SpintronicsSpintronics: : Materials and ApplicationsMaterials and Applications

Kelly M. WhitakerKelly M. WhitakerGamelinGamelin Research GroupResearch GroupUniversity of WashingtonUniversity of WashingtonDepartment of ChemistryDepartment of Chemistry

June 9, 2008June 9, 2008

Spin + Electronics = Spintronics Spin + Electronics = Spintronics

• Present day electronics: Transport of electric charges

• Spintronics (Spin-Electronics): Electronics based on thespin degree of freedom

e-e-e-

e-e-

e-

eUltrafast optical control!

SpintronicsSpintronics: An introduction: An introductionWhat is spinWhat is spin--based electronics?based electronics?

What are the advantages?What are the advantages?

What are the challenges?What are the challenges?

Spin-based electronics (spintronics) uses the spin of the electron in addition to the charge of the electron to carry information.

Increased data processing speedDecreased power consumptionIncreased data storage densities

Optimization of spin lifetimesDetection of spin coherenceTransport of spin-polarized carriers across long enough length scalesManipulation of spins on fast enough time scalesMaterials with high enough curie temperatures (Tc)

“If we can understand and control the spin degree of freedom in semiconductors, the potential for high-performance spin-based electronics will be excellent”

-Wolf et. al. Science 2001, 294, 1488-1495

SpintronicsSpintronics“…the impact of these semiconductors for spintronics applications is restricted by a relatively low Curie temperature.”

- Dietl, Nature Materials, 2003, 10, 646

What is a Quantum Dot?What is a Quantum Dot?Heisenberg’s Uncertainty Principle: Both the position and momentum of

an object cannot be simultaneously known to an arbitrary precision.

Quantum Confinement increases the precision to which the position of an electron can be known, increasing the range of possible momentums that

the electron can take on.

“quantum well”

“quantum wire”

Atkins, P.; de Paula, J., Physical Chemistry. 7th ed.; W.H. Freeman and Company: New York, 2002

Electronic Band StructureElectronic Band Structure

The band structure effects the degree of conductivity a material has

E

Metal Insulator Semi-conductor

Valence band

Conduction band

Band Gap

Quantum confinementQuantum confinement

In quantum dots,electronic structure is governed by size.

Smaller size = Larger band gap

Size‐tunable emission in CdSe QDs

Ferromagnetic DopingFerromagnetic DopingWhat do we mean when we say a semiconductor is What do we mean when we say a semiconductor is ‘‘dopeddoped’’??

Unmagnetized Material Magnetized Material

Diluted Magnetic Semiconductors

Adapted from Ohno, Science 1998, 281, 951

Nonmagnetic Semiconductor

Magnetic Semiconductor

Diluted Magnetic Semiconductor

Open Shelltransition metal ions

Host LatticeZinc Ions

Host Oxygen Ions

What are DMSWhat are DMS--QDs used for?QDs used for?

““SpintronicsSpintronics””: spin: spin‐‐based based electronicselectronics

Phosphor TechnologiesPhosphor Technologies

BioBio‐‐labelinglabeling

Studying fundamental Studying fundamental synthetic and physical synthetic and physical inorganic chemistries inorganic chemistries 

nucleation and growth in the nucleation and growth in the presence of impuritiespresence of impurities

Secret to great hair!Secret to great hair!

Quantum Computing: Theory to PracticeQuantum Computing: Theory to Practice

Electron spins in Quantum Dots is the workhorse for

proposed quantum computers… …but, not much is known

about how these spins behave under quantum confinement.

Cerletti, V.; Coish, W. A.; Gywat, O.; Loss, D., Nanotechnology 2005, 16, R27-R49Petta, J. R.; Johnson, A. C.; Taylor, J. M.; Laird, E. A.; Yacoby, A.; Lukin, M. D.; Marcus, C. M.; Hanson, M. P.; Gossard, A. C., Science 2005, 309, 2180-2184.

