Electrical control of magnetism in semiconductors

F. Matsukura1,2, D. Chiba2,1, Y. Nishitani1, M. Endo1, and H. Ohno1,2

1Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University

2Semiconductor Spintronics Project, ERATO-JST

JST-DFG Workshop on NanoelectronicsAachen, Germany, March 5-7, 2008

・Introduction (field effect transistor with a magnetic semiconductor channel)・Thickness dependence・Mn composition dependence・Summary

Outline

Discussion with M. Sawicki and T. Dietl(Polish Academy of Sciences)

Semiconductor Spin-electronics (Spintronics)

Spin-related phenomena in semiconductors →an additional degree of freedom (spin + charge → spintronics)

In order to enhance spin-related phenomena in semiconductors· alloy system with host semiconductor and guest magnetic ion

(diluted magnetic semiconductors; DMSs) Multifunctional materials

electronicselectronics opticsoptics

spinspin

new functional devices utilizing controllability of spin dynamics,

coherence, and magnetismlaser

electric fieldand/or current

nuclear spin

magnetic spin

carrier spin

semiconductor

III-V Based Magnetic Semiconductors

· Combine present day electronic device materials with magnetism· Low solubility of magnetic elements overcome by

low-temperature molecular beam epitaxy (LT-MBE)

GaMn

As

New materials can be synthesized under non-equilibrium growth condition

First III-V Based Magnetic Semiconductor(In,Mn)As: H. Munekata et al., Phys. Rev. Lett. 63, 1849 (1989).

Ferromagnetism(In,Mn)As: H. Ohno et al., Phys Rev. Lett. 68, 2664 (1992).

(Ga,Mn)As: H. Ohno et al., Appl. Phys. Lett. 69, 363 (1996).

Mn acts simultaneously as an acceptor and as a magnetic spin

III-V Based Magnetic Semiconductors

· Combine present day electronic device materials with magnetism· Low solubility of magnetic elements overcome by

low-temperature molecular beam epitaxy (LT-MBE)

Mn spinsM ≠ 0

β

carrier spins

T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 (2000).T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev. B 63, 195205 (2001).

p-d Zener Model

p-d exchangeinteraction

Interaction (p-d exchange interaction) between holes and Mn spins induces the spin-splitting of valence band

Energy gain by repopulation of holes between spin subbands stabilizes ferromagnetism

( ) ( )B

2FF0

c 121

kEASSxNT S βρ+=Curie temperature:

1019 10201

10

100 exp, cal, (Ga,Mn)As, x = 0.053

, (In,Mn)As, x = 0.03

, (Zn,Mn)Te, xeff = 0.015

T C (K

)

p (cm-3)

Comparison of Experimental and Calculated TC

exp.: (Ga,Mn)As:T. Omiya et al., Physica E 7, 976 (2000).(In,Mn)As: D. Chiba et al., J. Supercond. and Novel Mag.16, 179 (2003).(Zn,Mn)Te: D. Ferrand et al., Phys. Rev. B 63, 085201 (2001).

larger TC for larger pquantitative agreement between experiment and calculation

Control of magnetism of ferromagnetic semiconductors by external means

tem

pera

ture

, T

TC as a function of p

hole concentration, p

ferromagnetism

after irradiation

paramagnetism

before irradiation

isothermal control of magnetism

-0.3 0.0 0.3-80

-40

0

40

80

(In,Mn)Asx = 0.06

µ0H (T)

5 K

before after

irradiation

M (m

T)hν

hν

electron

hole

EC

EF

EV(In,Mn)As GaSb

optical means

photo-generatedcarriers

S. Koshihara et al., Phys. Rev. Lett. 78, 4617 (1997).

Control of magnetism of ferromagnetic semiconductors by external means

isothermal control of magnetism

electrical means

field-effecttransistor

H. Ohno et al., Nature 408, 944 (2000).

-1.0 -0.5 0.0 0.5 1.0-50

-25

0

25

50

(In,Mn)Asx = 0.03

µ0H (mT)

22.5 K

Vg (V) 0 +125 -125 0

RH

all (Ω

)

tem

pera

ture

, T

TC as a function of p

hole concentration, p

ferromagnetism

(EG < 0)

paramagnetism

(EG > 0)

Previous result on FET with (Ga,Mn)As channel

substrate

buffer

channel

gate Insulator

gate metal

S.I. GaAs (001) sub.GaAs100 nm

In0.13Ga0.87As500 nmAl0.8Ga0.2As30 nm

GaAs7 nmGa0.863Mn0.047As7 nm

Al2O350 nmAu / Cr100 nm

30 µm

sample structure

FET device with Hall-bar shape

Hall resistance

∆∆∆∆TC ~ 5 K

Arrott-plot analysis

D. Chiba et al., Appl Phys Lett 89, 162505 (2006).

This work

1. FETs with (Ga,Mn)As channel and gate insulator (Al2O3 or HfO2 ) deposited by atomic layer deposition (ALD)

2. TC & ∆TC of (Ga,Mn)As channels in FET

· Channel thickness dependence

· Mn composition dependence

Atomic Layer Deposition (ALD)

Au/insulator/Au capacitors

side view

top view

capacitor

SI-GaAs sub.

