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


Motivation

  • Applications:

    • Doped crystals lasers (radiation-resistant)

    • Optical memory

  • Difficulties of investigation


Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


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)


Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


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

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

Energy band structure

Transitions:

Reflectance spectrum

I

III

II


Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


Quantum wells

Eg1

Eg2

Energy levels for infinite well

Selection rule


Content

Dielectrics

Fluorites

Quantum wells

Nanostructures


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-KClMultilayers

  • Evaporation onto a quartz glass substrate

  • Specimens:

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


KIx-KBr1-x mixed crystal films

  • Blue shifts:

    • Quantum confinement effects

    • Interlayer mixed crystallization


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


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

- complex dielectric constant

- refractive index

- reflection coefficient

- reflectivity


CaF2 harmonic approximation

  • Oscillators parameters:


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

  • 1 CaF2 multilayer at 4500C

  • CaF2 nanoislands stretched along <110> direction

  • Bare Si surface between CaF2 islands


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 CaF2/CdF2superlattices

X-Ray diffraction

TEM

6ML

4ML

10ML

T/2

High crystalline quality


Thank you for attention


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


Reflectance spectrum of CaF2/CdF2superlattices


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