In situ monitoring damage density of GaN substrate surface in ICP
containing energetic electrons
Ying Guo12, Qinyu Yang12, Dexin Wang12, Jing Zhang12, Yoshitaka Nakano3, Hideo, Sugai3,
Keiji Nakamura3 and Jianjun Shi12
1, College of Sciences, Donghua University
2999 North Ren Min Road, Shanghai 201620, PR. China
2, Member of Magnetic Confinement Fusion Research Center, Ministry of Education, Donghua University,
2999 Renmin Rd., Songjiang District, Shanghai 201620, P.R .China
3, College of Engineering, Chubu University
11200 Matsumoto, Kasugai, Aichi 487-8501，Japan
Phone: +86-21-6779-2333, fax: +86-21-6779-2085
Abstract: In this paper we describe a new method for in situ monitoring damage density of GaN substrate
surface in inductively-coupled plasmas (ICP) containing energetic electrons. Such high energy electrons
(keV) are produced by sheath acceleration of secondary electrons at a negatively-biased electrode. A
current of a Langmuir probe located in such plasma is investigated to examine how the high energy
electrons behave in the plasma. A sample of n-type GaN film was observed to emit significant optical
fluorescence in the wavelength range of 370-390 nm corresponding to band gap energy of the GaN
when the plasma contains the high energy electrons, the fluorescence intensity of the GaN film increased
with the incident electron energy higher than a critical energy of ~5 keV. By XPS and PL results, we can
see that this method of online diagnosis GaN film on the surface does not cause significant damage.
These results suggested cathode luminescence technique will be used to detect a damage density of GaN
substrate surface even in plasma conditions.
Keywords: In situ, GaN, Damage density, ICP
1. Introduction
Plasma immersion ion implantation (PIII) has
been developed as a novel technique for threedimensional surface modification, and many
efforts have been made for various practical
applications 1 – 4). In the PIII processes,
negative pulse voltages are applied to a
processed target, and the ions accelerated at the
sheath are implanted onto the target surface.
Simultaneously, at the surface, secondary
electron emission is induced by the ion
bombardment. Since a sheath around the target
has a potential structure to accelerate the
secondary electrons, a large secondary electron
current flows at the target. From the secondary
electron point of view, sheath diagnostics has
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been performed4–5), and new techniques for
measuring
sheath-accelerated
energetic
secondary electrons have been developed 6).
On the other hand, development of highpower and high-efficiency GaN light-emitting
diodes (LEDs) is of technological importance
for the realization of solid-state lighting 7).
Recently in some applications, microfabrication
of the GaN film will require a plasma treatment
such as etching, but the treatment may have an
influence on the GaN surface.
In this paper, we investigate plasma interaction
between the GaN film and plasma containing
high energy electrons which is a similar plasma
situation of the PIII source. In order to detect the
high energy electrons, we introduce a Langmuir
probe, and the time variations of the probe
current are investigated to examine how the
high energy electrons influence on the plasma.
Furthermore, in-situ observation of optical
luminescence induced by the high energy
electrons are carried out for sample materials
(GaN) located in the plasma as a plasmamaterial interaction. Through the XPS and PL
results, we can see that this method of online
diagnosis GaN film on the surface does not
cause significant damage. These results
suggested cathode luminescence technique will
be used to detect a damage density of GaN
substrate surface even in plasma conditions.
2. Experimental Methods
The experimental device has been in the
previous article described 8). To generate high
energy electrons, negative high voltage pulses
are applied to the target. When applying the
pulse voltages to the target, significant
secondary electron emission is induced by
strong ion bombardment since incident ions are
accelerated at the sheath in front of the target
surface. The potential structure sheath also
accelerates the secondary electrons up to energy
approximately equal to the applied target
voltage. A wire-type Langmuir probe is used to
determine plasma parameters and sheath
evolution is estimated by depletion of electron
saturation current. The electron saturation
current is measured with the probe biased at
+20V higher the plasma potential of ~16 V. A
sample holder with a 2μm-thick n-type GaN
film deposited on a sapphire substrate is axially
introduced to the plasma and located at 6 cm
above the target.
