plzeň, 5.1.101 tato prezentace je spolufinancována evropským sociálním fondem a státním...
TRANSCRIPT
Plzeň, 5.1.10 1
Tato prezentace je spolufinancována Evropským sociálním fondem a státním rozpočtem České republiky.
MagnetickMagnetický polovodič (Ga,Mn)As:ý polovodič (Ga,Mn)As:technologie, možnosti aplikacetechnologie, možnosti aplikace
• Fyzikální ústav AV ČR, v.v.i.
• theory (Jugwirth, Sinova, ...)• MBE (Novák, Cukr, Olejník, ...)• SQUID, transport (Olejník, Novák, ...)
• Hitachi Lab Cambridge, UK
• lithography (Irvine, ...)• transport (Wunderlich, Owen, ...)
• University,of Nottingham, UK
• MBE (Foxon, Campion)
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• magnetic semiconductors
• (Ga,Mn)As
• technology issues
• optimized xMn-series
• gating GaMnAs
Outline
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• semiconductors • magnetism
(ferro)magnetic semiconductors
Eu-chalcogenides (EuO, EuGdS, ...)
problems: technology, TC , ...
diluted magnetic semiconductors (GaMnAs, GaMnP, ...)
Modern electronicsModern electronics
electrically tunable magnetic properties
spin degree of freedomspintronics !spintronics !
Plzeň, 5.1.10
GaGa1-x1-xMnMnxxAs - semiconductorAs - semiconductor
Mn : [Ar] 4s2 3d5
xMn < 0.1 % : EA ~ 100 meV
xMn > 1 % :
Jungwirth et al., PRB 76, 125206 (2007)
x=0.05%
1%
2%
7%
~100 meV
EG/2
E
DOSEF
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GaGa1-x1-xMnMnxxAs - ferromagnetAs - ferromagnet
xMn > 1 % : ~
carrier mediated FM
1 hole per Mn
~ 4.5 B per Mn
TC ~ M.p1/3
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GaGa1-x1-xMnMnxxAs - technologyAs - technology
hex. MnAs in cub. GaAs
Problem: solubility limit of Mn in GaAs (~ 0.1%)
Solution: Molecular Beam Epitaxy
low-temperature MBE
GaAs at TS > 150°C, but: defects , ,
growth parameters critical
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Molecular Beam EpitaxyMolecular 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 AVMBE in FZU AV ČR 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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GaGa1-x1-xMnMnxxAs - technologyAs - technology
hex. MnAs in cub. GaAs
Problem: solubility limit of Mn in GaAs (~ 0.1%)
Solution: Molecular Beam Epitaxy
low-temperature MBE
GaAs at TS > 150°C, but: defects , ,
growth parameters critical
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• crystal quality / surface morphology ?crystal quality / surface morphology ?
amorphous / poly / 2D / 3D ?
~ 240°C 3D
RHEED images (non-rotating)
LT-MBE of GaMnAsLT-MBE of GaMnAs
~ 220°C 2D
~ 7% Mn
~ 260°C polygrowth T: > <
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J. Appl. Phys. 102, 083536 (2007)
LT-MBE of GaMnAsLT-MBE of GaMnAs• crystal quality / surface morphologycrystal quality / surface morphology• temperature stability ?temperature stability ?
