Nanoimprint technology was used to synthesize a series of nanostructures with hexagonal holes on a n-GaN baseplate. The hydrothermal growth technique was then used to produce 1.5-μm n-type ZnO nanorods. Radio frequency reactive magnetron sputtering was employed to grow a 50-nm thick layer of cuprous oxide film over the nanorods to form a p-n Cu2O/ZnO core-shell structure. Based on the different imprint widths and intervals obtained, Cu2O/ZnO heterostructure samples A, B, C, and D showed aperture ratios of 0.0627, 0.0392, 0.0832, and 0.0537, respectively. Scanning electron microscopy and atomic force microscopy indicated that a 50-nm Cu2O film coated the ZnO nanorods, forming a core-shell structure. X-ray diffraction and x-ray rocking curve (XRC) analysis showed that the Cu2O lattice structure had polycrystalline characteristics. The lattice planes of Cu2O were (111) and (220), and Sample C exhibited the narrowest XRC half-height full-width value. Therefore, among the samples obtained, Sample C had the optimal material properties. Measurement of the optical properties of the samples demonstrated that their luminous peak did not change with variations in temperature. Sample C also showed optimal optical properties. High-resolution transmission electron microscopy indicated the presence of a midlayer in the Cu2O/ZnO junction that had a direct impact on the Cu2O lattice arrangement on the top, corner, and side faces of the ZnO nanorods. The sample with the largest aperture ratio exhibited the most favorable optical and material properties. The novel structure obtained can potentially be used in solar cell applications.
© 2014 Optical Society of America
The development of clean and sustainable energy has been a research objective for a long period of time. Current research has focused primarily on solar cells that use ZnO and CuO as key materials [1,2]. ZnO is a direct gap semiconductor with a hexagonal wurtzite structure, and thus exhibits excellent properties in ultraviolet light. At room temperature, it possesses an extremely wide energy gap of 3.37 eV and an extremely high exciton binding energy of approximately 60 meV [3–5]. Cuprous oxide is a red to red-brown crystal or powder that possesses a direct energy gap of 2.137 eV and a cubic lattice structure. Cuprous oxide films are nonpoisonous and have high absorption coefficients and rich materials. They are widely used in solar energy conversion, sensors, and catalysis. Cu2O films generally exhibit p-type semiconductor characteristics because of the presence of Cu vacancies [6–8]. In addition, the Cu2O film growth using the N plasma treatment has also been reported for solar cell application . These compounds are extensive, widely accessible, nonpolluting, and can be prepared at low cost. Thus, ZnO and Cu2O are often studied as materials for solar cells.
Cu2O/ZnO nanostructures include nanorod, nanowire, and nanopillar structures. Heterostructure construction is a method that potentially enhances the overall conversion efficiency of photovoltaic devices. When a heterostructure forms between Cu2O and ZnO, the separation and transportation of photoinduced charge carriers in Cu2O/ZnO film are facilitated. It also decreases the probability of recombination and subsequently enhances photovoltaic properties. Therefore, Cu2O/ZnO heterostructures are widely used in solar cells and photocatalysis.
Nanostructures (particularly core shells) are used to enhance the photovoltaic efficiency of solar cells. These structures present several key benefits, including decreased net reflectance (increased absorbance), increased junction area, and decreased carrier travel distance. The increased surface area of a nanowire array and its inherent surface features result in decreased reflection and, consequently, higher incident light absorption . This finding is largely a result of light trapping and scattering effects, where light obtained is reflected in between the nanowires; this process considerably reduces the likelihood that the light will be reflected outwards [10–13]. Theoretically, Cu2O/ZnO p-n heterojunction structures can achieve 20% photoelectric transformation efficiency . However, the single-crystal characteristics of Cu2O remain unclear  and various epitaxy and electrochemical technologies are unable to generate sizeable amounts of high-quality Cu2O films. Moreover, the lattice mismatch rate of Cu2O grown on ZnO is 7.1%; defects in the junction cause threading dislocation defects, resulting in misfit defects in the resultant Cu2O film . These defects can affect Cu2O thin film growth . Cu2O/ZnO internal defects occur in the Cu2O/ZnO boundary and in the Cu2O film. These defects cause the carrier to be captured by defects, resulting in a considerable drop in the open circuit voltage of the solar cell and low photoelectric transformation efficiency.