3. Results and Discussion
Figure 1. Time evolution of (a) target bias voltage (b) probe
current for the discharge power of 200W. The probe potential is
fixed at 20V higher than the plasma potential of 16 V.
The time evolution of the target voltage as well
as the probe current is shown in Figure 1. The
pulse voltage is applied after 20μs from the end
of the pulse modulated discharge. The probe
current increased approximately by a factor of
~2 during the application of the negative high
voltage (HV) pulse bias of -3.5 kV in amplitude,
and was gradually recovered after the end of the
HV pulse. This fact suggests the existence of
high-energy electrons in the plasma. The
energetic secondary electrons accelerated at the
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sheath during the HV pulse enhance ionization
of the plasma, and that the secondary electron
current is proportional to the plasma density 8).
For the GaN films exposed to the plasma
containing the high energy electrons described
in the previous section, the optical luminescence
was observed in the wavelength rage from 350
to 400 nm.
untreated sample. Considering that PL intensity
sensitively depends on surface recombination
velocity, the decreased PL intensity of the
present sample is indicative of a growth in the
surface recombination velocity. However,
detailed mechanisms for the high energy
electrons treatment dependence of the PL
behaviors are now under investigation.
Intensity (arb.unit)
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
-0.0005 0
untreated
treated
200
400
600
800
Wavelength (nm)
Figure3. PL spectra of the samples before and after treatment.
Figure2.Shows
a
typical
example
of
fluorescence spectrum of the GaN film for the
discharge power of 100W and the different
target voltages, the intensity of fluorescence
spectrum increases with the target voltage in the
voltage range higher than a critical value of -5
kV as shown in Figure 2. Such an
electroninduced
fluorescence
of
nitride
materials were observed in a vacuum
environment for scanning electron microscopy
measurement as a cathode luminescence (CL)
technique 9). Thus the incident electron energy
at the GaN surface is suggested to be a crucial
parameter to govern the fluorescence intensity.
The PL spectra of the samples before and after
exposed to the plasma containing the high
energy electrons shown in Figure 3. Both the
samples show peak energy at about 560nm. It is
shown that the exposed to the plasma containing
the high energy electrons results in a decrease in
the PL intensity, compared with that of the
To deeply study the damage of plasma for the
sample, XPS examination was performed.
Figure 4 shows the Ga 2p core-level spectra for
the GaN interfaces. It is evident that the Ga 2p
core-level spectra of the treated samples shift
toward the low-binding energy side, compared
with that of the untreated one. This indicates
that the high energy electrons and plasma
treatments give effect to a shift of the surface
Fermi level toward the conduction-band edge.
Intensity/ Counts per second
Figure 2. Fluorescence observation of GaN film in different
target voltages
900
800
700
600
500
400
300
200
100
0
untreated
treated
100
110
120
130
Binding Energy/ev
140
Figure 4. Photoemission spectra of the Ga 2p core level of GaN
samples.
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4. Conclusions
This work investigated the optical luminescence
of GaN films in plasma containing high energy
electrons similar to PIII sources. A step-like
increment of the probe current observed
immediately after the high voltage application
was proportional to the discharge power. In a
plasma environment, significant fluorescence of
GaN film was observed near 370–390 nm in
wavelength. The fluorescence intensity of the
GaN film increased with the incident electron
energy given by the target voltage, but there was
a critical energy of -5 keV. And the XPS and PL
results suggested cathode luminescence
technique can been used to detect a damage
density of nitrides surface even in plasma
conditions leading to in-situ monitoring for
plasma treatment like etching process.
Gardner, R. S. Kern, and S. A. Stockman,
Appl. Phys. Lett. 78(2001) 3379.
[8]. Ying Guo, Keiji Nakamura_, Jing Zhang,
Yoshitaka Nakano, and Hideo Sugai,
Japanese Journal of Applied Physics 50
(2011) 01AA02
[9]. S.-N. Lee, H. S. Paek, H. Kim, Y. M. Park,
T. Jang, and Y. Park: Appl. Phys. Lett. 92
(2008) 111106.
Acknowledgments
The authors are grateful to the Nature Science
Foundation of China (No. 11005017, 10775031).
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