Cr-induced structural phase transformation in sputter deposited poly-AlN thin film from wurtzite to rocksalt structure and their effect on the optical properties

We report Cr doping-induced structural phase transformation (PT) in sputter-deposited polycrystalline-AlN thin film from wurtzite (w) w ) to rocksalt (r) r ) structure. Rietveld analysis shows that the steady substitution of Al ions by Cr ions in the w-AlN lattice distorts and compresses the wurtzite lattice along the c-axis. The chemical pressure exerted in the w-AlN lattice by the compression from the doped Cr ions causes the PT, similar to the highpressure-induced PT in AlN. Post the PT, the r-Al 1-x Cr x N thin film continues to exhibit compressive residual stress, indicating that such sustained stress may also play a role in stabilizing the metastable r-Al 1-x Cr x N phase. The structural distortion and the PT also affected the optical absorption characteristics of the w-Al 1-x Cr x N thin films. The w-Al 1-x Cr x N exhibit a direct inter-band transition and the band gap (Eg) E g ) decreases from 6.06 eV for pure w-AlN to 4.15 eV for w-Al 0.73 Cr 0.27 N. After the PT, the r-Al 0.61 Cr 0.39 N thin film, on the other hand, exhibits an indirect inter-band transition with an E g of 1.84 eV. In addition to the change in E g , a prominent defect band appears at 4 eV at low Cr% and another band appears at 0.98 eV at higher Cr%.