The lattice arrangement in the Cu2O(111) direction is similar to that in the ZnO(0001) direction; therefore, Cu2O can be made to grow in the ZnO (0001) plane direction. The resulting Cu2O film can be expected to exhibit improved characteristics . The core-shell structures of ZnO nanorods and Cu2O thin film result in higher material quality, combined with a very low rate of photoelectrons. Core-shell structures of Cu2O/ZnO can be used to increase the crystallinity and single-crystal structure of Cu2O films and increase the current absorption conversion efficiency from 2% to 6% .
2. Experimental procedures and sample growth conditions
Figure 1 shows a schematic diagram detailing the preparation of the specimen. Figure 1(a) demonstrates that a 40 nm SiZx film layer grew on the n-type GaN base plate with SiH4 and N2 deposition gas, 350°C growth temperature, and 95 Pa chamber pressure by MOCVD. Figure 1(b) displays the nanoimprinting process. We imprinted four types of apertures on the SiNx layer. Figure 1(c) shows the transfer graphic that remained on the base plate after removal of the nanoimprint pattern. Figure 1(d) illustrates the aperture etching of an n-GaN layer using the dry reactive-ion etching (RIE) method. Etching conditions are using CF4 gas, 25°C etching temperature, 1.3 Pa chamber pressure, and 80W/h operation power. The hydrothermal technique was then used to grow 1.5-μm-long ZnO nanorods, as shown in Fig. 1(e). The hydrothermal technique is a simple method that easily generates oxide materials. The hydrothermal reaction solution was prepared from equimolar zinc nitrate hexahydrate [Zn(NO3)2∙6H2O] and hexamethylenetetramine [(CH2)6N4] at room temperature. Concentrations were varied from 0.05 M to 0.1 M to examine the concentration dependence of nanorod growth. The solution was stirred for 20 min and vibrated using an ultrasonic oscillator. Filter paper was then used to remove precipitates. The solution was subsequently preheated in an oven for 10 min at the appropriate growth temperature. After the growth solution had become adequately heated, the patterned Si3N4/GaN/Al2O3 template was immersed into the growth solution upside down in a sealed beaker. This template was kept at temperatures ranging from 65 °C to 95 °C for a given duration to examine the temperature and time dependencies of nanorod growth. The Cu2O film growth is illustrated in Fig. 1(f). Radio frequency reactive magnetron sputtering was utilized to grow Cu2O layers. The sputtering process was conducted using a pure copper target 2 inches in diameter and of 99.99% purity. The distance between the target and substrate was 15 cm. During Cu2O thin film deposition, the temperature of the substrate holder was maintained at 400 °C and the RF power was set to 60 W. Argon and oxygen were also introduced during the deposition as carrier gas and reactive gas, respectively. The flow rates of argon and oxygen were 18.4 and 1.6 sccm, respectively, which was the optimum condition to deposit high-quality Cu2O layers in our lab environment. The base pressure of the deposition chamber was maintained at approximately 5 × 10−6 torr, whereas the working pressure was 2 × 10−3 torr. Cu2O evenly coated the entire ZnO nanorod to form a core-shell nanostructure. Figure 1(g) displays the final Cu2O/ZnO nanostructure.
Table 1 lists the aperture size, pitch, and the aperture ratio of Samples A, B, C, and D, grown on GaN base plates during the nanoimprinting process. It also lists the ZnO nanorod weight and height that was observed during the hydrothermal growth process. SEM was performed using a JEOL JSM 6700F system. AFM images were obtained using a Veeco Metrology Group Dimension 3100 instrument. XRD measurements were performed using a Bede D1 system. PL spectra were obtained using the 325-nm line of a 50-mW He-Cd laser as the excitation source. Samples were placed in a cryostat for temperature-dependence measurements ranging from 10 K to 300 K. TEM investigations were performed using a JEOL TEM-300F field emission electron microscope with an accelerating voltage of 300 KV.
