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.
Effect of substrate temperature on the properties of ZrB2 film on Si(111) deposited by pulsed DC magnetron sputtering
The wurtzite GaN and related nitride films are typically grown on sapphire or SiC substrates. However, large lattice-mismatches between GaN and sapphire or SiC can lead to degradation of quality of grown GaN by 16 and 4%, respectively. ZrB2 is a semi-metal compound and has a hexagonal crystal structure. The a-axis lattice constant of ZrB2 is 3.168 Å, which is almost lattice-matched to GaN. The thickness of ZrB2 film is 228 nm which was prepared on a Si(111) substrate by pulsed DC magnetron sputtering deposition. The effects of substrate temperatures ranging from 400 to 550 °C on the microstructure, resistivity and surface roughness of the deposited ZrB2 films were investigated by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), the four-point probe technique and atomic force microscopy (AFM). XRD analysis revealed that the ZrB2 film had a polycrystalline structure and the preferred orientation was along (001) when the substrate temperature was above 450 °C. The relationship between the average grain size of the ZrB2 film and substrate temperatures ranging from 450 to 550 °C forms a downward parabolic function. The minimum average grain size of the ZrB2 film was 10.6 nm when the substrate temperature was 500 °C. The minimum resistivity of the ZrB2 film was 229 µΩ cm when the substrate temperature was 550 °C. The AFM measurements showed the ZrB2 films to have a uniform morphology with a very low surface roughness value of 0.2 nm. In this work, it was found that a uniform surface morphology and preferred orientation along (001) of the ZrB2 film on Si(111) could be obtained through pulsed DC magnetron sputtering when the substrate temperature was above 450 °C. This mirror-polished ZrB2 buffer layer is a good candidate for GaN film growth on Si(111) wafers.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.