Structural and optical characteristics designed by sputtering deposition conditions of the oxide thin films

The influence of film thickness on the structural and optical properties of silicon dioxide (SiO2) and zinc oxides (ZnO) thin films deposited by radio frequency magnetron sputtering on quartz substrates was investigated. Deposition conditions were optimized to achieve stoichiometric thin films. The orientation of crystallites, structure and composition were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), while the surface topography of samples was analyzed using scanning electron microscopy (SEM). The optical characteristics were measured for samples with the same composition but obtained with different deposition parameters, such as increasing thickness. Optical constants (i.e. refractive index n, extinction coefficient k, and absorption coefficient α) of the SiO2 and ZnO oxide films were determined using Swanepoel’s method from the transmission spectra recorded in the 2 range 190 2500 nm, while the energy bandgap was calculated from the absorption spectra. The influence of thickness on the structural and optical properties of the oxide films was investigated. A good optical quality and real performance was noticed, which makes these thin films deem worthy of integration into metamaterial structures.

Metamaterials applied in the field of space science come with a new dimension of microstructural representation of the advanced functional materials [10,11]. Materials with dielectric qualities, such as SiO2 and ZnO, are used to make devices with metasurface structures in the broad visible spectral domain. They are intensely investigated due to their versatile properties, such as: high transmissions in the visible range [12,13], energy bandgap [14][15][16], etc. Among the important applications of these oxides are: materials with dielectric properties into the metasurface structures; transparent conductive oxides and buffer layers in solar cells technology; materials into the sensor technology, etc. [6,8,[17][18][19][20][21]. As materials with dielectric properties, SiO2 and ZnO, exhibit a dependence of the electrical resistance with temperature [22,23].
The SiO2 and ZnO films are obtained by various deposition techniques, such as: matrix-assisted pulsed laser evaporation (MAPLE) [24,25], spin coating of sol-gel precursor solutions [26], radio frequency magnetron sputtering (rfMS) [27][28][29][30], vacuum thermal evaporation (VTE) [31][32][33], chemical methods [34], reactive ion beam sputter 3 deposition [35] etc. For example, SiO2 and ZnO films obtained by rfMS technique can be used as dielectric materials in metasurface structures, or as dielectric interfaces into the structure of a metamaterial. This paper reports the experimental conditions for deposition of ZnO and SiO2 films, as an improvement of rfMS vacuum deposition technique for dielectric layers (e.g. ZnO and SiO2) onto quartz substrates. Here we investigated SiO2 and ZnO thin films with thicknesses ranging from 200 nm to 300 nm. Thus, we analyzed the beneficial effect of increasing the film thickness on the composition, morphology, structure and spectral characteristics of the studied samples. This way of analyzing oxide thin film thickness dependence on the optical and structural characteristics, allows us to clearly point out their necessity in metamaterial structures.

Experimental
The VARIAN ER 3119 EletroRava (INFLPR) deposition installation is provided with a deposition chamber, two magnetrons and in situ thickness monitoring. Thus, radio frequency magnetron sputtering (rfMS) technique [27,36,37] was used to deposit the SiO2 and ZnO oxide films. This ensures deposition on large areas and quality thin films for multiple applications. They are achieved at room temperature on quartz substrates with thickness ranging from 200 nm to 300 nm, from targets in the form of a disk with 4″ diameter and 0.125″ thickness (SiO2 and ZnO individually sintered, 99.99% purity from Lesker). Working gases (i.e. argon, 95% and oxygen, 5%) were introduced in the deposition chamber via a circuit provided with flow meters (30 sccm and 1.5 sccm, respectively), in order to precisely control and regulate the flow of gases into the deposition chamber. Quartz (fused silica, NEGS2) slides with dimensions of 2 cm × 2 cm × 0.1 cm were used as substrates. Initially, all substrates were cleaned in an ultrasonic bath to ensure good reproducibility of the properties of thin films.
Subsequently, the substrates were kept in special mounts on the rotating metallic plate, above the deposition targets. The rfMS method results in uniform growth of the oxide films and a good control of their composition.
To characterize the structure and thickness of the deposited SiO2 and ZnO thin films several methods were applied, including X-ray photoelectron spectroscopy (XPS), Xray diffraction (XRD), and scanning electron microscopy (SEM). Hence, the ESCALAB 250+ XPS equipment was used to determine the surface composition of the samples, with the following specifications: monochromatic radiation Al Kα (1486.6 eV) and vacuum in the analysis chamber, p ~ 1.6 × 10 -10 mbar. The XRD analysis was performed using a Brucker D8 Advance diffractometer. The crystalline structure of oxide thin films was investigated by the standard XRD technique, using Cu Kα radiation (λ = 1.55418 Å) in the range of 2θ = 25 -80 degrees.
Using SEM technique, we studied the surface of the samples at different magnifications, by scanning them with a beam of accelerated electrons to very high energies (~ 20 keV). The structural quality and surfaces morphology were investigated using a scanning electron microscope (FEI Co., model Inspect S50). The system is equipped with X-ray source and EDX unit with elementary energy dispersion spectroscopy (EDS). These analyzes employ different magnifications depending on the quality of the thin films and structure of their surface. Using the cross-section imaging and a magnification of 20 000 ×, it was possible to gain information related to the thickness of our samples.
Optical transmission spectra were acquired using a UV-VIS-NIR Perkin-Elmer Lambda 950 Spectrophotometer, with a measuring range between 190 nm and 2500 nm, and a wavelength accuracy of 0.08 nm in UV-VIS and 0.3 nm in NIR band respectively.

