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