NRI
This presentation is the property of its rightful owner.
Sponsored Links
1 / 59

Making semiconductors magnetic: new materials properties, devices, and future PowerPoint PPT Presentation


  • 93 Views
  • Uploaded on
  • Presentation posted in: General

NRI SWAN. Making semiconductors magnetic: new materials properties, devices, and future. JAIRO SINOVA Texas A&M University Institute of Physics ASCR. Hitachi Cambridge Jorg Wunderlich , A. Irvine, et al. Institute of Physics ASCR Tomas Jungwirth , Vít Novák, et al.

Download Presentation

Making semiconductors magnetic: new materials properties, devices, and future

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Making semiconductors magnetic new materials properties devices and future

NRI

SWAN

Making semiconductors magnetic:

new materials properties, devices, and future

JAIRO SINOVA

Texas A&M University

Institute of Physics ASCR

Hitachi Cambridge

Jorg Wunderlich, A. Irvine,et al

Institute of Physics ASCR

Tomas Jungwirth, Vít Novák, et al

Texas A&ML. Zarbo

Ohio State University

Oct 2nd 2009

University of Nottingham

Bryan Gallagher, Tom Foxon,Richard Campion, et al.


Outline

OUTLINE

  • Motivation

  • Ferromagnetic semiconductor materials:

    • (Ga,Mn)As - general picture

    • Growth, physical limits on Tc

    • Related FS materials (searching for room temperature)

    • Understanding critical behavior in transport

  • Ferromagnetic semiconductors & spintronics

    • Tunneling anisotropic magnetoresistive device

    • Transistors (4 types)


Making semiconductors magnetic new materials properties devices and future

ENGINEERING OF QUANTUM MATERIALS

Technologically motivated and scientifically fueled

  • Generates new physics:

  • Tunneling AMR

  • Coulomb blockade AMR

  • Nanostructure magnetic anisotropy engineering

Incorporate magnetic properties with semiconductor tunability (MRAM, etc)

  • Understanding complex phenomena:

  • Spherical cow of ferromagnetic systems (still very complicated)

  • Engineered control of collective phenomena

More knobs than usual in semiconductors: density, strain, chemistry/pressure, SO coupling engineering


Making semiconductors magnetic new materials properties devices and future

Ferromagnetic semiconductor research :

strategies

  • Create a material that marriages the tunability of semiconductors and the collective behavior of ferromagnets; once created search for room temperature systems

  • Study new effects in this new material and utilize in metal-based spintronics

  • Develop a three-terminal gated spintronic device to progress from sensors & memories to transistors & logic


Making semiconductors magnetic new materials properties devices and future

(Ga,Mn)As

GENERAL PICTURE


Making semiconductors magnetic new materials properties devices and future

Ga

Mn

As

Mn

Ferromagnetic semiconductors

Need true FSs not FM inclusions in SCs

GaAs - standard III-V semiconductor

Group-II Mn - dilute magnetic moments & holes

(Ga,Mn)As - ferromagnetic semiconductor

+


Making semiconductors magnetic new materials properties devices and future

Ga

As

Mn

What happens when a Mn is placed in Ga sites:

Mn–hole spin-spin interaction

As-p

5 d-electrons with L=0

S=5/2 local moment

intermediate

acceptor (110 meV)

 hole

Mn-d

hybridization

Hybridization  like-spin level repulsion  Jpd SMn shole interaction

In addition to the Kinetic-exchange coupling, for a single Mn ion, the coulomb interaction gives a trapped hole (polaron) which resides just above the valence band


Making semiconductors magnetic new materials properties devices and future

Ga

Ga

Mn

As

As

Mn

Mn

Transition to a ferromagnet when Mn concentration increases

GaAs:Mn – extrinsic p-type semiconductor

EF

spin 

~1% Mn

<< 1% Mn

>2% Mn

DOS

Energy

spin 

onset of ferromagnetism near MIT

As-p-like holes localized on Mn acceptors

valence band As-p-like holes

Ga

Mn

As

Mn

FM due to p-d hybridization (Zener local-itinerant kinetic-exchange)


