Study of effect of thermal annealing on crystalline perfection of bismuth germanate single crystals grown by low thermal gradient Czochralski method

Abstract

Abstract Bismuth germanate single crystals containing low angle and very low angle boundaries were selected as specimen for annealing experiments at 1000 °C for varying lengths of time in the range: 8 h to 80 h. Specimen with larger surface along (111) and (100) planes were investigated. The tilt angles of the boundaries were found to be in the range: 16 arc sec to 88 arc sec. Half widths of the diffraction curves of different subgrains were in the range: 15 arc sec to 39 arc sec. High resolution X-ray diffractometry and topography were employed for structural characterization and secondary ion mass spectrometry (SIMS) was used for study of impurity distribution. Annealing led to significant improvement in the crystalline perfection of specimen and all but one of the boundaries could be removed. The very low angle boundaries, which could not be annealed out, had a tilt angle of 41 arc sec. The half widths of diffraction curves of the two remaining subgrains were: 10 arc sec and 17 arc sec. The growth of subgrains as a result of annealing was investigated. SIMS investigation of annealed specimen, in which a very low angle boundary persisted, showed the boundary was decorated with silicon impurity, which presumably acted as a pining center. Detailed diffractometric and topographic experiments helped in delineating the two subgrains around the very low angle boundary. The morphological features of the very low angle boundary were studied in detail.