Investigation of optical, structural, and electrical properties of heterostructure Fe 2 O 3 deposited by RF magnetron sputtering on ZnO layer by spray pyrolysis

In this study, ﬁrstly, ZnO thin ﬁlm was produced on glass substrate by chemical spray pyrolysis method at 450 (cid:3) C and then Fe 2 O 3 layer was produced on ZnO substrate thin ﬁlm by RF magnetron sputtering technique. Fe 2 O 3 /ZnO heterostructure was fabricated. The crystal structure, surface morphology and structure, chemical composition, optical and electronic properties, and electric properties of the ZnO and Fe 2 O 3 /ZnO samples were performed by X-ray diffraction (XRD), Raman spectrometer, ﬁeld emission scanning electron microscope (SEM), focused ion beam (FIB) microscope, atomic force microscope (AFM), energy-dispersive X-ray (EDX), ultraviolet–visible spectrophotometer (UV–VIS), and Hall measurements, respectively. XRD and Raman measurements showed that ZnO and Fe 2 O 3 thin ﬁlms have wurtzite hexagonal and cubic crystal structures, respectively, and both are in polycrystalline form. FE-SEM and cross-sectional FIB-SEM images indicate that Fe 2 O 3 ﬂake-like nano-sized structures


Introduction
Recently, multi-functional magnetic nanocrystalline films have encountered many potential applications due to their many interesting physical properties [1]. Nano-structured magnetic thin films attract the attention of researchers due to this wide application area. One of the most studied thin films are the magnetic thin films with multilayers with nanometer spacing, and they are the first metallic quantum structures that can be used in electronic devices, such as reading heads in hard discs [2]. Another type of thin-film sample developed by a group in literature is called magnetic/non-magnetic multilayers; they are known as 'spin valves' and are mostly used in magnetic storage devices [2].
Maghemite nanoparticles taking place among the magnetic materials are widely used in biomedical applications because their magnetism interacts easily with external fields, and they are also biocompatible and potentially non-toxic to humans [3,4]. In literature, there are other studies realized by two different groups. They have developed devices that can be used in the field of spintronics. Their studies have suggested that c-Fe 2 O 3 can be used as a magnetic tunneling barrier for room-temperature spin-filter devices [5,6].
On the other hand, researchers have studied the nanocomposites consisting of two or more metal oxide semiconductors, and they have developed thinfilm samples which can be used in photocatalysts [7], lithium batteries [8], solar cells [9], gas sensors [10], and spintronic applications [11]. Wang et al. have fabricated CdS/Fe3O4 and CdS/a-Fe2O3 heterostructures and investigated their magnetic, optical, and photocatalytic properties [12]. Hong et al. first fabricated Fe 3 O 4 /ZnO composite thin film and demonstrated that Fe3O4/ZnO photocatalytic activity is higher than ZnO [13]. Suryavanshi et al. have grown the ZnO semiconductor (Eg. * 3.37 eV) with a wide band gap and Fe 2 O 3 structure as a heterojunction on fluorine-doped tin oxide (FTO)-coated glass substrates. Thus the high efficiency of luminescence in the UV to have fabricated Fe 2 O 3 /ZnO structure on (FTO)-coated glass substrates [14]. They have investigated the ZnO layer effect on the film's crystal structure, morphological, optical, and photocatalytic properties [14]. In another study, Suryavanshi et [15]. Fe 2 O 3 (iron oxide phases) thin-film phases can involve devices with captivating magnetic, optical structure, and semiconducting properties. After all, deposition of quality and satisfying iron oxide phase thin films is tricky and requires vacuum processes. [16].
This study has formed ZnO thin film onto glass substrates by chemical pyrolysis and then Fe 2 O 3 nanocrystalline thin films have been grown on them by RF sputtering technique. The characteristics of the Fe 2 O 3 /ZnO structures were carried out by XRD, Raman Spectroscopy, and FE-SEM with EDX, AFM, and UV-Vis spectrophotometer techniques.
The present work aims to study the optical, structural, and electrical properties of Fe 2 O 3 /ZnO heterostructure. Firstly, it has been revealed that the hardly growing Fe 2 O 3 phase in iron oxides grows on another film at the nanoscale.

