Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates

Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates

Eli KaponLaboratory of Physics of Nanostructures

Swiss Federal Institute of Technology Lausanne (EPFL)

Introduction Self-ordering on nonplanar substartes Neutral and charged low-D excitons Contacting single QWRs and QDs Summary and outlook

ADMOL, Dresden, Germany, February 23-27, 2004

Quantum Confinement:Compound semiconductor heterostructures

Electron envelope functions :

Schrödinger equation with heterostructure potential :

Ψn,kr( )=un,kr( )φn,kr( )

− h22m*∇2+Vhetr( )

⎡

⎣

⎢ ⎢

⎤

⎦

⎥ ⎥ φn,kr( )=Eφn,kr( )

φr( )

Vhetr( )

AlGaAs GaAs AlGaAs

Quantum Well Heterostructure

AlGaAs

Confinedenvelopefunctions

AlGaAs GaAs

Potential well

Quantum Well Potential

Low-Dimensional Semiconductors:Quantum wells, wires and dots

Den

sit

y o

f st

ates

Quantum Well

Quantum DotQuantum Wire

Spontaneous Formation of Quantum Nanostructures:Self-formed quantum dots

400nmX400nm STM scan of MBE-grown GaAs (100) surface

R. Grousson et al., Phys. Rev. B 55, 5253 (1997)

« Natural » QDs

Zhuang et al., J. Crystal Growth 201/202, 1161 (1999)

TEM cross section of vertically-stackedSK-grown quantum dots

Stranski-Krastanow QDs

Surface fluxes of adatoms are not controlled: random nucleation and broad size distribution

€

μ i =μ0+ Δμstrain + Δμcapillarity + Δμmixing

=μ0+Ω 0

2Eστ (x)[ ]

2 +Ω0 γ(θ)+γ"(θ)[ ]κ(x)+kT lnXi (x)

Chemical potential:

€

ji (x) =−niDi

kT∂μi

∂xSurface flux:

Lateral Patterning during Epitaxial Growth:Controlling lateral fluxes with the surface chemical potential

Strain Capilarity Entropy of mixing

G. Biasiol and E. Kapon, Phys. Rev. Lett. 81, 2962 (1998); G. Biasiol et al., Phys. Rev. B 65, 205306 (2002)

V-Groove Quantum Wires:Size and shape control by growth adjustments

Surface Chemical Potentialrbrslbslµinitial, etched

profilefaceted profile

s

G. Biasiol et al., PRL 81, 2962 (1998);Phys. Rev. B 65, 205306 (2002)

Size and Shape Control

Nano-template width adjusted by surface diffusion length Wires/dots produced by switching surface diffusion length

lbsl =

2Ω0rsγLs2

ΔrkBT

⎛

⎝ ⎜ ⎞

⎠ ⎟

1/3

Self-limiting facet width

Excitons in Quantum Wires:Signatures of a 1D system

PLE, Excit. pol. ||

PLE, Excit. pol. ⊥

1.56 1.60 1.64

e1-h

1e2-h

2e3-h

3e4-h

4

e1-h

6+ "2 "s

( . )PLE optical spectra arb units

( )Photon energy eV

e2-h

6

Experiment: PL-excitation spectra

Excitonic transitions dominate (reduced Sommerfeld factor in 1D) Polarization anisotropy due to valence band mixing Enhanced exciton binding energy (14.5 meV) deduced

1.56 1.60 1.64

A1 exc.

B1 exc. (pol. ||)

A2 exc. (pol. ⊥)

( . )X optical absorption arb units

( )Photon energy eV

Theory: excitonic absorption

M.-A. Dupertuis et al., to be published

Contacting a Single Quantum Wire:1D Electron Gas in V-Groove QWRs

EtchedAreas

1 µm

Currentflow

QWR

wire

---- - -

--

--

---

--

+

++

+

++

+ +

+

+

+

+

++

+

++

++

+

-+

+

++

-- +

--QWs

QWR

D. Kaufmann et al., Phys. Rev. B 59, R10433.(1999)

Moduation-doped V-groove QWR structure Wire contacted via 2D electron gas on sidewalls Conductance quantized close to 2e2/h Discrepancy due to quantum contact resistance

012345678

-3,2-3 -2,8-2,6-2,4-2,2-2 -1,8

QWR Conductance0.3 um0.45 um1.0 um1.5 um1.9 um

Gate Voltage [V]

Groove axis (nm)

-6

-4

-2

0

2

4

0 200 400 600 800 1000 1200 1400

Hei

ght p

rofi

le (

nm)

Sidewalls

Bottom (100) facetMLs steps

•Long range (~1µm) variations induced by lithography imperfection

•Short range (~100nm) variations induced by monolayer steps

Structural Disorder Along a V-Groove QWR:Monolayer steps at the central (100) wire facet

2000nm0

Sidewalls

(100) bottom with ML steps

(311) facets(100) top

12nm-thickGaAs cap

layer

Charged Excitons in V-Groove QWR:Binding energies and localization

• Micro-PL spectra through sub-m apertures• Modulation doped QWRs for charging control• Sharp lines represent localized excitons

