# Sputter Power Influenced Structural, Electrical, and Optical Behaviour of Nanocrystalline CuNiO2 Films Formed by RF Magnetron Sputtering

Authors: A. Sreedhar; M. Hari Prasad Reddy; S. Uthanna; J. F. Pierson
Journal: ISRN Condensed Matter Physics
 (2013)

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## Abstract

Copper nickel oxide (CuNiO2) films were deposited on glass and silicon substrates using RF magnetron sputtering of equimolar Cu50Ni50 alloy target at different sputter powers in the range of 3.1–6.1 W/cm2. The effect of sputter power on the chemical composition, crystallographic structure, chemical binding configuration,  surface morphology, and electrical and optical properties of CuNiO2 films was investigated. The films formed at sputter power of 5.1 W/cm2 were of nearly stoichiometric CuNiO2. Fourier transform infrared spectroscopic studies indicated the presence of the characteristic vibrational bands of copper nickel oxide. The nanocrystalline CuNiO2 films were formed with the increase in grain size from 75 to 120 nm as the sputter power increased from 3.1 to 5.1 W/cm2. The stoichiometric CuNiO2 films formed at sputter power of 5.1 W/cm2 exhibited  electrical resistivity of 27 Ωcm, Hall mobility of 21 cm2/Vsec, and optical bandgap of 1.93 eV.


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## Body

## 1. Introduction

Cuprous oxide (Cu2O) is a p-type semiconducting oxide material with a direct bandgap of about 1.9–2.2 eV and has a wide range of applications in the field of gas sensors, solar cells, thin-film transistors, and catalysts [1–4]. Cu2O is the base for many p-type transparent conducting oxides (TCOs) to form CuMO2 delafossites (M = Al, Cr, B, In, Ga, etc.), which are thought to retain the valence band features and conduction mechanism of Cu2O [5]. Nickel oxide (NiO) thin films are very much attractive for the antiferromagnetic layers, electrochromic devices, and p-type transparent conducting oxide and functional layers in chemical sensors [6–8] because of their chemical stability along with suitable optical, electrical, and magnetic properties. Chen et al. [9] studied the change in the valence of Ni from Ni2+ to Ni3+ by incorporating copper into the NiO films formed by reactive sputtering of nickel target with varied copper concentrations of 10 and 18 at%. Moghe et al. [10] noticed that the crystallite size of the films decreased from 18.1 to 15.2 nm and that the optical bandgap decreased from 3.20 to 2.96 eV with the increase of copper concentration from 0 to 10% in NiO films formed by spray pyrolysis. The electrical resistivity of the films also decreased with the increase in the content of copper in NiO films. Yang et al. [11] reported electrical resistivity of 0.19 Ωcm and optical bandgap of 3.7 eV in p-type Ni0.9Cu0.1O films formed by pulsed plasma deposition. Miyata et al. [12] deposited p-type Cu2O–NiO films with various concentrations of Cu2O by RF magnetron sputtering and found electrical resistivity of 2 × 104 Ωcm, hall mobility of 2.5 cm2/Vsec, and optical bandgap of 2.9 eV in 50% of copper-doped NiO films. He et al. [13] achieved good electrochromic behaviour in sol-gel processed Cu0.2Ni0.8O films in  both the color and  the bleached states with optical modulation of 7.5% at 633 nm and response time less than 1 sec. Chen et al. [14] found that the electrical resistivity and optical transmittance decreased with the increase of copper concentration from 10 to 18 at% in RF magnetron-sputtered Cu-doped NiO films. Recently, Ashok Kumar Reddy et al. [15] reported on the (30 at%) Cu-doped NiO films formed by DC magnetron sputtering and that the electrical resistivity decreased from 64 to 18 Ωcm and found that optical bandgap increased from 2.20 to 3.30 eV with the increase of oxygen partial pressure from 9 × 10−3 to 4 × 10-2Pa, respectively. Among the thin-film deposition techniques, the reactive magnetron-sputtering is one of the most widely used techniques for preparation of oxide films on large-area substrates by sputtering of metallic or metal oxide target in reactive gas of oxygen and sputter gas of argon. Nanocrystalline CuNiO2 films with grain size of 95 nm, electrical resistivity of 1.5 Ωcm, and Hall mobility of 19 cm2/Vsec were achieved by RF magnetron sputtering technique at a substrate bias voltage of −60 V [16]. In this paper, copper nickel oxide films were formed by RF reactive magnetron sputtering technique by sputtering of equimolar Cu50Ni50 alloy target at various sputter powers, and we studied the influence of sputter power on the chemical composition, crystallographic structure, surface morphology, chemical binding configuration, and electrical and optical properties of the deposited films.


