Below-Band-Gap Index of Refraction

Typical index-of-refraction spectra of Mg Zni- O thin films with wurtzite and rocksalt structure are plotted in Figs. 3.21a-d. Data for 0 x 1 originate from thin film measurements, those for x = 0 and x = 1 are 

Sum rule considerations [140] imply that the discontinuity in the refractive index must be accompanied by a substantial increase of the oscillator strengths of higher energy transitions above the spectral range, from which data are available (E 10 eV). Otherwise the discontinuity of the lowest band-to-band transition energy (Sect. 3.7) would cause a decrease in the refractive index if the oscillator strengths within the rocksalt-structure part of the alloy would remain comparable to those within the wurtzite-structure part. 

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