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，具有较高的导热性和导电性。采用悬浮区法制备的ZrB_2晶体具有较高的熔点（3220℃），在其衬底上生长出了位错密度小于10^7 /cm^2的高质量GaN薄膜。因此，本研究对提高ZrB_2晶体的纯度和质量进行了实际应用的尝试。由商品粉末制备的晶体，由于纯度低，往往会造成一些麻烦，<99%。主要杂质有铁、碳等。例如，晶体中的碳杂质由于真空加热而在表面析出。因此，为了提高纯度，采用氧化锆与硼反应的方法合成了原料ZrO_2 + 4B。铁和碳的含量分别下降到30ppm。由于浮区生长，它们分别进一步下降到4 ppm和不到20 ppm。晶体的总纯度提高到>99.99%，足以抑制上述问题。通过与其它二硼化物晶体TiB_2、HfB_2、VB_2、NbB_2、TaB_2和CrB_2的比较，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降低。ZrB_2的蚀坑密度为5 × 10^6 /cm^2，随着生长温度降低200℃，蚀坑密度减小到原来的一半。亚晶界和裂纹的形成受热膨胀各向异性的影响。ZrB_2晶体的热膨胀系数小，各向异性也小，是二硼化物中质量最高的晶体。因此，为了进一步提高晶体质量，找到一种降低生长温度的助熔剂是非常重要的。

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.