Photonische Materialien5. Vorlesung

• Einführung in quantenmechanische Aspekte und experimentelle Verfahren (1)

•Lumineszenz-Label (1)•Supramolekulare und biologische Systeme (1)•Halbleiter Nanopartikel (2)•Quantum-Well-Strukturen (1) •Metallische Nanopartikel (2)•Solarzellen (1)•Organische Leuchtdioden (1)•Flüssige Kristalle (2)•Photonische Kristalle (2).

Halbleiter Nanopartikel• Warum Nanopartikel ?

– Volumen – Oberflächenverhältnis– Qualitative Änderung von Eigenschaften

• Beispiele• Vom Atom zum Festkörper

– „quantum size“ Effekte• Optische Eigenschaften

– Exzitonen– Transport

• Kolloidale Halbleiter– Oberflächen– Photolumineszenz-Eigenschaften

• Silizium Nanopartikel– Poröses Silizium– Nanokristalle

• Epitaktische Halbleiter

Warum Nanopartikel?• Nano entspricht

– 10 m = 1 nm– Atomdurchmesser Silizium (Si): 0,12 nm– Bindungslänge Si-Si: 0,233 nm– Grobe Abschätzung Würfel

• Durchmesser d =10 nm: ca 80.000 Si Atome• Durchmesser d= 1nm: ca 100 Si Atome, alle an

der Oberfläche– Atomgewicht: 28,09 g/mol

• 1 mg entspricht ca 2 10 Si Atomen– Kristallstruktur beachten

-9

19

Wied

erho

lung

Warum Nanopartikel?

• Qualitative Änderung von Eigenschaften als Funktion der Größe, z.B. – Leitfähigkeit– „Farbe“– Thermische Eigenschaften

• Oberflächen : Volumenverhältnis– (reaktive) Grenzflächen– Interface

Oberflächen

In the basic unit of a crystalline silicon solid, a silicon atomshares each of its four valence electrons with each of four neighboring atoms

Passivierung: H, O

reaktiv !!

Tunneln

Eigenschaften

• Optische Eigenschaften– Absorption

• Metalle• Halbleiter

– Emission• Halbleiter

– Streuung• Metalle

Semiconductor Quantum - Dots

CdSe Silicon

Linnros

Meier

Discrete States• Quantum confinement → discrete states• Energy levels from solutions to Schroedinger

Equation• Schroedinger equation:

• For 1D infinite potential well

• If confinement in only 1D (x), in the other 2 directions → energy continuum

Ψ=Ψ+Ψ∇− ErVm

)(2

22h

integer n ,)sin(~)( =Ψ Lxnx π

mp

mp

mLhn zy

228

22

2

22Energy Total ++=

x=0 x=L

V

Quantum Confinement

Photolumineszenz

Elektron-Loch Paare

Optische Anregung

Metall: Freie ElektronenHalbleiter: gebundene e-h Paare Exzitonen)

Photolumineszenz

ExzitonenWannier-

Exzitonen - Eigenschaften

Silizium:

Effektive Massen

1.44 me0.17meZnSe

1.21 me0.19 meZnO

II-VI

0.6 me0.013 meInSb

0.45 me0.067 meGaAs

III-V

0.37 me0.55 meGe

0.56 me1.08 meSi (4.2K)

Group IV

Hole effective massElectron effective massMaterial

h+

e-

aB

h+

e-

aB

h+

e-aB

Excitons:QuantumExcitons:Quantum ConfinementConfinement

Elektron – Loch Paar

Wannier Exciton: „H_Atom“: Halbleiter

Frenkel Exciton: „Lokalisierung“: Moleküle

Eg Eg Eg

)h(rhV)e(reVhrer2ε

2e2h

h2m

22e

e2m

2H ++

−−∇−∇−=

hh

*

2

2

2

22

10 248.0786.1112 Ryd

he

ERe

mmRE −−⎥

⎦

⎤⎢⎣

⎡+=

επh

HalbleiterHalbleiter Quantum Dots (QDQuantum Dots (QD’’s)s) ::QuantenpunkteQuantenpunkte

TOPO

ZnS Shell

CdSe Core

Kolloidale Quantum Dots - Oberfläche

Evident Technologies, Inc.

