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Wide Bandgap Nitride & Oxide
Semiconductors:
pulsed laser deposition, film characterization
& device applications
Dong-Sing Wuu, Prof.
Department of Materials Science and Engineering
Da-Yeh University, Taiwan
National Chung Hsing University, Taiwan
E-mail: [email protected]
http://www.nchu.edu.tw
2
Focus of this talk
Wide bandgap semiconductors, consisting of nitride- and oxide-based materials, have been widely explored for their universal applications such as electronics, optoelectronics, and spintronics.
These wide bandgap materials possess several advantages including high dielectric breakdown voltage, high electron mobility, and a bandgap in the blue and ultraviolet spectrum.
Among various growth methods, pulsed laser deposition (PLD) is more beneficial to prepare high-quality film because of its atomic-layer control and precision composition.
In this talk, the characterization and device applications of PLD-grown wide bandgap nitride (GaN-based) and oxide (ZnO- and Ga2O3-based) semiconductor films are presented.
PLD system GaN-baed LED
Ga2O3 photodetector
ZnO used in display
3
Introduction
Characteristics and growth mechanism of PLD
Wide bandgap films by PLD presented in this study
GaN-based materials
ZnO-based materials
Ga2O3-based materials
GaN-based films by PLD
A novel fabrication method of GaN-on-Si (100) template
High-indium-content InGaN films with high thermal stability
ZnO-based films by PLD
Al-doped ZnO (AZO) transparent contact layers for InGaN light emitters
Diluted magnetic cobalt-doped ZnO (CZO) as electron deceleration electrodes for
InGaN light emitters
Ga2O3 films grown by PLD
High performance solar-blind Ga2O3 photodetectors
Conclusions
Outline
4
Why using PLD to grow films?
PLD has a lot of advantages, such as:
Simple growth concept : Using a laser beam to vaporize a target,
generating a plasma plume.
Film’s composition consistent with target : When a high-energy
laser hits the target, a film with the same composition as the
target can be achieved (especially for the complex film’s growth).
Versatility : A lot of materials can be prepared in various gas
atmospheres over a wide range of gas pressures.
Cost-effective : One laser equipment can be served for many
vacuum systems.
High quality samples : Excellent electrical and optical properties
can be easily obtained in the films by PLD (especially for TCOs).
5
Disadvantages of PLD method
PLD has a lot of Disadvantages, such as:
Particulates: The large kinetic energy of some plume species
causes re-sputtering and likewise defects in the substrate surface
and growing film.
An inhomogeneous energy distribution in the laser beam profile
gives rise to an inhomogeneous energy profile and angular energy
distribution in the laser plume.
Light elements like oxygen or lithium have different expansion
velocities and angular distributions in a plume as compared to
heavier elements. To obtain the desired film composition, e.g. an
adapted target composition or a background gas is required.
Composition and thickness depend on deposition conditions.
Difficult scale-up to large wafers?
Source: Rev. Mod. Phys. 72 (1), 315-328 (2000).
6
Theory: Using a high-power laser pulse with an energy density of >108 W cm−2 to
melt, evaporate, excite, and ionize material from a single target.
Growth mechanism: Absorption of laser energy ablation of target material.
Plasma expansion, heating partial absorption of laser radiation transfer
of fast atoms, ions, clusters, and slow droplets, particles.
Condensation of plasma nucleation and growth.
Laser Energy
Temperature
Growth pressure
Pulse frequency
Target
Substrate 4000K-20000K
(e)
(d)
(c)
(b)
(a)
Growth mechanism of PLD
PLD Parameters
7
Laser source:
Excimer gas lasers are
usually ArF (193 nm),
or KrF (248 nm)
Optical components: 1. Lenses
2. Apertures
3. Beam Splitters
4. Laser Windows
Vacuum chamber: Operating at high vacuum
or ultra-high vacuum
Laser
Optical components
Vacuum chamber
Pulsed laser deposition system
PLD is an ideal technique since the atomic-layer control can be realized
by adjusting the laser repetition rate and the source particles possess
high energy which enhances the surface mobility of the ad-atoms.
8
Wide bandgap films by PLD presented in this study
In this study, several wide bandgap films consisting of GaN, high-indium-content InGaN, Al-doped ZnO (AZO), Co-doped ZnO (CZO), and Ga2O3 have been deposited and applied for optoelectronic devices. Moreover, PLD is a very suitable technique for these films’ growth. The reasons are described as follows.
For the complex nitride (InxGa1-xN, x=33-62%) and oxide (AZO and CZO) films, the film’s composition consistent with the target could be achieved by PLD.
For the transparent oxides growth (AZO, CZO, and Ga2O3), both excellent electrical and optical properties of the films can reach by PLD, which is helpful to fabricate the high-performance optoelectronic devices.
For the GaN growth on Si substrate, the serious drawback of melt-back etching occurred at MOCVD growth can be solved via PLD technique.
GaN-on-Si by PLD InGaN by PLD PLD-CZO n-electrode
in LED
PLD-Ga2O3 MSM
photodetector
9
InGaN applications
InGaN alloy has attracted significant interest of late due to its tunable
bandgap energy ranging from the ultraviolet region (GaN: 3.4eV) to the
near-infrared region (InN: 0.7eV).
This broad range of possible bandgap covers almost the entire solar
spectrum, giving the alloy significant potential for use in optoelectronic
devices, especially LEDs and full-spectrum solar cells.
Audi A4 1.8T
illumination Nightclubs
10
GaN-on-Si applications
High electron mobility transistor (HEMT) Maximum frequency of oscillations
Low specific on-resistance
High breakdown voltage
Light-emitting diode (LED)
Laser diode (LD)
http://powerelectronics.com
http://www.solarfeeds.com
http://www.phy.bris.ac.uk
http://www.canaleenergia.com
http://cdn2-b.examiner.com
11
ZnO-based materials
There have been many reports on the doping of ZnO with different
elements like Ga, In, Al, and so on. Recently, the ZnO-based film has
received much attention because it exhibits a wide band gap (~3.8 eV),
electrical and optical properties similar to ITO films.
Zinc oxide (ZnO) is a II–VI semiconductor, and it has a hexagonal
structure.
