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
2
Gallium Nitride
3
Widegap semiconductor
GaN: 3.4eV
cf. ZnSe, SiC, ZnO, CuCl
GaN has wide controllable range of bandgap
with ternary crystal semiconductor InN, AlN
Crystal growth is difficult
0.7eV~6.1eV
Blue- and UV-Light emitting diode and laser
Quantum dots
4
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
Advanced lecture on condensed matter physics
Single photon generator
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
NH3
heater
GaN/AlGaN
substrate (sapphire)
5
Growth regime of epitaxial method
Lattice mismatch between substrate and epitaxial layer
Frank-van der Merwe mode
6
Strain Energy
Monolayer growth
The strain energy is very small.
Stranski-Krastanov mode
Island on monolayer growth
The strain energy is small.
A few monolayer grow up.
The strain energy become large.
Nucleus grow up on the layer.
Volmer-Weber mode
epitaxial layer
substrate
Island growth
The strain energy is large.
Purpose
7
• 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
Al0.11Ga0.89N layer
8
9.1ML
Atomic Force Microscopic
GaN dot layer
Al0.11Ga0.89N layer
AlN layer
sapphire(1000)
10.9ML
9.1
10.9
13.6ML
13.6
GaN coverages(ML)
height/diameter(nm)
9.1
6.5/190
10.9
7.0/200
13.6
8.5/220
Photoluminescence of GaN dot
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
8.5nm
9
The activation energy
10
The activation energy means...
•Exciton binding energy
•Energy difference between QD state and...
♦barrier state
♦defect state
Energy
barrier state
Ebarrier
defect state
Edefect
QD state
height
Ebarrier
Edefect
Ea
6.5
114
43
43
7.0
131
69
70
8.5
173
104
106
Electron states associated with nitrogen vacancy
GaN
30meV
Al0.11Ga0.89N 50meV @ Ec
AlN
200meV
The nitrogen vacancy state of AlGaN
provides a carrier escape channel
for quenching the PL Intensity
Temperature dependence of PL peak energy
11
Temperature dependence of bandgap energy
was expressed by using Vashni’s equation.
E g (T )  E g ( 0 ) 
T
2
 T
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.
height
(nm)
Localization
energy (meV)
6.5
7±2
7.0
14±1
8.5
30±2
The PL structure is dominated from 1 state
Temperature dependence of PL intensity
The expression of the PL quenching
I (T ) 
I (0)
1  C 1 exp   E a kT   C 2 exp   E loc kT
activation energy
300K
60K
12
10K

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
7±2
7.0
14±1
8.5
30±2
Summary
13
• 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
ZnTe quantum dots
14
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
15