3. Results and discussion
3.1. Tilted scanning electron microscopy and atomic force microscopy results
Figures 2(a) to 2(d) show scanning electron microscopy (SEM) images of Samples A to D at a magnification of 10,000 × and a 30° inclination. The pattern obtained by the nanoimprinting technology displayed high regularity, evenness, and occupied a considerably large area. Compared to ZnO nanorods fabricated using other growth methods, such as those using a high-temperature furnace [20–22], an electrodeposition process , reactive magnetron sputtering , and chemical deposition , ZnO nanorods fabricated using nanoimprinting technology possess superior structural characteristics. Furthermore, ZnO nanorods fabricated using nanoimprinting technology use a layer of SiNx as the mask of the nanoimprint, blocking the impact of surface defects of n-GaN on the growth of the ZnO nanorods. The observed defects resulted from dislocations caused by mismatches between the sapphire base plate and the GaN lattice .
Figures 3(a) to 3(d) are SEM images of 50-nm-thick Cu2O/ZnO core-shell structures grown on ZnO nanorods at a magnification of 100,000 × and a 30° inclination. The Cu2O layer evenly coated the ZnO nanorod surface, forming a core-shell nanostructure with well-defined, hexagonal flat tops and straight shapes. However, the depth of the ZnO nanorod coated with the Cu2O layer is uncertain. Figures 4(a) to 4(d) are the atomic force microscopy (AFM) scanning results of Samples A to D. The root mean square values (RMS) of Samples A, B, C, and D were 169, 273, 183, and 300 nm, respectively. The AFM prober failed to penetrate 1.5 μm of the bottom, causing an inaccurate RMS value. Although the AFM scanning results confirmed excellent evenness at the top of the nanorods, the results could not confirm whether or not the Cu2O film filled the gaps between the nanorods .
3.2 X-ray diffraction and x-ray rocking curve results
Figure 5(a) shows the high-resolution x-ray diffraction (XRD) patterns of Cu2O/ZnO Sample C. The peak value of n-GaN (002) occurred at 17.28°, whereas that of n-GaN (102) occurred at 45.34°. The peak of the ZnO nanorod (101) was at 36.44°, that of the ZnO nanorod (103) at 63.0° , that of the Cu2O (111) at 36.28°, and that of the Cu2O (220) at 62.54° . ZnO nanorods that grew on n-GaN exhibited high single-crystal orientation and pure monocrystallinity. Cu2O grown on ZnO nanorods was affected by the ZnO lattice, and exhibited high single-crystal properties. Cu2O (111) and (220) presented single lattices in the film. Thus, two peaks of Cu2O, namely (111) and (220), may have resulted from the Cu2O growing in different ZnO nanorod directions (i.e., the top, lateral, and corner sides). The Cu2O film grew satisfactorily, with strong single-crystal diffraction signals. Previous research has provided no information regarding grain increases in this process [30,31].
Figure 5(b) displays the XRD patterns of Samples A, B, C, and D. Figures 5(c) and 5(d) are magnified images of the Cu2O(111) and Cu2O(220) peaks of XRD, respectively, that of the ZnO nanorods. These two figures indicate that the ZnO and Cu2O peak positions changed slightly. Shell growth on the ZnO nanorod surface may have induced compressive strain toward the ZnO nanorod’s core, decreasing the lattice constant of the core. The formation of Zn-containing alloys can reduce lattice mismatches between the ZnO core and the Cu2O shell. Therefore, the introduction of a CuZnO intermediate layer can reduce interfacial defects and promote the growth of single-crystalline layers on the ZnO surface. A small tilt angle between the CuZnO and ZnO (0001) directions inevitably appeared because of lattice mismatches; no alloy formation was observed, in contrast to the ZnO and CuZnO nanorod cores. The polycrystalline Cu2O shell can produce fairly low compressive strain toward the core of the ZnO nanorod [32–34].
