Optical properties and carrier dynamics of self-assembled GaN/AlGaN quantum dots

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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

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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

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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
0.7eV6.1eV
Crystal growth is difficult
Blue- and UV-Light emitting diode and laser
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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
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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)
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Growth regime of epitaxial method
Lattice mismatch between substrate and epitaxial
layer
Strain Energy
Volmer-Weber mode
Island growth
epitaxial layer
The strain energy is large.
substrate
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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
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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
GaN coverages(ML) height/diameter(nm)
9.1 6.5/190
10.9 7.0/200
13.6 8.5/220
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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
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The activation energy
The activation energy means...

• Exciton binding energy
• Energy difference between QD state and...
• barrier state
• defect state

barrier state
height Ebarrier Edefect Ea
6.5 114 43 43
7.0 131 69 70
8.5 173 104 106
Energy


defect state
Ebarrier
Edefect
QD state
Electron states associated with nitrogen vacancy
GaN 30meV
_at_ Ec
Al0.11Ga0.89N 50meV
The nitrogen vacancy state of AlGaN provides a
carrier escape channel for quenching the PL
Intensity
AlN 200meV
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Temperature dependence of PL peak energy
Temperature dependence of bandgap energy was
expressed by using Vashnis equation.
At high temperature (Tgt100K)
Shift follows the typical bandgap of bulk
semiconductor.
At low temperature (Tlt100K)
There are energy differences between the Vashnis
equation.
height (nm) Localization energy (meV)
6.5 72
7.0 141
8.5 302
The PL structure is dominated from 1 state
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Temperature dependence of PL intensity
The expression of the PL quenching
activation energy
localization energy
The activation energy is calculated at high
temperature regime.
height (nm) Activation energy (meV)
6.5 43
7.0 70
8.5 106
The localization energy is calculated at low
temperature regime.
height (nm) Localization energy (meV)
6.5 72
7.0 141
8.5 302
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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

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ZnTe quantum dots
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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