influence of al concentration on structural and optical properties of al-doped zno thin films
TRANSCRIPT
Influence of Al concentration on structural and optical propertiesof Al-doped ZnO thin films
Deniz Kadir Takci • Ebru Senadim Tuzemen •
Kamuran Kara • Sadi Yilmaz • Ramazan Esen •
Ozge Baglayan
Received: 30 December 2013 / Accepted: 27 February 2014
� Springer Science+Business Media New York 2014
Abstract Undoped ZnO and Al-doped zinc oxide
(ZnO:Al) thin films with different Al concentrations were
prepared onto Si (100) substrate by pulsed filtered cathodic
vacuum arc deposition system at room temperature. The
influence of doping on the structural and optical properties
of thin films was investigated. The preferential (002) ori-
entation was weakened by high aluminum doping in films.
Raman measurement was performed for the doping effects
in the ZnO. Atomic force microscopy images revealed that
the surface of undoped ZnO film grown at RT was
smoother than that of the Al-doped ZnO (ZnO:Al) films.
The reflectance of all films was studied as a function of
wavelength using UV–Vis–NIR spectrophotometer. Aver-
age total reflectance values of about 35 % in the wave-
length range of 400–800 nm were obtained. Optical band
gap of the films was determined using the reflectance
spectra by means of Kubelka–Munk formula. From optical
properties, the band gap energy was estimated for all films.
1 Introduction
As a large exciton binding energy (around 60 meV at room
temperature), a wide and direct band gap [1], ZnO thin
films have attracted a great attention. In recent years,
because of their properties, these films have been exten-
sively researched on various applications such as trans-
parent conducting films for varistors, gas-sensing, UV
light-emitters, solar cells, laser diodes.
Because of its native point defects, zinc oxide is not
efficient donors, it is highly resistive in the undoped state.
ZnO thin films doped with IIIrd group elements such as B, Al,
Ga, In, Ti have high quality, conductivity and transmittance
in the visible region. Among IIIrd group elements, aluminum
has the most promising element. It is a relatively durable,
soft, white, ductile, lightweight and malleable metal
depending on the surface roughness. Undoped and/or doped
ZnO films have attracted a lot of attention. During the last
decades, transparent conductive oxide (TCO) thin films have
been studied on their luminescence and electrical properties.
TCO doped by metal oxides, are used in optoelectronic
devices. TCO thin films have been generally used as trans-
parent conductive electrodes in photovoltaic applications
and display devices [2–6]. Aluminum-doped zinc-oxide
(AZO), indium-doped cadmium-oxide [7] and tin-doped
indium-oxide [8] have been proposed as alternative materials
for TCO. Among TCO films, AZO films are being consid-
ered as manufacturing transparent electrodes due to their
important properties, such as; inexpensive, nontoxic nature,
good electrical conductivity, good adhesion to substrate,
chemical inertness, high luminous transmittance and long
term environmental stability [9–12]. Aluminum doped ZnO
is TCO materials group that consisted of aluminum and zinc.
Al-doping of ZnO films have been studied by growing
ZnO with Al doping. In these studies, for the preparation of
D. K. Takci � S. Yilmaz � R. Esen
Department of Physics, Cukurova University,
01330 Adana, Turkey
E. Senadim Tuzemen (&)
Department of Physics, Nanotechnology Center,
Cumhuriyet University, 58140 Sivas, Turkey
e-mail: [email protected]
K. Kara
Department of Physics, Istanbul University,
34314 Istanbul, Turkey
O. Baglayan
Department of Physics, Anadolu University,
Eskisehir, Turkey
123
J Mater Sci: Mater Electron
DOI 10.1007/s10854-014-1843-0
Al doped ZnO films, many techniques including spray
pyrolysis process [13], sol gel method [14, 15], magnetron
sputtering method [16], hydrothermal method [17], filtered
cathodic vacuum arc technique [18], chemical vapor
deposition [19], direct current (DC) sputtering technique
[20] have been used.
The effects of structural, electrical and optical properties
of Al doped zinc oxide thin films have been widely studied by
filtered cathodic vacuum arc deposition system. Lee et al.
