Epitaxial growth of rhenium with sputtering

Abstract

We have grown epitaxial Rhenium (Re) (0001) films on α-Al₂O₃ (0001) substrates using sputter deposition in an ultra high vacuum system. We find that better epitaxy is achieved with DC rather than with RF sputtering. With DC sputtering, epitaxy is obtained with the substrate temperatures above 700 °C and deposition rates below 0.1 nm/s. The epitaxial Re films are typically composed of terraced hexagonal islands with screw dislocations, and island size gets larger with high temperature post-deposition annealing. The growth starts in a three dimensional mode but transforms into two dimensional mode as the film gets thicker. With a thin (∼2 nm) seed layer deposited at room temperature and annealed at a high temperature, the initial three dimensional growth can be suppressed. This results in larger islands when a thick film is grown at 850 °C on the seed layer. We also find that when a room temperature deposited Re film is annealed to higher temperatures, epitaxial features start to show up above ∼600 °C, but the film tends to be disordered.

Introduction

Epitaxial superconducting films of refractory metals are a promising new template for single crystal tunnel barriers in Josephson junction quantum bit (qubit) devices [1]. In existing Josephson junction qubits, it is believed that the widely used amorphous AlO_x tunnel barriers have undesirable two-state fluctuators. It is speculated that single-crystal tunnel barriers such as sapphire (α-Al₂O₃) may be free of such decoherence sources [1]. The refractory metals are appealing because preparation of a single-crystal tunnel barrier requires an epitaxial base electrode of high melting temperature with a good lattice match to the tunnel barrier. Along this line, Re is a good candidate because it has a very high melting temperature (3186 °C) and a hexagonal close packed (hcp) structure with a very good lattice match (a = 0.276 nm) to the oxygen sublattice (a = 0.277 nm) of α-Al₂O₃ (0001) [2]. Re also has a reasonably high superconducting critical temperature (T_c = 1.7 K) [3], which is compatible with the present qubit technology [1]. In addition, according to the free energy of oxide formation under the same oxidation condition, Re has much weaker tendency for oxidation than do most other elemental superconductors such as Nb [4], and thus a sharp interface between the base layer and the oxide tunnel barrier will be relatively easy to achieve with Re.

The most common epitaxial growth technique for refractory metals such as Re is the electron-beam (e-beam) based molecular beam epitaxy (MBE) technique [5], since the popular Knudsen cell based MBE [6] is not compatible with the high melting temperatures of refractory metals. On the other hand, the dominant thin film deposition method for device fabrication is sputtering [7]. While there are reports of epitaxial growth of Re by e-beam evaporation in the literature [3], [8] and epitaxial growth of other metals by sputtering [7], in this work we give a detailed report on the epitaxial growth of Re using sputter deposition technique. We describe the relationship between epitaxy and growth parameters using various analysis tools such as reflective high energy electron diffraction (RHEED), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), atomic force microscopy (AFM) and scanning tunneling microscopy (STM).