Preparation and Characterization of Electrochemically Deposited Cu2O/ZnO Heterojunctions on Porous Silicon

Cu2O/ZnO heterojunction was fabricated on porous silicon (PSi) by a two-step electrochemical deposition technique with changing current densities and deposition times, and then the PSi/Cu2O/ZnO nanostructure was systematically investigated. SEM investigation revealed that the morphologies of the ZnO nanostructures were significantly affected by the applied current density but not those of Cu2O nanostructures. It was observed that with the increase of current density from 0.1 to 0.9 mA/cm2, ZnO nanoparticles showed more intense deposition on the surface. In addition, when the deposition time increased from 10 to 80 min, at a constant current density, an intense ZnO accumulation occured on Cu2O structures. XRD analysis showed that both the polycrystallinity and the preferential orientation of ZnO nanostructures change with the deposition time. XRD analysis also revealed that Cu2O nanostructures are mostly in the polycrystalline structure. Several strong Cu2O peaks were observed for less deposition times, but those peaks diminish with increasing deposition time due to ZnO contents. According to XPS analysis, extending the deposition time from 10 to 80 min, the intensity of the Zn peaks increases, whereas the intensity of the Cu peaks decreases, which is verified by the XRD and SEM investigations. It was found from the I–V analysis that the PSi/Cu2O/ZnO samples exhibited rectifying junction and acted as a characteristical p-n heterojunction. Among the chosen experimental parameters, PSi/Cu2O/ZnO samples obtained at 0.5 mA current density and 80 min deposition times have the best junction quality and defect density.


INTRODUCTION
It has become increasingly common in optoelectronic device technology to combine the properties of different metal oxides (MOs) and/or alloys to produce and develop new materials with interesting and desirable properties. 1−3 Among MOs, cuprous oxide-zinc oxide (Cu 2 O/ZnO) binary compound stands out with its remarkable electrical, optical, and thermal properties. 1,4 In addition, they have several advantages such as low cost, material abundance, chemically stable, and low lattice mismatch (only 7.1%). 5 Cu 2 O is a p-type metal-oxide semiconductor (MOS) with a band gap of 2 eV, a high absorption coefficient (10 2 −10 6 cm −1 ), and a large exciton binding energy of 140 meV. 6 ZnO is also a well-known n-type MOS, with a band gap of 3.3 eV, high electron mobility (120 cm 2 /V s), and binding energy of exciton (60 meV). 1 Furthermore, the unique electrical and optical properties of these materials are important for solar cells and photodetector applications. 7,8 Cu 2 O/ZnO can be fabricated using different deposition techniques such as magnetron sputtering, 9 pulsed laser deposition, 10 electrochemical deposition, 11 and so forth. Compared to other production methods, the electrochemical deposition method is a simple, fast, low-cost process. Moreover, it provides the ability to tailor the size, shape, and morphology of the nanostructures deposited under a set of well-controlled synthesis parameters. Controlling the size and shape of the nanostructures is technologically crucial due to the close correlation between these parameters and optical, electrical, and catalytic features. 12,13 The most important problem in Cu 2 O/ ZnO-based device applications is interface defects. 4,11 In addition, obtaining high junction quality and low defect density is also a major challenge, so the experimental parameters must be well chosen. Fabricating good quality p-type ZnO semiconductor material is challenging due to oxygen vacancy and zinc interstitials in pure ZnO. 14 In addition, the limited chargetransport properties of the ZnO/Cu 2 O heterojunction could be affected by the interface states, imposing a significant limit on the maximum thickness of the junction, resulting in poor power conversion efficiency, and thus, the formation of an effective p− n junction is the most important key factor. 4,15 Up to now, although the growth and analysis of Cu 2 O/ZnO structures upon conventional substrates such as glass substrates, metallic substrates, and ITO-and FTO-substrates have been extensively studied, 16,17 the physical, electrical, and optical features of the Cu 2 O/ZnO grown on porous silicon by electrodeposition have not been reported in the literature yet. However porous silicon (PSi) has been extensively studied for its unique photoluminescence properties. PSi's importance has been increasing in the recent years because of its fascinating electrical properties. 13,18 However, due to the instability issue of porous silicon, developing stable porous silicon-based devices is a challenging process. For this reason, surface passivation with low-resistance, stable electrical contacts is required to increase the structural, optical, and electrical properties of the PSi structure and make it more stable as well as to develop porous silicon-based devices and their integration into electronic circuits. 13,19 Accordingly, the purpose of our work is to study the properties of the PSi/Cu 2 O/ZnO structure by varying the electrochemical deposition parameters (time and current density) by providing better control over the electronic structure, crystallinity, and morphology of PSi/Cu 2 O/ZnO.

