Carrier compensation and generation in Ca1−xYxTiO3 and CaTi1−xNbxO3 single crystals

Electrical and optical properties of Ca1−xYxTiO3 and CaTi1−xNbxO3 (x=0, 10−4, 10−3, 10−2) single crystals were investigated in order to understand electronic states of donors in oxide semiconductors. The single crystals were grown by FZ method in O2 flow with an infrared radiation furnace and they were annealed in Ar flow to avoid cracking. In both Y doped and Nb doped cases, the crystals of x=0, 10−4 and 10−3 were insulating and showed pale yellowish color. On the other hand, the crystal of x=10−2 was conductive and showed blue color. After H2 reduction at 1000°C, all of the doped samples (x=10−4, 10−3, 10−2) became conductive, and metallic behavior was observed by the resistivity measurement at low temperatures. Simultaneously with the remarkable increase in the conductivities, optical absorption due to the carrier generation appeared in the near infrared region. The carrier concentrations in the crystals were 1013–1015cm−3 for x=10−4, 5–6×1018 cm−3 for x=10−3 and 2–3×1019 cm−3 for x=10−2, and the Hall mobilities were found to be 2–4 cm2 V−1 s−1 for all of the doped crystals. The conductivity measurement under air at high temperatures found that the conductivity of the as-annealed crystals showed an unexpected dependence on x. The conductivity first decreased with an increase in the dopant concentration from x=0 to 10−3 and it remarkably increased with the further increase in the concentration to x=10−2. These results suggest that the crystal of x=0 already contained some defects acting as acceptors and the introduced donors were compensated by the defects up to x=10−3, and a large part of the acceptors could be removed by annealing under reducing atmosphere.