Advantages of ZnO:
1. Wide and direct band gap of about 3.37 eV
2. High melting point of about 1975 C and good thermal stability
3. Low temperature process
4. Low cost
5. Nontoxic feature
6. Abundant resource
12
ZnO-based materials: applications
Potential applications in various semiconductors
Light emitting diodes
Transparent electronic devices
Solar cells
Gas sensors
Photodetectors
13
Ga2O3-based materials
Advantages
Wide band-gap (4.5~4.9 eV) semiconductor material
Electrical characteristics vary from insulating to conductive
depending on growth conditions
Transparent semiconductor oxide from visible to ultraviolet
High thermal and chemical stabilities
M. Orita et al, Thin Solid Films 411,
134–139 (2002) Semiconductor Today, 16 Jan. 2012
Wide band-gap & large electrical variety
Good TCO material
14
Ga2O3-based materials: applications
Potential applications in various semiconductors
Field-effect devices
Transparent electronic devices
Flat-panel displays
Gas sensors
Photodetectors UV-TCO on LED
Field-effect transistor
Solarblind UV detectors in LYRA
onboard PROBA-2 satellite
Flat-panel display
Ga2O3 MESFET
15
GaN-based films by PLD
A novel GaN-on-Si(100) template
Source: OPTICAL EXPRESS Vol. 21, pp.26468-26474, 2013
16
Author Instrument Substrate interlayer GaN (μm)
S. Tripathy et.al, ASS, 253, 236 (2006) MOCVD SOI AlN 0.7-1.2 μm
A. Dadgar et.al, NJP, 9,389 (2007) MOVPE 4°cut off Si(100) AlN 0.5 μm
S. Joblot et.al, APL, 87,133505 (2005) MBE 5°cut off Si(100) AlN/AlGaN/GaN 0.6 μm
A. Soltani et.al, SST, 28, 094003 (2013) MBE 6°cut off Si(100) AlN/GaN 0.8 μm
N. C. Chen et.al, APL, 88,191110 (2006) MOCVD Si(111) TiN/AlN/AlGaN 0.4 μm
J. C. Gagnon et.al, JCG, 393, 98 (2014) MOCVD Si(111) AlN 1.0 μm
C. Mo et.al, JCG, 285, 312 (2005). MOCVD Si(111) AlN/ Ga-rich GaN 0.4 μm
S. Tripathy et.al, APL, 101, 082110 (2012) MOCVD Si(111) AlN/AlGaN 1.0 μm K.Radhakrishnan et.al, APL, 97, 232107 (2010)
PA-MBE Si(111) AlN 1.5 μm
X. Zhang et.al, APL, 74, 1984 (1999) MOCVD Si(100) a-Si/GaN/AlGaN 1.0 μm
J. Wan et.al, APL, 79, 1459 (2001) MOCVD Si(100) Sputtered AlN 0.4 μm
Our study PLD Si(100)/Si(111) None 4.0 μm
Since the phase of the deposited GaN on Si is depended on
the conformation of the used Si, the growth of thicker GaN
film (> 2 mm) on Si without crack will be a challenge.
Recent researches of GaN-on-Si growth
17
Review study: melt-back etching in GaN-on-Si
In typical GaN-on-Si by MOCVD without any interlayer, the Ga-Si meltback
etching usually occurred, creating a Ga-Si material during the GaN growth.
J. Crystal Growth, vol. 189-190, pp. 178-182,1998
18
GaN-on-Si(100) and GaN-on-Si(111)
What are the limitations of GaN-on-Si(111) devices?
1. For GaN-on-Si(111) LEDs:
2. For GaN-on-Si(111) HEMTs:
Integration of GaN HEMTs with advanced Si electronics is difficult.
Why hexagonal-GaN is so hard to be grown on Si(100)?
1. Hexagonal GaN is match to three-fold symmetry of Si(111)
2. Cubic GaN is more match to four-fold symmetry of Si(100)
J. Micromech. Microeng., 7, 137-140, 1997.
Removal of the Si(111)
substrate is 37 times slower
etching rate than of Si(100).
19
XRD of GaN-on-Si
2Theta (degree)
The growth principle of PLD
N2 plasma incorporation
The formation of stacking faults
Why there is no cubic GaN crystal ?
GaN-on-Si(111)
(J. Appl. Phys., vol.83, pp. 3800, 1998)
(Opt. Express, 20(14), pp.
15149–15156, 2012)
(J. Nanosci. Nanotechnol., vol. 7, pp.
2719–2725, 2007)
GaN-on-Si(100)
20
10 mins
2 hrs 4 hrs
1 hr
Hexagonal
grain
(e) 4 hrs cross section
After the GaN growth for 4 hours, the film surface became smooth.
It was not found the melt-back etching in the SEM images.
Time evolution of GaN grown on Si(100)
21
Time evolution of GaN grown on Si(111)
(e) 4 hrs cross section 10 mins (a) 1 hr (b)
2 hrs (c) 4 hrs (d)
Due to the good lattice match between GaN and Si(111), the
coalescence of GaN-on-Si(111) is faster than that of GaN-on-Si(100).
22
TEM observation of GaN-on-Si(100)
Meanwhile, the GaN-2 has two crystal directions of [0002] and [11-20].
The GaN-1 possesses two crystal directions of [0001] and [10-10].
23
TEM observation of GaN-on-Si(111)
The GaN-1 grains were not found in GaN-on-Si(111).
The GaN-2 has two crystal directions of [0002] and [11-20].
24
Mechanism of PLD GaN-on-Si growth
In GaN-on-Si(100), the GaN-2 formation dominated this growth, which was
attributed to the lateral growth rate of GaN-2 in the [11-20] direction being
faster than that of GaN-1 in the [10-10] direction.
Because the lattice match made the grains stay in a stable status, only the
GaN-2 grains were detected in the GaN-on-Si(111).
25
PL spectrum and XRD rocking curve
* The peak PL profile of the templates is stronger and sharper with increasing growth
time, and it is accompanied by a shift in peak position from 360 to 365 nm.
* The decrease of FWHM in XRD profile represented an increase of GaN grain size
according to Scherrer formula.
26
GaN-based films by PLD
High-indium-content InGaN films with high
thermal stability
Source: OPTICAL EXPRESS Vol. 20, pp. 21173-21180, 2012
Vol. 20, pp. 15149-15156, 2012
27
Issues of InGaN growth
Large difference in interatomic spacing between InN
and GaN.
Although the crystalline property was improved with
an increase of annealing temperature, In content in
InGaN was decreased due to the In lost from surface
via inward diffusion and some surface evaporation.
