J Mater Sci: Mater Electron DOI 10.1007/s10854-017-6371-2

Studies on copper oxide thin films prepared by simple nebulizer spray technique R. David Prabu1 · S. Valanarasu1 · I. Kulandaisamy1 · V. Ganesh2 · Mohd Shkir2 · A. Kathalingam3 

Received: 23 October 2016 / Accepted: 12 January 2017 © Springer Science+Business Media New York 2017

Abstract  Copper oxide films were deposited by simple nebulizer spray pyrolysis technique using aqueous solution of Copper(II) acetate monohydrate, high pure glucose and 20 vol% of 2-propanol. Structural, morphological, optical and electrical properties of the deposited films were characterized by XRD, Laser Raman, AFM, UV–Vis and Hall Effect measurements. The XRD study confirmed that the copper oxide films are in polycrystalline form of cuprous oxide ­(Cu2O) phase with cubic crystal structure for the films deposited using precursor volumes of 3 and 4  ml, whereas the films deposited using 5 ml precursor solution are in cupric oxide (CuO) phase with monoclinic crystal structure. The higher concentration films shows higher thickness (~ 600  nm for 5  ml) and that change the phase/ composition of the films. The prepared CuO films with 5  ml precursor solution are expected to show better properties. AFM studies revealed that the surfaces of the films are very smooth with uniformly distributed grains. The surface roughness of the film was increased with volume of the solution and the grain islands were coalesced with each other. UV visible spectrophotometer measurements showed that the band gap value of the prepared copper oxide thin films is varied between 1.63 and 1.23 eV due to change of volume of the solution. Hall Effect measurement showed * S. Valanarasu [email protected] 1

PG and Research Department of Physics, Arul Anandar College, Karumathur, Madurai, India

2

Department of Physics, Faculty of Science, King Khalid University, P.O. Box. 9004, Abha 61413, Saudi Arabia

3

Millimeter‑Wave Innovation Technology Research Center (MINT), Dongguk University, Seoul 100‑715, Republic of Korea



that the prepared films are in p-type conductivity with 8.21 ×102 Ω-cm resistivity (ρ) and 12.56 ×1015 cm−3 carrier concentration (n) for the films prepared at 5  ml solution. All the studied properties of CuO for 5 ml precursor solution are remarkably changed.

1 Introduction The energy crises increases globally and fossil fuels exhaust rapidly, drives towards the mandatory use of renewable energy in our day today life. Among the renewable sources of energy, the solar energy can be utilized to meet out the future global energy demands. So it is necessary to fabricate right and able material to manufacture good and efficient solar cells. Copper oxide is more suitable for photovoltaic devices as it is a non-toxic, inexpensive material with high carrier mobility and available abundantly in the earth’s crust. It is a p-type oxide semiconductor playing an important role in optoelectronic devices such as solar cells, gas sensors, field emitting diodes, optical switches and photo catalytic applications [1–8]. The low-cost and non-toxic nature of the copper oxide makes it as a suitable option for the various applications. The copper oxide is formed in two phases such as cupric oxide CuO (with monoclinic structure) and cuprous oxide ­Cu2O (with cubic structure) [9], and the particular phase has suitable properties for certain applications. Although, the preparation of a preferred phase of copper oxide is a challenging task. As per the available literature, there are many techniques have been generally used to prepare copper oxide thin films such as thermal oxidation, activated reactive evaporation, spray pyrolysis [10], electro-deposition [11], spin coating [12], chemical deposition [13], dip coating [14], chemical vapor deposition [15], electron beams deposition, pulsed laser

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Vol.:(0123456789)



deposition, sputtering [16] and nebulizer spray pyrolysis [17]. From the above mentioned thin film methods, large production of metal oxide thin films the nebulizer spray pyrolysis deposition method has been widely used, as it is facile and cost effective. In nebulizer spray pyrolysis (NSP) method optimization can be done by varying deposition parameters such as distance between nozzle and substrate, molar concentration of the solution, spray rate and air pressure. It is a convenient method, use low volume of spray solution and time fetching technique, by which small droplets of particles can be easily deposited. The merit of NSP is that it is handy for the production of huge area uniform film coating. In this work, preparation of copper oxide thin films by varying the precursor volume using nebulizer spray pyrolysis technique and their characterization was planned. The obtained results are discussed in this report.

