Characte rization and Comparison of GaN T hin Film s with Doping Pb [616956]

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Characte rization and Comparison of GaN T hin Film s with Doping Pb
Prepared by Thermionic Vacuum Arc
Soner Özen1, Suat Pat1, Volkan Șenay2, Șadan Korkmaz1
1Department of Physics, Eskișehir Osmangazi University Meșelik Campus, 26480, Turkey
2Primary Science Education Department, Bayburt University, 69000, Turkey
Corresponding author’s email: sonerozen55 @yahoo.co .uk

Abstract
The GaN thin film and its thin film with doping Pb were deposited by thermionic vacuum arc
method. The substrates were selected glass a nd polyethylene terephthalate (PET) which
optical transmittance materials. The structur al, optical and morphological properties of the
thin films were characterized . These physical properties are interpreted by comparison with
the related analysis methods. The crystalline structure of produced GaN thin film s is
hexagonal wurtzite structure. The optical band gap energy of the GaN thin films was found to
be 3.45 eV. The optical band gap energy of the GaN thin films with doping Pb was found to
be 3 .47. The produced thin films from AFM and FESEM images are nano -structured,
homogeny, and granular.
Keywords: thermionic vacuum arc ; Pb doping ; GaN ; thin film ; XRD ; dimensional analysis
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1. Introduction:
Gallium nitride (GaN) compounds is a III/V direct bandgap semiconductor with wurtzite, zinc
blend , and NaCl crystal structures [1-8]. It is used in the manufacture of technological devices
such as thin film transistor [9, 10] , light -emitting diodes [11-13], photodetector [14, 15] , high
power electr onic devices [16, 17] . The undoped and doped GaN thin films have been coated
on various crystal or amorphous substrates such as sapphire , glass, silicon , PET etc [12, 18 –
21]. GaN thin films undergo a change in optical, morphological, and electronic properties by
doping. There are studies on ba nd gap energy [22-28] and surface properties [23-26] of II-VI
semiconductor materials with doping Pb in the literature. Whereas, t here are several studies
on doping with IV g roup elements of the GaN in the literature [6, 7, 29 -33] but studies on Pb-
doped GaN film have not been reported so far. The novelty of this study is that Pb-doped GaN
thin films were deposited on insulator transpar ent substrates by using TVA technique for the
first time and it’s some physical properties is was compared with GaN thin film . The
produced thin films onto amorphous glass and semi -crystalline PET were produced and they
were deposited directly onto substra tes without exposing to any etching process.

2. Experimental Details
The GaN and Pb-doped GaN thin films on the transparent substrate s were coated by
thermionic vacuum arc method. The transparent substrates were used a commercial glass and
a commercial polyethylene terephthalate (PET). The thermionic vacuum arc (TVA) technique
is a plasma assisted thin film deposition technique whi ch generates pure metal and non -metal
plasma without using any buffer gas. TVA technique have a number of advantages such as
homogeneity, compact, nano -structure, low roughness, good adhesion, a short processing time Page 2 of 14 AUTHOR SUBMITTED MANUSCRIPT – MRX-104134
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and a high deposition rate [34-39]. TVA system consists of an anode and a cathode. The
distance between the anode an d the cath ode electrodes was d=4 mm. The cathode was made
from a tungsten wire with a diameter of 1 mm. The Pb piece ( Alfa Aesar, 99.95% ) only in the
second experiment and gallium nitride powder ( Alfa Aesar, 99.99% ) in both experiments
were placed on the anode elect rode in tungsten crucible. The doping process of GaN in the
TVA system is very simple compared to MOCVD and MBE. Contrary to other vacuum
techniques, used anode materials do not need special shapes like granule, target, rod, etc. The
vacuum was obtained by a coupled pumping down system with a mechanical and a diffusion
pump, ensuring an end pressure in the vessel close to 10-6 Torr. The coating process was
performed in 2 × 10− 5 Torr. The filament heating current was 18 A. The voltage applied to the
space b etween anode and cathode was 200 V. The ap plied voltage was suddenly dropped to 0
V and the discharge current increase to 600 mA from 0 A. A bright discharge occurs in the
interelectrodic space due to the effect that applied voltage. The produced plasma wa s
localized around the electrodes. Thus the material in the anode electrode to be coated starts to
be deposited on the transparent substrates. The deposition process was carried out for 40
seconds. All experiment conditions were the same in both production s.