The x-ray rocking curve (XRC) results of Samples A to D, obtained from the Cu2O (111) and (220) surfaces, are presented in Figs. 6(a) and 6(b), respectively. The full width at half maximum (FWHM) of XRC indicates the quality of film grown on a single-crystal surface. As shown in Fig. 6(a), the FWHMs of Samples A, B, C, and D were 606.1, 648.3, 530.2, and 633.6 arcsec, respectively. As shown in Fig. 6(b), the FWHMs of Samples A, B, C, and D were 409.4, 423.8, 391.7, and 414.9 arcsec, respectively. Similar trends of FWHM of Sample B > Sample D > Sample A > Sample C were obtained. These results indicated that the optimally grown Cu2O film was Sample C. The ZnO nanorod of Sample C exhibited optimal material quality among the samples. After calculating the nanoimprint parameters, the aperture ratio of the ZnO nanorod was found to be related to growth conditions. A sizeable aperture ratio implies a more high-quality ZnO nanorods. Figure 7(a) clarifies the aperture ratio definition. The three round holes in the figure (from the nanoimprint) result from magnification of the top right area. In a period structure with a hexagonal array, we used the triangle ABC as a unit cell. The AR was defined as the area of holes in a unit cell divided by the area of the unit cell, as presented in the top left of Fig. 7(a) [35,36]; the maximum ratio was 1. The AR values of Samples A, B, C, and D are presented in Table 1. A larger ratio indicated that the ZnO nanorod had a wider growth area for enhanced growth. The effect of AR on material growth is demonstrated in Fig. 7(b), which shows a unit cell diagram of three different AR values. The holes of four samples exhibited different features; thus, we normalized the area of a unit cell as 1. When the AR increases, the hole size increases and larger ZnO nanorods can be grown. According to the XRD and XRC results, Sample C exhibited the most favorable material properties and possessed the largest AR value. This finding indicated that the SiNx mask can block GaN dislocations. Furthermore, in areas without a mask (i.e., the ZnO nanorod area), the SiNx mask can block the penetration of dislocations  because nanorod growth blocks dislocations. Therefore, the unit cell of Sample C has a distinct arc, denoted as the blue line in Fig. 7(a). This arc is the boundary between SiNx and ZnO. In the boundary area, the ZnO strain was relaxed by tensile strain [37–41].
3.3 Photoluminescence results
Figures 8(a) to 8(d) depict the variable-temperature photoluminescence (PL) measurement results of Samples A to D. The luminescence band in 365 nm was attributed to the GaN base plate, the luminescence band at 375 nm was attributed to the ZnO nanorods [42,43], and the luminescence band at 500–900 nm was attributed to the Cu2O film [44,45]. A wide PL range was observed. Our references indicated that the direct energy gap of Cu2O is 2.0 eV and that its corresponding luminescence band is 620 nm. The luminescence wavelength generated from the interface energy states of Cu2O/ZnO was 640 nm. Therefore, the calculated interface electric energy gap was 1.93 eV. The direct energy gap of CuO was 1.83 eV, and the corresponding luminescence wavelength was 680 nm. However, because a CuO peak was not found in our XRD results, the contribution of CuO was not considered in our analysis.