Localization Effects

Self-Ordering of Pyramidal Quantum Dots:OMCVD growth on pyramidal patterns

1µm

(111)B

{111}A

GaAs substrate

{111}A

(111)B

(111B) substrates patterning

GaAs-support

Substrate removal

pump PL

1 m

AlGaAs

GaAsQD

Self-limited OMCVD growth

QDs self-formed at a dip in the surface chemical potential

>99% of QDs emit light Highly uniform dot arrays

Ground state CL image (7 meV window)

1 m

950 QDs7 meV

CL

Inte

nsity

(ar

b. u

nits

)

Photon Energy (eV)1.5 1.6 1.7 1.8

T = 7K CL spectrum

Dense Site-Controlled Pyramidal QD Arrays:Cathodoluminescene spectroscopy

Single Quantum Dot Spectroscopy:Origin of optical transitions

Back-Etched PyramidsMicro-PL of

Single Pyramids

Monochromatic CL Imaging

QD1.60 eVQWR1.70eV

QW1.94eV

10 K, 1W on single pyramid

QD~ 6 nm

QWR~ 3-4 nm

QW~ 1-1.5 nm

VQW

A. Hartmann et al., J. Phys.: Condens. Matter 11 5901 (1999)

Multi-Particle States in Quantum Dots:Excitonic states and charging mechanism

l = -1 0 +1

s

p

s

p

l = -1 0 +1

Energy

Em

iss

ion

X X- X- - 2X

2D harmonic oscillator model

QD

AlGaAs

n ~ 1017 cm-3 background doping

Chrage control by photoexcitation

Quantum Dots in an N-type Environment:Charged excitonic complexes

Sin

gle

exc

ito

n r

eg

ime

Mu

lti

exc

ito

n r

egim

e

X

2X

3e-2h

2e-h

3e-h

4e-h

5e-h

6e-h6e-h

5e-h

4e-h

3e-h

Theory

Full CI model

X

4e-h

5e-h5e-h

3e-h

4e-h

6e-h

6e-h

3e-h

2e-h

2X

3X

4X

laser = 2.42 eV

Experiment

30 pW

2.5 nW

600 nW

A. Hartmann et al., PRL 84, 5648 (2000)

TiSaLaser

DiodeLaser

c

unte

r

Laser

LaserPulse.Analyz.

i

l

QDsample

monochromator A

monochromator B

timedelay

ph

oto

n c

ou

nte

r

Single QDs are readily observed and probed Photon antibunching observed at X line

M. Baier et al., Appl. Phys. Lett. 84, 648-650 (2004)

Pyramidal QDs as Single-Photon Emitters:Hanbury Brown and Twiss correlation measurements

Controlled Photon Emission from 0D Excitons:Exciton dynamics probed by photon correlations

QD PL spectra

X-X correl.

X--X- X--X

2X-X 2X-X-

Carrier Transport into Quantum Wires:Preferential Injection via connected quantum wells

Vext0.5 µmn+ GaAsn+ AlGaAsp+ AlGaAs

Low-energy QWs form next to wires Carriers injected via QWs into quantum wires

600 650 700 750 800

Wavelength (nm)

AlGaAs VQW

QWR

(100) QW

(111)A QW

13 2

4EL:

PL:FB (0 mA)

051015202530

0 0.51 1.52 2.5Voltage (V)

T = 10 K•

•••

H. Weman et al., Appl. Phys. Lett. 73, 2959 (1998);79, 1402 (2001)

Electronic States in Pyramidal QDs:Finite element k.p modeling

ground stateground state

first excifirst excited stateted state

t

tqw

h

w

quantum dotquantum dot

lateral quantum wells

Z

Y

[112]

[111]

[110]

X

F. Michelini et al.

Electronic States in Pyramidal QDs:Impact of vertical quantum wire

ground state

second excited state

Without Wire With Wire80

40

0

840 ( )Dot height nm

with VQWR without VQWR

0.3

0.2

0.1

0.0108642

( )Dot height nm

first and second VQWR subbands

e3

e1

e2

without VQWR

with VQWR

F. Michelini et al.

Single Quantum Dot Light Emitting Diode:Preferential carrier injection into a single dot

quantumdot

Vertical Quantum wire

+

-

QWRsV

QW

R

Ga

As VQW QWs

QDPLEL

VQWR

Quantum dot light emitting diode structure Emission from vertical QWR and QD only (at low current)

QD

VQWR

M. Baier et al., APL, 2004 (in print)

QDs Embedded in Photonic Crystals:Energy tuning of ground and excited state transitions

QD in Hexagonal PhC « Defect »

S. Watanabe et al.

Wavelength-Dispersive CL images

QD positioned in a photonic crystal microcavity Emission energy tuned by epitaxial growth effect

Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates

Summary:

-Self-ordering during epitaxial growth on non-planar substrates is useful for producing high quality QWRs and QDs

-New excitonic states are made stable by lateral quantum confinement in QWRs and QDs

-Low-dimensional quantum nanostructures should be useful in novel optoelectronic devices such as single photon emitters and optically active photonic crystals

Collaborators:

Crystal growth:A. Rudra, E. Pelucchi

Nanofabrication and nanocharacterization:B. Dwir , K. Leifer, S. Watanabe, C. Constantin

Optical spectroscopy:D. Oberli, H. Weman, A. Malko, T. Otterburg, M. Baier

Theory:M.-A. Dupertuis, F. Michelini

Ordered Quantum Wire and Quantum Dot Heterostructures Grown on Patterned Substrates