Magnetický polovodič (Ga,Mn)As:
technologie, možnosti aplikace
• Fyzikální ústav AV ČR, v.v.i.
• theory (Jugwirth, Sinova, ...)
• MBE (Novák, Cukr, Olejník, ...)
• SQUID, transport (Olejník, Novák, ...)
• University,of Nottingham, UK
• MBE (Foxon, Campion)
• Hitachi Lab Cambridge, UK
• lithography (Irvine, ...)
• transport (Wunderlich, Owen, ...)
Tato prezentace je spolufinancována Evropským sociálním fondem a státním rozpočtem České republiky.
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Outline
• magnetic semiconductors
• (Ga,Mn)As
• technology issues
• optimized xMn-series
• gating GaMnAs
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Modern electronics
• semiconductors
• magnetism
(ferro)magnetic semiconductors
electrically tunable magnetic properties
spin degree of freedom
spintronics !
Eu-chalcogenides (EuO, EuGdS, ...)
problems: technology, TC , ...
diluted magnetic semiconductors (GaMnAs, GaMnP, ...)
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Ga1-xMnxAs - semiconductor
Mn : [Ar] 4s2 3d5
xMn < 0.1 % : EA ~ 100 meV
E
7%
2%
xMn > 1 % :
~100 meV 1%
DOS
EF
EG/2
x=0.05%
Jungwirth et al., PRB 76, 125206 (2007)
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Ga1-xMnxAs - ferromagnet
xMn ~> 1 % :
1 hole per Mn
carrier mediated FM
~ 4.5 mB per Mn
TC ~ M.p1/3
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Ga1-xMnxAs - technology
Problem: solubility limit of Mn in GaAs (~ 0.1%)
hex. MnAs in cub. GaAs
Solution: Molecular Beam Epitaxy
low-temperature MBE
GaAs at TS > 150°C, but: defects
, t
, s
growth parameters critical
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Molecular Beam Epitaxy
UHV growth chamber
growth kinetics
substrate
beams
sources
•
•
•
•
high crystallographic quality
low growth rate
atomically smooth interfaces
heterostructures, superlattices
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MBE in FZU AV ČR
• III-V semiconductors
• Kryovak
• Veeco Gen II
- 2” substrates
- 3 chambers (load-lock, preparation, growth)
- elements:
group V – As
group III – Ga, Al, In
dopants – Si, C, Mn
- in situ diagnostics: RHEED
band-edge thermometry
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Ga1-xMnxAs - technology
Problem: solubility limit of Mn in GaAs (~ 0.1%)
hex. MnAs in cub. GaAs
Solution: Molecular Beam Epitaxy
low-temperature MBE
GaAs at TS > 150°C, but: defects
, t
, s
growth parameters critical
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LT-MBE of GaMnAs
• crystal quality / surface morphology ?
amorphous / poly / 2D / 3D ?
RHEED images (non-rotating)
~ 7% Mn
growth T:
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>
~ 260°C  poly
~ 240°C  3D
<
~ 220°C  2D
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LT-MBE of GaMnAs
• crystal quality / surface morphology
• temperature stability ?
band-gap thermometry
doping-induced overheating
7 % Mn
5 % Mn
3 % Mn
J. Appl. Phys. 102, 083536 (2007)
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LT-MBE of GaMnAs
• surface morphology: 2D/3D best!
• temperature stability
3D
2D
also: Campion et al., J. Mater. Sci. 15, 727 (2004)
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LT-MBE of GaMnAs
• surface morphology : 2D/3D
• temperature stability
• As:(Ga+Mn) stoichiometry
3D
2D
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LT-MBE of GaMnAs
•
•
•
•
surface morphology : 2D/3D
temperature stability
As:(Ga+Mn) stoichiometry
annealing
Mn in interstitial position
(double donor, AF coupling)
Mni out-diffusion
increase in p, s, M, TC
8 h / 160°C
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LT-MBE of GaMnAs
•
•
•
•
surface morphology : 2D/3D
temperature stability
As-flux stoichiometric
optimal annealing
optimum time
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LT-MBE of GaMnAs
•
•
•
•
surface morphology : 2D/3D
temperature stability
As-flux stoichiometric
optimal annealing
optimum time
optimum temperature
... for given thickness
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LT-MBE of GaMnAs
•
•
•
•
•
surface morphology: 2D/3D
temperature stability
As-flux stoichiometric
optimal annealing
optimal sample thickness
12.0% Mn, 20 nm
188K
176K
e.g. PRB 78, 054403 (2008); APL 93, 132103 (2008), ...
room temperature in Antarctica ! (-89.2°C, Vostok, 21 July 1983)
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GaMnAs, xMn series
optimally grown/annealed samples (Ga1-xMnxAs, xMn=0.05 – 14 %, 20nm)
Curie temperature
magnetization
• characterization:
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- transport
- magnetometry
- IR absorption
- MO
- ...
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GaMnAs, gating
• Conventional MOS FET structure
~10-100 Volts (Ohno et al. Nature ’00, APL ’06, ...)
high-k dielectrics (Chiba et al., Nature ’08, Sawicky et al., Nature ’09, ...)
• alternatively ...
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GaMnAs, low voltage gating
• Built-in gate
AlGaAs barrier
LT-GaAs barrier
p-i-p, p-i-n, p-n structures
• Benefits
single technology
no surface states
high quality barrier (k ~ 10)
low gate voltage
• Problems !
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GaMnAs, low voltage gating
• Built-in gate problems
breakdown field ~ 1MV/cm @ 300 K
technology issues
p-type substrates in MBE 
unintentional Mn-doping at high TS
backward Mn diffusion
AsGa at low TS
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GaMnAs, low voltage gating
gate I-V
Corbino geometry
(gate leak reduction)
xMn = 2.0 %
barrier 20 nm
n~
2x1019
VG=+3 V
-1 V
cm-3
Olejník et al, PRB 78, 054403 (2008)
Owen et al, NJP 11, 023008 (2009)
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depletion possible
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GaMnAs, low voltage gating
DR ~ 100%
Corbino geometry
(gate leak reduction)
DTC ~ 2 K
Olejník et al, PRB 78, 054403 (2008)
Owen et al, NJP 11, 023008 (2009)
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GaMnAs, low voltage gating
tunable coercivity
switching by gate pulses
bistability :
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GaMnAs, low voltage gating
30% AMR tuneable
VG dependent competition of uniaxial
and cubic anisotropies
0
AMR(Vg) = R()/Rav
1.02
315
-1V
3V
45
1.00
0.98
0.96 270
90
0.98
1.00
1.02
225
135
180
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Summary
• technology optimization, “high” TC
• TC keeps increasing (although hardly)
• GaMnAs close to metals (but still semiconducting)
• gating control of AMR
• Thank you !
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