Mitigation of Dielectric Losses in NbN Resonators Using Thermal ALD with Hydrazine

The tremendous computation advantages promised by quantum processors are increasingly of interest due to the necessity of simulating complex quantum systems. One of the main issues for practical implementation of fault tolerant quantum processors based on superconducting qubits is the quantum decoherence processes emerging from materials-inherent defects. These impurities couple with microwave fields in quantum circuitry and result in loss of information by noise and dephasing. The origin of these defects is yet to be understood, posing great urgency with the rise of quantum information science. Dielectric losses introduced by formation of native oxide on Al and Nb are hypothesized to be one of the most predominant decoherence sources. In addition, thin films of Nb and Al with non-uniform thicknesses or impurities induce variations in the superconducting gap that are prone to trapping dissipative quasiparticles, breaking cooper pairs and therefore inducing decoherence in quantum states. Nitride-based compounds such as TiN and NbN with transition temperatures above 10K ideal for the replacement of archetypal Nb and Al to mitigate these effects in superconducting qubits.<br/>Plasma assisted atomic layer deposition is potentially a powerful growth method these nitrides, their alloys, and multilayers with excellent control of composition and uniformity providing a playground for material and device design for superconducting qubit platforms. The caveat for this process is potential variations in the growth phase which depends on the substrate crystallographic orientation, voltage bias, and temperature of the deposition technique. In addition, residual AC resistivity induced by oxygen incorporation during material growth can be critical. This is especially significant with quartz or sapphire plasma tubes which self-etch during long material growth process, introducing oxygen (~3%) into the nitrides. In this work, we address this problem by developing thermal ALD processes for NbN using hydrazine (N₂H₄) for nitridation. Sequential exposures of anhydrous N₂H₄ and the organometallic Nb precursor t-Butylimido tris(diethylamino) <i>niobium</i>(V) (<i>TBTDEN) were performed at various process temperatures and conditions.</i> Optimal conditions were found at 300C with a growth rate of 0.15 A per ALD cycle. XPS analysis indicates stoichiometric NbN with minimal oxygen content in comparison to plasma assisted ALD process. STEM-EELS analysis shows polycrystalline structure with grains up to 5nm and smooth interfaces with SiO₂. The bulk resistivity of 1.5 μΩ-m is calculated from 17 nm thick NbN films which presents transition temperature comparable to Nb ideal for superconducting qubits.