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

http://arxiv.org/abs/0912.4216

http://prb.aps.org/abstract/PRB/v81/i20/e205206

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



TEM

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


TEM

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

Mag

nitu

de o

f the

FT

, |χ(R

)| [Å

−3]

Distance, R [Å] − without phase correction

Rmin

Rmax

Mn I

TM

n IO Mn 3

GaN(b)

2 4 6 8 10 12

EX

AF

S s

igna

l, k2χ(k

)

Photoelectron wavevector, k [Å−1]

kmin kmax(a)

#490A Fit(MnGa) #100A

1 2 3 4 5 6

(c)

MnGaMn3GaN

MnIO

MnIT

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

Mag

nitu

de o

f the

FT

, |χ(R

)| [Å

−3]


Rmin

Rmax

Mn I

TM

n IO Mn 3

GaN(b)

2 4 6 8 10 12

EX

AF

S s

igna

l, k2χ(k

)

Photoelectron wavevector, k [Å−1]

kmin kmax(a)

#490A Fit(MnGa) #100A

1 2 3 4 5 6

(c)

MnGaMn3GaN

MnIO

MnIT

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

No

rm.

Ab

so

rptio

n C

oe

ffic

ien

t −

µ(E

)

Energy [eV]

(b) #100A#490A

Mn(3d4)

Mn(3d5)

6535 6540 6545 6550 6555 6560 6565 6570

No

rm.

µ(E

)

(a)#100A#490A

MnOMn2O3MnO2

6537 6540 6543 6546 6549

(c)

A1A2

A3A4

BkgFit

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

Fine-structure simulated withFDMNES⇒ MnGa


SQUID

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

SQUID

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

Nor

m. A

bsor

ptio

n C

oeffi

cien

t − µ

(E)

[arb

. shi

ft]

Energy [eV]

EF

800 °C

850 °C

950 °C

S691S680S690Fit 3Fit 2Fit 1

1 2 3 4 5

|χ(R

)|


Rm

in

Rm

ax

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

Nor

m. A

bsor

ptio

n C

oeffi

cien

t − µ

(E)

[arb

. shi

ft]

Energy [eV]

EF

800 °C

850 °C

950 °C

S691S680S690Fit 3Fit 2Fit 1

1 2 3 4 5

|χ(R

)|


Rm

in

Rm

ax

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 [email protected]

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)


[email protected]



http://link.aps.org/doi/10.1103/PhysRevB.79.195209

http://link.aps.org/doi/10.1103/PhysRevLett.101.135502

Technology

From DMS to CMS: a characterization protocol