Prezentace aplikace PowerPoint

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Magnetický polovodic (Ga,Mn)Astechnologie,
možnosti aplikace

• Fyzikální ústav AV CR, v.v.i.

• theory (Jugwirth, Sinova, ...)
• MBE (Novák, Cukr, Olejník, ...)
• SQUID, transport (Olejník, Novák, ...)

• University,of Nottingham, UK

• MBE (Foxon, Campion)

Tato prezentace je spolufinancována Evropským
sociálním fondem a státním rozpoctem Ceské
republiky.

• Hitachi Lab Cambridge, UK

• lithography (Irvine, ...)
• transport (Wunderlich, Owen, ...)

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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
spintronics !
spin degree of freedom
Eu-chalcogenides (EuO, EuGdS, ...)
problems technology, TC , ...
diluted magnetic semiconductors (GaMnAs, GaMnP,
...)
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Ga1-xMnxAs - semiconductor
Mn Ar 4s2 3d5
xMn lt 0.1 EA 100 meV
E
xMn gt 1
DOS
EF
Jungwirth et al., PRB 76, 125206 (2007)
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Ga1-xMnxAs - ferromagnet
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 gt 150C, 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 CR

• 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 gt 150C, 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
gt
lt
growth T
240C ? 3D
220C ? 2D
260C ? poly
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LT-MBE of GaMnAs

• crystal quality / surface morphology
• temperature stability ?

7 Mn
band-gap thermometry doping-induced overheating
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(GaMn) stoichiometry

3D
AsGa31
AsGa11
2D
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LT-MBE of GaMnAs

• surface morphology 2D/3D
• temperature stability
• As(GaMn) stoichiometry
• annealing

Mn in interstitial position (double donor, AF
coupling)
Mni out-diffusion
increase in p, s, M, TC
8 h / 160C
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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
e.g. PRB 78, 054403 (2008) APL 93, 132103
(2008), ...
room temperature in Antarctica ! (-89.2C,
Vostok, 21 July 1983)
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GaMnAs, xMn series

• optimally grown/annealed samples (Ga1-xMnxAs,
  xMn0.05 14 , 20nm)

Curie temperature
magnetization

• characterization

- 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 _at_ 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
VG3 V -1 V
n 2x1019 cm-3
Olejník et al, PRB 78, 054403 (2008) Owen et al,
NJP 11, 023008 (2009)
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
VG dependent competition of uniaxial and cubic
anisotropies
30 AMR tuneable
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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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