Semi Cond Spin Tronic s

7/28/2019 Semi Cond Spin Tronic s

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Department of Physics/CAPEM/LSRS

Semiconductor Spintronics:

Whats It All About?

ONR, DARPA and CAPEM

Bruce D. McCombe

Department of Physics and Center for AdvancedPhotonic and Electronic Materials

University at Buffalo

The State University of New York

Buffalo, NY 14260


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The Usual Suspects

(Collaborators)

Universi ty at Buffalo

H. Luo, X. Chen:growth(ideas)and structureK.P. Mooney, F. Gasparini:magnet ism

M. Na, C. Ruester: t ransport /magneto-transportG. Kioseoglou, Y.L. Soo, S. Kim,Y.H. Kao:x-rayM. Furis, G. Itskos, G. Kioseoglou, C. Meining, A. Petrou:opt ics and magnetoopt ics

G. Comanescu: IR

Notre Dame Universi ty

Y. Sasaki, X. Liu,J.K. Furdyna:growth

Penn State Universi ty

S. J. Potashnik andP. Schiffer: magnetism and magnetotransport


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DARPA Consortium

Organization (Consortium)

UB(lead Institution), Notre Dame U., U. of

Wuerzburg, Indiana U., Naval Research Lab.,Vanderbilt U., U. of Texas, N.C.State U., WPI,

Penn State U.

FocusIII-Mn-V Semiconductors and their

Heterostructures (GaMnAs, GaMnSb, InMnAs)


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Outline

BackgroundWhat is Spintronics Recent Developments -- Materials and Spin

Injection Ferromagnetic III-Vs -- Materials/Physics ---

Problems

Our Approach -- Digital Alloys:GaSb/InAs with Mn Some Selected Results - GaMnAs, GaMnSb Summary and Key Issues for the Future


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Spintronics

Conventional electronics:charge of electron used toachieve functionalitiese.g., diodes, transistors,

electro-optic devices (detectors and lasers.)

Spintronics: manipulate electron spin (or resultingmagnetism) to achieve new/improved functionalities --

spin transistors, memories, higher speed, lower power,

tunable detectors and lasers, bits (Q-bits) for quantum

computing.


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Conventional Electronics

Metal Gate

n+ n+

Ohmic contact Ohmic Contact

P-type Si

Oxide

Electron

Inversion layer

Metal Oxide Semiconductor Field Effect Transistor

MOSFET

Gate Voltage changes electron density

changes conductivity


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Spintronics

Spin Valves, Spin transistors, Switches, Modulators, MRAM,.

Datta and Das, APL 56, 665 (1990)

Inject polarized spin from one FM contact -- modulate current by

modifying spin precession via Rashba effect (Asymmetry - spin-orbit interact.)

Depends on perpendicular electric field on 2DEG; other FM contact is analyzer

Spin Transistor

Schottky GateFM Metal FM Metal

InGaAs

Modulation Doped AlGaAs

2DEG

SpinAnalyzer

B

SpinInjector


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BUILDING BLOCKS FOR SPINTRONICS

I

Spin filter

I

Nonvolatile

Spin valve

FM

FM

I

Exch. Bias

Material

AF

hard

soft


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New Ballistic Spin Filter Concept

NC STATE

UNIVERSITY

Quasi- 1D channels

from 2DEG, e.g.,

split-gate;

Back and front gates

to manipulate

SO-Interaction

T-shaped 2D

Structure

x

y

0.0 0.5 1.0 1.5 2.0 2.5 3.00.0

0.2

0.4

0.6

0.8

1.0

0

100

Spinpolarization(%)

Transmission(p

erchannel)

Kinetic energy (in E0

units)

Total

transmission

Totalpolarization

T-structure -- Goodpolarization, poor transmission

Asymmetric Square RingHigh Transmission

(approaching 100%) and high

polarization (60%)

Conventional approach externalmagnetic field -- micromagnets,

ferromagnetic films, .

