Transparent conducting lithium-doped nickel oxide thin films by spray pyrolysis technique

Article abstract of DOI:10.1023/A:1018657424566

Nickel oxide (NiO) and lithium-doped nickel oxide films were deposited by the spray pyrolysis technique using NiCl₂ and LiCl as starting materials. All the films were greenish-grey in colour and confirmed by X-ray analysis. The best NiO films were obtained when the substrate temperature, T_s = 480 °C where a conductivity of 2.1×10^-1 Ω^-1 cm^-1 and transparency above 80% in the visible region are achieved. High transparency (above 80%) and highly conducting NiO films were obtained when doped with lithium.

Full text of DOI:10.1023/A:1018657424566

JOURNAL OF MATERIALS SCIENCE 32 (1997) 3001—3006
Transparent conducting lithium-doped nickel oxide
thin films by spray pyrolysis technique
P . P U S P HARAJ AH, S . RADHAKRIS HN A
Institute of Advanced Studies, University of Malaya, 50603 Kuala Lum pur, Malaysia
A. K. AROF
Physics Division, Centre for Foundation Studies in Science, University of Malaya, 50603
Kuala Lum pur, Malaysia
Nickel oxide (NiO) and lithium -doped nickel oxide film s were deposited by the spray
pyrolysis technique using NiCl2 and LiCl as starting m aterials. All the film s were greenish-
grey in colour and confirm ed by X-ray analysis. The best NiO film s were obtained when the
substrate tem perature, Ts:480 °C where a conductivity of 2.1;10\1 )\1 cm \1 and
transparency above 80% in the visible region are achieved. High transparency (above 80%)
and highly conducting NiO film s were obtained when doped with lithium .
1. Introduction
2. Sample preparation
NiO is a candidate for p-type transparent conducting
films with a band gap energy from 3.6 to 4.0 eV [1].
Although stoichiometric NiO is an insulator with a
resistivity of the order of 10ꢀꢁ ) cm at room temper-
ature, its resistivity can be lowered by an increase of
Niꢁ> ions resulting from an addition of monovalent
atoms such as lithium or by the appearance of nickel
vacancies and/or interstitial oxygen in NiO crystalli-
ties [1, 2]. Transparent conducting nickel oxide
films have a wide range of applications in optoelec-
tronic and thermal devices. In the area of optoelec-
tronics, nickel oxide is well known as an electro-
chromic material [3, 4]. Another area where NiO
films are of great interest is in the new optical record-
ing material of a Ni—NiO heterogeneous system that
would realize a portable medium by using a laser
diode beam for recording and reading. The new me-
dium using a thin film of Ni—NiO needs no protecting
layer because of its stability under usage. This material
has advantages over conventional materials such
as tellurium which is easily oxidizable in high
humidity [5].
The nickel oxide and lithium-doped nickel oxide films
were prepared using the spray pyrolysis deposition
technique. Nickel chloride and lithium chloride were
used as the starting materials. The spraying solution
consists of 2 g of starting material, dissolved in a mix-
ture of ethyl alcohol and water with the volume ratio
of 3:1, respectively, which gives a total volume of
10 ml.
After spraying, the films were removed immediately
from the spraying chamber and allowed to cool in air
at room temperature to freeze the oxidation state.
Physical properties of the films were studied at dif-
ferent substrate temperature, ¹ , nozzle—substrate
ꢀ
distance, D , solution flow rate, F and lithium con-
ꢀꢁ
ꢂ
centrations. Further details of the experimental
methods are found in our earlier papers [8, 9].
3. Results and discussion
NiO films prepared by spray pyrolysis of NiCl solu-
tion were initially (all the films) yellowish in colour
ꢃ
There have been reports on nickel oxide films
produced by sputtering [2—5], by electron beam
evaporation [6] and by dip coating [7]. A resistivity
as low as 1.4;10\ꢀ ) cm and a hole concentration
of the order of 10ꢀꢂ cm\ꢁ were obtained by radio
frequency (r.f.) magetron sputtering. For a film
of thickness 110 nm an average transmittance of
about 40% was reported in the visible range [2].
Varkey and Fort [7] have deposited insulating
NiO films of 80 nm thickness using the dip
technique. An average transmittance of about
80% in the visible range and a band gap of 4.0 eV
were reported for the film produced using this
technique.
and became clear after cooling to room temperature.
At lower substrate temperatures (below 350 °C), the
films were physically unstable. These films absorb
water molecules from the air and form a watery sur-
face. At temperatures above 350 °C, the physical stab-
ility and adhesiveness to the substrate improves.
Cloudy greenish-grey films were obtained for films
deposited at substrate temperatures above 450 °C.
X-ray analysis shows that NiO films deposited by
spray pyrolysis are amorphous.
Fig. 1 shows scanning electron microscopy (SEM)
micrographs of the NiO films prepared at substrate
temperatures of 350 and 400 °C. It can be observed
that the droplets are uniformly distributed throughout
0022—2461 ( 1997 Chapman & Hall
3001