MATERIALS AND METHODS
PSi layer was derived via anodization technique by using n-type silicon wafer with orientation of (100) and resistivity of 1−10 Ω cm as defined in recent works. 13,20,21 Sigma Aldrich supplied all of the chemicals utilized in this study. Two-step deposition was performed to obtain PSi/Cu 2 O/ZnO nanostructures. First, PSi substrate (1 cm 2 surface area) as an anode and a Pt wire as a cathode were used, respectively, in a solution containing 0.4 M CuSO 4 , 0.5 M boric acid (H 3 BO 3 ), and 3 M lactic acid for the electrochemical deposition of Cu 2 O nanostructures. The pH of the solution was adjusted to 12 by adding 5 M KOH while maintaining a temperature of 65°C. Cu 2 O structures were obtained for different deposition times (varied from 10 to 80 min) at current densities of 0.1, 0.5, and 0.9 mA/cm 2 . Then, the samples were cleaned with distilled water. In the second step of electrochemical deposition, ZnO deposition was performed under the same current densities and deposition times in an electrolyte composed of an aqueous solution of 0.08 M Zn(NO 3 ) 2 ·6H 2 O and 0.05 M C 6 H 12 N 4 . The electrolyte temperature and pH were 85°C and 5.1, respectively. Finally, all samples were cleaned by distilled water.
Surface properties of PSi/Cu 2 O/ZnO were analyzed via field emission scanning electron microscopy (FE-SEM, Quanta-FEG). Patterns of X-ray diffraction (XRD) of PSi/Cu 2 O/ZnO were obtained by a Panalytical Empyrean diffraction system employing CuKα radiation (λ = 0.15418 nm). Composition analysis was carried out by X-ray photoelectron spectrometry (XPS; Thermo Scientific K-Alpha) in the depth direction of the Cu 2 O/ZnO deposited on porous silicon. The I−V analysis was carried out via a Keysight B2901A source meter (SMU) between −5 and +5 V in a dark environment at ambient temperature (300 K).

RESULTS AND DISCUSSION
Morphological and compositional properties of samples were analyzed by FE-SEM and EDX analyses. Figures Figure 1. As seen in Figure 1, for 10 min, Cu 2 O nanoparticles with the average diameter of ∼750 nm is obtained. At low current density and deposition times, especially in samples of 10−20 min, there are places where the substrate is exposed without a coating which is seen from the SEM analysis. This result shows that low current   Figure 3 when the current density was increased from 0.5 to 0.9 mA/cm 2 . For 10 min, the ZnO nanostructures are tiny and inhomogeneous around the octahedral and pyramidal shaped Cu 2 O structures (mean diameter 6.5 μm), though for 20 min deposition time, bigger ZnO nanoparticle clusters are detected on the surface (Figure 3b). SEM analysis indicate that with the increase in deposition time from 20 to 80 min, an intense ZnO deposition occured on Cu 2 O structures as observed at 0.1 and 0.5 mA/cm 2 . ZnO nanorod structures were observed in 80 min samples at all current densities (0.1, 0.5, and 0.9 mA/cm 2 ). The SEM image obtained by zooming is also given in Figure 4. We can summarize the microstructure as follows: When the current density and deposition time increase, the crystals look different just because of their different facet shapes, which varies with growth orientation. It can be seen from SEM analysis (Figures 123) that there are pyramid-like Cu 2 O structures at low current density and low deposition times. As the current density and deposition time increase, deposition of ZnO occurs on these structures (especially short rod structures are formed depending on the growth direction).