Alternative --- spin-orbit (SO)interaction couples spin and orbital

degrees of freedom (Rashba)

Difference between

polarization flux in

+y andy directionsCourtesy of K. W. Kim


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Polarized photons to spins

Photons modulate Magnetism

BUILDING BLOCKS FOR SPIN-PHOTONICS

Spins to Polarized Photons


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Why Semiconductor Spintronics?He who controls magnetism (in semiconductors) rules

the world Dick Tracy, ca. 1940

Possible Revolutionary Advances

Very fast, very dense memory and logic at extremely low power Spin Quantum Devices(Spin FETs, LEDs, RTDs) Quantum Computing at Room T Complete computer on a chip

Recent Work UCSB - RT opt.- induced, long lived quantum-coherent spin

state (Terahertz freq. Response- transported with small elec. fields)

Ferromag. in GaMnAs and InMnAs Optically and

Electrically induced ferromag. (Japanese groups and others) Ferromag. in GaMnAs, InGaMnAs, GaMnSb, etc. DIGITALALLOYs (UB/UCSB)


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Started by Munekata and Ohno (80s): IBM Japan Initially not widely received -- materials pretty bad (still are)

III1-x

Mnx

Vs without precipitates (grown below 300oC)Poor optical quality (no PL from LT GaAs)

All III1-xMnxVs are heavily p-typeNo excitonic absorption from heavily doped samples

Mn is dopant (acceptor) in III-Vs; generally not desirable for alloying

Mn2+

Tgrowth < 300oC Tgrowth > 300

oC

MnAs

III1-xMnxV Random Alloys(InMnAs and GaMnAs)


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MBE Phase DiagramGa1-xMnxAsRandom Alloys

H. Ohno , J. of Magnetism and Magnetic Materials 200(1999)

Mn composition x

Substr

atete

mperat

ure(oC)

0 0.02 0.04 0.06

100

200

300

Insulating (GaMn)AsInsulating (GaMn)As

Roughness

MnAs Precipitates

Polycrystalline

Metallic (GaMn)As

0.08

- Low concentration(< 1%

insulating, not FM)

- Higher Mn concentration

(1% - 7% - metallic, FM)

Clustering of a few Mn ions(not precipitates)

AF exchange between Mn ions

overcome by carrier-mediated

exchange

- Even Higher Mn Conc.

(> 8% - insulating, not FM)Complex situation Disorder,

Residual Magnetism, self-

compensation


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MBE-grown GaMnAs Random Alloys

Furdyna/Schiffer - U. of Notre Dame

Ga-Mn-As Ferromagnetic from about

3% - 7% Mn

I-M-I Transitions

Carrier mediated mechanism-Large density of holes (comeswith the territory - Mn on Ga

sites is an acceptor)

Highest TC

so far -- 110 K

TC 55 KRandom alloy -- 5.1% Mn

MR

HC

D f Ph i /CAPEM/ SRS


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Mechanism: Carrier-Mediated

Exchange Interaction

RKKY (Carrier Mediated Exchange)

Mn++ - Mn++ Direct interaction is AF

RKKY -- free carrier mediated Interaction between Mn++

can be eitherFMorAF

FM Mn++- Mn++ interaction mediated by holes depends on Mn and

carrier (hole)densities

Exchange Interaction Coulomb interaction plus Pauli Principle

can favor either: ferromagnetic or antiferromagnetic exchange)

Mn++ Separation

1/2kF

AF

FM FM

Average separation between Mn ions

Fermi wavelength ofholes

Length Scales

D t t f Ph i /CAPEM/LSRS


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Issues, Approach and Goals

Along the way

Magnetotransport and Magnetic measurements Learn about basic physics of FM

Effects of dimensionality

Random Alloys

MnAs Precipitates Poor Structural/Optical Properties

Issues

III-V/Mn digital alloys (MBE/ALE)Approach/Goals

Increase average Mn concentration and improve structural quality

2-D spin systems/carriers confined enhance TC

Improved Optical and transport properties (ordered alloy)



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Whats Needed?

