
Double exchange
Super exchange
The possible mechanism in DMS system:
However, the precise mechanism in the case of OxideDMS remains controversial!!
Origin of FM in DMS? Oxygen vacancies?
Intrinsic Ferromagnetism in Insulating Cobalt Doped Anatase TiO2
K. A. Griffin, PRL 22, 157204 (2005)
Vacuum anneal
•J. M. D. Coey et al., Nat. Mater. 4, 173
(2005)
Origin of ferromagnetism in oxide DMS
As a result of vacuum anneal, the resistivity decreases. This
change in resistivity is likely caused by the generation of
oxygen vacancies due to the reductive nature of the
vacuum anneal.
The Ms is often observed to increase with conductivity. The
dependence of the magnetic properties on resistivity is
presumably a result of the generation of carriers during the
vacuum anneal and is explained to be consistent with the
model of carrier induced ferromagnetism !?
? Role
Annealing
Oxygen vacancies
Structure
Electrical properties
Magnetism
x=1 sample (~5% doping) anneal in Ar at 1 atm)
Moment(emu/cm3)
50
as deposited
Tannealing=250oC
T
=500oC
The RT ferromagnetism of the
layers was also found to be
thermally stable to at least
750 °C.
annealing
Tannealing=750oC
But further studies are needed
to determine the origin of
ferromagnetism in these
annealing films.
0
-50
-1000
-500
0
Field(Oe)
500
1000
Annealing OxygenVacancy/Carriers
Ms(emu/cm3)
60
101
50
100
10-1
40
10-2
30
R(Ohm-cm)
102
10-3
20
0
200
400
600
10-4
800
Annealing Temperature (K)
Carrier mediated mechanism?
Chemical reaction in ZnO when
annealing in Ar
(1)lower temperature annealing
ZnO-Oo
Zn+e-+1/2O2(g)
increase electron concentration
(2)higher temperature annealing
O2(g)+2e-=2Odecrease electron concentration
To realize carrier-mediated magnetism, there needs to
be some means of introducing magnetic impurities as
well as carriers without disturbing the thermodynamic
stability.
D. H. Kim et al. PRB. 71, 01440 (2005)
After annealing the samples, larger clusters were found.
Systematic study of the annealing influence on oxide based
on DMS is necessary for discovering the mechanism for RT
ferromagnetism in oxide based DMS.
Annealing effect on the long range structure
ZnCoO
Intensity
(0002)
Al2O3(0006)
o
Ta=750 C
o
Ta=500 C
o
Ta=250 C
as deposited
30
40
50
2 theta(degree)
FWHM(theta)
0.48
0.42
0.36
0.30
0.24
0
200 400 600 800
Annealing Temperature(K)
The (0002) peak exhibits an
decreased line broadening as
annealing temperature increases,
except for the 500oC-annealed
sample.
It indicates a significant latticedefect formation for annealing at
500oC
This may indicate that TM in DMS
phase has escaped and formed
more TM-containing second
phases/clusters, thus the material
undergoes a structural transition
as a result of high-T annealing.
The low field magnetization(50 Oe) of CoFe(1Å)/ZnO MLs
Moment(emu/cm )
8
Tannealing=500oC
3
4
as deposited
0
0
100
200
8
3
Moment(emu/cm )
3
Moment(emu/cm )
Mixed satae
4
0
0
0
300
4
0
Temperature(K)
300
100
200
300
low spontaneous magnetization
3
Moment(emu/cm )
3
Moment(emu/cm )
Tannealiing=250oC
200
200
Temperature(K)
8
100
100
Temperature(K)
Temperature(K)
0
8
300
3
2
Tannealing=750oC
1
0
0
100
200
Temperature(K)
300
EXAFS-- RDF
Zn-O
Zn K-edge
Zn-Zn
pure ZnO
Co K-edge
FT(k )
as-deposited
Co-O
o
Co changes from
Zn substitution in
ZnO to that of Co
clusters phase.
3
250 C annealed
o
500 C annealed
Co-O
*
o
750 C annealed
Co-Co
0
Co-Co
Co clusters
aggregate
and oxidize
in the surface.
Co foil
2
4
6
8
Investigation of Co nanoparticles
with EXAFS and XANES
G. Cheng et al. Chem. Phys. Lett. , 400, 122
(2004)
R(Å)
The result is consistent with the chemical
reaction in ZnO annealing process.
