Optical properties of dielectric nanostructures Motivation Applications Doped

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 > 5 e. V � Transparent in visible region � Ionic crystals � When doped exhibit semiconductor properties (Cd. F 2)

Content Dielectrics Fluorites Quantum wells Nanostructures

Fluorites: lattice structure � � � Face-centered-cubic unit cell Oh 5 symmetry The crystal is not piezoelectric Close contact between different species of ions or atoms Materials: › The halides: Ca. F 2, Cd. F 2, Ba. F 2… › The oxides: Zr. O 2, Ce. O 2… › 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 m 0 ) Ion 1 s 2 s 2 p 3 s 3 p Ca 2+ 2 2 6 F- 2 2 6 The valence band: › Maximum is in Г-point The conduction band: › Minimum is located in Гpoint Energy band formation 3 d The energy bands of Ca. F 2

Energy band structure Reflectance spectrum Fluorites: optical properties Transitions: reflectivity interpretation Region I I II III 11, 2 Г 15 -Г 1 13, 9 X’ 2 -X 3 15, 5 X’ 5 -X 3 25, 1 Г’ 25 -Г 15 Region II Outermost core state of metal ion Region III Interband ionizing transitions of core e-

Content Dielectrics Fluorites Quantum wells Nanostructures

Quantum wells Eg 1 Eg 2 Energy levels for infinite well Selection rule

Content Dielectrics Fluorites Quantum wells Nanostructures

KCl-KBr Multilayer Quantum wells Evaporation on to cleaved Li. F substarate Specimens: › SL 2: KBr 100 A-layer › C-ML: a double structure of KBr 100 A on KCl 200 A › D-ML: a triple structure of KBr 100 A sandwiched by two KCl 200 A › E-ML: a 3, 5 periods one of KCl 150 A – KBr 50 A

KI-KBr and KI-KCl Multilayers Evaporation onto a quartz glass substrate Specimens: › KI layer is sandwiched between KBr layer of the same thickness

KIx-KBr 1 -x mixed crystal films Blue shifts: › Quantum confinement effects › Interlayer mixed crystallization

12. 1 e. V Ca. F 2 8 e. V Cd. F 2 Lattice constants: Ca. F 2 5, 46 A 0 Cd. F 2 5, 39 A 0 Si 5, 43 A 0 Ca. F 2 Cd. F 2 Si Ca. F 2 Energy difference at the interface Ca. F 2/Cd. F 2: 2, 9 e. V in the conduction band 1, 2 e. V in the valence band Ca. F 2 -Cd. F 2 heterostructures on Si Opposite sing of the fluoride lattice mismatch to Si flavors strain compensation and growth of pseudomorthic superlattices � Specimens: › › › › 1: Si-Ca. F 2(30 ML)-Ca. F 2 2: Si-Ca. F 2 -Cd. F 2(10 ML)-Ca. F 2 3: Si-Ca. F 2 -Cd. F 2(30 ML)-Ca. F 2 4: Si-Ca. F 2 -Cd. F 2(90 ML)-Ca. F 2 5: Si-Ca. F 2 -6 x[Ca. F 2(5 ML)-Cd. F 2(5 ML)]-Ca. F 2 6: Si-Ca. F 2 -14 x[Ca. F 2(2, 5 ML)-Cd. F 2(2 ML)]-Ca. F 2 7: Si-Ca. F 2 -3 x[Ca. F 2(10 ML)-Cd. F 2(10 ML)]-Ca. F 2

To add transitions Ca. F 2 -Cd. F 2 heterostructures on Si Ca. F 2 -Cd. F 2 10 ML Ca. F 2 -Cd. F 2 5 ML Ca. F 2 -Cd. F 2 2 ML Ca. F 2 -Cd. F 2 90 ML Ca. F 2 -Cd. F 2 30 ML Ca. F 2 -Cd. F 2 10 ML Ca. F 2

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

Ca. F 2 harmonic approximation Oscillators parameters: oscillator resonance energy dumping param amplitude 0 11, 49 0, 25 1, 24 1 0, 5 1, 9 2 0, 7 3, 8 3 1, 5 4, 4

Ca. F 2 -Cd. F 2 heterostructures 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 Ca. F 2 multilayer at 4500 C Ca. F 2 nanoislands stretched along <110> direction Bare Si surface between Ca. F 2 islands

Ca. F 2 stripes: high temperature growth 6 Ca. F 2 multilayers at 7000 C Ca. F 2 stripes are aligned along <110> direction Stripes length – several microns, height – 3 -6 nm Formation of wetting layer

MBE-growth of Ca. F 2/Cd. F 2 superlattices X-Ray diffraction TEM 6 ML 4 ML High crystalline quality 10 ML T/2

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The first Brillouin zone of the facecentered-cubic fluorite lattice

Reflectance spectrum of Ca. F 2/Cd. F 2 superlattices