J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7075-3

Structural, morphological, optical and electrical properties of spray deposited zinc doped copper oxide thin films Meherun Nesa1   · Mehnaz Sharmin1 · Khandker S. Hossain2 · A. H. Bhuiyan1 

Received: 19 February 2017 / Accepted: 3 May 2017 © Springer Science+Business Media New York 2017

Abstract  Nanostructured spray deposited zinc (Zn) doped copper oxide (CuO) thin films were characterized by employing X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), atomic force microscopy (AFM) and ultraviolet–visible–near infrared (UV–Vis–NIR) spectroscopy. XRD patterns of CuO and Zn doped CuO thin films indicated (monoclinic structure with the preferred orientation ) ̄ along 111 plane. Maximum value of crystallite size is found about 28.24 nm for 5 at% Zn doped CuO thin film. In FESEM images, nanoparticles were observed around the nucleation center. EDX analysis confirms the presence of all component elements in CuO and Zn doped CuO thin films. Analysis by AFM of CuO and Zn doped CuO thin films figured out decrease of surface roughness due to Zn doping. UV–Vis–NIR spectroscopy showed that CuO and Zn doped CuO thin films are highly transparent in the NIR region. Optical band gap of CuO thin films decreased with substrate temperature and that of Zn doped CuO thin films increased with Zn concentration. Refractive index of CuO and Zn doped CuO thin films raised with photon wavelength and became constant in the NIR region. 5 at% Zn doped CuO thin film showed the highest optical conductivity and the lowest electrical resistivity at room temperature.

* Meherun Nesa [email protected] 1

Department of Physics, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh

2

Department of Physics, University of Dhaka, Dhaka 1000, Bangladesh



1 Introduction Copper oxide (CuO) is a p-type oxide semiconductor due to its low band gap, nontoxicity, high melting and boiling points having monoclinic crystal structure [1, 2]. It has many applications such as in fabrication of photovoltaic cells [3], electro-chromic devices [4], gas sensors [5], catalytic materials [6], microwave dielectric materials [7], electrode materials for lithium batteries [8], etc. CuO thin films can be synthesized using various physical and chemical deposition techniques [9–16]. Properties of CuO thin films depend very strongly on the deposition techniques and deposition parameters. In recent years, many researchers have been studying the effect of transition metals such as nickel (Ni) [17], cobalt (Co) [18], manganese (Mn) [19], zinc (Zn) [20], etc. on the physio-chemical properties of CuO thin films. Baturay et al. [17] reported that optical band gap was increased for high-level (10%) Ni doped CuO thin films. Bayansal et al. [18] found that the crystallite size decreased from 22.7 to 12.2 nm with increasing Co concentration in nanostructured CuO thin films. Gulen et  al. [19] showed that the shape of the nanostructure was affected by Mn doping in the prepared CuO thin films. Faiz et  al. [20] noted that the optical band gap energy (­ Eg) increased from 0 to 3 atomic wt% and then ­Eg decreases for higher doping concentrations. Among these dopant metals, the ionic radii of ­Zn2+ (0.74 Å) is comparable to that of ­Cu2+ (0.73 Å) [21]. So, it is expected that incorporation of Zn into CuO will be advantageous for nice alloy formation. Besides this reason, limited information are available on the structural, optical and electrical properties of Zn doped CuO thin films. Very few works have been reported on Zn doped CuO thin films, rather most of the works on Zn doped CuO nanoparticles are published by others workers [22–24]. Since synthesized CuO thin film is transparent and the band gap can be tuned

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by suitable doping, it can be a useful material for optoelectronic applications. In this work, CuO thin films were prepared by spray pyrolysis technique (SPT). SPT represents a very simple and cost-effective technique [25]. In SPT, high quality substrate or chemical does not require as precursor materials. Dense, porous and multilayered films can be easily produced by SPT [26]. The effect of various substrate temperature ­(Ts) on the properties of CuO thin films have been studied by keeping other deposition parameters constant to find out the optimum ­Ts for preparation of good quality CuO thin films. It is observed that CuO thin film prepared at the ­Ts of 350 °C has better structural, electrical and optical properties than those of others. Zn doped CuO thin films have been deposited at ­Ts of 350 °C with Zn concentrations from 1 to 6 at%. The effect of Zn doping on structural, morphological, optical and electrical properties of Zn doped CuO thin films has been investigated by using different characterization techniques such as: X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive analysis of X-ray (EDX), atomic force microscopy (AFM), ultra-violet–visible–near infrared (UV–Vis–NIR) spectroscopy and DC electrical measurements.