AuAu

insulator

AuAu

Device structure

Measurement

100 mV 1 kHz

VC

10 kΩ

ALD Sample

lock-inVR

lock-in

0.00 0.05 0.10 0.15 0.200.0

0.5

1.0

Al2O3

HfO2

C (n

F)

A (mm2)

C /A

C

R

2 VRfVC

π=

0 10 20 300

1

2

3

tchannel = 5.0 nm

∆p (x

1020

cm

-3)

κ

∆E = ±5 MV/cm∆pideal

Al2O3

κ ~ 7.47 HfO2κ ~ 20.17

ACd ⋅=

0εκ

M. J. Biercuk et al. Appl. Phys. Lett. 83, 2405 (2003). A: area of capacitor, d: thickness of insulator

(Ga,Mn)As FETs

In0.15Ga0.85As500 nmGaAs

Al0.80Ga0.20AsGaAs

Ga0.949Mn0.051AsAl2O3 or HfO2

Au/Cr

S.I. GaAs substrate (001)30 nm

30 nm5.0 nm5.0 nm

50 or 40 nm100/5 nm

substrate

buffer

channel

insulator

metal

VG

typical sample structure

MBE

ALDEvaporation

FET fabrication

·Mesa structure with Hall bar geometryphotolithography and wet etching

·Gate insulatorALD

·Metal gateevaporation and lift-off 30 µm

FET device with Hall-bar shape

S DG

strain inducedperpendicularmagnetic easy axis

consistent withthe p-d Zener model

(Ga,Mn)As FETs

S

D

G

V

V

µµµµ0Hext

ISD

VHall

VGate

Vsheet

Measurement

-5.0 -2.5 0.0 2.5 5.0

3

4

5

Al2O3

T=20 K

Rsh

eet-1

(x10

-5 Ω

-1)

E (MV/cm)

slope αααα

E dependence of Rsheet

κεαµ0

−=

( ))(

11

sheet ERetEp

µ=

Eet

Ep 0)( κε∆ =

( )[ ]( )Eept

EppetR

0

1sheet

κεµ∆µ

−=+=−

⊥+= Mt

RHt

RR S0

0Hall µ t: channel thickness

ISD = 1 µAT = 10 ~ 250 K|µ0H| ≤ 0.5 T|E| ≤ 5 MV/cm

anomalous Hall effect

x = 0.05t = 5 nm

(Ga,Mn)As FET Magnetotransport properties

0.0 0.2 0.4 0.6 0.80.0

0.2

0.4

0.6

58 K57 K

56 K55 K

RH

all2 (k

Ω2 )

µ0H/RHall (T/kΩ)

E = 0 V/cm

54 KAl2O3

Arrott plots

-0.50 -0.25 0.00 0.25 0.50-1.0

-0.5

0.0

0.5

1.0

µ0H (T)

80 K

70 K60 K

50 K

RH

all (

kΩ)

40 KAl2O3

E = 0

⊥+= Mt

RHt

RR S0

0Hall µ

44 46 48 50 52 54 56 58 60 620.0

0.5

1.0

RH

all (k

Ω)

0 MV/cm

T (K)

5 MV/cm

-5 MV/cm

Al2O3

TC = 56.4 K

∆TC = 6.1 K

x = 0.05t = 5 nm

channel thickness dependence

(Ga,Mn)As FETs with different channel thickness

In0.15Ga0.85As500 nmGaAs

Al0.75Ga0.25AsGaAs

Ga0.935Mn0.065AsAl2O3

Au/Cr

S.I. GaAs substrate (001)30 nm

30 nm5.0 nmt nm40 nm

100/5 nm

substrate

buffer

channel

insulator

metal

Sample structure Rsheet-T

0 100 200

10-2

10-1

ρ (Ω

cm)

T (K)

3.5, 4.0, 4.5, 5.0 nm

t = 3.5, 4.0, 4.5, and 5.0 nm

insulating metallic

E = 0

Eet

Ep 0)( κε∆ =

∆p for thinner layer → larger ∆TC

x = 0.065

-0.1 0.0 0.1

-1.0

-0.5

0.0

0.5

1.0

80 K

70 K60 K

50 K

RH

all (k

Ω)

µ0H (T)

10 K

0 V/cm

-50 0 50-1.0

-0.5

0.0

0.5

1.0

RH

all (k

Ω)

4.5 nm

60 K

-5 MV/cm

0 V/cm

+5 MV/cm

µ0H (mT)

4.5 nm

(Ga,Mn)As FETs with different channel thicknessTypical results

Rsheet-T E dependence

x = 0.065

70 75 80 85 900.0

0.4 50 55 60 65 700.0

1.1 40 45 50 55 600.0

0.8 30 35 40 45 500.0

0.6

5.0 nm

4.5 nm

Rs H

all (k

Ω)

