Characterization of sputtered hafnium thin films for high quality factor microwave kinetic inductance detectors

Abstract

Hafnium is an elemental superconductor which crystallizes in a hexagonal close packed structure, has a transition temperature T C 400 mK, and has a high normal state resistivity around 90 µΩ cm. In Microwave Kinetic Inductance Detectors (MKIDs), these properties are advantageous since they allow for creating detectors sensitive to optical and near infra-red radiation. In this work, we study how sputter conditions and especially the power applied to the target during the deposition, affect the hafnium T C , resistivity, stress, texture and preferred crystal orientation. We find that the position of the target with respect to the substrate strongly affects the orientation of the crystallites in the films and the internal quality factor, Q i , of MKIDs fabricated from the films. In particular, we demonstrate that a DC magnetron sputter deposition at a normal angle of incidence, low pressure, and low plasma power promotes the growth of compressive (002)-oriented films and that such films can be used to make high quality factor MKIDs with Q i up to 600,000.

Material matters in superconducting qubits

“…hafnium [185], indium [186], molybdenium-rhenium alloys [187], compounds that exhibit the A15 crystal structure [188] and even heavily-doped silicon [189], that have been demonstrated or proposed for use in quantum computing. While some of these materials also possess superconducting transition temperatures above 10 K, the primary motivation of their incorporation is not necessarily associated with Tc, since the temperature necessary to reach the qubit ground state (kBT << hf) is substantially lower.…”

The impact of kinetic inductance on the critical current oscillations of nanobridge SQUIDs

“…L K scales linearly with the nanobridge's aspect ratio L/W . This tunability and magnitude of L K , together with its nonlinearity make our MoGe nanobridges extremely suitable for many applications, ranging from magnetic memories [11] to microwave detectors [12,13]. Furthermore, the measured L K can be maximized further by limiting the device thickness.…”

“…Moreover, the kinetic inductance is nonlinear in both current and temperature. These unique properties of high kinetic inductance nanobridges and nanowires result in their application as scalable key elements in many recently demonstrated device applications ranging from single-photon detectors [4] to qubit readout and qubit architectures [5][6][7][8][9], magnetic memories and sensors [10,11] and superconductor microwave detectors [12,13]. Despite the technical relevance and many applications of high kinetic inductance devices, it is complicated to precisely measure the kinetic inductance value.…”

Model and Measurements of an Optical Stack for Broadband Visible to Near-IR Absorption in TiN KIDs

“…For β -Ta, TiN, PtSi, and Hf absorption is limited to around 50% in the visible and 30% in the IR range, as illustrated by Fig. 1 and presented in [2]. To increase the power absorbed in the KID, we embed it in an optical stack consisting of a back short (cavity) and one or multiple matching layers.…”