Speaker
Description
Hafnium (Hf) is a superconducting material that has been gaining popularity among the superconducting detector community – for e.g. TES bolometers (Rotermund et al. in prep), TES calorimeters (Lita et al. 2009, Safonova et al. 2024), optical and near-IR MKIDs (Zobrist et al. 2019, Coiffard et al. 2020), phonon-sensitive MKIDs (Li et al. in prep), STJ (STAR Cryoelectronics SBIR awarded 2022), and QPDs (Ramanathan et al. 2024). Hf is an attractive superconducting film for many reasons, including that its bulk critical temperature (Tc) is near 128 mK, the London penetration depth is estimated to be 20 nm [Kraft et al. 1998], and the surface kinetic inductance is high at around 15-20 pH/◻ for a 125 nm film [Coiffard et al. 2020], thus making it well-matched to needs across many experiments.
Here we present a study of hafnium’s material properties that lends itself to being a good superconductor across many detector efforts. We investigate empirical relations of critical temperature, normal resistance, internal quality factor, and how they in turn affect the kinetic inductance. We consider film properties such as the phases present through XRD measurements and how they are affected by film thickness and deposition temperature.
One key difference between past Hf detector efforts and our own, is our use of a heated sputter deposition. The heated sputter deposition has 2-fold consequences 1) it enables us to precisely tune Tc to our desired target value due to its linear dependence on deposition temperature and 2) it ensures that the Tc is robust against subsequent exposure to heat as long as the initial deposition temperature is not exceeded.
While Hf is an attractive superconductor for many detector applications, we observed that making reliable electrical contact requires a detailed understanding of the Hf interface. We discuss the challenges and solutions for the superconducting interfaces with films such as niobium, aluminum, and niobium nitride.