From SEM analysis, it was understood that there was no significant effect of current density on the shapes of Cu 2 O nanostructures at constant deposition time, but the dimensions and shapes of ZnO nanostructures were significantly affected. It was observed that with the increase of current density from 0.1 to 0.9 mA/cm 2 , ZnO nanoparticles thickened and showed more intense deposition on the surface. The cross-sectional SEM (X-SEM) images of PSi/Cu 2 O/ZnO nanostructures prepared under different conditions are presented in Figure 5. The X-SEM images show that the film thickness formed on the surface increases with increasing deposition time. It has also been discovered that certain channels are continually loaded from the bottom of the pore to the PSi surface, while others are unfilled or partially loaded. The second occurrence is attributed to the bottleneck effect, 22,23 which occurs when the pore mouth closes until it has been entirely loaded with the Cu 2 O nanoparticles. This could be caused by the closing of the pore mouth owing to size of the particle, which increases with the current density. It is worth noting that throughout the cleavage process, some cluster of nanoparticles can fall from the surface. X-SEM analysis further shows that the film thickness of the Cu 2 O/ZnO layer increases with increasing duration from 10 to 80 min. As seen in Figure 4a, with increasing deposition time, the film thicknesses of Cu 2 O/ ZnO layer increase from 2 to 10 μm for 0.1 mA/cm 2 , from 3 to 18 μm for 0.5 mA/cm 2 , and from 5 to 15 μm for 0.9 mA/cm 2 .
The PSi/Cu 2 O/ZnO samples were also investigated via XRD to determine the purity of samples. The impact of deposition duration and current density on the XRD pattern of Psi/Cu 2 O/ ZnO nanostructures is shown in Figure 6. In the case of 10 min deposition time, the diffractogram is dominated by Cu 2 O (Figure 6a). As seen in Figure 6   (200) planes of metallic Cu. 13,27 It should be noted that the amount of OH − in the solution may affect the metallic copper formation in the structure. 13 According to XRD analysis, Cu 2 O nanostructures are generally polycrystalline, cubic, and preferentially oriented along the plane of (111) for 10 min. Some high Cu 2 O peaks were found in the PSi/Cu 2 O/ZnO sample prepared for 10 min, but those intensities were reduced with increasing deposition time due to increasing ZnO content. XRD patterns of the PSi/Cu 2 O/ZnO sample prepared for 80 min reveal that the crystal structure of ZnO phases is polycrystalline hexagonal wurtzite with (100), (002), (101), (102), (110), and (103) orientations corresponding to 2θ = 31.76°, 34.41°, 36.23°, 47.50°, 56.56°, and 62.84°according to JCPDS card No. 01-074-0534. 17,21,28 It has been also found that ZnO nanostructures have a polycrystalline, hexagonal wurtzite crystal structure, and its preferential orientation changed from (102) to (002) with increasing deposition time. This might be attributed to the fact that ZnO deposition on the Cu 2 O nanostructure increased with increasing deposition time.
