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Optical properties and carrier dynamics of self-assembled GaN/AlGaN quantum dots Nanotechnology 17 (2006) 2609-2613 Ashida lab. Nawaki Yohei
Contents • Gallium Nitride • Quantum dots • Fabrication of quantum dots • Growth regime of Self-assembled QDs • Fabricated sample • Photoluminescence spectra • Results • Temperature dependence of PL intensity • Temperature dependence of peak energy level • Summary
Gallium Nitride Widegap semiconductor cf. ZnSe, SiC, ZnO, CuCl GaN: 3.4eV GaN has wide controllable range of bandgap with ternary crystal semiconductor InN, AlN 0.7eV~6.1eV Crystal growth is difficult Blue- and UV-Light emitting diode and laser
Quantum dots Quantum Dots (QD) have three-dimensional carrier confinement The effect of QDs The confinement effect of carrier The alternation of density of state The restraint of kinetic momentum of carrier Application Quantum dot laser low threshold good thermal property Single photon generator Advanced lecture on condensed matter physics
Fabrication of QD Techniques to fabricate QDs (semiconductor) • laser ablation • precipitation of particles in solid • synthesis in organic solution • self-assembled particles by epitaxial growth • Molecular Beam Epitaxy • Metal Organic Chemical Vapor Deposition MOCVD heater Tri-Methyl Ga Tri-Methyl Al GaN/AlGaN NH3 substrate (sapphire)
Monolayer growth Frank-van der Merwe mode The strain energy is very small. Island on monolayer growth Stranski-Krastanov mode The strain energy is small. A few monolayer grow up. The strain energy become large. Nucleus grow up on the layer. Growth regime of epitaxial method Strain Energy Lattice mismatch between substrate and epitaxial layer Volmer-Weber mode Island growth epitaxial layer The strain energy is large. substrate
Purpose • To reveal carrier dynamics of GaN QDs Time-resolved spectroscopy Temperature dependence of photoluminescence spectra The authors use this method PL Intensity PL peak energy
Fabricated samples 9.1ML Al0.11Ga0.89N layer Atomic Force Microscopic GaN dot layer Al0.11Ga0.89N layer AlN layer sapphire(1000) 10.9ML 13.6ML 9.1 13.6 10.9
Photoluminescence of GaN dot 8.5nm 7nm He-Cd laser 325nm monochromator objective lens Al0.11Ga0.89N cap layer GaN dot layer Al0.11Ga0.89N layer AlN layer sapphire(1000) Inbe : Al0.11Ga0.89N near-band-edge emission Idefect : defect-related emission IQD : GaN QDs emission
The activation energy The activation energy means... • Exciton binding energy • Energy difference between QD state and... • barrier state • defect state barrier state Energy defect state Ebarrier Edefect QD state Electron states associated with nitrogen vacancy GaN 30meV @ Ec Al0.11Ga0.89N 50meV The nitrogen vacancy state of AlGaN provides a carrier escape channel for quenching the PL Intensity AlN 200meV
Temperature dependence of PL peak energy Temperature dependence of bandgap energy was expressed by using Vashni’s equation. At high temperature (T>100K) Shift follows the typical bandgap of bulk semiconductor. At low temperature (T<100K) There are energy differences between the Vashni’s equation. The PL structure is dominated from 1 state
300K 10K 60K Temperature dependence of PL intensity The expression of the PL quenching activation energy localization energy The activation energy is calculated at high temperature regime. The localization energy is calculated at low temperature regime.
Summary • The authors revealed carrier dynamics of GaN QDs. • The localization energy • There are temperature activated hopping of excitons/carriers in the quantum dots having the large diameter/height ratio. • The activation energy • The carrier escaped to the nitrogen vacancy state of AlGaN barrier layer
The Localization energy J. Appl. Phys. 97,033514(2005) I: The localized carrier at lower temperature II: The expanding carrier at higher temperature III: The barrier layer