Ferromagnetic Materials (semiconductors)

Spin Polarizers/Aligners Spin Injectors (and Long Spin lifetimes)

Means of manipulating spins (B, E, light)


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GaN

ZnO

GaAsGaSb

InAs

Predicted Curie Temperatures

Room

Temp

Dietl et al., Science , (2000)

Weird

Materials

with lowTC


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Photo-induced Ferromagnetism

Munekata et al, PRL 78, 4617 (1997) (InMnAs)

Basic idea

Heterostructure Band offsets Holes go to Incipient Ferromagnetic Material Mechanism for Ferromagnetism is Carrier-induced

Type-II Band Alignment

AlGaMnSb

No Illumination

InAs

Eg (InAs) < h < Eg (AlGaMnSb)

InAs

AlGaMnSb

h



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Ferromagnetic Resonant Interband

Tunneling Diode (FRITD)

Unique Band

Alignment -- Novel

Device Structures

Possible

Carriers (electrons)

have high mobility and

longer spin-coherence

lifetimes

Spin Polarizer or SpinFilter -- can be bias

controlledVB

CB

EFh

L1'

L1

InAs

collector

InAs

emitter

Ferro-

magnetic

GaMnSb

AlSbAlSb



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Digital alloys2 Types

Atomic Layer Epitaxy

Mn

GaAs:Mn

As orSb

Ga or In

GaAs:GaMn

Mn

Ga or In

As or Sb



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GaMnAs Digital Alloys



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TEM of GaMnAs Digital Alloys(Two series of baseline samples)

GaAs Spacer 8 monolayers( 2.3 nm)

8 seconds exposure ALE( 0.2 monolayer of Mn) Clear Superlattice Structure

No evidence of 3D Clusters

10 nm

Series of identical samples

parameter varied is Mn exposure timefrom 1 sec to 22 sec (0.02 - 0.5 ML)



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Magnetization MeasurementsGaMnAs Digital Alloys

Easy Axis in-plane

HC 100 Oe

0.2 ML Mn

MR< 1/3 MR,in-plane



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Summary of Magnetic

Measurements

Local (defect) structure important2D

lslands, AF spin-spin coupling, phases?

Hopping Conductivity in all samplesESD = Effective Spin density

from saturation Magnetization

(S = 2.5, g = 2)

5 10 15 20 25 30 35 40 455

10

15

20

2530

35

40

45

50

5

10

15

20

2530

35

40

45

50

TC

(K)

TC

EDS

EDS(101

3/cm

2)

Mn % in layer

ESD/TC

Correlated

Different Growth

Conditions

1 2 3 4 5 6 7 8 9 10 11 12

5

10

15

20

25

30

35

40

45

50

CurieTemp

erature(K)

Effect Spin Density (1014

/cm2

TC

(K

)

ESD (1013/cm2)



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Anomalous Hall Effect

Rhall = R0B + RSM

Normal

(dominates

at high B)

Anomalous

Magnetic Moments plus spin-orbit

interaction

Skew Scattering (RSsheet) Side-jump Scattering (RS2sheet)

RH

B

BI

VH



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-8 -6 -4 -2 0 2 4 6 81000

2000

3000

4000

5000

6000

7000

8000

TC

approx. 35K

Sample 01222C

52K

12K

21K

32K40K

Rxx(

ohms)

Magnetic Field (T)

MagnetoTransport Measurements

Digital GaMnAs Alloys

Thermally activated resistanceSamples in this series all Ferromagnetic

8 sec exposure, Higher flux -- 0.4 ML

-8 -6 -4 -2 0 2 4 6 8-150

-100

-50

0

50

100

150

52K40K

21K12K

32K

Rhall

vs.B)

sample 01222C

Rhall

()

Magnetic Field (T)

Anomalous

Hall Effect

Large Neg.

Mag. Res.

Pos. MR



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Correlation between Positive

Magnetoresistance and FM

8 sec Mn -- 0.4 ML

Bat Ears



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Thermally activated resistanceSamples in this series all Ferromagnetic

10 sec exposure, lower flux -- 0.5 ML

-8 -6 -4 -2 0 2 4 6 81

10

45K40K30K20K

10K

Rsheet

vs. B

Sample 01222B

4.6K

Rshee

t(k)

Magnetic Field (T)

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

-500

-400

-300

-200

-100

0

100

200

300

400

500

45K40K

30K20K10K

4.5KR

hallvs. B

Sample 01222B

Rhall

()

Magnetic Field (T)

Anomalous

Hall Effect

Pos. MR

Dominated by

Rsheet B dep.