3. Results and Discussion
Rigaku MiniFlex 600 XRD with SmartLab goniometer in the 2 range of 10 –80° was used for
crystallographic data of thin films deposited on the transparent substrates at room
temperature. The X -ray diffraction (XRD) patterns of t he produced samples are presented in
Figure 1. Determining peaks are compatible with the hexagonal wurtzite GaN (lattice constant
of alloy =3.68 )[33, 40, 41] , the strain crystallize s polyethylene terephthalate (PET) [33, 42] ,
the cubic structure of lead (Pb) [43-45]. The GaN thin film produced on the PET substrate Page 3 of 14 AUTHOR SUBMITTED MANUSCRIPT – MRX-104134
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observed an additional peak with (103) orientatio n according to the GaN thin film produced
on the glass substrate . However, this peak has been eliminated with Pb doping. For production
onto both substrates, polycrystalline thin films were also formed with Pb doping. XRD
analysis of the Pb-doped GaN on th e glass substrate observed four separate Pb peak, but only
one Pb peak was observed for the PET substrate.

Figure 1 XRD patterns of the thin films produced on the transparent (a) glass and (b) PET

Figure 2 Tauc plots of
vs.
to estimate the optical band gap s of the thin films
produced on the transparent (a) glass and (b) PET substrates .
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Optical absorption study of materials is a simple method to determine their band gap energy.
Figure 2 shows the Tauc plot which gives the di rect band gap energy. The optical band gap
energy
of the GaN and Pb-doped GaN thin films produced on the transparent substrates
were found to be 3.45 eV and 3.47, respectively .
The spectral dependencies of reflectance and refractive index of the produced thin films were
obtained by using Filmetrics F20 Thin Film Analyzer. The reflectance spectrum in the
wavelength range of 400 –1000 nm for the produced thin films are presented in Fig ure 3a,
indicating that the thin films have a reflectance ran ging between 1 1% and 25% in the
observed wavelength range. The film thickness es obtained from the spectral analysis of
reflectance are about 50 nm for GaN thin film onto glass substrate, 30 nm for GaN thin film
onto PET substrate, 45 nm for Pb-doped GaN th in film onto glass substrate, and 40 nm for
Pb-doped GaN thin film onto PET substrate .

Figure 3 (a) Reflectance and (b) refractive index spectra of the produced thin films on the
transparent substrates.
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For GaN samples, the reflectance values increased almost twice with the increase in thickness.
Besides, a significant change in the reflectance values was not observed due to small changes
in thickness for Pb-doped GaN samples. The spectral curves of refractive index are shown in
Figure 3b for all sample s. The Pb-doped GaN thin film on the glass substrate has lower
refractive index value than the other samples. As well as its reflectance curve showed a
distinct change than others.

Figure 4 FESEM images of the (a) GaN thin film produced on the glass sub strate, (b) Pb-
doped GaN thin film produced on the glass substrate, (c) GaN thin film produced on the PET
substrate, (d) Pb-doped GaN thin film produced on the PET substrate
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Figure 4 shows top view FESEM images of the thin films at 50 kx magnification. It can be
noticed from the images that film is composed of regular round grains except Pb-doped GaN
thin film on the glass substrate. The grain structures were observed to grow by doping Pb.

Figure 5 Dimensional analysis of the (a) GaN thin film, (b) Pb-doped GaN thin film on the
glass substrate

The surface characteristics were calculated by the Ambios Q -Scope atomic force microscope
(AFM). The AFM photomicrographs are compatible with FESEM images. The dimensional
analysis images are shown in Figure 5 for thin films on the glass substrates. The
photomicrographs of produced thin films are homogeny, nano -structured and granular. The
large nano -particle size of GaN thin film is determined as 175 nm. The grain size formed on
large nano -particle is 85 nm. The n ano-particle size of GaN thin film with Pb doping grew out
to 245 nm without the small grains. The dimensional analysis images are shown in Figure 6
for thin films on the PET substrates. The nano -particle size of GaN thin film is determined as Page 7 of 14 AUTHOR SUBMITTED MANUSCRIPT – MRX-104134
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106 nm. GaN thin film on the PET substrate exhibited relatively smooth image. The grain size
formed on large nano -particles of GaN thin film with Pb doping is determined as 90 nm.

Figure 6 Dimensional analysis of the (a) GaN thin film, (b) Pb-doped GaN thin film on the
PET substrate

4. Conclusion
For the first time, the thermionic vacuum arc (TVA) technique was used to produce thin films
of undoped and Pb-doped GaN. The doping process of GaN in the TVA technique is very
simple and rapid. The structural, morphologic al, and optical properties are investigated by
relevant analysis techniques. The XRD analysis results showed that the polycrystalline GaN
thin films were also formed with Pb doping . The XRD patterns of produced thin films have
hexagonal wurtzite structure with preferred (103) orientation. The dimensional analysis of
produced thin films was observed by AFM scope operating software. The obtained data was Page 8 of 14 AUTHOR SUBMITTED MANUSCRIPT – MRX-104134
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in nature supports FESEM micrographs and XRD results. TVA technique is an alternative
layer deposition sys tem for doped GaN thin film production.

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60