Introduction to Quantum Dots and Single Quantum Dot ... · ROYAL INSTITUTE OF TECHNOLOGY Aim of...

ROYAL INSTITUTEOF TECHNOLOGY

Introduction to Quantum Dots andSingle Quantum Dot Spectroscopy

Jan Linnros

Materials Physics, ICT School, KTHKista

ADOPT Winter School 2012

200 nm



Aim of lecture

Short review of physics of quantum dots

What can be learnt fom single quantum dot spectroscopy?

Our own work on silicon QDs – compared to direct bandgap QD’s

BenNiklas

Ilya Jan VMahtab(Fatemeh)

Acknowledgements:

Funding: Swedish Research Counsil (VR), VR project + Linné centre


Outline

IntroductionQuantum dot – ”artificial atom”The zoo of quantum dots: III-V/ II-VI / Si

TheoryQuantum confinement - Schrödinger equationRecombination, surface passivation

Single-dot spectroscopyTechniques to probe single dotsThe unexpected: Spectral diffusion, blinking High excitationsSTM of single quantum dots

Spectroscopy of single silicon quantum dots (our work)QDs from lithographyLinewidth, blinkingTime resolved and high excitations measurements

EpilogueSummaryApplications


Ene

rgy

Atom Bulk semiconductor

Quantum dot – ‘artificial atom’

λλλ

kT

e-

h+

Quantum dot


Metals - semiconductors

Density of states

Ene

rgy

Fermi level

Density of states

Ene

rgy

Fermi level

Conductionband

Valenceband

Metal Semiconductor

When size is reduced quantized states begin to form at band edges. This means that a semiconductor nanocrystal forms descrete states while a metal particle still has a very high density of states near Fermi level. Thus, metals turn to quantum dots only at very small sizes (< 1 nm)


Quantum confinement

Size - d [nm]B

and

gap

[eV

]Quantum dot

Effectivebandgap


Photon emission/absorption – quantum size effect

2.0

nm

2.4

nm

2.8

nm

3.2

nm

4.1

nm

5.0

nm

Diameter

Bulkbandgap

Emission Absorption

Murray, Norris, Bawendi, MIT


’zoo’ of nanocrystals/QDs


Nanocrystals


• In(Ga)As – grown on GaAs• Lattice mismatch ~7.2 %• Compressive strained layers

- up to a few monolayers• Relaxation => island growth• Size: lateral: 10 – 20 nm; height: 5 – 8 nm

GaAs

InAs

Stranski-Krastanov growth: In(Ga)As QDs

ROYAL INSTITUTEOF TECHNOLOGYKouwenhoven et al., 1991

Quantum dot defined by top gates

Gates 1,2,3,4 are reverse-biased. This forms restrictions inan underlying 2-dimensional electron gas (GaAs layer embedded in AlGaAs). Thus, a quantum dot is formed below electrodes!

Quantum dot(in bured layer)

Top electrodes


Porous silicon

Leigh Canham 1990, APL

Porous Si structureTEM, from Cullis et al.


Outline







Schrödinger eq. for quantum dot


Energy states in a spherical well

Gaponenko ”Optical properties ofsemiconductor nanocrystals, Cambridge


Exciton

Excitation

Luminescence

EC

EV

k

E

Coulombattraction

=>Bindingenergy!

Excitation of electron-hole pair: Exciton

-

+Exciton:


Exciton – weak/strong confinement

-

+

-

+

-

+a aB

Weak confinement Strong confinement

a > aB

with exciton Bohr radius: aB ~ εħ2 / µe2

where µ is the electron-hole reduced mass:µ-1 = me*-1 + mh*-1

a < aB


Exciton energy

,MR2n

RyEE 2

2ml

2

2

*

gnmlχ

+−=h

mlχ

B

2*

a2eRyε

= .**he mmM +=

The energy of an exciton in the weak confinement regime is:

where are roots of the spherical Bessel functions (n – number of the root, l – order of the function)Ry* is the exciton Rydberg energy given by:

, and

Thus, the exciton levels in the quantum dot are characterized by the quantum numbers n, m and l.

Weak confinement: R > aB

Strong confinement: R < aB

The confined electron and hole in such a case have no bound states correspondingto the hydrogen-like exciton. Then, position of energy levels become:

.R2

EE 2nl2

2

gnl χμ

+=h

1*1*1 −−− += he mmμwhere is the electron-hole reduced mass.


