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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 !

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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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