J.K. Furdyna J.K. Furdyna University of Notre Dame

CollaboratorsCollaborators::

X. Liu and M. X. Liu and M. DobrowolskaDobrowolska ,, University of Notre DameT. Wojtowicz, T. Wojtowicz, Institute of Physics, Polish Academy of Sciences

W.W. Walukiewicz and K.M. Yu,Walukiewicz and K.M. Yu, Lawrence Berkeley Nat’l Lab

Making Semiconductors Ferromagnetic: Making Semiconductors Ferromagnetic: Opportunities and ChallengesOpportunities and Challenges

University of Virginia, June 19-23, 2006

Ferromagnetic (FM) Semiconductors: Ferromagnetic (FM) Semiconductors: OutlineOutline

• Why make semiconductors ferromagnetic?• Properties of III1-xMnxV alloys• Mn interstitials: their role and detection• Effect of the Fermi energy on the growth of

III1-xMnxV alloys• Future directions• FM semiconductor devices

Motivation: semiconductorMotivation: semiconductor--based “based “spintronicsspintronics””

Today’s electronics and data processing uses semiconductorchips that take advantage of the electrical charge of electrons.

Recently a new field of semiconductor technology began to emerge, with the hope of using electron using electron spin spin in addition to its in addition to its charge charge in semiconductor devices (“(“spintronicsspintronics”)”).

This could combine many functionalitiesThis could combine many functionalities (information storage, logic and data processing, communications) in a single chipin a single chip.

Ultimate goal: quantum computation via spin entanglementquantum computation via spin entanglement

Examples of ferromagnetic semiconductors

•Ga1-xMnxAs (Tc ~170K)•In1-xMnxAs (Tc ~ 60K)•Ga1-xMnxSb (Tc ~ 30K)•In1-xMnxSb (Tc ~12K)

Note: Mn goes into the III-V lattice as divalent Mn++

substitutionally for the Group-III element.In this situation it is both a magnetic moment

and an acceptor.

How to make IIIHow to make III--MnMn--V ferromagnetic?V ferromagnetic?

RudermanRuderman--KittelKittel--KasuyaKasuya--YosidaYosida (RKKY) interaction (RKKY) interaction ––carrier mediated carrier mediated –– can be ferromagneticcan be ferromagnetic

Requirements:

Large Mn conc. xvery high p

II-Mn-VIs difficult to dope

III-Mn-Vs easier,since Mn also provides holes

Ga1-xMnxAs TC=170K

In practice, high Curie temperature TC will be needed for many applications;Mean field theory predicts:

2 * 1/3C MnT CN m pβ=

NMn : concentration of uncompensated Mn spins;β : coupling constant between localized Mn spins

and the free holes (p-d coupling),m* : effective mass of the holes,p : free hole concentration.

K. C. Ku et al., APL 82, 2302 (2003).F. Matsukura et al., PRB 57, R2037 (1998)

The The xx and and pp dependence of Tdependence of TCC in in GaMnAsGaMnAs

H. Ohno et al., Nature 2000

CarrierCarrier--controlled controlled ferromagnetism ferromagnetism in Inin In11--xxMnMnxxAsAs

FabricationFabrication

MBE Facility at Notre DameMBE Facility at Notre Dame

Typical growth temperature for Ga1-xMnxAs: 260 oC

Structures used in our studiesStructures used in our studiesGrowth method: Low Temperature (LT) MBEGrowth method: Low Temperature (LT) MBE

~450nm GaAs bufferTs = 590 ºC

~3nm LT−GaAs

Ts~ 265 ºC

(100) GaAs substrate

100−250nm Ga1-xMnxAs; Ga1-yBeyAs; Ga1-y-zBeyAlzAs or Ga1-x-yMnxBeyAs

orGa1-xMnxAs/Ga1-zAlzAs:BeTs~ 260−270 ºC

2nd MBEprocess

(100) GaAs substrate

~4.5 µm CdTe buffer

100 nm LT-InSbTs ≈ 210 ºC

230 nm In1-xMnxSb Ts ≈ 160 ºC

1st MBEprocess (Poland)

Ion Implantation & Pulsed Laser Melting (II-PLM)Ions

substrate

Ga1-xMnxAsdamaged layer

Substrate

Regrown Layer

excimer laser pulseexcimer laser pulse

Substrate

Damaged layer

Substrate

Liquid

Ion ImplantationIon Implantation•• kinetic process kinetic process →→ exceed equilibrium solubility exceed equilibrium solubility •• crystal damaged (annealing necessary)crystal damaged (annealing necessary)

Pulsed Laser MeltingPulsed Laser Melting•• ultraultra--fast solidificationfast solidification•• nonnon--thermal equilibrium growththermal equilibrium growth•• can model PLM & Mn incorporationcan model PLM & Mn incorporation

