Properties of NbTiN thin films prepared by reactive DC magnetron sputtering

Abstract

We have prepared Nbl-,Ti,N (NbTiN) thin films by reactive dc magnetron sputtering without intentional heating. Superconducting properties were strongly related to sputtering conditions. Lattice parameters of NbTiN films approached that of bulk NbTiN with decreasing the Nz mole fraction in Ar and Nz sputtering gas mixture. The film orientation was also strongly related with the sputtering conditions such as gas pressure. NbN thin films could grow epitaxially on MgO(100) substrates and showed very smooth surfaces. We found that smooth NbTiN films could be obtained on MgO(100) substrates with epitaxially grown NbN template layer.Indez T e m sNbl-,Ti,N, film orientation, surface morphology, epitaxial template layer. I. INTRODUCT~ONIOBIUM -titanium nitride, NliI-,Ti,N (NbTiN), N has a transition temperature between 1GK and 17K[1], [2], is one candidate of the superconducting materials for the superconducting circuits operated at 10K using a cryocooler [3]. This material is also very attractive for THz applications because of its large energy gap and low surface resistance[4], [5]. For both applications, it is necessary to deposit films with smooth surfaces and high transition temperatures.One possible way to deposit smooth films is epitaxial growth on a lattice-matched substrate. It is well known that NbN can be grown epitaxially on an MgO(100) single crystal substrate without intentional heating[G], [7].NbTiN has the same lattice structure of MgO and lattice parameter between 0.438nm (x=O) and 0.424nm (x=l), close to 0.421nm for MgO. This seems to lie very siiitable for preparing epitaxially grown NbTiN thin films on MgO(100) substrates. It has been, however, shown that the deposition of high quality NbTiN films requires the stimulation of surfnce diffusion during growth [8].In this study, we have prepared NbTiN thin films on both MgO(100) and Si(100) substrates by reactive dc magnetron sputtering without intentiond heating. We have examined NbTiN film orientation and surface morphology. An NbN template layer has been used for preparing smooth NbTiN thin films. Takeshi Iiziika and Susumii Takada EXPERIMENTALNbTiN thin films were prepared by reactive dc magnetron sputtering in a mixture of Ar and Nz. Dc splittering system had three targets in the sputtering chamber. We used a Cinch-diam Nb target (99.95% purity) and NliI-,Ti, (x=0.34, 0.62) alloy targets (99.9% piirity). Typical sputtering conditions are listed in Talile I. An arc control unit was used to stabilize the discharge state. The unit reversed the applied dc voltage for 5ps at 3OkHz. A substrate holder was floated from the ground to promote the stimulation of surface diffiision by s u b strate self-bias[8]. The measmred floating substrate self bias was around -4OV.After film deposition, we examined the orientation of the films using an X-ray diffraction (XItD) method and the in-plme orientation by reflection high-energy electron diffraction (RHEED). We observed surface morphology of the films by atomic force microscopy (AFM). Resistivity-temperature curves w...

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