2 Experimental detail 2.1 Preparation of precursor solution Schematic representation of copper oxide thin films preparation by the nebulizer spray pyrolysis (NSP) technique is shown in Fig.  1. For the preparation of precursor solution the copper(II) acetate monohydrate (­C4H6O4·H2O) of concentration 0.04  M was taken as a raw material, 0.1  M of high pure glucose ­(C6H12O6) was dissolved into distilled water and 20  vol% of 2-propanol [(CH3)2CHOH] was added to the precursor solution and stirred for 30 min

J Mater Sci: Mater Electron

at room temperature. The glucose was used as a reducing agent for the precipitation of copper particles [18]. The use of 20  vol% of 2-propanol is to enhance the wet-ability of the droplet in the substrate by reducing the surface tension of the solution. This can increase the homogeneity of the deposited films [19]. The volume of the solutions used for the deposition of films is varied as 3, 4, and 5 ml respectively. Substrate was kept at a constant temperature (i.e. 300 °C) maintained by a highly stable temperature controller. The nozzle was kept at ~5  cm from the substrate and the air flow rate was ~1.0 Kg cm−2. After the desired cycles of spray the substrate was allowed to cool to room temperature. The thicknesses of the prepared films was measured by SEM (JSM 6360 LA, Japan) and found to be in the range of 300–600 nm. 2.2 Characterization X-ray diffraction (XRD) patterns of the prepared copper oxide thin films were recorded using CuKα (λ = 1.5406 nm) radiation. The diffracting angle was scanned from 5 degree to 70 degree continuously with a rate two degree per minute. Raman spectra were recorded using Raman (Princeton Acton SP 2500) instrument at room temperature. Morphological variation and surface roughness of the films were studied using atomic force microscopy (AFM). Optical transition of the thin films was studied using UV visible spectrometer (Hitatchi-330) in the wavelength range of 500–2500  nm. Electrical properties of the films were

Fig. 1  Schematic diagram of nebulizer spray pyrolysis setup. 1 Air compressor 2 pressure meter 3 oil tap 4 air flow tube 5 solution 6 nebulizer 7 stand 8 class tube 9 furnace 10 thermocouple 11 temperature 12 substrate 13 substrate holder

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analyzed by Hall Effect measurement with the use of four probe method.

3 Result and discussion 3.1 Structural studies XRD pattern of copper oxide thin films deposited at 300 °C under different volumes (3–5 ml) is shown in Fig. 2. Peaks corresponding to reflection of (1 1 1) plane is observed for the samples prepared with the volume of 3 and 4  ml. This signifies that the films are polycrystalline cubic crystal structure of C ­ u2O phase which is in good agreement with the standard characteristic peaks (JCPDS 77-0199). The intensity of (111) peak is found to be enhanced with increasing the volume of precursor. Whereas the precursor volume of 5  ml has given cupric oxide (CuO) phase, the peak corresponding to (1 1 1) plane is present in the 5 ml XRD pattern. This result is in accordance with the standard data (JCPDS 89-2531) with monoclinic crystal structure cupric oxide. These results of XRD analysis of copper oxide thin films confirmed the phase transformation from cuprous ­(Cu2O) to cupric (CuO) on increasing the volume of the solution. During film formation ions are formed in preferred orientation, additional ions occupy in the unoccupied interstitial sites depending upon the availability of the ions in solution. .Hence, the change of volume leads to the change of preferred growth. Previous reports are also admit the crystalline growth rely on the deposition condition of the film [20]. The increase of XRD peak intensity with the increase of precursor solution volume indicates crystalline

improvement at higher volumes. This improvement can be understood by studying the crystallite size variation due to the volume change. Crystalline size (D) of the films was calculated using the full width at half maximum (FWHM) value of XRD peak employing the Debye Scherer formula as given below [21].

