Growth of High Purity Crystals of Niobium Triselenide

本研究的目的是制备高纯度的准一维导体三硒化铌单晶。它们用于研究低缺陷浓度介质中电荷密度波的动力学性质。在仔细选择气相反应生长的起始材料和石英管后，在不同的条件下重复晶体生长。使用了纯度为99.9997%的Nb。0.2-0.4 mm厚度的板材在高真空中进行退火热处理，以降低氧含量。所用硒的纯度为99.9999%。使用低浓度铁和铜的石英管。在密封起始材料之前，石英管在高温和高真空中进行退火热处理。为了密封管道，建造了清洁的真空系统，使用了一个冷冻泵和一个涡轮分子泵，后者由于使用了磁轴承而没有残留的油气。在650-700℃和合适的温度梯度下，沿石英管生长出横向尺寸为几十微米的较大晶体。当原料用量控制在0.01摩尔时，残余电阻比小于250，且其大小随样品长度的变化而变化。研究发现，残余电阻比与石英管中碱/羟基杂质的浓度无关。为了生长大晶体，需要适当大小的温度梯度；当使用温度均匀的高温炉时，只能生长出微小的晶体。

The purpose of this study is to prepare high purity single crystals of niobium triselenide, the typical quasi-one dimensional conductor. They are used for the study of the dynamical properties of charge density waves in media with low defect concentrations. Crystal growth were repeated under various conditions, after careful selection of startinal materials and quartz tubes for gas phase reaction growth. Niobium of 99.9997% purity was used. Sheets of 0.2-0.4 mm thickness were annealed in high vacuum to reduce the oxygen content. Selenium used was of 99.9999% purity. Quartz tubes, with low concentration of Fe and Cu were used. Before sealing the starting materials, quartz tubes were annealed at high temperature and in high vacuum. Clean evacuation system was constructed for sealing the tube, using a cryo-pump and a turbo-molecular pump, the latter being free from residual oil gas because magnetic bearings are used. Relatively large crystals, with the transverse dimensions several tens of micron, were grown at 650-700C with a suitable temperature gradient along the quartz tube. As long as the quantity of starting materials were kept to 0.01 mole, the residual resistance ratio was lower than 250, and its magnitude shows a variation along the sample length. It has been found that the residual resistance ratio has no relation to the concentration of alkaline/OH impurities of quartz tubes. To grow large crystals, suitable magnitude of temperature gradient is necessary; only tiny crystals have been grown when a furnace with high temperature homogeneity was used.