Growth of ZrB_2 Single Crystals for Substrates for GaN

ZrB_2具有与氮化镓GaN几乎相同的晶格常数和热膨胀系数，并且具有更高的热导率和电导率。由于ZrB_2晶体的熔点较高，为3220℃，采用浮区法在ZrB_2晶体衬底上生长了位错密度小于10^7 /cm^2的高质量GaN薄膜。因此，本研究试图提高ZrB_2晶体的纯度和质量，以满足实际应用的需要。主要杂质为铁、碳等，如晶体中的碳杂质在真空中加热时会在表面析出。因此，为了提高纯度，我们采用了ZrO_2 + 4 B的方法合成了ZrO_2 + 4 B。铁和碳的含量分别降低到30 ppm。由于浮区生长，它们分别进一步降低至4ppm和小于20 ppm。晶体的总纯度提高到99.99%以上，克服了上述问题，并与其它二硼化物晶体TiB_2、HfB_2、VB_2、NbB_2、TaB_2和CrB_2进行了比较，考察了影响晶体质量的因素。腐蚀坑密度随生长温度的降低而减小。在ZrB_2的情况下，腐蚀坑密度为5 × 10^6 /cm ^2，这将减少一半，降低200℃的生长温度。亚晶界和裂纹的形成受到热膨胀各向异性的影响。在硼化物晶体中，质量最好的ZrB_2晶体具有较小的热膨胀系数和较小的晶体各向异性。因此，为了进一步提高晶体质量，发现找到降低生长温度的助熔剂是重要的。

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.