Influence of the N2 gas flow on optical and structural properties of reactively sputtered ZrN films

Abstract

We present optical and structural properties of reactively sputtered zirconium nitride (ZrN) films for application as back reflectors in Cu(In,Ga)Se2 solar cells with sub-micrometer absorbers. In this study, ZrN films were deposited by reactive DC sputtering on blank, Mo-coated and Zr-coated soda-lime glass at two different process pressures and various ratios of nitrogen mixed in the argon working gas. When characterised by x-ray diffraction (XRD), the majority of the films were found to consist of single-phase cubic ZrN. All peaks corresponding to the ZrN phase were present in the diffractograms with intensities similar to those obtained from bulk ZrN, indicating that the films were randomly oriented. No significant differences were found between films grown on different substrate types. Films sputtered with lower nitrogen partial pressures displayed a spectral optical reflectance similar to metallic Zr, while films prepared with higher N2 flows showed the pronounced Drude-like reflectance characteristic of the nitride. The best ZrN films were achieved with a process pressure of 2.5 mTorr and a N2/(Ar+N2) flow ratio of 26.5%. At a wavelength of 800 nm the reflectance of these reached 85%, as compared to a typical value of 58% in the case of molybdenum.