[18] showed that the lowest resistivity of 8 9 10-4 ohm cm
was obtained for the Al-doped (5 at.%) film prepared at a
substrate temperature of 150 �C. Furthermore, they showed
that the optical absorption edge was found to shift to the
shorter wavelength with a reduction in substrate tempera-
ture. Gontijo et al. [21] reported that the films prepared from
the electrodes doped with 6 at.% of aluminum, showed the
best results regarding the structure, resistivity, transmittance
and FM; however, indicated that the mean free time and
mobility of the films deposited with electrodes with 4 at.% of
aluminum determined the highest values. Anders et al.
investigated the optical and electrical properties of AZO thin
films deposited on glass and silicon by pulsed filtered
cathodic arc deposition. They showed that the quality of the
AZO films were strongly depended on growth temperature
while marginal improvements were only obtained with post-
deposition annealing. They reported that the best films,
grown at a temperature of about 200 �C, had resistivities in
the low to mid 10-4 X cm range with a transmittance better
than 85 % in the visible part of the spectrum [22]. Gao et al.
reported that approximately 200 nm thick ZnO:Al films that
have high visible and infrared transmittance properties, were
prepared on glass substrates by the pulsed filtered cathodic
arc technique and rapid thermal annealing. They showed that
as the annealing temperature increases from 500 to 650 �C,
the visible transmittance remains nearly constant (*85 %)
while the infrared (780–2,500 nm) transmittance consider-
ably improves from 22 % for the as-deposited film to 58 % at
600 �C and 71 % at 650 �C at 2,500 nm for the annealed
films. The high-temperature annealing improved the crys-
tallinity and transmittance of the films [23]. In this paper, we
performed the deposition of undoped ZnO and ZnO:Al thin
films by pulsed filtered cathodic vacuum arc deposition
(PFCVAD) technique at room temperature, their structural
and optical properties.
2 Experimental details
Doped and undoped ZnO thin films were grown on Si (100)
substrates by pulsed filtered cathodic vacuum arc deposi-
tion. To examine the effect of varying Al content, metallic
targets with Al content of 0, 2, 4, 6 and 10 at.% were used.
During the growths, the substrate temperature was kept at
room temperature, the base pressure of the deposition
chamber was kept around 10-5 Torr and the working
pressure was about 7 9 10-2 Torr. The base pressure and
working pressure were controlled by SRS Stanford
Research Systems 4 Model PPM 100. High purity
(99.999 % pure) oxygen gas was introduced into the
chamber and controlled by multi gas controller. During the
deposition process trigger voltage and arc voltage were set
to be 20 kV and 600 V, respectively.
XRD studies were carried out using Rigaku Miniflex
system with CuKa radiation (k = 1.54059 A). Surface
morphology of the samples was investigated by atomic
force microscopy (AFM) (XE-70) measurements. Raman
measurements were carried out by Bruker Senterra Dis-
persive Raman Microscope using the 532 nm laser as an
excitation source. Diffuse reflectance of the ZnO:Al films
were measured wavelength range of 300-800 nm, using a
double-beam UV–Vis–NIR spectrophotometer (Cary 5000,
Varian) with a Cary 5000 Internal Diffuse Reflectance
Accessory consisting of a integrating sphere of 110 mm in
diameter. Baseline was recorded with the polytetrafluoro-
ethylene (PTFE) reference disk covering the reflectance
port. Data were collected at a scan rate of 600 nm/min with
a data interval of 1.0 nm, a signal band width of 2.0 nm
and signal-averaging time of 0.1 s in the UV–Vis range.
3 Results
3.1 XRD results of films
For comparison of the structural quality of undoped and Al
doped ZnO films on Si (100) substrate, XRD studies were
carried out. XRD measurements were taken from 20� to
70� with a 0.02� step size. The crystal lattice constant c and
the interplanar distances of the diffracting planes d were
identified using the Bragg equation nk = 2d sin h, where
n is the order of the diffracted beam, k is the wavelength of
the X-ray and h is the angle between the incoming X-ray
and the normal of the diffracting planes. The grain sizes of
the crystallites were determined from X-ray diffraction
data. The crystallite grain size D can be estimated using the
Scherrer Formula [24]
D ¼ 0:9kbcosh
ð1Þ
where k is the X-ray wavelength, h is the Bragg diffraction
angle, b is the FWHM in radians.
The X-ray diffraction patterns of the undoped ZnO and
Al-doped ZnO (AZO) thin films with different Al con-
centrations (2–10 at.%) are shown Fig. 1. One point should
be noted that all the films exhibit preferential (002) ori-
entation with c-axis perpendicular to the substrate surface,
J Mater Sci: Mater Electron
123
which indicates the ZnO thin films are of hexagonal
wurtzite crystal structure. The intensity of (002) peak
decreases by increasing of doping levels from 0 to 10 at.%
Al. When doping rate increases, the crystallinity of films
deteriorates. This may be appeared depending on the for-
mation of stresses. Diffraction peaks position is slightly
shifted towards higher angle increasing of doping levels
from 0 to 4 at.%. The crystalline quality of the films is
reduced in this range. The FWHM (full width at half-
maximum) of the peaks is rised by increasing doping
concentration, as given in Table 1. An increase in Al
doping may cause a decrease in crystallite size and the film
crystallinity is deteriorated.