4.0 nm

T (K)

0 MV/cm5 MV/cm

3.5 nm-5 MV/cm

(Ga,Mn)As FETs with different channel thicknessCurie temperature

larger TC and smaller ∆TC for thicker chnnael

x = 0.065

5 4.5 4 3.50.6

0.7

0.8

0.9

0

1

2

3

t (nm)

∆p (c

m-3)

p (c

m-3)

(x1020)

40

60

80

100

5 4.5 4 3.50

4

8

12

16

T C (K

)

t (nm)

∆TC (K

)

(Ga,Mn)As FETs with different channel thicknessHole concentration & Curie temperature

FETs with thinner channel have larger ∆TC as well as ∆p

x = 0.065

Mn composition dependence

Growth of Ga1-xMnxAs with High x (~ 0.2 )

D.Chiba et al., Appl. Phys. Lett. 90, 122503 (2007).

S. Ohya et al., Appl. Phys. Lett. 90, 112503 (2007).

decrease TS

thinner layer

Single phase Ga1-xMnxAs with higher x

Ga0.8Mn0.2As

T=20 K

-0.4 -0.2 0.2 0.40.0µ0H (T)

1.0

-1.0

-0.5

0.5

0.0

M/M

Set

c.

Substrate

Buffer layer

Channel layer

S.I. GaAs (001) sub.GaAs30 nm

In0.15Ga0.85As420 nmAl0.75Ga0.25As30 nm

GaAs4 nmGa0.8Mn0.2As5 nm [-110]

azimuth

In0.15Ga0.85As420 nm

MBE

ALDevaporation

GaAs

Al0.75Ga0.25AsGaAs

Ga1-xMnxAsHfO2

Cr / Au

S. I. GaAs substrate (001)30 nm

30 nm4.0 nm4.0 nm40 nm100 nm

Substrate

Buffer layer

Channel layerGate insulatorGate electrode

VG

0.0750.1000.1250.1750.200

x =

S

D

G

V

V

µµµµ0Hext

ISD(const.)

VHall

VGate

Vsheet

(Ga,Mn)As FETs with various Mn compositions

annealed at 180oC for 5 min

0 50 100 150 200

104

2x104

3x104

x = 0.175

x = 0.200

x = 0.125

x = 0.100

T (K)

R

shee

t (Ω

)

x = 0.075

E = 0

(Ga,Mn)As FETs with various Mn compositions

0.0 0.1 0.20

5

10

15

20

p (x

1020

cm

-3)

x

t = 4.0 nm

higher x → higher p> 1021 cm-3

(Ga,Mn)As FETs with various Mn compositions

0.0 0.4 0.8 1.2 1.60.00

0.04

0.08

77.6 K

R

Hal

l2 (kΩ

2 )

µ0H/RHall (T/kΩ)

73.5 K 74.4 K75.6 K76.6 K

x = 0.075E = 0

66 68 70 72 74 76 78 800.0

0.1

0.2

0.3

T (K)

RH

allS (k

Ω)

-5 MV/cm

0 V/cm+5 MV/cm

x = 0.075

∆TC

0.0 0.1 0.20

2

4

6

8

100

50

100

150

200

∆TC (K

)

x

T C (K

)

t = 4.0 nm

TC

Summary of experimental results

(Ga,Mn)As channel itself

・ larger TC and pfor thicker channel andhigher x

(Ga,Mn)As channel in FET

・larger ∆TCfor thinner channel and smaller x

0.0 0.5 1.0 1.50.0

0.1

0.2

0.3

(0.065, 5)

(0.065, 5)

(0.065, 3.5)

(0.065, 4)

(0.200, 4)(0.175, 4) (0.125, 4)

(0.100, 4)

∆TC/T

C

∆p / p

(0.075, 4)

(x, t nm)∆TC/TC ~ 0.2 ∆p/p

∆TC/TC ~ 0.6∆p/p is expected from the p-d Zener model

Other effects?

0 2 4 6 8 100

50

100

150

200

2D p-d Zener5 nm

p-d Zener

RKKY

T C

(K)

p (cm-3) (x1020)

xeff = 0.05N0β = -1.2 eVAF = 1.2

0.0 0.1 0.20

50

100

150

200

x

T C (K

)

Etchedby 1 nm

4 nm

· two-dimensional effect· RKKY oscillation· nonuniformity along

growth direction· ……

SummaryFETs with a (Ga,Mn)As channel· Control of TC is indeed possible by gating· Larger ∆p/p results in larger ∆TC/TC; ∆TC/TC ~ 0.2∆p/p· Quantitative description ?magnetization, channeling, distribution of Mn etc.

VGferromagnetic QWRs

VG

J

ferromagnetic QDs arrayanomalousHall effect

magnetic anisotropy

predicted p dependentproperties

T. Jungwirth et al., Phys. Rev.Lett. 88, 207208 (2002).

T. Dietl et al., Phys. Rev. B 63, 195205 (2001).