This is a study of the growth of titanium-nitride (TiN) by plasma assisted molecular beam epitaxy (MBE) for applications in low loss quantum circuits. Titanium nitride is a material known to have low loss at microwave frequencies which sees practical use in many emerging quantum device architectures. The goal of this research is to investigate improvements in TiN thin films utilizing MBE to produce highly pure, highly crystalline films. A review of the theory of superconducting qubits is given and recent developments in superconducting qubit research are summarized to elucidate how loss has been reduced historically and the importance of materials improvements to the future of scalable quantum computation. The dynamics of epitaxial film growth are discussed, focusing on the formation of a minimum energy surface, the thermally driven kinetics of atoms deposited on the surface, and how these lead to the formation of large scale crystalline domains in the film. An overview of the MBE growth chamber is given and the growth procedure for TiN is discussed. Transport and morphology measurement results are discussed for MBE TiN where, for optimized growth conditions, films are highly crystalline with superconducting transition temperatures(Tc) as high as Tc = 6.1K, matching the value for bulk crystals and exceeding all reported thin film values. Microwave loss measurements of transmission line resonators fabricated from these films are shown to have low-power quality factors(Q) as high as 200,000 and an increase to Q = 400,000 at high power, consistent with the saturation of two-level-systems (TLS) at high power. These results confirm a connection between the high quality crystalline materials produced in this work and low loss in quantum circuits.
Titanium nitride (TiN) is a known superconducting material that is attractive for use as passive components in superconducting circuits for both conventional and quantum information devices. In contrast to conventional synthesis techniques, here, plasma-assisted molecular beam epitaxy is reported to produce high-quality TiN on bare silicon wafers. Using a rf-plasma source to crack the nitrogen molecules and a conventional high-temperature effusion cell for titanium, TiN growth is completed under nitrogen-rich conditions. The growth and nucleation is monitored in situ, while the structure and composition are characterized using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and scanning transmission electron microscopy. The stoichiometric TiN (111) films sit on an amorphous nitride layer with low impurity concentrations. The films superconduct with Tc=5.4 K, and coplanar waveguide resonators are fabricated with a small center width of 6 μm that demonstrate single-photon quality factors approaching 1M and high-power quality factors over 5M without observing saturation.
Titanium nitride is an attractive material for a range of superconducting quantum-circuit applications owing to its low microwave losses, high surface inductance, and chemical stability. The physical properties and device performance, nevertheless, depend strongly on the quality of the materials. In this paper, we focus on the highly crystalline and epitaxial titanium nitride thin films deposited on sapphire substrates using magnetron sputtering at an intermediate temperature (${300}^{\ensuremath{\circ}}\mathrm{C}$). We performed a set of systematic and comprehensive material characterizations to thoroughly understand the structural, chemical, and transport properties. Microwave losses at low temperatures were studied using patterned microwave resonators, where the best internal quality factor in the single-photon regime is measured to be $3.3\ifmmode\times\else\texttimes\fi{}{10}^{6}$ and that in the high-power regime $>1.0\ifmmode\times\else\texttimes\fi{}{10}^{7}$. Adjusted with the material filling factor of the resonators, the microwave loss-tangent here compares well with the previously reported best values for superconducting resonators. This work lays the foundation of using epitaxial titanium nitride for low-loss superconducting quantum circuits.
Using molecular beam epitaxy, we have realized thin films of titanium nitride (TiN) on c-plane sapphire that exhibit the lowest observed full-width at half maximum X-ray rocking curve width of 18 arcsec. Though the (111) oriented TiN exhibits an abrupt and crystalline interface with sapphire, for the first time we observe sub-surface defects in the sapphire substrate, which nucleate structural defects in the epitaxial TiN layer. Using quarter-wavelength coplanar waveguide (CPW) resonators in a 3 \textmu m/6 \textmu m/3 \textmu m gap/strip/gap lines in a hanger geometry, we find the internal quality factor of the TiN resonators to be
>106 in the single-photon
⟨n⟩∼1 limit at 5.8 GHz and 10 mK, rising to
>20×106 at
⟨n⟩∼106. The results are of high interest for applications of superconducting TiN in several areas, and provide a path towards epitaxial Josephson junctions with crystalline barriers in the future for high coherence qubits.
