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Magnetick ý polovodič (Ga,Mn)As: technologie, možnosti aplikace

Magnetick ý polovodič (Ga,Mn)As: technologie, možnosti aplikace. Fyzik ální ústav AV ČR, v.v.i. theory (Jugwirth, Sinova, ...) MBE (Nov ák, Cukr, Olejník, ...) SQUID , transport (Olejník, Novák, ...). University,of Nottingham, UK. MBE (Foxon, Campion).

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Magnetick ý polovodič (Ga,Mn)As: technologie, možnosti aplikace

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  1. 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) Tato prezentace je spolufinancována Evropským sociálním fondem a státním rozpočtem České republiky. • Hitachi Lab Cambridge, UK • lithography (Irvine, ...) • transport (Wunderlich, Owen, ...)

  2. Outline • magnetic semiconductors • (Ga,Mn)As • technology issues • optimized xMn-series • gating GaMnAs

  3. 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, ...)

  4. 7% 2% 1% ~100 meV EG/2 x=0.05% Ga1-xMnxAs - semiconductor Mn : [Ar] 4s2 3d5 xMn < 0.1 % : EA ~ 100 meV E xMn > 1 % : DOS EF Jungwirth et al., PRB 76, 125206 (2007)

  5. xMn > 1 % : ~ Ga1-xMnxAs - ferromagnet 1 hole per Mn carrier mediated FM ~ 4.5 mB per Mn TC ~ M.p1/3

  6. 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

  7. Molecular Beam Epitaxy UHV growth chamber growth kinetics substrate beams sources • high crystallographic quality • low growth rate • atomically smooth interfaces • heterostructures, superlattices

  8. 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

  9. 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

  10. LT-MBE of GaMnAs • crystal quality / surface morphology ? amorphous / poly / 2D / 3D ? RHEED images (non-rotating) ~ 7% Mn > < growth T: ~ 260°C poly ~ 240°C 3D ~ 220°C 2D

  11. 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)

  12. LT-MBE of GaMnAs • surface morphology: 2D/3D best! • temperature stability 3D 2D also: Campion et al., J. Mater. Sci. 15, 727 (2004)

  13. LT-MBE of GaMnAs • surface morphology : 2D/3D • temperature stability • As:(Ga+Mn) stoichiometry 3D As:Ga=3:1 As:Ga=1:1 2D

  14. LT-MBE of GaMnAs • surface morphology : 2D/3D • temperature stability • As:(Ga+Mn) stoichiometry • annealing Mn in interstitial position (double donor, AF coupling) Mniout-diffusion increase in p, s, M, TC 8 h / 160°C

  15. LT-MBE of GaMnAs • surface morphology : 2D/3D • temperature stability • As-flux stoichiometric • optimal annealing optimum time

  16. LT-MBE of GaMnAs • surface morphology : 2D/3D • temperature stability • As-flux stoichiometric • optimal annealing optimum time optimum temperature ... for given thickness

  17. 188K 176K 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 ! (-89.2°C, Vostok,21 July 1983)

  18. GaMnAs, xMn series • optimally grown/annealed samples (Ga1-xMnxAs, xMn=0.05 – 14 %, 20nm) Curie temperature magnetization • characterization: - transport - magnetometry - IR absorption - MO - ...

  19. 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 ...

  20. 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 !

  21. 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

  22. GaMnAs, low voltage gating gate I-V Corbino geometry (gate leak reduction) xMn = 2.0 % barrier 20 nm VG=+3 V -1 V n ~ 2x1019 cm-3 Olejník et al, PRB 78, 054403 (2008) Owen et al, NJP 11, 023008 (2009) depletion possible

  23. 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)

  24. GaMnAs, low voltage gating tunable coercivity switching by gate pulses bistability :

  25. GaMnAs, low voltage gating VG dependent competition of uniaxial and cubic anisotropies 30% AMR tuneable

  26. 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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