XRD measurements have been made with a Bruker D8 Advance diffractometer (Germany) using X-rays of wavelength 1.5406 Å from C ­ uKα target and a secondary graphite monochromator. Bragg’s equation has been used to calculate the interplanar distance (d) and the results have been compared with JCPDS data card no. 01-073-6023. FESEM images of the synthesized thin films have been taken at ×50 k magnifications with a JEOL JSM-7600F field emission scanning electron microscope (USA). The elemental analysis has been performed by a JEOL EX-37001 electron dispersive spectrometer attached to the FESEM. Topographic AFM examinations have been carried out by Nanosurf FlexAFM (Switzerland). Average surface roughness ­(Ra) and root-mean square (rms) surface roughness ­(Rq) have been measured from AFM image with SPM control software version 3.1. Optical properties have been studied at room temperature by a dual beam UV–Vis–NIR spectrophotometer (Model SHIMADZU UV-1601, Japan) in the wavelength range of 300–1100  nm. DC electrical characterization of the synthesized thin films has been carried out using a four-point probe set up.

2 Experimental details

3.1 Structural analysis

Copper(II) acetate monohydrate [Cu(CH3COO)2·H2O] (purity 99%) has been used as a precursor material to synthesize CuO thin films because it is non-toxic, nonflammable and water-soluble. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] (purity 99%) has been used as dopant material to prepare Zn doped CuO thin films. Both the precursor and dopant materials were manufactured by MERCK, KGaA, 64271 Darmstadt, Germany and was collected from local market. CuO thin films have been deposited onto cleaned glass substrates at various T ­ s of 300, 350, 370 and 400 °C by SPT from 0.1  M aqueous solution of Cu(CH3COO)2·H2O. In order to prepare Zn doped CuO thin films with the desired compositions, the amount of Cu(CH3COO)2·H2O and Zn(CH3COO)2·2H2O were adjusted relatively in atomic percentage (at%) unit during preparation of the spray solution. Zn concentration was changed from 1 to 6 at% in Zn doped CuO thin films. Zn doped CuO thin films were synthesized at the ­Ts of 350 °C. For both cases distilled water and ethanol have been used as solvents. Spray gun used in this work is made of ebonite. The aperture of the outlet of the gun is 0.001 m in diameter. The distance between the nozzle and the substrate after optimization has been maintained at 25  cm during spraying. Air pressure has been fixed at 1  bar. Spray rate was kept constant at 0.08  mL/min. Deposition time has been kept constant at 5 min.

XRD patterns of CuO thin films observed at the 2θ angles between 30° and 60° are shown in Fig. 1. The characteristic peaks observed in XRD patterns of CuO are identified as

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3 Results and discussion

Fig. 1  XRD patterns of CuO thin films synthesized at the T ­ s of a 300, b 350 and c 400 °C

J Mater Sci: Mater Electron

( ) ̄ the reflections from 111 , (111) and (200) planes, which represents monoclinic crystal structure. From Fig.  1, it is seen that the thin film synthesized at 400 °C has both CuO (monoclinic) phase and ­Cu2O (cubic) phase. It is also seen from Fig.  1 that the peak intensity slightly increases with the increase of ­Ts. Peak intensity in a XRD pattern is often influenced by the increase of film thickness, but in this work thickness of CuO thin films prepared at various T ­ s are almost equal (about 150 nm). Because the deposition time was short (5  min) and other deposition parameters were kept constant during deposition of CuO thin films at various ­Ts. The slight increase in the intensity cannot be attributed to film thickness effect; rather it may have occurred as a consequence of gaining enough energy by the crystallites