Making semiconductors magnetic new materials properties devices and future

(Ga,Mn)As GROWTH

high-T growth

  • Low-T MBE to avoid precipitation

  • High enough T to maintain 2D growth

  • need to optimize T & stoichiometry

  • for eachMn-doping

  • Inevitable formation of interstitial Mn-double-donors compensating holes and moments

  •  need to anneal out but without loosing MnGa

optimal-T growth


Making semiconductors magnetic new materials properties devices and future

Interstitial Mn out-diffusion limited by surface-oxide

Polyscrystalline

20% shorter bonds

O

GaMnAs-oxide

x-ray photoemission

MnI++

GaMnAs

Olejnik et al, ‘08

10x shorther annealing with etch

Optimizing annealing-T another key factor

Rushforth et al, ‘08


Making semiconductors magnetic new materials properties devices and future

(Ga,Mn)As

GENERAL THEORY


Making semiconductors magnetic new materials properties devices and future

HOW DOES ONE GO ABOUT

UNDERSTANDING SUCH SYSTEMS

  • One could solve the full many body S.E.: not possible AND not fun

  • Combining phenomenological models (low degrees of freedom) and approximations and comparison to other computational technieques while checking against experiments

“This is the art of condensed matter science, an intricate tango

between theory and experiment whose conclusion can only be

guessed at while the dance is in progress”

A.H.M et al., in “Electronic Structure and Magnetism in Complex Materials” (2002).


Making semiconductors magnetic new materials properties devices and future

Theoretical Approaches to DMSs

Jungwirth, Sinova, Masek, Kucera, MacDonald, Rev. of Mod. Phys. 78, 809 (2006)

  • First Principles LSDA

PROS: No initial assumptions, effective Heisenberg model can be extracted, good for determining chemical trends

CONS:Size limitation, difficulty dealing with long range interactions, lack of quantitative predictability, neglects SO coupling (usually)

  • Microscopic TB models

PROS: “Unbiased” microscopic approach, correct capture of band structure and hybridization, treats disorder microscopically (combined with CPA), very good agreement with LDA+U calculations

CONS: neglects (usually) coulomb interaction effects, difficult to capture non-tabulated chemical trends, hard to reach large system sizes

  • k.p  Local Moment

PROS:simplicity of description, lots of computational ability, SO coupling can be incorporated,

CONS: applicable only for metallic weakly hybridized systems (e.g. optimally doped GaMnAs), over simplicity (e.g. constant Jpd), no good for deep impurity levels (e.g. GaMnN)


Making semiconductors magnetic new materials properties devices and future

Magnetism in systems with coupled dilute moments

and delocalized band electrons

coupling strength / Fermi energy

band-electron density / local-moment density

(Ga,Mn)As


Making semiconductors magnetic new materials properties devices and future

Impurity bandit vs Valence Joe

KP Eastwood

Fast principles Jack

Which theory is right?


How well do we understand ga mn as

How well do we understand (Ga,Mn)As?

In the metallic optimally doped regime GaMnAs is well described by a disordered-valence band picture: both dc-data and ac-data are consistent with this scenario.

The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe (III,Mn)V metallic DMSs very well in the optimally annealed regime:

  • Ferromagnetic transition temperatures 

  •  Magneto-crystalline anisotropy and coercively 

  •  Domain structure 

  •  Anisotropic magneto-resistance 

  •  Anomalous Hall effect 

  •  MO in the visible range 

  •  Non-Drude peak in longitudinal ac-conductivity 

  • Ferromagnetic resonance 

  • Domain wall resistance 

  • TAMR 

  • Transport critical behaviour 

  • Infrared MO effects

TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping


Making semiconductors magnetic new materials properties devices and future

EXAMPLE: MAGNETO-OPTICAL EFFECTS IN THE INFRARED


Making semiconductors magnetic new materials properties devices and future

Tc LIMITS AND STRATEGIES


Making semiconductors magnetic new materials properties devices and future

Mn

Mn

Mn

As

Ga

Problems for GaMnAs (late 2002)

  • Curie temperature limited to ~110K.