D=

0.9𝜆 𝛽 cos 𝜃

(1)

where, D is the crystallite size of the film, λ is the wavelength of the incident ray, K is a shape factor, β is the full width half maxima in radians of the (111) and (111) diffraction peaks and θ is the Bragg diffraction angle. The calculated crystallite size of the films prepared for the volumes of 3, 4 and 5 ml are 30, 32 and 36 nm respectively. It indicates that there is a crystalline size improvement with the increase of spray solution volume. The micro-strain and dislocation density of the films deposited at different volumes of solution were also calculated. Micro-strain and dislocation density of the films were calculated using the Eqs. (2) and (3), respectively [22, 23] and the values are given in  Table 1.

𝛿=

1 D2

(2)

𝜀=

𝛽 cos 𝜃 4

(3)

The dislocation density and micro-strain of the films are found to decrease with the increase of crystalline size indicating the indicating the crystalline improvement at increased solution volume. This decrease in micro strain and dislocation density attributes the reduction of lattice defects along the grain boundaries higher volumes. 3.2 Raman analysis The prepared copper oxide thin films were analyzed using Raman spectroscopy to know phase information of the films. Figure  3 shows the observed frequencies as a function of Raman intensity and Raman shift of the copper oxide thin films of deposited with 3, 4, and 5  ml solution volumes. Figure 3a gives the response for the Raman shift wavelength 100–400  cm−1, whereas the Fig.  3b gives for Table 1  Structural parameters of copper oxide thin films

Fig. 2  XRD pattern of copper oxide thin films

Volume (ml)

Crystallite size (nm)

Dislocation density ­(1015 lines/m2)

Microstrain ­(Lines2 m4)

3 4 5

30 32 36

1.10 0.93 0.74

1.14 1.05 0.94

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Fig. 3  Raman spectra of copper oxide thin films

200–2000  cm−1, The Raman spectra of 3 and 4  ml solutions show the characteristic phonon frequencies of the crystalline cuprous oxide (Cu2O). In Fig.  3a the samples grown with 3 and 4 ml solutions display peaks corresponding to cuprous oxide ­(Cu2O) at 148 and 218 cm−1, whereas the samples grown with 5  ml solution shows peaks corresponding to cupric oxide (CuO) at 273 and 327  cm−1 [24]. The 218 cm−1 line is attributed to two Г−12 phonons [25–27], whereas the line at 148  cm−1 can be recognized as Raman scattering from phonons of symmetry LO Г−15 [26–28]. The peak at 218  cm−1 corresponds to secondorder allowed Raman mode of cuprous ­(Cu2O) crystals. The peak obtained at 109 cm−1 is assigned to the inactive Raman mode [27]. These observations infer that the phase of copper oxide is transformed to cupric oxide phase at the volume of 5 ml. The peak shift 1600 cm−1 shown in Fig. 3b for the volume of 5 ml is caused by the lattice defects and lattice disorder The peak at 1250  cm−1 for 3  ml solution can be assigned to longitudinal optical (LO) mode, due to some defects of oxygen vacancy and Cu interstitial. Comparing the peaks of 3 and 4 ml the intensity peaks for 5 ml is higher; these are due to the point defects created at higher volumes. Both the XRD and Raman spectroscopy results confirm the change phase due to the change of volume of the solution. The C ­ u2O phase obtained at the volumes of 3 and 4 ml solution is changed to CuO phase at 5 ml volume of solution. 3.3 Morphological studies Figure  4a–c show the 2D and 3D AFM images of copper oxide thin films deposited using different volumes via

