This work describes the characteristics of zinc oxide (ZnO) thin films formed on a polycrystalline (poly) 3C-SiC buffer layer using a sol-gel process. The deposited ZnO films were characterized using X-ray diffraction, scanning electron microscopy, and photoluminescence (PL) spectra. ZnO thin films grown on the poly 3C-SiC buffer layer had a nanoparticle structure and porous film. The effects of post-annealing on ZnO film were also studied. The PL spectra at room temperature confirmed the crystal quality and optical properties of ZnO thin films formed on the 3C-SiC buffer layer were improved due to close lattice mismatch in the ZnO/3C-SiC interface.
Zinc oxide (ZnO) is an II-IV compound semiconductor with a wide direct band gap of 3.3 eV at room temperature. ZnO applications are found in solar cells, surface acoustic devices, optical waveguides, gas sensors, piezoelectric transducers, and varistors . Techniques used for the deposition of ZnO thin films include sputtering, chemical vapor deposition (CVD), pulsed laser deposition and sol-gel. The sol-gel technique has distinct advantages due to being the simplest deposition method, its excellent compositional control, homogeneity on the molecular level due to the mixing of liquid precursors, and lower synthesis temperature . The sol-gel method was reported as having a high potential to produce ZnO nanoparticles [2,3] that has demonstrated the high surface to volume ratio and enhances sensor qualities in gas sensing fields  or ultraviolet detection .
The crystalline and optical properties of the ZnO films were influenced by strains, residual stresses, and defects in the thin films. A variety of buffer layers were used in the growth of the ZnO films, including MgO , CaF2, and ZnS , to reduce the residual stress and film defects. The resulting buffer layers improved the crystalline quality and optical properties of ZnO.The ZnO thin film growth directly on Si substrates has induced high stress in interfacial ZnO/Si, because the lattice mismatch between ZnO and Si (100) is 39% and the difference in their thermal expansion coefficients is about 68%. The 3C-SiC, with its excellent properties described in , is also a good buffer layer, as the lattice mismatch and thermal expansion mismatch between the ZnO and the 3C-SiC, which are 5% and 38%, respectively, are smaller than those between ZnO and Si.
This study presented the growth of ZnO thin films on the polycrystalline(poly) 3C-SiC using the sol-gel process. The structural properties of ZnO/3C-SiC were investigated using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD)and photoluminescence (PL) spectra.
A poly 3C-SiC buffer layer was deposited onto Si (100) substrates using atmospheric pressure CVD with an Ar + H2 mixture as the carrier gas and hexamethyl-disilane [(CH3)6Si2] as the precursor.Poly 3C-SiC thin films 0.2 ㎛ thick were grown at a deposition temperature of 1,100℃, as described in our previous work.
Zinc acetate dehydrate [Zn(CH3COO)2.2H2O] was used as a starting material to produce the ZnO coating solution for the
sol-gel method. The precursor was dissolved in 2-methoxyethanol,and an appropriate amount of monoethanolamine(MEA: NH2C2H4OH) was added as a stabilizer. The solution was stirred with a magnetic stirrer for 2 hours until a transparent sol was obtained. The solution was aged at room temperature for three days. The ZnO thin films prepared via the sol-gel method were deposited onto the poly 3C-SiC/Si substrate by spin coating at 1,000-2,500 rpm (10-30 seconds). The films were heated on a hotplate at 300℃ for 10 minutes after each deposition. Eight to ten layers of the ZnO thin films were deposited to obtain a 300-350 nm thickness, and the samples were annealed in a furnace
[Fig. 3.] (a) Field emission scanning electron microscopy (FE-SEM)cross-sectional of ZnO/SiC at 600℃ and SEM images of ZnO/3CSiC/Si at various annealing temperatures (b) no annealing (c) 600℃and (d) 700℃.
at 600℃ and 700℃. The crystalline characteristics of the ZnO films on the 3C-SiC buffer layer were investigated using XRD with CuKα radiation (1.5406 Ao) by XPERT-PRO. The surfaces of the thin films were characterized using a JSM-6500F FE-SEM and atomic force microscopy. The PL measurements were acquired using a Shimadzu RF-5301PC at room temperature with a Xenon laser excitation and a 254 nm emission to study the optical properties of the ZnO film.