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-- Digital GaMnAs Alloys

Lower flux, short exposure -- 0.15 ML

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8-400

-200

0

200

400

10K15K

50K40K

20K30K

Rhall

()

Magnetic Field (T)

Rhall

vs.B

Sample 01221A

Anomalous

Hall Effect

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 81

10

15K

50K

40K30K

20K

10K

Rsheet(k

)

Magnetic Field

Rsheet

vs. B

Sample 01221A Pos. MR

(T)



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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

8

9

10

11

12

1/T1/4

1/T1/3

1/T1/2

1/T

Ln(R

sheet)

1/Tn

Ln (Rsheet

) vs. 1/Tn

Sample 01221A

T Dependence ofSheet Resistance0.15 ML Sample (shorter exposure)

TC

CriticalScattering



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epa t e t o ys cs/C / S S

Temperature Dependence of

Sheet ResistanceDigital Alloys

0.1 0.2 0.3 0.4 0.5 0.6 0.7

8.0

8.5

9.0

9.5

10.0

10.5

1/T1/4

01222B Sample

1/T1/3

1/T1/2

Ln(R

shee

t)

T-n

(K-n)

10 sec exposure, Lower flux

Similar resistance, Similar magnetotransport,Similar Curie TC , Different activation behavior

8 sec exposure, Higher flux

0.1 0.2 0.3 0.4 0.5 0.6 0.77.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

To

1/4=8.2(K

1/4)

1/T1/4

To

1/3=7.6(K

1/3)

To

1/2=7.8(K

1/2)

01222C Sample

1/T1/3

1/T

1/2

Ln(R

sh

eet)

(I/T)-n(K

-n)T

-n (K-n)



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p y

Summary - GaMnAs Digital Alloys

Systematic study of digital GaAs:Mn alloys Mn layer

coverages < 0.5 monolayer -- Good Structural

properties (TEM, X-ray)

ESD and TC correlated -- maximum TC vs. Mn layer

coverage local structure important -- Clustering, AF

spin-spin coupling, phases?

Anomalous Hall Effect. Magnetoresistance -- initially

positive followed by large negative MR in FM regime

Thermally activated (hopping) conductivity in all

samples -- lnR T-1/2, T-1/4 observed Modified mechanism for FM -- no free holes



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p y

GaMnSb Digital Alloys



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p y

GaMnSb Digital Layers

MBE/ALE - Growth temperature = 250oC

Vary Mn layer spacing

Vary Mn layer coverage

GaSb

cap

GaAs (100)

substrate

GaSb/MnAlSb GaSb

Structure

-1000 -500 0 500 1000

-8

-6

-4

-2

0

2

4

6

8

T = 5K

B in plane

M

(10-6e

mu)

Magnetic Field (T)

SQUIDMeasurements for

a sample with

Easy Axis in-plane

20 nm

TEM

Good structuralquality no

evidence of

precipitates


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y

GaMnSb Digital Alloys-

Magnetotransport

0.5 ML Mn; 10 ML GaSb Spacer

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8

-2

0

2

290K

200K

100K

p = 1.95 x 1013

cm-2

/ Mn layer

100K

70K

20K

60K50K

30K

10K

4.5K

40K

Rhall

()

Magnetic Field (T)

Sample 10630J

Hall Resistance

Large AHE

Persists to HiT

Magnetoresistance

-8 -6 -4 -2 0 2 4 6 8

36

38

40

42

44

46290K

70K

Sample 10630J (GaSb:Mn)

Magnetic Field (T)

Rshee

t()

200K

100K60K

50K40K

30K

20K

10K

4.5K

Crossover

from neg.

to Pos. MR



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TC about

50 K

Arrott Plot from

Magnetotransport

0 50 100 150 200 250 3000.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

"MSat

" vs. T from Arrot Plots (McC)

Sample 10630J

MSat

(arb

.units)

T (K)

Magnetization

Persists to very

High T



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GaMnSb Digital Alloys-

Magnetotransport

-8 -6 -4 -2 0 2 4 6 843

44

45

46

47

48

49

50

51

52 320K

200K

100K

70K

60K50K

40K

30K

20K

10K

4.9K

Rsheet(

)

Magnetic Field (T)

10630B (GaMnSb)

