Optical properties of dielectric nanostructures
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Optical properties of dielectric nanostructures. Motivation. Applications: Doped crystals lasers (radiation-resistant) Optical memory Difficulties of investigation. Content. Dielectrics Fluorites Quantum wells Nanostructures. Content. Dielectrics Fluorites Quantum wells

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Optical properties of dielectric nanostructures

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Optical properties of dielectric nanostructures

Optical properties of dielectric nanostructures


Motivation

Motivation

  • Applications:

    • Doped crystals lasers (radiation-resistant)

    • Optical memory

  • Difficulties of investigation


Content

Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Content1

Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Dielectrics

Dielectrics

  • Non-conducting substance

  • “dia-electric” from Greek “dia” – “through”

  • No free charge carriers

  • Wide band gap > 5eV

  • Transparent in visible region

  • Ionic crystals

  • When doped exhibit semiconductor properties (CdF2)


Content2

Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Fluorites lattice structure

Fluorites: lattice structure

  • Face-centered-cubic unit cell

  • Oh5 symmetry

  • The crystal is not piezoelectric

  • Close contact between different species of ions or atoms

  • Materials:

    • The halides: CaF2, CdF2, BaF2…

    • The oxides: ZrO2, CeO2…

    • The others…

- Ca

- F


Fluorites energy band structure

Fluorites: energy-band structure

Electronic configuration of ions

  • Main features:

    • Electron effective mass is comparable to free electron mass (0,5~1,0 m0)

  • The valence band:

    • Maximum is in Г-point

  • The conduction band:

    • Minimum is located in Г-point

The energy bands of CaF2

Energy band formation


Fluorites optical properties

Fluorites: optical properties

Energy band structure

Transitions:

Reflectance spectrum

I

III

II


Content3

Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Quantum wells

Quantum wells

Eg1

Eg2

Energy levels for infinite well

Selection rule


Content4

Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Kcl kbr multilayer quantum wells

KCl-KBr Multilayer Quantum wells

  • Evaporation on to cleaved LiFsubstarate

  • Specimens:

    • SL2: KBr 100A-layer

    • C-ML: a double structure of KBr 100A on KCl 200A

    • D-ML: a triple structure of KBr 100A sandwiched by two KCl 200A

    • E-ML: a 3,5 periods one of KCl 150A – KBr 50A


Ki kbr and ki kcl multilayers

KI-KBr and KI-KClMultilayers

  • Evaporation onto a quartz glass substrate

  • Specimens:

    • KI layer is sandwiched between KBr layer of the same thickness


Ki x kbr 1 x mixed crystal films

KIx-KBr1-x mixed crystal films

  • Blue shifts:

    • Quantum confinement effects

    • Interlayer mixed crystallization


Caf 2 cdf 2 heterostructures on si

CaF2-CdF2heterostructures on Si

Energy difference atthe interface CaF2/CdF2:2,9 eV in the conduction band1,2 eV in the valence band

  • Specimens:

    • 1: Si-CaF2(30ML)-CaF2

    • 2: Si-CaF2-CdF2(10ML)-CaF2

    • 3: Si-CaF2-CdF2(30ML)-CaF2

    • 4: Si-CaF2-CdF2(90ML)-CaF2

    • 5: Si-CaF2-6x[CaF2(5ML)-CdF2(5ML)]-CaF2

    • 6: Si-CaF2-14x[CaF2(2,5ML)-CdF2(2ML)]-CaF2

    • 7: Si-CaF2-3x[CaF2(10ML)-CdF2(10ML)]-CaF2

12.1eV

8eV

Opposite sing of the fluoride lattice mismatch to Si flavors strain compensation and growth of pseudomorthic superlattices

CaF2

CdF2

Lattice constants:CaF2 5,46 A0CdF2 5,39 A0Si 5,43 A0

Si

CaF2

CaF2

CdF2

CaF2

CdF2

CaF2

CdF2

CaF2


Caf 2 cdf 2 heterostructures on si1

To add transitions

CaF2-CdF2heterostructures on Si

CaF2-CdF2 10ML

CaF2-CdF2 5ML

CaF2-CdF2 2ML

CaF2-CdF2 90ML

CaF2-CdF2 30ML

CaF2-CdF2 10ML

CaF2


The harmonic oscillator model

The harmonic oscillator model

- complex dielectric constant

- refractive index

- reflection coefficient

- reflectivity


Caf 2 harmonic approximation

CaF2 harmonic approximation

  • Oscillators parameters:


Caf 2 cdf 2 heterostructures on si2

CaF2-CdF2heterostructures on Si

  • Future plans:

    • To calculate phase of reflective index

    • To calculate reflectance spectrum from thin films

    • To estimate energy level shifts in quantum well

    • To estimate interface effects


Nanoislands low temperature growth

Nanoislands: low temperature growth

  • 1 CaF2 multilayer at 4500C

  • CaF2 nanoislands stretched along <110> direction

  • Bare Si surface between CaF2 islands


Caf 2 stripes high temperature growth

CaF2 stripes: high temperature growth

  • 6 CaF2 multilayers at 7000C

  • CaF2 stripes are aligned along <110> direction

  • Stripes length – several microns, height – 3-6 nm

  • Formation of wetting layer


Mbe growth of caf 2 cdf 2 superlattices

MBE-growth of CaF2/CdF2superlattices

X-Ray diffraction

TEM

6ML

4ML

10ML

T/2

High crystalline quality


Thank you for attention

Thank you for attention


The first brillouin zone of the face centered cubic fluorite lattice

The first Brillouin zone of the face-centered-cubic fluorite lattice


Reflectance spectrum of caf 2 cdf 2 superlattices

Reflectance spectrum of CaF2/CdF2superlattices


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