01-Low temperature processed CO2 laser-assisted RF-sputtered GaN thin film for wide bandgap semic

Text Material Preview

Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=tace20
Journal of Asian Ceramic Societies
ISSN: (Print) (Online) Journal homepage: www.tandfonline.com/journals/tace20
Low temperature processed CO2 laser-assisted
RF-sputtered GaN thin film for wide bandgap
semiconductors
Seoung-Hyoun Kim, Chang-Hyeon Jo, Min-Sung Bae, Masaya Ichimura &
Jung-Hyuk Koh
To cite this article: Seoung-Hyoun Kim, Chang-Hyeon Jo, Min-Sung Bae, Masaya Ichimura &
Jung-Hyuk Koh (2023) Low temperature processed CO2 laser-assisted RF-sputtered GaN thin
film for wide bandgap semiconductors, Journal of Asian Ceramic Societies, 11:1, 68-79, DOI:
10.1080/21870764.2022.2151102
To link to this article: https://doi.org/10.1080/21870764.2022.2151102
© 2022 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Group on behalf of The Korean Ceramic
Society and The Ceramic Society of Japan.
Published online: 13 Dec 2022.
Submit your article to this journal 
Article views: 848
View related articles 
View Crossmark data
https://www.tandfonline.com/action/journalInformation?journalCode=tace20
https://www.tandfonline.com/journals/tace20?src=pdf
https://www.tandfonline.com/action/showCitFormats?doi=10.1080/21870764.2022.2151102
https://doi.org/10.1080/21870764.2022.2151102
https://www.tandfonline.com/action/authorSubmission?journalCode=tace20&show=instructions&src=pdf
https://www.tandfonline.com/action/authorSubmission?journalCode=tace20&show=instructions&src=pdf
https://www.tandfonline.com/doi/mlt/10.1080/21870764.2022.2151102?src=pdf
https://www.tandfonline.com/doi/mlt/10.1080/21870764.2022.2151102?src=pdf
http://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2022.2151102&domain=pdf&date_stamp=13 Dec 2022
http://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2022.2151102&domain=pdf&date_stamp=13 Dec 2022
FULL LENGTH ARTICLE
Low temperature processed CO2 laser-assisted RF-sputtered GaN thin film for 
wide bandgap semiconductors
Seoung-Hyoun Kima, Chang-Hyeon Job, Min-Sung Baeb, Masaya Ichimurac and Jung-Hyuk Koha
aSchool of Electrical and Electronics Engineering, Chung-Ang University, Seoul, Republic of Korea; bGraduate School of Intelligent Energy 
and Industry, Chung-Ang University, Seoul, Republc of Korea; cDepartment of Electrical and Mechanical Engineering, Nagoya Institute of 
Technology, Nagoya, Japan
ABSTRACT
Owing to its wide bandgap (3.4 eV) and high electron mobility, GaN has attracted significant 
attention for applications in solar cells, power transistors, and high-electron-mobility transis-
tors. Crystallized GaN thin film can be hardly prepared in thin film form by employing physical 
vapor deposition processes, such as reactive RF sputtering and pulsed laser deposition, 
because a high driving energy is required to deposit a thin film due to its high binding energy. 
Herein, GaN thin films were prepared by CO2 laser-assisted RF sputtering at a relatively low 
temperature of 200°C. The CO2 laser with a 10,600 nm wavelength shows excellent conversion 
efficiency from optical energy to thermal energy. At the optimized laser energy density of 
0.98 W/mm2, GaN thin film can have a (0002) orientation with a bandgap energy of 3.26 eV. The 
crystalline, surface morphological, and optical properties of the fabricated GaN thin films were 
evaluated using X-ray diffraction, FE-SEM, X-ray photoelectron (XPS), and photoluminescence 
(PL) spectroscopy, and UV-vis spectrometry. The energy bandgap of the fabricated GaN thin 
film was measured using the Tauc plot and confirmed via PL. The film composition thus 
obtained was analyzed using XPS.
ARTICLE HISTORY 
Received 1 July 2022 
Accepted 20 November 2022 
KEYWORDS 
CO2 laser; RF-magnetron 
sputtering; thin films; 
crystallization; GaN
1. Introduction
Wide energy bandgap semiconductors, such as 4 H-SiC 
(3.23 eV), β-Ga2O3 (4.8–4.9 eV), GaAs (1.43 eV), and GaN 
(3.4 eV), have been extensively investigated. In particular, 
gallium nitride (GaN) has attracted attention for power 
device applications because of its wide energy bandgap 
(3.4 eV), high thermal conductivity (1.3 W/(cm·K) at 300 K), 
high lateral breakdown voltage (10 kV), and high electron 
mobility (1000 cm2/(V·s at 300 K)) [1–3]. They have there-
fore been used in various applications such as LED, high- 
power transistors, and high-frequency devices [4–7]. Wide 
energy bandgap semiconductors are especially promis-
ing for power device applications that can withstand high 
voltages and currents even at high temperatures. Such 
devices should exhibit low leakage currents at high elec-
tric fields, low thermal currents at high temperatures, and 
low switching currents at high frequencies. A high current 
or fast switching can sometimes induce significant heat-
ing problems during operation; therefore, power devices 
should not be influenced by the surrounding thermal 
atmosphere.
