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AlN bulk single crystals grown by the physical vapor transport method may be beneficially applied as substrates for deep ultraviolet light emitting devices or as a basic material for piezoelectric resonators operating at high temperatures. Identification of point defects which deteriorate the optical, electrical, and electromechanical properties of AlN crystals for such applications is the subject of the present work. Using Raman spectroscopy, two local vibrational modes (LVMs) were discovered at wave numbers of 1189 cm−1 and 1148 cm−1. By analyzing an AlN crystal intentionally enriched with the carbon isotope 13C, it is unambiguously shown that the two LVMs originate from two different, but in each case carbon-related defects. Furthermore, it is evidenced that the defect underlying the LVM at 1189 cm−1 contains exactly two carbon atoms. The tricarbon defect-related LVM reported earlier in an infrared absorption study is found to be Raman active at 1772 cm−1. The Raman scattering intensity of all three LVMs strongly depends on the photon energy of the exciting light what is interpreted as a resonance Raman effect. This allows linking the identified defects with their contribution to the strong, carbon-related ultraviolet absorption around 4.7 eV and proves that these defects introduce optically and electrically active deep levels in the bandgap of AlN. [ABSTRACT FROM AUTHOR]

We report on the identification of a tri-carbon defect in AlN bulk crystals grown by physical vapor transport. The defect gives rise to a single infrared absorption line at 1769 cm-1 in unintentionally carbon doped crystals. This line splits into eight lines in crystals enriched with the carbon isotope 13C. The observed line patterns can unambiguously be assigned to a local vibrational mode of a defect that contains exactly three carbon atoms. The most probable arrangement of the three carbon atoms is on nearest-neighbor substitutional sites, replacing two nitrogen atoms and one aluminum atom, whereby one carbon-carbon bond is directed non-parallel and the other parallel to the crystal's c axis. It is suggested that the tri-carbon defect can exist in three different charge states (neutral, singly negative, and doubly negative) and hence possesses two transition levels within the band gap. This energy level scheme explains the appearance and disappearance of the local vibrational mode in dependence on the Fermi level position as well as a similar appearance-disappearance behavior of a strong ultraviolet absorption band at 4.7 eV that has been repeatedly reported in the literature. We propose that the singly negative charge state of the tri-carbon defect essentially contributes to that ultraviolet absorption. [ABSTRACT FROM AUTHOR]

Terbium titanate (Tb2Ti2O7) is a spin-ice material with remarkable magneto-optical properties. It has a high Verdet constant and is a promising substrate crystal for the epitaxy of quantum materials with the pyrochlore structure. Large single crystals with adequate quality of Tb2Ti2O7 or any pyrochlore are not available so far. Here we report the growth of high-quality bulk crystals using the Czochralski method to pull crystals from the melt. Prior work using the automated Czochralski method has suffered from growth instabilities like diameter fluctuation, foot formation and subsequent spiraling shortly after the seeding stage. In this study, the volumes of the crystals were strongly increased to several cubic centimeters by means of manual growth control, leading to crystal diameters of up to 40 mm and crystal lengths of up to 10 mm. Rocking curve measurements revealed full width at half maximum values between 28 and 40′′ for 222 reflections. The specific heat capacity cp was measured between room temperature and 1573 K by dynamic differential scanning calorimetry and shows the typical slow parabolic rise. In contrast, the thermal conductivity κ(T) shows a minimum near 700 K and increases at higher temperature T. Optical spectroscopy was performed at room temperature from the ultraviolet to the near infrared region, and additionally in the near infrared region up to 1623 K. The optical transmission properties and the crystal color are interpreted to be influenced by partial oxidation of Tb3+ to Tb4+. [ABSTRACT FROM AUTHOR]