## 2. Experimentation

Copper nickel oxide (CuNiO2) films were deposited on glass and p-silicon substrates using RF magnetron sputtering technique by sputtering of equimolar (Cu50Ni50) copper nickel alloy target of 50 mm diameter and 3 mm thickness. The films were formed on the substrates held at temperature of 523 K and at different sputter powers in the range of 3.1–6.1 W/cm2. The box-type sputter chamber was evacuated to ultimate pressure of 5 × 10−4Pa using diffusion pump backed by rotary pump. Oxygen and argon gases were used as reactive and sputter gases and were admitted into the sputter chamber by fine control needle valves. The duration of the deposition of the films was 90 min. The deposition conditions maintained for the preparation of CuNiO2 films are given in Table 1.Table 1
Deposition conditions fixed for the growth of CuNiO2 films.
Sputter target
Cu50Ni50
Target-to-substrate distance
50 mm
Ultimate pressure
5 × 10−4 Pa
Oxygen partial pressure (pO2)
2 × 10−2 Pa
Sputtering pressure
4 Pa
Substrate temperature
523 K
Sputter power (Pw)
3.1–6.1 W/cm2The thickness of the deposited CuNiO2 films was determined by using the Veeco Dektak depth profilometer. The chemical composition of the deposited films was analyzed with energy-dispersive X-ray analysis (Oxford Instruments INCA Penta FETx3) attached to the scanning electron microscope (Carl Zeiss model EVO MA 15). The crystallographic structure of the films was determined by X-ray diffractometer (Siefert Diffractometer model 3003 TT) using monochromatic CuKα radiation with wavelength of 0.15406 nm. The surface morphology of the deposited films was analyzed with atomic force microscope (Seiko Instruments model SPA 400). The chemical binding configuration of the films formed on silicon substrates was analyzed by the Fourier transform infrared spectroscope (Nicolet model 5700 FTIR). The electrical properties of the films were determined with a Hall measurement system (Ecopia HMS, 3000 VER 3.51.3). The optical transmittance of the films formed on glass substrate was recorded in the wavelength range of 300–2500 nm using the Perkin-Elmer double-beam spectrophotometer in order to determine the optical bandgap of the deposited films.


## 3. Results and Discussion

The deposition rate of the CuNiO2 films was calculated from the thickness and duration of the deposition. Figure 1 shows the variation of deposition rate of the films with the sputter power applied to the target in the range of  3.1–6.1 W/cm2. The deposition rate of the deposited films increased from 3.6 to 13.8 nm/min with the increase of sputter power from 3.1 to 6.1 W/cm2, respectively. The deposition rate of the films increased almost linearly with the increase of sputter power. As the sputter power increased, the bombardment of argon ion on the sputter target increased; hence, the deposition rate of the films increased. Such an increase in the deposition rate with the increase of sputter power was observed by Gagaoudakis et al. [17] in indium oxide films and was observed by Sivasankar Reddy et al. [18] in copper oxide films deposited by DC reactive magnetron sputtering.Figure 1
Variation of deposition rate with sputter power of CuNiO2 films.


### 3.1. Chemical Composition

The chemical composition of the deposited films was determined by energy-dispersive X-ray analysis. The chemical constituents present in the films were influenced by the sputter power fed to the target during the growth of the films. The chemical composition of the deposited films was determined from the intensity of the constituent elements present in the EDAX spectra. Figure2 shows the EDAX spectra of copper nickel oxide films formed at different sputter powers. The chemical composition of the films formed at different sputter powers is given in Table 2. It is seen from the table that the films deposited at sputter power of 5.1 W/cm2 were of nearly stoichiometric, CuNiO2, that is with copper of 25.2 at%, nickel of 24.0 at% and oxygen of 50.8 at%. The films while deposited at higher sputter power of 6.1 W/cm2 being contained excess of metallic content, that is, copper of 31.1 at%, nickel of 30.7 at%, and oxygen of 38.2 at%. At sputter power of 5.1 W/cm2, the sputter yield of metallic species was low, and the sputtered species of copper and nickel effectively react with the available oxygen in the sputter chamber and form stoichiometric CuNiO2 films. At higher sputter power of 6.1 W/cm2 the ejected sputter atoms of copper and nickel were increased due to high sputter yield which reacts with the insufficient oxygen in the sputter chamber, hence achieving metal-rich oxide films. From these studies, it is revealed that the films formed at sputter power of 5.1 W/cm2, were of nearly stoichiometric CuNiO2.Table 2
Chemical composition of CuNiO2 films formed at different sputter powers determined with the energy-dispersive X-ray analysis.
Sputter power (W/cm2)
Atomic concentration (%)
Cu
Ni
O
3.1
24.2
22.9
52.9
4.1
24.8
23.6
51.6
5.1
25.2
24.0
50.8
6.1
31.1
30.7
38.2EDAX spectra of CuNiO2 films formed at different sputter powers: (a) 3.1 W/cm2, (b) 5.1 W/cm2 and (c) 6.1 W/cm2.
(a)
(b)
(c)