For GaN materials, single crystals or epitaxial thin films 
are difficult to prepare. Therefore, heteroepitaxial wafers 
are typically prepared using hydride vapor phase epitaxy 
(HVPE), metal–organic chemical vapor deposition 
(MOCVD), or molecular beam epitaxy (MBE) processes 
on GaAs heterogeneous substrates. To deposit a GaN 
epitaxial thin film on a single-crystal substrate, HVPE and 
MOCVD are typically employed. However, owing to the 
toxicity of CVD precursors, physical vapor deposition 
(PVD) processes are preferred for growing GaN thin films 
[8–10]. In Table 1, the different processes for preparing 
GaN thin films are summarized. As presented in the table, 
an extremely high substrate temperature surpassing 
500°C is typically required to grow GaN thin films using 
CVD, while even higher temperatures exceeding 800°C 
are required for PVD. These conditions suggest the diffi-
culty of growing GaN thin films because of their high 
melting temperature and high energy bandgap. To 
date, many PVD processes, including sputtering, MBE, 
and pulsed laser deposition, have been used to grow 
GaN thin films [11–13]. Among these processes, radio 
frequency (RF) magnetron sputtering is a representative 
PVD process [14–18]. Owing to its high melting tempera-
ture of 1600°C, GaN requires high energy to form a thin 
film. Therefore, high processing energy with high vacuum 
pressure are key requirements in the processing chamber 
system. Owing to its reliability and controllability, the 
substrate heating method is preferred to provide suffi-
cient energy for the thin film process during PVD. 
However, high thermal energy cannot be transferred to 
the thin films in a vacuum chamber system with a direct 
substrate heating process because high vacuum condi-
tions cannot be sustained at the high temperatures 
required in the processing chamber. In addition, re- 
evaporation occasionally occurs because of the high 
CONTACT Jung-Hyuk Koh jhkoh@cau.ac.kr Chung-Ang University, 84 Heokseok-Dong, 84 Heokseok-Dong, Seoul, Republic of Korea
JOURNAL OF ASIAN CERAMIC SOCIETIES 
2023, VOL. 11, NO. 1, 68–79 
https://doi.org/10.1080/21870764.2022.2151102
© 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. 
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits 
unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
http://www.tandfonline.com
https://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2022.2151102&domain=pdf&date_stamp=2023-03-02
substrate temperature in the chamber. Therefore, high- 
quality thin films cannot be grown using a standard mag-
netron sputtering process at elevated temperatures.
High quality of GaN thin films is very important for the 
power devices applications. Especially, single crystalline 
thin film is attractive for deviceapplications due to their 
low defects. Therefore, the main purpose of this experi-
ment is to obtain a high-quality polycrystalline thin film 
for power devices with characteristics similar to that of 
a single-crystal thin film. To obtain a high-quality GaN thin 
film, a CO2 laser (wavelength 10,600 nm, beam diameter 
3.8 mm) was used in the RF sputtering process. In addi-
tion, the Ar-dependent plasma conditions were varied to 
optimize the gas processing conditions. The crystallinity 
of the GaN thin films was controlled by varying the pro-
cessing conditions. The crystalline structure and surface 
were analyzed using X-ray diffraction (XRD) patterns and 
scanning electron microscopy (SEM) images. The Tauc 
plot constructed using the transmittance values and 
photoluminescence (PL) spectra was analyzed and com-
pared to extract the energy bandgap values.
2. Experimental
A CO2 laser-assisted RF sputtering process was used to 
deposit GaN thin films on sapphire substrates. A pre- 
sputtering process was performed to clean the target 
and sapphire (c-plane) substrate. To supply sufficient 
energy during thin-film growth for GaN wide bandgap 
materials, a CO2 laser (wavelength of 10,600 nm, beam 
diameter of 3.8 mm) was used to irradiate the substrate 
during RF sputtering. Irradiated optical energy can pro-
mote thin-film growth with sputtered adatoms in the 
substrate. A schematic of the CO2 laser-assisted RF sput-
tering system used in this study is shown in Figure 1. 