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8-4

-3

-2

-1

0

1

2

3

4

p=3.5*1013/cm

2per Mn layer

320K

200K

60K

70K

100K

30K

50K40K20K

10K4.9K

RHall

()

Magnetic Field (T)

10630B (GaMnSb)

Magnetoresistance Hall Resistance




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Magnetic Force Microscopy

Before annealing

500nm 500nm

After annealing at 500 C

3D Precipitates




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Possible Model

Small (< 20-30 nm) 2D metallic

FM Islands of MnSb (TC > 500 K)

embedded in Random Matrix

of Mn Substituting for Ga

Large Hole density (about 10%

of Mn density) due to random

isolated Mn acceptors near the

MI transition -- holes interact

with magnetic islands at high T --

Two critical temperatures

Mn ion

MnSb Island

A Single

Layer



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Calculations of Curie

Temperature in Digital Alloys

Large VBO localizes holewavefunctions in vicinity

of ferromagnetic layers

MFT predicts Tc up to RT

GaMnAs/GaAs SL:LargeTcon ly if P > 10

20cm-3

GaMnAs/Al(Ga)As SL :Strong Tcenhancement

even fo r P < 1019cm-3!

Digital Layers areGood

StrongestHole

Confinement

1018

1019

1020

0

50

100

150

200

250

300 = 5%

9 ML AlAs/

1 ML Ga0.5

Mn0.5As SL

9 ML GaAs/

1 ML GaMnAs

SL

Bulk

Ga0.95

Mn0.05

As

CurieTemperature(K)

Avera e Hole Densit cm-3

EBOMwith sel f-con sistent mean-

f ield th eory

Courtesy of Jerry Meyer, NRL



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Summary/Future Directions

Promising Results on Digital-layer Alloys

(particularly GaSb-based)

Interesting Transport Results -- Activated (2D?)

Conductivity -- Localized holes -- FerromagneticBasic mechanisms

Understand High T behavior of GaMnSb

Correlation between Effective Spin Density and

TC Growth conditions very important

Heterostuctures (III-Mn-IIIV) for Devices

Interfaces important and need to be studied



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B In-plane

Magnetization MeasurementsGaMnAs Digital Alloys

B Perpendicular to plane

Easy Axis in- plane

HC 100 Oe



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Temperature Dependence of

Sheet Resistance

0 50 100

44

46

48

50

52

Rsheet vs. Temperature with B Sample 10630B (GaSb/Mn) 24sec Mn

Temperature (K)

Rsheet

()

0T

1T

2T

3T

4T

5T

6T

7T

0 50 100

36

38

40

42

44

46

48

10630J (20sec Mn)

Rsheet(

)

Temperature (K)

0T

1T

2T

3T

4T

5T

CrossoverCrossover



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Magnetotransport Measurements

Digital GaMnSb Alloys

-8 -6 -4 -2 0 2 4 6 8

8

9

10

40K

30K

20K

Sample 01223M (GaSb:Mn)

10K

4.3K

Rsheet

()

Magnetic Field(T)-8 -6 -4 -2 0 2 4 6 8

-1.8

-1.2

-0.6

0.0

0.6

1.2

1.8

2.4

40K30K

10K

20K

4.3K

Sample 01223M (GaSb:Mn)

Rhall()

Magnetic Field(T)

TG = 273C, 50 repetitions; 9 ML

GaSb spacer; 0.2 ML Mn; p-type

Neg.

Magnetores.

much smaller

than

GaMnAs


f


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MBE GaAs:Mn epilayer - doped at high density

(TS = 590 C) Metallic: n = 2.4 x 1018 cm-3

Infrared Absorption

Measurements

Sample 01223C

95 100 105 110 115 120 125 130 135 140 145

95

96

97

98

99

100

101

102

low Mn density

77 Kelvin 10222A

NormalizedTransmittan

ce

Photon Energy (meV)

95 100 105 110 115 120 125 130 135 140 14596

97

98

99

100

101

102

103

2.5 x 1018

cm-3

77 K 1223C

Normalizedtransmittance

NormalizedTra

nsmittance

Photon Energy (meV)

01223C77 K

77 K 10222A

n = 2.5 x 1018 cm-3

Lo Mn Density

GaAs:Mn diffused(Linn arsson et al .

