Electrical, optical and nonlinear optical process in spray pyrolyzed Zn:CuO nanostructures for optoelectronic device applications

Nanostructured pure and Zn doped CuO thin films were deposited on a glass substrate at 400 °C using the chemical spray pyrolysis method. The fabricated thin films were characterized to study the compositional, structural, morphological, optical and electrical properties. X-ray diffraction spectra show the polycrystalline nature of the sample and confirm the monoclinic phase of copper oxide. Raman analysis further confirms the absence of cuprous oxide phases and impurities. High absorbance in the visible region was observed for the films with bandgap values ranging from 1.7–2.0 eV. A near-band edge emission peak in the red region is recorded in the photoluminescence spectra. Uniformly distributed nanoparticles are observed in SEM images. Hall effect measurements indicate p-type conductivity and 5% Zn doped copper oxide showed the highest conductivity and carrier concentration. The non-linear absorption coefficient (β eff) of the samples was obtained with the help of z-scan method with a Helium-Neon laser under the CW regime. Zn doping results in an increase in nonlinear absorption, supporting the use of Zn:CuO for optoelectronic devices.


Introduction
Recent advancement in the area of nonlinear optical materials with enhanced optical properties which are responsive to applied electric fields has resulted in an increased demand for the same.These materials can be used in nonlinear spectroscopy, Q-switching, optical distortion correction, optical switching, optical data processing, waveguide switches, optical logic gates, mode-locking, optical limiting, optical communications and modulators due to their behavior in the applied field.Metal oxide thin films have been extremely interesting and widely researched topics in recent years.Metal oxide semiconductor based nonlinear materials have gained much attention in optoelectronic device applications.Among metal oxide semiconductors Cupric oxide is a prominent material due to its good structural, optical, and electrical properties.CuO is a low energy bandgap material ranging from 1.2-2 eV with a p-type nature and has a high boiling point and melting point adding to which it is also non-toxic and abundant in nature [1].Various applications of CuO have been reported in the field of semiconductor electronics ranging from photovoltaics, gas sensing, and photocatalysts [2].Many researchers in the past decades have studied the effects of doping different elements like In [3], Fe [4], Co [5], Mn [6], Zn [7], and Ag [8] on cupric oxide.Kose et al [3] reported that Indium doping increased the bandgap of CuO and decreased its roughness.Fathima et al [4] reported an increase in the transmittance of CuO when doped with Fe ions.Shabu et al [8] synthesized Ag doped CuO and reported low resistivity values on increasing doping concentration.Doping of CuO is observed to have drastic changes in its structural, electrical & optical properties which can be tuned for specific applications.Transition metal doping has also been extensively studied in recent times and has proven to have outstanding utility in solid state electronics [9].Additionally, this is considered to be a feasible option for doping due to its inexpensive nature.Faiz et al examined the effects of Zn doping using the PLD method and reported a better crystalline nature of the samples [10].Meherun nesa et al reported an increase in bandgap values of CuO with Zn doping [7].Zn 2+ has an ionic radius comparable to Cu 2+ among other transition metals hence it is expected to have effective incorporation in the host lattice.Apart from this minimal data is available on Zn doped CuO thin films specifically using the spray pyrolysis method.
Spray pyrolysis is a cost-effective yet simple method to achieve evenly distributed chemical deposition for large-scale production.Multilayered deposition on thin films can be easily obtained by this process.A. Chen et al studied the third-order nonlinearity CuO thin films deposited using the PLD method.Open aperture z-scan technique reported that the films showed saturable absorption with a negative absorption signature [11].Ashour et al investigated the nonlinear properties of CuO thin films deposited using direct current magnetron sputtering and reported saturable absorption [12].Although there have been numerous works related to nonlinear studies of CuO, there is inadequate data on nonlinear optical properties of doped CuO thin films.From a thorough survey of scientific studies, there are no known works that focus on the non-linear optical properties of thin films deposited with Zn doped CuO specifically by chemical spray pyrolysis.This work presents an elaborate study on the morphological, structural, electrical, non-linear and linear optical properties of pure and different concentration of Zn doped CuO thin films developed by spray pyrolysis process.Additionally, the suitability of the material in optoelectronic applications using open aperture Z-scan technique is also conducted.

Experimental details 2.1. Chemicals used
Copper chloride dihydrate (CuCl 2 .2H 2 O) and zinc acetate dihydrate (ZnCH 3 CO.2H 2 O) were purchased from Merck, India.Deionized water along with ethanol are the solvents used.

