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

Abstract

We report Cr doping-induced structural phase transformation (PT) in sputter-deposited polycrystalline-AlN thin film from wurtzite (w) to rocksalt (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 high-pressure-induced PT in AlN. Post the PT, the r-Al_1-xCr_xN 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-xCr_xN phase. The structural distortion and the PT also affected the optical absorption characteristics of the w-Al_1-xCr_xN thin films. The w-Al_1-xCr_xN exhibit a direct inter-band transition and the band gap (E_g) decreases from ∼ 6.06 eV for pure w-AlN to ∼ 4.15 eV for w-Al_0.73Cr_0.27N. After the PT, the r-Al_0.61Cr_0.39N 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%.