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Busan City in the World
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DONGEUI UNIVERSITY
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Dongeui Unversity
Students : 20,000
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My Lab.
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My Lab.
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Busan Metropolitan City Hall
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Busan Port
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Beaches in Busan
<Hayundae>
<Songjung Beach>
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<Kwangan Beach>
<Dadaepo Beach>
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Mountains and River
<Kumjung Mountain>
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<Great Nakdong River>
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2004 Korea – Australia Photonics Workshop
Epitaxial Growth of ZnO Thin Films for
LED by Pulsed Laser Deposition
YUNSIK YU
1. Laser Spectroscopy Technology Research Center,
2. Research Institute of Basic Science and Department of Physics,
3. Electronic Ceramics Center, Dongeui University, Busan 614-714, Korea
[email protected]
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Contents
I.
Introduction
II. What is the PLD?
III. Experiment
IV. Results and Discussion
V. Conclusion
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I. Introduction
ZnO has band-gap and
structure compatible to
that of GaN
The exciton binding
energy (60meV) of ZnO
is much larger than that
of GaN (28meV)
Potential applications in
blue and ultraviolet
LEDs and LDs.
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What is ZnO?
• Structure : Hexagonal (Wurtzite)
• a=3.249Å, c=5.207 Å
• Exciton Binding Energy : 60 meV
• Packing Fraction : 44 %
• Pure ZnO : n-type
• Refractive Index : 2.008 (at 632.8nm)
• High Piezoelectric Effect: c-direction
•Wide-band gap semiconductor, 3.37eV
(GaN~3.5, SiC 3.0eV)
• ZnO ~ GaN, UV / violet / blue, LED & LD
• Technique: MBE, PLD, Laser MBE,
MOCVD and HVPE …
Crystal structure of ZnO
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Why shall we study on Ga doped ZnO(GZO)?
A kind of low-resistivity n-type transparent conducting
oxides (TCO)
Ga and N co-doping has been proven to be one of the most
promising way to make p-type ZnO from theory calculations
The epitaxial GZO films and their luminescence
properties need to be investigated in more detail
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II. What is the PLD?
History of PLD
1. 1960s
: Ruby Laser
PLD experiment
2. 1970s
: High energy density Laser
(Q-switching)
Short wavelength Laser (SHG)
3. 1980s
: Laser MBE
Good Quality
4. Present : Oxide Superconductor
Artificial Super-lattice
New Functional Material
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Thin Film Formation by Pulsed Laser Deposition
Substrate
Excimer Laser
Thin film
Plasma
Target
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Explosive Laser – Target Interactions
Time
0
Absorption
I=I0exp(-x)
Thermal Conduction
Surface Melting
Vaporization
Plasma Production
Plasma Emission
30nS
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Advantage and Application of PLD
Process:
1. Vaporization of target materials
2. Transport of the vapor plume
3. Nucleation and growth of films
on the substrate
Advantages:
Simplicity; Versatility; Efficiency; Low cost; Controllability
Applications:
Films growth: superconductors, ferroelectrics, piezo-electrics,
magnetoresistive materials, semiconductors
Nanomaterials preparation: nanopowder, nanorods, nanotubes
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Thin Film Formation Mechanism by PLD
Laser
Deposition
Migration
Thin Film Formation
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Kinds of Thin Film Formation
Two Dimensional
Mono-layer Growth
Three Dimensional
Island Growth
Two and Three Dimensional
Complex Growth
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On-Axis Method of PLD
Laser
Heater
Plasma
Substrate
Target
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Masked On-Axis Method of PLD
Laser
Plasma
Substrate
Micro Filter
Target
Heater
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Off-Axis Method of PLD
Laser
Heater
Substrate
Plasma
Target
Rotation
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Our PLD System
Laser
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Structure of PLD Chamber
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Deposition Parameters of PLD
d
Laser Beam
E, 
Adsorption
T
RPR
Substrate
Target
P
O2
O
Dissociation
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Desorption
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Plume Shape with Oxygen Pressure
Schematic diagram of the PLD
experimental setup
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500 mTorr
300 mTorr
250 mTorr
200 mTorr
180 mTorr
100 mTorr
Plume Shape with Oxygen Pressure
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Principle of Excimer Laser
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III. Experiment
Preparation of PLD Target
Pure ZnO
Ga doped ZnO
Form Disk
From Disk
CIP
200Mpa
CIP
200Mpa
Sintering
600 ℃ 2h
Sintering
600 ℃ 2h
Sintering
1200 ℃ 4h
Sintering
1200 ℃ 4h
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Experimental conditions of PLD
1. Processing
Pulsed laser deposition: KrF excimer laser 248 nm
Laser energy density: ~1J/cm2
Laser repetition rate: 5Hz
Substrates: sapphire (0001)
Target-substrate distance: 5cm
Substrates temperature: 400℃
Ambient oxygen pressure: 100~300mTorr
Ga doping concentration: 0.4at.%~3.0at.%
2. Characterization
XRD, HRXRD, AFM, PL
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0.40
ZnO(0004)
0.35
FWHM (degree)
ZnO(0002)
o
500 C
Sapphire(0006)
IV. Results and Discussion
o
400 C
0.30
0.25
0.20
0.15
Intensity (a. u.)
