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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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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 transitionsCaF2-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

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