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Understanding Optical Properties of Dielectric Nanostructures

Explore the characteristics and applications of dielectrics like fluorites and quantum wells, studying their energy band structure, crystal lattice, and optical properties. Discuss the challenges and future research plans in investigating these systems.

christopherstanford
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Understanding Optical Properties of Dielectric Nanostructures

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  1. Optical properties of dielectric nanostructures

  2. Motivation • Applications: • Doped crystals lasers (radiation-resistant) • Optical memory • Difficulties of investigation

  3. Content Dielectrics Fluorites Quantum wells Nanostructures

  4. Content Dielectrics Fluorites Quantum wells Nanostructures

  5. 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)

  6. Content Dielectrics Fluorites Quantum wells Nanostructures

  7. 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

  8. 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

  9. Fluorites: optical properties Energy band structure Transitions: Reflectance spectrum I III II

  10. Content Dielectrics Fluorites Quantum wells Nanostructures

  11. Quantum wells Eg1 Eg2 Energy levels for infinite well Selection rule

  12. Content Dielectrics Fluorites Quantum wells Nanostructures

  13. 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

  14. KI-KBr and KI-KClMultilayers • Evaporation onto a quartz glass substrate • Specimens: • KI layer is sandwiched between KBr layer of the same thickness

  15. KIx-KBr1-x mixed crystal films • Blue shifts: • Quantum confinement effects • Interlayer mixed crystallization

  16. 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

  17. 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

  18. The harmonic oscillator model - complex dielectric constant - refractive index - reflection coefficient - reflectivity

  19. CaF2 harmonic approximation • Oscillators parameters:

  20. 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

  21. Nanoislands: low temperature growth • 1 CaF2 multilayer at 4500C • CaF2 nanoislands stretched along <110> direction • Bare Si surface between CaF2 islands

  22. 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

  23. MBE-growth of CaF2/CdF2superlattices X-Ray diffraction TEM 6ML 4ML 10ML T/2 High crystalline quality

  24. Thank you for attention

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

  26. Reflectance spectrum of CaF2/CdF2superlattices

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