From DMS to CMS: a characterization protocol

From dilute to condensed magnetic semiconductors:a protocol for systematic characterization

Mauro Rovezzi1

1Institute for Semiconductors and Solid State Physics, Johannes Kepler University, Linz, Austria

[W. Stefanowicz et al., arXiv.org 0912.4216 (2010)][A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010)]

M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 1 / 32

Co-authors

A. Navarro-Quezada, B. Faina, T. Devillers, T. Li, A. rois, T. Winkler,A. BonanniInstitute for Semiconductors and Solid State Physics, Johannes KeplerUniversity, Linz, Austria

W. Stefanowicz, D. Sztenkiel, R. Jakiela, M. Sawicki, T. DietlInstitute of Physics, Polish Academy of Sciences, Warsaw, Poland

R. T. Lechner, G. BauerInstitute for Semiconductors and Solid State Physics, Johannes KeplerUniversity, Linz, Austria

F. d’AcapitoItalian National Research Council, IOM-OGG, c/o ESRF GILDA,Grenoble, France

Outline

1 Introduction

2 Growth characterization protocol

3 Case studiesDilute (Ga,Mn)NFe segregation in GaN

4 Conclusions

IntroductionBuilding-blocks for Spintronics devices

[A. Bonanni and T. Dietl, Chem. Soc. Rev. (2010)]M. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 4 / 32

DMS→ (Ga,Mn)N

With holes→ sp-d Zener model

x=5% and 3.5×1020 h/cm3

[T. Dietl et al., Science (2000)]

Without holes→ double exchange

[K. Sato et al., Phys. Rev. B (2004)]

[L. Berqvist et al., Phys. Rev. Lett. (2004)]

Controversial experimental reports⇒ accurate nanoscale characterization

DMS→ (Ga,Mn)N

x=5% and 3.5×1020 h/cm3

DMS→ (Ga,Mn)N

x=5% and 3.5×1020 h/cm3

CMS→ (Ga,Fe)N

TMs solubility in (most) semiconductors at thermal equilibrium is low⇒ chemical/crystallographic phase separation⇒ a priori unknown magnetic properties⇒ mostly high TC FM

Examples of chemical phase separation (spinodal decomposition in DMS literature)

(Ga,Mn)As annealing

[M. Moreno et al., J. Appl.Phys. (2002)]

(Ga,Mn)N clusters

[G. Martinez-Criado et al., Appl.Phys. Lett. (2005)]

(Ge,Mn) nano-columns

[M. Jamet et al., Nature Mat.(2006)]

Detection and control of phase separation⇒ nano-characterization tools

CMS→ (Ga,Fe)N

(Ga,Mn)As annealing

(Ga,Mn)N clusters

CMS→ (Ga,Fe)N

(Ga,Mn)As annealing

(Ga,Mn)N clusters

CMS→ (Ga,Fe)N

(Ga,Mn)As annealing

(Ga,Mn)N clusters

CMS→ (Ga,Fe)N

(Ga,Mn)As annealing

(Ga,Mn)N clusters

Growth characterization protocol

Metalorganic Vapour Phase Epitaxy(Ga,Mn)N and (Ga,Fe)N[:Si]

AIXTRON 200RF horizontal reactorc-plane sapphire substratesPrecursors: TMGa, NH3, Cp2Fe, Cp2Mn, SiH4

Monitoring: ellipsometry

Well established growth procedure

1 Substrate nitridation2 LT (540 ◦C) GaN nucl. layer3 Annealing/recrystallization4 1 µm HT (1050 ◦C) GaN5 0.5 – 1 µm GaN:(Fe/Mn)[:Si]

800 – 950 ◦C50 – 490 sccm (→ 0.05 – 1 %)

Limits of conventional high-resolution XRD

No evidence of second phases GaN(0002) peak broadening

⇒ Synchrotron radiation

Limits of conventional high-resolution XRD

No evidence of second phases GaN(0002) peak broadening

⇒ Synchrotron radiation

European Synchrotron Radiation Facility, Grenoble

Case studies

1 Dilute (Ga,Mn)N [W. Stefanowicz et al., arXiv.org 0912.4216 (2010)]

2 Fe segregation in GaN

Wide range of Mn concentration up to ≈ 1 at. %

Low-resolution High-resolution

Energy dispersive X-rayspectroscopy

No evidence of crystallographic phase separationNo Mn-induced defects

Is Mn really dilute? → SXRD

Wide range of Mn concentration up to ≈ 1 at. %

Low-resolution High-resolution

Energy dispersive X-rayspectroscopy

No evidence of crystallographic phase separationNo Mn-induced defects

Is Mn really dilute? → SXRD

SXRDMeasurements conducted on BM20 at 10 KeV

High crystal quality [from the FWHMof rocking curves around GaN(0002)]