The XRD spectra in Fig. 1(a) illustrate that the poly 3C-SiC film grown on the Si substrate had a (111) preferred orientation
at 2θ = 35.6o, resulting in a SiC thin film with the highest Young’s modulus of the studied thin films . Figure 1(b) illustrates the XRD spectra of the ZnO thin films deposited on the SiC buffer layer using the sol-gel method. The major peaks were observed at 2θ = 31.8o, 34.4o, 36.3o, 47.5o, 56.6o, and 62.9o, which correspond to the (100), (002), (101), (102), (110), and (103) plane reflections of a hexagonal ZnO with a = 3.25 Ao and c = 5.21 Ao, as denoted by International Center for Diffraction Data (JCPDS 00-036-1451)for ZnO. The ZnO thin film prepared via the sol-gel method had a polycrystalline hexagonal wurtzite structure with no preferred orientation. SiC (111) did not appear in Figure 1(b), as its peak degree, 2θ = 35.6o, was too close to the ZnO (101) peak at 2θ =36.3o. Instead, the peak at 2θ = 41.4o represented the SiC (200)plane (JCPDS 01-074-2307) and confirmed the presence of the SiC buffer layer in the structure. As shown in Fig. 1(b), the peaks in the XRD spectra did not appear clearly without annealing and only appeared when the annealing temperature increased, such that the intensities of the peaks at (002) and (101) were greater at 700 ℃ than at 600 ℃.
Figure 2 shows the transverse optical modes in the Fourier transform infrared spectroscopy spectra of the ZnO films grown on 3C-SiC using sol-gel technologies at 600℃ annealing. Two strong peaks occur from Zn-O (412 cm-1) and Si-C (805 cm-1), and two weaker peaks occur due to the Si substrate (611 cm-1) and the Si-O (1,100 cm-1).
Figure 3(a) shows the cross-sectional FE-SEM image of the ZnO/3C-SiC heterostructure. Two layers were easily distinguished due to the different structures. ZnO films had nanosized powders and no preferred orientation of crystallites with a 300 nm thickness. The round poly 3C-SiC grains were stacked on top of one another. The inset picture of Fig. 3(a) shows the higher magnification of the interface between ZnO and 3C-SiC.
Figures 3(b-d) show the SEM images of the samples at various annealing temperatures for films prepared using sol-gel. In general, smaller grain sizes and a porous film were obtained with the sol-gel technique. The average grain sizes of ZnO powder are 40 nm and 60 nm with post-annealing temperature 600℃and 700℃, respectively. Grain growth following post-annealing was observed in Figures 3(b-d). The small crystallites coalesced to form larger crystallites, when the annealing temperature was increased , corroborating the reduction in the full width half maximum in the XRD results of the higher annealing temperature.
Figure 4 shows the PL spectra of the ZnO thin films at room temperature consists of the UV emission band and the visible emission broadband. The UV emission at 375-381 nm is attributed to exciton recombination and is strongly related to the quality of the films . The intensity of the UV emission increased with the presence of a 3C-SiC buffer layer; therefore, the optical properties of ZnO improved due to the SiC layer. These improvements can be explained by the close lattice mismatch of the ZnO and SiC layers . The origin of the visible broadband at 500 nm of the ZnO film is controversial, with some researchers reporting that it is caused by impurities and structural defects in the crystal,such as oxygen vacancies and zinc interstitials . However,other researchers concluded that the visible emission broadband is the characteristic of ZnO nanostructures . From observing the PL spectra of many forms of ZnO, it is believed that nanosized ZnO will generate strong peaks in the visible emission band. The green emission observed in Fig. 4 for both the ZnO/Si and ZnO/3C-SiC/Si structures prepared by sol-gel demonstrates the nanocrystalline ZnO structures.
The increased intensity of PL peaks at the UV band (380 nm)following annealing of films were observed in Fig. 4 and corroborate the results of other reports [2,12]. This increase was caused by the enhanced quality of films at a higher post-heating temperature,since the atoms of ZnO films have energy to arrange again,move into the suitable lattice sites during the post-annealing process. This will reduce the dislocations and defects in films.The increased green emission peak at the 510 nm wavelength for the sample annealed at 700℃ was caused by a higher density of oxygen vacancies in ZnO due to the reductive environment when annealing at a high temperature .
This work described the formation of ZnO thin films on a 3CSiC buffer layer by the sol-gel process. The nanocrystalline ZnO on the 3C-SiC layer prepared by sol-gel showed a strong intensity in the PL spectra. The 3C-SiC buffer layer improved crystalline and optical properties of the ZnO film due to the close lattice mismatch between ZnO/SiC interfaces. The ZnO prepared by sol-gel on 3C-SiC buffer layer was suitable for optoelectronic or UV/blue devices in the nanoscale.