Seiji Sarayama

Conditions for seeded growth of GaN crystals by the Na flux method

Abstract Conditions for seeded growth of GaN crystals by the Na flux method were investigated. Seeded growth in the Na flux could be achieved under temperature–N2 pressure conditions between the condition of GaN formation without seeding and the condition of GaN decomposition. GaN single crystals with a maximum area of 3.0×1.5 mm2 and thickness of 1.0 mm were grown from seeds at 850 °C and 2 MPa of N2 for 200 h. The crystal growth occurred selectively on the (0001) Ga-polar surface of the platelet seed crystals. The growth rate in the c direction was higher than those of other directions and increased with growth temperature and N2 pressure. The maximum growth rate in the c direction was about 4 μm/h at 850 °C and 2 MPa of N2.

Influence of 3d-Transition-Metal Additives on Single Crystal Growth of GaN by the Na Flux Method

The influence of 3d-transition-metal additives (Cr, Mn, Fe, Co and Ni) on crystal growth of GaN at 750–850°C by the Na flux method was investigated. In the case of Cr addition, CrN single crystals were deposited and no significant difference in the crystal growth of GaN was found. In the case of Mn addition, growth of GaN single crystals in the -c direction was enhanced and prismatic crystals were obtained. Mn of 0.10–0.35 at.% was contained in GaN crystals. The Mn-doped GaN crystals were red or orange in color and showed Curie-like paramagnetism. Fe, Co and Ni additives also enhanced the growth of prismatic crystals. These transition-metal elements as well as Cr were not detected in the GaN crystals by inductively coupled plasma (ICP) atomic emission spectroscopy. Colorless transparent prismatic crystals having 1.5 mm length were prepared using the Ni additive.

Time Dependence of the Growth Morphology of GaN Single Crystals Prepared in a Na?Ga Melt

The yields of GaN prepared in a Na–Ga melt at 700–800°C and 1–5 MPa of N2 for 200 h were measured. The morphology of the GaN crystals changed from pyramidal (yields 6–13%) to prismatic (yields 19–100%), and finally to thin platelets (yields 61–100%) with increasing temperature and N2 pressure. A time dependence of the morphology was observed for the sample prepared at 750°C and 5 MPa of N2. The morphology changed from pyramidal, prismatic to thick platelets with heating times up to 50 h. The yield of GaN increased linearly during this period. The formation rate of GaN increased after 50 h, and the crystal growth perpendicular to the c axis was enhanced. The crystal growth was completed within 200 h, and thin platelet single crystals with a size of 1–2 mm were formed. Microphotoluminescence spectra were measured at the cross section of a thin platelet GaN crystal. A large broad luminescence peak at 3.26 eV, probably associated with Mg or Si acceptors, was observed in the spectra obtained from the regions near the (0001) Ga polar plane.

Dissolution and Recrystallization of GaN in Molten Na

The dissolution mass of GaN in molten Na at 650–800°C and 3–5 MPa of N2 was determined by weight loss of polycrystalline GaN fragments immersed in Na and by chemical analysis of the amount of Ga dissolved in Na. The dissolution of GaN in Na was equilibrated within 100 h and the dissolution mass increased with increasing temperature but decreased with increasing N2 pressure. It was found that the addition of a small amount of Li3N into the Na melt increased the dissolution mass of GaN. Transparent platelet single crystals of GaN with sizes over 300 µm were grown from nutrient GaN powder by cooling a Na solution with a Li3N additives from 800°C to 700°C at 1°C/h.