### 3.2. X-Ray Diffraction Studies

X-ray diffraction profiles of the films deposited at different sputter powers recorded in the 2θ range of 15–80° are shown in Figure 3. It is seen from the figure that the films deposited at all sputter powers showed broad diffraction peak centered around 2θ = 41.4° related to the (002) reflection of CuNiO2 [19]. The presence of broader diffraction peak indicated that the grown films were of nanocrystalline in nature. It is seen that the broadness of the peak slightly decreased with the increase of sputter power up to 5.1 W/cm2 which revealed that the crystallite size of the films enhanced with the increase of sputter power. The films formed at sputter power of 5.1 W/cm2, exhibited another weak diffraction peak at about 2θ = 36.9° related to the (111) reflection of CuNiO2. Further increase of sputter power to 6.1 W/cm2, the sharpness of the (002) reflection decrease, whereas the (111) reflection disappeared. The reduction in the sharpness of the peak may indicate the presence of unreacted copper and nickel along with copper nickel oxide. This revealed that the films formed with sputter power of 5.1 W/cm2 were of single-phase CuNiO2, which was also confirmed by the chemical composition determined by the energy-dispersive analysis. It is to be noted that Miyata et al. [12] achieved  the polycrystalline multicomponent Cu2O–NiO films with the content of nickel in the range of 30–60 at% in RF magnetron-sputtered films by sputtering of Cu2O–NiO powder targets.Figure 3
X-ray diffraction profiles of CuNiO2 films formed at various sputter powers.The crystallite size (L) of the deposited CuNiO2 films was calculated from the (002) reflection using the following Debye-Scherrer relation:
(1)L=Kλβcosθ,whereK is a constant with a value about 0.89 for copper target, β is the full width at half maximum (FWHM) of the peak measured in radians, and θ is the diffraction peak angle. The variation in full width at half maximum and the crystallite size of the films formed at different sputter powers are shown in Figure 4. The full width at half maximum of the films decreased from 4.7 to 4.0° with the  increase of sputter power from 3.1 to 5.1 W/cm2; thereafter, it increased to 4.46° at higher sputter power of 6.1 W/cm2. The crystallite size of the films increased from 1.9 to 2.6 nm with the increase of sputter power from 3.1 to 5.1 W/cm2, later, it decreased to 2.0 nm at higher sputter power of 6.1 W/cm2. This  clearly revealed that the grown films were of nanocrystalline CuNiO2 with the crystallite size in the range of 1.9–2.6 nm.Figure 4
Variation of FWHM and crystallite size of CuNiO2 films with sputter power.The dislocation density (δ) is a measure of dislocations present in a unit volume of the film. The dislocation density of the films was calculated from the crystallite size using the following relation:
(2)δ=1L2(lines/m2).The strain (ε) developed in the films was determined from the full width at half maximum of X-ray diffraction peak using the following relation:
(3)ε=βcosθ4(lines-2/m4).The dislocation density and the strain of the films as a function of sputter power are shown in Figure5. The dislocation density of the films decreased from 27 × 1016 to 15 × 1016 lines/m2 with the  increase of sputter power from 3.1 to 5.1 W/cm2, respectively, and at higher sputter powers it further increased to 25 × 1016 lines/m2. The strain developed in the films decreased from 1.6 × 10−2 to 1.3 × 10−2 lines−2/m4 with the increase of sputter power from 3.1 to 5.1 W/cm2, and at higher sputter power of 6.1 W/cm2 it increased to 1.45 × 10−2 lines−2/m4. From these studies, it is revealed that the increase of crystallinity is the main reason for the decrease of dislocation density and the strain in the films.Figure 5
Variation of dislocation density and strain of CuNiO2 films with sputter power.