We report on the fabrication and characterization of superconducting coplanar waveguide (CPW) resonators based on titanium nitride (TiN) thin films deposited on 200\,mm diameter high-resistivity Si(100) substrates. We systematically investigate how deposition conditions, dry-etch power and in-situ resist strip temperature affect morphology, superconducting properties and dielectric losses. By tuning reactive sputtering conditions, three distinct preferred out-of-plane crystal orientations - (111), (200), and a mix of both are achieved. Our results demonstrate that all films exhibit similar minimal two-level system (TLS) losses, with TiN111 exhibiting the lowest median TLS losses
δ~TLS, and greater robustness against reoxidation. The applied structuring process, in contrast, has a far greater influence on the TLS loss than the crystal orientation of the TiN film and, consequently, the intrinsic material properties of the superconducting layer. The lowest TLS losses for all TiN depositions were achieved with a low power etch and low temperature resist strip. An additional buffered oxide etch (BOE) treatment could remove high-loss interfacial oxides at the metal-air (MA) and substrate-air (SA) interface and recover the etch-induced TLS losses. Consequently, TiN resonators exhibiting
δ~TLS values as low as
9.67×10−7 were realized. The corresponding median low-power loss,
δ~LP, amounts to
11.04×10−7, which translates to an internal quality factor approaching one million. These findings highlight the critical role of process induced oxide formation at the MA and SA interfaces in limiting the performance of TiN resonators and provide a scalable, low-loss process compatible with industry-grade 200\,mm CMOS qubit fabrication workflows.
Abstract Low microwave loss and regulatable kinetic inductance are two key characteristics of superconducting films to manufacture high-performance quantum devices. Epitaxial titanium nitride (TiN) thin films have been found to exhibit significant advantages in these two aspects. In this paper, we investigated into the dielectric loss and kinetic inductance of epitaxial TiN films, which were prepared on both silicon and sapphire substrates with different temperatures. Crystal structure, surface morphology, and superconducting properties were characterized systematically. The radio-frequency response of microwave resonators made of TiN films was studied at low temperatures to extract the information of kinetic inductance and quality factor. We obtained the highest internal quality factor up to 1.6 × 10 6 at single photon regime and achieved a regulation of kinetic inductance exceeding 16 times by altering the substrate and the sputtering temperature of the TiN films. This work not only provides an essential foundation for the precise design and preparation of low-loss TiN resonators for superconducting quantum bits, but also demonstrates TiN film as an excellent material platform with both low microwave loss and high kinetic inductance that can be applied in other fields such as microwave kinetic inductance detectors.
Motivated by recent advancements highlighting
Ta as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have explored the effect of cryogenic temperatures on the growth of
Ta and its integration in superconducting circuits. Cryogenic growth of
Ta using a low-temperature molecular beam epitaxy (MBE) system is found to stabilize single-phase $\ensuremath{\alpha}$-
Ta on several different substrates, which include
Al2O3(0001),
Si(001),
Si(111),
SiNx, and
GaAs(001). The substrates are actively cooled down to cryogenic temperatures and remain 20 K during the
Ta deposition. X-ray $\ensuremath{\theta}$-$2\ensuremath{\theta}$ diffraction (XRD) after warming to room temperature indicates the formation of polycrystalline $\ensuremath{\alpha}$-
Ta. The 50-nm $\ensuremath{\alpha}$-
Ta films grown on
Al2O3(0001) at a substrate manipulator temperature of 7 K have a room temperature resistivity (${\ensuremath{\rho}}_{300\phantom{\rule{0.1em}{0ex}}\mathrm{K}}$) of $13.4\phantom{\rule{0.1em}{0ex}}\text{\ensuremath{\mu}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}$, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature (
TC) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an rms roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher ${\ensuremath{\rho}}_{300\phantom{\rule{0.1em}{0ex}}\mathrm{K}}$, lower RRR and
Tc, and increased $\ensuremath{\beta}$-
Ta content. Coplanar waveguide resonators with a gap width of $3\phantom{\rule{0.1em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$ fabricated from cryogenically grown
Ta on
Si(111) and
Al2O3(0001) show low-power
Qi of 1.9 million and 0.7 million, respectively, indicating polycrystalline $\ensuremath{\alpha}$-
Ta films may be promising for superconducting qubit applications even though they are not fully epitaxial.