Transcript Document 7588372

Making semiconductors magnetic:
new materials properties, devices, and future
JAIRO SINOVA
Texas A&M University
Institute of Physics ASCR
Texas A&M L. Zarbo
Institute of Physics ASCR
Tomas Jungwirth, Vít Novák, et al
215th ECS Meeting - San Francisco, CA
May 27th 2009
Hitachi Cambridge
Jorg Wunderlich, A. Irvine, et al
University of Nottingham
Bryan Gallagher, Tom Foxon,
Richard Campion, et al.
NRI
SWAN
OUTLINE
• Motivation
• Ferromagnetic semiconductor materials:
–
–
–
–
(Ga,Mn)As - general picture
Growth, physical limits on Tc
Related FS materials (searching for room temperature)
Understanding critical behavior in transport
• Ferromagnetic semiconductors & spintronics
– Tunneling anisotropic magnetoresistive device
– Transistors (4 types)
Ferromagnetic semiconductor research :
Motivations and strategies
1. Create a material that marriages the tunability of semiconductors
and the collective behavior of ferromagnets; once created search
for room temperature systems
2. Study new effects in this new material and utilize in metal-based
spintronics
3. Develop a three-terminal gated spintronic device to progress from
sensors & memories to transistors & logic
(Ga,Mn)As
GENERAL PICTURE
Ferromagnetic semiconductors
Need true FSs not FM inclusions in SCs
Ga
GaAs - standard III-V semiconductor
As
+
Group-II Mn - dilute magnetic moments & holes
Mn
(Ga,Mn)As - ferromagnetic semiconductor
Mn
What happens when a Mn is placed in Ga sites:
Mn–hole spin-spin interaction
Ga
As
Mn
As-p
Mn-d
hybridization
5 d-electrons with L=0
 S=5/2 local moment
intermediate
acceptor (110 meV)
 hole
Hybridization  like-spin level repulsion  Jpd SMn
 shole interaction
In addition to the Kinetic-exchange coupling, for a single Mn ion, the coulomb
interaction gives a trapped hole (polaron) which resides just above the valence band
Transition to a ferromagnet when Mn concentration increases
GaAs:Mn – extrinsic p-type semiconductor
EF
spin 
~1% Mn
DOS
<< 1% Mn
>2% Mn
Energy
spin 
onset of ferromagnetism near MIT
As-p-like holes localized on Mn acceptors
valence band As-p-like holes
Ga
Ga
As
As
Mn
Mn
Ga
Mn
Mn
As
Mn
FM due to p-d hybridization (Zener local-itinerant kinetic-exchange)
(Ga,Mn)As GROWTH
high-T growth
•Low-T MBE to avoid precipitation
•High enough T to maintain 2D growth
 need to optimize T & stoichiometry
for each Mn-doping
•Inevitable formation of interstitial Mn-doubledonors compensating holes and moments
 need to anneal out but without loosing MnGa
optimal-T growth
Polyscrystalline
20% shorter bonds
Interstitial Mn out-diffusion limited by surface-oxide
O
GaMnAs-oxide
x-ray photoemission
GaMnAs
MnI++
Olejnik et al, ‘08
10x shorther annealing with etch
Optimizing annealing-T another key factor
Rushforth et al, ‘08
Tc LIMITS AND
STRATEGIES
188K!!
“... Ohno’s ‘98 Tc=110 K is the fundamental
upper limit ..” Yu et al. ‘03
160
140
120
2008
Olejnik et al
100
TC(K)
Tc limit in (Ga,Mn)As
remains open
180
80
60
40
“…Tc =150-165 K independent of
xMn>10% contradicting Zener kinetic
exchange ...” Mack et al. ‘08
“Combinatorial” approach to growth
with fixed growth and annealing T’s
20
0
0
1
2
3
4
5
6
Mntotal(%)
7
8
9
10
Can we have high Tc in Diluted Magnetic Semicondcutors?
NO IDENTIFICATION OF AN INTRINSIC LIMIT
NO EXTRINSIC LIMIT
Tc linear in MnGa local (uncompensated)
moment concentration; falls rapidly with
decreasing hole density in heavily
compensated samples.
(lines – theory, Masek et al 05)
Relative Mn concentrations obtained through
hole density measurements and saturation
moment densities measurements.