The luminescence band of Cu2O covering the 500–900 nm range can be attributed to several factors. First, the core-shell structure exhibits varying energy gap widths in different areas of ZnO nanorods, such as the top, corner, and lateral sides. This phenomenon is caused by the growth of the Cu2O thin film on different ZnO lattices. Second, the Cu2O thin film is of different thicknesses on the top and lateral side of the ZnO nanorods; these differences generate quantum effects and cause wide energy gaps . Third, the energy band of the Cu2O/ZnO heterostructure is bent at the interface, resulting in a wide energy gap. The bent heterostructural interface results from the different lattice constants of Cu2O and ZnO causing a mismatched lattice; this facilitates gaps and defects in the interface. This mismatched lattice can induce differences in energy levels and bend the energy band . The work function of Cu2O is greater than that of ZnO; in other words, the Fermi energy level of Cu2O is lower than that of ZnO. When these compounds interact, electrons in ZnO are transferred to Cu2O and an interface electric field is generated between the ZnO/Cu2O films, resulting in energy band bending. Comparing Figs. 8(a) to 8(d), the PL width in Fig. 8(c) was observed to be the largest. Therefore, Sample C possesses the optimal optical absorption properties, and the prepared solar cell possesses the optimal absorption waveband. Figure 9 shows the integrated PL intensity ratio of the samples as a function of temperature. The normalized ratio of the integrated intensity of the near-band-edge emission (NBE; 1.38–2.48 eV) to that at 10 K was used as a basis for evaluation. The NBE reflects the emission of various exciton recombinations in Cu2O, such as donor-bound excitons, receptor-bound excitons, and free excitons. A strong NBE intensity indicates that the emission of various excitons is safe, and a higher ratio results in higher Cu2O quality. The optical quality of Sample C was generally higher than that of any other sample. Therefore, Sample C exhibited the optimal optical characteristics.
We found that in variable temperature PL measurements of Cu2O/ZnO samples, peak positions of PL did not change as the temperature changed. This finding may be explained in several ways. First, the core-shell nanostructure inhibits the effect of band gap shrinkage . Second, Cu2O shell layers of different thicknesses are found on the top and lateral sides of the ZnO nanorods and generate quantum effects that inhibit the generation of band shrinkage. Finally, we found that the luminescence of the electric field of interface states did not decrease with increasing temperature. This finding indicates that the electric field does not cause defects in luminescence features [49,50]. The schematic band diagram at the Cu2O-ZnO interface is presented in Fig. 10.As observed in the figure, photoinduced electrons in Cu2O could easily move into ZnO along the band bending, avoiding recombination with holes, exhibiting an interesting property on new wavelength luminescence because of nanoheterojunctions.
3.4 High-resolution transmission electron microscopy results
Figure 11(a) displays cross-sectional images of Sample C. As shown in this figure, the ZnO nanorods were fully coated by the Cu2O layer, and an intermediate layer between Cu2O and ZnO can be observed. To demonstrate that the lattice structure of Cu2O thin films grows on ZnO nanorods in different directions, we obtained high-resolution transmission electron microscopy (TEM) images, displayed in Fig. 11(b). Figures 11(c), (d), (e), and (f) are enlarged partial images of the lattice in Fig. 11(b). The inset of these figures present images of the diffraction patterns of Cu2O, ZnO, the intermediate layer, and the lateral side of Cu2O/ZnO. Figure 11(c) demonstrates that the ZnO nanorod lattice was evenly arranged, without dislocation or stacking faults. The inset shows the diffraction pattern of the ZnO hexagonal structure. Figure 11(d) presents a 14-nm-thick intermediate layer with amorphous characteristics (marked with a red arrow). The Cu2O layer is approximately 40-nm thick and exhibits grain distribution characteristics. The growth condition of Cu2O film is 50-nm thick, so we can speculate that the intermediate layer is the O from Cu2O, which is captured by a dangling band of ZnO, leading to the formation of a CuO layer. The inset refers to the diffraction pattern of Cu2O and CuO. Figure 11(e) demonstrates that the ZnO nanorod lattice is arranged unevenly at the corner. The thickness of the intermediate layer is approximately 12 nm, evidencing amorphous features. The inset of the figure depicts an overlapping diffraction lattice image of the area between Cu2O and the intermediate layer. Compared with Fig. 11(d), more grain distribution characteristics are observed in Cu2O. In Fig. 11(f), the intermediate layer is 7-nm-thick and its material characteristics were consistent with those in the two other directions. Less grain distribution characteristics were found in the Cu2O layer. The inset displayed an overlapping diffraction lattice image of the lateral sides of ZnO and Cu2O. The thickness of the intermediate layer decreased in the top, corner, and lateral-side growth directions. This change may be explained by the reduction in the material sedimentation rate in the top, corner and later-side growth directions during Cu2O film sedimentation. We can observe from Figs. 11(a) and 11(b) that the thickness of the Cu2O film was also reduced in the top, corner, and later-side growth directions, thereby producing a series of samples with excellent optical properties. We used the ZnO and Cu2O lattice structures to further investigate the growth of Cu2O films on the ZnO nanorods in different directions. Figure 12(a) displays the ZnO (0001), (10-11), and (1-100) lattice directions, and Fig. 12(b) displays the Cu2O (111) and (220) lattice directions. The directions of Cu2O (111) and ZnO (0001) provided the most stable sedimentation growth conditions [51,52]. The Cu2O (220) and ZnO (1-100) directions became another possible growth condition. No relative Cu2O epitaxy was observed on the semipolar lattice face of ZnO. This finding was consistent with the XRD data.