Experimental
In this study, the fabrication of the Fe 2 O 3 /ZnO structure is performed in two steps as follows: The first step involved formation of ZnO thin film onto a glass substrate by spray pyrolysis method and then to obtain Fe 2 O 3 /ZnO structure, Fe 2 O 3 thin film was grown on ZnO thin film by RF magnetron sputtering technique.

Growing of ZnO thin film on the glass substrate
In the first step, to grow the ZnO thin-film layer on the glass substrate, the methenamine (CH 2 ) 6  Then, two solutions were obtained stirring these two mixtures with magnetic mixers for 40 min. And then, these two solutions were poured into another glass beaker, stirred with the magnetic mixer, and obtained the final solution of 100 ml, which will be used to fabricate ZnO thin-film samples. This solution was loaded into the syringe of the chemical spraying apparatus. After these processes, ZnO thinfilm sample preparation proceeded. Initially, ten pieces 1 9 1 cm 2 glass substrates were chemically cleaned using a known method, Piranha [17], on the heater plate was used in chemical pyrolysis experimental set and heated the heater plate until 450°C.
Finally, the pump was adjusted to pump the solution through the nozzle with a solution flow rate of 2.5 ml/min and then it started the spraying process of the solution on the substrates and obtained ten numbers of ZnO thin-film samples. Then it divided these ten thin-film samples into two groups, each group including five thin-film samples. They used one of these group samples to characterize the ZnO thin-film samples and the other group to fabricate the Fe 2 O 3 layer on ZnO.

Formation of Fe 2 O 3 /ZnO heterostructure
After completion of the ZnO thin-film samples, the second step started. Fe 2 O 3 thin films were placed with a 2-inch radius prepared by us and ZnO thinfilm samples on the RF magnetron sputtering system's anode and cathode plates, respectively. It then adjusted the pressure inside the system and controlled the flow rates of argon and oxygen gases (O 2 /Ar = 1/14) and total combustion chamber of the system 1.4 9 10 -6 Torr, 25 mTorr. Oxygen partial pressure during the growth of the Fe 2 O 3 layer on ZnO thin-film substrates was accepted as the fundamental parameter for determining optical and structural properties of the samples. After adjusting the power supply, temperature of the substrates, and taking the distance of the target (source material) and substrates as 100 W, 290°C, and 4.5 cm, the system was started and fabricated the Fe 2 O 3 /ZnO thin-film samples.
Finally, completing the heterostructure production, the characterization of heterostructure samples was performed.

Characterization measurements
Crystal structure-crystal energy bond structure, surface morphology, qualitative analysis, and optical properties of Fe 2 O 3 heterostructure samples were examined using X-ray diffractometer (XRD Bruker D 2 , Ka, k = 1.54 A, Scanning angle 70°], Micro-Raman Spectroscopy (WITec alpha 300 Series Raman High-Resolution Optical and Scanning Probe Microscopy Systems measurements), scanning electron (FE-SEM) (Sigma 300 Model Zeiss Gemini), Electron diffusion X-ray (EDX associated with FE-SEM), atomic force microscope (AFM) (AFM 5000 II Model), FIB-SEM (Zeiss Gemini 2 Cross Beam 540), and UV-VIS spectrometer (PerkinElmer Lambda 2S UV-Visible spectrometer) techniques, respectively. Also, the electronic properties of Fe 2 O 3 heterostructure samples were examined by the Hall measurement method.  According to these results, the ZnO-pure thin-film sample has a hexagonal phase [18] and a polycrystalline structure. This result was also identified and matched with the PDF No: 01-075-1526 pattern corresponding to ZnO hexagonal crystal phase.

