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Infrared study on low temperature atomic layer deposition of GaN using trimethylgallium and plasma-excited ammonia

Authors:
  • Kasetsart University (KU-SRC)
Infrared study on low temperature atomic layer deposition of GaN using trimethylgallium and
plasma-excited ammonia
P. Pansila, K. Kanomata, S. Kubota, B. Ahmmad, F. Hirose
Graduate School of Science and Engineering, Yamagata University,
4-3-16 Jonan, Yonezawa 992-8510, Japan
Email: fhirose@yz.yamagata-u.ac.jp
GaN attracts much attention due to its wide band gap of ~3.4 eV. Many conventional fabrication
methods have been reported for the GaN deposition such as molecular beam epitaxy (MBE) and
metalorganic vapor phase epitaxy (MOVPE), although most methods require high-temperature
processes in excess of 700 °C. In future, however, to achieve the selective GaN epitaxy on the device
fabricated wafers, the low-temperature growth might be demanded especially on the flexible
substrates for light emitting applications [1].
Recently, atomic layer deposition (ALD) has been examined for the deposition of GaN. ALD is a
method that was developed for depositing the conformal oxide and nitride thin films onto substrates of
varying compositions at low temperatures. If the deposition temperature is decreased to near RT with
the ALD technique, the grown film might be amorphous. However, by using the temperature
programmed rapid annealing, the amorphous GaN films might be crystallized with the minimum
thermal budget, which might be a merit for realizing the GaN devices on the flexible substrates.
To establish the low temperature GaN ALD, we investigated the surface reactions of TMG
adsorption at RT and nitridation using plasma excited NH3 at 115°C. The plasma excited NH3 was
used as the N source since it has a potential to provide the sufficient N coverage. In this paper, we first
report the investigation of fundamental reactions in TMG adsorption at RT and its reaction with
plasma excited NH3 on GaN surfaces at 115°C by MIR-IRAS and XPS. We demonstrate the GaN
ALD at 115°C. We also discuss the method to achieve the low temperature ALD of GaN at near RT.
For the experiment, an ALD analytical system was used with in situ infrared absorption
spectroscopy with a multiple internal reflection mode (MIR-IRAS) as shown in Fig. 1. We
investigated the adsorption of TMG and its reaction with plasma excited NH3 on GaN surfaces. A Si
(100) substrate was used as the sample. A sample was cut to the size of 0.5×1×40 mm3 with a 45°
bevel on each of the short edges for TMG adsorption analysis. Before setting a sample in the analysis
chamber, a sample was cleaned by the RCA method, followed by being dipped in hydrofluoric acid to
remove the native oxide on the surface. Before the experiment of the TMG adsorption on the GaN
surface, we prepared a GaN amorphous film on the Si surface with the present ALD method with a
thickness of 0.23 nm. TMG was irradiated onto the GaN surface with exposures from 200 to 2.0×105
L at RT where 1 L corresponds to 1.33×10-4 Pas (= 1.0×10-6 Torrs). The IR absorbance spectra are
shown in Fig. 2. We can see that TMG is possible to adsorb on the GaN surface. To investigate the
nitridation, plasma excited NH3 was irradiated to the TMG saturated GaN surface from 1-50 s at 115°
C. The IR absorbance spectra on the C-H removal after the plasma excited NH3 treatment are shown
in Fig. 3. It is found that the plasma excited NH3 can remove hyrdrocarbon in the TMG saturated GaN
surface at 115° C. Figure 4 shows a variation of C-H absorbance in the course of the TMG adsorption
and the plasma excited NH3 treatment. It is found that TMG is saturated at RT on the GaN surfaces
with the TMG exposures exceeding 8×104 L while the removal of hydrocarbon is completed with the
plasma treatment time exceeding 10 s at 115 ° C.
Base on the IR analysis, the low temperature ALD of GaN was examined by using RT-TMG
adsorption and plasma excited NH3 at 115°C. The ALD sequence includes the TMG irradiation step
of 1.5x105 L and the plasma excited NH3 treatment for 90 s. We can confirm the chemical state of
GaN from N 1s spectra at a binding energy of 397.54 [2]. Figure 5 shows the XPS spectrum of N 1s
of GaN obtained from this experiment. The thickness of GaN films were estimated from XPS spectra
of Si 2p measured from the GaN deposited Si sample as shown in Fig. 6. It was found that GaN films
were grown by ALD at process temperatures around 115°C with a growth per cycle of 0.045 nm/cycle.
[1] J. Chun, Y. Hwang, Y. Choi, J. Kim, T. Jeong, J. H. Baek, H. Ch. Ko, and S. Park, Scripta. Mater.
77, 13, (2014).
[2] C. Ozgit, I. Donmez, and N. Biyikli, Acta. Phys. Pol. A. 120, A-55, (2011).
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2-7
IR
source
Substrate
Mirror
unit
DC
TMP
NH
plasma
InSb
detector N2
TMG
MFC
N2
Variable
valve
PC
Gate valve
RF
13.56 MHz
CaF
window
2
CaF
window
2
MFC
Ar
MFC
3
IR
Si
prism
NH3
Fig. 1 Schematic diagram of ALD analysis
system.
Fig. 2 IR absorbance spectra of TMG adsorption on
the GaN surface.
Fig. 3 IR spectra of TMG adsorbed GaN at
RT, followed by the plasma excited NH3
treatment at 115°C.
Fig. 4 Variation of C-H absorbance on the GaN
surface in the course of the TMG adsorption and
plasma treatment at 115°C.
Fig. 5 N 1s XPS spectrum obtained from GaN
film with Ar etching for 30 min.
Fig. 6 GaN thickness as a function of the ALD
cycle.
8x10-3
6
4
2
0
Absorbance
3000 2900 2800 2700
Wavenumber (cm-1)
5 s
25 s
50 s
15 s
20 s
10 s
TMG adsorption
200000L at RT
CH3
Plasma excited NH3 treatment
at 115°C
2968 2912
1.6x10-3
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
ΔAbsorbance of CH 3
2.5x105
2.01.51.00.50.0 TMG irradiation (L)
-1.6x10-3
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
ΔAbsorbance of CH 3
50403020100
Plasma treatment time (s)
TMG adsorption at RT
Plasma treatment at 115°C
Photoelectron intensity
402 400 398 396 394
Binding energy (eV)
N1s GaN
397.54eV
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Thickness (nm)
6050403020100
ALD cycle
GPC = 0.045 nm/cycle
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