Drawbacks of InGaN film growth by MOCVD
Phase separation
Composition inhomogeneity
Indium droplets
Poor crystalline property
Advantages of PLD system
Growth temperature from 25 -1000 oC
Evaporation atomic contained high energy
Thickness of film controlled by laser pulse
Growth rate controlled by repetition rate
Improvement method for MOCVD
High V/III flux ratio
Low growth rate
Low growth temperature
Low growth pressure
28
Indium content: 33%
Sapphire
InGaN
Surface morphologies of InxGa1-xN
2 min
Deposition times of 2, 10,
30, 60 and 300 min
10 min 30 min
60 min 300 min
Initially, only a few InN nanoparticles are deposited on the sapphire surface for 2 min,
and the film was composed at this time of two distinct grains with few vacancies as the
deposition time increased from 10 to 30 min.
The grains of the film have merged with smooth surface for deposition time of 300 min.
When InN and InGaN grains are distributed on the sapphire surface, some grains act as
nucleation sites, facilitating further thin film growth.
29
XRD and roughness of InxGa1-xN
Indium content: 33%
Sapphire
InGaN
Deposition times of 2, 10,
30, 60 and 300 min
(a) XRD patterns of the InGaN film following 2, 10, 30, 60, and 300 min of deposition.
(b) The ratio of the integrated intensities of the InGaN peak to the InN peak and the RMS roughness as a function
of deposition time.
When a pulse from the ablation laser impacted the PLD target, indium vapor was
generated and allowed to react with the nitrogen plasma, resulting in the
formation of InN on the sapphire substrate.
Following 300 min deposition time, the sample roughness decreased and the ratio
of the XRD peak intensities achieved a 112% enhancement over sample measured
at 10 min, which indicates that the growth mode had completely transferred to a
layer-by-layer process.
30
InN
InNInGaN
void
2 min deposition time
10 min deposition time
30 min deposition time
300 min deposition time
Only InN deposited
InGaN involved
InGaN reaction dominated
Layer-by-layer growth
Sapphire
The co-deposition behavior and the growth mode gradually
transfers from island growth to layer growth with increasing
deposition time.
InxGa1-xN co-deposition behavior on sapphire
31
XRD of InxGa1-xN
XRD peak of InGaN from 33.43o shifted to 32.51o with F factor decreased from 68.2%
to 36.4%.
Indium content of the film could be modulated through controlling the concentration
of both the indium and GaN vapor, which in turn is controlled by the composition of
the dual-compositing target.
The intensities of the InGaN XRD peak increases upon annealing, due to Indium
produced from the decomposition of InN within the InGaN films tends to react with
surrounding InGaN grains to yield InGaN film.
Indium content: 33-62%
Sapphire
InGaN
32
Electrical properties and roughness of InxGa1-xN
Mobility increases from 25.5 to 73.8 cm2/V·s, then decreases to 22.4 cm2/V·s as the
indium content increases from 33% to 62%.
The trend in mobility was attributed to the low effective electron mass of the InGaN
films and the larger lattice mismatch between the film and the sapphire substrate.
Root-mean-squared roughness of annealed InGaN films with Indium content of 33-
62% was 1.43, 1.4, 1.36 and 4.42 nm, respectively.
Indium content: 33-62%
Sapphire
InGaN
33
Sapphire
InGaN
u-GaN
PLD
MOCVD
Indium content: 33 and 60%
Annealed at 500oC-800oC
for 15 min in N2 ambient
(vacuum system)
XRD TEM
AES AFM
Effects of vacuum annealing on In content of InGaN
34
Crystallinity of InGaN before and after annealing
In: 33%
Full width at half maximum (FWHM) of the InGaN peaks with indium content of 33%
and 60% were 0.31o and 0.39o, respectively.
Weak InN peak of both indium contents were gradually decomposed with the
annealing temperature increased to 800oC, and no indium droplets were discernible in
the XRD patterns.
Sapphire
InGaN
u-GaN
Indium content: 33 and 60%
In: 60%
35
High resolution cross-sectional TEM images
D-spacing of InGaN with 33 and 60% indium
before and after annealing are 2.678 Å and
2.751 Å , which correspond to the In content of
33% and 60% in XRD results.
The existence of nanoscale InN alloy indicates
that the post-annealing time was too short to
fully decompose InN.
The varying InN orientations embedded within
InGaN creates mixture polarity interfaces
between InGaN and InN and causes different
decomposition rates for each InN domain.
Particularly, a number of stacking faults were
found near the interface between InN domain
and InGaN, which enhances the activation
energy of interface and maintains the structural
stability.
Sapphire
InGaN
u-GaN
Indium content: 33 and 60%
In-33% as-dep. In-60% as-dep.
In-33%-800oC In-60%-800oC
36
InN dissociation caused a small variation in the indium concentration near the InGaN/
GaN interface.
In contrast to InGaN with 33% Indium, a clear decrease in indium concentration was
found in 60% indium.
Root-mean-squared of indium content of 33 and 60% were increased from 1.35 nm to
3.34 nm and 4.3 to 7.24 nm, respectively.
Auger spectra of InGaN with various In contents
Sapphire
InGaN
u-GaN
Indium content: 33 and 60% In-33% as-dep. In-33%-600oC In-33%-800oC
In-60% as-dep. In-60%-600oC In-60%-800oC
37
The growth mechanism and crystal quality of the PLD GaN-on-Si
template was verified through the XRD, SEM, and TEM
measurements.
The growth mode of GaN-on-Si gradually changed from island growth
to layer growth as the growth thickness exceeded 2 mm. With
increasing the GaN thickness to 4 mm, a smooth surface was formed
via the full coalescence of GaN grains.
We have demonstrated the fabrication of InGaN films with indium
concentration of 33, 39, 49 and 62% by low-temperature PLD using a
controllable InGaN target.
The high thermal stability of InGaN films with indium contents of 33
and 60% was demonstrated through the measurements of structural
and optical characteristics after high-temperature annealing.
Summary
38
Al-doped ZnO films by PLD
Al-doped ZnO (AZO) TCLs for InGaN light emitters
Source: OPTICAL EXPRESS Vol. 19, pp.16244-16251, 2011
39
Comparison of various TCLs for LEDs
ITO (200 nm) AZO (200 nm) Ni/Au (5nm/5nm)
Transmission (400~700 nm) > 80 % > 90 % < 70 %
Resistivity (Ω-cm) 10-4 10-4 10-6
Mobility (cm2/Vs) 30~40 30~40 ~160
Carrier concentration (cm-3) ~1021 ~1021 ~1023
Cost High Low High
Resource Indium
seldom Abundant Gold seldom
Toxic Yes Non Non
Thermal stability Medium Excellent Medium
In our study, AZO films with good transmittance (>90%) form UV (365 nm) to
visible region and low resistivity were achieved by PLD.
