Stable and low loss oxide layer on {\alpha}-Ta (110) film for superconducting qubits

The presence of amorphous oxide layers can significantly affect the coherent time of superconducting qubits due to their high dielectric loss. Typically, the surface oxides of superconductor films exhibit lossy and unstable behavior when exposed to air. To increase the coherence time, it is essential for qubits to have stable and low dielectric loss oxides, either as barrier or passivation layers. In this study, we highlight the robust and stable nature of an amorphous tantalum oxide layer formed on {\alpha}-Ta (110) film by employing chemical and structural analyses. Such kind of oxide layer forms in a self-limiting process on the surface of {\alpha}-Ta (110) film in piranha solution, yielding stable thickness and steady chemical composition. Quarter wavelength coplanar waveguide resonators are made to study the loss of this oxide. One resonator has a Qi of 3.0x10^6 in the single photon region. The Qi of most devices are higher than 2.0x10^6. Moreover, most of them are still over 1x10^6 even after exposed to air for months. Based on these findings, we propose an all-tantalum superconducting qubit utilizing such oxide as passivation layers, which possess low dielectric loss and improved stability.


I. INTRODUCTION
Superconducting quantum computing has gained substantial experimental progresses over the last two decades.It is now one of the leading candidates to build a fault-tolerant quantum computer. 1F. Arute et al. achieved quantum "supremacy" in 2019 with a chip of 53 X-mon qubits. 2 To demonstrate real quantum advantage, the quality and quantity of qubits need to increase simultaneously.4][5][6][7] In 2020, A. P. M. Place et al. reported coherence time of exceeding 0.3 ms for the Transmon fabricated with alphaphase tantalum films. 8Following this report, C. Wang et al. increased the coherence time to 0.5 ms in 2021. 9 is believed that most lossy channels emerged in the material growth and device fabrication processes. 10,11Specifically, the dielectric loss arises from three interfaces, 12 i.e., the amorphous oxide layer at the superconductor-air (MA) interface, the amorphous oxide layer at the substrate-air (SA) interface, and the amorphous layer at the superconductor-substrate (MS) interface.Substrate cleaning, epitaxial film growth and subsequently etching method for the qubit fabrication can improve the MS interface. 13- 17 owever, the amorphous oxide layers in the MA interface is inherent with material properties and hard to manage.Most superconductors used in the qubits are metals which are easy to be oxidized in air, and the thicknesses of these oxides increase over time.Sustained efforts have been paid to reduce loss of the MA interface.Verjauw et al. discovered that removing the niobium (Nb) oxides can increase the internal quality factor (Qi) with half magnitude. 18Other groups also found that passivation of the superconductor surface can improve the quality of the devices. 191][22] Even though the detailed analysis of the chemical composition of the amorphous oxide layer mainly focuses on the α-Ta (111) films on C-plane sapphire substrates. 22These studies show that the surface oxides of Ta film are a major source of two level system (TLS) losses, especially some of these sub-oxides, and that these losses can be reduced by chemical etching. 21,23Besides the three interfaces, the Josephson junction can also be lossy and noisy.The best insulator layer in the Josephson junction is still amorphous alumina, which is unstable and has large dielectric loss.
In this paper, we investigate the surface oxide of α-Ta (110) films prepared in piranha.
As to the α-Ta (110) film, the oxide layer thickness is about 2.24 nm, containing not only pentavalent tantalum (Ta 5+ ), but also trivalent tantalum (Ta 3+ ) after exposing to air.
With the piranha solution treatment, the surface oxide layer is mainly composed of pentavalent tantalum (Ta2O5), with an oxide layer thickness of about 2.61 nm.Multiple piranha treatments result in little change in thickness.The CPW resonators are made of α-Ta (110) film with this kind of Ta2O5 surface oxide.The highest Qi is about three million in the single-photon regime at 10 mK.The Qi of most devices are higher than 2.0×10 6 .Moreover, most of them are still over one million even after exposed to air for months.The high Qi and robustness of the resonators indicate that such Ta2O5 layer is of low dielectric loss and stable.Thus, we suggest an all-tantalum superconducting qubit incorporating such oxide as passivation layers, which exhibit low dielectric loss and enhanced stability.

