Session M47: Superconducting Qubits: Metal Films I

Reducing Losses in Transmon Qubits Using Fluorine-Based Etches

Superconducting qubits have developed from proof-of principle single-bit demonstrations to mature deployments of many-qubit quantum processors. Reducing materials- and processing-induced decoherence in superconducting qubit circuits is critical to further the development of large-scale quantum architectures. In this talk we discuss the results of applying selective fluorine-based etches, targeting lossy silicon oxides, in close proximity to sensitive aluminum circuit elements such as Josephson Junctions, resonators and crossover tethers. These fabrication improvements can be implemented with little to no damage to existing structures. The impact that these have on transmon qubit coherence will be discussed.   

Single-crystal aluminum superconducting ultra-thin films grown on GaAs(111)A with low twin ratios and the impact on microwave properties

The disordered grain boundaries of superconducting thin films and interface inhomogeneity to adjacent layers are sources of energy relaxation channels that limit the performance of superconducting quantum devices. In this work, we have demonstrated single-crystal aluminum (Al) ultra-thin films on GaAs(111)A substrates with high crystallinity and very low twin ratios. The transport properties of these Al films were measured to characterize the thin-film quality.

Synchrotron radiation X-ray diffraction scans on the Al films with thickness from 1.5 to 20 nm exhibited clear Pendellösung fringes. The full width at half maximum (FWHM) of θ-rocking curves of the Al films is 0.018°-0.027°. The in-plane scans revealed that the Al(111) films have low twin ratios, and the FWHM of in-plane Al(111̅) peaks is very low of 0.76°-1.88°. The surface morphology characterized by atomic force microscopy is smooth and exhibits terrace steps. The scanning transmission electron microscopy images have revealed the atomically sharp Al/GaAs interface. These nm-thick Al films exhibited high residual resistance ratios from 1.11 to 4.46, indicating the high quality of Al films with low twin ratios. Microstrip resonators were fabricated using these ultra-thin Al films to measure the internal quality factors. Such single-crystal Al is a potential candidate for building high-coherence superconducting quantum circuits.

The support from the Natl. Sci. Technol. Council in Taiwan through NSTC 112-2119-M-007-009 is acknowledged.   

Microstrip resonators with internal quality factors over one million made from MBE-Al films capped with an in-situ deposited oxide layer

Superconducting resonators are used for dispersive readout on superconducting qubit chips for quantum computing. The internal quality factor (Q_i) of a resonator is a measure of the ability to store microwave energy and is closely related to the coherence time of qubits. Substantial efforts have been made to improve the Q_i of resonators, which includes improving material quality by enhancing the crystallinity of the film and optimizing the fabrication process. However, most reported Q_i’s for Al resonators patterned on sapphire are lower than 1x10⁶.

Here, we present high-quality Al resonators fabricated on sapphire. The single-crystal Al films were grown by molecular beam epitaxy (MBE) and were immediately capped with a stoichiometric Al₂O₃ layer which was in-situ deposited on the Al film using e-beam evaporation. This unique approach is appealing since it provides an oxide layer with uniform and controllable thickness to protect the underlying superconducting film, which cannot be achieved by the conventional oxidation method. The high-quality Al₂O₃ layer suppressed the formation of two-level systems and thus reduced the energy relaxation channels. With this approach and an optimized fabrication process, we demonstrated resonators with Q_i exceeding one million at 5.1 GHz in a single photon limit.   

Investigating loss in NbN and NbTi resonators on high resistivity Si substrates

Developing low loss, magnetic field compatible superconducting microwave circuits opens the door to exploring novel systems in quantum computing and quantum sensing. Superconducting resonators with such properties can be fabricated using superconducting thin films with relatively high critical temperature (Tc) and high critical field (Bc). In this work, we study the superconducting coplanar waveguide resonators made from thin films of NbN and NbTi sputtered on silicon substrates. We conduct a systematic investigation on the effect of growth conditions, and film thickness on the internal quality factor and magnetic field compatibility of NbN and  NbTi in order to study two level system losses and dissipation from vortices. We integrate these resonators with InAs-Gatemon qubits and study the performance in presence of magnetic fields.    

Impact of Deposition Method on Structure and Performance of Nb/Si-based Superconducting Coplanar Waveguide Resonators

Structural defects on surfaces and interfaces in superconducting qubits can lead to decoherence and two-level systems (TLS) loss. Using various electron microscopy, electronic transport, and magnetic characterization methods, we reveal the effects deposition methods have on planar Nb on Si resonator performance and TLS loss, and correlate these to material property variations on average grain size, surface roughness, interface chemistry, magnetic and electronic property differences.  We find that differences in microstructure and metal-substrate interface correlate with changes in Qi and TLS losses. Our findings highlight the importance of carefully selecting the deposition method to optimize the microstructure and performance of Nb/Si-based superconducting resonators.    