The AZO films were applied for the transparent contact layers (TCLs) of GaN-
based blue LEDs to improve the light extraction.
40
XRD of AZO films
For AZO films grown in Ar atmosphere, compared with the (002) peak of pure ZnO
(34.45), a little shift of AZO (002) to the higher angle is observed. This could be
attributed that the length of the c-axis is expected to be shorter when the Al atoms
were substituted into the Zn site in the crystal.
All the (002) peaks of AZO films grown in O2 ambient were observed at
approximately 34.48, indicating that the crystal structures of AZO films grown with
abundant O2 atmospheres were similar to the ZnO films.
20 30 40 50 60 70 80
Sapphire(006)
AZO(004)
700 oC
500 oC
300 oC
100 oC
Inte
ns
ity
(a
.u.) AZO(002)
Two theta (degree)
20 30 40 50 60 70 80
Sapphire(006)
AZO(004)
700 oC
500 oC
300 oC
100 oC
Inte
ns
ity
(a
.u.)
Two theta (degree)
AZO(002)
AZO grown in Ar atmosphere AZO grown in O2 atmosphere
41
Electrical properties of AZO films
Optimum electrical properties occurred at the 100C-grown AZO film (Ar
atmosphere), which had a lowest resistivity (2.33 × 10-4 Ω-cm) and a highest
mobility (38.8 cm2/V-s).
Based on the XPS results, the 100C-grown AZO film (Ar atmosphere)
possessed a higher Zn/O ratio of 2.03.
AZO film with a higher Zn/O ratio has the low resistivity and high carrier
concentration due to the Zn interstitials and oxygen vacancies.
100 200 300 400 500 600 7000
2
4
6
8
10
10-4
10-3
10-2
10-1
100
15
20
25
30
35
40
Concentration
Re
sis
tivity
(-c
m)M
ob
ilit
y (
cm
2/V
s)
Ca
rrie
r c
on
ce
ntr
ati
on
(1
02
0 c
m-3)
Substrate temperatur (oC)
Resistivity
Mobility
Ar atmosphere
100 200 300 400 500 600 70010
0
101
102
10-3
10-2
10-1
100
5
10
15
20
25
30
Mo
bil
ity
(c
m2/V
s) R
es
istiv
ity (
-cm
)
Concentration
Ca
rrie
r c
on
ce
ntr
ati
on
(1
01
9 c
m-3)
Substrate temperature (oC)
Resistivity
O2 atmosphere
Mobility
42
Transmittance and bandgap of AZO
200 400 600 800 10000
20
40
60
80
100
Tra
ns
mit
tan
ce
(%
)
Wavelength (nm)
100 oC
300 oC
500 oC
700 oC
Sapphire
AZO-200 nm
2.0 2.5 3.0 3.5 4.0 4.5 5.00
2
4
6
8
10
100 200 300 400 500 600 7003.40
3.45
3.50
3.55
3.60
3.65
3.70
3.75
3.80
3.85
Op
tical b
an
dg
ap
(eV
)
Substrate temperature (oC)
2 (
10
9 c
m-2
)
Photon Energy (eV)
100 oC
300 oC
500 oC
700 oC
All films exhibit high transmittance spectra of 92.7-99.2% in the
range of visible wavelength (400-700 nm).
At the substrate temperature of 100 C, the transmittance of AZO film
can reach 69% and 91% at the wavelength of 325 nm and 365 nm,
respectively.
The band gap of AZO films decreased from 3.8 to 3.5 eV as the
substrate temperature increased from 100 to 700C.
AZO grown in Ar atmosphere
43
Device process of lateral-type GaN-based LED
Sapphire
p-GaN
u-GaN
n-GaN
MQW
Sapphire
p-GaN
u-GaN
n-GaN
MQW
PR
Sapphire
p-GaN
u-GaN
n-GaN
MQW
PR
Sapphire
p-GaN
u-GaN
n-GaN
MQW
Sapphire
p-GaN
u-GaN
n-GaN
MQW
AZO
Sapphire
p-GaN
u-GaN
n-GaN
MQW
PR AZO
Sapphire
p-GaN
u-GaN
n-GaN
MQW
AZO
PR
Sapphire
p-GaN
u-GaN
n-GaN
MQW
AZO
PR
Sapphire
p-GaN
u-GaN
n-GaN
MQW
AZO
MESA
PLD-AZO
ITO
Deposition
TCL
TCL and Annealing
HF Etching AZO
Thermal
Cr/Au PAD
The 100 C-grown AZO film (Ar atmosphere) was
used as the transparent conducting layer
44
Characteristics of ITO/AZO transparent
conducting layers
200 400 600 800 10000
20
40
60
80
100
Wavelength (nm)
Tra
ns
mit
tan
ce
(%
) ITO (200 nm)
ITO (1000 nm)
ITO/AZO (200 nm)
ITO/AZO (460 nm)
ITO/AZO (1000 nm)
Various thicknesses of
ITO/AZO films
Transmittance at
465 nm (%)
Sheet resistance
(Ω/)
Contact
resistance (Ω-cm2)
ITO (50 nm)/AZO (200 nm) 94 28.1 1.33
ITO (50 nm)/AZO (460 nm) 96 10.8 3.00×10-1
ITO (50 nm)/AZO (1000 nm) 90 3.5 1.32×10-3
45
I-V characteristic & output power
The turn-on voltages of LEDs with ITO/AZO (200-1000 nm) TCLs are just a little higher than that with ITO (200 nm) TCL, indicating that the thin ITO film with thickness of 50 nm plays a role of ohmic contact layer successfully.
The output powers for the LEDs fabricated with ITO/AZO (200 nm), ITO/AZO (460 nm) and ITO/AZO (1000 nm) TCLs had 45%, 63%, and 71% enhancement compared to that fabricated with ITO (200 nm) TCL at a 20 mA operating current, respectively.
46
The thicker AZO window layer can
improve the light extraction by allowing
additional light to escape through the side
facets.
Trace-Pro simulation results
47
Photographs of LEDs with various TCLs
The brightness of LED increased with increasing the AZO thickness, especially as the AZO thickness was 1000 nm.
Chip Size : 12 mil x 24 mil ; Wavelength : 465 nm
48
Co-doped ZnO films by PLD
Diluted magnetic cobalt-doped ZnO (CZO)
as electron deceleration electrodes
for InGaN light emitters
Source: APPLIED PHYSICS LETTERS Vol. 109, 021110, 2016
49
Dilute magnetic CZO films
Among various magnetic elements (Fe, Co, Mn, etc) doped into ZnO, Co metal
is a promising material since the Co-doped sample can exhibit a remarkable
magnetization per Co ion for very low substitutions.
