Plasma-enhanced atomic layer deposition of elemental niobium thin films

We report atomic layer deposition (ALD) of superconducting elemental niobium films, where we have been able to improve the critical superconducting temperatures (Tc) of the films by optimization of deposition conditions, with Tc as low as 2 K for deposition at 300 °C and Tc up to 5.7 K for films deposited at 400 °C, which are substantially higher values than other literature reports of elemental ALD superconductors such as Ti and Al below 2 K. Our process employs plasma-enhanced atomic layer deposition using cycles of niobium (V) fluoride (NbF5) pulses and hydrogen plasma, depositing onto silicon substrates at temperatures ranging from 300 to 400 °C. X-ray photoelectron spectroscopy (XPS) depth profiling of these niobium films reveals residual concentrations of fluorine (≤5%), oxygen (≤10%), and carbon (≤9%). X-ray diffraction (XRD) scans of a film deposited at 300 °C, with oxygen content measured to be 10%, showed a small broad peak, suggesting an equiaxed niobium film consisting of nanoscale grains. Applying a radio frequency bias to the substrate during the hydrogen plasma half cycle increases the intensity of measured XRD peaks and raises the Tc of the deposited niobium films. Increasing the deposition temperature to 400 °C and increasing the hydrogen plasma exposure time led to lower oxygen concentration in the niobium film measured by XPS depth profiling and resulted in a Tc of 5.7 K, which we attribute to the reduction of niobium oxides by the hydrogen. XRD also indicated a more ordered film where the feature, attributed to (110) orientation, increased substantially relative to the peak measured for the films deposited at lower temperatures. The corresponding room temperature resistivities for the niobium thin films described in this Letter were in the range of 50–120 µΩ cm.