No evidence for second phasesIncrement of the latticeparameters with Mn concentrationIn situ annealing (under N-richatmosphere) experiments→samples stable up to 900 ◦C

Where is Mn? → EXAFS

SXRDMeasurements conducted on BM20 at 10 KeV

High crystal quality [from the FWHMof rocking curves around GaN(0002)]

No evidence for second phasesIncrement of the latticeparameters with Mn concentrationIn situ annealing (under N-richatmosphere) experiments→samples stable up to 900 ◦C

Where is Mn? → EXAFS

EXAFSMeasurements conducted on BM08 at Mn K-edge

1 2 3 4 5 6 7 8

, |χ(R

)| [Å

Distance, R [Å] − without phase correction

n IO Mn 3

GaN(b)

2 4 6 8 10 12

l, k2χ(k

Photoelectron wavevector, k [Å−1]

kmin kmax(a)

#490A Fit(MnGa) #100A

1 2 3 4 5 6

MnGaMn3GaN

Mn substitutional in GaN at Ga sites(MnGa)Low local disorderMn-Ga first shell expansion∆R=2.5(5)%Additional contributions can be ruledout in the 5% limitMn interstitial defects and Mn3GaNsimulated via ab initio codes[VASP+FEFF8]

What Mn charge state? → XANES

EXAFSMeasurements conducted on BM08 at Mn K-edge

1 2 3 4 5 6 7 8

, |χ(R

)| [Å

n IO Mn 3

GaN(b)

2 4 6 8 10 12

l, k2χ(k

Photoelectron wavevector, k [Å−1]

kmin kmax(a)

#490A Fit(MnGa) #100A

1 2 3 4 5 6

MnGaMn3GaN

Mn substitutional in GaN at Ga sites(MnGa)Low local disorderMn-Ga first shell expansion∆R=2.5(5)%Additional contributions can be ruledout in the 5% limitMn interstitial defects and Mn3GaNsimulated via ab initio codes[VASP+FEFF8]

What Mn charge state? → XANES

Charge state

Role of TMs charge state in DMSExchange interactionsCarriers and their localizationInter-atomic Coulomb interactions

Mn in GaNMn2+ → 3d5

Mn3+ → 3d4

Charge state

Role of TMs charge state in DMSExchange interactionsCarriers and their localizationInter-atomic Coulomb interactions

Mn in GaNMn2+ → 3d5

Mn3+ → 3d4

XANESMeasurements conducted on BM08 at Mn K-edge

6530 6540 6550 6560 6570 6580 6590 6600 6610

Energy [eV]

(b) #100A#490A

Mn(3d4)

Mn(3d5)

6535 6540 6545 6550 6555 6560 6565 6570

(a)#100A#490A

MnOMn2O3MnO2

6537 6540 6543 6546 6549

BkgFit

Absorption edge shifts with thecharge state⇒ Mn3+ ⇒confirmed by pre-edge peaks

Fine-structure simulated withFDMNES⇒ MnGa

Accurate subtraction of the substrate diamagnetic signal and extra m(H,T)

Magnetization(T) Magnetization(H)

⇒ Paramagnetism from non-interacting Mn3+ ionsM. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 17 / 32

Accurate subtraction of the substrate diamagnetic signal and extra m(H,T)

Magnetization(T) Magnetization(H)

⇒ Paramagnetism from non-interacting Mn3+ ionsM. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 17 / 32

Summary

Dilute (Ga,Mn)N - x ≤ 1%Mn substitutional in GaNMn3+ charge stateParamagnetic

OutlookIncrease Mn concentration (in the DMS limit)Co-doping:

(Ga,Mn)N:Si → n-type → Mn2+

(Ga,Mn)N:Mg → p-type → itinerant h

Summary

Dilute (Ga,Mn)N - x ≤ 1%Mn substitutional in GaNMn3+ charge stateParamagnetic

OutlookIncrease Mn concentration (in the DMS limit)Co-doping:

(Ga,Mn)N:Si → n-type → Mn2+

(Ga,Mn)N:Mg → p-type → itinerant h

Case studies

1 Dilute (Ga,Mn)N

2 Fe segregation in GaN [A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010)]

(Ga,Fe)N: a high TC magnetic semiconductor?

Paramagnetism from isolated Fe3+ Ferromagnetism persisting aboveroom temperature

What is the origin of the ferromagnetism?

(Ga,Fe)N: a high TC magnetic semiconductor?

