多晶TaN_(1-δ)薄膜的电输运性质研究

Electrical transport properties of polycrystalline TaN_(1-δ) films

利用射频溅射法在石英玻璃基底上制备了一系列面心立方结构的多晶TaN1-δ薄膜,并对其晶体结构和2—350 K温度范围的电子输运性质进行了系统研究.薄膜呈多晶结构,并且平均晶粒尺寸随着基底温度的升高逐渐增大.电输运测量结果表明,TaN1-δ薄膜在5 K以下表现出类似超导体-绝缘体颗粒膜的电输运性质;随着温度的升高,薄膜在10—30 K表现出类似金属-绝缘体颗粒膜的性质;在70 K以上,热涨落诱导的遂穿导电机理主导着电阻率的温度行为.我们的结果表明:TaN1-δ多晶薄膜的类颗粒膜属性使其具有较高的电阻率和负的电阻温度系数.

Tantalum nitride with a face-centered cubic structure （TaN1-δ） has received much attention due to its high hard-ness, 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 experi-mental results show that the carrier concentration in TaN1-δ films are comparable to that of normal metals. However, semiconductor-like temperature behavior of resistivity is often observed in polycrystalline TaN1-δ film. In the present paper, we systematically study the crystal structures and electrical transport properties of a series of TaN1-δ 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 （logρ） 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-δ 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-δ polycrystalline film can be reasonably ascribed to the similarity in microstructures between TaN1-δ polycrystalline film and metal-insulator granular film.