spintronics: a new twist in electronics · 2019-04-09 · spintronics: dream … • personal...
TRANSCRIPT
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Electronics all around …
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Trend in electronic devices …
Smaller size
Added functionality
Better performance
Less power consumption
Reduced cost
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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!
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Limitation to miniaturization …
• Heating
• Leakage current
• Limitation of lithography
• Cost effectiveness
• Laws of quantum mechanics takes over
• Alternatives required!!
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Electron has spin!
02/07/09
Tiny magnets!
Spins are randomized in nonmagnetic materialsIts presence was ignored in conventional electronics
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Spintronics: spin‐based‐electronics
Utilize “spin‐polarized” electrons to• Store
• Process
• Transmit
information
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
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MoreMiniaturization;Higher speed
New features
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Spintronics device already in market
• Nonvolatile magnetic memory (MRAM)
• Magnetic read‐head (uses GMR device)
• Magnetic sensors
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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.
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Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
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Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
We study spin dynamics in single electron doped InP QDs using
optical spectroscopy
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Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Photon polarization electron spin
Band structure and symmetry of crystal
Selection rule of optical transition
σ+ polarization σ− polarization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Photon polarization electron spin1) Spin dynamics can be studied using polarization
selective optical spectroscopy2) Electron spin may be used for quantum information
processing
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Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Time‐resolved measurement of P
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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 + −
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
A simple model of PL polarization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
A simple model of PL polarization
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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 valuePhoto‐excitation converted A‐type QDs to P‐type QDs!!
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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 + −
PL polarization remained constant up to PL decay time spin memory longer than electron‐hole recombination lifetime !!
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Pump‐probe setup
02/07/09
PMT
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Pump‐probe scheme
02/07/09
Detect probe PL by time‐gated measurement
ProbePump
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Pump‐probe scheme
02/07/09
Detect probe PL by time‐gated measurement
Probe
Pump
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Summery
• Introduced spintronics = spin based electronics
• Boost to the electronics industry
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
ElectronCharge
Photon Polarisation
ElectronSpin
Semiconductor Spintronics
Summery
Benefits: Fast, small, low dissipation devices
Quantum computation?
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Model of optical pumping
Optical pumping is conversion of aresident electron spin parallel tothe helicity of incident light
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Time synchronization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Time synchronization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Time synchronization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Time synchronization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
Time synchronization
02/07/09
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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
Semiconductor Nanostructures
Ultrafast Laser Spectroscopy
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