Deposition of pure and Zn:CuO thin films
Soda lime glass substrates are used to deposit pure and Zn:CuO thin films using the chemical spray pyrolysis method (HOLMARC).0.05 M of pristine solution is prepared by taking copper chloride dihydrate along with deionized water (75%) and ethanol (25%), for other weight percentages (1,3,5) of Zn doped CuO solutions, zinc acetate dihydrate is added.The solutions prepared are stirred using a magnetic stirrer for about 20 min.The substrate temperature is maintained at 400 °C and a 15 s spray time is set, the flow rate is maintained at 4000 μl min −1 and Pressure is kept at 0.3 mbar.Before deposition, the glass substrates are washed with acetone and Isopropyl alcohol and ultrasonicated for 20 min.

Characterization techniques
Rigaku Miniflex 600 5th gen equipment is used to determine the crystalline nature of the thin films with a scanning rate of 1°/min, Cu-k α1 radiation 1.54 A°and 2θ range between 20°and 80°, structural confirmation from Raman analysis using Renishaw Raman spectrometer.Optical absorbance spectra were performed in the range of 450-1000 nm using Shimadzu UV-1900I.Defect studies are done using Photoluminescence spectroscopy JASCO fp-8300 spectrofluorometer.SEM analysis is done using (EVO MA18) along with the Oxford instrument for EDAX.Surface morphology is investigated by AFM using Innova SPM Atomic Force Microscope.I-V characteristics of the sample are found using (2636B KEITHLEY).The nonlinear absorption of the film was calculated using the z-scan method with a Helium-Neon continuous laser at a wavelength of 633 nm.The input intensity was constantly maintained at 20 m, and the Rayleigh length was approximately 6.7 mm.
The structural parameters for the samples is found using the Scherrer method is given in table 1. Scherrer equation is used to find the Crystallite size D, D k cos , l b q = / strain ε is found using he equation, cos 4 e b q = / and using the equation, Here k -Scherrer constant, λ -wavelength of the incident radiation, β -Full width at half maximum θ -Bragg's angle.From the XRD spectra of the Zn:CuO films, it was observed that the introduction of dopants caused a shift in the preferred orientation from the (002) to the (111) plane [14].This change is likely attributed to the variation in thickness of the doped films, as indicated by Malik et al [15].The thickness of the films is in the range of 280-330 nm; this change in the thickness might be attributed to the addition of dopants, which leads to condensation and agglomeration in the film, resulting in an alteration of thickness compared to the pure sample [16].Additionally, this agglomeration of the films is evident in the SEM images.Since the purpose of this study was to focus on the change in properties as we increase the concentration of dopants, the amount of solution sprayed for each concentration was maintained at the same level throughout the deposition of the films.On doping, the crystallite size has a decreasing trend, which reduces from 25.05 nm to 15.67 nm; as zinc ions provide strain to the lattice, it hinders the growth of CuO crystal, resulting in a decrease in crystallite size [17].The increase in dislocation when doping with zinc might be due to the introduction of lattice imperfection, increased strain, and disorder in the films.Although the ionic radii of copper and zinc are similar and do not significantly alter the crystal structure, the addition of Zn to CuO increases lattice strain, leading to distortions and higher density of dislocations [18].This is observed in the lattice strain values provided in table 1.

Raman analysis
Structural confirmation of pure and doped samples is obtained from Raman spectroscopy.Cupric oxide belongs to the space group C 6 2h (C 2/c) due to it having monoclinic structure in nature.CuO has 3 raman active modes 1 A g and 2 B g modes [19].Figure 2 displays the raman spectra of the doped and clean samples.
A 532 nm laser at 10 mW power was used to excite the samples.The spectra were recorded with 5 s accumulation time.From the spectra, we can observe three peaks at 293.5 cm −1 (A g mode) 340.7 cm −1 (B 1g mode) and 627.6 cm −1 (B 2g mode).The Cu 2 O phase-corresponding peaks are absent, therefore confirming the monoclinic structure.Peaks related to any impurities and substrate are absent as seen in XRD spectra suggesting the sample purity.A multi-phonon band at 1109.6 cm −1 is formed, which relates to the inharmonic coupling between the phonons.The 2B g phonon band corresponds to the stretching vibration in the x 2 -y 2 plane which is produced due to variation in electron density [20].

