Electrical control of magnetism in semiconductors F. Matsukura 1,2 , - - PowerPoint PPT Presentation

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 Nanoelectronics Aachen, 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

electronics electronics

• ptics
• ptics

spin spin

new functional devices utilizing controllability of spin dynamics, coherence, and magnetism

laser electric field and/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) Ga Mn 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 spins M ≠ 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 exchange interaction

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 2 F F c

12 1 k E A S S xN T

S

β ρ + =

Curie temperature:

10

19

10

20

1 10 100 exp, cal

, (Ga,Mn)As, x = 0.053 , (In,Mn)As, x = 0.03 , (Zn,Mn)Te, xeff = 0.015

TC (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 p quantitative agreement between experiment and calculation

Control of magnetism of ferromagnetic semiconductors by external means

temperature, 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

40 80

(In,Mn)As x = 0.06 µ0H (T)

5 K

before after irradiation

M (mT)

hν hν

electron hole EC EF EV (In,Mn)As GaSb

• ptical means

photo-generated carriers

• 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-effect transistor

• H. Ohno et al., Nature 408, 944 (2000).
• 1.0
• 0.5

0.0 0.5 1.0

• 50
• 25

25 50

(In,Mn)As x = 0.03 µ0H (mT) 22.5 K

Vg (V) +125

• 125

RHall (Ω)

temperature, 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. GaAs 100 nm In0.13Ga0.87As 500 nm Al0.8Ga0.2As 30 nm GaAs 7 nm Ga0.863Mn0.047As 7 nm Al2O3 50 nm Au / Cr 100 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.

Au Au insulator Au Au

Device structure Measurement 100 mV 1 kHz VC 10 kΩ ALD Sample lock-in VR lock-in

0.00 0.05 0.10 0.15 0.20 0.0 0.5 1.0 Al2O3 HfO2 C (nF) A (mm

2)

C /A

C R

2 V R f V C π =

10 20 30 1 2 3

tchannel = 5.0 nm

∆p (x10

20 cm

• 3)

κ

∆E = ±5 MV/cm

∆pideal

Al2O3

κ ~ 7.47

HfO2

κ ~ 20.17

A C d ⋅ = ε κ

• M. J. Biercuk et al. Appl. Phys. Lett. 83, 2405 (2003).

A: area of capacitor, d: thickness of insulator

(Ga,Mn)As FETs

In0.15Ga0.85As 500 nm GaAs Al0.80Ga0.20As GaAs Ga0.949Mn0.051As Al2O3 or HfO2 Au/Cr S.I. GaAs substrate (001) 30 nm 30 nm 5.0 nm 5.0 nm

50 or 40 nm

100/5 nm

substrate buffer

channel insulator

metal

VG

typical sample structure

MBE ALD Evaporation

FET fabrication ·Mesa structure with Hall bar geometry photolithography and wet etching ·Gate insulator ALD ·Metal gate evaporation and lift-off

30 µm

FET device with Hall-bar shape

S D G

strain induced perpendicular magnetic easy axis consistent with the p-d Zener model

(Ga,Mn)As FETs

S D G V V µ µ µ µ0Hext ISD VHall VGate Vsheet

Measurement

• 5.0
• 2.5

0.0 2.5 5.0 3 4 5 Al2O3 T=20 K Rsheet

• 1 (x10
• 5 Ω
• 1)

E (MV/cm) slope α

α α α

E dependence of Rsheet

κ ε α µ − =

( )

) ( 1 1

sheet E

R et E p µ =

E et E p ) ( κε ∆ =

( ) [ ] ( )

E ept E p p et R

1 sheet

κε µ ∆ µ − = + =

−

⊥

+ = M t R H t R R

S Hall

µ

t: channel thickness

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

anomalous Hall effect x = 0.05 t = 5 nm

(Ga,Mn)As FET

Magnetotransport properties

0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 58 K 57 K 56 K 55 K

RHall

2 (kΩ 2)

µ0H/RHall (T/kΩ)

E = 0 V/cm

54 K

Al2O3

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 K 60 K 50 K

RHall (kΩ)

40 K

Al2O3

E = 0

⊥

+ = M t R H t R R

S Hall

µ

44 46 48 50 52 54 56 58 60 62 0.0 0.5 1.0

RHall (kΩ) 0 MV/cm T (K) 5 MV/cm

• 5 MV/cm

Al2O3

TC = 56.4 K ∆TC = 6.1 K x = 0.05 t = 5 nm

channel thickness dependence

(Ga,Mn)As FETs with different channel thickness

In0.15Ga0.85As 500 nm GaAs Al0.75Ga0.25As GaAs Ga0.935Mn0.065As Al2O3 Au/Cr S.I. GaAs substrate (001) 30 nm 30 nm 5.0 nm t nm

