Nickel-lithium oxide alloy transparent conducting films deposited by spray pyrolysis technique

Article abstract of DOI:10.1016/j.jallcom.2010.11.075

In this research, nickel oxide (NiO) transparent semiconducting films are prepared by spray pyrolysis technique on glass substrates. The effect of Ni concentration in initial solution and substrate temperature on the structural, electrical, thermoelectric

Full text of DOI:10.1016/j.jallcom.2010.11.075

Journal of Alloys and Compounds 509 (2011) 2770–2775
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Nickel–lithium oxide alloy transparent conducting films deposited by spray
pyrolysis technique
a
a
b,∗
Hasan Azimi Juybari , Mohammad-Mehdi Bagheri-Mohagheghi , Mehrdad Shokooh-Saremi
a
School of Physics, Damghan University, Damghan, Iran
Department of Electrical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2010
Received in revised form 2 November 2010
Accepted 14 November 2010
Available online 21 November 2010
In this research, nickel oxide (NiO) transparent semiconducting films are prepared by spray pyrolysis
technique on glass substrates. The effect of Ni concentration in initial solution and substrate temperature
on the structural, electrical, thermoelectrical, optical and photoconductivity properties of NiO thin films
are studied. The results of investigations show that optimum Ni concentration and suitable substrate
temperature for preparation of basic undoped NiO thin films with p-type conductivity and high optical
◦
transparency is 0.1 M and 450 C, respectively. Then, by using these optimized deposition parameters,
Keywords:
Transparent conducting oxides
Spray pyrolysis
Nickel oxide
Lithium doping
nickel–lithium oxide ((Li:Ni)Ox) alloy films are prepared. The XRD structural analysis indicate the for-
mation of the cubic structure of NiO and (Li:Ni)Ox alloy films. Also, in high Li doping levels, Ni2O3 and
NiCl2 phases are observed. The electrical measurements show that the resistance of the films decreases
with increasing Li level up to 50 at%. For these films, the optical band gap and carrier concentration are
1
5
18
−3
obtained to be 3.6 eV and 10 –10 cm , respectively.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
sol–gel, and spray pyrolysis technique [19–25]. The spray pyrolysis
SP) technique is a very important method for preparation of trans-
(
Transparent conducting oxides (TCOs) have been extensively
parent conducting oxide films. Spray pyrolysis is a relatively simple,
atmospheric pressure deposition method and an inexpensive tech-
nique for large-area coating. However, few efforts have been made
to systematically investigate the effect of deposition parameters
and Li impurity on the electrical and thermo-electrical properties
of the NiO thin films fabricated by spray pyrolysis technique.
In this research, we investigate the effect of the deposition
parameters, such as substrate temperature and Li concentration
level (in precursor solution), on the physical properties of the
NiO films prepared by spray pyrolysis method. The electrical,
thermo-electrical, structural and optical properties of these films
are studied using Hall effect and Seebeck measurements, X-ray
diffraction (XRD), Scanning electron microscopy (SEM) analysis and
UV–vis absorption spectroscopy.
studied in recent years since they not only exhibit high optical
transparency in the visible region but also the high electrical con-
ductivity [1–3]. TCOs, such as ITO, ZnO and SnO2 are widely used
in a variety of optoelectronic devices [4–6].
In contrast to n-type TCOs (like SnO , In O , and ZnO), nickel
2
2
3
oxide (NiO) shows p-type semiconductivity and has attracted much
attention due to its excellent chemical stability and unique optical,
electrical and magnetic properties. Nickel oxide has found impor-
tant applications in electro-chromic devices, organic light emitting
diodes, chemical sensors, dye sensitized solar cells, and thin film
p–n junctions [7–11]. Undoped NiO has a wide direct band gap of
3
.5–4 eV [12,13] and exhibits low p-type conductivity. Its resistiv-
ity can be decreased by doping with monovalent impurities, such
as lithium (Li) [14–16]. In 2003, Ohta et al. fabricated an ultravi-
olet detector based on lithium-doped NiO (NiO:Li) and ZnO films
2. Experimental procedure
[
17]. Recently, NiO thin films with switching properties suitable for
Undoped NiO and (Li:Ni)Ox alloy thin films were deposited on glass substrate by
spray pyrolysis technique. For deposition of undoped films, certain amount of nickel
nitrate hexa-hydrate (Ni(NO3)2·6H2O) was dissolved in distilled water to make ini-
tial (precursor) solution. For alloy films, the precursor solution was prepared by
dissolving Ni(NO3)2·6H2O and different amounts of lithium chloride (LiCl) in dis-
tilled water. The [Li]/[Ni] atomic ratio was 0–100 at%. The NiO and (Li:Ni)Ox films
were deposited under the similar conditions: solution volume (V) = 30 ml, deposition
rate (R) = 10 ml/min, nozzle to substrate distance (d) = 35 cm. Compressed air was
used as the carrier gas. We optimized the Ni concentration and the substrate tem-
perature for deposition of the basic undoped NiO films, which will be presented in
Section 3.
memory devices have been fabricated by reactive sputtering [18].
NiO films have been prepared by variety of physical and chemi-
cal techniques, such as sputtering, pulsed laser deposition (PLD),
chemical vapor deposition (CVD), electron beam evaporation,
∗ Corresponding author.
E-mail address: mehrdad.s.saremi@gmail.com (M. Shokooh-Saremi).
0
925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.11.075