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nebulizer spray pyrolysis. The surfaces of the prepared copper oxide thin films are very smooth and the grains are uniformly distributed. Surface roughness and particle sizes of the films are listed in Table 2. Grain islands are coalesced with each other and the surface roughness is increased with increase in volume. Surface roughness of the film depends on the volume of the solution. As seen from the images the higher volume of 5  ml leads to larger particle size while 3  ml volume exhibits smaller particle size. The measured surface roughness of the films are 23, 26, 29 nm and particle size values are 44, 47, 52  nm for the 3, 4, and 5  ml respectively. Thicker film is found to show higher surface roughness. 3.4 Optical analysis Optical transmittance of copper oxide films deposited using different volumes (3, 4, and 5 ml) of solution keeping the substrate at 300 °C are shown in Fig. 5. All the films show transparency in the visible region after 500 nm. In case of 3 and 4 ml, the transmittance spectrum reveal an interference pattern with a rapid fall of transmittance at the band edge which specify that the copper oxide films are uniform in nature. The transmittance data are used to calculate refractive index and band gap of the thin films in which transmittance values are varying with increase in the different volumes of the solution. The absorption spectrums of copper oxide thin films are shown in Fig. 6. The absorption is decreased as the wavelength is increased. The optical constants (n and k) of polycrystalline thin films are determined using the formulas (4) and (5) respectively.

J Mater Sci: Mater Electron

Fig. 4  2D and 3D images for copper oxide thin films for a 3, b 4, c 5 ml solution

Refractive index √ n = 4R∕(R − 1)2 − K2 − (R − 1)∕(R − 1) Extinction coefficient

k= (4)

𝛼𝜆 4𝜋

(5)

The refractive index and extinction coefficient of the copper oxide thin films of different precursor volumes are

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Table 2  Surface roughness and particle size of copper oxide thin films Volume (ml)

Particle size (nm)

Surface roughness (nm)

3 4 5

44 47 52

23 26 29

Fig. 7  Refractive index versus wavelength of copper oxide thin films for different volumes

Fig. 5  Transmittance spectrum of copper oxide thin films

Fig. 8  Extinction coefficient of copper oxide thin films for different volumes

Fig. 6  Absorption spectrum of copper oxide thin films

shown in Figs.  7 and 8 respectively. The refractive index is important parameter as it has link with the electronic polarizability of ions and the local field inside the material. This variations of extinction coefficient (k) and refractive index (n) of the copper oxide films are found similar to the variation of k and n of absorbing semiconducting materials. The reduction of k and n with the change of volumes

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indicates growth of crack free surface of copper oxide films. The existence of an absorption edge at 800 nm confirms the sudden increase in refractive index (n). Figure 7 admits that the index of refraction increases with decrease in wavelength and becomes constant at higher wavelengths. The rate of increase in refractive index (n) becomes higher below the absorption edge (500  nm) whereas ‘n’ is found to be same at wavelengths above 1000  nm. The precursor solution of 5 ml which belongs to cupric (CuO) phase shows difference in the refractive index and extinction coefficient. From the figure, it is seen that the refractive index of copper oxide thin films is very much related to the volume of the solution.

J Mater Sci: Mater Electron

The transmission results helps to estimate the optical band gap ­(Eg) of the copper oxide thin films. Being a direct semiconducting material, the ­ Eg of copper oxide is determined by extrapolating the full line of the plot to the abscissa axis. By employing Tauc’s plot method, Eg is found to be around 1.63  eV for the first two films (3 and 4 ml) and 1.23 eV for the last film (5 ml) solution (Fig. 9). From literature the band gap of C ­ u2O film is given in the range 1.8–2.5  eV depending upon growth condition [29]. This decrease of optical band gap values of C ­ u2O films may be connected to the low electrical conductivity of the films. To calculate the value of skin depth (χ) the formula (6) was used.

𝜒=

𝜆 2𝜋k

(6)

Figure 10 represent the relationship between skin depth (X) versus wavelength. From this picture, it can be seen that the intensity of light penetrated from the thin film is transmittance oriented. Optical conductivity values of the films can be found from the formula given in Eq. (7).