Most researches of DMSs focused on improvement in magnetic characteristic.
The TCO-related properties such as resistivity and transmittance are usually
neglected.
PLD-grown cobalt-doped ZnO (CZO) films proposed in our work possess good
magnetic characteristic, excellent transmittance and low electrical resistivity.
The CZO film was grown on the n-GaN layer to serve as a n-electrode of InGaN
light emitters to enhance the device performance.
Incorporation of CZO
film into LED structure Conventional LED
50
Experimental: CZO growth by PLD
Fixed parameters
(1) Laser source: KrF laser 248 nm
(2) Substrate: sapphire
(3) Laser energy: 1 J/cm2
(4) Frequency: 2 (Hz) pulse/ sec
(5) Base pressure: ~ 1 × 10-6 torr
(6) Working pressure: 1 × 10-3 torr
(7) Film thickness: 120 nm
(8) Target: 95 at.% ZnO + 5 at.% Co
Independent variables
(1) Gas atmosphere：Ar 15 sccm or
O2 15 sccm or Ar/O2 15 sccm
(2) Substrate temperature (Ts):
100~700 C
51
Experimental: device process of LED
Sapphire
u-GaN
n-GaN
p-GaN
MQWs
(a)
Sapphire
u-GaN
n-GaN
p-GaN
MQWs
ITO
(b)
Sapphire
u-GaN
n-GaN
p-GaN
MQWs
ITO
(c)
Sapphire
u-GaN
n-GaN
p-GaN
MQWs
ITO
CZO
(d)
Sapphire
u-GaN
n-GaN
p-GaN
MQWs
ITO
pad
pad
CZO
(e)
Device process:
(a) Epitaxial growth by MOCVD
(b) Deposition of ITO TCL
(c) Preparation of mesa
(d) Growth of CZO on n-GaN
(e) Preparation of Ti/Au pads
45 mil × 45 mil
52
XRD of CZO films
20 30 40 50 60 70 80
100C
Two theta (degree)In
ten
sit
y (
a.u
.)
700C
600C
500C
400C
300C
200C
Sapphire(006)CZO(002) CZO(004)
CZO grown in Ar atmosphere
20 30 40 50 60 70 80
100C
700C
600C
500C
400C
300C
200C
Inte
ns
ity
(a
.u.)
Two theta (degree)
CZO(002) Sapphire(006) CZO(004)
CZO grown in O2 atmosphere
All samples present the CZO(002), CZO(004) and sapphire(006) diffraction
peaks, and there is no peak with other planes in CZO films, indicating these
films possess a single crystalline phase.
The CZO(002)-family peaks existed in the films can be preferred as stacking
along (001) plane with the lowest surface free energy on sapphire surface.
53
Ts
(Ar atmosphere)
FWHM
(degree)
d
(Å )
Co dopant
(at.%)
Ts
(O2 atmosphere)
FWHM
(degree)
d
(Å )
Co dopant
(at.%)
100 C 2.870 2.620 1.8 100 C 3.860 2.627 1.4
200 C 1.861 2.613 2.5 200 C 3.490 2.617 1.9
300 C 1.358 2.607 2.5 300 C 3.070 2.610 2.0
400 C 0.926 2.604 2.6 400 C 0.700 2.603 2.9
500 C 0.464 2.602 3.1 500 C 0.462 2.604 3.0
600 C 0.381 2.603 4.9 600 C 0.340 2.605 2.9
700 C 0.298 2.602 4.9 700 C 0.206 2.605 3.3
Crystal and compositional properties
With increasing the Ts from 100 to 700 C, the crystal quality of PLD-CZO
film was improved, and the d-spacing value of CZO(002) plane was reduced
gradually to that of bulk material.
Based on the XPS results, the Co concentration increased gradually when
the Ts was increased.
54
(a)Ar
100C
200C
300C
400C
500C
600C
700C
oxygen vacancy
350 400 450 500 550 600
(b) O2 100C
200C
300C
400C
500C
600C
700C
Wavelength (nm)
PL
in
ten
sit
y (
a.u
.)
PL characteristics of CZO films
Two dominant emission bands in the
ultraviolet (~400 nm) and green (~525 nm)
regions were found in the PL spectra.
The ultraviolet emission is the exciton
recombination associated with near-
band edge emission of ZnO material.
The intense emission at around 525 nm
is because of the defects related to the
oxygen vacancy with 0 charge state (VO0).
With increasing Ts to 400 C, the CZO
films prepared in Ar and O2 atmospheres
both possessed a larger amount of VO0
than that of the other films.
55
Analysis of electrical resistivity – 1
100 200 300 400 500 600 700
0.0
0.5
1.0
1.5
2.0
2.5
Re
sis
tiv
ity
(o
hm
-cm
)
Temperature (C)
in argon
in oxygen
The lowest resistivities of CZO films prepared in Ar and O2
atmospheres both occurred at the Ts of 400C. The results
are in good agreement with the PL spectra, which reveal
that the 400C-grown CZO films have more VO0.
56
Analysis of electrical resistivity – 2
0.0 0.2 0.4 0.6 0.8 1.0
0.04
0.08
0.12
0.16
0.20
Re
sis
tiv
ity
(o
hm
-cm
)
[Ar/(Ar+O2)] (flow ratio)
Sub. temp.: 400 C
Total flow: 15 sccm
The CZO film prepared in pure Ar atmosphere possessed the lowest electrical resistivity
than that in the other atmospheres. In addition, the resistivity increased gradually with
decreasing the [Ar/(Ar+O2)] flow ratio.
Especially, when the pure O2 atmosphere was used, there was a significant increment in
the resistivity of CZO film. This is due to the higher formation energy of VO0 for the ZnO-
based films deposited in O2-rich atmosphere.
The CZO films grown in pure Ar atmosphere are highly potential in optoelectronic
applications. Then the microstructures, transmittances, and magnetic properties of the
CZO films deposited in pure Ar atmosphere were analyzed in detail.
57
Cross-sectional TEM observation – 1
The interfaces of CZO/sapphire in these two samples can be clearly identified.
Based on our observation, the 100 C-grown CZO film exhibited a dense
columnar structure.
When the Ts was increased, the columnar structure was transformed to
featureless structure, which was similar to the 400 C-grown CZO film.