Paramagnetism from isolated Fe3+ Ferromagnetism persisting aboveroom temperature

What is the origin of the ferromagnetism?

Presence of ε-Fe3N nano-crystalsBainite structure (hexagonal), ≈ 15 nm average size, TC = 575 K

⇒ Solubility limit at ≈ 0.4 % Fe in our growth conditions

Presence of ε-Fe3N nano-crystalsBainite structure (hexagonal), ≈ 15 nm average size, TC = 575 K

⇒ Solubility limit at ≈ 0.4 % Fe in our growth conditions

How to control Fe incorporation in GaN?

Growth rate

Co-doping

Growth temperature

Control by growth rate

⇒ Solubility limit increased by increasing the growth rate (TMGa)

Control by growth rate

⇒ Solubility limit increased by increasing the growth rate (TMGa)

Fermi-level engineering by co-doping with SiPartial charge state reduction, Fe3+ → Fe2+, demonstrated by XANES

Fe 3d states resides in the GaN band gapCo-doping with Si→ tuning Fermi level→ Fecharge state modificationCoulomb repulsion between Fe ions hindersaggregation [T. Dietl, Nature Mat. (2006)]

Do this really hinders the aggregation?

Control by co-doping

⇒ Solubility limit increased by co-doping with SiM. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 25 / 32

Control by co-doping

⇒ Solubility limit increased by co-doping with SiM. Rovezzi (HFP/JKU) DMS & CMS characterization DEMATEN 2nd workshop (June 2010) 25 / 32

Growth temperatureSXRD and TEM

Additional Fe-rich phases:γ′-Fe4N, ζ-Fe2N, α-Fe, γ-FeNano-crystals size: 10-20 nm

Growth temperatureXANES & EXAFS

7110 7120 7130 7140 7150 7160 7170 7180 7190

t − µ

Energy [eV]

800 °C

850 °C

950 °C

S691S680S690Fit 3Fit 2Fit 1

1 2 3 4 5

Fe−NFe−Fe

Fe−Ga

XANES: linearcombination fits oftheoretical spectra(FDMNES)⇒ identificationand concentration ofdifferent phasesEXAFS: refinement of thelocal structure

SXRD, (HR)TEM and XAS⇒ phase-diagram of FexNy in GaN

Growth temperatureXANES & EXAFS

7110 7120 7130 7140 7150 7160 7170 7180 7190

t − µ

Energy [eV]

800 °C

850 °C

950 °C

S691S680S690Fit 3Fit 2Fit 1

1 2 3 4 5

Fe−NFe−Fe

Fe−Ga

XANES: linearcombination fits oftheoretical spectra(FDMNES)⇒ identificationand concentration ofdifferent phasesEXAFS: refinement of thelocal structure

SXRD, (HR)TEM and XAS⇒ phase-diagram of FexNy in GaN

Growth temperatureSQUID

Paramagnetic part⇒dilute and non-interactingFeGa

Superparamagnetic-likepart⇒ ferromagneticε-Fe3N, γ′-Fe4N α-Fe andγ-FeAntiferromagneticcomponent⇒ ζ-Fe2N andhighly-nitrated FexN (x≤2)phases

Summary

Segregated (Ga,Fe)NHigh TC FM→ ε-Fe3N clustersSolubility limit controlled by→ growth-rate→ co-doping

Growth temperature→ additional FexNy phases→ wide range of magnetic behaviors

OutlookControl of single-phase FexNy in/on GaN

Summary

Segregated (Ga,Fe)NHigh TC FM→ ε-Fe3N clustersSolubility limit controlled by→ growth-rate→ co-doping

Growth temperature→ additional FexNy phases→ wide range of magnetic behaviors

OutlookControl of single-phase FexNy in/on GaN

CMSSecond generation spintronics devices

Spin battery

Electromotive force and hugemagnetoresistance observed inmagnetic tunnel junctions withzb-MnAs clustersDemonstrated at 30 K

[N.H. Pham et al., Nature (2009)]

Conclusions

Growth characterization protocolSelf-consistent loop

1 Growth/optimization2 Structural characterization3 Magnetic characterization4 Modeling/simulation

Synchrotron radiation as required tool

Thank you for your attention!

Contact/further discussionmauro.rovezzi@jku.at

References for this work

W. Stefanowicz et al.,arXiv.org 0912.4216 (2010)→ soon in Phys. Rev. BA. Navarro-Quezada et al.,Phys. Rev. B 81, 205206 (2010)M. Rovezzi et al.,Phys. Rev. B 79, 195209 (2009)A. Bonanni et al.,Phys. Rev. Lett. 101, 135502 (2008)

From DMS to CMS: a characterization protocol

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