SpintronicsSpintronics for Quantum Computingfor Quantum Computing

Engel, Recher, Loss, Sol. State Comm., 2001, 119, 229

Example Example SpintronicSpintronic DevicesDevices

Ohno et. al., Nature, 1999, 402, 790

The downside: This device operates most efficiently at 6 K and stops working all together at 50 K.

Examples of Examples of SpintronicsSpintronics DevicesDevices

Uses magnetic Uses magnetic hysteresis/magnetoresistance to hysteresis/magnetoresistance to write/readwrite/readFunctions like semiconductor RAM Functions like semiconductor RAM ANDAND would retain data with the power would retain data with the power off.off.→→ 1000x faster with no wearout1000x faster with no wearout→→ low cost/powerlow cost/power

Datta, S.; Das, B. Appl. Phys. Lett. 1990, 56, 665.Awschalom, D. D.; Flatté, M. E.; Samarth, N. Sci. Amer. 2002, 286, 66-73.Pearton, Norton, Frazier, Han, Abernathy, Zavada IEE Proc.-Circuits Devices Syst. 2005, 152 (4), 312-322

4 Mb MRAM – Now Available!!

PrinciplePrinciple –– electric field from gate causes electric field from gate causes spins to precess. Channel impedance spins to precess. Channel impedance depends on extent of spin rotationdepends on extent of spin rotation

AdvantageAdvantage –– much less energy and time much less energy and time required to flip spins than to depopulate required to flip spins than to depopulate channelchannel

Small or no effect when metallic contacts Small or no effect when metallic contacts are used because of conductivity are used because of conductivity mismatch mismatch –– NEED MAGNETIC NEED MAGNETIC SEMICONDUCTORS FOR SPIN SEMICONDUCTORS FOR SPIN INJECTION!INJECTION!

Spin Field-effect Transistor

semiconductor channel

antialigned electrons are rejected!

Current Problems with Current Problems with SpintronicSpintronicDevicesDevices

What are the problems with these devices?What are the problems with these devices?Only work at LTOnly work at LT‘‘TheoreticalTheoretical’’Detection/optimization of coherence timeDetection/optimization of coherence timeEfficiency of spin injectionEfficiency of spin injectionCompatibility with current electronicsCompatibility with current electronics

Nobel Prize in Physics 2007Nobel Prize in Physics 2007Giant Giant MagnetoresistanceMagnetoresistance

Albert Albert FertFert and Peter and Peter GrGrüünbergnberg

Electrical resistance from electrons scattering against irregularities in a material so that movement is obstructed.

Magnetic resistance from opposing directions of electron spins.

http://nobelprize.org/nobel_prizes/physics/laureates/2007/index.html

GMR for Data StorageGMR for Data Storage

Magnetization can change the resistance/conductivity of semiconductor materials.

Data can be stored on hard disks in the form of differently magnetized areas.

http://nobelprize.org/nobel_prizes/physics/laureates/2007/index.html

What comes next?What comes next?

How do we make the jump to the next level How do we make the jump to the next level and overcome some of these problems? and overcome some of these problems?

Our approach Our approach –– developing, chemically developing, chemically synthesizing, and characterizing new synthesizing, and characterizing new materials designed for these applications: materials designed for these applications: diluted magnetic semiconductors.diluted magnetic semiconductors.

High High integrationintegration densitiesdensities, , nonvolatilitynonvolatility

Incompatible technologies in present day electronics:Incompatible technologies in present day electronics:

LogicLogic: semiconductors: semiconductors Data storageData storage: : ferromagnetsferromagnets

Ferromagnetic semiconductors ?Ferromagnetic semiconductors ?