### 3.3. The Fourier Transform Infrared Spectroscopic Studies

A Fourier transform infrared spectroscopic study gives the information about the chemical binding configuration in the films. Figure6 shows the Fourier transform infrared transmittance spectra of CuNiO2 films formed at different sputter powers. The films formed at low sputter power of 3.1 W/cm2 exhibited an absorption band at 616 cm−1 and a broad absorption peak at 437 cm−1. Further increase of sputter power to 4.1 W/cm2 made the absorption band intensity increase. The band observed at 616 cm−1 was related to the vibration of Cu2O [20]. To  be noted are the vibrations of CuO and Cu2O located at 510 and 630 cm−1, respectively, the vibration of nickel oxide (Ni2O3), located at 450 cm−1 [21], and the vibration of NiO located at 426 cm−1 [22]. Hence, the vibration band seen at 616 cm−1 was related to the Cu2O. The band located at 437 cm−1 was related to the copper nickel oxide phase. This  revealed that the films formed up at  sputter power of 4.1 W/cm2 contained the vibration bands of Cu2O and copper nickel oxide. The films formed at 5.1 W/cm2 exhibited copper nickel oxide without the presence of Cu2O. Further increase of sputter power to 6.1 W/cm2 once again made the vibration band related to Cu2O observable. It is to be noted that the vibration band related to copper nickel oxide shifted from 437 to 443 cm−1 with the increase of sputter power from 3.1 to 5.1 W/cm2. Shobha et al. [23] also noticed the absorption band at 447 cm−1 in the electrodeposited Ni–Cu oxide films. From these studies, it is revealed that the films formed at sputter power of 5.1 W/cm2 were of single-phase CuNiO2 without presence of the additional phase of Cu2O.Figure 6
FTIR transmittance spectra of CuNiO2 films formed at different sputter powers.


### 3.4. Atomic Force Micrograph Studies

The surface morphology of CuNiO2 films was influenced by the sputter power fed to the sputter target. Figure 7 shows three- and two-dimensional atomic force micrographs of films formed at different sputter powers. The films formed at low sputter power of 3.1 W/cm2, showed fine grains uniformly spread on the surface of the substrates. As the sputter power increased to 5.1 W/cm2 the shape of the grains were spherical and larger in size. In the case of the films deposited at higher sputter power of 6.1 W/cm2, the shapes of the grains were in rod like with decrease in the size of the grains. The grain size increased from 75 to 120 nm with the  increase of sputter power from 3.1 to 5.1 W/cm2 due to the increase of crystallinity of the films. The films formed at higher sputter power exhibited smaller size grains (90 nm) with needle-like shape which may be due to the presence of CuNiO2 along with metallic copper and nickel as noticed in the EDAX data. The root mean square (RMS) roughness of the films increased from 2 to 8 nm with the increase of sputter power from 3.1 to 5.1 W/cm2 and then it decreased to 4.7 nm at sputter power of 6.1 W/cm2. Such a variation in the surface morphology was observed by Jin et al. [24] in DC magnetron-sputtered titanium films.Three- and two-dimensional atomic force micrographs of CuNiO2 films formed at           different sputter powers: (a) 3.1 W/cm2, (b) 5.1 W/cm2, and (c) 6.1 W/cm2.
(a)
(b)
(c)


### 3.5. Electrical Properties

Figure8 shows the variation of electrical resistivity (ρ) of the CuNiO2 films with the sputter power. The electrical resistivity  was highly influenced by the sputter power maintained during the growth of the films. It is seen that the electrical resistivity of the films decreased with the  increase of sputter power. The electrical resistivity of the films formed at low sputter power of 3.1 W/cm2, was 130 Ωcm. As the sputter power increased to 5.1 W/cm2 the electrical resistivity decreased to 27 Ωcm. The decrease in the electrical resistivity with the  increase of sputter power was due to improvement in the crystallinity of the films. The electrical resistivity of the films reduced to 21 Ωcm in the case of the films formed at high sputter power of 6.1 W/cm2, which may be due to presence of metallic nickel and copper in copper nickel oxide. It is to be mention that the reported electrical resistivity of RF magnetron-sputtered Cu2O and NiO films was 9.6 × 101 Ωcm and 8.0 × 105 Ωcm, and the equimolar Cu–Ni oxide films, it was 3.0 × 104 Ωcm [12]. Yang et al. [11] achieved low electrical resistivity of 0.2 Ωcm in Ni0.9Cu0.1O films formed by pulsed plasma deposition and Chen et al. [9] noticed that the resistivity decreased from 0.12 to 0.07 Ωcm with the  increase of copper from 10 to 18 at% in RF magnetron-sputtered NiO films. In contrast to this, Kikuchi et al. [25] reported that the electrical resistivity of the Cu2O films increased from 2 × 102  to 8 × 102 Ωcm with the  increase of the Ni content from 0 to 2 at%. The large variations in the electrical resistivity with the doping concentration of the Ni in CuO films were mainly due to the deposition method employed and the process parameters fixed during the formation of the films.Figure 8
Variation of electrical resistivity of CuNiO2 films with sputter power.The Hall measurements indicated that the deposited films were of p-type in electrical conduction. The Hall mobility (μ) of the films increased from 3.9 to 21 cm2/Vsec the with the  increase of sputter power from 3.1 to 5.1 W/cm2, thereafter, it decreased to 20 cm2/Vsec at higher sputter power of 6.1 W/cm2 as shown in Figure 9. The increase in the Hall mobility with the  increase of sputter power was due to, improvement in the crystallinity and maybe substitution of Ni2+ by Cu+ lead to dislocation of holes in the valance band hence increase in the Hall mobility [26]. The hole concentration (p) in the films increased from 1.1 × 1015 to 1.1 × 1016 cm−3 with the  increase of sputter power from 3.1 to 5.1 W/cm2 while at higher sputter power of 6.1 W/cm2 it further enhanced to 1.5 × 1016 cm−3 due to the presence of metallic phase along with CuNiO2. Miyata et al. [12] achieved low Hall mobility of 2 cm2/Vsec and low carrier concentration of about 8 × 1013 cm−3 due to high resistivity of the films. Chen et al. [14] obtained Hall mobility of 1 cm2/Vsec and carrier concentration of 2.5 × 1020 cm−3 in 18 at% copper doped NiO films formed by RF magnetron sputtering.Figure 9
Variation of hole concentration and Hall mobility of CuNiO2 films with sputter           power.