Qualitative consistent picture
within LDA, TB, and k.p
Define Mneff = Mnsub-MnInt
Closed symbols annealed
140
TC(K)
TTC(K)
(K)
120
C
TC(K)
100
120
100
100
100
80
80
80
60
40
20
0
0
1
2
3
4
5
6
Mntotal(%)
7
8
80
60
60
1.7%Mn
Mn Linear increase of Tc
1.7%
2.2%
Mn Mn = Mn -Mn
1.7%Mn
2.2%
eff
sub
Int
3.4%
Mn
2.2%
Mn
3.4% Mn
4.5%Mn
Mn 180
4.5%
3.4%
Mn
5.6%Mn
Mn 160
5.6%
4.5%
Mn
6.7%Mn
Mn 140
6.7%
5.6%
Mn
9%
Mn
9%Mn
Mn
6.7%
Mn 120
8%
9% Mn 100
TC(K)
180
180
Tc as grown and annealed samples
180
160
160
160
140
180
140
140
160 Open symbols as grown.
120
120
60
40
40
40
20
20
20
0 10
90
0
1
1
0 0
0
1
80
60
40
20
22
33
2
3
with
0
04 4
15 5 26
As-Grown
As-Grown
Annealed
As-Grown
Annealed
High
Annealed
compensation
7
6 37 7 4 8 85 9 9610 10
Mn
(%)
Mneff(%)
(%)
total
4Mn
5
6
7
8
9
10
total
Mntotal(%)
●
●
●
Concentration of uncompensated MnGa moments has to reach ~10%. Only 6.2%
in the current record Tc=173K sample
Charge compensation not so important unless > 40%
No indication from theory or experiment that the problem is other than
technological - better control of growth-T, stoichiometry
How well do we understand (Ga,Mn)As?
In the metallic optimally doped regime GaMnAs is well described by a disordered-valence
band picture: both dc-data and ac-data are consistent with this scenario.
The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe
(III,Mn)V metallic DMSs very well in the optimally annealed regime:
• Ferromagnetic transition temperatures 
 Magneto-crystalline anisotropy and coercively 
 Domain structure 
 Anisotropic magneto-resistance 
 Anomalous Hall effect 
 MO in the visible range 
 Non-Drude peak in longitudinal ac-conductivity 
• Ferromagnetic resonance 
• Domain wall resistance 
• TAMR 
•Transport critical behaviour 
TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity
formation energies, lattice constant variations upon doping
III = I + II  Ga = Li + Zn
GaAs and LiZnAs are twin SC
LDA+U says that Mn-doped
are also twin DMSs
n and p type doping through
Li/Zn stoichiometry
No solubility limit
for group-II Mn
substituting for
group-II Zn !!!!
Masek, et al. PRB (2006)
UNDERSTANDING
CRITICAL BEHAVIOUR
IN TRANSPORT
Solving a puzzle in (Ga,Mn)As: FM & transport
Dense-moment MS
F<< d-
Dilute-moment MS
F~ d-
Eu - chalcogenides
Critical contribution to resistivity at Tc
~ magnetic susceptibility
Broad peak near Tc disappeares with
annealing (higher uniformity)???

When density of carriers is smaller than
density of local moments what matters
is the long range behavior of Γ (which
goes as susceptibility)
uncor  small
When density of carriers is similar to
density of local moments what matters
is the short range behavior of Γ (which
goes as the energy)
Tc
(k ~ k F ~ 1 / d )
EuCdSe
(k ~ kF ~ 0) ~ 
Ni
d / dT ~ d / dT ~ cv
Tc
d/dT singularity at Tc – consistent with kF~d-
Optimized materials with x=4-12.5% and
Tc=80-185K
Annealing sequence
Remarkably universal
both below and above
Tc
V. Novak, et al “Singularity in temperature derivative of resistivity in
(Ga,Mn)As at the Curie point”, Phys. Rev. Lett. 101, 077201 (2008).
OUTLINE
• Motivation
• Ferromagnetic semiconductor materials:
–
–
–
–
(Ga,Mn)As - general picture
Growth, physical limits on Tc
Related FS materials (searching for room temperature)
Understanding critical behavior in transport
• Ferromagnetic semiconductors & spintronics
– Tunneling anisotropic magnetoresistive device
– Transistors (4 types)
AMR
TMR
~ 1% MR effect
~ 100% MR effect
Exchange split & SO-coupled bands:
 ~ v g ( M vs. I )
TAMR

 ~ TDOS ( M )
Au
Exchange split bands:
 ~ TDOS ()  TDOS ()
discovered in (Ga,Mn)As Gold et al. PRL’04
TAMR in metal structures
experiment
ab intio theory
Shick, et al, PRB '06, Park, et al, PRL '08
Park, et al, PRL '08
Also studied by Parkin et al., Weiss et al., etc.