to orient in proper equilibrium sites at relatively higher ­ s [27]. Since, the objective of this work was to prepare T CuO thin films with single monoclinic phase, 350 °C was selected as the optimum T ­ s for preparation of Zn doped CuO thin films. XRD patterns of Zn doped CuO thin films in Fig.  2 showed monoclinic structure similar to that of CuO. No peak corresponding to Zn impurity is observed. It may be possible due to the substitution of ­Cu2+ ions in the lattice structure of CuO by ­Zn2+ ions. The reason of substitution of ­Cu2+ by Z ­ n2+ is due to their comparable ionic radii. It is observed from Fig.  2 that the intensity of the XRD peaks decreases with increasing Zn concentration may be due to less crystallinity. Lattice constants (ɑ, b and c) were calculated from the predominant XRD peak of monoclinic phase CuO and Zn doped CuO thin films. The formula used to calculated the lattice parameter is given by, [ 2 ]1 2hkcosγ h k2 l2 2 1 = a2 sin2 γ + b2 sin2 γ − absin2 γ + c2 . The angle γ is d hkl

99.54° for monoclinic structure. The lattice constant of ­Cu2O (cubic) phase for the CuO thin film deposited at √ 1 h2 +k2 +l2 ­Ts = 400 °C was calculated by the formula, d = . a hkl

The values of the lattice constants for CuO and Zn doped CuO thin films are represented in Table  1. The standard values of lattice parameters are ɑ = 4.6837 Å, b = 3.4226 Å, c = 5.1288  Å and ratios ɑ/b = 1.3684 and c/b = 1.4985 according to JCPDS card file no. no. 01-073-6023. Table 1 shows that there is not much difference between the standard and calculated values of the lattice constants. The structural parameters of CuO and Zn doped CuO thin films like crystallite size (D), microstrain ( (ε)) and dislō cation density (δ) have been calculated for 111 plane and are recorded in Table 2. The average D for the films were kλ determined by using Scherrer formula [28], D = βcosθ , Fig. 2  XRD patterns of Zn doped CuO thin films synthesized with Zn concentration of a 0, b 2, c 4, d 5 and e 6 at%

where k is a constant known as shape factor and is taken as

Table 1  Lattice constants for CuO and Zn doped CuO thin films Substrate temperature, T ­ s (°C)

ɑ (Å)

b (Å)

c (Å)

ɑ/b

c/b

300 350 400 (CuO) 400 ­(Cu2O)

4.6811 4.6915 4.6811 4.2608

3.5130 3.4127 3.8532 4.2608

4.9174 5.2527 4.1048 4.2608

1.3325 1.3747 1.2152

1.399 1.5392 1.0653

Zn concentration (at%)

ɑ (Å)

b (Å)

c (Å)

ɑ/b

c/b

2 4 5 6

4.6825 4.6827 4.6824 4.6895

3.4736 3.5242 3.4107 3.3628

5.0381 4.8984 5.1858 5.6095

1.3480 1.3287 1.3786 1.3945

1.4504 1.3899 1.5204 1.6681

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Table 2  Structural parameters of CuO and Zn doped CuO thin films for preferred plane in the XRD patterns Substrate temperature, T ­ s (°C)

Crystallite size, D (nm)

Microstrain, ε × 10−3

Dislocation density, δ (­ 103/ nm2)

300 350 400 ­(Cu2O)

7.37 7.90 8.99

6.20 4.58 4.03

18.37 16.01 12.35

Zn concentration (at%)

D (nm)

ε × 10−3

δ ­(103/nm2)

2 4 5 6

7.53 10.07 28.24 12.23

4.80 3.42 1.28 2.95

17.62 9.85 1.25 6.67

0.94, λ is the wavelength of X-rays, β is the FWHM and θ is the Bragg angle. The ε values were calculated using the βcosθ relation, ε = 4 [29]. The δ values are the dislocation lines per unit area of the crystal, can be evaluated from the crystallite size ‘D’ using the Williamson and Smallman’s formula [30], δ = D12 . In Table  2, it is observed that D of CuO thin films slightly increase with increasing ­Ts. It implies that at higher ­Ts smaller crystallites agglomerated to form slightly larger crystallites and hence causes a change in D distribution. ε and δ decrease with increasing T ­ s for CuO thin films. It can be said that crystallites are less strained and dislocations along the preferred direction have been reduced with the change of crystalline phase at ­Ts = 400 °C. Some reports [31, 32] are available where D was found between 8 and 26 nm for CuO which is in good agreement with the result shown in Table  2. In Zn doped CuO thin films, D increases with the increase of Zn concentration up to 5 at% and ϵ and δ exhibit the reverse behavior of that of D. It is observed that the maximum value of D is about 28.24 nm and minimum values of ϵ and δ are about 1.28 × 10− 3 and 1.25 × 10− 3/nm2, respectively for 5 at% Zn doped CuO thin film. Larger D indicates the less dislocation per unit area

