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.
Growth and Characterization of Thick GaN by Sublimation Method and Homoepitaxial Growth by Metalorganic Chemical Vapor Deposition
Thick GaN, 10-30 µm, was grown on sapphire substrate by a sublimation method for the first time. GaN was homoepitaxially grown on this thick layer by metalorganic chemical vapor deposition to obtain a high-quality layer. Stimulated emission from a photopumped homoepitaxial GaN grown on thick GaN prepared by the sublimation method at room temperature was demonstrated. The threshold of the stimulated emission was estimated to be 1.04 MW/cm2.
Brushite-collagen composites for bone regeneration.
Brushite-based biomaterials are of special interest in bone regeneration due to their biocompatibility and biodegradability; on the other hand, collagen is a well-known osteoconductive biomaterial. In the present study a new brushite-collagen composite biomaterial is reported. This new biomaterial was prepared by combining citric acid/collagen type I solutions with a brushite cement powder. The obtained biomaterial was a cement paste, with improved handling properties. The effect of collagen on the setting reaction of brushite cement was studied, and was found to speed up the cement setting reaction. The cement paste set into a hard ceramic material within 18.5+/-2.1min and had compressive strength similar to that of spongeous bone (48.9+/-5.9MPa in dry conditions and 12.7+/-1.5MPa in humid conditions). The combination of collagen with citric acid revealed an interesting synergistic effect on the compressive strength of the composite material. Moreover, this new biomaterial had excellent cohesion properties (ninefold better than brushite cement), and high cellular adhesion capacity (threefold higher than brushite cement). The composite biomaterial described in this study combines good handling properties, compressive strength, cohesion and cell adhesion capacity, along with the osteoconductive and biodegradable properties inherent in brushite and in collagen-based biomaterials.