Effect of heat treatment on properties of melt-grown bulk In2O3 single crystals

Abstract

Melt-grown bulk In₂O₃ single crystals were utilized for systematic heat-treatment experiments, which we performed at temperatures of 200–1400 °C, for 2–300 h and in eight different atmospheres, from oxidizing through vacuum to reducing atmospheres. We found that melt-grown In₂O₃ single crystals are chemically stable up to 1200–1300 °C in neutral and oxidizing atmospheres and up to 1000 °C in vacuum but they readily decompose at 600 °C in a highly reducing atmosphere. Electron concentrations of as-grown In₂O₃ single crystals were just above 10¹⁸ cm⁻³, which decreased by one order of magnitude after annealing under non-reducing conditions. The electron mobility stayed between 140–180 cm² V⁻¹ s⁻¹ as long as no decomposition occurred. On the other hand, annealing of the melt-grown In₂O₃ crystals in hydrogen-containing atmosphere led to an increase of the electron concentration by up to two orders of magnitude (10²⁰ cm⁻³) accompanied by a mobility drop. Moreover, annealing conditions were identified, within which electron concentrations can be tuned quite precisely. Transmittance spectra revealed a sharp absorption edge at 440 nm. High transparency in the visible range was observed after annealing under non-reducing conditions, whereas in the near infrared region free-carrier absorption played a role in case of high electron concentrations. Further, a broad emission peak has been found by cathodoluminescence at RT in all melt-grown In₂O₃ crystals, which is independent of any annealing conditions.