  • Only metallic for ~3% to 6% Mn

  • High degree of compensation

  • Unusual magnetization (temperature dep.)

  • Significant magnetization deficit

“110K could be a fundamental limit on TC”

But are these intrinsic properties of GaMnAs ??


Making semiconductors magnetic new materials properties devices and future

Can a dilute moment ferromagnet have a high Curie temperature ?

EXAMPLE OF THE PHYSICS TANGO

  • The questions that we need to answer are:

  • Is there an intrinsic limit in the theory models (from the physics of the phase diagram) ?

  • Is there an extrinsic limit from the ability to create the material and its growth (prevents one to reach the optimal spot in the phase diagram)?


Making semiconductors magnetic new materials properties devices and future

Intrinsic properties of (Ga,Mn)As

COMBINATION OF THEORY APPROACHES PREDICTS:

Tc linear in MnGa local moment concentration; falls rapidly with decreasing hole density in more than 50% compensated samples; nearly independent of hole density for compensation < 50%.

Jungwirth, Wang, et al. Phys. Rev. B 72, 165204 (2005)


Extrinsic effects interstitial mn a magnetism killer

Extrinsic effects: Interstitial Mn - a magnetism killer

Mn

As

Interstitial Mn is detrimental to magnetic order:

compensating double-donor – reduces carrier density

couples antiferromagnetically to substitutional Mneven in

low compensation samplesBlinowski PRB ‘03, Mašek, Máca PRB '03

Yu et al., PRB ’02:

~10-20% of total Mn concentration is incorporated as interstitials

Increased TC on annealing corresponds to removal of these defects.


Making semiconductors magnetic new materials properties devices and future

MnGa and MnI partial concentrations

As grown

Materials calculation

Jungwirth, Wang, et al.

Phys. Rev. B 72, 165204 (2005)

Microscopic defect formation energy calculations:

No signs of saturation in the dependence of MnGa concentration

on total Mn doping


Making semiconductors magnetic new materials properties devices and future

Experimental hole densities: measured by ordinary Hall effect

Open symbols & half closed as grown. Closed symbols annealed

Low Compensation

Obtain Mnsub assuming change in hole density due to Mn out diffusion

High compensation

Jungwirth, Wang, et al.

Phys. Rev. B 72, 165204 (2005)

Annealing can vary significantly increases hole densities.


Making semiconductors magnetic new materials properties devices and future

Mnsub

MnInt

Obtain Mnsub & MnInt assuming change in hole density due to Mn out diffusion

Experimental partial concentrations of MnGa and MnI in as grown samples

Theoretical linear dependence of Mnsub on total Mn confirmed experimentally

Jungwirth, Wang, et al.

Phys. Rev. B 72, 165204 (2005)

SIMS: measures total Mn concentration.

Interstitials only compensation assumed


Can we have high tc in diluted magnetic semicondcutors

Can we have high Tc in Diluted Magnetic Semicondcutors?

NO INTRINSIC LIMIT

NO EXTRINSIC LIMIT

Tc linear in MnGa local (uncompensated) moment concentration; falls rapidly with decreasing hole density in heavily compensated samples.

There is no observable limit to the amount of substitutional Mn we can put in

Define Mneff = Mnsub-MnInt


Making semiconductors magnetic new materials properties devices and future

Linear increase of Tc with Mneff = Mnsub-MnInt

High compensation

8% Mn

Tc as grown and annealed samples

Open symbols as grown. Closed symbols annealed

  • Concentration of uncompensated MnGa moments has to reach ~10%. Only 6.2% in the current record Tc=173K sample

  • Charge compensation not so important unless > 40%

  • No indication from theory or experiment that the problem is other than technological - better control of growth-T, stoichiometry


Making semiconductors magnetic new materials properties devices and future

188K!!