### 3.6. Optical Properties

Figure10 shows the wavelength dependence optical transmittance of the CuNiO2 films formed at different sputter powers. it is seen from the figure that the optical transmittance of the films decreased with the  increase of sputter power. The optical absorption edge of the films shifted towards higher wavelengths side with the increase of sputter power. The decrease in the transmittance at higher sputter power of 6.1 W/cm2 may be due to the clusters (growth of copper and nickel) reflecting the incident photons or scatter by the grain boundaries [15]. It is to be noted that the optical transmittance of Cu doped NiO films decreased from 72 to 43% with the  increase of Cu content from 2.3 to 18.2 at% respectively in RF sputter deposited from Cu2O–NiO mixed powder target [14]. The absorption coefficient (α) of the films was calculated from the optical transmittance (T) data using the following relation [27]:
(4)α=-(1t)lnT,
where t is the film thickness. The optical bandgap of the films was determined from the absorption coefficient (α) and photon energy (hν) by using the following Tauc relation:
(5)(αhν)=A(hν-Eg)1/2
assuming that the direct transitions take place from the valance band to the conduction band. The optical bandgap of the films decreased from 2.0 to 1.41 eV with the  increase of sputter power from 3.1 to 6.1 W/cm2 respectively. The decrease of bandgap with the  increase of sputter power may be due to improved crystallinity. Miyata et al. [12] achieved an optical bandgap of 2.75 eV in 50 at% Ni in Cu–Ni–O films formed by RF magnetron sputtering, whereas Moghe et al. [10] reported a value of 3.04 eV in 50 at% copper-doped NiO films formed by spray pyrolysis. The different values of optical bandgap achieved in the films are maybe due to the difference in the formation of the compound.Figure 10
Optical transmittance spectra of CuNiO2 films formed at different sputter powers.


## 3.1. Chemical Composition

The chemical composition of the deposited films was determined by energy-dispersive X-ray analysis. The chemical constituents present in the films were influenced by the sputter power fed to the target during the growth of the films. The chemical composition of the deposited films was determined from the intensity of the constituent elements present in the EDAX spectra. Figure2 shows the EDAX spectra of copper nickel oxide films formed at different sputter powers. The chemical composition of the films formed at different sputter powers is given in Table 2. It is seen from the table that the films deposited at sputter power of 5.1 W/cm2 were of nearly stoichiometric, CuNiO2, that is with copper of 25.2 at%, nickel of 24.0 at% and oxygen of 50.8 at%. The films while deposited at higher sputter power of 6.1 W/cm2 being contained excess of metallic content, that is, copper of 31.1 at%, nickel of 30.7 at%, and oxygen of 38.2 at%. At sputter power of 5.1 W/cm2, the sputter yield of metallic species was low, and the sputtered species of copper and nickel effectively react with the available oxygen in the sputter chamber and form stoichiometric CuNiO2 films. At higher sputter power of 6.1 W/cm2 the ejected sputter atoms of copper and nickel were increased due to high sputter yield which reacts with the insufficient oxygen in the sputter chamber, hence achieving metal-rich oxide films. From these studies, it is revealed that the films formed at sputter power of 5.1 W/cm2, were of nearly stoichiometric CuNiO2.Table 2
Chemical composition of CuNiO2 films formed at different sputter powers determined with the energy-dispersive X-ray analysis.
Sputter power (W/cm2)
Atomic concentration (%)
Cu
Ni
O
3.1
24.2
22.9
52.9
4.1
24.8
23.6
51.6
5.1
25.2
24.0
50.8
6.1
31.1
30.7
38.2EDAX spectra of CuNiO2 films formed at different sputter powers: (a) 3.1 W/cm2, (b) 5.1 W/cm2 and (c) 6.1 W/cm2.
(a)
(b)
(c)