II. EXPERIMENTAL
The α-Ta (110) films with thickness of 200 nm were deposited on 2-inch C-plane sapphire substrates by DC magnetron sputtering in a high vacuum chamber.Prior to the film deposition, the substrate was thermally cleaned inside the sputtering chamber at 700 ℃ for 30 minutes, and then slowly cooled down to 400 ℃ at a rate of 30 ℃ per minute.During the optimal deposition process, the substrate was maintained at 400 ℃.
The sputtering pressure of Ar gas was kept constant at 15 mTorr, and the DC sputtering power was set to 200 W. X-ray Diffraction (XRD) confirmed the film crystallographic plane is shown in Fig. S1 in the supplementary material [URL will be inserted by AIP Publishing].For comparison, four different α-Ta (110) samples from the same wafer were prepared, i.e., one untreated with piranha, two soaked in piranha for 1 and 3 times and 20 minutes each time, and one exposed to air for 4 months.The piranha solution in our experiment is a mixture of sulfuric acid and 30% hydrogen peroxide in a volume ratio of 2:1.Surface morphology and roughness are measured by atomic force microscopy (AFM).The thicknesses and chemical compositions of oxide layers are investigated using scanning transmission electron microscope (TEM/STEM) and angleresolved X-ray photoelectron spectroscopy (XPS).The XPS spectrum at different angles is presented in Fig. S2 in the supplementary material [URL will be inserted by AIP Publishing].Qi of the coplanar waveguide (CPW) resonators made from α-Ta (110) film are measured at 10 mK.The measurement configuration is shown in Fig. S3 in the supplementary material [URL will be inserted by AIP Publishing].The input signal path incorporates a variable attenuator at room temperature, a 20 dB attenuator at 4 K, a 6 dB attenuator at the still plate, and a 30 dB attenuator and an infrared filter at 10 mK.
At 4 K on the output signal path, a HEMT amplifier with a gain of 40 dB and a room temperature amplifier with a gain of 40 dB are employed.The output signal path also incorporates three isolators and a low-pass filter.The test chips are encapsulated in high-purity Al-made sample boxes with a μ-metal magnetic shielded.The measurement is done by a vector network analyzer (VNA) with variable output power.The excitation power is applied from -160 dBm to -100 dBm by adjusting the output power of VNA and the variable attenuator.The photon number in the CPW resonators can be estimated from the input power of the pump using the following equation: where n is the estimated photon number, Pin is the pump power, h is Planck's constant, f is the resonance frequency of the CPW resonators, Q is the loaded (total) quality factor of the resonator, Qc is the value which is related to the coupling capacitance between the CPW resonator and driveline, and Qi is the intrinsic quality factors.In order to ensure the acquisition of high-quality Qi, the designing value of Qc was set at 300,000.
Due to the use of wet etching process, there will be some differences in the actual devices.The Qi was extracted by fitting the S21 vs. frequency curve.The corresponding fitting formula can been seen in section Ⅳ the supplementary material [URL will be inserted by AIP Publishing].Moreover, an example of a fitting curve is shown in Fig. S4 in the supplementary material [URL will be inserted by AIP Publishing].