Attach to the surface

Some TOPO desorbP

CH3 CH

3

CH3

O

Poröses Silizium

Stranski-Krastanow Wachstum z.B. InGaAs on GaAs SiGe on Si

Oliver G. Schmidt

Uncapped indium arsenide (InAs) self-assembled quantum Dots grown on gallium arsenide (GaAs).

images.pennnet.com

Epitaxy: Self-Organized Growth• Self-organized QDs through epitaxial growth • Stranski-Krastanov growth mode (use MBE, MOCVD)

• Islands formed on wetting layer due to lattice mismatch (size ~10s nm)

• Disadvantage: size and shape fluctuations, ordering• Control island initiation • Induce local strain, grow on dislocation, vary growth

conditions, combine with patterning

AFM images of islands epitaxiall grown on GaAs substrate.

(a) InAs islands randomly nucleate.

(b) Random distribution of InxGa1-xAs ring-shaped islands.

(c) A 2D lattice of InAsislands on a GaAssubstrate. P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.

Epitaxy: Patterned Growth

• Growth on patterned substrates – Grow QDs in pyramid-

shaped recesses– Recesses formed by

selective ion etching– Disadvantage: density

of QDs limited by mask pattern

T. Fukui et al. GaAs tetrahedral quantum dot structures fabricated using selective area metal organic chemical vapor deposition. Appl. Phys. Lett. May, 1991

Optische Anregung in Halbleitern

• Exciton: bound electron-hole pair (EHP)• Excite semiconductor → creation of EHP

– There is an attractive potential between electron and hole– mh

* > me * ⇒ hydrogenic system

– Binding energy determined from Bohr Theory

• In QDs, excitons generated inside the dot• The excitons confined to the dot

– Degree of confinement determined by dot size– Discrete energies

• Exciton absorption ⇒ δ function-like peaks in absorption

mass reduced ;4

;2 22

2

020

2

==−= μμπ

εε e

hana

eEn

Emission as a function of time (25 ns)

Einzelne Halbleiter Quantendots

Imaging

Lifetime (200 ps)

Spectra

Silizium Quantum - Dots

Linnrosc. Meier

Si Quantendots

Christian von Borczyskowski

Extended Red Emission (ERE) of Interstellar Dust

• ERE from Silicon Nanocrystals ?

• Photophysics of Silicon Nanocrystals

Dots of size selected Si - Nanocrystals

Bleaching by Laserirradiation

2 um

after

before

Si-NC Bleaching and Spectral Shifts

Size selected

Non-size selected

Wavelength Bleaching and shift) !!!

Optical Properties of Silicon QD2 R

• Indirect optical band transition of silicon

• Increasing energy gap with decreasing size

• Change of transition probability with size

• Remaining size distribution

2 3 4 51

4

3

2

1

particle seize R [nm]

ener

gy g

ap E

g[e

V]

Eg = EgBulk +

h2π 2

2R21

me* +

1mh

*

⎛

⎝ ⎜

⎞

⎠ ⎟ −

1.8e2

εR

E(k)

k

Übergangänge zwischen Bändern

Calculation of Localisation

e

h

Wolkin et al. PRL 1999

Übergangänge zwischen Bändern

erlaubtverboten

1000 nm485 nm

electronic band structure of silicon(with quantum confinement)

Dark excitons cannot couple to electric field→ Optically forbidden!

Temperaturabhängigkeit

Temperaturabhängigkeit

Si Photolumineszenz

Oszillatorenstärke (Absorption)

Si- Photolumineszenzlebensdauer

Single NC sSingle QD

Nanoparticlesmatrix

core

shell/interface

Examples:- Colloidal II/VI semiconductors (CdSe/ZnS)

- Silicon: Interstellar dust, optoelectronic material

Einzelne Kristalle

Si-NC Bleaching and Spectral Shifts

Single Si - NC

Si – NC Ensemble

Wavelength500 550 600 650 700 750 800

3000

4000

5000

6000 Single SiNC/gas

Flu

ores

cenc

e (a

. u.)

Wavelength (nm)

Line narrowing of Photoluminescence

Single Si-Particle Spectra

SiO2

1,6 1,8 2,0 2,2 2,4

0

20

40

60

80

100

Obs

erva

tion

Tim

e (s

)

50

40

30

20

10

0Lum

ines

cenc

e (a

. u.)

Photon Energy (eV)

Series of Lum inescence SpectraSeries of Lum inescence Spectra

fluctuations in intensity and in peak positionline shape of spectra determ ined by dynam ics of environm ent

in time

Single Si-NC

Herstellungsmethoden

• Gasphasensynthese• Elektrochemisches Ätzen • Implantation• Lithographie

Beispiel: PL ZerfallPoröses SiliziumMikrowelleninduzierte Silizium Nanopartikel

L.Pavesi, Phys. Rev. B 48, 17625 (1993)

Beta: dispersion factor → disorder

Cedrik Meier Experimental Physics

Gas-Phasen Synthese

J. Knipping, H. Wiggers, B. Rellinghaus, P. Roth, D. Konjhodzic, and C. Meier, J. Nanosci. Nanotech. 4, 1039 (2004).V. G. Kravets, C. Meier, D. Konjhodzic, A. Lorke, and H. Wiggers, J. Appl. Phys. 97, 084306 (2005).