A ZnSe single-crystal viewport was employed to transfer 
photon energy from the laser to the vacuum chamber in 
the sputtering system. The substrate temperature and 
working pressure during sputtering were 200°C and 15 
mTorr, respectively. The processing energy, Ar partial 
pressure, and laser energy density during the CO2 laser 
assisted RF sputtering process ranged between 100 and 
200 W, 20% and 80%, and 0 and 1.16 W/mm2, respec-
tively. The crystal structure of the prepared GaN thin film 
was determined through XRD analysis, and New D8- 
Advance (Bruker-AXS) using CuKα radiation was ana-
lyzed under 40 kV and 40 mA conditions. The composi-
tion of the prepared thin film was determined through 
X-ray photoelectron spectroscopy (XPS) and analyzed 
using the AlKα line of K-alpha+ (Thermo Fisher 
Scientific). The transmittance was measured using a UV 
spectrometer Lambda 35 (Perkin Elmer), and a Tauc plot 
was used to extract the light energy bandgap through 
the transmittance value. The extracted energy bandgap 
was compared with the simulated energy bandgap 
measured by PL spectroscopy. Photoluminescence 
spectra were measured using a HeAg 30 Deep UV laser 
source at room temperature.
Table 1. Deposition methods, substrates, and corresponding substrate temperatures.
Type Deposition method Substrate
Substrate 
Temperature 
(°C) Phase
Chemical vapor deposition Metalorganic chemical vapor deposition c-plane sapphire 500 Polycrystalline[8]
1000 Single crystal[20]
Hydride vapor phase epitaxy c-plane sapphire 600 Polycrystalline[9,10]
1000- Single crystal[19]
Physical vapor deposition Pulsed laser deposition c-plane sapphire 700 Polycrystalline[11,12]
Molecular beam epitaxy c-plane sapphire 600 Polycrystalline[13]
750 Single crystal[20]
RF magnetron sputtering Liquid Ga target Quartz, c-plane sapphire 860 Polycrystalline[17,18]
GaAs target Quartz 850 Polycrystalline[15,16]
GaN target Glass 200–300 Amorphous[14]
Figure 1. Schematic of the CO2 laser-assisted RF sputtering system.
JOURNAL OF ASIAN CERAMIC SOCIETIES 69
3. Results and discussion
The X-ray diffraction patterns of the RF-sputtered GaN 
thin films fabricated by varying the Ar partial pressure are 
shown in Figure 2. The RF sputtering process conditions 
were as follows: a substrate temperature of 200°C, RF 
power of 200 W, and working pressure of 15 mTorr. The 
Ar partial pressure was varied from 20% to 80% in 20% 
increments. The optimized partial pressure was consid-
ered and discussed to improve crystalline properties. As 
the Ar partial pressure increased to 60%, the (1010) peak 
of GaN was observed. The observed (1010) diffraction 
peaks of the RF-sputtered GaN thin films were attributed 
to the nonpolar planes of wurtzite GaN, which suggested 
the successful fabrication of the wurtzite GaN film. Black 
GaN films with poor crystallinity can be produced under 
low processing energy conditions during sputtering, 
which may result in the destruction of the atomic struc-
ture [14]. However, the GaN thin films fabricated in this 
study were not black in color. The intensity of the (1010) 
diffraction peak related to the nonpolar plane showed 
the peak when the ratio of reactive gas N2 and inert gas 
Ar was properly mixed. The Ar-to-N2 gas ratio depends on 
the plasma density; thus, the Ar partial pressure in the 
plasma can affect the crystallinity of GaN thin films 
because the plasma quality is affected by the discharge 
current with Ar ions . This is consistent with previous 
reports stating that plasma density and discharge current 
increase with increasing Ar partial pressure [21]. In 
Figure 2, the GaN(1010) orientation diffraction peak (C. 
P.S) values were (a) 689, (b) 1030, (C) 722, (d) 709, and (e) 
sapphire substrate 688. Therefore, we confirmed that 
optimizing the Ar partial pressure during RF sputtering 
is an important step in the GaN thin-film process. The 
maximum intensity of the (1010) diffraction peak near 32° 
is observed at an Ar partial pressure of 60%.
Figure 3 shows the X-ray diffraction patterns of the RF- 
sputtered GaN thin films fabricated under different RF 
power conditions. The generated power in the RF 
Figure 2. X-ray diffraction patterns of the RF-sputtered GaN thin films deposited on sapphire substrates at the temperature of 
200°C, RF power of 200 W, and working pressure of 15 mTorr, with different Ar partial pressure conditions (a) – (d) and without 
thin-film fabrication process (e).
70 S.-H. KIM ET AL.
sputtering process was varied from 100 W to 200 W. 
Owing to the limitations of the RF power source, the 
generated output power was restricted to 200 W. The 
processing temperature, working pressure, and Ar partial 
pressure were 200°C, 15 mTorr, and 60% of Ar/(N2+ Ar), 
respectively. The intensity of the (1010) reflection peak 
increased with the increase in the applied RF power, indi-
cating that a high energy is necessary to effectively grow 
GaN thin films through RF sputtering. A high RF power can 
help introduce high-quality plasma with high kinetic 
energy into the chamber during sputtering. 