PRB 55, 6938 (1997).

MBE GaAs:Mn epilayer low density (TS = 590

C) semiconducting: p = ?

Impurity Band Transitions ?



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Photoluminescence Measurements

Random GaAs:Mn

0 5 10 15 20 25 30

11050

11100

11150

11200

11250

11300

11350

11400

11450

11500

11550

T = 4.2K

S = 00618A less Mn

Energy

(cm

-1)

Magnetic Field (T)

10400 10600 10800 11000 11200 11400 11600 11800 12000 12200

0

20000

40000

60000

80000

100000

HeNe = 5mW

window 11.300cm-1

B = 0T

S = 00618A

No.ofcounts

ENERGY (cm-1)

10400 10600 10800 11000 11200 11400 11600 11800 12000 12200

0

20000

40000

60000

80000

HeNe = 5mW

window 11.300cm-1

B = 30T

S = 00618A

No.ofcounts

ENERGY (cm-1)

Data taken at NHMFL

Mn

Acceptor

LO



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Random GaAs:Mn

11150 11200 11250 11300 11350 11400 11450 11500 115500

5000

D-Mn

CB - Mn

500/8000/500

g = 600 gr/mm

T = 10K

laser = 632.8nm/5mW

tau = 1 sec

S = 000618B More MnIntensity(#o

fcounts)

Energy(cm-1)

11950 12000 12050 12100 12150 12200 12250 12300 12350 12400 124500

500

1000

1500

2000

2500

3000

BULK

CB - C(acc) 500/8000/500

g = 600 gr/mm

T = 10K

laser = 632.8nm/5mW

tau = 1 sec

S = 000618B More MnIntensity(#o

fcounts)

Energy(cm-1

)

Band Edge RegionMn Acceptor Region



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Mn Acceptor Region

11150 11200 11250 11300 11350 11400 11450 11500 115501000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

500/8000/500

g = 600 gr/mm

T= 10K

laser = 632.8nm/5mW

tau = 10 sec

S = 000614B - 12monolayers

Intensity(#o

fcounts)

Energy(cm-1)

Band Edge Region

11950 12000 12050 12100 12150 12200 12250 12300 12350 12400 124500

5000

10000

15000

BULK

CB-C(acc)

500/8000/500

g = 600 gr/mm

T= 10K

laser = 632.8nm/5mW

tau = 10 sec

S = 000614B - 12monolayers

Energy(cm-1)



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REFLECTIVITY of GaMnAs DIGITAL ALLOYS

12100 12150 12200 12250 12300 12350

12 monolayers GaAsReflectance(a

.u.)

Energy(cm-1)

8 monolayers GaAs

16 monolayers GaAs

Optical, magnetic and transport

properties are Correlated

Curie temperature around 40 K

Highly resistive; Sheet hole density

~ 2-3 x 1010 cm-2 at room T

Curie temperature around 30 K

Highly resistive; cant estimate

sheet hole density

Not Ferromagnetic; insulating



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Magnetization MeasurementsDependence on Bonding

-1000 -500 0 500 1000

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

1.0x10-5

As/Mn/As

Ga/Mn/Ga

MagneticMomentperArea

(emu/mm

2)

Magnetic Field (Gauss)

Digital Alloys



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Low concentration(< 1% - insulating; not Ferromagnetic)- Magnetic properties determined by spins of individual Mn2+(S = 5/2 )- Paramagnetic

Higher Mn concentration(between 1% and 8% - metallic;Ferromagnetic)

- Clustering of a few Mn ions (not precipitates which are larger)

- Antiferromagnetic exchange between Mn ions overcome by carrier-

mediated exchange

MAGNETISM in Ga-Mn-As(Random Alloys)

Even Higher Mn Concentration ( > 8% - insulating; notFerromagnetic)

- Complex situation Disorder; Residual Magnetism; self-

compensation



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GaMnAs Phase Diagram

(H. Ohno , J. of Magnetism and Magnetic Materials 200,(1999))

Su

bstrateTem

p.