and minimum strain inside the crystallite. Decrease of ϵ and δ with Zn concentration indicates less deformation of the crystallites at high er Zn concentration. D values of Zn doped CuO thin film were comparable to that found by other worker [20]. 3.2 Surface morphology Figure  3 presents the surface morphology of the deposited CuO films observed by FESEM. FESEM micrographs reveal that sprayed particles are adsorbed onto the glass substrate into clusters, as the primary stage of nucleation. Amorphous grains covered the entire surface of the film and no well-defined grain boundaries are observed. It is seen that the thin films were crack free with nano-sized particle agglomerations. At lower T ­ s, the agglomerated regions of particles are not uniform in their shape. At higher ­Ts, those regions are uniform and clear enough to be identified. It is observed in the FESEM images that particles of larger sizes are formed in the CuO thin film deposited at the ­Ts of 350 °C compared to those deposited at other ­Ts. During deposition of CuO thin films at moderately high ­Ts (upto 300 °C), the

Fig. 3  The FESEM images of CuO thin films synthesized at the T ­ s of a 300, b 350 and c 400 °C

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deposition process is controlled by surface diffusion and the film surface becomes relatively smooth with a collection of closely spaced particles [33]. At the ­Ts higher than 300 °C, the precursor reaction occurs in the gaseous phase and the growth mechanism is controlled by the formation of precursor crystallites in the gaseous phase [34]. Thus the CuO thin films deposited at the ­Ts of 350 and 400 °C show different morphology compared to that deposited at 300 °C. Similar result was found by Sing et al. [35] for aerosol spray deposited CuO thin films. FESEM images of Zn doped CuO thin films, shown in Fig. 4, expose the presence of particle agglomerates. Film surfaces are homogeneous and crack-free. Particle size is increased with the increase of Zn concentration upto 5 at% in Zn doped CuO thin films. Rise of particle size may be accredited to the successful incorporation of Zn into CuO lattice. The particle size is decreased for 6 at% Zn doped CuO thin film. The reason of decrease of particle size after 5 at% Zn concentration may be an indication of exceeding the solubility limit of Zn into CuO lattice [36]. Formation of higher sized particles at 5 at% Zn concentration in CuO may be due to the optimum Zn concentration into CuO upto which Zn is soluble [36]. Variations of particle size of Zn doped CuO thin films seen in FESEM images follow the same nature as the variations of crystallite size calculated from the XRD data.

3.3 Compositional analysis The elements in CuO and Zn doped CuO thin films were confirmed by EDX analysis and the EDX spectra are shown in Fig.  5. In Fig.  5a two strong peaks are observed corresponding to Cu and O that confirms the formation of CuO thin films. The ratio of Cu and O in EDX analysis is almost 1:1 for the CuO thin films. In Fig.  5b not only Cu and O peaks are present but also small peaks are present for Zn impurity that confirms the formation of Zn doped CuO thin films. Table 3 shows that the atomic percentage (at%) and mass percentage (mass%) of Zn increase with the increase of Zn concentrations in CuO thin films up to 5 at% doping and then these decreased for 6 at%. EDX report reveals that the highest value of Zn atomic percentage is 5.42 for 5 at% Zn concentration. The amount in at% of O is larger in Zn doped CuO thin films than that in CuO thin films. 3.4 Topological analysis AFM images (100 × 100 nm) of CuO and 5 at% Zn doped CuO thin films deposited at ­Ts of 350 °C are shown in Fig.  6a, b, respectively. AFM images reveal that the film surface is homogeneous. The surface growth of the films showed that “peaks and valley” are uniformly distributed over the substrate in the scanned area of the thin film. The

Fig. 4  The FESEM images of Zn doped CuO thin films synthesized with Zn Concentration of a 1, b 2, c 3, d 4, e 5 and f 6 at%

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Fig. 5  EDX spectra of a CuO and b Zn doped CuO thin films. (Color figure online) Table 3  EDX report of CuO and Zn doped CuO thin films