Consequently, the high kinetic energy in the plasma can 
be transferred to the GaN thin films during fabrication.
Figure 4 displays X-ray diffraction patterns of the CO2- 
laser-assisted RF-sputtered GaN thin films deposited on 
a sapphire substrate at a low temperature of 200°C. The 
laser energy density was varied from 0.44 W/mm2 to 
1.16 W/mm2 in 0.18 W/mm2 increments. The CO2 laser 
with a wavelength of 10,600 nm employed to assist the 
RF-sputtering process has a photon energy to 0.12 eV. 
The intensity of the (0002) reflection of GaN significantly 
increased as the laser energy density increased from 0.8 
W/mm2 to 0.98 W/mm2. At a laser energy density of 
0.98 W/mm2, the (0002) peak of GaN was the dominant 
single film peak. The surface energy of the (0002) plane 
was higher than those of the other planes of wurtzite 
GaN, [22] indicating that the required nucleation energy 
of the c-plane of the GaN thin films was the highest 
among those of the other planes. By employing the 
CO2 laser for RF sputtering, the fabricated GaN thin 
films were able to acquire sufficient nucleation and 
growth energies during the CO2 laser-assisted RF sputter-
ing because the CO2 laser could supply sufficient forma-
tion energy density at the CO2 laser energy density of 
0.98 W/mm2. In comparing the fabrication CO2 laser 
energy densities, the peak intensities of (1010) and 
(1011) were found to be lower under the energydensity conditions of 0.62 and 0.8 W/mm2 com-
pared with that at 0.98 W/mm2. This finding indi-
cates that a CO2 laser energy density of 0.98 W/ 
Figure 3. X-ray diffraction patterns of the RF-sputtered GaN thin films deposited on sapphire substrates at the temperature of 
200°C, working pressure of 15 mTorr, with different RF power conditions (a) – (c) and without thin-film fabrication process (d).
JOURNAL OF ASIAN CERAMIC SOCIETIES 71
mm2 can supply sufficient energy for the nucleation 
and growth of reflections of the (0002) plane 
of GaN.
The peak intensity of the (1010) plane of GaN showed 
similar values up to a laser energy density of 0.8 W/mm2. 
However, as the laser energy surpassed 0.98 W/mm2, the 
(1010) peak of GaN suddenly disappeared, and the (0002) 
peak of GaN was formed. In addition, the XRD profiles of 
the sapphire substrate (non-laser) (f) and CO2-laser- 
assisted RF-sputtered GaN thin film fabricated at a laser 
energy density of 1.16 W/mm2 (a) were similar to those of 
an undeposited sapphire substrate. At a laser energy 
density of 1.16 W/mm2, the high laser energy density re- 
evaporated the GaN thin films on sapphire substrates. To 
estimate the surface energy of the (0002) plane of the 
GaN thin film, the Gibbs free energy per unit volume of 
the GaN thin films can be calculated using the following 
equation (1) 
ΔG� ¼
16πγvf
3 ΔGvð Þ
2
( )
2 � 3cosθþ cos3θ
4
� �
; (1) 
where γvf is the surface energy of the nucleus–sub-
strate and ΔGv is the Gibbs free energy per unit 
volume of the solid phase. Further, θ is the contact 
angle at the thin film–substrate interface, which is 
Figure 4. X-ray diffraction patterns of the CO2-laser-assisted RF-sputtered GaN thin films deposited on a sapphire substrate by 
varying the energy density of the CO2 laser. Process conditions: Ar partial pressure 60%, RF power 200 W, working pressure 15 
mTorr, and substrate temperature 200°C (a) – (e). GaN thin film without CO2 laser assistance (f) and without thin-film fabrication 
process (g).
72 S.-H. KIM ET AL.
equal to 180° for unattached thin films [23]. Owing 
to the wurtzite structure of GaN, γvf depends on the 
direction of the plane. The surface energies of GaN 
semiconductors with wurtzite structures are sum-
marized in Table 2. The surface energies of the 
(1010), (1011), and (0002) planes of the fabricated 
GaN thin films are 1.40, 1.76, and 2.64 J/m2 respec-
tively [22]. The GaN (0002) surface energy was 
about 2.64 J/m2, which means that the growth of 
the (0002) orientation reached the highest surface 
energy. As shown in Figure 4, the GaN (0002) dif-
fraction peak is the highest at a laser energy density 
of 0.98 W/mm2.
Figure 5 shows the (0002), (1010), and (1011) plane 
peak ratios according to the laser energy density calcu-
lated by Equation (2) [24]. 