band-gap thermometry
doping-induced overheating
3 % Mn
5 % Mn
7 % Mn
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3D
2D
also: Campion et al., J. Mater. Sci. 15, 727 (2004)
LT-MBE of GaMnAsLT-MBE of GaMnAs• surface morphology: surface morphology: 2D/3D 2D/3D bestbest!!• temperature stabilitytemperature stability
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3D
2D
As:Ga=3:1As:Ga=1:1
LT-MBE of GaMnAsLT-MBE of GaMnAs• surface morphology : 2D/3Dsurface morphology : 2D/3D• temperature stabilitytemperature stability• As:(Ga+Mn) stoichiometryAs:(Ga+Mn) stoichiometry
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LT-MBE of GaMnAsLT-MBE of GaMnAs• surface morphology : 2D/3Dsurface morphology : 2D/3D• temperature stabilitytemperature stability• As:(Ga+Mn) stoichiometryAs:(Ga+Mn) stoichiometry• annealingannealing
Mn in interstitial position
(double donor, AF coupling)
8 h / 160°C
Mni out-diffusion
increase in p, , M, TC
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optimum time
LT-MBE of GaMnAsLT-MBE of GaMnAs• surface morphology : 2D/3Dsurface morphology : 2D/3D• temperature stabilitytemperature stability• As-flux stoichiometricAs-flux stoichiometric• optimal annealingoptimal annealing
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optimum temperature
LT-MBE of GaMnAsLT-MBE of GaMnAs• surface morphology : 2D/3Dsurface morphology : 2D/3D• temperature stabilitytemperature stability• As-flux stoichiometricAs-flux stoichiometric• optimal annealingoptimal annealing
optimum time
... for given thickness
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176K
12.12.00% Mn, 2% Mn, 200 nm nm
188K
e.g. PRB 78, 054403 (2008); APL 93, 132103 (2008), ...
LT-MBE of GaMnAsLT-MBE of GaMnAs• surface morphology: 2D/3Dsurface morphology: 2D/3D• temperature stabilitytemperature stability• As-flux stoichiometricAs-flux stoichiometric• optimal annealingoptimal annealing• optimal sample thicknessoptimal sample thickness
room temperature in Antarctica ! (-89.2°C, Vostok, 21 July 1983)
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GaMnAs, xGaMnAs, xMn Mn seriesseries
optimally grown/annealed samples (Gaoptimally grown/annealed samples (Ga1-x1-xMnMnxxAs, xAs, xMnMn=0.05 – 14 %, 20nm)=0.05 – 14 %, 20nm)
Curie temperaturemagnetization
- transport - magnetometry - IR absorption - MO - ...
• characterization:
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• Conventional MOS FET structure
~10-100 Volts (Ohno et al. Nature ’00, APL ’06, ...)
high-dielectrics (Chiba et al., Nature ’08, Sawicky et al., Nature ’09, ...)
GaMnAs, gatingGaMnAs, gating
• alternatively ...
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GaMnAs, low voltage gatingGaMnAs, 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 ( ~ 10)
low gate voltage
• Problems !
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GaMnAs, low voltage gatingGaMnAs, 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 gatingGaMnAs, low voltage gating
Corbino geometryCorbino geometry(gate leak reduction)(gate leak reduction)
Olejník et al, PRB 78, 054403 (2008)
Owen et al, NJP 11, 023008 (2009)
gate I-Vgate I-V
n ~ 2x1019 cm-3
barrier 20 nm
xMn = 2.0 %
depletion possible
VG=+3 V -1 V
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GaMnAs, low voltage gatingGaMnAs, low voltage gatingR ~ 100%R ~ 100%
TTCC ~ 2 K ~ 2 K
Olejník et al, PRB 78, 054403 (2008)
Owen et al, NJP 11, 023008 (2009)
Corbino geometryCorbino geometry(gate leak reduction)(gate leak reduction)
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GaMnAs, low voltage gatingGaMnAs, low voltage gating
tunable coercivitytunable coercivity switching by gate pulsesswitching by gate pulses
bistability :
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GaMnAs, low voltage gatingGaMnAs, low voltage gating
0.96
0.98
1.00
1.02
0
45
90
135
180
225
270
315
0.96
0.98
1.00
1.02
AM
R(V
g) =
R(
)/R
av
-1V 3V
30% AMR tuneable30% AMR tuneableVVGG dependent competition of uniaxial dependent competition of uniaxial
and cubic anisotropiesand cubic anisotropies
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SummarySummary
• 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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