III. RESULTS AND DISCUSSION
The surface of as grown α-Ta (110) film with optimal deposition condition is shown in surface in air and piranha are self-limiting processes which probably evolve logarithmically with time. 24,25The difference is that, in air, the surface oxide layer grows slowly and takes longer time to saturate; but in the piranha, which is a strong oxidizer, the surface oxide layer reaches its limited thickness in a shorter time.
The chemical composition of the α-Ta (110) surface oxide layer is measured by the XPS.(c) Considering the factor that the oxide prepared in piranha is almost pure Ta2O5 in a dense amorphous structure, no wonder it is very stable in air and in piranha solution.From the angle-resolved data, 26 the thicknesses of the surface oxide layer are also obtained.
They are 2.45 nm for the one exposed to the air and 2.74 nm for the one immersed in piranha solution, which is consistent with the TEM data, with deviation of about 0.20 nm.The agreement of thicknesses measured by two distinct methods suggest again that the surface oxide on α-Ta (110) film formed in piranha solution is uniform.
As to the superconducting device, the metallic and semiconductor nature of sub-oxides lead to conductivity losses. 5the losses as a whole can be estimated from Qi of resonators, we fabricated CPW resonators using the α-Ta (110) film covered with such oxides across different positions on the wafer scale.The devices were designed with center conductor and insulating gap widths of w = 10 µm and g = 6 µm, respectively.
Fig. S5 and Fig. S6 in the supplementary material [URL will be inserted by AIP Publishing] show the dimensions and etching edge structure of the resonator.The resonance frequency was 6.5 GHz.To begin with, the sapphire wafer with α-Ta (110) film was soaked in piranha.The resonators were defined with optical lithography and a wet etching process.Then, the wafer was cut into 8 mm × 8 mm square chips.Last, the chips were rinsed in piranha again.
The Qi of the new resonators and resonators these had been exposed to air for four months are measured and shown in Fig. 3.One resonator has a Qi of 3.0×10 6 in the single photon region.This value is lower than that in the Ref. 23.The observed discrepancy can be attributed to several factors.Firstly, the CPW resonator design could be distinct.Secondly, the use of pure Al sample boxes for packaging the test chips may result in a higher actual sample temperature compared to the displayed temperature of the dilution refrigerator.Furthermore, the absence of infrared shielding covers for the test samples could also contribute to the discrepancy.From the participation ratio analysis, the up limited loss tangent of the amorphous Ta2O5 is about 1.5×10 -2 .The relevant calculation formulas and values of dielectric constants are provided in Section Ⅶ of the supplementary materials [URL will be inserted by AIP Publishing].The Qi 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 5 2.0x10 6 4.0x10 6 6.0x10 6 8.0x10 6 1.0x10 7 after 4 months New resonators FIG. 3. The Qi vs. photon number curves of series of resonators made from α-Ta (110) film in different area on wafer scale measured immediately and after 4 months.
of most devices are higher than that (2.0×10 6 ) of the resonator made from an aluminum (Al) film deposited by molecular beam epitaxy on sapphire substrate, 14 and that (1.0×10 6 ) of the resonator made from a Nb film deposited by sputtering on silicon substrate. 18Regarding the surface roughness (Such as 0.40 nm for Al/Sapphire and 0.58 nm for Nb/Si) in these references and 1.09 nm for the α-Ta (110)/Sapphire here, it is fair to infer that Ta2O5 layer has a lower dielectric loss than that of the amorphous oxide layer formed in air on Al and Nb film surfaces.For the resonators left in air for four months, a slight degradation is observed.In view of many factors, like surface contamination, can substantially plague the resonators, the Qi above million is quite remarkable.The robustness of the resonators further imply that the amorphous Ta2O5 layer is stable in atmosphere.
With the stability and low loss properties of the Ta2O5, an all-tantalum superconducting qubit using the Ta2O5 as the dielectric and passivation layer is proposed.

IV. CONCLUSIONS
In conclusion, we have investigated the surface oxides of α-Ta (110) films prepared in piranha.The oxide layer thickness on the α-Ta (110) surface is

Fig. 1 (
Fig. 1(a).The grains of Ta are elongated with tetragonal symmetry and tightly packed

Fig. 4 FIG. 4 .
FIG.4.The process flow for the preparation of superconducting qubits based on Ta2O5 as dielectric and passivation layers.(a) Preparing the substrate; (b) Depositing α-Ta (110) film for the bottom electrode (BE) layer of Josephson junctions and other circuit structures; (c) Preparing the Ta2O5 tunnel barrier layer for the Josephson junctions; (d) Depositing α-Ta (110) film for the top electrode (TE) of Josephson junctions; (e) Patterning the junction area; (f) Surface passivation to obtain Ta2O5 passivation layer; (g) Patterning the resonators and other circuit structures, (h) Surface passivation to obtain Ta2O5 passivation layer.

approximately 2 .
24 nm after exposing to air for four hours, containing various oxidization states of Ta.Treatment with the piranha solution primarily composed of pentavalent tantalum, with a thickness of approximately 2.61 nm.CPW resonators are fabricated using α-Ta (110) film with Ta2O5 surface oxide, exhibiting high Qi and robustness even after exposure to the atmosphere for months.Guided by these research results, we suggest the development of a full tantalum superconducting qubit, employing oxides with reduced dielectric loss and enhanced stability as passivation layers.This is consistent with the observations made in the fabrication of CPW resonators using α-Ta (110) film with Ta2O5 surface oxide.ACKNOWLEDGMENTS K. L. X acknowledges support from the Youth Innovation Promotion Association of Chinese Academy of Sciences (2019319).J. G. F. acknowledges support from the Startup foundation of Suzhou Institute of Nano-Tech and Nano-Bionics, CAS, Suzhou