Spintronics: A new twist in electronics · 2019-04-09 · Spintronics: dream … • Personal...

Semiconductor Nanostructures

Ultrafast Laser Spectroscopy

Spintronics: A new twist in electronics

Bipul PalIndian Institute of Science Education & Research – Kolkata

02/07/09

1st Platinum Jubilee Meeting of the Indian Academy of Sciences



Electronics all around …

02/07/09



Trend in electronic devices …

Smaller size

Added functionality

Better performance

Less power consumption

Reduced cost

02/07/09



Trend in electronic devices …

Smaller sizeAdded functionalityBetter performanceLess power consumptionReduced cost

02/07/09

Electronic diary, Radio, Music player, Television, Camera, Web surfing, GPS, Money transaction

Just in size of a small paper back novel, weight ~ 1.1 Kg, 1.6 GHz CPU speed, 160 GB Hard disk, Battery life ~ 9.5 hours!



Moor’s law ‐ 1965

No. of transistor that can be cost‐effectively placed on a IC chip willdouble approximately every two years!

02/07/09



Moor’s law ‐ 1965

02/07/09

Feature size reducingNow 65 nm node2010: 45 nm node2013: 32 nm node2016: 22 nm node

1 nm = 10‐6 mmHuman hair 0.1 mm



Limitation to miniaturization …

• Heating

• Leakage current

• Limitation of lithography

• Cost effectiveness

• Laws of quantum mechanics takes over

• Alternatives required!!

02/07/09



Electron has spin!

02/07/09

Tiny magnets!

Spins are randomized in nonmagnetic materialsIts presence was ignored in conventional electronics



Spintronics: spin‐based‐electronics

Utilize “spin‐polarized” electrons to• Store

• Process

• Transmit

information

02/07/09



Spintronics: benefits …

• Spin interaction weaker than Coulomb interaction – Less interference from environment

• Spin current flows almost without dissipation– Less heating

• Spin can flip very fast; requires small energy– Fast operation, Less power consumption

• Non‐volatile memory– Storage and manipulation in single chip

• Manipulation by polarized photons– Easy external control

02/07/09

MoreMiniaturization;Higher speed

New features



Spintronics device already in market

• Nonvolatile magnetic memory (MRAM)

• Magnetic read‐head (uses GMR device)

• Magnetic sensors

02/07/09



Spintronics: dream …

• Personal computers mayBecome small and super fast and yet cheapBoast terabytes of memoryKeep data in active memory even when switched off Boot up instantlyConsume less power

• Futuristic application …– Quantum computation– Quantum information processing

02/07/09



Quantum computation: spin as qubit

Quantum computer

Classical bits (0 or 1) replaced by quantum bits (qubits) that can be in

a superposition of states.

Use spin ½ as a qubit.

02/07/09



Quantum dot based quantum computers

• Coupled-GaAs quantum dots containing one electron per dot

• D. Loss and D.P. DiVincenzo, Phys. Rev. A 57, 120 (1998)

Long spin lifetime is essential for quantum computation!

• Quantum dot defined in 2DEG by side gates• Coulomb blocked used to get one electron per dot• Spin of electron is qubit• Controllable coupling of dots by point‐contact gate voltage• Readout by gatable magnetic barrier

02/07/09



We study spin dynamics in single electron doped InP QDs using

optical spectroscopy

02/07/09



Photon polarization electron spin

Band structure and symmetry of crystal

Selection rule of optical transition

σ+ polarization σ− polarization

02/07/09



Photon polarization electron spin1) Spin dynamics can be studied using polarization

selective optical spectroscopy2) Electron spin may be used for quantum information

processing

02/07/09



Why QDs are interesting?

QDs are called artificial atoms Carriers spatially confined in all 3D within ~10 nm

3D quantum confinement discrete statesMany spin relaxation mechanisms suppressed

Long spin lifetime expected [†]

[†] See, e.g., Khaetskii et al., PRB 61, 12639 (2000); PRB 64, 125316 (2001);Woods et al., PRB 66, 161318 (2002).