Quantum confinement

r=0.7 nm

Exciton Bohr radius: red arrowsGaponenko ”Optical properties ofsemiconductor nanocrystals, Cambridge

Si


Surface states - passivation

”dangling bonds”

”core shell quantum dot”

Quantum dot surface


Recombination mechanisms

Augereeh(hhe)

hν

Radiative

ET

SRH:Defect/impurity

EC

EV

Shockley-Read-Hallrecombinationdominating in Si, Ge etc(ET= trap state energy)

dominates at highcarrier densities(e.g. in lasers)

common indirect-bandgapsemiconductors =>LEDs, lasers

In a perfect nanocrystal at low excitation


”Phonon bottleneck” ???

Bulk – conduction band Quantum dot – descrete states


Outline







Need for single quantum dot studies

Energy

Ensemble ofnanocrystals

Inhomogenous broadening!

Energy

Single nanocrystal

Sharp linewidth


Techniques to probe single QD

Adapted after M. Sugisaki in’Semiconductor quantum dots’, Springer 2002

quantumdots

substrate

microscope SNOM metal mask mesa etching tip-enhanced

fiber

lens

(dot spacing > λ)

AFM metal tip


Empedocles, Norris, BawendiMIT, PRL 77, 3873, -96

Width lessthan kT !

Ensemble average

Single dot

Spectroscopy of single CdSe nanocrystals

Energy (eV)


To see individual quantum dots....

disperse QDs > ~1 um

use high numerical aperture microscope lens

in cryostat – window corrected lens

filter out excitation light and background fluorescence

cooled CCD camera (< -90 C)

mechanically stable system

You need to:


The unexpected.....

On/Off blinking


Single CdSe nanocrystals

Shimizu et al.MIT, PRB 2001

Blinking

on

off

On/Off blinking observed in:

• II-VI nanocrystals• single molecules• in III-V Q-dots - rarely

Time


The unexpected.......

spectral diffusion


Empedocles, Norris, BawendiMIT, PRL 77, 3873, -96E=0

Spectraldiffusion Conclusions:

• Randomly oriented local fieldschanging over time

• Increasing by high excitation

• Charges at the dot surface?

+ -

E

E in kV/cm

Starkshift

CdSe nanocrystals: Spectral diffusion


high excitations


Bayer et al.Nature -00

In0.6Ga0.4As self-assembled dots on GaAs

Single QD

Multi-excitonic emission


STM of single quantum dots

Martin Janson, NANOSPECTRA2004

Wave-Function Mapping of Strain-Induced InAs QDsSTS imaging - |ψ|2(x, y, V)

0.8 0.9 1.0 1.1 1.2 1.3 1.4

01234567

dI/d

V [a

rb. u

nits

]

sample voltage [V]

QD wetting layer

STS 1.14 V

(100)

STS 0.89 V

(000)

Martin Janson, NANOSPECTRA2004

Wave-Function Mapping of Strain-Induced InAs QDsSTS imaging - |ψ|2(x, y, V)

14 nm

STM H=9.4 nm

14 nm1.0 1.2 1.4 1.6 1.8 2.0

0.0

0.2

0.4

0.6

0.8

Vstab=2.4 VIstab=70 pAdI

/dV

[arb

. uni

ts]

sample voltage [V]

ROYAL INSTITUTEOF TECHNOLOGYU. Banin et al. Nature 1999

Size evolutionSTM I-V curve

InAs QD: atomic-like electronic states


Outline







fabrication – our approachto see individual Si QD’s


Single Si quantum dots by lithography

Fabrication1. e-beam lithography2. plasma etching3. oxidation (self limiting)4. PL?5. repeat 3 & 4.....

35 Kpillars0D dots?

walls - 1D lines?planes - 2D quantum wells?

SOI wafer


Pillar array – PL image

White light reflection image(arrays of 30x30 pillars)

PL imagePumping: 325 nm cw (HeCd)

Each single light source can be traced down to a certain pillar


200 nm


Some structures after oxidation

100 nm


Make one or two dots?

Schematic of a wall with varying thickness (repeated continuously along line):

Oxi

datio

n

Double dots

Single dots

Bruhn et al, PSS (2010)


New setup for combined AFM/PL


Single-dot spectra – room temperature


35 K => 3 meV

Width less than kT!

Descrete lines!– Quantum dot!