Our approach:
Samples: [ZnO(20)/Co(1)]25 multilayers under
different annealing condition
1. As-deposited
2. Annealing at 250 oC in Ar (ar250)
3. Annealing at 250 oC in Ar(90%)/H2(10%) (hy250)
4. Annealing at 250 oC in air (air250)
The aim of this work:
Probe the role of oxygen vacancies and search for
the mechanism of ferromagnetism in TM doped
ZnO by annealing process
Magnetic properties:
Mhy250>Mar250>Mas-deposited>Mair250
M(emu/cm3)
120
60
in Ar
in Ar/ H2
in Air
As deposited
0
-60
-120
-1500 -1000 -500
0
500 1000 1500
H(Oe)
air-annealed sample:
Insulator ~do not support carrier-mediated
Sturcture:
Log intensity(arb. units)
All samples show ZnO(002) peak.
Al2O3(0006)
ZnO(002)
hy250
air250
ar250
as deposited
30
35
40
45
50
2 theta (degree)
55
Electronic and local structures surrounding Co :
a.Co atoms of all these samples are +2 state.
Co K-edge
Co-O
Co-Zn
as deposited
FT(k x)
Ar250
2
Normalized Absorption
Coefficient
b.All show Zn substitution structure without phase segregation
after annealing at 250oC under different condition.
hy250
as-deposited
Ar250
hy250
air250
7710
7725
7740
Energy(eV)
air250
0
2
4
6
R(angstrom)
8
Electronic structures of Zn:
Pre-edge peaks appear in ar250 and hy250 films.
These peaks may be associated with oxygen vacancies.(?)
Zn K-edge
pure ZnO
air250
hy250
ar250
as deposited
First derivative, eV
-1
Normalized Absorption
Coefficient
pure ZnO
air250
hy250
ar250
as deposited
9650
9660
9680
Energy(eV)
9700
9660
9670
9680
Energy(eV)
9690
XANES simulation
Using ab initio multiple scattering code
O
O vacancy
Zn absorber
Zn or
Co substitution
FEFF 8.2
2.0
Zn-O(1 Shell) Oxygen 1
Zn-O(2 Shell) Oxygen 4
3
Zn-O(2 Shell) Oxygen 1
ZnO
0
9650
9660
9670
9680
Energy(eV)
9690
First derivative,eV-1
Absorption Coefficient
Zn-O(1 Shell) Oxygen 1
1.5
Zn-O(2 Shell) Oxygen 4
1.0
Zn-O(2 Shell) Oxygen 1
0.5
ZnO
0.0
9700
9650
9660
9670
9680
9690
9700
Energy(eV)
The appearance of pre-edge peak is caused by oxygen vacancies.
XANES simulation result--Derivative
Using ab initio multiple scattering code FEFF 8.2
A1
First derivative, eV
-1
C1
hydrogen-annealed
B1
A1
C1 B
1
A1
Ar-annealed
C1 B1
A1
as-deposited
First derivative, eV-1
B1
C2 B2
A2
y=0.12
A2
C2 B2
y=0.06
A2
B2
y=0
ZnO film
9660
9680
9700
9660
9680
Energy(eV)
Energy(eV)
The appearance of pre-edge peak is caused by oxygen vacancies.
9700
Conclusion of Part II
1. Co/ZnO DMS multilayers under different LT annealing
at 250oC show different saturated magnetization
moment. Mhy250>Mar250>Mas-deposited>Mair250
2. By measuring the electronic structure and local
structure surrounding Co atoms, all samples reveal
intrinsic DMS phase without showing second phase
formation. LT anneal can change the magnetic
properties without disturbing the thermodynamic
stability.
3. The distinct difference in Zn K-edge for these
annealing samples can be associated with oxygen
vacancies using XANES simulation.
This result implies the oxygen vacancies play an
important role of the ferromagnetism in oxide-based
DMS system.
Reference
J. C. A. Huang, H. S. Hsu, Y.M. Hu, and C.H. Lee, Appl. Phys. Lett. 85, 3815 (2004) .
H. S. Hsu, J.C.A. Huang, L. Horng, C.H. Lee, and Y.H. Huang, IEEE. Trans. Magn. 41,
903 (2005).
J. C. A. Huang and H. S. Hsu, , Appl. Phys. Lett. 87 , 132503 (2005) .
Acknowledgement
C. H. Lee, Y. H. Huang, Y. F. Liao, M. Z. Lin
Scattering and Nano technology Lab.
Department of Engineering and System Science, National Tsing-Hua University
Lance Horng,
Superconductivity and Magnetism Lab.
Department of Physics, National Changhua University of Education
C. R. Lin,
Department of Mechanical Engineering. Southern Taiwan University of Technology
This work has been supported by the National Science Council of the ROC.
Thanks for your attention !