H.A. Juybari et al. / Journal of Alloys and Compounds 509 (2011) 2770–2775
2771
◦
Fig. 1. (a) The XRD patterns of NiO films prepared with different Ni concentration in precursor solution and Tsub = 450 C. (b) The XRD patterns of NiO films deposited at
various substrate temperatures for 0.1 M Ni concentration.
The sheet resistance (RS) of the films was measured by two-point probe method
using thermally evaporated aluminum electrodes. The concentration and type of
the majority carriers was determined by Hall effect experiment. Majority carrier
concentration was determined using the following equation [26]:
Ni O phase is observed. Based on this figure, films prepared with
2 3
0
.1 M of Ni in solution show the best single phase characteristics.
The X-ray diffraction patterns of NiO films deposited at different
substrate temperatures are shown in Fig. 1(b), keeping Ni concen-
tration at 0.1 M. The films show cubic NiO crystallographic lattice
with preferred orientation along (1 1 1) direction. The intensity of
the (1 1 1) peak increases with increasing the substrate tempera-
IB
Nn,p =
(1)
|
q|VH t
where I, B, t, q and VH are the measured current, magnetic flux density, film thickness,
electron charge and Hall voltage, respectively. By applying a temperature gradient
between the two ends of the samples, the thermoelectric e.m.f. of the prepared films
was measured and then the Seebeck coefficients were determined by calculating the
slope of the thermoelectric e.m.f. versus temperature difference.
For structural study of the films, XRD patterns of the NiO and (Li:Ni)Ox films were
recorded by D8 Advance Bruker system using Cu K␣ (ꢀ = 0.15406 nm) radiation. The
average crystallite size (D) was calculated using the Scherrer’s formula [26]:
◦
◦
ture from 350 C to 450 C and then decreases [29]. For preparation
of the alloy films, we kept Ni concentration and substrate temper-
◦
ature at 0.1 M and 450 C (as the optimal deposition parameters),
respectively.
Fig. 2 shows the XRD patterns of the deposited (Li:Ni)Ox alloy
films with different Li doping levels from 0 at% to 100 at% in the
solution. As seen, all films are polycrystalline and all the crys-
tallographic peaks belong to the cubic NiO phase with preferred
orientation along (1 1 1) direction. At 50 at% Li doping level, the
intensity of the peak corresponding to the (1 1 1) plane is the
strongest, however, at higher doping levels, other phases such as
Ni O and NiCl2 are observed.
kꢀ
D =
(2)
ıw cos ꢁ
where ıw is the full width at half maximum (FWHM) of the corresponding XRD
peak, k is a constant (∼1) and ꢁ is the Bragg angle.
Surface morphology of the films was observed by Philips XL-30 SEM system. The
optical measurements were carried out in the range of 190–1100 nm using Unico
2
3
4
802 spectrophotometer system. The direct band gap (Eg) of the prepared films was
The XRD parameters and mean grain size of the undoped NiO
and (Li:Ni)Ox alloy films for (1 1 1) crystallographic orientation,
have been summarized in Table 1.
Scanning electron microscopy (SEM) micrographs of the
(Li:Ni)Ox alloy films are shown in Fig. 3(a)–(f). The micrographs
show that the nanostructure of the films exhibits the particle-
cluster type growth. The undoped NiO film has a nearly smooth
surface; however the (Li:Ni)Ox films have a porous structure and
their corresponding grain size increases with increasing the doping
level.
2
obtained from the extrapolation of the linear part of the (˛hꢂ) curve versus photon
energy (hꢂ) and using the following equation [27]:
2
(
˛hv) = A(hv − Eg )
(3)
where ˛ and Eg are the absorption coefficient and the energy gap, respectively,
and A is a constant.
3
. Results and discussion
3.1. Structural and electrical properties
To find the optimal parameters for deposition of the basic
undoped NiO films (Ni concentration in solution and substrate tem-
perature), two sets of experiments were carried out. To obtain the
appropriate concentration, Ni molarities in solution was changed
◦
from 0.05 to 0.2 M and substrate temperature kept at 450 C. Then,
in order to study the effect of substrate temperature, NiO thin films
◦
were prepared in different temperatures (350–550 C) in optimum
concentration found from the first set of experiments. After that
(
Li:Ni)Ox films were deposited on glass substrate with precursors
with optimal Ni concentration and optimal substrate temperature.
Fig. 1(a) shows the XRD patterns of the NiO films prepared with
different nickel concentrations. As seen, all films have polycrys-
talline NiO phase with a cubic structure and preferred orientation
along (1 1 1) direction. The intensity of the peaks corresponding to
the (1 1 1), (2 0 0) and (2 2 0) orientations increases with Ni molarity
increase in the solution. In addition, this increase in the intensity of
the peaks may be attributed either to the grain growth associated
with larger thicknesses, or the increase in the degree of crystallinity
by increasing the solution molarity [28]. Also, in the XRD patterns
corresponding to the higher Ni concentrations (0.15 M and 0.2 M),
Fig. 2. X-ray diffraction patterns of various (Li:Ni)Ox alloy films: (a) undoped NiO,
(
b) NiO:Li (10 at%), (c) NiO:Li (20 at%), (d) NiO:Li (30 at%), (e) NiO:Li (40 at%), (f)
NiO:Li (50 at%), (g) NiO:Li (60 at%), (h) NiO:Li (80 at%) and (i) NiO:Li (100 at%). Ni
◦
concentration is 0.1 M and Tsub = 450 C.