Sychugov et al. PRL (2005)

Low temperature spectra


Narrow spectra - phonons


k-conservation relaxed

Si

Phononline (TOL)

Zerophononline (ZPL)

Δx • Δp ≥ h

Silicon nanocrystals– still indirect bandgap

CdSenanocrystals

EmpedoclesPRL 1996

Sinanocrystals

SychugovPRL 2005

TO


Time resolved imaging

Delay: 0 µs 5 µs 10 µs 15 µs

100 µs

20 µs

Delay

5 µs

Laser modulation

Image intensifier

on

off

open

shut


Lifetimes

τ ~30 µs


Summary – single dot studies on Si

100 excitons/nanocrystal??

1 exciton/nanocrystal


On/Off blinking


Blinking


Blinking - stability

Data for one Si nanocrystal


Model for blinking

-

+

-

+

QD trapstates

Light state


Model for blinking

-

+

-

+

QD trapstates

-+

-

+

Dark state


Model for blinking

-

+

-

+

QD trapstates

Light state

P(t) = A * t –α

with α = 1.5

For Gaussiantrap distribution:


Blinking at high excitation


Supression of blinking using graded shell

Wang et al.Nature 2009


Outline







Comparison - quantum dots

ZnSe nanocrystals InAs quantum dots Si quantum dots

type of bandgap

lifetime

on/off blinking

quantum efficiency

tuning

direct

ns - range

YES

10 – 60 %

visible

direct

ns - range

NO (rarely)

?

IR

indirect

10 µs - range

YES

10 – 80 % ?

visible - IR


Summary – single dot spectroscopy

excellent for studying the physics of quantum dotshas revealed new phenomenaindividual variation

But…..

rich scenario of individual properties may exist:size, shape, stress, passivation, local environment

=> large number of dots must be analyzed forproper conclusions

Silicon quantum dots:

Similar physics as direct bandgap QD – less intenseHigh quantum efficiencies?Nontoxic


• Labelling of (bio-) molecules(fluoresence tagging)

• Single-photon source:Quantum cryptography

• Luminescent devicesLED, laser

• ”Dots in well” infrared detectors

• Phosphor for new lighting

Time

Ideal singlephoton source

Low thresholdcurrent

Applications of quantum dots


Rhodamine Green Dextran

CdSe quantum dot

Dubertret et al.Nature -02

Bleaching – QD’s vs dyes


nano silicon image gallery


nanosilicon group

seniors

PhD students

M.Sc. students

Jan Linnros(prof)

Ilya Sychugov(ass prof)

Torsten Schmidt(postdoc)

Benjamin Bruhn ’Mahtab’ Sangghaleh

Roodabeh Afrasiabi Yashar Hormozan Karolis Gulbinas

Viktor Tullgren Fatjon Qejvanaj

Miao Zhang(starting March)


Thank you for your attention!


Bibliography

Books:’Optical properties of semiconductor nanocrystals’

S.V. Gaponenko (Cambridge Univ. Press 1998)’Semiconductor quantum dots: Physics, spectroscopy and applications’

Y.Masumoto, T. Takagahara (Eds) (Springer 2002)’Quantum dots’ L.Jacak, P. Hawrylak, A. Wojs (Springer 1998)‘The physics of low-dimensional semiconductors’

John Davies (Cambridge 1998)

Articles:‘Semiconductor clusters, nanocrystals and quantum dots’ A.P. Alivisatos, Science 271, 933 (1996)‘The use of nanocrystals in biological detection’ A.P. Alivisatos, Nature biotech.22, 47 (2004)‘Biologists join the dots’ News feature, Nature 413, 450 (2001)‘Artificial atoms’ M.A. Kastner, Physics Today January 1993, p 24‘Electrons in artificial atoms’ R.C. Ashoori, Nature 379, 413 (1996)‘Identification of atomic-like states in……’ U. Banin et al., Nature 400, 542 (1999)‘The structural and luminescence properties of porous silicon’

Cullis, Canham, Calcott, J. Appl. Phys. 82, 909 (1997)‘Overview of fundamentals and appl. of electrons, excitons and photons in confined structures’

C. Weisbuch, H. Benisty, R. Houdre, J. of Luminescence 85, 271 (2000)‘Excitons in nanoscale systems’ G.D. Scholes, G. Rumbles, Nature Materials 5, 683 (2006).

Home page:Evident technologies (selling QD’s): http://www.evidenttech.com/

http://www.evidenttech.com/

Introduction to Quantum Dots and Single Quantum Dot ... · ROYAL INSTITUTE OF TECHNOLOGY Aim of...

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