Exploring the effect of growth and surface quality of niobium thin-films for superconducting devices

We explore the potential improvement of coherence in niobium-based superconducting devices with a focus on two aspects. The first aspect is the role of the niobium and substrate surfaces. We use an approach for sample packaging where devices can be quickly isolated in vacuum after metal oxide removal, using a special purpose sealable microwave cavity. The second aspect is the role of film growth, with both sputtering and electron cyclotron resonance growth being considered.

In this talk we will present the design of several types of test structures, including lumped and distributed resonators and transmon qubits, used to probe various contributions to loss, including surface and interface loss and quasiparticle losses. We will present our preliminary results on the fabrication and characterization of these devices.   

Understanding loss mechanisms in tantalum-based superconducting circuits

Tantalum as a material platform for superconducting qubits has shown longer relaxation times on average than other commonly used materials, such as Al, TiN, and Nb. In this work, we explore the loss mechanisms in tantalum circuits including loss tangent and defect coupling. We grow niobium and alpha-tantalum on silicon substrate, and fabricate lumped-element resonators with varying characteristics and composition in our experiments. Materials properties at each interface are extracted through two-step finite-element simulation and correlate with the results from extensive materials characterization including XPS, TEM and EDS. The work here will provide a pathway to understand the origin of the loss channels in group V element superconductors, and ultimately enable us to improve the resonator quality factor and qubit relaxation time in superconducting quantum circuits.   

Tantalum thin films on a-plane sapphire for low-loss superconducting circuits

Superconducting quantum circuits are a promising hardware platform in the fields of quantum computing. Ways to reduce noise and dissipation are of major importance in order to improve device performance and particularly promising in this respect is the exploration of novel materials. α-Tantalum (Ta) thin films grown on c-plane sapphire have been shown to improve Transmon qubit lifetimes when used for their shunting capacitor. However, achieving the high film quality needed to fabricate low loss devices, requires high substrate temperature during sputter deposition. At intermediate sample temperatures (< 650°C), that can be realized in many deposition systems, the film growth is characterized by the competing growth of (110) Ta and (111) Ta . The resulting films, when patterned into superconducting resonators, exhibit loss tangents larger than observed for resonators patterned from films grown at higher temperature. This can be avoided by growing the films on the a-plane surface of sapphire, instead. There, only (110) Ta is formed at intermediate temperatures. We correlate the film properties with quality factors of lumped element resonators fabricated from films grown on the different sapphire surfaces.   

Low-Temperature MBE growth of superconducting Ta films on Silicon and Sapphire substrates

Recent advances in tantalum-based Transmon qubits on sapphire have demonstrated coherence times up to 0.5 milliseconds^1,2 The growth of alpha-Ta at elevated temperatures is required for the realization of desirable superconducting properties. Alternatively, superconducting tantalum qubits can be grown on Si substrates by heating the substrate, but the potential formation of silicides at the Si-Ta interface could impact qubit coherence. 

Here, we demonstrate low-temperature MBE growth (<20K) to stabilize alpha-Ta independent of the substrate without using a seed layer enabling its integration with various material systems. We characterize the growth of alpha-Ta on several substrates including amorphous SiN_x, Si(111), GaAs(001), and Al₂O₃(0001). Both structural and electrical measurements indicate the successful growth of α-Ta with good superconducting properties independent of the substrate. Finally, we fabricate CPW resonator circuits and 2D Transmon qubits to compare α-Ta on Si and Sapphire substrates under the same growth conditions.

1) Place, A.P.M., et al., Nat Commun 12, 1779 (2021). 

2) Wang, C., et al., npj Quantum Inf 8, 3 (2022).   

Growth of ordered Tantalum films and Integration into Quantum Circuits

We investigate the growth of tantalum on Si and sapphire to understand the limits of atomically-ordered superconducting qubit construction.  On c-plane sapphire we observe (111)-oriented bcc Ta films with residual resistivity ratios exceeding 60 in 200 nm films.  We present x-ray diffraction, low temperature transport, and transmission electron microscopy studies of these films and further demonstrate resonators with low-power quality factors exceeding 10⁶ at 10 mK, 2D and 3D transmon qubits with coherence times up to 400 us, and the production of bcc Ta airbridges.   