## 3.2. X-Ray Diffraction Studies

X-ray diffraction profiles of the films deposited at different sputter powers recorded in the 2θ range of 15–80° are shown in Figure 3. It is seen from the figure that the films deposited at all sputter powers showed broad diffraction peak centered around 2θ = 41.4° related to the (002) reflection of CuNiO2 [19]. The presence of broader diffraction peak indicated that the grown films were of nanocrystalline in nature. It is seen that the broadness of the peak slightly decreased with the increase of sputter power up to 5.1 W/cm2 which revealed that the crystallite size of the films enhanced with the increase of sputter power. The films formed at sputter power of 5.1 W/cm2, exhibited another weak diffraction peak at about 2θ = 36.9° related to the (111) reflection of CuNiO2. Further increase of sputter power to 6.1 W/cm2, the sharpness of the (002) reflection decrease, whereas the (111) reflection disappeared. The reduction in the sharpness of the peak may indicate the presence of unreacted copper and nickel along with copper nickel oxide. This revealed that the films formed with sputter power of 5.1 W/cm2 were of single-phase CuNiO2, which was also confirmed by the chemical composition determined by the energy-dispersive analysis. It is to be noted that Miyata et al. [12] achieved  the polycrystalline multicomponent Cu2O–NiO films with the content of nickel in the range of 30–60 at% in RF magnetron-sputtered films by sputtering of Cu2O–NiO powder targets.Figure 3
X-ray diffraction profiles of CuNiO2 films formed at various sputter powers.The crystallite size (L) of the deposited CuNiO2 films was calculated from the (002) reflection using the following Debye-Scherrer relation:
(1)L=Kλβcosθ,whereK is a constant with a value about 0.89 for copper target, β is the full width at half maximum (FWHM) of the peak measured in radians, and θ is the diffraction peak angle. The variation in full width at half maximum and the crystallite size of the films formed at different sputter powers are shown in Figure 4. The full width at half maximum of the films decreased from 4.7 to 4.0° with the  increase of sputter power from 3.1 to 5.1 W/cm2; thereafter, it increased to 4.46° at higher sputter power of 6.1 W/cm2. The crystallite size of the films increased from 1.9 to 2.6 nm with the increase of sputter power from 3.1 to 5.1 W/cm2, later, it decreased to 2.0 nm at higher sputter power of 6.1 W/cm2. This  clearly revealed that the grown films were of nanocrystalline CuNiO2 with the crystallite size in the range of 1.9–2.6 nm.Figure 4
Variation of FWHM and crystallite size of CuNiO2 films with sputter power.The dislocation density (δ) is a measure of dislocations present in a unit volume of the film. The dislocation density of the films was calculated from the crystallite size using the following relation:
(2)δ=1L2(lines/m2).The strain (ε) developed in the films was determined from the full width at half maximum of X-ray diffraction peak using the following relation:
(3)ε=βcosθ4(lines-2/m4).The dislocation density and the strain of the films as a function of sputter power are shown in Figure5. The dislocation density of the films decreased from 27 × 1016 to 15 × 1016 lines/m2 with the  increase of sputter power from 3.1 to 5.1 W/cm2, respectively, and at higher sputter powers it further increased to 25 × 1016 lines/m2. The strain developed in the films decreased from 1.6 × 10−2 to 1.3 × 10−2 lines−2/m4 with the increase of sputter power from 3.1 to 5.1 W/cm2, and at higher sputter power of 6.1 W/cm2 it increased to 1.45 × 10−2 lines−2/m4. From these studies, it is revealed that the increase of crystallinity is the main reason for the decrease of dislocation density and the strain in the films.Figure 5
Variation of dislocation density and strain of CuNiO2 films with sputter power.