58
300 450 600 750 9000
20
40
60
80
100
Tra
ns
mit
tan
ce
(%
)
Wavelength (nm)
100C
200C
300C
400C
500C
600C
700C
=567 611 657 nm
200 nm
Ts：400 oC, Ar
sapphire
CZO (120 nm)
Optical transmittance of CZO films
Three absorption peaks at 567, 611 and 657 nm were ascribed to 4A2(F)→2A1(G), 4A2(F)→4T1(P) and 4A2(F)→2E(G) transitions, respectively, resulting from the crystal-field
transitions in the high spin state of Co2+ (3d7) at tetrahedral sites. The results confirmed
that Co existed in the ZnO lattice of the wurtzite structure when Co2+ ions were well
substituted for Zn2+ at the tetrahedral sites.
At the visible region, the transmittance of CZO film increased with increasing the Ts. The
optical transmittances of CZO films can reach 73.4%, 78.4%, 80.9%, 84.1%, 88.6%, 91.1%
and 95.8% at blue wavelength (450 nm) as the Ts is increased to 100, 200, 300, 400, 500,
600 and 700C, respectively.
59
-10000 -5000 0 5000 10000-6
-4
-2
0
2
4
6
Ar
Ma
gn
eti
za
tio
n (
em
u)
10
-5
Sub. temp.
100C
400C
700C
Magnetic field (Oe)
CZO (120 nm thick)
Magnetic characteristics of CZO films
The saturation magnetization (Ms) values of the CZO films prepared at 100, 400 and 700
C are determined to be 2.74 × 10–5, 3.37 × 10–5 and 5.33 × 10–5 emu, respectively.
CoO and Co3O4 possess antiferromagnetic and paramagnetic behaviors, respectively. If
CoO or Co3O4 phase was generated in the samples, the variation trend of Ms of CZO films
with rising the Ts could become irregular. Consequently, the fact that no secondary phase
of Co-oxide formed in the CZO films can be verified again.
60
Comparisons with other researches Deposition
method Structure Co (at.%) T (%) ρ Ms Ref.
Sol–gel method Nanoparticle 3 at.% -- -- 8 × 10–6
(emu) 1
Spin coating Nanostructure
film 5 at.%
80%
(@450 nm)
9 × 105
(KΩ/sqr) -- 2
Sputtering Polycrystalline
film 1~5 at.%
70%
(@450 nm)
0.01~0.05
(Ω-cm) -- 3
Ultrasonic
spray
Polycrystalline
film 2 wt.%
55-84%
(@450 nm)
0.148
(Ω-cm) -- 4
PLD Single
crystalline film 0~5 at.%
68~84%
(@450 nm) --
0.26~0.4
(μB/Co) 5
PLD Single
crystalline film 5 at.%
75-85%
(@475 nm)
0.313
(Ω-cm) 0.27 (μB/Co) 6
PLD Single
crystalline film 1~5 at.%
84~95%
(@450 nm)
0.043
(Ω-cm)
0.91
(μB/Co)
Our
study
1. F. Ahmed et.al , Microelectron. Eng. 89, 129–131 (2012).
2. H. Gu et.al , Appl. Phys. Lett. 100, 202401 (2012).
3. L.E. Mir et.al , Thin Solid Films 517, 6007–6011 (2009).
4. S. Benramache et.al , Superlattices Microstruct. 52, 807–815 (2012).
5. S. Yang et.al , J. Alloys. Compd. 579, 628–632 (2013).
6. L. Zhang et.al , J. Alloy. Compd. 509, 2149–2153 (2011).
P.S.: The Ms value of 3.37 × 10–5 emu for the 400 C-grown CZO film can be transferred to 0.91 μB/Co.
The PLD-CZO films proposed in this study possess lower electrical resistivity,
higher mobility, good magnetic characteristic and excellent transmittance.
61
I-V characteristic & output power (blue LED)
Based on the above-mentioned results,
the CZO film deposited at 400 C in Ar
atmosphere was chosen to serve as the
n-electrode of blue LED structure.
The forward voltages at various
currents of these two LEDs are similar
to each other, indicating an addition of
the CZO film does not influence the
electrical properties of the LED.
When an injection current of 350 mA
was applied, the output powers of the
blue LEDs with and without a CZO film
on the n-GaN layer were measured to
be 246.7 and 212.9 mW, respectively. In
comparison to the conventional blue
LED, there was 15.9% enhancement in
the output power (@350 mA).
At an injection current of 350 mA, the
EQE droops of the blue LEDs with and
without a CZO film on the n-GaN layer
were 31.0% and 35.8%, respectively.
0 100 200 300 400 5000
100
200
300
0
1
2
3
4
5
(b)
(a)
Ou
tpu
t p
ow
er
(mW
)
Current (mA)
Blue LED W/O CZO
Blue LED with CZO
Vo
lta
ge
(V
)
15
20
25
30
35
40
EQ
E (
%)
400 450 500
@350 mA
Inte
ns
ity
(a
.u.)
Wavelength (nm)
62
I-V characteristic & output power (green LED)
The CZO film deposited at 400 C in Ar
atmosphere was also chosen to to
serve as the n-electrode of green LED
structure.
The forward voltages (@20-500 mA) of
these two green LEDs were almost the
same to each other.
The output powers (@350 mA) of green
LEDs with and without inserting the
CZO film were 103.1 and 87.6 mW,
respectively. The output power (@350
mA) of the green LED inserted with the
CZO film showed 17.7% improvement
as compared with that of the
conventional green LED.
At an injection current of 350 mA, the
EQE droops of the green LEDs with and
without a CZO film on the n-GaN layer
were 55.6% and 57.8%, respectively.
0 100 200 300 400 5000
50
100
0
1
2
3
4
(b)
Ou
tpu
t p
ow
er
(mW
)
Current (mA)
(a)
Green LED W/O CZO
Green LED with CZO
Vo
lta
ge
(V
)
5
10
15
20
25
30
EQ
E (
%)
450 500 550 600
@350 mA
Inte
ns
ity
(a
.u.)
Wavelength (nm)
63
Mechanism of CZO/n-GaN on LED – 1
Hall measurement for n-GaN Hall measurement for TCO/n-GaN
The mobility of n-GaN layer was 176 cm2/V-s. By adding the 120-nm-thick ITO, ZnO, 400
C-grown CZO, and 700 C-grown CZO films on the n-GaN layers, their mobilities can be
reduced to 170, 160, 141, and 155 cm2/V-s , respectively.
Obviously, the efficient reduction in the electron mobility can both occur in the patterned-
CZO films (Ts: 400 and 700 °C) on n-GaN, revealing the doping of magnetic Co atoms into
ZnO film is helpful to reduce the electron mobility of patterned-TCO/n-GaN.