Peak ratio 0002ð Þ ¼
Peak intensity 0002ð Þ½ �
Peak intensity uvtwð Þ½ �
; (2) 
The peak ratio of the (1010) plane with the lowest surface 
energy was calculated to be 84% at a laser energy density 
of 0.44 W/mm2. At a laser energy density of 0.62 W/mm2, 
the peak ratio of the (1010) planes increased to 47%, and 
the peak ratio of the (0002) planes with the highest sur-
face energy increased to 31.5%. At a laser energy density 
of 0.80 W/mm2, the peak ratio of (0002) planes increased 
to 77.7%, and the peak ratio of (1010) and (1011) planes 
with relatively low surface energy decreased. At a laser 
energy density of 0.98 W/mm2, the peak ratio of (0002) 
plane is 95.2%, indicating that most of the grains grow in 
(0002) orientation. As a result, this figure showed that 
higher peaks of (0002), (1011), and (1010) were observed 
at laser energy density of 0.98, 0.62, 0.44 W/mm2, respec-
tively. It means that the required energies for (0002), 
Table 2. Surface energy of the (0001), (1120), (1011), and (1010) planes of GaN 
wurtzite.
GaN plane (0001) (1120) (1011) (1010)
Surface energy (J/m2)[22] 2.64 1.53 1.76 1.40
Figure 5. X-ray diffraction pattern peak ratios in (0002), (1010) 
and (1011) orientations of CO2 laser-assisted RF sputtered GaN 
thin films fabricated at different laser energy densities.
Figure 6. Transmittance and Tauc plot of the CO2 laser-assisted RF-sputtered GaN thin films fabricated at different laser energy 
densities. The insert shows the simulated bandgaps of the GaN thin films from the transmittance values.
JOURNAL OF ASIAN CERAMIC SOCIETIES 73
(1011), and (1010) planes were well coincide with esti-
mated surface energy in Table 2.
The optical transmission spectra of the CO2-laser- 
assisted RF-sputtered GaN thin films fabricated at different 
laser energy densities in the wavelength range of 200– 
800 nm are shown in Figure 6. By increasing the CO2 laser 
energy density, the transmittance decreased from 90% to 
80% within the wavelengths range near 500 nm. In addi-
tion, the wavelength edge of the transmittance increased 
from 267 nm to 345 nm with the increase in the CO2 laser 
energy density. Therefore, the estimated energy bandgap 
of CO2 laser-assisted RF-sputtered GaN thin films shifted to 
lower values at approximately 3.32 eV as the CO2 laser 
energy increased. The black line in Figure 6 corresponds to 
the CO2-laser-assisted RF-sputtered GaN thin film fabri-
cated at a laser energy density of 0.98 W/mm2. For the 
laser energy density of 1.16 W/mm2, the transmittance 
value was not obtained. Apparently, the thin films evapo-
rated during deposition, indicating that the increasing 
laser energy density aided the crystal growth of wurtzite 
GaN. The simulated data of the energy band gap from the 
Tauc plots of the CO2-laser-assisted RF-sputtered GaN thin 
films are presented in the insets of Figure 6. The transmit-
tance of the GaN thin films was measured using a UV-vis 
spectrometer, and the (ahν)2 vs. hν curve was subse-
quently plotted: [25] 
ahvð Þ
2
¼ A hv � Eg
� �
; ahνð Þ
2
¼ A hν � Eg
� �
; (3) 
where a is the absorption coefficient, Eg is the optical 
bandgap of the GaN thin film, hν is photon energy, and 
A is a constant.
Figure 7. Bandgaps of the CO2 laser-assisted RF-sputtered GaN 
thin films fabricated at different laser energy densities.
Figure 8. Scanning electron microscopy (SEM) images of the CO2 laser-assisted RF-sputtered GaN thin films fabricated at different 
laser energy densities.
74 S.-H. KIM ET AL.
The relationship between the laser energy density and 
bandgap of the CO2-laser-assisted RF-sputtered GaN thin 
films is illustrated in Figure 7. The energy bandgap 
decreased with increasing laser energy density. When 
the laser energy density approached 0.98 W/mm2, the 
energy bandgap of the CO2 laser-assisted RF-sputtered 
GaN thin films was 3.32 eV, which was similar to the 
reported bandgap of GaN thin films. When the laser 
energy density exceeded 1.14 W/mm2, delamination 
and re-evaporation of the GaN thin film occurred owing 
to the excessively high energy density of the CO2 laser. 
Therefore, the optimized photon energy of 0.98 W/mm2 
can enhance the crystalline properties of the GaN thin 
film along the c-plane. Hence, CO2-laser-assisted RF sput-
tering is an effective method for growing GaN thin films.
Figure 9. Scanning electron microscopy (SEM) surface images (a)-(e), cross-sectional images (f) of CO2 laser-assisted RF sputtered 
GaN thin films fabricated at a laser energy density of 0.98 W/mm2.