The interplanar distances of the diffracting planes
d decreased with increasing of doping levels from 0 to
4 at.%. This is due to the smaller ionic bond radius of Al–O
than that of Zn–O [25]. Depending on the doping levels,
the grain size decreases as seen in Table 1. This indicates
an increase in lattice strain [26]. Also in Fig. 2, grain size
versus different Al concentration is plotted.
3.2 AFM results of films
The surface properties of the undoped and doped films
have an important role on their optical and electrical
properties. The morphology of the films was characterized
by atomic force microscopy (AFM). Figures 3 and 4 show
the AFM images of 2D and 3D, respectively.
It was observed that the Al concentration increases, also
increasing the RMS (see also Table 2). Smooth of the
coating surface decreases as a result of increasing Al
doping. These results indicate that the surface of undoped
ZnO film is smoother than that of the Al-doped ZnO films.
3.3 Raman results of films
Figure 5 shows the Raman scattering (RS) spectra of films,
excited by the 532 nm laser lines using laser power of 5 mW. It
is observed at one mode about 439 cm-1 of the undoped ZnO
and Al-doped ZnO (AZO) thin films by different Al concen-
trations. This mode corresponds to E2 (high) mode of wurtzite
ZnO [27, 28]. The increasing of the doping concentration is
caused to broaden in the FWHM of the E2 (high) peak and
decrease in intensity of its, which is contributed to Al ions in
the ZnO lattice substituting for Al in the Zn position [28].
3.4 Optical results of films
The optical properties of undoped ZnO and Al-doped ZnO
(AZO) thin films with different Al concentrations on Si (100)
substrate were examined by using a Varian Carry 5000
model UV–Vis–NIR spectrophotometer with an integrating
sphere. In this analysis, it was used PTFE as the reference
Al 2%
Al 4%
Si
20 30 40 50 60 70
2θ (Degree)
0
2000
4000
6000
8000
10000In
tens
ity (
a.u.
)
Al 10%
Al 6%
Al 0%
Fig. 1 The X-ray diffraction patterns of the undoped ZnO and
Al-doped ZnO (AZO) thin films with different Al concentrations
0 2 4 6 8 10
Al %
0
4
8
12
16
20
FW
HM
(Deg
ree)
0
4
8
12
16
20
Gra
insi
ze (
nm)
FWHM (Degree)Grain size (nm)
Fig. 2 FWHM and grain size versus different Al concentration
Table 1 Comparison of the structural characteristics of the undoped
ZnO and Al-doped ZnO (AZO) thin films with different Al
concentrations
2 h (�) d (nm) c (nm) FWHM (�) D (nm)
ZnO 0 at.% 34.44 0.2601 0.5202 0.466 17.85
Al 2 at.% 34.53 0.2595 0.5190 1.452 5.732
Al 4 at.% 35.00 0.2561 0.5122 1.571 5.304
Al 6 at.% 34.55 0.2593 0.5186 1.830 4.546
Al 10 at.% 34.22 0.2617 0.5234 1.868 4.451
J Mater Sci: Mater Electron
123
disk. Diffuse and total reflectance spectra were taken in the
range of 300–800 nm in air at room temperature.
The Kubelka–Munk function was used to convert
reflectance measurements into equivalent absorption
spectra. The Kubelka–Munk theory allows us to calculate
the energy gap of the thin films on nontransparent sub-
strates. The diffuse reflectance of the sample is related to
the Kubelka–Munk function F(R). Diffuse reflectance data
was calculated using Kubelka–Munk function by the
relation
Fig. 3 AFM 2D images of the
undoped ZnO and Al-doped
ZnO (AZO) thin films with
different Al concentrations
J Mater Sci: Mater Electron
123
FðRÞ ¼ ð1� RÞ2
2R¼ K
s; ð2Þ
where R is the diffuse reflectance of the sample, F(R) is
the Kubelka–Munk function which corresponds to the
absorbance, K is the absorption coefficient and s is the
scattering coefficient. Absorption coefficient of a direct
band gap semiconductor is related with Tauc’s equation
[29, 30]:
ahm ¼ A hm� Eg
� �n ð3Þ
where a is the linear absorption coefficient of the material,
hv is the photon energy, A is a proportionality constant, n is
a constant accounting for the type of optical transition,
n = 1/2 indicates direct allowed transition. Kubelka–Munk
function is directly proportional to the absorbance
(/¼ FðRÞt
, where t is the thickness of film) [31].
Figure 6 shows the total reflectance (specular and dif-
fuse) spectrum of undoped ZnO and Al-doped ZnO (AZO)
thin films by different Al concentrations.