Pulsed Laser Melting (PLM)

Substrate

excimer laser pulseexcimer laser pulse

Regrown LayerDamaged layerLiquidLiquid

MeltFront

As implanted50 keV 2x1016 /cm2

0.2 J/cm2

1.1x1016 /cm2

retained

Equilibrium Solubility

GaGa11--xxMnMnxxAs SIMSAs SIMS

MeltDepth

•• ultraultra--fast solidificationfast solidification•• m/sm/s front velocity front velocity →→ solute trappingsolute trapping•• can model PLM & Mn incorporationcan model PLM & Mn incorporation

PLM: repair lattice & maintain supersaturation PLM: repair lattice & maintain supersaturation

Magnetic AnisotropyMagnetic Anisotropy

Magnetic Anisotropy and Strain – an Overview

GaMnAs

GaAs

GaMnAs

GaInAs

H

MM

H

M

H

M

Perpendicular Uniaxial Anisotropy

Origin of magnetic anisotropy in III-Mn-V:– strain induced uniaxial anisotropy

EF

hh

lh

e

Biaxial tensile strain induces uniaxial anisotropy: Easy axis is typically out of plane

hh

lh

e

hh

lh

e

Biaxial compressive strain induces uniaxial anisotropy: Easy axis is typically in plane

EF

hh

lh

e

Dietl, Ohno, Matsukura, PRB 2001

GaMnAs / GaAs GaMnAs / GaInAstensilecompressive

-1.0 -0.5 0.0 0.5 1.00.0

0.2

0.4

0.6

0.8

1.0

Ms

Ms

1.5x1020 cm-3

3.5x1020 cm-3A

NIS

OTR

OPY

FIE

LD [

T]

BIAXIAL STRAIN εxx [%]

Magnetic Domain StructureMagnetic Domain Structure

Method for mapping domains in IIIMethod for mapping domains in III--MnMn--V alloys V alloys **

* Collaboration with Ulrich Welp and V. K. Vlasko-Vlasov at Argonne National Laboratory

MagnetoMagneto--optical images of domain boundaries in Gaoptical images of domain boundaries in Ga11--xxMnMnxxAsAs

H || [100] , T=15 K (left) and 35 K (right); each square is 1 mm × 1 mmU. Welp et al. PRL (2003).

(b)

[1-10]

[100]

[110][010]

[-110]

[-100]

[-1-10]

[0-10]

H

I

ϕ=88o

M1

2

34

5

(c)

12

34

5

(a)

kOe

H1 H2

Planar Hall Effect (PHE) in Planar Hall Effect (PHE) in GaMnAsSbGaMnAsSb alloyalloy

Attempts to increase TAttempts to increase TCC by doping to by doping to increase the hole concentrationincrease the hole concentration

and the role of and the role of MnMn ions in ions in Interstitial lattice positionsInterstitial lattice positions

Temperature dependence of Temperature dependence of resistivityresistivity of of GaGa11--xx--yyMnMnxxBeBeyyAs with x=0.055As with x=0.055

0 50 100 150 200 250 300

10-2

TC<11K

TC=19K

TC=61K

TBe=1010C

TBe=1017CTC=24K

TBe=1045C

TBe=1038C

TBe=1025C

TC=30K

TBe=0C

TC=42K

Log 10

(ρ(0

) [Ω

cm])

Temperature [K]

11011cte 11010ete 11011bte 11011ate 11010ate 11010dte

Does TCdecreasewith Be co-doping?

Substitutional Mn

MnMnIIIIII acceptoracceptorMnMnGaGa with Eact=110 meV

Interstitial Mn

MnMnII double donordouble donor(passivates 2 MnIIIacceptors)

Random clustersnot commensurate

Electrically inactive Mn-related small clusters (and possibleMnV precipitates)III (Ga, In)

V (As, Sb)Mn No direct measurement of No direct measurement of MnMn location in IIIlocation in III11--xxMnMnxxVV

Role of lattice location of Role of lattice location of MnMn in IIIin III11--xxMnMnxxV alloysV alloys

RBS

PIXE

<110>

<111>

2 MeV4He+

<111>

<110>

Combined channeling Rutherford backscattering (RBS) and particle induced x–ray emission (PIXE)

Lattice location determined by angular scanLattice location determined by angular scanof cof c--RBS/PIXERBS/PIXE

PIXE/RBS angular scans for GaPIXE/RBS angular scans for Ga11--xx--yyMnMnxxBeBeyyAsAs

0.0

0.2

0.4

0.6

0.8

1.0

1.2<110>y=0

TC=61K

<110>y~0.05T

C<5K

G a A s (R B S )M n (P IX E )

<110>y~0. 01T

C=30K

GaAs (RBS)Mn (PIXE)