Other evidence:X-ray reflectivity
Intensity (arb. units)
10000
1000
ZnO/CoFe(20:1)25
100
ZnO/CoFe(20:2)25
10
ZnO/CoFe(20:5)25
1
0.1
20:1
0.01
1E-3
1E-4
20:2
1E-5
1E-6
20:5
1E-7
1E-8
0.0
0.1
0.2
q (1/Å)
0.3
0.4
0.5
Co 2p XAS on ZnO/Co MLs
Co L-edge
Intensity
20:1
770
780
790
800
Energy(eV)
Wi et al, APL, 84, 4233
Model:Co atoms substitute for Zn in ZnO matrix.
Other evidence:d-d. transitions of the Co2+
Transmission(%)
1.0
undoped ZnO
as-deposited Co:ZnO
Ar/H2-annealed Co:ZnO
0.9
570nm 615nm
0.8
400
500
600
665nm
700
Wavelength(nm)
Z. W. Jin, ibid. 237 (2002) 548
This result indicates that our Co2+ are in the tetrahedral
oxygen coordination and in substitution of zinc Zn2+.
Other evidence: TEM analysis
1.Zn94.6Co5.4O1-ઠ
2.Zn92.2Co7.8O1-ઠ
3.Zn92.1Co7.9O1-ઠ
4.Zn93.3Co6.7O1-ઠ
5.Zn92.1Co7.9O1-ઠ
No Co clusters or Co-rich
regions are detected by TEM.
Log intensity(arb. units)
Structure:XRD result
Al2O3(0006)
ZnO(0002)
ZnO
x=5
x=2
x=1
25
30
35
40
45
50
55
2 theta (degree)
Epitaxial growth of TM:ZnO (0001) oriented films
Note that epitaxial growth of the (0001) oriented [ZnO(20
Å)/Co0.7Fe0.3(x Å)]25 cannot maintain (thus polycrystalline
appears) for MLs with x reaches about 9
MCD effect
The Ti1-xCrxO exhibits neither an anomalous Hall effect nor x-ray magnetic
circular dichroism at the Cr L edge, in contrast to the prototypical DMS,
Mn-doped GaAs. Thus, the material is not expected to exhibit spin
polarization in its semiconducting state and is fundamentally different from
Zener-type ferromagnetic DMSs such as Mn doped GaAs.
T. C. Kaspar, et al. PRL 95, 217203 (2005)
Our results
0.4
Intensity
0.3
+
-
0.2
0.1
0.0
780
790
Photon energy(eV)
Log intensity(arb. units)
XRD results
ZnO(002)
Al2O3(0006)
air250
ar250
as deposited
30
35
40
45
50
2 theta (degree)
55
2.0
XANES(ZnO)
pDOS(ZnO)
The origin of the additional peak
1.5
1.0
0.5
0.0
-30
-20
-10
0
10
20
30
40
50
60
Energy(eV)
2.5
XANES(1O)
pDOS(1O)
2.0
1.5
1.0
0.5
0.0
-30
-20
-10
0
10
20
Energy(eV)
30
40
50
60
GaN:Cr
14
12
10
G. T. Thaler et al,APL. 86,
131901(2005)
8
FC
ZFC
6
4
ZnO:Cu
2
0
0
100
200
300
Temperature(K)
D. B. Buchholz et al,
APL. 87, 082504 (2005)
Si
1-x
Mnx
substitutional sites
interstitial sites
G. M. Dalpian et al., PRB, 68,
113310 (2003)
The formation energy of a Mn impurity on the bare surface is
very much reduced when compared to the bulk value.
Therefore, it seems that the growth temperature would have to
be relatively small.
Andrej Mihelie, XANES spectroscopy
(2002)
Zn K-edge
A
1.5
ZnO
as-deposited
Ar-annealed
Ar/H2-annealed
1.0
0.5
air-annealed
C
C
9660
Ar/H2-annealed
-1.0
as-deposited
9680
9670
Engery(eV)
air-annealed
-1.5
9700
9680
as deposited
Ar-annealed
Ar/H2-annealed
-0.5
Ar-annealed
9660
9650
0.0
B A
B
First deverivative
Normalized Absorption
Coefficient
Our results:
9690
-1500 -1000 -500
0
500 1000 1500
H(Oe)
The XAS spectra present narrow multiplet structures which are not
observed in the corresponding bulk metal spectra. This is a clear
indication of 3d localization on the transition metal impurities.
P. Gambardella, et al., PRL, 88,047202
Co metal (which is 1.76 μB)
Co2+:3d74s0
3 μB
1 μB