UV–Visible diffuse reflectance spectroscopy (UV/DRS)
was used to investigate the optical properties of ZnO and
spectroscopy measurements were continued at room
Fig. 4 AFM 3D images of the
undoped ZnO and Al-doped
ZnO (AZO) thin films with
different Al concentrations
Table 2 Roughness statistical
parameters: Rq = standard
deviation of the height value
(RMS: the root mean squared
roughness), Ra = average
roughness
Rq (nm) Ra (nm)
Al 0 at.% 0.77 0.53
Al 2 at.% 0.86 0.62
Al 4 at.% 0.91 0.70
Al 6 at.% 1.68 1.29
Al 10 at.% 2.06 1.72
J Mater Sci: Mater Electron
123
temperature in the wavelength range of 300–800 nm.
Diffuse reflectance spectra of undoped ZnO and Al-doped
ZnO (AZO) thin films with different Al concentrations
(2–10 at.%) is shown in Fig. 7.
According to graph of F(R) versus wavelength values
(Fig. 8), the UV-absorption band of undoped ZnO and Al-
doped ZnO (AZO) thin films with different Al concentra-
tions (2–10 at.%) was around 380–420 nm. The UV
absorption edge is blue, shifted as increasing Al doping
concentration from 0 to 4 at.%. The absorption edge shifts
towards higher wavelength region at the Al concentration
of 6 and 10 at.%.
The energy band gap of films can be calculated by
plotting the square of the Kubelka–Munk function versus
energy. According to this calculation, all of thin films that
prepared in our studies, have a direct energy gap. Value of
the linear portion of the curve onto the x-axis gives us the
energy band gap of the films by Fig. 9.
300 400 500 600 700 800
Wavelength (nm)
0
20
40
60
80
Tot
alre
flec
tanc
e(R
%)
SiAl 0%Al 2%Al 4%Al 6%Al 10%
Fig. 6 The total reflectance (specular and diffuse) spectra
300 400 500 600 700 800
Wavelength (nm)
0.4
0.6
0.8
1
Dif
fuse
refl
ecta
nce
(R%
)
SiAl 0%Al 2%Al 4%Al 6%Al 10%
Fig. 7 Diffuse reflectance spectra
300 400 500 600 700 800
Wave length (nm)
40
60
80
100
120
140
160
F(R
)
Al 0%Al 2%Al 4%Al 6%Al 10%
Fig. 8 F(R) versus wavelength
Al 10%
Si
200 400 600 800 1000 1200
Wavenumber (cm-1)
0
1000
2000
3000
Ram
anin
tens
ity
(a.u
.)
Al 6%
Al 4%
Al 2%
Al 0%
Fig. 5 The Raman spectra of Si, undoped ZnO and Al doped ZnO
films
J Mater Sci: Mater Electron
123
As seen in Figs. 9 and 10, the band gap energy estimated
for undoped ZnO and Al-doped ZnO (AZO) thin films with
different Al concentrations (2–10 at.%) were found to be
3.090, 3.225, 3.30, 3.225 and 3.112 eV, respectively.
The Al doping strongly affects the optical properties of the
ZnO thin films. Several factors can be effective to the energy
band gap dependence to doping. Stress/strain can also affect
the band gap energy [32]. In this study, the band gap value
increases with an increase of Al concentration from 0 to
4 at.%. The increase of Eg value by increasing Al contents
can be attributed to the well-known Burstein Moss effect
[33]. The optical band gap decreases at the Al concentration
of 6 and 10 at.%. The excess Al atoms were segregated into
the grain boundaries at the doping concentration of 6 and
10 at.%. Venkatachalam et al. [34] reported that the segre-
gated Al atoms did not act as a dopant. Also, due to
increasing of carrier concentration, it may be attributed to the
band shrinkage effect [35]. The optical band gap of doped
zinc oxide is broader than that of undoped zinc oxide films.
Wang et al. [36] explained that it can be attributed to the
according to the Burstein Moss theory.
4 Conclusions
Undoped ZnO and Al-doped ZnO thin films were success-
fully grown by PFCVAD technique. The effects of doping
that is different Al concentrations on the structural, mor-
phology and optical properties of ZnO were examined. One
point should be noted that all the films exhibit preferential
(002) orientation with c-axis perpendicular to the substrate
surface, which indicates the ZnO thin films are of hexagonal
wurtzite crystal structure. It was found that high aluminum
doping in films was weakened the crystallinity of the films.
The surface properties of the undoped and doped films are
important factors for technological applications. The surface
property of the films was characterized by atomic force
microscopy (AFM). It is found that RMS increases by
increasing of Al concentration. It is observed at one mode
about 439 cm-1 of the undoped ZnO and Al-doped ZnO
(AZO) thin films that are prepared different Al concentra-
tions. While the band gap value increases depending on
increase of Al concentration from 0 to 4 at.%, it decreases at
the Al concentration of 6 and 10 at.%. Al-doped ZnO thin
films grown on p-type Si substrate by PFCVAD technique at
room temperature may be potential candidate for optoelec-
tronic device applications.
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