<110>y~0.01T

C=30K

<110>y~0.11

0.0

0.2

0.4

0.6

0.8

1.0

-2.0 -1.0 0.0 1.0

<111>

-2.0 -1.0 0.0 1.0

<111>

-2.0 -1.0 0.0 1.0

<111>

-2.0 -1.0 0.0 1.0 2.0

<111>

tilt angle (degree)

norm

aliz

ed y

ield

, χ

Huge Huge increaseincreaseof of MnMnIIwith Bewith Becontentcontent

• [MnI] and random Mn-related clusters increase with Be content. • MnI are unstable and form random clusters after LT-annealing• Hole concentration relatively constant at ~5x1020cm-3

Correlation of Correlation of MnMn location, location, pp and Tand TC C in in GaGa11--xx--yyMnMnxxBeBeyyAsAs

0.0

0.2

0.4

0.6

0.8

1.0

1.2Ga

1-xMn

xAs x=0.02

<110>

0.0

0.2

0.4

0.6

0.8

1.0

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5tilt angle (o)

<111>

-2.0 -1.0 0.0 1.0 2.0tilt angle (degree)

<111>

<110>

GaAs (RBS)Mn (PIXE)

Ga1-x

MnxAs x=0.08

• Random fraction MnRan (clusters) typically ≤0.05

• [MnI] up to ~0.15-0.2 for high x (>0.05)Attempts to correlate TC with x not meaningfulXRD cannot accurately determine x

Angular scans for GaAngular scans for Ga11--xxMnMnxxAsAs

Low Temperature annealingLow Temperature annealing

LTLT--annealing: annealing: resistivityresistivity and magnetizationand magnetization

TTC C increases for optimal annealingincreases for optimal annealing

0 50 100 150 2000

5

10

15

20

25

30

35

-400 -200 0 200 400

-30

-20

-10

0

10

20

30

in plane fieldH=10 Gs

as-grown annealed

Mag

netiz

atio

n (e

mu/

cm3 )

Temperature (K)

T=5 K

Mag

netiz

atio

n (e

mu/

cm3 )

Magnetic field (Gs)

•• resistivityresistivity decreasesdecreases

•• saturation magnetization Msaturation magnetization MS S increases when the samples are increases when the samples are annealed at around 280annealed at around 280°°C C (indicates that the concentration of (indicates that the concentration of magneticallymagnetically––activeactive MnMn ions increases).ions increases).

• LT-annealing (280ºC) breaks antiferromagnetically coupled MnI-MnGa pairs ⇒ Mn clusters, increases• p = 1x1021 cm-3 ([MnGa]-2[MnI])• [MnGa] active spins

⇒ higher TC=110K• 350ºC annealing drives the system

towards equilibrium⇒ random Mn precipitates and/or clusters from MnGa

Changes observed are unlikely due to AsGa defects (stable up to 450ºC)

LTLT--annealing: annealing: MnMn location, location, pp and Tand TCC

• [MnI] and random Mn-related clusters increase with Be content. • MnI are unstable and form random clusters after LT-annealing• Hole concentration relatively constant at ~5x1020cm-3

Correlation of Correlation of MnMn location, location, pp and Tand TC C in in GaGa11--xx--yyMnMnxxBeBeyyAsAs

•Electrical compensation by interstitial MnIleads to a reduction of the hole concentration (and TC ~ p1/3 )

•MnI donors tend to drift toward MnIII to form MnIII-MnI pairs in III1-xMnxV alloys

•Such MnIII-MnI pairs then couple antiferromagnetically

Role of interstitials in limiting (reducing) TRole of interstitials in limiting (reducing) TCC

Direct evidence of FermiDirect evidence of Fermi--energyenergy--dependent formation of dependent formation of MnMninterstitials in Gainterstitials in Ga11--xxMnMnxxAs:As:Studies of modulation doped Studies of modulation doped GaGa11--xxMnMnxxAs/GaAs/Ga11--yyAlAlyyAs:Be As:Be heterostructuresheterostructures

GaMnAs Quantum Well Geometry

20 ml (Ga,Mn)As

100-nm GaAs Buffer

(001) GaAs Substrate

50 ml AlGaAs:Be

Temperature dependence of Temperature dependence of resistivityresistivity of MD of MD GaGa11--xxMnMnxxAs/GaAs/Ga11--yyAlAlyyAs:Be As:Be heterostructuresheterostructures

0 50 100 150 200 250 3006x10-3

7x10-3

8x10-3

9x10-3

10-2

1.1x10-2

1.2x10-2

1.3x10-2

Tρ=110 K

dBe=13.2 nm

Tρ=99 K

dBe=5.3 nm

Tρ=97 K

dBe=5.3 nm

dBe=0 nmT

ρ=77 K

Log 10

(ρ(0

) (Ω

cm))