Tc limit in (Ga,Mn)As remains open

2008

Olejnik et al

“... Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” Yu et al. ‘03

“…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...” Mack et al. ‘08

`

“Combinatorial” approach to growth

with fixed growth and annealing T’s


Making semiconductors magnetic new materials properties devices and future

Getting to higher Tc: Strategy A

- Effective concentration of uncompensated MnGa moments has to increase

beyond 6% of the current record Tc=173K sample. A factor of 2 needed

 12% Mn would still be a DMS

- Low solubility of group-II Mn in III-V-host GaAs makes growth difficult

Low-temperature MBE

Strategy A:stick to (Ga,Mn)As

- alternative growth modes (i.e. with proper

substrate/interface material) allowing for larger

and still uniform incorporation of Mn in zincblende GaAs

More Mn - problem with solubility


Making semiconductors magnetic new materials properties devices and future

Getting to higher Tc: Strategy B

  • Find DMS system as closely related to (Ga,Mn)As as possible with

  • larger hole-Mn spin-spin interaction

  • lower tendency to self-compensation by interstitial Mn

  • larger Mn solubility

  • independent control of local-moment and carrier doping (p- & n-type)


Making semiconductors magnetic new materials properties devices and future

Other (III,Mn)V’s DMSs

Kudrnovsky et al. PRB 07

Delocalized holes

long-range coupl.

Weak hybrid.

Mean-field but

low TcMF

InSb

d5

Impurity-band holes

short-range coupl.

Strong hybrid.

Large TcMF but

low stiffness

GaP

(Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host


Making semiconductors magnetic new materials properties devices and future

  • Steps so far in strategy B:

  • larger hole-Mn spin-spin interaction : DONE BUT DANGER IN PHASE DIAGRAM

  • lower tendency to self-compensation by interstitial Mn: DONE

  • larger Mn solubility ?

  • independent control of local-moment and carrier doping (p- & n-type)?

Using DEEP mathematics to find a new material

3=1+2


Making semiconductors magnetic new materials properties devices and future

EF

L

As p-orb.

Ga s-orb.

As p-orb.

III = I + II  Ga = Li + Zn

GaAs and LiZnAs are twin SC

LDA+U says that Mn-doped are also twin DMSs

It can be n and p doped!!!

No solubility limit for group-II Mn

substituting for group-II Zn !!!!

Masek, et al. PRB (2006)


Making semiconductors magnetic new materials properties devices and future

UNDERSTANDING CRITICAL BEHAVIOUR IN TRANSPORT


Making semiconductors magnetic new materials properties devices and future

Towards spintronics in (Ga,Mn)As: FM & transport

Dilute-moment MS

F~ d-

Dense-moment MS

F<< d-

Eu - chalcogenides

Broad peak near Tc disappeares with annealing (higher uniformity)???

Critical contribution to resistivity at Tc

~ magnetic susceptibility


Making semiconductors magnetic new materials properties devices and future

When density of carriers is smaller than density of local moments what matters is the long range behavior of Γ (which goes as susceptibility)

When density of carriers is similar to density of local moments what matters is the short range behavior of Γ (which goes as the energy)

Tc

Ni

EuCdSe

Tc


Making semiconductors magnetic new materials properties devices and future

d/dT singularity at Tc – consistent with kF~d-

Annealing sequence

Optimized materials with x=4-12.5% and Tc=80-185K

Remarkably universal both below and above Tc

V. Novak, et al “Singularity in temperature derivative of resistivity in (Ga,Mn)As at the Curie point”, Phys. Rev. Lett. 101, 077201 (2008).