Figure 1 SEM micrograph of NiO films obtained at (a) ¹ "300 °C
ꢀ
and (b) ¹ "400 °C.
ꢀ
Figure 2 Variation of transmittance (ଙ), thickness (#), refractive
the films. Grain size is influenced by the substrate temp-
index (E) and band gap (*) with substrate temperature, ¹ .
ꢀ
erature, ¹ . From the micrographs it is observed that
ꢀ
large grains can be obtained by depositing at higher
¹ . This is because when the substrate temperature is
ꢀ
increased, the mobility of the molecules on the surface
is improved and causes an increase in grain size.
Microchemical analysis of the NiO film by electron
dispersive spectroscopy (EDAX) shows the decrease in
chlorine content in the films as the substrate temper-
ature is increased. This shows that at low temper-
energies than those for polycrystalline or bulk crystal-
line materials.
The transmission, refractive index, film thickness
and band gap of the NiO films are shown in Fig. 3 for
samples prepared at various nozzle—substrate distan-
ces. It can be observed that in general an increase in
nozzle—substrate distance improves the transmission
up to a nozzle—substrate distance of 29 cm beyond
which transmission begins to decrease and signifi-
cantly falls off beyond a nozzle—substrate distance of
30 cm. This improvement may be attributed to the
decrease in the band gap, which is observed to de-
crease from +2.85 eV for the sample prepared at
a nozzle—substrate distance of 26 cm to +2.50 eV for
the sample prepared at a nozzle—substrate distance of
29 cm. At nozzle—substrate distances greater than
29 cm, the band gap is observed to increase and it
increases significantly to +3.70 eV when the trans-
mission falls off significantly to just above 20% at
a nozzle—substrate distance of 34 cm.
atures only partial conversion of NiCl to NiO takes
ꢃ
place. The conversion of NiCl to NiO increases with
ꢃ
increasing substrate temperature.
The optical transmission of NiO films in the
wavelength range of 290—800 nm for different substra-
te temperatures, ¹ , is shown in Fig. 2. Transparency
ꢀ
of the film decreases with increasing substrate temper-
ature. Above 450 °C, the films turned greenish-grey
and cloudy [7]. The average transmission of very
cloudy films have been observed to be as low as 40%.
High transmittance in the visible region of the spec-
trum at lower ¹ suggests the possible use of these
ꢀ
films as glazing materials [2].
Extrapolation of the linear portion of the transmit-
tance curve to zero absorbance (100% transmission)
shows that the NiO films have an optical band gap
between 3.15—3.50 eV. Bulk crystalline NiO has
a band gap of &4.0 eV [1, 2], much higher than the
value obtained for sprayed films. The difference could
be mainly due to the amorphous structure. In
amorphous materials the 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 [10]. This leads to lower
Fig. 4 shows the relationship between the transmit-
tance, film thickness, refractive index and band gap
with the flow rate, F . It can be observed that transmit-
ꢂ
tance does not vary much as the solution flow rate is
doubled from 8 to 16 ml min\ꢀ. From the same figure
it can be seen that the film thickness increases by
about 20% as F is doubled. The increase in thickness
ꢂ
of NiO films may be understood by considering the
deposition rates. The deposition rate, R, depends on
3002