XRD and micro-Raman measurements
On the other side, the first broad peak in XRD patterns of Fe 2 O 3 /ZnO thin films appears at 2 h angle 30.26 degree corresponding to (220) directional plane. This peak corresponds to Fe 2 O 3 nano-crystallization; this result was identified and matched well with the PDF No: 01-083-0112 pattern corresponding to a cubic Fe 2 O 3 (Maghemite) structure.
In Fig. 1 (red line), only one of the peaks belonging to the Fe 2 O 3 /ZnO structure is seen to belong to the Fe 2 O 3 structure and the rest of the peaks correspond to the ZnO-pure thin film. This peak belonging to the Fe 2 O 3 structure is quite broad and has a low-intensity value indicating that its crystal structure is nanostructured [19]. Micro-Raman results also support these results.
On the other side, it calculated the crystal sizes of the thin-film samples corresponding to the angles that the peaks appear using the Debye-Scherrer formula, where h, b, and k are Bragg's angle, the full width at half maximum (FWHM) of XRD peak, and the wavelength of the incident X-ray (k = 1.5405 Å ), respectively. K is a shape factor. Since the exact value of the Scherrer constant is not known for the present material system, K = 0.9 was used, and all D-calculations are estimates. Crystal structure parameters of the Fe 2 O 3 /ZnO heterostructure thin-film sample are given in Table 1. The calculated crystal size values of these two thinfilm samples confirm that they also have a polycrystalline structure with different sizes.
Raman spectrum is one of the most applied methods to characterize the materials' bond structure.
Electron-phonon interaction has a significant effect on the electronic and optical structure of the materials. Thus, Raman spectra give information about materials' energy relaxation rate of excited carriers and phonon reproduction of excitons in the luminescence [20]. Therefore, the Raman scattering technique was applied on the Fe 2 O 3 /ZnO thin-film sample since this technique is more sensitive to crystallization, structural disorder, and defects in nanocrystalline thin films than other techniques. In Fig. 2, Raman spectra of the heterostructure Fe 2 O 3 / ZnO and ZnO thin-film samples obtained in the range of 0-1500 cm -1 and at the room temperature are given with the graphics red-black, respectively.
As seen from Fig. 2 (black line), the three typical Raman modes of ZnO thin film are observed in the spectra at 438 cm -1 , 574 cm -1 , and 583 cm -1 . These bands observed at 438 cm -1 are attributed to the E 2high mode of wurtzite ZnO structure, and the other bands 574 cm -1 and 583 cm -1 are attributed to A 1 (LO) and E 1 (LO) modes, respectively [21][22][23][24]. The appearance of the 'E 2 -high mode' peak with the highest intensity indicates that the wurtzite structure of the ZnO thin film is predominantly observed in this structure. In addition, these results show that ZnO thin film has quality crystallization.
On the other hand, as seen in Fig. 2 (red line), it is observed that the Raman mode of ZnO thin film at 438 cm -1 has not existed in Fe 2 O 3 /ZnO structure.
Here, it shows that the Fe 2 O 3 nanosheets film layer covers the ZnO substrate film, and the ZnO thin film prevents the 'E1-high mode' vibration frequency. However, two weak Raman modes are shown in red spectra at 574 cm -1 -A 1 (LO) and 583 cm -1 -E 1 (LO) [21]. It is seen that the intensity of these peaks corresponding to ZnO thin film decreases. Here, the decrease in the Raman peak intensity of the ZnO film of the substrate may indicate that this molecule concentration decreases with the effect of the other layer on it. The a-Fe 2 O 3 structure is the most crystallized earth among many iron oxide phases. In the literature, seven phonon vibration lines, A g1 (1), E g1 , E g2, E g3 , E g4 , E g5 , and A g1 (2), are detected in the Raman scattering spectra for a-Fe 2 O 3 [20,24]. As shown in Fig. 2 (red line), the typical Raman peaks of a-Fe 2 O 3 for a-Fe 2 O 3 /ZnO are observed at 225 cm -1 -A g1 (1) and 288 cm -1 -E g1 . However, small shifts were observed due to differences in size and shape of the particles.
In the literature, there are many Raman spectra studies of the a-Fe 2 O 3 structure [21,23,[25][26][27][28]. According to previous research, as the grain size decreases in the a-Fe 2 O 3 structure, the Raman peaks become broader, and the intensity decreases [23,28].
In this study, 225 cm -1 -A g1 (1) and 288 cm -1 -E g1 , two broad and small peaks, indicate that the a-Fe 2 O 3 structure is nano-sized. These results confirm the XRD analysis.