H. Nienhaus, V. Kravets, S. Koutouzov, C. Meier, A. Lorke, H. Wiggers, M. Kennedy, and E. Kruis, J. Vac. Sci. Techn. B 24, 1156 (2006).

Low-pressure plasma process.

Silane (SiH4) as precursor gas.

Power/pressure control particle size.

TEM micrograph

Mostly spherical particles

Single crystalline.

Particles covered by native oxide shell.

Si - Kristallstruktur

Gasphasen Pyrolyse

2 – 4 nm

Elektrochemisches ÄtzenPorous silicon - LT Canham, Appl. Phys. Lett. 57 , 1046 (1990)

• Etching of porous structures• Structure contains nanocrystallites• Distribution of crystallite sizes 70 µm

+ - Confocal Image of Porous Siliconλem>530 nm

HF/H2O2

Poröses Silizium: TEM

Scanning electron micrograph of a n-type porous silicon surface

Spincoating

Si Nc filter

Toluene Solution

3-4 hours

Si Nc in Toluene

+

Toluene

PMMA powder

=

Si Nc in Tolueneand PMMA

Spin coating Clean Si-Sio2 substrate

“Porous” Single NanoparticlesConfocal image of single

silicon nanoparticles

70 µm 10 µm

Confocal image of a poroussilicon structure (λem>530 nm)

UltrasonicTreatment

• Ensemble of Si Nanocrystals • Isolated Si Nanocrystals

Spincoating

Emission Intermittency – Blinking of Single NC

0 20 40 600.0

1.0

2.0

time [s]

coun

t rat

e [k

Hz]

Single Particle PL Intensity TraceSingle Particle PL Intensity TraceConfocalConfocal ImagesImages

off

on

Ionen Implantation

Schematic of ion implantation with scanning probe alignment: the ion beamis transported through an electrostatic lens element to a precollimatingaperture, then through the final aperture in the cantilever. The metal lineson the workpiece illustrated here are 20 nanometers high, one micrometer wide, and spaced three micrometers apart.

Implantation

Image: Max Planck Institute of Microstructure Physics

Lithography

• Etch pillars in quantum well heterostructures– Quantum well heterostructures give 1D confinement– Pillars provide confinement in the other 2 dimensions

• Electron beam lithography• Disadvantages: Slow, contamination, low density, defect formation

A. Scherer and H.G. Craighead. Fabrication of small laterally patterned multiple quantum wells. Appl. Phys. Lett., Nov 1986.

Lithographie

Optisches Filterelement

Laterally graded optical filter made of porous Silicon

Spiegel

Illustration of how a porous silicon multi-layer acts as a filter-mirror

Photonische Kristalle

Scanning electron micrograph of a porous silicon photonic crystal. The sample is prepared from an aqueous suspension

of small silicon particles about 10 nanometers in diameter and polystyrene spheres about one micrometer in diameter. When the water evaporates from the suspension, the polystyrene spheres self-assemble into a face-centered cubic (FCC) colloidal crystal, and the silicon particles are trapped in the voids. After all the water evaporates, the sample is heated to burnoff the polystyrene, leaving a silicon matrix with an ordered FCC arrangement of pores. The final material is a photonic crystal—it diffracts light across a range of angles and wavelengths.

20 µm

Periodic structure of ensembles of siliconnanocrystals

deposition from a particle beam withnarrow size distribution (Δd~1nm)

• 2.5 µm dot spacing• 1.2 µm dot diameter• ~200000 particles per dot

Photovoltaisches Element

Elektrolumineszenz

Zeitverhalten Elektrolumineszenz

Strom-Spannungscharakteristik

QD Lasers

• Advantages– More efficient, higher material

gain, lower threshold• Concentration of carriers near

band edge

– Less thermal dependence, spectral broadening

• Material gain – Theoretical prediction:

• G=104 cm-1, Jth=5A/cm2 at RT

– Compared to bulk InGaAsP:• N~1018, G~102 cm-1

Ledenstov et al. Quantum-dot heterostructure lasers. JSTQE, May 2000.