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.
GaN films annealed under high pressure
Abstract We present in this manuscript the results of a comparison study between MOCVD-grown GaN films annealed in a novel high-pressure annealing furnace and those annealed in a conventional tube furnace under flowing argon gas at similar temperatures for similar times. Atomic force microscopy, scanning electron microscopy, X-ray diffraction, photoluminescence and cathodoluminescence were used to investigate the affects of the high-pressure anneals in comparison to those which were annealed with the more conventional method. We demonstrate that high pressure annealing has successfully prevented the degradation of GaN films at high temperatures.
Epitaxial films of YBa2Cu3O7−δ on NdGaO3, LaGaO3, and SrTiO3 substrates deposited by laser ablation
Frequency‐doubled Nd:YAG laser (532 nm) pulses of 1.7 J/cm2 and 10 ns duration were used to deposit thin films of YBa2 Cu3 O7−δ by laser ablation on NdGaO3 , LaGaO3, and SrTiO3 substrates held at 725±5 °C in 0.2 Torr of O2 ambient. Electrical resistivities versus temperature of all films show normal metallic behavior and sharp superconducting transitions with Tc (R=0) at 92–93 K. Critical current densities in 0.3–0.6 μm thick, 200 μm long, and 10–30 μm wide strips were measured to be 106 A/cm2 at 60, 77, and 80 K in the films on LaGaO3, NdGaO3, and SrTiO3, respectively. X‐ray diffraction patterns show that all films grew epitaxially, with domains of only two crystalline orientations rotated 90° with respect to each other in the a‐b plane (consistent with twins), and the c axis perpendicular to the substrates. The closely matched lattice constants of the film and substrates (0.8–2.1%) result in epitaxial growth of the films.