## 3.3. The Fourier Transform Infrared Spectroscopic Studies

A Fourier transform infrared spectroscopic study gives the information about the chemical binding configuration in the films. Figure6 shows the Fourier transform infrared transmittance spectra of CuNiO2 films formed at different sputter powers. The films formed at low sputter power of 3.1 W/cm2 exhibited an absorption band at 616 cm−1 and a broad absorption peak at 437 cm−1. Further increase of sputter power to 4.1 W/cm2 made the absorption band intensity increase. The band observed at 616 cm−1 was related to the vibration of Cu2O [20]. To  be noted are the vibrations of CuO and Cu2O located at 510 and 630 cm−1, respectively, the vibration of nickel oxide (Ni2O3), located at 450 cm−1 [21], and the vibration of NiO located at 426 cm−1 [22]. Hence, the vibration band seen at 616 cm−1 was related to the Cu2O. The band located at 437 cm−1 was related to the copper nickel oxide phase. This  revealed that the films formed up at  sputter power of 4.1 W/cm2 contained the vibration bands of Cu2O and copper nickel oxide. The films formed at 5.1 W/cm2 exhibited copper nickel oxide without the presence of Cu2O. Further increase of sputter power to 6.1 W/cm2 once again made the vibration band related to Cu2O observable. It is to be noted that the vibration band related to copper nickel oxide shifted from 437 to 443 cm−1 with the increase of sputter power from 3.1 to 5.1 W/cm2. Shobha et al. [23] also noticed the absorption band at 447 cm−1 in the electrodeposited Ni–Cu oxide films. From these studies, it is revealed that the films formed at sputter power of 5.1 W/cm2 were of single-phase CuNiO2 without presence of the additional phase of Cu2O.Figure 6
FTIR transmittance spectra of CuNiO2 films formed at different sputter powers.


## 3.4. Atomic Force Micrograph Studies

The surface morphology of CuNiO2 films was influenced by the sputter power fed to the sputter target. Figure 7 shows three- and two-dimensional atomic force micrographs of films formed at different sputter powers. The films formed at low sputter power of 3.1 W/cm2, showed fine grains uniformly spread on the surface of the substrates. As the sputter power increased to 5.1 W/cm2 the shape of the grains were spherical and larger in size. In the case of the films deposited at higher sputter power of 6.1 W/cm2, the shapes of the grains were in rod like with decrease in the size of the grains. The grain size increased from 75 to 120 nm with the  increase of sputter power from 3.1 to 5.1 W/cm2 due to the increase of crystallinity of the films. The films formed at higher sputter power exhibited smaller size grains (90 nm) with needle-like shape which may be due to the presence of CuNiO2 along with metallic copper and nickel as noticed in the EDAX data. The root mean square (RMS) roughness of the films increased from 2 to 8 nm with the increase of sputter power from 3.1 to 5.1 W/cm2 and then it decreased to 4.7 nm at sputter power of 6.1 W/cm2. Such a variation in the surface morphology was observed by Jin et al. [24] in DC magnetron-sputtered titanium films.Three- and two-dimensional atomic force micrographs of CuNiO2 films formed at           different sputter powers: (a) 3.1 W/cm2, (b) 5.1 W/cm2, and (c) 6.1 W/cm2.
(a)
(b)
(c)


## 3.5. Electrical Properties

Figure8 shows the variation of electrical resistivity (ρ) of the CuNiO2 films with the sputter power. The electrical resistivity  was highly influenced by the sputter power maintained during the growth of the films. It is seen that the electrical resistivity of the films decreased with the  increase of sputter power. The electrical resistivity of the films formed at low sputter power of 3.1 W/cm2, was 130 Ωcm. As the sputter power increased to 5.1 W/cm2 the electrical resistivity decreased to 27 Ωcm. The decrease in the electrical resistivity with the  increase of sputter power was due to improvement in the crystallinity of the films. The electrical resistivity of the films reduced to 21 Ωcm in the case of the films formed at high sputter power of 6.1 W/cm2, which may be due to presence of metallic nickel and copper in copper nickel oxide. It is to be mention that the reported electrical resistivity of RF magnetron-sputtered Cu2O and NiO films was 9.6 × 101 Ωcm and 8.0 × 105 Ωcm, and the equimolar Cu–Ni oxide films, it was 3.0 × 104 Ωcm [12]. Yang et al. [11] achieved low electrical resistivity of 0.2 Ωcm in Ni0.9Cu0.1O films formed by pulsed plasma deposition and Chen et al. [9] noticed that the resistivity decreased from 0.12 to 0.07 Ωcm with the  increase of copper from 10 to 18 at% in RF magnetron-sputtered NiO films. In contrast to this, Kikuchi et al. [25] reported that the electrical resistivity of the Cu2O films increased from 2 × 102  to 8 × 102 Ωcm with the  increase of the Ni content from 0 to 2 at%. The large variations in the electrical resistivity with the doping concentration of the Ni in CuO films were mainly due to the deposition method employed and the process parameters fixed during the formation of the films.Figure 8
Variation of electrical resistivity of CuNiO2 films with sputter power.The Hall measurements indicated that the deposited films were of p-type in electrical conduction. The Hall mobility (μ) of the films increased from 3.9 to 21 cm2/Vsec the with the  increase of sputter power from 3.1 to 5.1 W/cm2, thereafter, it decreased to 20 cm2/Vsec at higher sputter power of 6.1 W/cm2 as shown in Figure 9. The increase in the Hall mobility with the  increase of sputter power was due to, improvement in the crystallinity and maybe substitution of Ni2+ by Cu+ lead to dislocation of holes in the valance band hence increase in the Hall mobility [26]. The hole concentration (p) in the films increased from 1.1 × 1015 to 1.1 × 1016 cm−3 with the  increase of sputter power from 3.1 to 5.1 W/cm2 while at higher sputter power of 6.1 W/cm2 it further enhanced to 1.5 × 1016 cm−3 due to the presence of metallic phase along with CuNiO2. Miyata et al. [12] achieved low Hall mobility of 2 cm2/Vsec and low carrier concentration of about 8 × 1013 cm−3 due to high resistivity of the films. Chen et al. [14] obtained Hall mobility of 1 cm2/Vsec and carrier concentration of 2.5 × 1020 cm−3 in 18 at% copper doped NiO films formed by RF magnetron sputtering.Figure 9
Variation of hole concentration and Hall mobility of CuNiO2 films with sputter           power.