Results
X-ray diffraction analysis of oxide samples was realized in the range of angles 25 -80 degrees ( Fig.1). It was performed to determine the type of structure (e.g. polycrystalline or amorphous) and orientation of the thin films. Figure 1 shows typical XRD patterns of ZnO thin films with increasing thickness and prepared by rfMS process. Following the effect of deposition parameters of the oxide films we found that diffractograms show increasing intensities of the peaks with thickness, determining an improvement of their crystalline structure. In the case of ZnO samples, the crystalline phases and peaks were identified as corresponding to (002) and (004) planes, according to JCPDS XRD the standard diffractograms [38]. These diffractograms indicate good crystalline quality and the analyzed films show two diffraction peaks, characteristic of the ZnO hexagonal structure (wurtzite). The studied thin films have a crystalline structure with a strong orientation of the planes (002) parallel to the surface of the substrates. In the diffractogram of the 200 nm thick film, diffraction peaks corresponding to a single-phase growth of the film were initially identified. Low intensities of the diffraction lines (004) are due to growth stress, which is unevenly distributed in the film. In the diffractogram corresponding to the 250 nm thick film it can be seen how the film grew oriented with the c-axis perpendicular to the substrate surface, a phenomenon that is specific to depositions made at room temperature.
The values of the (002) plane, corresponding to the multiple reflections on the substrate surface, for ZnO thin films are presented in Table 1. One of the films of the highest quality in terms of structure was the one deposited at a thickness of 300 nm; this is consistent with the value of the lattice parameter c = 5.2090 Å, indicating a good oxygenation. proved to be essentially amorphous [28,37]   To keep the contamination layer from the surface of the films, they were not sputtered using the ion gun. This is because sometimes the sputtering affects the stoichiometry of the samples by depleting the films of oxygen. Figure 2 shows the general oxide spectra for three SiO2 samples.
The high resolution (HR) analysis Si 2p3 and O 1s spectra recorded [43,44] for the SiO2 samples are shown in Figs. 3a and 3b. Using this analysis, we determined the elemental composition as well as the chemical and electronic states of the elements that exist in the SiO2 films. Although Si 2p3 shows small chemical changes, the binding energy value of 103.7 eV indicates a completely oxidized Si for the SiO2 films (Fig. 3a). [43][44][45][46]. Experimental data reveal that there is good stoichiometry for this film.   The measured thickness values for oxide films are shown in Table 2, and they are found to be similar with the predefined ones. EDS distribution in all investigated samples caused the appearance of chemical elements like Zn L, Si K and O K ( Table   2). It is found that SiO2 and ZnO films show a better stoichiometry with increasing film thickness (from 200 nm to 300 nm). This is evidenced by the values of atomic concentrations of the analyzed films.  The presence of maxima and minima in the transmission spectra of SiO2 and ZnO thin films allows for determination of their optical constants, with the help of the envelope method proposed by Swanepoel [50,51]. It was found that thinner samples have a transmission in the range 78 -92 % while thicker ones in the range 82 -65 %, for radiation with wavelength of 600 nm. Stronger absorption of the 300 nm thickness thin films is influenced by the increase in the volume of inter-crystalline regions [52].
Optical constants were calculated for both high and low absorption ranges. In the case of transparent SiO2 and ZnO thin films, between the extinction coefficient, k, (Figs. 9 a) and b)) and the absorption coefficient, α, there is the following relation: where λ is the wavelength. Values of the dielectric constant were obtained by using the Drude method [56,57] and spectral absorption of the oxide films. This allowed the assessment of the permittivity and polarizability of the material, as well as the density of states in the band interval.
Based on calculus, the value of the real dielectric constant (εr) has the formula: and the relationship to compute the imaginary dielectric constant is as follows: where n is the refractive index and k is the extinction coefficient. constants of ZnO and SiO2 films with photon energy. It can be seen that the value of the real part is higher than the imaginary one, which is more evident in the 300 nm thick sample. The attractive characteristics of the thickest sample (i.e., 300 nm) suggest that optimal deposition conditions have been found for real performance, and possibility to be integrated in the metamaterial structures.
(i) (ii) Thus, increasing the thickness of ZnO and SiO2 films by 100 nm, from 200 nm to 300 nm, improves the quality of samples by 12% -15%, resulting in a better stoichiometry, increased crystallinity, improved optical and dielectric properties.

Conclusions
SiO2 and ZnO oxide thin films of various thicknesses were deposited by radio frequency magnetron sputtering technique. Films with a good stoichiometry were obtained as related to the target of provenience. In the case of SiO2 thin films, they were confirmed by X-ray diffraction measurements that all structures are amorphous.
Following X-ray diffraction analyzes, it was proved that the ZnO films show an orientation with the c-axis perpendicular to the substrate surface. The results indicate that the ZnO thin films are crystalline with a hexagonal structure, and with increasing film thickness the crystallinity of the films as well as the size of the crystallites for the (002) plane increases.
Transmission spectra of the studied oxide films are strongly influenced by the deposition conditions. Smaller values for the transmission coefficient were obtained in the case of thicker samples, such as 74% for ZnO 300 and 68% for SiO2 300 respectively. The results show that the Swanepoel model describes very well the optical properties of SiO2 and ZnO films. Also, an improvement of the optical properties of the thin films with increasing thickness was noticed. In conclusion, some of the best quality SiO2 and ZnO films in terms of structure and optical properties were the 300 nm thick ones. Thus, the attractive characteristics of the thickest sample suggest that optimal deposition conditions have been found, allowing us to obtain samples with real performance to be integrated into metamaterial structures.