40 nm

100/5 nm

substrate buffer

channel insulator

metal

Sample structure Rsheet-T

100 200 10

• 2

10

• 1

ρ (Ωcm)

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

E et E p ) ( 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 K 60 K 50 K

RHall (kΩ)

µ0H (T)

10 K 0 V/cm

• 50

50

• 1.0
• 0.5

0.0 0.5 1.0

RHall (kΩ)

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 thickness Typical results Rsheet-T E dependence x = 0.065

70 75 80 85 90 0.0 0.4 50 55 60 65 70 0.0 1.1 40 45 50 55 60 0.0 0.8 30 35 40 45 50 0.0 0.6

5.0 nm 4.5 nm R

s Hall (kΩ)

4.0 nm T (K) 0 MV/cm 5 MV/cm 3.5 nm

• 5 MV/cm

(Ga,Mn)As FETs with different channel thickness Curie temperature larger TC and smaller ∆TC for thicker chnnael x = 0.065

5 4.5 4 3.5 0.6 0.7 0.8 0.9 1 2 3

t (nm)

∆p (cm

• 3)

p (cm

• 3)

(x10

20)

40 60 80 100 5 4.5 4 3.5 4 8 12 16

TC (K) t (nm)

∆TC (K)

(Ga,Mn)As FETs with different channel thickness Hole 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 0.4 0.0

µ0H (T)

1.0

• 1.0
• 0.5

0.5 0.0

M / MS etc.

Substrate Buffer layer Channel layer S.I. GaAs (001) sub. GaAs 30 nm In0.15Ga0.85As 420 nm Al0.75Ga0.25As 30 nm GaAs 4 nm Ga0.8Mn0.2As 5 nm

[-110] azimuth

In0.15Ga0.85As 420 nm MBE ALD evaporation GaAs Al0.75Ga0.25As GaAs Ga1-xMnxAs HfO2 Cr / Au

• S. I. GaAs substrate (001)

30 nm 30 nm 4.0 nm 4.0 nm 40 nm 100 nm Substrate Buffer layer Channel layer Gate insulator Gate electrode VG

0.075 0.100 0.125 0.175 0.200 x =

S D G V V µ µ µ µ0Hext ISD(const.) VHall VGate Vsheet

(Ga,Mn)As FETs with various Mn compositions annealed at 180oC for 5 min

50 100 150 200 10

4

2x10

4

3x10

4

x = 0.175 x = 0.200 x = 0.125 x = 0.100

T (K) Rsheet (Ω)

x = 0.075

E = 0

(Ga,Mn)As FETs with various Mn compositions

0.0 0.1 0.2 5 10 15 20

p (x10

20 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 1.6 0.00 0.04 0.08 77.6 K

RHall

2 (kΩ 2)

µ0H/RHall (T/kΩ)

73.5 K 74.4 K 75.6 K 76.6 K

x = 0.075 E = 0

66 68 70 72 74 76 78 80 0.0 0.1 0.2 0.3

T (K) RHall

S (kΩ)

• 5 MV/cm

0 V/cm +5 MV/cm

x = 0.075

∆TC

0.0 0.1 0.2 2 4 6 8 10 50 100 150 200

∆TC (K)

x TC (K)

t = 4.0 nm TC

Summary of experimental results (Ga,Mn)As channel itself ・ larger TC and p for thicker channel and higher x (Ga,Mn)As channel in FET ・larger ∆TC for thinner channel and smaller x

0.0 0.5 1.0 1.5 0.0 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/TC ∆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?

2 4 6 8 10 50 100 150 200

2D p-d Zener 5 nm

p-d Zener

RKKY TC (K) p (cm

• 3)

(x10

20)

xeff = 0.05 N0β = -1.2 eV AF = 1.2

0.0 0.1 0.2 50 100 150 200

x TC (K)

Etched by 1 nm

4 nm

· two-dimensional effect · RKKY oscillation · nonuniformity along growth direction · ……

Summary FETs 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.

VG

ferromagnetic QWRs

VG J

ferromagnetic QDs array

anomalous Hall effect magnetic anisotropy

predicted p dependent properties

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

63, 195205 (2001).