不同溅射条件对超薄外延氮化铌薄膜超导性能的影响

Influence of different sputtering conditions on superconductivity of ultra-thin epitaxial niobium nitride film

氮化铌(NbN)由于其较高的超导转变温度、较窄的转变宽度以及良好的稳定性一直被广泛地应用于低温超导器件中.因此,生长高质量的超导Nb N薄膜是制备高性能薄膜超导器件的关键及难点.本文主要介绍了利用磁控溅射法在氧化镁(MgO)<100>基底上生长厚度为5 nm的超薄超导外延NbN薄膜,并系统分析了溅射参数对薄膜性能的影响.实验结果表明,在合适的氮氩比或功率下,高真空、高溅射温度、低工作气压与NbN薄膜超导转变温度成正相关.采用综合物性测量系统(physical property measurement system, PPMS)进行电学分析,所制备的NbN薄膜超导最高转变温度为12.5 K.该工作为进一步研究超薄NbN薄膜的生长提供了理论指导,也为后期在其他基底上制备NbN超导器件提供了基础.

Superconductors are used in military and commercial applications due to the full diamagnetic and zero resistance properties. Hereinto, niobium nitride(NbN) has been extensively studied for its excellent superconducting properties, such as high transition temperature(Tc), large superconducting energy gap, narrow transition width and good stability.Superconducting NbN films are widely used in low temperature superconducting devices, for instance, hot electron bolometer, superconducting quantum interference devices and rapid single flux quantum. Especially, in 2001 Gol’tsman has successfully detected a single photon response of 810 nm near-infrared using an ultra-thin superconducting NbN,which proved that the high-quality growth of ultra-thin NbN film was the key step for high resolution and sensitivity of the superconducting detector. Up to now, atomic layer deposition(ALD), molecular beam epitaxy(MBE), pulsed laser deposition(PLD) and magnetron sputtering have been successfully used to grow NbN thin films. However, Tcis decreasing gradually as the reduced of thickness. Therefore, there is still a challenge to grow ultra-thin superconducting NbN film with high performance.In this work, 5 nm thickness of superconducting epitaxial NbN film has been grown on MgO<100> substrate by magnetron sputtering. We systematically study the effect of different sputtering parameters on the superconductivity properties of the NbN film. The experiment results show that the high vacuum, high sputtering temperature and low working pressure are positively correlated with the superconducting transition temperature of NbN film under the premise of suitable argon-nitrogen ratio or magnetron power. The analyses of physical property measurement system(PPMS)indicate that as-grown 5 nm-thick NbN film has high performance with a superconducting transition temperature as high as12.5 K under the optimum condition. Morphologic characterizations show that the ultra-thin NbN films have high density and smoothness with 0.116 nm of the surface roughness. Microstructural measurements also illustrate the epitaxial growth of NbN on MgO substrate. This work demonstrates the ultra-thin NbN films can be well controllably grown with high quality and offers a platform to fabricate the ultra-thin superconducting devices in the coming future.