Optical studies 3.2.1. UV-visible analysis
Uv visible spectroscopy is used to analyze the samples behavior in response to light absorption.Figure 3 displays the absorption spectra of the pristine and Zn doped samples.The spectra reveal that the samples have high absorbance in the visible region.As compared to doped samples, pure CuO has a lower absorbance.Absorbance increases with doping, reaching a maximum of 5% Zn:CuO.
The bandgap of these thin films is found using Tauc's equation given by, h A h Eg n a n n -= ( ) ( ) [21] where, α-absorption coefficient, hν-the photo energy, A-band edge constant, E g -energy band gap, n-transition dependent factor.The Tauc's plot is shown in figure 3.
Table 2 displays the bandgaps of pure and Zn: CuO thin films.From Tauc's plot, we can infer that the bandgap increases on doping with zinc till 3%.The bandgap for a pure sample is 1.85 eV, and it rises to 2 eV for 3% Zn:CuO.This increase in bandgap is due to the decrease in crystallite size as seen in the XRD results.Singh et al has reported equations that give relation between crystallite size and bandgap energy [22].Since in the nanoscale domain, the size of particles largely influences the properties such as bandgap, this increase in bandgap can be deduced as a quantum size effect where doping influences reduction in crystallite size, thereby increasing the bandgap energy [23].For 5% Zn:CuO there is a decreasing trend as the bandgap reduces to 1.75 eV, and this can be attributed to the fact that there is an overlap between the acceptor level and the valence band as doping is increased which lowers the bandgap [24].

Photoluminescence studies
Studies for photoluminescence are carried out on both pure and Zn:doped samples.The excitation wavelength used is 350 nm.The photoluminescence spectra of the films are shown in figure 4. From the spectra, five peaks are observed in violet, blue, green, and red regions.Similar peaks in these regions have been reported by Elseman et al [25], Parui et al [26], Ansari et al [27].The radiative transition between the oxygen interstitial states and subband of the conduction band produces a peak at 3.08 eV (402 nm) in the violet region.The violet peak observed at 2.87 eV (432 nm) could be attributed to the transition from conduction band sub-bands to valence band Cu-d shells.The blue peak at 2.65 eV (467 nm) corresponds to the electronic transition from oxygen interstitial acceptor level to copper vacancy donor level.The peak at 2.25 eV (551 nm) in the green region corresponds to electronic transitions between oxygen vacancies [28].The peak at 2 eV (620 nm) corresponds to the near band edge emission [29].This is close to the bandgap values reported in uv results.As we dope cupric oxide, PL quenching can be observed in the spectra.This drop in intensity is due to non-radiative charge carrier relaxation at defect states.Moreover, due to the rise in the number of defects in doping, which can be seen from XRD results, the PL intensity decreases further as doping is performed [30].

Morphological studies 3.3.1. Scanning electron microscopy (SEM)
2-D images at 50kX magnification of the films are taken using SEM to get morphological information.Figure 5 displays pictures of pristine and Zn:CuO films.The pure sample has visible grains uniformly distributed with no definite shape.For 1% Zn:CuO the sample has become denser and agglomeration has taken place.At 3% and 5% doping there is no significant change but it is denser compared to 1% Zn:CuO.The scale of SEM images obtained is 200 nm, and grain size is calculated using ZEISS SmartSEM software.The grain size for a pure sample is around 300 nm, and it reduces as we dope.For 5% Zn:CuO grain size around 180 nm, this decrease in grain size is similar to the trend in crystallite size observed in XRD results.

Atomic force microscopy (AFM)
AFM in tapping mode is used to obtain 3-D images of the material with a surface area of 5 × 5 microns.Sample images are shown in figure 6.The images show that grains are uniformly distributed, similar to SEM images.The images are analyzed using nano scope analysis to get the roughness values.The roughness of the samples varies with doping and the roughness values of the sample are given in table 3.For 1% Zn:CuO the roughness of the films decreases.But for 3% and 5% doping, there is an increase in the roughness.

Compositional analysis 3.4.1. Energy dispersive xrays (EDX)
EDX is used to study the samples chemical composition.The presence of only Cu, O, and Zn is reported indicating absence of impurities.The EDX spectra depicted in figure 7 and the table containing atomic percentages are given in table 4. From the table, we can conclude that the doping has been efficiently performed.

Electrical analysis 3.5.1. I-V Characteristics
Current-Voltage characteristics of samples are conducted to study their electrical properties.The currentvoltage plots were taken in the bias range of −3 to +3 V.The characteristic of each film is plotted in the graph shown in figure 8.The plots are linear, passing through the origin which confirms the ohmic contact of the samples.5% Zn:CuO has the highest current value and hence can be explored for photodiode applications.