## 3.6. Optical Properties

Figure10 shows the wavelength dependence optical transmittance of the CuNiO2 films formed at different sputter powers. it is seen from the figure that the optical transmittance of the films decreased with the  increase of sputter power. The optical absorption edge of the films shifted towards higher wavelengths side with the increase of sputter power. The decrease in the transmittance at higher sputter power of 6.1 W/cm2 may be due to the clusters (growth of copper and nickel) reflecting the incident photons or scatter by the grain boundaries [15]. It is to be noted that the optical transmittance of Cu doped NiO films decreased from 72 to 43% with the  increase of Cu content from 2.3 to 18.2 at% respectively in RF sputter deposited from Cu2O–NiO mixed powder target [14]. The absorption coefficient (α) of the films was calculated from the optical transmittance (T) data using the following relation [27]:
(4)α=-(1t)lnT,
where t is the film thickness. The optical bandgap of the films was determined from the absorption coefficient (α) and photon energy (hν) by using the following Tauc relation:
(5)(αhν)=A(hν-Eg)1/2
assuming that the direct transitions take place from the valance band to the conduction band. The optical bandgap of the films decreased from 2.0 to 1.41 eV with the  increase of sputter power from 3.1 to 6.1 W/cm2 respectively. The decrease of bandgap with the  increase of sputter power may be due to improved crystallinity. Miyata et al. [12] achieved an optical bandgap of 2.75 eV in 50 at% Ni in Cu–Ni–O films formed by RF magnetron sputtering, whereas Moghe et al. [10] reported a value of 3.04 eV in 50 at% copper-doped NiO films formed by spray pyrolysis. The different values of optical bandgap achieved in the films are maybe due to the difference in the formation of the compound.Figure 10
Optical transmittance spectra of CuNiO2 films formed at different sputter powers.


## 4. Conclusions

Thin films of CuNiO2 were formed on glass and silicon substrates held at 523 K by sputtering of equimolar (Cu50Ni50) target at various sputter powers in the range of 3.1–6.1 W/cm2 employing RF magnetron sputtering method. The influence of sputter power on the chemical composition, chemical binding configuration, structure, and electrical and optical properties was systematically investigated. The deposition rate of the films was almost linearly increased with the  increase of sputter power. The energy-dispersive X-ray analysis indicated that the films formed at sputter power of 5.1 W/cm2, were of nearly stoichiometric CuNiO2. The grown films were of nanocrystalline with the crystallite size of about 3 nm. The atomic force micrographs exhibited that the grains size of the films increased from 75 to 120 nm with the  increase of sputter power from 3.1 to 5.1 W/cm2 and the shape of the grains depends on the sputter power. The Fourier transform infrared spectroscopic studies showed the characteristic vibration bands of CuNiO2 in the films. electrical resistivity and Hall mobility of the films increased with the increase of sputter power. The deposited films were of p-type electrical conductivity. The optical transmittance of the films decreased and the absorption edge shifted towards higher wavelengths side with the  increase of sputter power. The optical bandgap decreased from 2.0 to 1.41 eV with the  increase of sputter power from 3.1 to 5.1 W/cm2. In conclusion, the films deposited at sputter power of 5.1 W/cm2 were nearly stoichiometric CuNiO2 with nanocrystalline in nature with crystallite size of 2.6 nm. These films showed the electrical resistivity of 27 Ωcm, Hall mobility of 21 cm2/Vsec, and optical bandgap of 1.93 eV.



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*Source: 527341-2013-08-25.xml*