Effect of thickness on the structure, morphology and optical properties of sputter deposited Nb2O5 films
Introduction
Nb2O5 films have been applied in various modern technologies as a consequence of their unique physical and chemical properties such as high refractive index, wide band gap, excellent chemical stability and corrosion resistance. Based on these novel properties, Nb2O5 films have been widely used as optical interference filters [1], [2], electrochromic films [3], gas sensors materials [4], etc. The properties of films closely rely on the sputtered materials, deposition techniques, deposition parameters, as well as the thickness of films. The structure and morphology of the film may be changed with the increase of growing thickness [5], which will affect the properties of the film. For example, the increase of surface roughness [6] will enhance the optical scattering. Therefore, it is important to gain more knowledge of the film thickness effects on its properties. Additionally, these effects must be taken into account in order to assess the practical usefulness of the films.
In previous studies, there are a lot of reports about the effect of thickness on the properties of the films. For example, Ma et al. reported the thickness dependence of electrical and optical properties of SnO2:Sb films [7]. Brüggemann et al. measured the optical scattering and surface roughness in microcrystalline silicon with different film thickness [8]. Kim et al. studied the effect of thickness on the electrical and optical properties of indium tin oxide film [9]. In our previous work [10], the substrate temperature effects on the optical properties of Nb2O5 films were investigated. It is found that the change of substrate temperature results in inhomogeneity of the films. In present study, we focus on the effect of thickness (d) on the structure, morphology and optical properties of Nb2O5 films. The films were deposited on BK-7 substrates by the reactive magnetron sputtering. From the experimental results, it is found that the thickness affects significantly the properties of film. The films turn into crystallization as d > 2010 nm. The defects on the film surface increase with increasing thickness. Moreover, the refractive index decreases, and extinction coefficient increases with increasing thickness when d > 2010 nm.
Section snippets
Experimental details
Nb2O5 films were deposited on BK-7 substrates at room temperature by using a low frequency reactive magnetron sputtering system. The sputter target was niobium metal (99.9% purity). The distance between target and substrate was about 12 cm, the substrate rotated at a speed of 600 rpm during the deposition to provide good uniformity of film and the sputter power was 3.98 kW. The base pressure of vacuum chamber was 2.0 × 10−4 Pa and total pressure was kept at 0.17 Pa during deposition. O2 and Ar flow
Crystal structure
Fig. 1 shows the XRD patterns of Nb2O5 films with different thicknesses. From Fig. 1, it is found that, the structures of the films are amorphous as d < 2010 nm, and the crystalline diffraction peak starts to appear for the film with d = 2400 nm. Additionally, the XRD peak intensity increases with increasing thickness. Comparing the diffraction peak position with the data from JCPDS international diffraction data base [12], we find the diffraction peak corresponding to the (1 0 0) plan of Nb2O5 with
Conclusion
Nb2O5 films with thicknesses ranging from 55 to 2900 nm were deposited on BK-7 substrates by a reactive magnetron sputtering system at room temperature. The structure of film is amorphous as d < 2010 nm, and turns into crystallization as d > 2010 nm. RMS surface roughness slightly decreases with increasing thickness as d < 1080 nm, and then increases when d > 1080 nm. Width and depth of the holes on film surface increase monotonously with increasing thickness. Refractive index decreases and extinction
Acknowledgement
This work was supported by Natural Science Foundation of Fujian Province of China (E0320002, K04023).
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