Sample
(120-nm-thick)
Mobility
(cm2/V-S)
Concentration
(/cm3)
ITO 24.5 1.21×1021
ZnO 33.2 1.79×1020
CZO (Ts=400 C) 24.7 1.01×1019
CZO (Ts=700 C) 27.7 6.73×1018
Sample Mobility
(cm2/V-S)
n-GaN 176
ITO/ n-GaN 170
ZnO/ n-GaN 160
CZO (Ts=400 C)/ n-GaN 141
CZO (Ts=700 C)/ n-GaN 155
In ballSapphire
n-GaN
Sapphire
n-GaN
TCO
In ball
64
Actually, the CZO films possess homogeneous
microstructure, and the direction of magnetic
field formed in the films is random.
As the CZO film was deposited on the n-GaN
layer, the electrons were scattered via the spin-
orbit interaction of Co2+ ions, causing the
reduction in the mobility of electron carrier.
The 400 C-grown CZO can reduce the mobility
of TCO/n-GaN more efficiently than the other
TCO films. However, its electrical and magnetic
properties are not both the best among these
films.
This could be attributed that the 400C-grown
CZO had the lowest electrical resistivity (4.3 ×
10–2 Ω-cm) in comparison to the other CZO films.
Mechanism of CZO/n-GaN on LED – 2
100 200 300 400 500 600 700
140
150
160
170
180
5
10
15
Patterned-CZO/n-GaN/sapphire
Mo
bil
ity
(c
m2/V
s)
Substrate temperature (C)
n-GaN=176
CZO/sapphire(a)
Re
sis
tiv
ity
1
0-2
(o
hm
-cm
)
(b)
65
Mechanism of CZO/n-GaN on LED – 3
As the CZO was deposited at 400 C, it had a better conductivity than that of
700C-grown film. Thus, when the light emitter was operated, a more uniform
current injection can be achieved in the 400C-grown CZO film, and there is a
higher probability of encountering between electrons and Co ions.
By growing the 400C-grown CZO film on the n-GaN layer, more electrons
can be scattered because of the collisions between the magnetic atoms and
the electrons as the device is driven, leading to the efficient reduction of the
electron mobility.
n-pad
20-500 mACo
2+
CZO
400 °C-grown CZO film(a) (b) 700 °C-grown CZO film
ρ = 4.3 x 10-2
(Ω-cm) ρ = 1.5 x10-1
(Ω-cm)
66
Mechanism of CZO/n-GaN on LED – 4
Formula of carrier recombination rate in the LED device:
Factors of non-recombination rate:
According to the formula, when the large difference between electron and hole mobilities
occurs in the device, the CDL and DDL will be increased, leading to an increment in the non-
recombination rate and a degration in the luminous performance of LED device.
In the conventional GaN-based LED, the electron mobility is much higher than the hole
mobility. By introducing the 400 C-grown CZO film into the LED structure, the electron
mobility can be decreased significantly, resulting in the reduction of the difference between
electron and hole mobilities. Thus, the non-recombination rate is reduced, and the luminous
performance of the LED can be improved.
67
The 200-nm-thick PLD-AZO film deposited at 100 C in Ar atmosphere
shows the lowest resistivity of 2.33 × 10-4 Ω-cm and high transmittance
in both visible and UV regions.
Compared to the conventional LED with ITO (200 nm) TCL, the light
output power of the LEDs fabricated with ITO/AZO TCLs can be
improved, especially as the thickness of AZO layer is 1000 nm.
The PLD-CZO films prepared in Ar atmosphere have the lower
resistivity in comparison to those prepared in O2 and Ar/O2-mixed gas
atmospheres.
The growth of CZO n-electrode on n-GaN would lead to the reduction
of electron mobility, the decrease in the mobility difference between
electron and hole carriers, the increment of carrier recombination rate,
and the improvement of optoelectronic performance for the light
emitters.
Summary
68
Ga2O3 films grown by PLD
High performance solar-blind Ga2O3 photodetectors
Source: OPTICAL MATERIALS EXPRESS Vol. 5, pp.1240-1249, 2015
69
X-ray diffraction of Ga2O3 films
20 30 40 50 60
1000 °C
-Ga2O
3 phase
(-801)
(-603)(-402)
800 °C
600 °C
Inte
ns
ity
(a
.u.)
Two theta (degree)
400 °C
(-201)(400)
Al2O
3(0006)
-2000 -1000 0 1000 2000
-Ga2O
3 phase
(-201) plane
402 arcsec1000 C
359 arcsec800 C
516 arcsec600 C
Inte
ns
ity
(a
.u.)
Omega (arcsec)
By increasing the substrate temperature to 600 and 800C, the XRD patterns presented
three peaks indexed to the (–201) plane family. These peaks were associated to the β-
Ga2O3 phase.
When the substrate was heated to 1000C, the other diffraction peaks indexed to (400)
and (–801) planes of β-Ga2O3 phase were generated, indicating that this film had a
polycrystalline nature.
Based on the rocking curve at (–201) plane, the 800C-grown film possesses higher
crystal quality than the others.
70
200 400 600 800
Tra
ns
mit
tan
ce
(%
)
1000 C
800 C
600 C
Binding Energy(eV)
400 C
Wavelength (nm)
0
20
40
60
80
100
Transmittance of Ga2O3 films
Ga2O3 films deposited at 600, 800, and 1000C show an obvious absorption edge at the
DUV region near a wavelength of 250 nm.
The 400C-grown film possessed a relatively lower transmittance (<30%) in the measured
wavelength range. This is owing to the amorphous structure within the 400C-grown film.
71
Rutherford backscattering spectroscopy
Substrate
temperature Ga (%) O (%) O/Ga
400 C 46.08 53.92 1.17
600 C 43.67 56.33 1.29
800 C 40.65 59.35 1.46
1000 C 40.32 59.68 1.48
The O/Ga ratios of these four films grown at 400, 600, 800, and 1000C were
determined to be 1.17, 1.29, 1.46, and 1.48, respectively. Apparently, the O/Ga ratio
increases with increasing substrate temperature, which indicates that the composition of
gallium oxide film is close to the formation of Ga2O3, especially for the samples prepared
at the substrate temperatures of 800 and 1000C.
With increasing the growth temperature, the amount of oxygen vacancy reduced
gradually.
72
SEM and TEM observations of Ga2O3 films
The d-spacings of the films prepared at 800 and 1000C were evaluated to be 4.62
and 4.65 Å , respectively, which corresponded to the β-Ga2O3 (–201).