the mass transfer coefficient, K , the diffusion poten-
'
tial, *½, the molecular weight, M, and the density, ꢀ,
of the starting material [11]:
R"(M/ꢀ)K *½
'
The increase in solution flow rate increases the depos-
ition rate which leads to the increase in thickness but
does not affect the transparency of the film, since the
variation in transparency is less than 10%. The small
variation in transparency can be understood from the
small variation in refractive index and possibly to the
lower refractive index of the sample (compare refrac-
tive index values in Figs 3 and 4) obtained as the
solution flow rate is doubled. The reciprocal relation
between transmittance and band gap is once again
demonstrated although the variation of these para-
meters with the solution flow rate is quite small.
Fig. 5 shows variation of transmission, optical
constant, film thickness and band gap with lithium
concentration. Lithium-doped films show high trans-
parency throughout the visible region with average
transmission above 80%. With increasing lithium
concentration, the film thickness is observed to de-
crease. This can be attributed to the displacement of
Ni atoms by Li atoms which are smaller than Ni
atoms. Differences in orientation of the Ni, Li and
O atoms may result in slightly thicker films for sam-
ples doped with higher Li concentrations. Since the
transparency of the films is quite constant for different
Li concentrations, the refractive index also does not
show much variation with Li concentration.
Figure 3 Variation of transmittance (ଙ), thickness (#), refractive
index (E) and band gap (*) with nozzle—substrate distance, D .
ꢀꢁ
Resistivity was measured at room temperature for
all the films. The dependence of resistivity as a func-
tion of substrate temperature for nickel oxide films is
shown in Fig. 6. The film resistivity decreases with
increasing substrate temperature. The decrease can be
due to either a kinetic reaction in the deposition pro-
cess or to the increase in film thickness. At higher
substrate temperatures, the droplets will absorb more
thermal energy such that all the solvent will vaporize
just before reaching the substrate and approach
homogeneous to heterogeneous reaction conditions.
This heterogeneous reaction reported in [12] will im-
prove the growth rate and the carrier density.
The variation in resistivity, q, with D (Fig. 7) for
ꢀꢁ
the NiO films also suggests that with increasing D
ꢀꢁ
there is the possibility of a change from a homogene-
ous reaction to a heterogeneous reaction. At lower D
(up to 30 cm), the resistivity is low and shows a small
ꢀꢁ
increase with increasing D . On increasing D , the
ꢀꢁ
ꢀꢁ
droplet travels a greater distance in the pyrolytic zone
such that all the solvent will be vaporized and de-
composition of nickel oxide will occur at the surface of
the substrate. Under this condition it approaches
a hetereogeneous reaction. Since the droplet travels
much greater distance in the furnace, it will absorb
more heat and the decomposition may occur in the
gas phase itself, thus affecting the growth rate. This
has an effect on the film thickness and increases the
resistivity 10 times higher than D at 30 cm. This
ꢀꢁ
result is quite consistent with that shown in Fig. 3,
where, for D greater than 30 cm, the film thickness of
the samples is observed to decrease.
Figure 4 Variation of transmittance (ଙ), thickness (#), refractive
ꢀꢁ
index (E) and band gap (*) with solution flow rate, F .
ꢂ
3003

Figure 7 Variation of electrical resistivity of NiO film with sub-
strate—nozzle distance, D .
ꢀꢁ
Figure 5 Variation of transmittance (ଙ), thickness (#), refractive
index (E) and band gap (*) with lithium concentration.
Figure 8 Variation of electrical resistivity of NiO film with solution
flow rate, F .
ꢂ
the incorporation of monovalent atoms. Introduction
of Li> into the NiO films gives rise to acceptor centres
Li>—Niꢁ>. At low temperatures, the Niꢁ> holes for-
med in this way are bound to a lithium ion, which has
an effective negative charge. At higher temperatures
these holes can detach from the Li>—Niꢁ> centres and
move quasi-freely in the sample by exchange of elec-
trons between Niꢁ> and adjacent Niꢃ> ions [1].
The variation in resistivity with the lithium concen-
tration is shown in Fig. 9. The resistivity of the films
shows a minimum value of 0.7 ) cm at 25% of lithium
concentration. The decrease in resistivity of Li-doped
NiO film depends mainly on the increase in the num-
ber of Niꢁ> ions and possibly to the overall decrease
in band gap as the Li concentration increases from
5 to 25%.
Figure 6 Variation of electrical resistivity of NiO film with sub-
strate temperature, ¹ .
ꢀ
The electrical properties of NiO films depend
largely on their structure and composition, and
consequently on the deposition environment. The
impedance was measured and analysed in the complex
plane for various parameters which assist in obtaining
a deep insight into the mechanism controlling the
conductivity of the NiO material in thin film form.
Impedance spectroscopy was carried out for the
amorphous NiO sample in the frequency range
Fig. 8 shows resistivity, q, of NiO films at different
solution flow rates, F . The resistivity decreases with
ꢂ
F and at a flow rate 16 ml min\ꢀ, the resistivity is
ꢂ
8.3 ) cm. This improvement can be attributed to the
increase in thickness, as shown in Fig. 4.
The conduction in nickel oxide is due to the pres-
ence of Niꢁ> ions [1, 2]. These ions may be formed
either by the appearance of NiO nickel vacancies or by
3004