0.10
o
300 C
0.05
100
200
300
400
500
o
Substrate temperature ( C)
o
200 C
o
100 C
20
30
40
50
60
70
80
X-Ray diffraction (θ/2θ) patterns
and the FWHM values of ω-scans
for ZnO(0002) films deposited on
sapphire(0001)
substrates
at
different temperatures
2-Theta (degree)
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φ-scan of ZnO Thin Film
Intensity (a. u.)
a)
XRD in-plane φ-scan of a) plane of
sapphire substrate; b) plane of ZnO film
grown at 400℃ under 200 mTorr oxygen
pressure
b)
Epitaxial relationship:
Out-plane: ZnO(0001) // Sapphire(0001)
In-plane: ZnO [101 0] // Saphire [1120]
-180 -150 -120 -90
-60
-30
0
30
60
90
120 150 180
f (degree)
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o
FWHM=1.23
Sapphire (0006)
14
16

18
20
GZO (0004)
Intensity (a. u.)
GZO (0002)
θ /2θ Scan and Rocking-curve of GZO Thin Film
20
30
40
50
60
70
80
2 (degrees)
θ /2θ scan of 2.0% Ga doped ZnO prepared at 400℃ and
200mTorr. In-set: Rocking-curve of (0002) peak of GZO films
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φ-scan of GZO Film and Sapphire Substrate
Intensity(a.u.)
Sapphire
GZO
-180 -150 -120 -90 -60 -30
0
30
60
90
120 150 180
f (degrees)
φ-scan of GZO films prepared at 400 ℃ and 200mTorr
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AFM Images of GZO Films
0.4%
2.0%
1.2%
0.8%
2.5%
1.6%
3.0%
AFM of GZO films grown at 400 ℃ and 200 mTorr
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SEM Image of ZnO Thin Film
ZnO film deposited on sapphire (0001) substrates at 400 ℃
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PL of ZnO / sapphire (0001) Thin Films
o
Intensity (a. u.)
500 C
o
400 C
o
300 C
o
200 C
o
100 C
350
400
450
500
550
600
650
Wavelength (nm)
Photoluminescence of ZnO film deposited on sapphire (0001)
substrates at different temperatures
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PL of GZO films with Ga Concentration
X 1/4
PL Intensity (a.u.)
[Ga]=0
[Ga]=0.4 at%
[Ga]=0.8 at%
[Ga]=1.2 at%
[Ga]=1.6 at%
[Ga]=2.0 at%
[Ga]=2.5 at%
[Ga]=3.0 at%
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Photon energy (eV)
PL of GZO films with different Ga concentrations grown on
sapphire (0001) at 400℃ and 200 mTorr.
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Variation of NBE Emission with Ga Concentration
3.290
3.285
band-to-band emission
deep-level emission
PL Intensity (a. u.)
Ep (eV)
3.280
3.275
3.270
3.265
3.260
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
1.5
2.0
2.5
3.0
Ga doping concentration(at%)
Ga doping concentration (at%)
Shift of NBE emission peak position
with Ga concentration
The change of emission intensity
with the Ga concentration
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V. Conclusion
ZnO films have been grown on Sapphire(0001) substrates by
the pulsed laser deposition method. High quality epitaxial ZnO
films have been obtained at 400 ℃ of substrate temperature
under 200mTorr ambient pressure;
The epitaxial relationship is:
Out-of plane: ZnO(0001) // Sapphire(0001)
In-plane: ZnO[101 0] // Sapphire [1120] ;
The intensities of photoluminescence of ZnO films increase
with the increasing of crystallization. Strong UV luminescence
is obtained from the expitaxial ZnO films;
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V. Conclusion
Epitaxial GZO films have been grown on sapphire(0001)
substrates by PLD method;
The epitaxial growth is possible at 400 ℃. There exists an
epitaxial relationship: Out-of plane: GZO(0001) // sapphire(0001)
In-plane:GZO[101 0] // sapphire [1120] ;
NBE emission of GZO films shift to higher energy with the
increase of Ga concentration;
NBE emission of GZO film quenches due to the occurrence of
the nonradiative transitions. This is related to the grain size of
the film.
All GZO films show orange deep-level emission. The orange
emission is related to oxygen vacancies and Zn intertistials.
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