Because of the similar oxygen atom arrangements on a c-plane sapphire substrate
and a β-Ga2O3 (–201) plane, the gallium oxide films mainly consisted of (–201)-
oriented planes.
Within the 1000C-grown film, the other d-spacing of 1.52 Å indexed to β-Ga2O3 (–
801) plane was also found, which agreed well with the XRD result.
73
Dark current of Ga2O3 MSM PD
0 10 20 30 40 50 60 70 8010
-12
10-11
10-10
10-9
10-8
10-7
10-6
Bias voltage (V)
D
ark
cu
rre
nt
(A)
PD fabricated with 800 C-grown gallium oxide
PD fabricated with 600 C-grown gallium oxide
When the substrate temperature was increased to 800C, the crystal quality of the gallium
oxide film in comparison to that deposited at 600 C was obviously enhanced.
The 800C-grown film had much fewer oxygen vacancies than that of 600C-grown film.
The oxygen vacancies in the gallium oxide film would result in many free electrons from
gallium atoms.
The 800C-grown film had lower leakage current than that of 600C-grown film. At an
applied bias of 5 V, the measured dark currents of these two devices fabricated with 600C-
and 800C-grown films were 3.9 × 10-10 and 1.2 × 10-11 A, respectively.
74
Photoresponsivity of Ga2O3 MSM PD
240 260 280 300 320 340 360
10-4
10-3
10-2
10-1
100
R
es
po
ns
ivit
y (
A/W
)
Wavelength (nm)
PD fabricated with 600 C-grown gallium oxide
1V
2V
3V
4V
5V
240 260 280 300 320 340 36010
-5
10-4
10-3
10-2
10-1
100
101
Wavelength (nm)
Re
sp
on
siv
ity
(A
/W)
PD fabricated with 800 C-grown gallium oxide
1V
2V
3V
4V
5V
These two devices both exhibited a maximum responsivity around 250 nm, which
confirmed that the gallium oxide PDs were really solar-blind.
Under a bias voltage of 5 V, the peak responsivity of the device with 600 C-grown film
was 0.359 A/W, and the contrast ratio between 250 and 350 nm was 833. When the
MSM PD was prepared with 800 C-grown film, its peak responsivity (@5 V) and
contrast ratio were 0.903 A/W and 7867, respectively.
The higher responsivity and larger contrast ratio of the latter device can be attributed to
the better crystal quality and fewer O vacancies in the 800C-grown film.
75
Time-dependent response of Ga2O3 MSM PD
0 100 200 300 400 500 600 70010
-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Biased at 5VUV off
UV on
PD fabricated with 800 C-grown gallium oxide
Cu
rre
nt
(A)
Time (sec)
Time-dependent response of the MSM PD fabricated with the 800C-grown gallium oxide
film biased at 5 V as the 254 nm exciting light was switched on and off.
The dark current was around of 1 × 10-11 A, and the current increased instantaneously to a
stable value of approximately 1.8 × 10-6 A under 254 nm illumination. The on/off current
contrast ratio was about 105.
By turning off the exciting light, a relatively slow response occurred in the device. This slight
decay in response was probably ascribed to the oxygen-related hole-trap states generated
at the surface of gallium oxide film. These hole-trap states would reduce charge carrier
recombination because some carriers are captured as the traps empty.
76
Comparisons with other researches
Deposition method Crystalline state
of Ga2O3
Device performance Ref.
Sol-gel method Polycrystalline Max. responsivity (@10 V): 8 × 10-5 A/W 1
Furnace oxidization
of GaN Polycrystalline
Max. responsivity (@5 V): 0.453 A/W
Dark current (@5 V): 1.39 × 10-10 A 2
MBE Single crystalline Max. responsivity (@10 V): 0.037 A/W
Dark current (@10 V): 1.4 × 10-9 A 3
Laser MBE Single crystalline Dark current (@1 V): 3.1 × 10-10 A 4
MOCVD Single crystalline Dark current (@5 V): 4 × 10-12 A 5
PLD Single crystalline Max. responsivity (@5 V): 0.903 A/W
Dark current (@5 V): 1.2 × 10-11 A
Our
study
1. Y. Kokubun et.al , Appl. Phys. Lett. 90, 031912 (2007).
2. W. Y. Weng et.al , IEEE Sens. J. 11,999–1003 (2011).
3. T. Oshima et.al , Jpn. J. Appl. Phys. 46, 7217–7220 (2007).
4. D. Y. Guo et.al , Appl. Phys. Lett. 105, 023507 (2014).
5. P. Ravadgar et.al , Opt. Express 21, 24599–24610 (2013).
Table. Solar-blind MSM PDs fabricated with the gallium oxide films grown by
various techniques.
77
A variety of PLD-grown wide bandgap nitride (GaN-based) and
oxide (ZnO-based and Ga2O3-based) semiconductor films are
presented.
The GaN-on-Si technology via the PLD possesses several
advantages including the prevention of melt-back etching without
any interlayer and the GaN growth on Si(100) substrate.
We have demonstrated the fabrication of highly thermal stable
InGaN films with In content of 33-62% on sapphire by low-
temperature PLD using a controllable InGaN target.
The ITO/AZO films deposited by PLD at 100C in Ar atmosphere
were utilized as TCLs for the fabrication of InGaN blue LEDs. The
light extraction of LED with ITO/AZO TCL is increased as the AZO
thickness increased.
Conclusions (2-1)
78
In comparison to previous researches on CZO DMSs, the PLD-
CZO films proposed in our study possess lower electrical
resistivity, good magnetic characteristic and excellent
transmittance.
By incorporating the 400 C-grown CZO film in the LED structure,
the difference of mobility between the electron and hole carriers
in the device can be decreased efficiently, resulting in the
increment of carrier recombination rate and the improvement of
the light output power.
Ga2O3 films were grown at various substrate temperatures ranging
from 400 to 1000 C by PLD. The better device performance of 800 C-grown Ga2O3 MSM photodetector can be attributed to the higher
crystal quality and fewer O vacancies in this film.
This indicates the Ga2O3 films presented in our study have high
potential for solar-blind photodetector applications.
Conclusions (2-2)
79
Acknowledgements
The financial support from the Ministry of
Science and Technology (Taiwan R.O.C.),
MOST Grant # 101-2221-E-005-023-MY3 is
gratefully acknowledged.
I would like to thank Professors Ray-Hua
Horng (NCTU), and Sin-Liang Ou (DYU) for
research cooperation.
I would like to thank all the Postdoc, Ph.D
and master students in my lab for their
efforts.
80
Thanks for your attention!