Outline1

OUTLINE

  • Motivation

  • Ferromagnetic semiconductor materials:

    • (Ga,Mn)As - general picture

    • Growth, physical limits on Tc

    • Related FS materials (searching for room temperature)

    • Understanding critical behavior in transport

  • Ferromagnetic semiconductors & spintronics

    • Tunneling anisotropic magnetoresistive device

    • Transistors (4 types)


Making semiconductors magnetic new materials properties devices and future

Au

AMR

TMR

~ 1% MR effect

~ 100% MR effect

Exchange split & SO-coupled bands:

TAMR

Exchange split bands:

discovered in (Ga,Mn)As Gold et al. PRL’04


Making semiconductors magnetic new materials properties devices and future

TAMR in metal structures

experiment

Park, et al, PRL '08

ab intio theory

Shick, et al, PRB '06, Park, et al, PRL '08

Also studied by Parkin et al., Weiss et al., etc.


Making semiconductors magnetic new materials properties devices and future

DMS DEVICES


Making semiconductors magnetic new materials properties devices and future

Gating of highly doped (Ga,Mn)As: p-n junction FET

(Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06

p-n junction depletion estimates

~25% depletion feasible at low voltages

Olejnik et al., ‘08


Making semiconductors magnetic new materials properties devices and future

Increasing  and decreasing AMR and Tc with depletion

Tc

Tc

AMR


Making semiconductors magnetic new materials properties devices and future

Persistent variations of magnetic properties with ferroelectric gates

Stolichnov et al.,

Nat. Mat.‘08


Making semiconductors magnetic new materials properties devices and future

Electro-mechanical gating with piezo-stressors

Strain & SO 

Rushforth et al., ‘08

Electrically controlled magnetic anisotropies via strain


Making semiconductors magnetic new materials properties devices and future

(Ga,Mn)As spintronic single-electron transistor

Wunderlich et al. PRL ‘06

Huge, gatable, and hysteretic MR

Single-electron transistor

Two "gates": electric and magnetic


Making semiconductors magnetic new materials properties devices and future

Single-electron charging energy controlled by Vg and M

M

[010]

[110]

F

Q

VD

[100]

Source

Drain

[110]

[010]

Gate

VG

Q0

Q0

e2/2C

magnetic

electric &

SO-coupling (M)

control of Coulomb blockade oscillations

Theory confirms chemical potential anisotropies in (Ga,Mn)As

& predicts CBAMR in SO-coupled room-Tc metal FMs


Making semiconductors magnetic new materials properties devices and future

V

DD

V

V

A

B

V

B

V

A

Nonvolatile programmable logic

1

0

ON

OFF

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device

1

0

ON

OFF

1

1

0

0

0

1

1

0

OFF

ON

OFF

ON

ON

OFF

ON

0

1

1

1

0

0

1

1

Vout

AB Vout

00 0

10 1

01 1

11 1

OFF

ON

OFF

1

1

0

0

“OR”

OFF

ON

OFF

ON


Making semiconductors magnetic new materials properties devices and future

V

DD

V

V

A

B

V

B

V

A

Nonvolatile programmable logic

1

0

ON

OFF

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device

1

0

ON

OFF

Vout

AB Vout

00 0

10 1

01 1

11 1

“OR”


Making semiconductors magnetic new materials properties devices and future

Device design

Physics of SO & exchange

Materials

FSs and metal FS with strong SO

Chemical potential

 CBAMR

SET

FSs

Tunneling DOS

 TAMR

Tunneling device

metal FMs

Group velocity & lifetime

 AMR

Resistor


Conclusion

Conclusion

No intrinsic or extrinsic limit to Tc so far: it is a materials growth issue

In the metallic optimally doped regime GaMnAs is well described by a disordered-valence band picture: both dc-data and ac-data are consistent with this scenario.

