Growth of Niobium Nitride Whiskers by Chemical Vapor Deposition and Superconducting Properties

Abstract

The growth of niobium nitride(NbN) whiskers on various substrates from the NbCl₄N₂-H₂ system was investigated in the temperature range of 900∼1400°C, and the superconducting transition temperature (T_c) of the whiskers was measured. Ni/graphite, (Pd +Si)/mullite, and Pd/quartz were effective for the growth of NbN whiskers. In the growth on Ni/graphite substrate, pillar or needle whiskers grew with the maximum axial growth rate of ca.1 mm/h. The tip of pillar whiskers were surrounded with facets, while the tip of a needle whisker was semisphere which contained Ni. Needle whiskers are considered to grow by the VLS mechanism. Besides NbN whiskers, NbN_xC_1-x whiskers grew on Ni/graphite. In the case of a Pd/quartz substrate, pillar niobium nitride whiskers grew, the phases and growth rates of which changed with the gas vapor compositions. With a composition [NbCl₄]=1.0∼1.5 vol%, [N₂]=30∼70 vol%, and [H₂] =10 vol%, the deposit was a pure δ-NbN phase. The axial growth rates showed a maximum for [NbCl₄] or [H₂]. The T_c of the whisker grown on Pd/quartz was 15.3 K, while the whisker grown on Ni/graphite showed two-step transitions at ca.15 K and 17.3 K.