Single Chip ComputerSingle Chip Computer

Vacuum Deposition MethodsVacuum Deposition Methods

Molecular Beam Epitaxy (MBE)

Pulsed Laser Deposition (PLD)

Instruments of Scott Chambers, PNNL

Synthesis of TMSynthesis of TM2+2+:ZnO :ZnO QDsQDs2

2 2 4 2 4(1 ) ( ) ( ) 2 : 2x Zn OAc xCo OAc NMe OH Co ZnO H O NMe OAc+− + + → + +

Hiltunen, Leskela, Makela, Niinisto, Acta.Chem.Scand.A 1987, 41, 548 Schwartz, Norberg, Nguyen, Parker, Gamelin, JACS 2003, 125, 13208Basic Zinc Acetate Cluster

Transmission Electron MicroscopyTransmission Electron MicroscopyA focused electron beam is bombarded and transmitted through

the sample and an image of the sample is projected onto the phosphor screen.

Characterization of TMCharacterization of TM2+2+:ZnO:ZnO

What we learn: Particles sizes, shapes, and morphologies.

SpectroscopySpectroscopy

We can get different info based We can get different info based on what kind of light/detectors we use.on what kind of light/detectors we use.

Spectroscopy is the interaction of radiation with matter as a function of energy or frequency

Ground State

Excited State

cE h

hcE

λνν

λ

==

= Energy of radiation must equal the energy level

difference. We are measuring some property of

this energy difference (energy to absorb, emit, etc.)

Electronic Absorption SpectroscopyElectronic Absorption SpectroscopyUV/visible light source, detect energies of light being absorbed.

E

Semi-conductor

Valence band

Conduction band

Band Gap

Electron

Hole

+ = Exciton

http://www.varianinc.com/cgi-bin/nav?/

Characterization of TMCharacterization of TM2+2+:ZnO:ZnO

What we learn: local environments of dopant ions, band gap, etc.

1st Excitonic Transition (Band Gap)

Ligand Field Transitions (signature of dopant ion)

Schwartz, Norberg, Nguyen, Parker, Gamelin, JACS 2003, 125, 13208

Luminescence SpectroscopyLuminescence SpectroscopyUV source, detect energies of light being emittedUV source, detect energies of light being emitted

Ground State

Excited State

Characterization of TMCharacterization of TM2+2+:ZnO:ZnO

Visible ‘green’ trap emission UV Excitonic emission

What we learn: defects, surface effects, etc.van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2000, 104, 1715

ZeemanZeeman EffectEffect

E = hν

H=0 H>0

MS = +

S = ±12

12MS = -

Energy1

2

Magnetic energy levels are degenerate with no applied field. The energy levels split linearly with applied magnetic field.

This results in the splitting of spectral lines.

f

H > 0H = 0

a, b, c

d, e, fd

e

ab

c

7 Tesla 7 Tesla MagnetoMagneto‐‐opticaloptical

CryostatCryostat DetectionDetectionOpticsOptics

Field andField andTemperatureTemperatureControllersControllers

MMagnetic agnetic CCircular ircular DDichroism Spectroscopyichroism Spectroscopy

Source,Source,Monochromator,Monochromator,Polarization OpticsPolarization Optics

MCD Selection Rules:

Ener

gy

H

0

-1+1

Ψg (J = 0)

Ψe (J = 1)

MJ

LCP, σ-RCP, σ+

∆ΑMCD = ΑLCP - ΑRCP

∆MJ = ±1

0hν

∆MJ = +1

RC

P ∆Α

LCP

∆MJ = -1

0hν

Close-up

∆Α

geff μΒH

MMagnetic agnetic CCircular ircular DDichroism ichroism SpectroscopySpectroscopy

Zeeman Zeeman EffectEffect

Characterization of TMCharacterization of TM2+2+:ZnO:ZnO

What we learn: interactions between dopant ions and host lattice

Use UV/Vis source, measure difference between left and right circularly polarized light

Schwartz, Norberg, Nguyen, Parker, Gamelin, JACS 2003, 125, 13208

ElectronElectron Paramagnetic ResonanceParamagnetic Resonance

Selection Rule (transverse mode):