02/07/09



Sample structure

Charge state of the QDs is controlled by external electric bias (Ub). For Ub= −0.1V, QDs are singly negatively charged [ ]

Schematic of the sample growth structure.

100 nm

100 nm

300 nm

[ ] Kozin et al., PRB 65, 241312 (20002)

Dot density ~1010 /cm2

02/07/09



How to probe spin dynamics?

Measure the degree of circular polarization (P)of photoluminescence (PL) to probe the spindynamics

Here is the PL intensity for excitation and detection

−+++

−+++

+−

=IIIIP

)(−++I +σ)(−+σ

02/07/09



How to probe spin dynamics?

Degree of PL circular polarization: where is PL

intensity for excitation and detection −+++

−+++

+−

=IIIIP )(−++I

+σ )(−+σ

σ+ polarization σ− polarization

P maximum P zeroP reduced

02/07/09



Time‐resolved measurement of P

02/07/09



Time‐resolved polarized PL

0.0 0.2 0.4 0.6 0.8 1.0-60

-40

-20

0

20

40

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

Pol

ariz

ed P

L (a

rb. u

nits

)

Time (ns)

Ι + −

Ι + +

T = 2 K, Ub = −0.1 VB = 0.1 T, P = 5 mW Ι + ++ Ι + −

Time (ns)

Deg

ree

of P

Lci

rcul

ar p

olar

izat

ion

(%)

P = Ι + +− Ι + −

Degree of PL circular polarization reached a negative value and remained constant up to the PL decay time

ANCP

02/07/09




0.0 0.2 0.4 0.6 0.8 1.0-60

-40

-20

0

20

40

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

Pol

ariz

ed P

L (a

rb. u

nits

)

Time (ns)

Ι + −

Ι + +

T = 2 K, Ub = −0.1 VB = 0.1 T, P = 5 mW Ι + ++ Ι + −

Time (ns)

Deg

ree

of P

Lci

rcul

ar p

olar

izat

ion

(%)

P = Ι + +− Ι + −

Excitation: ~1.77 eV → e‐h pair in excited stateDetection: ~1.73 eV → ground state luminescence

ANCP is maximum under this condition

ANCP

02/07/09



Excitation and detection energy

1.70 1.75 1.80 1.85 1.90 1.950

2

4

6

exci

tatio

n

Energy (eV)

PL s

igna

l (ar

b. u

nits

)

dete

ctio

n

InPQDs

InGaPbarrier

PL spectrumT = 4.2 Kλexc = 532 nm

Excitation: ~1.77 eV→ e‐h pair in excited stateDetection: ~1.73 eV→ ground state luminescence

02/07/09



A simple model of PL polarization

Depending on parallel or antiparallel orientation of photo‐created and resident electron spins, two types of QDs: P‐typeand A‐type QDs

02/07/09




02/07/09




Degree of PL circular polarization is negative (positive) for P‐type (A‐type) QDs

Net PL polarization from the QD ensemble is determined by the ratio of P‐ & A‐type QDs

02/07/09




0.0 0.2 0.4 0.6 0.8 1.0-60

-40

-20

0

20

40

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

Pol

ariz

ed P

L (a

rb. u

nits

)

Time (ns)

Ι + −

Ι + +

T = 2 K, Ub = −0.1 VB = 0.1 T, P = 5 mW Ι + ++ Ι + −

Time (ns)

Deg

ree

of P

Lci

rcul

ar p

olar

izat

ion

(%)

P = Ι + +− Ι + −

Degree of PL circular polarization reached a negative valuePhoto‐excitation converted A‐type QDs to P‐type QDs!!

02/07/09




0.0 0.2 0.4 0.6 0.8 1.0-60

-40

-20

0

20

40

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

Pol

ariz

ed P

L (a

rb. u

nits

)

Time (ns)

Ι + −

Ι + +

T = 2 K, Ub = −0.1 VB = 0.1 T, P = 5 mW Ι + ++ Ι + −

Time (ns)

Deg

ree

of P

Lci

rcul

ar p

olar

izat

ion

(%)

P = Ι + +− Ι + −

PL polarization remained constant up to PL decay time spin memory longer than electron‐hole recombination lifetime !!