2
772
H.A. Juybari et al. / Journal of Alloys and Compounds 509 (2011) 2770–2775
Table 1
Summary of XRD parameters and mean grain size of all sample for (1 1 1) orientation.
◦
◦
Sample
2ꢁ ( )
Lattice distance (Å)
FWHM ( )
Mean grain size (nm)
Identification with (hkl) value
◦
(
a) The effect of Ni concentration in solution (Tsub = 450 C)
0
0
0
0
.05 M
.1 M
.15 M
.2 M
–
–
–
–
–
37.35
37.38
37.40
2.409
2.417
2.415
0.724
0.551
0.499
12.38
16.23
17.90
Cubic-NiO
Cubic-NiO
Cubic-NiO
(
b) The effect of substrate temperature (Ni concentration = 0.1 M)
◦
3
4
4
5
5
50 C
37.41
37.50
37.48
–
2.402
2.394
2.397
–
1.112
0.972
0.696
–
8.03
9.11
12.76
–
Cubic-NiO
Cubic-NiO
Cubic-NiO
–
–
◦
00 C
◦
50 C
◦
00 C
◦
50 C
–
–
–
–
◦
(
c) The effect of Li doping (Ni concentration = 0.1 M and Tsub = 450 C)
Un-doped NiO
NiO:Li (10 at%)
NiO:Li (20 at%)
NiO:Li (30 at%)
NiO:Li (40 at%)
NiO:Li (50 at%)
NiO:Li (60 at%)
NiO:Li (80 at%)
NiO:Li (100 at%)
37.40
37.50
37.36
37.46
37.44
37.52
37.56
37.44
37.42
2.403
2.396
2.405
2.399
2.400
2.395
2.393
2.400
2.402
0.787
0.674
0.655
0.621
0.575
0.496
0.464
0.425
0.404
11.35
13.17
13.68
14.32
15.49
17.88
19.08
20.95
22.07
Cubic-NiO
Cubic-NiO
Cubic-NiO
Cubic-NiO
Cubic-NiO
Cubic-NiO
Cubic-NiO
Cubic-NiO
Cubic-NiO
The results of the electrical measurements for the undoped and
alloy films have been summarized in Table 2. The thickness of
the films was determined from transmission data using the Puma
software [30]. As seen in this table, undoped NiO films deposited
at the optimal condition exhibit the lowest sheet resistance. The
electrical measurements for (Li:Ni)Ox alloy films show that the
resistance of the films decreases with increasing Li-doping up to
reveal that the majority carriers are holes for all samples (p-type
conductivity).
3.2. Thermoelectrical properties
Fig. 4(a)–(c) shows the measured Seebeck coefficients versus
temperature for undoped NiO and (Li:Ni)Ox alloy films. The posi-
tive sign of the Seebeck coefficients of the films confirms the p-type
conductivity. As shown in Fig. 4(a), the highest value of the Seebeck
5
0 at% and then increases. The lowest sheet resistance was obtained
for doping level of 50 at%. The Hall effect experiment results
Fig. 3. SEM images of undoped and (Li:Ni)Ox alloy films: (a) Undoped NiO, (b) NiO:Li (20 at%), (c) NiO:Li (40 at%), (d) NiO:Li (50 at%), (e) NiO:Li (60 at%) and (f) NiO:Li (100 at%).