The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe (III,Mn)V metallic DMSs very well in the optimally annealed regime:

  • Ferromagnetic transition temperatures 

  •  Magneto-crystalline anisotropy and coercively 

  •  Domain structure 

  •  Anisotropic magneto-resistance 

  •  Anomalous Hall effect 

  •  MO in the visible range 

  •  Non-Drude peak in longitudinal ac-conductivity 

  • Ferromagnetic resonance 

  • Domain wall resistance 

  • TAMR 

  • Transport critical behaviour 

  • Infrared MO effects

BUT it is only a peace of the theoretical mosaic with many remaining challenges!!

TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping


Making semiconductors magnetic new materials properties devices and future

Tomas Jungwirth

Inst. of Phys. ASCR

U. of Nottingham

Tomesz Dietl

Institute of Physics, Polish Academy of Sciences

Ewelina Hankiewicz

Fordham Univesrsity

Allan MacDonald

U of Texas

Hideo Ohno

Tohoku Univ.

Laurens Molenkamp

Wuerzburg

Joerg Wunderlich

Cambridge-Hitachi

Bryan Gallagher

U. Of Nottingham

Other collaborators: John Cerne, Jan Masek,Karel Vyborny, Bernd Kästner, Carten Timm, Charles Gould, Tom Fox, Richard Campion, Laurence Eaves, Eric Yang, Andy Rushforth, Viet Novak


Making semiconductors magnetic new materials properties devices and future

k.p  Local Moment - Hamiltonian

·Model Anderson Hamiltonian:

(s - orbitals: conduction band; p - orbitals: valence band)

+ (Mn d - orbitals: strong on-site Hubbard int. local moment)

+ (s,p - d hybridization)

· Semi-phenomenological Kohn-Luttinger model for

heavy, light, and spin-orbit split-off band holes

·Local exchange coupling:

Mn: S=5/2; valence-band hole: s=1/2; Jpd > 0

ELECTRONS

Mn

ELECTRONS-Mn

Large S:

treat classically


Making semiconductors magnetic new materials properties devices and future

Tc LIMITS AND STRATEGIES


Can we have high tc in diluted magnetic semicondcutors1

Can we have high Tc in Diluted Magnetic Semicondcutors?

NO IDENTIFICATION OF AN INTRINSIC LIMIT

NO EXTRINSIC LIMIT

Tc linear in MnGa local (uncompensated) moment concentration; falls rapidly with decreasing hole density in heavily compensated samples.

(lines – theory, Masek et al 05)

Relative Mn concentrations obtained through hole density measurements and saturation moment densities measurements.

Qualitative consistent picture within LDA, TB, and k.p

Define Mneff = Mnsub-MnInt


Making semiconductors magnetic new materials properties devices and future

Linear increase of Tc with Mneff = Mnsub-MnInt

High compensation

8% Mn

Tc as grown and annealed samples

Open symbols as grown. Closed symbols annealed

  • Concentration of uncompensated MnGa moments has to reach ~10%. Only 6.2% in the current record Tc=173K sample

  • Charge compensation not so important unless > 40%

  • No indication from theory or experiment that the problem is other than technological - better control of growth-T, stoichiometry


Making semiconductors magnetic new materials properties devices and future

III = I + II  Ga = Li + Zn

GaAs and LiZnAs are twin SC

n and p type doping through Li/Zn stoichiometry

LDA+U says that Mn-doped are also twin DMSs

No solubility limit for group-II Mn

substituting for group-II Zn !!!!

Masek, et al. PRB (2006)


Making semiconductors magnetic new materials properties devices and future

?

Tc limit in (Ga,Mn)As remains open

Indiana & California (‘03): “ .. Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..”

Yu et al. ‘03

Nottingham & Prague (’08): Tc up to 185K

so far

California (‘08): “…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...”

Mack et al. ‘08

“Combinatorial” approach to growth

with fixed growth and annealing T’s


  • Login