Sample

CuO 1at%CuO: Zn 2 at% CuO: Zn 3 at% CuO: Zn 4 at% CuO: Zn 5 at% CuO: Zn 6 at% CuO: Zn

Elements in at% Cu

O

Zn

Cu

O

Zn

44.72 27.71 24.02 23.47 10.58 8.03 15.49

55.28 72.05 73.80 73.50 85.40 86.55 80.41

– 0.24 2.18 3.03 4.02 5.42 4.10

76.26 60.10 53.57 52.04 42.38 22.70 38.76

23.74 39.35 41.44 41.04 50.11 61.56 50.68

– 0.55 4.99 6.92 7.51 15.74 10.56

values of R ­ a and R ­ q of the CuO and 5 at% Zn doped CuO thin film surfaces are documented in Table 4. Since, ­Ra and ­Rq are lower for Zn doped CuO thin film, it may be said that Zn incorporation has significant influence on the surface roughness of CuO thin film. XRD and FESEM analyses showed that crystallite and particle sizes of 5 at% Zn doped CuO thin film were larger than those of the Zn doped CuO thin films with other Zn concentrations. Larger size particle may have caused the reduction of surface roughness which can be beneficial for the reduction of the short-circuit effect when used in solar cells [37]. 3.5 Optical properties Figure  7 shows the optical transmittance (T %) of CuO thin films deposited at T ­ s of 300, 350, 370 and 400 °C. T increases sharply with the increase of wavelength in the visible region and are found to be transparent in the NIR region of spectra. The highest value of T is found to be 77.72% for ­Ts = 350 °C at 1100 nm. The reason of increase

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Elements in mass%

of T for the CuO thin film deposited at ­Ts of 350 °C may be due to relatively less scattering because of the lessening of the degree of irregularity in the particle size distribution [38]. Figure  8 represents the optical transmittance versus wavelength graph for 0–6 at% Zn doped CuO thin films. In the visible region, T increases with the increase of doping concentration up to 5 at% of Zn and then it decreases for 6 at% of Zn. Zn doped CuO thin films are highly transparent in NIR region and the maximum T is about 87% for 5 at% Zn concentration. Increase of transparency with the rise of Zn concentration in CuO thin films may be attributed to increase of particle size up to 5 at% Zn concentration. Because, larger particle size allowed the films to decrease the irregularity in the grain boundary and causes less scattering due to which the films become more transparent. Evaluation of E ­ g of CuO and Zn doped CuO thin films was done by using Tauc relation, αhν = B (hν − Eopt)n, where hν is the energy of absorbed light, n is the parameter connected to the distribution of the density of states B,

J Mater Sci: Mater Electron

Fig. 6  AFM images of a CuO and b 5 at% Zn doped CuO thin films

Table 4  Roughness values of CuO and Zn doped CuO thin films Sample

Image area

Ra (nm)

Rq (nm)

CuO 5 at% Zn doped CuO CuO 5 at% Zn doped CuO

1 × 1 µm

10.084 2.178 0.609 0.517

12.952 2.666 0.757 0.643

100 × 100 nm

a constant or Tauc parameter [39]. Here n = 1/2 for direct and n = 2 for indirect transitions. The ­Eg for CuO and Zn doped CuO thin films were obtained from (αhν)2 versus hν plots and are shown in Figs.  9 and 10, respectively. The ­Eg values of synthsized CuO and Zn doped CuO thin films are presented in Figs. 11 and 12, respectively.

Figure 11 shows that the values of ­Eg for CuO thin films vary between 1.81 and 2.63  eV which are consistent with the previous reports [40–42]. In Fig. 11 for CuO thin films, ­g the value of E ­ g decreases with the increase of ­Ts. Fall of E ­ g may be with ­Ts may be a cosequence of increase in D. E affected a little by the lattice phonon-free electron and lattice phonon-hole interactions [9]. ­Eg of Zn doped CuO thin films is found to be between 2.69 and 2.88  eV and these values are in agreement with that obtained from 2.64 to 2.78 eV for Zn doped CuO thin films prepared by spin coating technique [43]. Figure 12 shows that E ­ g increases with Zn concentration up to 5 at% Zn and then it is dereased for 6 at% Zn. In this case, the ­Eg broadening may have occurred due to the presence of plenty of oxygen observed in EDX analysis of Zn doped CuO thin films (Table 3). Especially for 4–6 at% Zn doped CuO the amounts of oxygen are more