Average crystal size was calculated from the dominant peaks where β, λ, and θ are the full width at half-maximum, the wavelength of X-ray, and the diffraction angle, respectively. The wavelength of the Cu K α beam was taken as 1.54 Å. The calculated crystal sizes are listed in Table 1. Table 1 indicates that when the current density increases, the average size of Cu 2 O crystallite declines. According to Çetinel 13     XPS analysis of PSi/Cu 2 O/ZnO samples was carried out to identify the surface atomic states. Figure 7 shows the XPS spectra obtained on PSi/Cu 2 O/ZnO samples prepared for 10 and 80 min deposition times. While Figure 7a represents the survey scan peaks for elemental Zn, Cu, and O, their highresolution spectra are shown in Figure 7b−h. Because of spin− orbit coupling, the Zn and Cu peaks form as doublets of 2p 3/2 and 2p 1/2 , respectively. For higher energy range, the peaks at 1045.3 and 1022.3 eV could be linked to Zn 2p 1/2 and Zn 2p 3/2 , respectively, observed for Zn element (Figure 7b). The spin− orbit separation is about 23.0 eV. The peak of 2p 3/2 detected at 1022.3 eV could be ascribed to the presence of Zn 2+ . 24,31,32 Both Zn 2p spin−orbit components can be deconvoluted into a single peak located at 1022.3 and 1045.3 eV. In Figure 7c, two important peaks were detected at 952.6 eV for Cu 2p 1/2 and 932.7 eV for Cu 2p 3/2 (19.9 eV peak splitting) in the lower energy range, which belonged to Cu, which is consistent with the literature. 24,31−33 While the well-known Cu 2+ satellites around 942 and 962 eV, a clue to the existence of CuO, were not detected for 10 min deposition time (Figure 7e), for 80 min deposition time, there were weak satellite peaks at around 934 and 954 eV signifying the presence of the chemical state of Cu 2+ (Figure 7f). As stated in the study by Han et al., 34 Cu 2+ peaks in Cu 2 O films can be caused by two sources: oxidation of the outmost surface of Cu 2 O films kept in the ambient environment and Cu 2+ adsorbed on unstable Cu vacancy sites during electrodeposition. According to XPS analysis, when the deposition time increases to 80 min, the Zn peak intensity increases, whereas the intensity of the Cu peaks decreases, which is supported by the XRD and SEM investigations. In addition, as can be seen in Figure 7d 7,9,35 Electrical properties of PSi/Cu 2 O/ZnO samples were determined at 300 K under reverse and forward bias cases. Typical I−V for PSi/Cu 2 O/ZnO heterojunctions in both semilog and linear scales are represented in Figures 8 and 9. It is clearly seen from Figure 8 that PSi/Cu 2 O/ZnO heterojunctions reveal rectifier property and act as a typical p-n junction. 36−38 The rectifying nature of the heterojunction was stable regardless of the deposition conditions. In reverse bias, the current decreases up to nA values due to the tunneling of electrons from the crystal silicon (c-Si) to the PSi/Cu 2 O structure. In forward bias, the achieved asymmetric characteristic in the I−V curve indicates the effective p-n heterojunctions between the p-Cu 2 O and n-ZnO layers. It is also worth noting that the PSi and PSi/ZnO samples exhibited a linear I−V characteristic ranging from −5 to +5 V (inset fin Figure 8), indicating that the p-n feature derives from the ZnO/Cu 2 O heterojunctions.
The semilog I−V plot was used to get the significant characteristics of the heterojunction diode, such as saturation current (I 0 ), barrier height (φ b ), and ideality factor (n), presented in Table 2 calculating from the well-known diode equation: 37 where I 0 is the saturation current, k is the Boltzmann constant, q is the electronic charge, T is the absolute temperature in K (at room temperature, 300 K), V is the bias voltage, A and A* are the effective diode area and Richardson constant (112 A/cm 2 K 2 for n-type Si), respectively. 18 Eq 3 may be used to calculate the φ b using the I 0 data. According to Hussain et al., 39 12,13,40 The low current value is also related to the high resistance at the PSi/Cu 2 O junction. At the current density of 0.5 mA/cm 2 , the ideality factor (n) value of PSi/ Cu 2 O/ZnO samples decreased from 10 to 80 min. It has been also found that among the experimental parameters selected to produce the nanostructure, the sample fabricated at 0.5 mA/cm 2 for 80 min has the lowest ideality factor (n = 1.36) due to a space charge effect at the grain boundary or interface. 41 Thus, it is expected that the samples fabricated at the current density of 0.5 mA/cm 2 will make a great contribution to advanced technological applications and literature as a functional material whose structural and electrical properties can be adjusted and which is easy to adapt to existing silicon technology.

CONCLUSIONS
In this study, an economical two-step electrochemical deposition approach was used to create Cu 2 O/ZnO nanoparticles with varied durations and current densities. SEM examination indicated that the Cu 2 O structures in the octahedron and truncated pyramid shapes were obtained, while ZnO nanostructures changed from spherical to hexagonal