𝜎=

𝛼nc 4𝜋

(7)

Figure  11 shows the optical conductivity (σ) versus wavelength. The optical conductivity of copper oxide thin films is reduced up to 800  nm and afterward it becomes constant for all the films. This reduction is due to the low absorbance of the films in low photon energy region. The 5 ml solution is slightly increased in the low photon energy region. The Fig.  12 shows the reflectance spectra of copper oxide thin films. This reflectance value of the prepared

Fig. 9  Direct band gap of optimized Copper oxide thin films

Fig. 10  Skin depth of copper oxide thin films for different volumes

Copper oxide thin film makes suitable material for antireflection coating. Reflectance is 40, 45 and 78% for the 3, 4 and 5 ml respectively. The reflectance spectra of copper oxide thin film for 5  ml solution do not possess any different variations for wavelength between 300 and 550  nm which signifies the presence of CuO rather than ­Cu2O. 3.5 Electrical analysis Figure  13 gives the carrier concentration, Hall mobility and electrical resistivity for ­Cu2O and CuO thin films which were measured by a Hall Effect measurement and the results are listed in Table  3. All the three sample shows p-type conductivity. Hall effect studies show a positive Hall constant at room temperature and all the

Fig. 11  Optical conductivity of copper oxide thin films for different volumes

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The resistivity values vary from 45.2 × 102 to 8.21 × 102 (Ω cm), these result was good agreement with [30].

4 Conclusion

Fig. 12  Reflectance spectrum of copper oxide thin films

Fig. 13  Electrical resistively (ρ), carrier concentration (n) and carrier mobility (μ) of the copper oxide thin films

Table 3  Electrical properties of copper oxide thin films Volume (ml)

Resistivity (Ω cm) ×102

Carrier concentration ­(cm-3) ×1015

Hall mobility ­cm2/vs

3 4 5

45.2 17.4 8.21

3.85 7.43 12.56

0.35 0.48 0.60

samples can be considered p-type having carrier concentrations in the range around 1­ 015 or 1­ 016 ­cm−3. The carrier concentration increases with increasing precursor volume from 3 to 5 ml. The change in carrier concentration is due to the phase transformation from cuprous oxide to cupric oxide. The resistivity decreased along with increase of different precursor volumes at 300 °C which indicates the insufficient copper vacancies of copper oxide films.

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Copper oxide thin films were successesfully deposited on a glass substrate using nebulizer spray pyrolysis method employing different volumes of precursor solutions at 300 °C. Grown films were characterized for structural, morphological, optical and electrical properties. X-ray diffraction analysis showed the copper oxide thin films are polycrystalline with cuprous oxide (­Cu2O) for 3, 4 ml solution and cupric oxide (CuO) for the 5 ml solution. The crystallite size is increased with the change in precursor volume. Raman analysis showed that the 3 and 4 ml solution have significant peaks corresponding to cuprous oxide ­(Cu2O) (148 and 218  cm−1). The 5  ml solution showed the peaks corresponding to CuO at 273, 298  cm−1. The AFM study showed the growth of very smooth film with grains distributed uniformly over the surface. The surface roughness calculated are 23, 26, 29 nm and particle size values are 44, 47, 52 nm for the 3, 4 and 5 ml respectively. From the optical studies the band gap of 1.63  eV was observed for cuprous oxide ­(Cu2O) thin film and 1.23 eV for cupric oxide (CuO) thin film. The Hall Effect studies unveiled a significant increase in mobility, carrier concentration and conductivity of Copper oxide thin films for increasing precursor volume and it confirms that the grown films are P type in nature with the carrier concentration range of 1­ 015/ cm3. Further work is in progress to optimize other parameters, which will be presented in future publications. Acknowledgements  This work was supported by DST, India, under the scheme of Science and Engineering Research Board (SERB). DST.No. SB/FTP/PS-131/2013.

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