02/07/09



A pump‐probe schemeNet PL polarization determined by ratio of P‐ & A‐type QDs.

A pump pulse create optical orientation which is thenmonitored by measuring PL polarization of a delayed probe

When pump and probe are co‐circularly polarized more P‐type QDs at zero pump‐probe delay (τ)When pump and probe are cross‐circularly polarized moreA‐type QDs at τ = 0Difference (PCR – PCO) is a good measure of pump induced spinpolarization

02/07/09



Pump‐probe setup

02/07/09

PMT



Pump‐probe scheme

02/07/09

Detect probe PL by time‐gated measurement

ProbePump



Pump‐probe scheme

02/07/09

Detect probe PL by time‐gated measurement

Probe

Pump



Millisecond‐range spin memory

0.0 0.4 0.8 1.2 1.6 2.0

1

10

100T = 2 K B = 0.1 TUbias= −0.1 V

Delay (ms)

P CR−

P CO (%

) Wpump= 0.5 W/cm2

Wprobe= 0.05 W/cm2

Spin memory decays by 1 order of magnitude in 1 ms

Nonexponential decay distribution of decay rates due to size distribution of QDs, presence of paramagnetic defects…

02/07/09



More spin dynamics study

• “Sub‐millisecond electron spin relaxation in InP quantum dots”, Phys. Rev. B 72, 153302 (2005).

• “Millisecond‐range electron spin memory in singly charged InP quantum dots”, J. Phys. Soc. Jpn. 75, 054702 (2006).

• “Spin dephasing of doped electrons in charge‐tunable InP quantum dots: Hanle‐effect measurements”, Phys. Rev. B 74, 205332 (2006).

• “Nuclear‐spin effects in singly negatively charged InP quantum dots”, Phys. Rev. B 75, 125322 (2007).

• Collaborators• Y. Masumoto and M. Ikezawa, Univeristy of Tsukuba, Japan• I. Ignatiev and S. Yu. Vervin, St. Petersburg State University,

Russia

02/07/09



Summery

• Introduced spintronics = spin based electronics

• Boost to the electronics industry

02/07/09



ElectronCharge

Photon Polarisation

ElectronSpin

Semiconductor Spintronics

Summery

Benefits: Fast, small, low dissipation devices

Quantum computation?

02/07/09



Summery

• Optical study of spin dynamics in QDs

• Long spin lifetime in charged InP QDs

02/07/09

0.0 0.4 0.8 1.2 1.6 2.0

1

10

100T = 2 K B = 0.1 TUbias= −0.1 V

Delay (ms)

P CR−

P CO (%

)

Wpump= 0.5 W/cm2

Wprobe= 0.05 W/cm2


Ultrafast Laser Spectroscopy02/07/09



Model of optical pumping

Optical pumping is conversion of aresident electron spin parallel tothe helicity of incident light

02/07/09



Time synchronization

02/07/09




02/07/09




02/07/09




02/07/09




02/07/09



PL spectrum

1.70 1.75 1.80 1.85 1.90 1.950

2

4

6

Energy (eV)

PL

sign

al (a

rb. u

nits

) InPQDs

InGaPbarrier

PL spectrumT = 4.2 Kλexc = 532 nm

02/07/09



Why charged QDs?

Neutral QDs

Spin lifetime (τs) ≤ recombination lifetime (τr)

eh

Charged QDs

Resident electron:infinite lifetime

Optical pumping

eh

Photo‐generated electron

Resident electron

τs not limited by τr

QDs – artificial atom – discrete energy states

Bulk spin relaxation mechanisms suppressed

Long electron spin relaxation time expected

02/07/09

Date post:	09-Aug-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

Spintronics: A new twist in electronics · 2019-04-09 · Spintronics: dream … • Personal...

Documents