| Research Abstract |
ZrB_2 has almost the same lattice constant and thermal expansion coefficient as those of gallium nitride, GaN, and further high thermal and electric conductivity. High quality GaN films with dislocation density of less than 10^7 /cm^2 were grown on the substrates of ZrB_2 crystals, which were prepared by the floating zone method because of high melting point, 3220℃. Therefore, in this research, improvement in the purity and quality of ZrB_2 crystals was tried for the practical use.
The crystals prepared from the commercial powders, often cause some troubles because of the low purity, <99%. The main impurities are iron, carbon and so on. For example, carbon impurity in the crystal precipitates on the surface due to heating in vacuum. Therefore, in order to improve the purity, the starting material was synthesized by reaction of zirconia and boron, ZrO_2 + 4B. The contents of iron and carbon decreased to 30 ppm, respectively. They further decreased to 4 ppm and less than 20 ppm, respectively, due to the floating zone growth. The total purity of the crystal increased to >99.99%, high enough to suppress the above problems.
Factors which determine the crystal quality were examined by comparison with the other diboride crystals, TiB_2, HfB_2, VB_2, NbB_2, TaB_2 and CrB_2. The etch pit densities decreased with decreasing the growth temperature. In the case of ZrB_2, the etch pit density was 5 x 10^6 /cm^2, which would decrease to half, decreasing the growth temperature by 200℃. The formation of the sub grain boundaries and cracks were influenced by the anisotropy of the thermal expansion. ZrB_2 crystals with the highest quality among the diborides had the small thermal expansion coefficients and the small anisothoropy. Therefore, to improve the crystal quality further, it was found to be important to find a flux which decreases the growth temperature.
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