Growth and Interface Engineering of Superconducting TiN on Sapphire by Thermal-Laser Epitaxy

Abstract

Thermal-laser epitaxy (TLE) extends the accessible pressure, temperature, and growth-rate regimes of conventional molecular-beam epitaxy, enabling in situ laser annealing, high-purity buffer-layer growth, and efficient evaporation of refractory elements. These capabilities make TLE a promising platform for engineering low-loss superconducting resonators and Josephson-junction heterostructures. Here, we study the TLE growth of TiN on sapphire and observe improved transport properties with increasing growth temperature up to 1150 C. Unfortunately, we observe that the high ammonia pressure and elevated substrate temperatures required for optimal TiN properties promote reactions at the sapphire surface, resulting in voids at the TiN-substrate interface. These defects increase interfacial surface area, introduce dangling bonds, and could compromise tunnel-barrier heterostructures. We mitigate this degradation using an initial TiN seed layer grown at 850 C. With the seed layer, we achieve a superconducting transition temperature of 5.8 K, a residual resistivity ratio of 12.2, and a resistivity of 1.19 micro-ohm cm at 10 K, which, to our knowledge, is the lowest reported for TiN grown on sapphire.