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h

1. GaAs
QUANTUM DOT COM
Ray Murray

2. Why Quantum Dots?
•Novel “atom-like” electronic structure
•Immunity to environment
•Epitaxial growth
•Well established device fabrication
•Scalable
Single Photon SourcesPotential as qubits

3. Density of states
bulk QW
QWi QD

4. Molecular Beam Epitaxy
substrate
As
InGa
Growth of Quantum Dots
t< 1.7 ML
GaAs
‘capped’
t> 1.7 ML
Scanning TEM image

5. Optical Properties
Relaxation Escape
E
0
E
1
E
2
Wavelength (Å)
100001050011000115001200012500 13000
I
n
t
e
n
s
i
t
y
(
a
r
b
. u
n
i
t
s
)
E
0
E
1
E
2
Time (ps)
0200040006000
Time (ps)
0200040006000
1 Wcm
-2
Time (ps)
0200040006000
P
L

I
n
t
e
n
s
i
t
y

(
a
r
b
.

u
n
i
t
s
)
350 Wcm
-2
12 Wcm
-2
Time (ps)
0200040006000
Time (ps)
0200040006000
1 Wcm
-2
Time (ps)
0200040006000
P
L

I
n
t
e
n
s
i
t
y

(
a
r
b
.

u
n
i
t
s
)
350 Wcm
-2
12 Wcm
-2
0 2 4 6
P
L

i
n
t
e
n
s
i
t
y
time (ns)

6. Single photon sources
Santori et al. Phys Rev Lett 86, 1502 (2001)

7. 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
mesa
aperture
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

8. Controlling dot density
•InAs/GaAs QD growth
under typical conditions
yields QD densities of
~2-5 x 10
10
cm
-2
•For single photon
devices need QD
density of ~10
8
cm
-2
•Reduction in InAs
deposition rate leads to
reduction in QD density
Alloing et al. Appl Phys Lett 86, 101908 (2005)

9. 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

10. x
V(x,y)
-a a
S
1 S
2
B(z)
E(x)
y
a
B
QD
Electron spin S as “qubit”
Why Spin?
•QM property – interaction only with
QM forces
•No interaction with electrostatic forces
•Easy to create, manipulate and detect
spins in semiconductors
Burkard, Loss and DiVincenzo Phys.Rev.B 1999

11. Spin states in III-V semiconductors
p
s
p-antibonding
s-antibonding
s-bonding
p-bonding
CB
VB
E
g
Energy
k
hh
lh
so
Energy
k
J=3/2
J=1/2
so
-3/2 +3/2
hh hh
-1/2 +1/2
σ+ σ-
-1/2 +1/2
lh lh

12. H
N
: no spin conservation
-Spin is irrelevant to the dynamics
-Spin need not be conserved during relaxation
H
S: spin is always conserved
-Spin lifetimes are long compared to radiative
lifetimes
-Spin is conserved during relaxation
X1
GS
X1
GS
X1
GS
H
N H
S
)(
11
q
)(
20q
)(
21q
Spin conservation in QDs- Pauli blocking
T=10 K
Integrated PL Intensity (a. u.)
R
a
t
io

I
(
X
1
)

/

I
(
G
S
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
T=10 K
Energy (eV)
0.951.001.051.101.15
Le Ru et al. Phys.Stat.Sol. (2003)

13. Probing spin states with light

rad
~500ps

rel
<100ps
Ts~900ps
Spin lifetime reduced by acoustic phonon scattering
Gotoh et al. J.J.Appl.Phys. 42 (2003)

14. Spin-LED structure
InAs QDs

Fe
Emission
Fe
n-AlGaAs
InAs/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)

15. 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

16. The oblique Hanle effect
45° B field
S
S
z
•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
S
0x

17. Experiment
1/4
monochromator
lin pol
•Spin injection from Fe into semiconductorSpin lifetime of the ground state exciton

18. •Spin polarisation in the dots ~ 7.5%
•From Hanle half-width B
1/2 obtain
using g* =-1.7, obtain spin lifetime of ~300 ps
S
x
T
S

)%.7.05.7(
0

2/1
*
Bg
T
B
S



• Spin injection from the Fe to AlGaAs of 20 ± 3%

19. Spin relaxation mechanisms
1.D’yakonov-Perel – k
3
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

20. 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

21. 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

22. 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 Dorpe
IMEC
Martin Ward and Andrew Shields
Toshiba Research Europe