Optical properties and carrier dynamics of self-assembled GaN/AlGaN quantum dots Ashida lab. Nawaki Yohei Nanotechnology 17 (2006)

2 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

3 Gallium Nitride Widegap semiconductor GaN: 3.4eV cf. ZnSe, SiC, ZnO, CuCl 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

4 Quantum dots Quantum Dots (QD) have three-dimensional carrier confinement The confinement effect of carrier The alternation of density of state The restraint of kinetic momentum of carrier Advanced lecture on condensed matter physics Application Quantum dot laser low threshold good thermal property The effect of QDs Single photon generator

5 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 Tri-Methyl Ga Tri-Methyl Al NH 3 substrate (sapphire) heater GaN/AlGaN

6 Growth regime of epitaxial method Volmer-Weber mode Island growth The strain energy is large. Lattice mismatch between substrate and epitaxial layer Strain Energy Frank-van der Merwe mode Monolayer growth The strain energy is very small. Stranski-Krastanov mode Island on monolayer growth The strain energy is small. The strain energy become large. A few monolayer grow up. Nucleus grow up on the layer. substrate epitaxial layer

7 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

8 Fabricated samples 9.1ML 10.9ML 13.6ML Al 0.11 Ga 0.89 N layer sapphire(1000) Al 0.11 Ga 0.89 N layer AlN layer GaN dot layer GaN coverages(ML)height/diameter(nm) / / / Atomic Force Microscopic

9 Photoluminescence of GaN dot 7nm 8.5nm sapphire(1000) Al 0.11 Ga 0.89 N layer AlN layer Al 0.11 Ga 0.89 N cap layer GaN dot layer I nbe : Al 0.11 Ga 0.89 N near-band-edge emission I defect : defect-related emission I QD : GaN QDs emission monochromator He-Cd laser 325nm objective lens

10 The activation energy The activation energy means... Exciton binding energy Energy difference between QD state and... ♦barrier state ♦defect state barrier state defect state QD state E barrier E defect Energy heightE barrier E defect EaEa Electron states associated with nitrogen vacancy GaN30meV AlN200meV Al 0.11 Ga 0.89 N 50meV  E c The nitrogen vacancy state of AlGaN provides a carrier escape channel for quenching the PL Intensity

11 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 height (nm) Localization energy (meV) 6.57± ± ±2

12 Temperature dependence of PL intensity 10K300K 60K The activation energy is calculated at high temperature regime. activation energy localization energy The localization energy is calculated at low temperature regime. height (nm) Localization energy (meV) 6.57± ± ±2 height (nm) Activation energy (meV) The expression of the PL quenching

13 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

14 ZnTe quantum dots

15 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