FE-SEM (with EDX), FIB-SEM, and AFM measurements
The prepared samples' thin-film surface morphologies and structures were obtained from the FE-SEM, FIB-SEM, and AFM techniques.  Figure 3a shows that the ZnO FE-SEM micrograph exhibits three-dimensional (3D) flowerlike structures with nanometer scale, and it also has a surface morphology with the nanocrystals formed by spherical-shaped particles coming together. As mentioned above, the aggregation of these particles is due to the thermal heat reaction during the thin-film growth. At the same time, the dispersal of the crystallites is well ordered and uniform structure. This result is confirmed in the literature [29,30]. Figure 3b displays that the distribution of flake-like Fe 2 O 3 structure forms over the subsurface area of ZnO thin film with granular morphology, and there are no pinholes on the surface area of the thin film. These nanosheets, taking place in the ZnO, are seen as the sheets in which 3D flower-like structures are distributed. Figure 3c  Nanosheets originate from the Fe 2 O 3 nanoscale thin film grown on ZnO thin film by RF magnetron sputtering. Therefore, these Fe 2 O 3 structures should appear on the ZnO thin film as nanosheets with nanoscale.
As shown in Fig. 3d, e, and f, it has obtained the substrate surface morphology, thickness, and average roughness values of ZnO and Fe 2 O 3 /ZnO thin films by FIB-SEM surface image and FIB-SEM cross-sectional area images, respectively. Figure 3d shows the FIB-SEM surface image of ZnO substrate thin film. ZnO thin-film FIB-SEM image is the scale at 400 nm scale, and it is shown that the regular distribution of three-dimensional (3D) flower-like spherical-shaped particles have an average diameter of 180 nm. Figure 3a and Fig. 3b confirm each other. Figure 3e and Fig. 3f show FIB-SEM cross-sectional images of Fe 2 O 3 /ZnO structure with different scales. Figure 3e shows that the film thicknesses obtained for both layers are found to be 236.7 nm and 661 nm for the Fe 2 O 3 nanolayer and ZnO substrate thin film, respectively. The average size of each Fe 2 O 3 nanosheet is found to be 220 nm. It is clear from the cross-sectional area FIB-SEM images that the distribution of each nanosheet on the surface of the ZnO thin-film substrate is relatively regular. However, overlapping deposits of nanosheets are also observed in places.
The average roughness value in Fig. 3f is measured at 273.6 nm for the substrate ZnO thin film and FE-SEM is associated with an EDX analysis system to make compositional analysis on material localization surface area. The typical EDX spectrum seen in Fig. 4 shows the chemical compositions of Fe 2 O 3 / ZnO heterostructure. As seen from Fig. 4 However, the nominal rate of Au and Si elements is existent in the composition of this thin-film sample. It is considered that the Si element comes from the glass substrate, and the presence of Au element takes its source from Au used to overlay the surface of the Fe 2 O 3 /ZnO sample before the FE-SEM measurement. Figure 5a and b displays the 3D and 2D RMS (459 nm) AFM micrograph images of the Fe 2 O 3 /ZnO heterostructure, respectively. The surface roughness plays a vital role in charge transportation in thin films and determines any inter surface layer thin-film device application features. As seen from Fig. 5a, the image of AFM topographic shows that ZnO crystallites forming in the first layer have a smooth distribution, and the flake-like Fe 2 O 3 crystallites are distributed at a lower density on ZnO thin film. This result is also confirmed by the XRD, FE-SEM, and FIB-SEM measurements.
Additionally, the mean roughness of the Fe 2 O 3 / ZnO sample was found at 459 nm. As a result, it can be said that the fabricated Fe 2 O 3 /ZnO heterostructure thin-film sample has a highly smooth and almost uniform surface structure. The roughness values of the structure obtained by two different methods also confirm each other.