Figure 13(a) includes plane-view TEM images of Sample C and large-scale TEM images of a core-shell. To clearly comprehend the material properties of the Cu2O layers grown on the top and side faces of ZnO, we obtained plane-view TEM measurements. Plane-view TEM can be used to determine defects on the growth direction of the material and lattice domain . In this figure, we selected (b), (c), and (d), marked with red rings, to analyze the Cu2O layer structure. The inset of Fig. 13(a) depicts the atomic lattice diffraction pattern of the figure. Imaging results in the ZnO (0001) direction are clearly observable in this figure. In Figs 13(b) and 13(c), a 2–5 nm-thick intermediate layer is observed at the corner face. The plane lattice structure of the ZnO nanorods was extremely clear, and different blocks with contrasting dark and light features can be observed. These blocks were caused by the differing thicknesses of the samples. Figure 13(d) demonstrates that the ZnO nanorod lattice possessed an even arrangement. In the middle of the lattice, an intermediate layer that as 3- to 4-nm-thick was also found. The Cu2O core-shell layer exhibits a clear grain lattice arrangement, as indicated by the green ring. The results shown in Fig. 13(d) indicated that the lattice arrangement of ZnO differed from that of Cu2O. According to the lattice direction in Fig. 13(d), the Cu2O layers grown in the lateral sides and corners of the nanorods [Figs. 13(b) and 13(c)] were confirmed to have different growth lattices. This finding was consistent with the XRD data.
Nanoimprinting technology and the hydrothermal technique were used to grow four types of ZnO nanorods of different sizes, and radio frequency reactive magnetron sputtering was used to deposit a 50-nm-thick Cu2O film over them. A new type of p-n heterojunction core-shell Cu2O/ZnO structure was obtained, and higher nanoimprint aperture ratios were observed to improve material properties in the ZnO nanorod. The Cu2O film clearly grew in only two directions, (111) and (220). The crystallized quality of Cu2O thin film depends on the ZnO nanorod growth. Since the growth in the heterogeneous materials, the lattice mismatch between the underlying and the upper materials will affect the crystal quality of the upper material layer. Thin Cu2O films generated different grain growth rates because these films are grown in different ZnO lattice growth faces. We confirmed that Sample C exhibited the largest imprint aperture ratio and demonstrated, using numerous analyses and observation methods, that the Cu2O film sediment on Sample C was of the highest quality. This type of evenly arranged Cu2O/ZnO nanostructure has a wide luminous bandwidth (about 200 nm) and optical properties that do not change with temperature. The use of Cu2O/ZnO nano-core-shells in solar cells is expected to improve the photoelectric transformation efficiency of the resultant device. The results of this work contribute to research on the transformation efficiency of Cu2O.
This research was supported by the National Science Council, Republic of China, under grants NSC 100-2221-E-194-043, 101-2221-E-194-049, 102-2221-E-194-045, 102-2221-E-110-064, and 102-2622-E-194-004-CC3.
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