JOURNAL OF ASIAN CERAMIC SOCIETIES 75
Figure 8 shows the SEM surface images of the CO2 
laser-assisted RF-sputtered GaN thin films fabricated at 
different laser energy densities ranging from 0 W/mm2 to 
1.16 W/mm2. In the SEM images of GaN thin films without 
laser (f) and with CO2 laser assistance with small energy 
density of 0.44 W/mm2 (e), a partially grown grain struc-
ture was observed. As the laser energy density was 
increased beyond 0.44 W/mm2, highly dense grains 
with a uniform size were formed. When the CO2 laser 
energy density approached 0.8 and 0.98 W/mm2, the 
grain size further increased. GaN thin films(b) deposited 
at a CO2 laser energy density of 0.98 W/mm2 were well 
crystallized in their thin film form. Therefore, CO2 laser- 
assisted RF-sputtered GaN thin films fabricated at a laser 
energy density between 0.8 and 0.98 W/mm2 exhibited 
relatively large grains and a uniform structure. Moreover, 
we determined that the grain size increased with increas-
ing laser energy density. Finally, the partially damaged 
surface of the CO2 laser-assisted RF-sputtered GaN thin 
films fabricated at a high incident laser energy density of 
1.14 W/mm2 (Figure 8(a)) suggested that high laser 
energy densities can damage the thin film quality of the 
fabricated GaN [26].
In Figure 9(a-e), the average grain size of the central 
region of the GaN thin film was 259 nm, and the average 
grain size of the edge region was 246–254 nm. The aver-
age grain size of the whole thin film was 252 nm, indicat-
ing that the film quality in the central region and the edge 
region was almost uniform. In Figure 9(f), the deposited 
GaN thin film is growing in the C-axis direction, and the 
Figure 10. High-resolution scanning electron microscopy (SEM) images of the CO2 laser-assisted RF sputtered GaN thin films 
fabricated at different laser energy densities.
76 S.-H. KIM ET AL.
thickness of the thin film has grown around 1 μm. It 
means that crystalline size of vertically grown grains 
(c-axis direction) were much larger than horizontally 
grown grains. In the experiments of X-ray photoelectron 
(XPS) and photoluminescence (PL) spectroscopy, and UV- 
vis spectrometry, a 1 μm-thick thin film was used.
Figure 10 shows high-resolution SEM surface images 
of CO2 laser-assisted RF sputtered GaN thin films fabri-
cated at different laser energy densities. In the GaN thin 
film (f) fabricated without laser assistance, small and non- 
uniformly grown crystal grains were observed. The GaN 
thin film fabricated with the assistance of a CO2 laser 
formed high-density particles at a laser energy density 
of 0.44 W/mm2. As a result of measuring the grain size of 
the GaN thin film fabricated with the assistance of a CO2 
laser, (e): 41 nm, (d): 68 nm, (c): 122 nm, (b): 247 nm. 
Figure 10(a) shows the surface damaged by the high laser 
energy density. It shows that grain size increases with 
increasing laser energy density in the range 0–0.98 W/ 
mm2. The optimal laser energy for obtaining a large grain 
size and high-density GaN thin film is 0.98 W/mm2.
Figure 11 displays the PL spectra of the CO2 laser 
assisted RF-sputtered GaN thin films with different laser 
energy densities. In the PL measurement, a He-Ag source 
with UV laser system of 224 nm wavelength was 
employed as the excitation source, and a charge coupled 
device detector was used to record the PL data from the 
excited electron emission. In general, the PL peak of GaN 
is located at approximately 360–380 nm [29,30]. By vary-
ing the CO2 laser energy density from 0.62 W/mm2 to 
0.98 W/mm2, the PL peak position shifted from 325 nm to 
380 nm, which corresponded to 3.81 and 3.26 eV, 
respectively. These PL data show similar energy bandgap 
behavior in Figure 6 and 7. When high laser energy 
densities of approximately 0.98 W/mm2 were applied in 
the thin film fabrication process, GaN thin film exhibited 
appropriate PL spectra at 3.26 eV. This PL spectrum value 
corresponds to the reported energy band gap of GaN 
thin films [27,28]. When a low energy density of approxi-
mately 0.44 W/mm2 or higher energy density exceeding 
1.16 W/mm2 was applied, thin films showed no PL spec-
tra peaks within the range of 325–380 nm. An appropri-
ate energy density of the CO2 laser can promote the 
formation of the wurtzite GaN structure with energy 
band gap of 3.3–3.4 eV. Without CO2 laser assistance, 
the GaN structure was not properly formed, as evidenced 
by the absence of PL data near the 360–380 nm range.