Figure 9 Variation of electrical resistivity of NiO film with lithium
concentration.
Figure 10 A.c. conductivity curves of NiO films prepared at (a)
¹ "300 °C and (b) ¹ "400 °C.
ꢀ
ꢀ
between 40 Hz to 100 kHz at room temperature to
obtain the real and imaginary impedances. The plot of
imaginary impedance versus real impedance may take
the shape of a semicircle which ends at the origin of
the imaginary impedance and real impedance axes. If
this is the case, then the intercept of the plot at the
Figure 11 A.c. conductivity curves of lithium doped NiO films
prepared at (a) 5%, (b) 10%, (c) 15% and (d) 20% lithium concen-
tration.
lower frequency end will give the bulk resistance, R .
"
In the case where two semicircles can be observed, the
intercept of the second semicircle with the real impe-
dance axis (furthest from the origin) will give the value
of R #R , where R is the grain boundary resist-
plots for some of the NiO films. Extrapolating the
high and low frequency limits of the arcs, the plots
"
'"
'"
ance. Figs 10 and 11 show the complex impedance
intercept the real axis (Zꢀ) at resistance values of R
"
3005

and R #R , respectively. For all the substrate tem-
films and the conductivity is reasonably stable when
samples are stored at room temperature. From the
Cole—Cole plot we can conclude that the resistance of
lithium-doped NiO films do not show any dependence
on frequency in the high frequency range. NiO and
lithium-doped NiO films fabricated by spray pyrolysis
should be useful as a less expensive p-type transparent
electrode for solar cells and displays.
"
'"
peratures, the NiO films show a single semicircle with
R and R #R at the real axis intercepts. The de-
"
"
'"
crease in R and R with substrate temperature may
"
'"
be attributed to the growth of the film. Knowing R ,
the resistivity of the films can be calculated from the
"
equation p"R A/l where A is the surface area of the
"
sample and l its distance between electrodes.
Fig. 11 shows the Cole—Cole plot of NiO films at
different lithium, concentrations. Semicircles inter-
cepting the real impedance axis at R and R #R
References
"
"
'"
were obtained for all the films. For films with 5%
lithium concentration another semicircle which inter-
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Solid Films 236 (1993) 27
cepts the real axis at the origin and R is expected.
"
This semicircle is due to a parallel combination of the
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bulk resistance, R , and the bulk capacitance, C , of
"
"
the sample. The conduction mechanism of this sample
is ruled by two parallel R—C circuits [13]. Films with
more than 5% lithium concentration do not show the
dependence of the real part of impedance with fre-
quency after the bulk resistance but the imaginary
part of impedance shows an abrupt change with fre-
quency. For these samples, the conduction mechanism
may be controlled in the same way as that of an
equivalent circuit comprising a serial combination of
a resistor with resistance R and a capacitor with
"
capacitance C in series with a parallel R—C circuit
"
where the resistance is R and capacitance is C .
'"
'"
4. Conclusions
12. Idem, ibid. 193/194 (1990) 696.
13. S. CHANDRA, ‘‘Superionic Solids: Principle and Applications’’
(North-Holland, Amsterdam, 1981).
Highly transparent and well conducting amorphous
thin films of NiO and lithium-doped NiO have been
prepared by the spray pyrolysis technique. Films
obtained by this method are adherent and have a
uniform surface. NiO films have been prepared with
resistivities as low as 5 ) cm and with almost 90%
transmission in the visible spectrum for 2 lm thick
Received 3 January
and accepted 19 November 1996
.
3006