Temperature (K)

x=0.062dGaMnAs=5.6 nm

TBe=1040 oC

vs. thickness of doped region (vs. thickness of doped region (ddBeBe)) vs. Be doping level (vs. Be doping level (TTBeBe))

dBeTBe

0 50 100 150 200 250 3004x10-3

5x10-3

6x10-3

7x10-3

8x10-3

9x10-3

10-2

x=0.066dGaMnAs=5.6 nmdBe=13.3 nm

Log 10

(ρ(0

) (Ω

cm))

Temperature (K)

layer

Tρ=110 K

TBe=1050 oC

Tρ=95 K

TBe=1020oC

Tρ=73 K

TBe=0 oC

Tρ=72 K

TBe=0 oC

Temperature dependence of Temperature dependence of remanentremanent magnetization magnetization of MD Gaof MD Ga11--xxMnMnxxAs/GaAs/Ga11--yyAlAlyyAs:Be As:Be heterostructuresheterostructures

decrease of TCconsistent with strong dependence of MnI formation on Fermi energy during growth

increase of TCin qualitative agreement with self-consistent calculations (Vurgaftmanand Meyer)

v.b.

growth direction

GaMnAsGaAlAs

v.b.

growth direction

GaMnAs GaAlAs

0 20 40 60 80 100 120 140 160 1800

10

20

30

40

second barrier

undoped

second barrier doped undoped first barrier doped

Mag

netiz

atio

n (e

mu/

cm3 )

Temperature (K)

TC=98 K TC=80 K TC=62 K

In-plane fieldB = 100 Gs

0 50 100 150 200 250

10

15

20

first barrier

Res

istiv

ity (1

0−3 Ω

cm)

Temperature (K)

Tρ=87 K

Tρ=68 K

Tρ=110 K

ConclusionsConclusions

Directly identified MnI in Ga1-xMnxAs and In1-xMnxSb thin films

MnI are responsible for creation of AF pairs and for the limiting the Curie temperature in ferromagnetic Ga1-xMnxAs and In1-xMnxSb

LT- annealing drives MnI to random clusters

⇒ increase of p, of active spins, and of TC

Incorporation of nonmagnetic acceptors increases concentration of MnI and decreases TC.

Counter doping with donors as a method to enhance TC.

Modulation doping – first experimental results that confirms our model of Fermi-energy dependent creation of MnI

Bottom lineBottom line

Incorporation of MnGa (and MnIn) and creation of MnI results from the value of EF during the growth!

This is what limits TC

Devices Devices

Giant domain wall magnetoresistance in planar devices with nano-constrictions

C. Rüester et al., Phys. Rev. Lett. 91, 216602 (2003).

Giant Planar Hall Effect in Epitaxial (Ga,Mn)As Devices

H.X. Tang et al., PRL 90, 107201 (2003)

Electrical Manipulation of Magnetization Reversal in a Ferromagnetic Semiconductor

D. Chiba, M. Yamanouchi, F. Matsukura, H. Ohno, Science 301, 943 (2003)

Current-driven magnetization reversal in a ferromagnetic semiconductor based(Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction

D. Chiba, Y. Sato, T. Kita, F. Matsukura, and H. Ohno , PRL 93, 216602 (2004)

Current-induced domain-wall switching in a ferromagnetic semiconductor structure

M. Yamanouch, D. Chiba, F. Matsukura and H. Ohno, Nature 428, 539 (2004),

(a)H

θ

[110]z

[110]α

[001]

x I

[110][001]

[110]

α

GaMnAs

I

(b)

10 1000.4

0.6

0.8

1.0

rxx(T) / rxx(T

c)

Tc~ 60K

Tem perature (K)

0o

2o

4o

5o

Magneto-transport devices on vicinal GaMnAs

GaAs

+Iα = 5°

[010]

[110]

[100]

M close to +H8°

a

Mo || [010]

H = 0

+I

b

[010]

Mo ||[100]

-H +I

c

_[010] M close to -H

+Ie

[100]

-H +Id

Mo || [010]

-0.5 0.0 0.5-100

-50

0

50

100R

xy (Ω

)

H (kG)

P

Q

ab

c

de

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0

2

4

6

0.00 0.02 0.04 0.06

0.00

0.02

0.04

0.06

5o

4o

2o

dr / DR

PHE

a (deg)

DR

AHE / DR

PHE

(b)

2 sin(a)/

5o

4o

2o

dr / DR

AHE

(a)

Ω

Ω

Ω

Counter doping with donors as a method to enhance TC.

Modulation doping – first experimental results that confirms our model of Fermi-energy dependent creation of MnI

Major challenge:Major challenge:Need for strategies to increase TNeed for strategies to increase TCC