DMS DEVICES
Gating of highly doped (Ga,Mn)As:
p-n junction FET
(Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06
p-n junction depletion estimates
-3
carrier density [ 10 cm ]
10
8
19
0V
3V
5V
10V
6
4
2
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
GaMnAs layer thickness [nm]
~25% depletion feasible at low voltages
Olejnik et al., ‘08
Increasing  and decreasing AMR and Tc with depletion
Vg = 0V
Vg = 3V
24.5
-3
 [10 cm]
19.4
24.0
19.2
19.0
23.5
18.8
23.0
18.6
20
22
24
26
28
30
32
34
22.5
T [K]
-6
d/dT [10 
AMR
100
Tc Tc
0
0
-100
-100
-200
-300
-200
20
22
24
26
28
T [K]
30
32
34
Persistent variations of magnetic
properties with ferroelectric gates
Stolichnov et al.,
Nat. Mat.‘08
62K
65K
depletion
accumulation
dR/dT
200
100
30
40
50
60
T (K)
70
80
90
100
Electro-mechanical gating with piezo-stressors
exy = 0.1%
exy = 0%
Strain & SO 
Rushforth et al., ‘08
Electrically controlled magnetic anisotropies via strain
(Ga,Mn)As spintronic single-electron transistor
Wunderlich et al. PRL ‘06
Huge, gatable, and hysteretic MR
Single-electron transistor
Two "gates": electric and magnetic
Single-electron charging energy controlled by Vg and M
Source
QQ
ind0 = (n+1/2)e
Q VD
Drain
QQ0 ind = ne
Gate
VG
eE2/2C
C
n-1
Q
U   dQ 'VD (Q ' ) 
0
Q ( M )
e
(Q  Q0 ) 2
 ( M ) C
U
& Q0  CG [VG  VM ( M )] & VM 
2C
e
CG
electric
& magnetic
control of Coulomb blockade oscillations
n
n+1
[110]

n+2
[010] M
F
[100]
[110]
[010]
SO-coupling (M)
Theory confirms chemical potential anisotropies in (Ga,Mn)As
& predicts CBAMR in SO-coupled room-Tc metal FMs
Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device
V DD
VA
ON
OFF
ON
VB
ON
OFF
OFF
ON
1
VB
ON
OFF
10 Vout
10
ON
OFF
1
0
VA
1
0
1
0
0
1
0
OFF
ON
OFF
“OR”
A
0
1
0
1
B
0
0
1
1
Vout
0
1
1
1
Nonvolatile programmable logic
Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device
V DD
0
1
OFF
ON
1
0
VA
VB
ON
OFF
Vout
VB
VA
“OR”
A
0
1
0
1
B
0
0
1
1
Vout
0
1
1
1
Device design
Physics of SO & exchange
Materials
FSs and metal FS
with strong SO
Chemical potential
 CBAMR
SET
FSs
Tunneling DOS
 TAMR
Tunneling device
metal FMs
Resistor
Group velocity & lifetime
 AMR
Mario Borunda
Texas A&M U.
Liviu Zarbo
Texas A&M U.
Alexey Kovalev
Texas A&M U.
Xin Liu
Texas A&M U.
Matching
TAMU funds
Tomas Jungwirth
Inst. of Phys. ASCR
U. of Nottingham
Allan MacDonald
U of Texas
Joerg Wunderlich
Cambridge-Hitachi
Laurens Molenkamp
Wuerzburg
Bryan Gallagher
U. Of Nottingham
Other collaborators: Bernd Kästner, Satofumi Souma, Liviu Zarbo, Dimitri Culcer ,
Qian Niu, S-Q Shen, Brian Gallagher, Tom Fox, Richard Campton
33
EXTRAS
AMR nature of the effect
normal AMR
Coulomb blockade AMR
Theory confirms chemical potential anisotropies in (Ga,Mn)As
& predicts CBAMR in SO-coupled room-Tc metal FMs
•CBAMR if change of |(M)| ~ e2/2C
•In our (Ga,Mn)As ~ meV (~ 10 Kelvin)
•In room-T ferromagnet change of
|(M)|~100K
•Room-T conventional SET
(e2/2C >300K) possible
Strong exchange splitting & SO coupling in (Ga,Mn)As
Ga
As-p-like holes
Mn
As
Mn
Standard MBE techniques for high-quality tunneling structures