Electrical transport properties of polycrystalline TaN1- films

Abstract

Tantalum nitride with a face-centered cubic structure (TaN1-) has received much attention due to its high hardness, good wear resistance, chemical inertness, thermodynamic stability, and low temperature coefficients of resistivity. First-principles calculations have indicated that cubic-TaN possesses metallic energy band structure, and the experimental results show that the carrier concentration in TaN1- films are comparable to that of normal metals. However, semiconductor-like temperature behavior of resistivity is often observed in polycrystalline TaN1- film. In the present paper, we systematically study the crystal structures and electrical transport properties of a series of TaN1- thin films, deposited on quartz glass substrates at different temperatures by the rf sputtering method. Both X-ray diffraction patterns and scanning electron microscope images indicate that the films are polycrystalline and have face-centered cubic structure. It is also found that the mean grain sizes of the films gradually increase with increasing depositing temperature. The temperature dependence of resistivity is measured from 350 K down to 2 K. The films with large grain sizes have a superconductor-insulator transition below ～ 5 K, while the films with small grain sizes retain the semiconductor characteristics down to the minimum measuring temperature, 2 K. These phenomena are similar to that observed in superconductor-insulator granular composites. Above 5 K, the temperature coefficients of the resistivities of the films are all negative. In the temperature range between 10 and 30 K, the films show hopping transport properties which are often seen in metal-insulator granular systems, i. e. the logarithm of the resistivity (log ) varies linearly with T-1/2, where T represents the measured temperature. The thermal fluctuation-induced tunneling conductive mechanism dominates the temperature behaviors of resistivities from 70 K up to 350 K. It can be seen that the thermal fluctuation induced tunneling conductive mechanism is also the main conductive mechanism in metal-insulator granular systems in the higher temperature regions. Our results indicate that the electrical transport properties of the polycrystalline TaN1- films are similar to that of metal-insulator granular films with different volume fractions of metal, where the metal possesses superconductivity at low temperatures. Hence the high resistivity and negative temperature coefficient of resistivity of TaN1- polycrystalline film can be reasonably ascribed to the similarity in microstructures between TaN1- polycrystalline film and metal-insulator granular film.