Study on relationship between device and material characteristics for Bi2212-THz emitters*

THz-waves can be generated by processing Bi₂Sr₂CaCu₂O_8+δ (Bi2212) single crystals into mesa structures and applying dc bias voltages across the intrinsic Josephson junctions (IJJs) ^[1]. In this study, we prepared Bi2212 crystals with different δ values, and compared the crystal characteristics and device characteristics to obtain further understanding of the role of excess oxygen for the emitter. To adjust  δ values, the grown crystals were annealed either under 0.2%-O₂ gas flow at 650℃ or under N₂ gas flow at 600℃. By using these crystals, a wet etching method ^[2] and a sandwich structure ^[3], we fabricated Bi2212 THz-emitters. To evaluate the homogeneity of the crystals and the unit cell parameter of the samples, X-ray diffraction was performed. As for the device characteristics, the current-voltage characteristics and emission characteristics were measured. As a result, the unit cell parameter c becomes short with increase the oxygen contents. The 0.2%-O₂-annealed samples show larger critical currents than those of N₂ samples and clear THz emission. The details of these results will be discussed in the meeting.  [1] L. Ozyuzer et al, Science 318, 1291 (2007).  [2] T. Imai et al., Jpn. J. Appl. Phys. 60, 126501 (2021). [3] T. Kashiwagi et al., Phys. Rev. Applied 4, 054018 (2015).