Figure 12 shows the XPS spectra of the Ga 3d core level 
of the CO2 laser-assisted RF-sputtered GaN thin films at 
different laser energy densities from 0.44 W/mm2 to 
1.16 W/mm2 and films RF-sputtered without CO2 laser 
assistance. Without CO2 laser assistance, the Ga 3d core 
level showed a faint peak intensity, meaning that the GaN 
thin film was hardly formed during RF sputtering. However, 
by introducing CO2 laser irradiation during RF sputtering, 
the Ga 3d core level showed increased peak intensity. The 
envelope, Ga 3d core level peak comprised three peaks 
originating from Ga-N, Ga-metal, and Ga-O bonds, which 
corresponded to 19.9, 19.0, and 20.7 eV, respectively [27]. 
As shown in the Figure 12(b-e), by increasing the laser 
energy density from 0.44 W/mm2 to 0.98 W/mm2, the 
number of Ga-N bonds increased, while those of Ga- 
metal and Ga-O were virtually unchanged. This finding 
Figure 11. Photoluminescence (PL) wavelength of the CO2 laser-assisted RF-sputtered GaN thin films fabricated at different laser 
energy densities.
JOURNAL OF ASIAN CERAMIC SOCIETIES 77
means that near the laser energy density of 0.98 W/mm2, 
the GaN thin film grew properly. However, as the laser 
energy density was increased to 1.16 W/mm2 
(Figure 12(a)), the peak intensity of Ga-N bonds decreased. 
As previously discussed, the GaN structure can be 
destroyed by high laser energy densities. As shown in the 
Figure 12, the envelope peak center shifted to low energy 
level with increasing laser energy density due to the 
increased number of G-N bonds at the energy density of 
0.98 W/mm2. At the assisted CO2 laser energy density of 
approximately 0.98 W/mm2, the Ga 3d core level showed 
the highest envelope intensity and Ga-N binding energy. 
This finding means that at the laser energy density of 
0.98 W/mm2, GaN thin film has the highest number of 
Ga-N bonds and a well-grown crystalline structure.
4. Conclusion
For the first time, CO2 laser-assisted RF-sputtered 
GaN thin films were prepared by varying the laser 
energy density at a relatively low substrate tem-
perature of approximately 200°C. To assist in the 
growth of crystallized GaN thin films on sapphire 
substrates, a CO2 laser was employed during the RF 
sputtering process. By employing CO2 laser with 
energy density of 0.98 W/mm2 to assist RF sputter-
ing at a low substrate temperature of 200°C, (0002) 
oriented, well-grown crystalline-structured GaN thin 
films were obtained. Both the measurement meth-
ods of Tauc plot and PL spectra proved that GaN 
thin films processed with the CO2 laser and RF 
sputtering system yielded a well-grown (0002) 
orientation with a wide energy bandgap of 
Figure 12. X-ray photoelectron spectroscopy (XPS) spectra of the Ga 3d core level of the CO2 laser-assisted RF-sputtered GaN thin 
films deposited on a sapphire substrate by varying the energy density of the CO2 laser. Process conditions: Ar partial pressure 60%, 
RF power 200 W, working pressure 15 mTorr, substrate temperature 200°C.
78 S.-H. KIM ET AL.
3.26 eV when the CO2 laser was irradiated at an 
energy density of 0.98 W/mm2 during the RF sput-
tering process.
Disclosure statement
No potential conflict of interest was reported by the 
author(s).
Funding
This research was supported by the MOTIE (Ministry of Trade, 
Industry, and Energy) in Korea, under the Fostering Global 
Talents for Innovative Growth Program(P0017308) super-
vised by the Korea Institute for Advancement of 
Technology (KIAT) and Human Resources Development 
(No.20214000000280) of the Korea Institute of Energy 
Technology Evaluation and Planning(KETEP) grant funded 
by the Korea government Ministry of Trade, Industry and 
Energy.
References
[1] Sichel EK, Pankove JI. Thermal conductivity of GaN, 
25-360K. J Phys Chem Solids. 1977;38:330.
[2] Kim M, Seo J, Singietti U, et al. Recent advances in 
free-standing single crystalline wide band-gap 
semiconductors and their applications: gaN, SiC, 
ZnO, β -Ga2O3, and diamond. J Mater Chem C. 
2017;5:8338.
[3] Chow T. High-voltage SiC and GaN power devices. 
Microelectron Eng. 2006;83:112–122.
[4] Shchekin O,Epler J, Trottier T, et al. High performance 
thin-film flip-chip InGaN–GaN light-emitting diodes. 
J Appl Phys Lett. 2006;89:071109.
[5] Uemoto Y, Hikita M, Ueno H, et al., Gate Injection 
Transistor (GIT)—A normally-off AlGaN/GaN power 
transistor using conductivity modulation. IEEE Trans 
Electron Devices. 2007;54:12.
[6] Jogai B. Influence of surface states on the 
two-dimensional electron gas in AlGaN/GaN hetero-
junction field-effect transistors. J Appl Phys. 
2003;93:3.
