Developments of GaN Bulk Substrates for GaN Based LEDs and LDs
In the present work, we discuss the importance of bulk GaN substrates for GaN based LEDs and LDs. As crystal growth methods for growing bulk GaN, we study the hydride vapor phase epitaxial (HVPE) method and the pressure-controlled solution growth (PC-SG) method. The recent results obtained by these two methods are presented. The maximum size of the grown single crystals was 3 × 4 cm2 for the HVPE method and 12 mm diameter for the PC-SG method. Both methods can provide high quality GaN substrates, with a low dislocation density of the level of less than 106 cm–2.
Hydride Vapor Phase Epitaxy of GaN on NdGaO3 Substrate and Realization of Freestanding GaN Wafers with 2-inch Scale
GaN thick film is grown on NdGaO3 substrate using hydride vapor phase epitaxy. To avoid decomposition of NdGaO3 substrate by NH3, a low-temperature grown GaN protective layer is adopted. Using NdGaO3 substrate and a low-temperature GaN protective layer simultaneously, 2-inch-scale GaN wafers are realized. The photoluminescence (PL) spectrum indicates strong band-edge emission without deep-level related emission. Both plan-view transmission electron microscope and cathode luminescence images suggest that the dislocation density of freestanding GaN wafers is expected to be as low as 106 cm-2.
Thermal expansion coefficients of high‐Tc superconductor substrate NdGaO3 single crystal
We report the temperature dependence of lattice constants for a NdGaO3 single crystal grown by the Czochralski method and thermal expansion coefficients which were obtained for the first time. Lattice mismatch between the NdGaO3 substrate and YBa2Cu3Ox superconducting film is 0.27% at elevated temperatures and the appropriate expansivities are similar. The crystalline quality of deposited YBa2Cu3Ox films on NdGaO3 by the laser ablation method was extremely good. NdGaO3 is, therefore, an excellent candidate for epitaxial high‐Tc superconducting films.