∆MS=±1hν=gβH

H=0 H>0

MS = +

S = ± 12

12

12

MS = -

Energy

The derivative signal is detected by locking onto the oscillating magnetic

field that is superimposed on the static sweeping

field. Absorptive Mode

Derivative Mode

Microwave radiation, detect resonance signal of Microwave radiation, detect resonance signal of unpaired electronunpaired electron

What we learn: environment of electrons, coherence times

ElectronElectron Paramagnetic ResonanceParamagnetic Resonance

Diamagnetic: No unpaired electrons

Paramagnetic: Unpaired electrons

http://www.bruker-biospin.com/

Spin CoherenceSpin CoherenceMagnetization as superposition of parallel and antiMagnetization as superposition of parallel and anti--parallel parallel

states: how long will it stay between the two states?states: how long will it stay between the two states?

The processing of quantum information based on the electron spin degree of freedom requires fast and coherent

manipulation of local spins.http://www.physics.ucsb.edu/~awschalom/http://www.bruker-biospin.com/epr.html

Relatively long spin dephasing times (~1ns) have been Relatively long spin dephasing times (~1ns) have been measured recently on bulk materials and measured recently on bulk materials and epitaxialepitaxial films, but films, but

no work has been done on ZnO quantum dots.no work has been done on ZnO quantum dots.

Spin Dephasing in ZnOSpin Dephasing in ZnO

Ghosh, Sih, Lau, Awschalom, Bae, Wang, Vaidya, and Chapline, Applied Physics Letters, 86, 232507, 2005

Three main mechanisms contribute to spin dephasing times in QDs:

Electron spin - electron spin interactions

Electron spin - phonon interactions

Electron spin - nuclear spin interactions

Photochemical Reduction: Photochemical Reduction: experiments and expected results experiments and expected results

Donor+

Donor

Band gap bleaching

hν

CB

VB

e-

e-

Intraconductionband transition

2

33

3232323

2323

2 HH

HCHOCHCHOHCH

CHOHCHOHCHCHOHCHCHOCHCH

HOCHCHhOHCHCH VB

→

+→

+→+

+→+

•

••

••

+•+

Shim, M.; Guyot-Sionnest, P. J. Am. Chem. Soc. 2001, 123, 11651.

hv

Liu, W.K., Whitaker, K.M., Smith, A.L., Kittilstved, K.R., Robinson, B.H., Gamelin, D.R. Phys. Rev. Lett., 2007, 98, 186804 .

Photochemical Charging Results Photochemical Charging Results in in UndopedUndoped ZnOZnO

Photochemical Charging Results in Photochemical Charging Results in UndopedUndoped ZnOZnO

ZnO = Zn2+ : 3d10

O2- : 2p6

No unpaired electrons in ZnO as-prepared

96.1* ≈g

g* is proportional to field, frequency and energy. It gives information on the local environment of the

electron.Liu, W. K.; Whitaker, K. M.; Smith, A. L.; Kittilstved, K. R.; Robinson, B. H.; Gamelin, D. R. Phys. Rev. Lett.2007, 98, 186804.

Measuring Coherence Times by EPR

g* (frequency) can be transformed into the time domain to get coherence times.

Times of ~25 ns are measured (compared to 0.19 ns before)Liu, W. K.; Whitaker, K. M.; Smith, A. L.; Kittilstved, K. R.; Robinson, B. H.; Gamelin, D. R. Phys. Rev. Lett. 2007, 98, 186804.

SummarySummary

Semiconductor Semiconductor QDsQDs have are great have are great candidates for future candidates for future spintronicspintronic and and quantum computing devices.quantum computing devices.

Long spin coherence times have been Long spin coherence times have been measured by EPR on these materials.measured by EPR on these materials.

Much more work is needed to gain a full Much more work is needed to gain a full understanding of how these materials can understanding of how these materials can be used in be used in spintronicspintronic device application.device application.