HYDROGEN ANNEALING EFFECTS ON THE OPTICAL ABSORPTION OF Bi-SUBSTITUTED IRON GARNET FILMS

Abstract

Reducing treatment effects on the optical absorption loss in the near-infrared region were studied for Ca-free and -doped Bi-substituted iron garnet films. (BiGd)3(FeAlGa)5O12 films were grown by the conventional LPE technique and annealed in hydrogen (H2) atmosphere at a step of 50°C between 150 and 400°C for 22 hours. The absorption loss at wavelength λ=0.8μm reached minimum values of 250 ∼ 300 dB/cm by annealing at 250°C for both Ca-free and -doped films. The decrease in the absorption loss at λ=0.8μm by this annealing is mainly ascribed to the reduction of Pb4+ to Pb2+ on the dodecahedral sites in garnets. The wavelength dependence of the difference of absorption loss between as-grown and 250°C-H2-annealed films has a peak around λ=0.56μm and the difference abruptly decreases in the longer wavelength side. This peak is thought to be due to a charge transfer transition from the crystal field split level of Fe3+ to Pb4+. Absorption loss at λ=1.15μm decreased for both Ca-free and -doped films by annealing at 250°C. The values reached ∑2 dB/cm for Ca-doped films and ∼9 dB/cm for Ca-free films. It is thought that the absorption loss of ∼9 dB/cm for the Ca-free films is due to Fe2+ which exists at the as-grown state. The increase in the absorption loss by annealing above 250°C may be due to Fe2+ ions created by the reduction of Fe3+→Fe2+.