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Fig. 7  Transmittance versus wavelength graph of CuO thin films synthesized at the T ­ s of a 300, b 350, c 370 and d 400 °C

Fig. 8  Transmittance versus wavelength graph of Zn doped CuO thin films synthesized with Zn concentration of a 0, b 1, c 2, d 3, e 4, f 5 and g 6 at%

than 80%. The amount of oxygen is the maximum for 5 at% Zn doped CuO thin film for which ­Eg is the highest. Since ­Eg depends on the composition and stoichimetry of the deposited film, increase of ­Eg can be attributed as the effect of variation of composition of elemental components in Zn doped CuO [43]. Refractive index (η) of CuO and Zn doped CuO thin films were √calculated by the formula [44], ) ( ) ( 4R 2 . Variation of η with waveη = 1+R − k + 1−R (1−R)2 length for deposited thin films is shown in Figs.  13 and

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Fig. 9  Variation of (αhν)2 with hν for CuO thin films deposited at the ­Ts of a 300, b 350, c 370 and d 400 °C

Fig. 10  Variation of (αhν)2 with hν for Zn doped CuO thin films deposited with Zn concentration of a 0, b 1, c 2, d 3, e 4, f 5 and g 6 at%

14. η rises from 1.33 to 2.60 for CuO and that of Zn doped CuO thin films increases from 1.19 to 2.62. It is evident from Fig. 13 that η has lowest value for CuO thin film prepared at ­ Ts  =  350  °C in the NIR region (750–1100 nm). When the incident light interacts with a material which has low number of particles, the amount of refraction may be low, and thus the refractivity of the film may be decreased [11]. Figure 14 also exhibits that η decreases with the increase of Zn incorporation up to 5 at% between the wavelengths 750 and 1100  nm. In that wavelength region, higher η is found for 6 at% Zn doped CuO thin films. Lower value of η in Zn doped CuO thin film with higher concentration of Zn reveals that light moves faster through these thin films.

J Mater Sci: Mater Electron

Fig. 11  Variation of opitcal band gap for CuO thin films with ­Ts

Fig. 13  Variation of refractive index for CuO thin films prepared at the ­Ts of a 300, b 350, c 370 and d 400 °C

Fig. 12  Variation of opitcal band gap for Zn doped CuO thin films with Zn concentration

Fig. 14  Variation of refractive index for Zn doped CuO thin films prepared with Zn concentration of a 0, b 1, c 2, d 3, e 4, f 5 and g 6 at%

Extinction coefficient (k) was calculated from α data by using the equation, k = αλ . The variation of k with 4π wavelength for CuO and Zn doped CuO thin films is shown in Figs. 15 and 16, respectively. Both Figs. 15 and 16 show that k decreases with increasing wavelength. The values of k are varied between 0.14 and 0.93. The rise and fall in k is directly related to the absorption of light. The fall in k may be due to the absorption of light at the grain boundaries. In Fig.  15, the lower value of k is observed for CuO thin film synthesized at 350 °C. In Fig. 16, the k decreases due to Zn doping and lower values of k is found for 5 at% Zn doped CuO thin film up to 800 nm and above that the lower values of k are observed

for 4 at% Zn doped CuO thin film. The lower value of k indicates the low surface roughness of the deposited samples. Calculated k value strongly supports the AFM analysis result. The optical conductivity (σopt) of the thin films was αηc calculated using the equation, σopt = 4π . Figs.  17 and 18 exhibit the variation of σopt with hν for the deposited thin films. σopt is increased at high photon energies due to the high absorbance in that region [45]. It is a frquency dependent quantity which is inversely related to the E ­ g values. As 5 at% Zn doped CuO thin film has high E ­ g, it shows minimum conductivity at lower photon energy region.