H.A. Juybari et al. / Journal of Alloys and Compounds 509 (2011) 2770–2775
2773
Table 2
The electrical measurement results for the undoped and Li-doped NiO films. Thickness determination error is ∼± 5%.
Sample
RS (Mꢃ/ꢀ)
Carrier concentration (cm−3)
Thickness (t), (nm)
◦
(
a) The effect of solution concentration (Tsub = 450 C)
1
1
1
1
5
5
5
5
0
0
0
0
.05 M
.1 M
.15 M
.2 M
82.5
25.6
29.3
36.2
2.21 × 10
6.35 × 10
6.67 × 10
7.11 × 10
170
190
220
240
(b) The effect of substrate temperature (Ni concentration = 0.1 M)
◦
15
15
15
14
14
3
4
4
5
5
50 C
34.3
23.7
20.6
53.1
71.8
2.37 × 10
3.38 × 10
6.93 × 10
8.72 × 10
7.96 × 10
260
230
195
135
95
◦
00 C
◦
50 C
◦
00 C
◦
50 C
◦
c) The effect of Li doping (Tsub = 450 C and Ni concentration = 0.1 M)
(
15
16
17
18
17
17
15
15
15
Un-doped NiO
NiO:Li (10 at%)
NiO:Li (20 at%)
NiO:Li (30 at%)
NiO:Li (40 at%)
NiO:Li (50 at%)
NiO:Li (60 at%)
NiO:Li (80 at%)
NiO:Li (100) at%)
21.9
18.9
15.4
13.6
13.3
4.7
5.3
7.7
16.6
6.50 × 10
2.16 × 10
1.02 × 10
1.14 × 10
4.20 × 10
1.91 × 10
4.87 × 10
4.15 × 10
3.58 × 10
187
190
194
197
202
205
209
214
219
coefficients belongs to the undoped sample prepared with 0.1 M
Ni concentration in the precursor solution. Fig. 4(b) shows that
the undoped sample deposited at 450 C has the largest Seebeck
coefficient among the ones prepared with other substrate tem-
peratures. In case of the Li-doped films, Fig. 4(c) shows that
for the sample with 50 at% doping level, the value of Seebeck
coefficient is the highest among the samples with other doping
levels.
3.3. Optical properties
◦
Optical transmittance of the undoped NiO and (Li:Ni)Ox films in
300–1100 nm range is shown in Fig. 6(a)–(c). In case of undoped
films deposited with different Ni concentration (Fig. 6(a)), the
average transmittance decreases from ∼90% to ∼70% when the
◦
Ni molarity changes from 0.05 M to 0.2 M (T = 450 C) due to
sub
increase in the film thickness [28]. In contrast, Fig. 6(b) shows that
The variation of thermoelectric e.m.f. versus temperature dif-
ference between the two ends of samples (␦T) is shown in
Fig. 5(a)–(c). Fig. 5(a) and (b) belongs to the undoped NiO
samples. Fig. 5(c) shows that the highest thermoelectric e.m.f.
is ∼80 mV (ıT = 160 K) corresponding to the 50 at% Li-doped
sample.
the optical transparency of undoped films increases from ∼60% to
◦
∼95% when the substrate temperature increases from 350 C to
◦
550 C (Ni concentration = 0.1 M). Transparency of the Li-doped NiO
films decreases from ∼80% to ∼50% when the doping level increases
from 0 at% to 100 at% under optimal deposition condition (Fig. 6(c)).
This can be also attributed to the increase in the film thickness [14].
◦
Fig. 4. Seebeck coefficient (S) versus temperature for: (a) Undoped samples with different Ni concentration in the solution (Tsub = 450 C), (b) undoped samples deposited on
substrates with different temperatures (Ni concentration = 0.1 M), and (c) Li-doped NiO films with different doping levels.