Microscopic source of quasiparticle loss in tantalum resonators

Tantalum (Ta) based superconducting circuits have been demonstrated to enable world-record qubit coherence times (T₂) and quality factors [1,2], motivating a careful study of the microscopic origin of the remaining losses that limit their performance. We have recently shown [2] that temperature-dependent loss measurements reveal that some devices display behavior consistent with a lower superconducting critical temperature (T_c) than can be predicted by direct film characterization. Specifically, their constituent films have a single crystal structure associated with the high-T_c BCC (α) phase of Ta, and dc resistivity measurements show a T_c of over 4 K, while some resonators fabricated from these films show quasiparticle losses consistent with a T_c as low as 0.4 K.  In addition, scanning SQUID measurements did not reveal any evidence of inhomogeneity in these films at the micrometer scale. Here, we present a comparative study of the structural and thermodynamic properties of Ta-based resonators with high- and low-T_c characteristics via X-ray diffraction and resistivity measurements. X-ray diffraction shows a clear correlation between the α-Ta (222) peak position and the apparent T_c obtained from microwave loss measurements. The α-Ta (222) peak associated with high-T_c resonators is consistently shifted to higher angles by ~0.6° compared to the low-apparent T_c films. Furthermore, resistivity as a function of temperature under applied magnetic field reveals that the magnetic field required to completely suppress the superconductivity is around twice as large in high-T_c films compared to low-apparent T_c ones. These results provide material proxies for optimizing Ta device quality, and demonstrate that detailed materials measurements can provide proxies for device performance.

 

[1] Nature Communications, 12(1), 1779 (2021)

[2] Physical Review X 13, 041005 (2023)   

Dielectric Loss from Josephson Junctions in Superconducting Qubits

Superconducting qubits are a leading platform for realizing fault-tolerant quantum computation and have enabled demonstrations of both quantum error correction and quantum simulation. The lifetimes of current superconducting qubits are limited by dielectric loss that is orders of magnitude higher than would be expected from the bulk properties of the constituent materials, suggesting uncontrolled interfaces may be limiting their performance. Recently, tantalum (Ta) has been shown to be a promising material platform for superconducting qubits, and it is believed that its promise is related to both its stoichiometric oxide and chemical robustness. Additional studies of Ta superconducting resonators suggest that dielectric loss from interfaces and bulk materials are comparable for the longest-lived devices, meaning both dielectric loss sources must be addressed if further improvements in device performance are to be achieved. However, qubits based on tantalum still utilize aluminum josephson junctions, which has been suggested to be a dominant source of dielectric loss. Here, we study the dependence of transmon lifetime on the size of the junction leads in order to quantify the contribution of aluminum oxide to dielectric loss in superconducting qubits.   

Interphase engineering in tantalum and aluminium superconducting resonators

The performance of cutting-edge superconducting quantum devices is primarily limited by microwave dielectric losses at various surface and interface regions. Potential enhancements in device performance may be achieved through the strategic modification of these interfaces, specifically through the incorporation of additional layers or the application of specific surface treatments. This will facilitate the understanding of the contributions of each interface to the overall device loss. In this study, we investigate the performance of alpha-tantalum and aluminum resonators, which have been fabricated utilizing various seed layers. We conduct an extensive characterization study of the device material using transmission electron microscopy, X-ray photoemission spectroscopy, X-ray diffraction, atomic force microscopy, and critical temperature measurements. Depending on the specific seed layer material employed, there is either a notable reduction in both low and high internal quality factors, or no discernible variation in comparison to the reference resonators.   

Controlling Surface Oxidation of Superconducting Circuit Materials

Achieving large-scale quantum computations with superconducting quantum circuits, particularly those based on transmon qubits, demands significant improvements in qubit coherence time. In recent advancements, tantalum (Ta) has emerged as a leading candidate, outperforming traditional counterparts in terms of coherence time. Despite its promise, the presence of an amorphous surface Ta oxide layer poses a challenge, potentially introducing dielectric loss and limiting the coherence time. In this talk, we will present a novel approach for suppressing the formation of surface Ta oxide, aiming to unlock the full potential of Ta-based quantum circuits for high-performance quantum computing applications.   

Heteroepitaxial oxide/α-Ta(110) films on sapphire(11-20) – growth, structure, transport, and microwave properties

       For building high-performance superconducting qubits and supportive circuits like a high-Q microwave resonator, we need to precisely control material properties in film crystallinity, hetero-interfaces, and capped oxide layer. Ta superconducting films are usually exposed to air, inevitably forming a native oxide layer, which may enhance energy relaxation channels such as two-level systems (TLS) for superconducting circuits.¹ Here, growth of heteroepitaxial deposited-oxide/α-Ta films on a-plane sapphire substrates was achieved in a multi-chamber UHV system.^2,3 The research aims to investigate the microwave properties of the in-situ deposited heterostructures to understand dielectric losses in superconducting quantum circuits.

       α-Ta films 35 nm in thickness grew epitaxially in (110) orientation on (11-20) sapphire substrates, followed with in-situ growth of epitaxial and amorphous Al₂O₃ films. The presence of Pendellösung fringes indicates a high degree of structural coherence over the entire film thickness. The root mean square roughness of the Ta films is 0.8 nm. After the in-situ oxide deposition, the surface roughness decreases to 0.47 nm. The Ta film has a critical temperature of 4.29 K and a high residual resistance ratio of 16.

 

1. Crowley et al., PRX 13, 041005 (2023).

2. Kwo et al., Appl. Phys. Lett. 49, 319 (1986). 

3. Lin et al., J. Cryst. Growth 512, 223 (2019).