Superconducting V$_3$Si for quantum circuit applications

TL;DR

This study investigates the structural and superconducting properties of V$_3$Si thin films on silicon and sapphire substrates, addressing challenges related to thermal strain and silicide formation for potential quantum circuit integration.

Contribution

It provides insights into the thermal stress development and identifies a process window to optimize V$_3$Si film quality while minimizing silicide formation on silicon.

Findings

Non-linear stress development during heating due to crystallization and grain growth.

Thermoelastic behavior of V$_3$Si during cooling.

A process window for V$_3$Si film thicknesses to balance quality and silicide formation.

Abstract

V$_3$Si thin films are known to be superconducting with transition temperatures up to 15 K, depending on the annealing temperature and the properties of the substrate underneath. Here we investigate the film structural properties with the prospect of further integration in silicon technology for quantum circuits. Two challenges have been identified: (i) the large difference in thermal expansion rate between V$_3$Si and the Si substrate leads to large thermal strains after thermal processing, and (ii) the undesired silicide phase VSi$_2$ forms when V$_3$Si is deposited on silicon. The first of these is studied by depositing layers of 200 nm V$_3$Si on wafers of sapphire and oxidized silicon, neither of which react with the silicide. These samples are then heated and cooled between room temperature and 860{\deg}C, during which in-situ XRD measurements are performed. Analysis reveals a highly non-linear stress development during heating with contributions from crystallization and subsequent grain growth, as well as the thermal expansion mismatch between silicide and substrate, while the film behaves thermoelastically during cooling. The second challenge is explored by depositing films of 20, 50, 100 and 200 nm of V$_3$Si on bulk silicon. For each thickness, six samples are prepared, which are then annealed at temperatures between 500 and 750{\deg}C, followed by measurements of their resistivity, residual resistance ratio and superconducting critical temperature. A process window is identified for silicide thicknesses of at least 100 nm, within which a trade-off needs to be made between the quality of the V$_3$Si film and its consumption by the formation of VSi$_2$.