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We present optical and structural properties of reactively sputtered zirconium nitride (ZrN) films.
ZrN films have previously been proposed to replace molybdenum films as back contacts for Cu(In,Ga)Se2-based solar cell devices with absorbers thinner than one micrometer.[1]
It has been found that thin devices on molybdenum back contacts typically suffer from considerable absorption losses.
With a back contact of ZrN, which exhibits a stronger long-wavelength reflectance than Mo, more light passes the absorber a second time, yielding higher generation and thereby potentially higher efficiencies.
In this study, ZrN films were deposited by reactive DC sputtering on soda-lime glass (SLG), Mo-coated SLG and Zr-coated SLG.
The process pressure was either 2.5 mTorr or 5 mTorr, respectively, and the percentage of N2 in the Ar+N2 sputter-gas mixture was altered in the range of 14% through 32%. Process durations varied between 10s and 600s.
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 difractograms with intensities similar to those obtained from bulk ZrN, indicating that the films were randomly oriented.
No 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 2.5mTorr and a N2 gas-flow portion of 26.5%; for these the reflectance at a wavelength of 800nm reached 83.5%, as compared to a typical value of 58% in the case of molybdenum.
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