(oC)

0 0.02 0.04 0.06

100

200

300

Mn composition x

Insulating (GaMn)AsInsulating (GaMn)As

Roughness

MnAs Precipitates

Polycrystalline

Metallic (GaMn)As

0.08



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Spin Injection

Spin injection into semiconductorsAbstract

Spin injection (spin polarized current) results from thepassage of a current through a contact between a ferromagnetand a semiconductor. Depending on the type of contacts, eitherthe majority or the minority carriers may be polarized. Ananalysis is made of the influence of a magnetic field on such

spin injection and conditions for its observation are discussed.Soviet Physics - Sem iconduc tors10, 698(1976).

A. G. Aronov and G. E. Pikus

B. P. Konstantinov Institute of Nuclear Physics

and A. F. Ioffe Physicotechnical Institute

Academy of Sciences of the USSR, Leningrad



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Substrates:(100) GaAs

Materials/Structures: 8-16 ML GaAs/4 ML MnGa

Growth temperature:275oC

Deposition rate:monitored with RHEED oscillations

Growth Mode:MBE for GaAs/Atomic Layer Epitaxy for MnGa

GaAs/MnGa Superlattices

4 ML of MnGa (2 periods of Mn

and Ga depositions)

substrate

GaAs



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InSb interface bonds GaAs Interface Bonds

Two types (and combinations) of Interfaces: InSb and GaAs

Can Control Type During Growth

Interface type affects Electrical and Optical Properties

GaSb

GaAs

Interface Formation in InAs/GaSb

InAs InAs

GaSb

InSb


Interface Effects on Band coupling


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Interface Effects on Band coupling

The k p coupling across the interface depends on Overlap Integral of

the electron and hole subband envelope functions

InAs GaSbGaAs

0.3 nm

0.81eV

0.15eV

0.45eV

CB

VB

InAs GaSbInSb

0.3 nm

0.81eV

0.15eV

0.45eV

CB

VB

Very simple(minded)PictureInterface layer

-- additional barrier

Interface layer

-- lower barrier



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Our Approach III-V/Mn digital alloys- Increase Mn concentration and improve structural quality

- 2-D spin systems and interlayer coupling

- Optical and transport properties in ordered alloy (in 1D)

systems

Intrinsic Problem:- Presence of precipitates

for high Mn

concentrations

Past lessons- -doping is a highly

effective method forincreased dopingconcentration

- Digital alloys result inhigh quality materials

Mn INCORPORATION



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TEM of GaMnSb Digital Alloy20 nm

High resolution

Low resolution

5 nm


R T G M A Di it l All


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R vs. T: GaMnAs Digital Alloy(16 ML GaAs/


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MATERIALS ISSUES

Poor understanding of

spin polarization(Complex Valence Bands)

Poor spin injection (2%)Holes heavily

compensated by defects

Poor optical response

Poor crystal quality

Low Curie Temperature

Mn is a p-type dopant for III-Vs (good and bad)

Low growth temperature required (275oC)



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Magnetization Measurements


ESD = Effective Spin density

from saturation Magnetization

(S = 2.5, g = 2)

5 10 15 20 25 30 35 40 455

10

15

20

25

30

35

40

45

50

510

15

20

25

30

35

40

45

50

TC

(K)

TC

EDS

EDS(10

13

/cm

2)

Mn % in layer

Correlated

Behavior



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0.0 0.1 0.2 0.3 0.4 0.5

[(GaAs)8Mn]

50

Log(Reflectivity)(arb.units)

qz(-1)

[(GaAs)12

Mn]50

[(GaAs)16

Mn]50

X-RAY REFLECTIVITY

Weakly ferromagnetic

sample -- 12- monolayer

GaAs spacers -- showsbest crystal quality

Periods of digital

alloys from Bragg

peaks agree well with

thickness measured

in situ



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Magnetization (Squid)

Easy Axis in-plane



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Approach III-V/Mn and III-V/GaMn digital alloys

Anticipated Outcomes Increased Mn concentration and improved structural quality

Quasi 2-D carrier system in region of ion spins enhanced TC

Improved Optical and transport properties (ordered alloy)

Materials Issues Precipitates for high Mn

concentration

Poor Structural/Optical

Properties

Past Lessons -doping highly effective for

increasing concentration

Digital alloys result in

high quality materials

III1-x MnxVs for Spintronics

Documents

Semi Cond Spin Tronic s