[7] Osipov K, Ina Ostermay I, Bodduluri M, et al., Local 
2DEG density control in heterostructures of piezoelec-
tric materials and its application in GaN HEMT fabrica-
tion technology. IEEE Trans Electron Devices. 2018, 65, 
8.
[8] Dissanayake A, Lin JY, Jiang HX, et al. Low tem-
perature epitaxial growth and photoluminescence 
characterization of GaN. J Appl Phys Lett. 
1994;65:2317.
[9] Richter E, Gramlich S, Klein A, et al. Direct growth of 
GaN on (0001) sapphire. J Phys Stat Sol. 
2001;188:439.
[10] Lee JW, Paek HS, BYoo J, et al. Low temperature buffer 
growth to improve hydride vapor phase epitaxy of 
GaN. J Mater Sci Eng. 1999;59:12.
[11] Huang TF, Marshall A, Spruytte S, et al. Optical and 
structural properties of epitaxial GaN films grown 
by pulsed laser deposition. J Cryst Growth. 
1999;200:362.
[12] Kawwam M, Lebbou K. The influence of deposition 
parameters on the structural quality of PLD-grown 
GaN/sapphire thin films. J Appl Surf Sci. 
2014;292:906.
[13] Li W, Li A. Post growth thermal annealing of GaN 
grown by RF plasma MBE. J Cryst Growth. 
2001;227:415.
[14] Maruyama T, Miyake H. Gallium nitride thin films 
deposited by radio-frequency magnetron sputtering. 
J Vac Sci Technol A. 2006;24:1096.
[15] Yadav BS, Singh S, Ganguli T, et al. Highly oriented GaN 
films grown on ZnO buffer layer over quartz substrates 
by reactive sputtering of GaAs target. Thin Solid Films. 
2008;517:488–493.
[16] Nisha Preschilla A, Major S. Nanocrystalline gallium 
nitride thin films. Appl Phys Lett. 2000;77:1861.
[17] Kikuma T, Tominaga K, Furutani K, et al. GaN films 
deposited by planar magnetron sputtering. Vacuum. 
2002;66:233–237.
[18] Shinoda H, Mutsukura N. Structural properties of GaN 
and related alloys grown by radio frequency magne-
tron sputter epitaxy. Thin Solid Films. 
2008;516:2837–2842.
[19] Naniwae K, Itoh S, Amano H, et al. Growth of single 
crystal GaN substrate using hydride vapor phase 
epitaxy. J Cryst Growth. 1990;99:1–4.
[20] Pearton SJ, Ren F, Zhang AP, et al. Fabrication and 
performance of GaN electronic devices. Mater Sci Eng 
R Rep. 2000;30:3–6.
[21] Borah SM, Bailung H, Pal AR, et al. Study on the influ-
ence of nitrogen on titanium nitride in a dc post 
magnetron sputtering plasma system. J Phys D: App 
Phys. 2008;41:195205.9.
[22] Hong YJ, Yoo J, Doh YJ, et al. Controlled epitaxial 
growth modes of ZnO nanostructures using different 
substrate crystal planes. J Mater Chem. 
2009;19:941–947.
[23] Cao G Nanostructures & nanomaterials: synthesis, 
properties & applications. Imperial college press, 
2004. 93p.
[24] Jin SD, Koh J-H. Effects of calcination temperature on 
the piezoelectric properties of lead-free Ag doped (Na, 
K) NbO3–LiTaO3 piezoelectric ceramics. J Alloys 
Compd. 2013;555:390–394.
[25] Lin K, Kuo DH. Characterization of quaternary AlInGaN 
films obtained by incorporating Al into InGaN film with 
the RF reactive magnetron sputtering technology. 
J Mater Sci: Mater Electron. 2017;28:43–51.
[26] Yoo JH, Menor MG, Adams JJ, et al. Laser damage 
mechanisms in conductive widegap semiconductor 
films. Opt Express. 2016;24:16.
[27] Kumar M, Pasha SK, Krishna TC, et al. Facile synth-
esis and photoluminescence spectroscopy of 
3D-triangular GaN nano prism islands. J Dalton 
Trans. 2014;43:11855.
[28] Kim D, Song KM, Jung U, et al. Effects of different 
InGaN/GaN electron emission layers/interlayers on 
performance of a UV-A LED. Appl Sci. 2020.10.1514.
[29] Wu S, Wang L, Yi X, et al. Influence of lateral 
growth on the optical properties of GaN nanowires 
grown by hydride vapor phase epitaxy. J Appl Phys. 
2017;20:122.
[30] Jiao W, Kong W, Li J, et al. Characterization of MBE-grown 
InAlN/GaN heterostructure valence band offsets with 
varying In composition. J AIP Adv. 2015;6:3.
JOURNAL OF ASIAN CERAMIC SOCIETIES 79
	Abstract
	1. Introduction
	2. Experimental
	3. Results and discussion
	4. Conclusion
	Disclosure statement
	Funding
	References