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

Ashida lab.

Nawaki Yohei

Nanotechnology 17 (2006) 2609-2613

2Contents

• 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

3Gallium Nitride

Widegap semiconductor

GaN: 3.4eV cf. ZnSe, SiC, ZnO, CuCl

GaN has wide controllable range of bandgap

with ternary crystal semiconductor InN, AlN0.7eV~6.1eV

Crystal growth is difficult

Blue- and UV-Light emitting diode and laser

4Quantum 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 thresholdgood thermal property

The effect of QDs

Single photon generator

5Fabrication of QDTechniques 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 substrate (sapphire)

heater

GaN/AlGaN

6Growth 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

7Purpose

• To reveal carrier dynamics of GaN QDs

Time-resolved spectroscopy

Temperature dependence of photoluminescence spectra

The authors use this method PL IntensityPL peak energy

8Fabricated samples9.1ML

10.9ML 13.6ML

Al0.11Ga0.89N layer

sapphire(1000)

Al0.11Ga0.89N layer

AlN layer

GaN dot layer

GaN coverages(ML) height/diameter(nm)

9.1 6.5/190

10.9 7.0/200

13.6 8.5/220

9.110.9

13.6

Atomic Force Microscopic

9Photoluminescence of GaN dot7nm 8.5nm

sapphire(1000)

Al0.11Ga0.89N layer

AlN layer

Al0.11Ga0.89N cap layer

GaN dot layer

Inbe : Al0.11Ga0.89N near-band-edge emission

Idefect : defect-related emission

IQD : GaN QDs emission

monochromator

He-Cd laser325nm

objective lens

10The 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

Ebarrier

Edefect

Ene

rgy

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

AlN 200meV

Al0.11Ga0.89N 50meV Ec The nitrogen vacancy state of AlGaNprovides a carrier escape channel

for quenching the PL Intensity

11Temperature dependence of PL peak energy

T

TETE gg

2

)0()(

Temperature dependence of bandgap energywas 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 stateheight (nm)

Localization energy (meV)

6.5 7±2

7.0 14±1

8.5 30±2

12Temperature dependence of PL intensity

kTECkTEC

ITI

loca

expexp1

)0()(

21

10K300K 60KThe 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)

6.5 7±2

7.0 14±1

8.5 30±2

height (nm)

Activation energy (meV)

6.5 43

7.0 70

8.5 106

The expression of the PL quenching

13Summary

• 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

14ZnTe 　 quantum dots

15The 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