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Fig. 15  Variation of k with λ for CuO thin films prepared at the ­Ts of a 300, b 350, c 370 and d 400 °C

Fig. 16  Variation of k with λ for Zn doped CuO thin films prepared with Zn concentration of a 0, b 1, c 2, d 3, e 4, f 5 and g 6 at%

J Mater Sci: Mater Electron

Fig. 17  Variation of optical conductivity with photon energy for CuO thin films prepared at the ­Ts of a 300, b 350, c 370 and d 400 °C

Fig. 18  Variation of optical conductivity with photon energy for Zn doped CuO thin films prepared with Zn concentration of a 0, b 1, c 2, d 3, e 4, f 5 and g 6 at%

3.6 Electrical properties Dielectric loss is a loss of energy due to heating a dielectric material in a varying electric field. The dielectric loss tangent (tanδ) was calculated using the relation tanδ = εi/εr. tanδ of CuO and Zn doped CuO thin films are plotted against wavelength in Figs. 19 and 20, respectively. Figure 19 shows lowest value of tanδ for CuO thin film prepared at ­Ts = 350 °C. In Fig.  20, tanδ decreases with the increase of Zn doping up to 5 at% and then it increases in the case of 6 at% Zn concentration. From tanδ versus wavelength curves it is also observed that the values of tanδ gradually decrease with the increase of wavelength. tanδ exhibits the similar nature to that of k.

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Room temperature resistivity (ρ) of CuO and Zn doped CuO thin films are shown in Figs.  21 and 22, respectively. ρ values are found in the order of ­103 Ω-m which are consistent with that reported for spray deposited CuO by some worker [46] under some different deposition conditions. It is seen that the CuO thin film prepared at ­Ts = 350 °C has the lowest value of ρ which is 1.54 × 103 Ω-m. After a certain ­Ts, the absorption of oxygen from air increases which causes the increase of ρ. In SPT, oxygen is the most significant background impurity that can easily be diffused into the crystal lattice while deposited at higher ­Ts [47]. Figure  22 shows that ρ increases with

J Mater Sci: Mater Electron

Fig. 19  Variation of dielectric loss with wavelength for CuO thin films synthesized at the T ­ s of a 300, b 350, c 370 and d 400 °C Fig. 21  Variation of resistivity for CuO thin films with ­Ts

Fig. 20  Variation of dielectric loss with wavelength for Zn doped CuO thin films synthesized with Zn concentration of a 0, b 1, c 2, d 3, e 4, f 5 and g 6 at%

the increase of Zn concentration up to 4 at% in Zn doped CuO thin films. It can be said that due to the incorporation of Zn as donor impurity in CuO, a p-type semiconductor, free carriers are compensated. So, there may be an increase of free electron concentration in the conduction band and subsequent decrease of hole concentration in the valance band with Zn incorporation [48]. The lowest value of ρ is found to be 1.51 × 103 Ω-m for 5 at% Zn doped CuO thin film. Increase of Zn concentration beyond 5 at% CuO thin films results in a reduction of ρ which may be attributed to the solubility limit of Zn into the CuO lattice [36]. ρ increases for 6 at% Zn

Fig. 22  Variation of resistivity for Zn doped CuO thin films with Zn concentration

concentration in CuO thin film which may be due to the decrease of free carriers in the film.

4 Conclusions In this work, the optimum ­Ts for deposition of CuO thin film on to glass substrate from copper acetate precursor

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by SPT was found to be 350 °C. CuO thin film synthesized at ­Ts = 350 °C was more crystalline and transparent and had larger particle size and lower resistivity than those deposited at other ­Ts. Although Zn doping did not improve the crystallinity of CuO thin films, it caused significant improvement to the surface morphology, optical and electrical properties. Film surface was more homogeneous and surface roughness reduced to a significant extent due to Zn doping. Shape and distribution of particles were more uniform and particle size increased after Zn doping. Transparency of CuO was raised to a maximum of 87% after Zn doping. It was interesting to see that although band gap increased with Zn concentration resistivity decreased and it was comparable to that of pure CuO thin film. So, it can be concluded that band gap can tuned by keeping resistivity constant. Thus, it can be inferred from the outcome of this research work that the Zn doped CuO films may be useful in solar cell and other optoelectronic devices. Acknowledgements  The authors are thankful to the authorities of Bangladesh University of Engineering and Technology and Ministry of Science and Technology, Bangladesh, for providing necessary support to this research work. The authors express sincere gratitude to Prof. Dr. Jiban Podder, Dept. of Physics, BUET for fruitful discussion. One of the authors gratefully acknowledges support from International Science Program (ISP), Uppsala University, Sweden.

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