Hall effect measurements
Hall effect measurement with van der Paw method were performed to get the electrical parameters.The conductivity type of the samples was found to be p-type as reported [31].Table 5 contains the list of the parameters.The resistivity of the materials is found to be reduced with an increase in Zn concentration and is lowest for 5% Zn:CuO.This may be attributed to stoichiometric changes due to the addition of Zn dopants causing defects.Since carriers use thermal energy to conduct and there is less carrier scattering effect, the sharp decrease in resistivity can be associated with a rise in mobility [32].5% Zn:CuO has the highest carrier concentration and hence has the highest current value as reported in I-V results.

Non-linear optical studies
Studying the optical changes of a material when exposed to intense (laser) light is called non-linear optical studies.In nonlinear materials, the dielectric polarization (P) of the light reacts nonlinearly to the applied electric field.The z-scan apparatus is used to study the 3rd order nonlinearity of a material which uses the theory of spatial beam splitting.Z-scan technique in open aperture mode gives the nonlinear absorption coefficient β eff .

Z-scan technique
Nonlinear absorption measurements are done by z-scan method using a CW Helium-Neon laser of wave length 633 nm with 21 mW input power and the Rayleigh length (Z R ) is 6.7 mm.Under open aperture mode he normalized equation of transmission is, Where the free factor q 0 (Z) can be obtained by, where Z is the sample position, Io is the laser beam intensity at the focus, L eff is the thin film length under scan, and Z R is the Rayleigh length [33].Figure 9 displays the open aperture traces of samples.Reverse saturable absorption (RSA) is shown by an open aperture curve that has a valley at the focus, whereas saturated absorption (SA) is indicated by a curve that has a peak at the focus.While SA characterizes an increase in transmittance with an increase in incident energy, RSA characterizes a reduction in transmittance.RSA mechanisms are usually due to the phenomenon, namely: Nonlinear scattering processes, free carrier absorption (FCA), and two-photon absorption (TPA), or a combination of all these processes.The normalized curve for pure, 3%, and 5% Zn:CuO refers to the character of nonlinear absorption, which is relates to reverse saturable absorption (RSA).1% Zn:CuO shows negative value of absorption nonlinearity which relates to saturable absorption (SA) [21].The transition of the nonlinear absorption signature from RSA to SA was due to variations in the absorption cross-section of the ground state as well as excited state with the doping concentration [1].For 3% and 5% Zn:CuO, which switches the absorption signature back to positive showing the RSA mechanism [34].The nonlinear absorption coefficients of the sample are given in table 6.As zinc concentration grows, it is evident that cupric oxide's nonlinear absorption increases, indicating potential uses in optoelectronic devices.

Conclusions
In this work, we have synthesized pristine and (1,3,5) % Zn:CuO thin films by chemical spray pyrolysis technique.XRD spectra showed the monoclinic phase which is confirmed by raman spectra.Linear optical studies report an increase in visible light absorption on doping.SEM and AFM images show uniformly distributed grains.Electrical studies show a rise in the conductivity of the samples on doping which is highest for 5% Zn:CuO.An increase in nonlinear absorption has been observed on increasing doping concentration by using the z-scan technique in open aperture mode.By controlling the deposition parameters, the bandgap of the films can be tuned for their applications in LEDs and absorber layers in photovoltaic cells.Since the nonlinear absorption is increasing on Zn doping it can be explored for optical limiting applications.The turnover property from RSA to SA from pure CuO to 1% Zn:CuO can be applicable in optical switches, logic gates and controlling laser pulse width.

Figure 1 .
Figure 1.XRD spectra of pure and Zn doped thin films.

Figure 2 .
Figure 2. Raman spectra for CuO and different concentrations of Zn:CuO thin films.

Figure 3 .
Figure 3. Absorbance spectra and Taucs's plot of pristine and varying Zn:CuO concentrated thin films.

Figure 8 .
Figure 8. Current-voltage characteristics of both Pure and Zn:CuO samples.

Figure 9 .
Figure 9. Open aperture z-scan traces of pure and doped Zn:CuO thin films.

Table 1 .
Structural parameters of deposited thin films.

Table 2 .
Bandgap values of both pure and varying concentration of Zn:CuO thin films.

Table 3 .
Thickness and Roughness values of pure and Zn:CuO thin films.

Table 4 .
Atomic percentages of thin film samples.

Table 5 .
Electrical parameters of films.

Table 6 .
Nonlinear absorption coefficients of pure and Zinc doped CuO.