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GaAs
QUANTUM DOT COM
Ray Murray
Why Quantum Dots?
•Novel “atom-like” electronic structure
•Immunity to environment
•Epitaxial growth
•Well established device fabrication
•Scalable
Single Photon Sources Potential as qubits
Density of states
bulk QW
QWi QD
Molecular Beam Epitaxy
substrate
AsInGa
Growth of Quantum Dots
t< 1.7 MLGaAs
‘capped’
t> 1.7 ML
Scanning TEM image
Optical Properties
Relaxation Escape
E0
E1
E2
Wavelength (Å)
10000 10500 11000 11500 12000 12500 13000
Inte
nsi
ty (
arb
. un
its)
E0E1
E2
Time (ps)
0 2000 4000 6000
Time (ps)
0 2000 4000 6000
1 Wcm-2
Time (ps)
0 2000 4000 6000
PL
In
ten
sity
(ar
b. u
nit
s)
350 Wcm-2
12 Wcm-2
Time (ps)
0 2000 4000 6000
Time (ps)
0 2000 4000 6000
1 Wcm-2
Time (ps)
0 2000 4000 6000
PL
In
ten
sity
(ar
b. u
nit
s)
350 Wcm-2
12 Wcm-2
0 2 4 6
PL
inte
nsi
ty
time (ns)
Single photon sources
Santori et al. Phys Rev Lett 86, 1502 (2001)
image of quantum dot layer in an Atomic Force Microscope
n-contact
p-contact
electron injector
quantum dot layer
substrate/buffer
hole injector
insulator
single photon emission
mesaaperture
n-contact
Conventional p-i-n diode containing layer of quantum dots
Science 295, 102 (2002)
1 m
quantum dots
15 x 5 nm
Single Photon Emitting Diode
1. Electrically driven (easy to use)
2. Fab. similar to LED(cheap)
Toshiba Research
p-contact
Controlling dot density
• InAs/GaAs QD growth under typical conditions yields QD densities of ~2-5 x 1010 cm-2
• For single photon devices need QD density of ~108 cm-2
• Reduction in InAs deposition rate leads to reduction in QD density
Alloing et al. Appl Phys Lett 86, 101908 (2005)
PL from etched mesas
4.2 K PL from a 2-µm diameter etched pillar incorporating a low density QD layer emission from single QDs can be resolved
X
300 K Reflectivity from planar cavity
x
V(x,y)
-a a
S1 S
2
B(z)
E(x)
y
aB
QD
Electron spin S as “qubit”
Why Spin?
•QM property – interaction only withQM forces•No interaction with electrostatic forces•Easy to create, manipulate and detectspins in semiconductors
Burkard, Loss and DiVincenzo Phys.Rev.B 1999
Spin states in III-V semiconductors
p
s
p-antibonding
s-antibonding
s-bonding
p-bonding
CB
VB
Eg
Energy
k
hh
lh
so
Energy
k
J=3/2
J=1/2
so
-3/2 +3/2hh hh
-1/2 +1/2
σ+ σ-
-1/2 +1/2
lh lh
HN: no spin conservation
- Spin is irrelevant to the dynamics
- Spin need not be conserved during relaxation
HS: spin is always conserved
- Spin lifetimes are long compared to radiative lifetimes
- Spin is conserved during relaxation
X1
GS
X1
GS
X1
GS
HN HS
)( 11q
)( 20q
)( 21q
Spin conservation in QDs- Pauli blocking
T=10 K
Integrated PL Intensity (a. u.)
Ra
tio I(
X1
) / I
(GS
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
T=10 K
Energy (eV)0.95 1.00 1.05 1.10 1.15
Le Ru et al. Phys.Stat.Sol. (2003)
Probing spin states with light
rad~500ps
rel<100psTs~900ps
Spin lifetime reduced by acoustic phonon scattering
Gotoh et al. J.J.Appl.Phys. 42 (2003)
Spin-LED structure
InAs QDs
Fe
Emission
Fe
n-AlGaAsInAs/GaAs
QDs
p-AlGaAs
Inject electrons through Schottky diode into n-i-p LED (injected polarisation from Fe ~ 45%)
Ballistic transport: AlGaAs barriers
Itskos et al. Appl.Phys.Lett. 88 (2006)
Rotating the spins
Faraday Geometry
B=0
Oblique Hanle Geometry
B>1.4T B<<1T
Faraday geometry rotates spins in the metal
Oblique Hanle geometry rotates spins in the semiconductor
Magnetisation axis
Injected spin
The oblique Hanle effect
45° B field
SSz
•Initially, no overall component of the spin in the direction of the emission
•Apply oblique magnetic field: spin precesses about the field
•Introduces a component of the spin in z-direction
•Leading to circularly polarised emission
S0x
Experiment1/4
monochromator
lin pol
•Spin injection from Fe into semiconductorSpin lifetime of the ground state exciton
• Spin polarisation in the dots ~ 7.5%
•From Hanle half-width B1/2 obtain
using g* =-1.7, obtain spin lifetime of ~300 ps
Sx T
S
)%.7.05.7(0
2/1* Bg
TB
S
• Spin injection from the Fe to AlGaAs of 20 ± 3%
Spin relaxation mechanisms
1. D’yakonov-Perel – k3 term splits the conduction band
2. Elliott-Yafet – band mixing through k.p interaction
3. Exchange interaction connecting electrons/holes of opposite spin
4. Hyperfine interaction with nucleii
Investigating spin decoherence
• A similar device emits at lower currents
• Oscillations with magnetic field
• Cascade process• Further work
needed: PL data
D’yakonov and Perel, in Optical Orientation
Further work
• Faraday geometry measurements• Current dependence• Temperature dependence• Optical injection: oblique Hanle effect• P-doped quantum dots
Single Photon Sources•Lower dot density•Investigate regular arrays of QDs•Target 10% efficient fibre compatible sources
Spin LED
Acknowledgements
Steve Clowes and Lesley Cohen
Grigorios Itskos, Edmund Clarke, Patrick Howe,
Edmund Harbord, Peter Spencer, Richard Hubbard and Matthew Lumb
Paul Stavrinou
Wim Van Roy and Peter Van DorpeIMEC
Martin Ward and Andrew ShieldsToshiba Research Europe