Optical properties
Optical absorption spectra of ZnO-pure and Fe 2 O 3 / ZnO heterostructure thin-film samples varying in the range between 300 and 900 nm are given in Fig. 6. As seen from Fig. 6, the absorption of light in the UV range is high for these two samples. The optical band gap energy values of thin-film samples can be calculated using the absorption data. For this purpose, firstly, the absorption coefficient of the sample (a) corresponding to the wavelength of the light is obtained using the expression: where I, I 0, and d are the transmitted radiation intensity, incident radiation intensity, and thickness of the thin film. The optical band gap energy (E g ) value of the sample is found by using the following equation: where E g , ht, and h are band gap energy of the sample, energy of the photon, and Planck's constant, respectively.
A is a constant for the effective masses of charge carriers, and n is the power based on the optical transition character (n = 1/2 and 2 for direct and indirect transition, respectively) [30]. Substituting 1/2 for n in Eq. 2, since our sample has a direct transition property and then taking the square of both sides of this equation following equation is obtained: The plotted graphs of (aht) 2 versus ht for ZnOpure and Fe 2 O 3 /ZnO heterostructure thin-film samples are given in Fig. 7. After this, the extrapolation method is applied to determine the band gap energy values of ZnO-pure thin film and Fe 2 O 3 /ZnO heterostructure. According to this method, the point where the line drawn from the part of the curve with constant slope intersects the ht-axis corresponding to the band gap energy of the sample. As seen from Fig. 7, the intercepts corresponding to the band gap energy of ZnO-pure thin film and Fe 2 O 3 /ZnO structure are 3.277 eV and 3.24 eV, respectively. Due to doping and ensuing defect, the band structure and absorption edge change [31]. Significantly, the drop in the band gap with the Fe 2 O 3 deposition of another

Hall measurements
The numerical values of the electrical and magnetic characteristic parameters of the Fe 2 O 3 /ZnO thin-film sample were obtained with the help of the full automation cryogenic hall measuring system whose schematic diagram presenting its operation principle is given in Fig. 8. As seen from Fig. 8, the thin-film sample was placed in a magnetic field produced by the experimental set-up, and the Hall voltage, V H , and the current passing through the sample in the x-direction, I x , is measured automatically by the system. It first entered the numerical values of the magnetic field, B, width, w, length, L, and thickness of the sample as 2.5 T, 0.5 cm, 1.5 cm, and 897.7 nm, respectively, and then was operated in the experimental set-up. In order to obtain the numerical values of the Hall coefficients (R H ), carrier density (n), resistivity (q), conductivity (r), Hall mobility (l H ), and magnetoresistance coefficient (MRc) of the Fe 2 O 3 /ZnO sample, a computer program was used. This computer program calculated the numerical value of each of the parameters mentioned above by substituting the numerical values of B, V H , and I x (given in Table 2) in the following equations.
Substituting the measured values of V H (0.455 V) and I x (1.2 9 10 -4 A) and given values of B (2.5 T) and thickness of the thin film, t (897.7 nm) in the equation, The calculated value of R H is 9.1 9 10 -4 m 3 /As. Electron density, n, is obtained, substituting À 1.6 9 10 -19 C (charge of the electron) and the calculated value of R H as above, in the following equation, Moreover, the value for n is 6.88 9 10 21 m -3 . The conductivity of the sample, r, is calculated using the following equation, Furthermore, the calculated value of the conductivity of the sample is found at 4.39 9 10 2 X -1 m -1 .
Substituting the values of R H and r in the following equation, the mobility value of the sample finds

Conclusion
Firstly, this study has realized that Fe 2 O 3 thin film could be formed by magnetron sputtering technique on ZnO thin film produced by spray pyrolysis method successfully. XRD, Raman Analysis, FE-SEM, EDX, FIB-SEM, and AFM studies show ZnO and Fe 2 O 3 structures in the prepared Fe 2 O 3 /ZnO structure. Especially, FE-SEM analysis revealed that Fe 2 O 3 nanosheets are constructed in a 3D flake-like ZnO structure. These Fe 2 O 3 nanosheets are observed in AFM images, and FE-SEM and FIB-SEM analyses results confirm this result.
The electrical properties of the Fe 2 O 3 /ZnO structure have been investigated using galvano-magnetic measurements. Therefore, using the four-point method in the Hall experiment, Hall coefficient, carrier density (n), resistivity (q), conductivity (r), hall mobility (l), and also magnetoresistance coefficient (MRc) have been determined for Fe 2 O 3 /ZnO structure. Due to the positive R H (9.1 9 10 -4 m 3 /As) and MRc (7.99 9 10 -1 ) values, Fe 2 O 3 /ZnO structure exhibits p-type electrical properties. Finally, these obtained results have shown that this fabricated thinfilm sample can be an excellent promising candidate for spintronic applications.
Finally, these results have shown that this fabricated thin-film sample can be an excellent promising candidate for spintronic and photo-electrochemical applications.