2
774
H.A. Juybari et al. / Journal of Alloys and Compounds 509 (2011) 2770–2775
◦
Fig. 5. The variation of thermoelectric e.m.f. with temperature difference (ıT) for: (a) undoped samples with different Ni concentration in the solution (Tsub = 450 C),
b) undoped samples deposited on substrates with different temperatures (Ni concentration = 0.1 M), and (c) Li-doped NiO films deposited with different level of
doping.
(
Fig. 7(a)–(c) presents the variation of (˛hꢂ)2 versus hꢂ (pho-
films decreases from 3.704 eV to 3.627 eV when the substrate
◦
◦
ton energy) for the films, in which ˛ is the absorption coefficient.
The optical band gap has been calculated by extrapolation of
linear part of the curve. As seen in Fig. 7(a), the optical band
gap gradually decreases from 3.718 eV to 3.515 eV by increas-
ing Ni concentration in the solution from 0.05 M to 0.2 M when
temperature increases from 350 C to 450 C (due to grain size
◦
increase [29]) and then increases for temperatures up to 550 .
This increase could be related to the amorphous structure of films
deposited at high substrate temperature. In amorphous materi-
als, electron transitions may be either from localized states in the
conduction band or from extended states in the valence band to
localized states at the conduction edge. This leads to lower ener-
gies than those for polycrystalline or bulk crystalline materials
◦
Tsub = 450 C. This may be attributed to the increase in the film
thickness as well as the crystalline order and increasing grain
size [28]. Fig. 7(b) shows that the band gap of the undoped NiO
◦
Fig. 6. Optical transparency of: (a) undoped samples with different Ni concentration in the solution (Tsub = 450 C), (b) undoped samples deposited on substrates with different
temperatures (Ni concentration = 0.1 M), and (c) Li-doped films deposited with different Li concentration.

H.A. Juybari et al. / Journal of Alloys and Compounds 509 (2011) 2770–2775
2775
and various Li-doping levels have been investigated. It is observed
that the physical properties of films strongly depend on the depo-
sition conditions and Li-doping levels. The obtained results lead to
the following conclusions:
(
1) The prepared films exhibit a preferential growth along the
1 1 1) direction with a cubic NiO phase. With increasing the
(
Ni concentration in precursor solution and substrate tempera-
ture, the (1 1 1) peak intensity is enhanced and crystallite size
increases. Also, at 60 at%, 80 at% and 100 at% Li doping levels,
other phases such as Ni O3 and NiCl2 are observed.
2
(
2) Electrical measurements of the samples show that the sheet
resistance of the films decreases with increasing Ni concen-
◦
tration up to 0.1 M and substrate temperature up to 450 C.
Also, the lowest resistance was obtained to be 4.7 (Mꢃ/ꢀ) for
doping level of 50 at%. The Hall effect and thermoelectrical mea-
surements have shown p-type conductivity in all films. The
highest Seebeck coefficient was 503 ␮V/K at 400 K for 50 at%
Li-doping.
(
3) The transparency of the films decreases with increasing Ni
concentration and increases with increasing the substrate tem-
perature. In case of (Li:Ni)Ox alloy films, transparency decreases
from ∼80% to ∼50% when the doping level increases from 0 at%
to 100 at%.
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