Epitaxial superconducting films of refractory metals are a promising new template for single crystal tunnel barriers in Josephson junction quantum bit (qubit) devices. In existing Josephson junction qubits, it is believed that the widely-used amorphous AlOx 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.
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. Rhenium (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 = 2.76 Å) to the oxygen sublattice (a = 2.77 Å) of α-Al2O3 (0001). Re also has a reasonably high superconducting critical temperature (Tc = 1.7 K), which is compatible with the present qubit technology.
The most common epitaxial growth technique for refractory metals such as Re is the electron-beam (e-beam) based molecular beam epitaxy (MBE) technique, since the popular Knudsen cell based MBE 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. The relationship between epitaxy and growth parameters is explained using various in situ and ex situ 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).
The films were all grown in an ultrahigh vacuum (UHV) sputtering chamber and transferred into various analysis chambers for the RHEED, LEED, AES and STM studies without breaking vacuum. Base pressure of the system is about 1×10-10 Torr, and the system is composed of three isolated chambers and a load-lock. The sample is transferred between chambers using magnetically driven sample transfer rods. Among the three UHV chambers, the first one is dedicated to Re sputtering, equipped with a radiative sample heating stage (maximum 850°C continuous), and pumped by a turbomolecular pump (TMP). The other two chambers are each pumped by an ion pump and a titanium sublimation pump in addition to a TMP, and are used for RHEED, LEED, AES and STM analysis. The epitaxy of the films was checked by RHEED and LEED, and the morphology studied by use of in situ STM and ex situ AFM (tapping mode).
RHEED and LEED are taken with 5.0 keV and up-to 1 keV e-beams, respectively. AES was used to check the cleanliness of the substrate and impurity levels of the film.
Sputtering was performed in an Ar environment (~5 mTorr) by use of a magnetron sputtering gun that is capable of both DC and RF operation and is fitted with a 1" diameter Re target (99.9 % purity). AES on the as-sputtered films showed no trace of Ar. This shows that Ar incorporation is negligible.
The substrate was an epi-ready α-Al2O3 (0001) cut from a commercial c-plane sapphire wafer. The substrate showed atomically flat terraces as measured by AFM. The substrate was scratch-free and the measured root mean square (RMS) roughness was about 2 Å. After ultrasonically degreasing the substrate in acetone and isopropyl alcohol, it was then cleaned in situ by annealing at 850°C for one hour. There was no measurable trace of carbon or any other contaminants in the AES spectrum.
As a first step for the epitaxial growth, RF and DC operation were compared using the same sputtering gun. Figures (a) & (b) show the difference between RF and DC sputtered films grown both at 850°C, respectively. The DC sputtered film shows better epitaxy even with a higher deposition rate than the RF sputtered film. AFM of these samples shows that the island sizes are much larger in DC sputtered films (> 100 nm in diameter) than for RF sputtered films (< 50 nm in diameter). In addition, DC sputtered films show atomic step terraces, while RF sputtered films show no terraces within the AFM resolution. Finally, the DC sputtered films show sharper hexagonal LEED patterns than the RF sputtered films.
This work was done by Seongshik Oh, Dustin A. Hite, K. Cicak, Kevin D. Osborn, Raymond W. Simmonds, Robert McDermott, Ken B. Cooper, Matthias Steffen, John M. Martinis, and David P. Pappas for the National Institute of Standards and Technology (NIST). NIST-0001