Correlative Reciprocal-Space and Spectral Mapping of the Quantum Microstructure in High-Performance Epitaxial Materials for Superconducting Qubits
Peter G. Lim, David A. Garcia Wetten, Mitchell J. Walker +6 more
Superconducting qubit technology is among the most promising hardware platform for quantum computing due to its fast gate operation, manufacturing scalability, and architectural flexibility [1]. A key figure of merit that dictates quantum computing functionality is the qubit’s coherence time, which needs to be maximized in order to bring quantum computing to the practical and scalable realm [2]. In particular, the T1 relaxation time—now on the order of milliseconds—can be enhanced with materials engineering, as amorphous dielectrics and crystallographic defects, among other factors, are known to induce decoherence [3-7]. Current state-of-the-art superconducting qubit devices use amorphous AlOx sandwiched between two superconducting Al electrodes as the Josephson junction (JJ), a key element in the transmon qubit, but amorphous dielectrics introduce two-level system (TLS) loss that underlie decoherence [8]. For this reason, numerous efforts have explored crystalline dielectrics as a replacement for amorphous AlOx, since single-crystal materials do not suffer from the lack of long-range order and sub-stoichiometry that comprise TLS defects [9-12]. However, none of these alternative materials choices have been able to surpass, let alone replicate, the performance of Al-AlOx-Al JJs. In this work, we fabricate superconducting quantum devices using a novel heteroepitaxial TiN/γ-Al2O3/TiN tri-layer stack grown on sapphire (Fig. 1a) and find that the measured microwave loss of the single-crystal γ-Al2O3 dielectric is lower than that of amorphous AlOx by two orders of magnitude. For qubits made with this tri-layer to transcend the coherence of incumbent Al-AlOx-Al, the structural, chemical, and electronic defects that induce TLS loss in these materials need to be well understood. Towards this end, we combine scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) methods to highlight structural and interfacial complexity across length scales, ultimately identifying loss sources in the TiN/γ-Al2O3/TiN heteroepitaxial tri-layer. With atomic-resolution imaging and four-dimensional (4D-)STEM nanobeam electron diffraction (NBED), we find that the dielectric γ-Al2O3 layer, though single-crystalline (Fig. 1b), suffers from the formation of antiphase boundaries (Fig. 1c), which can host strong local electric fields and enhance lossy dielectric response [13]. Correlated with these nanoscale planar defects is uniformly distributed elastic strain, on the order of 6%, between the lattice-mismatched γ-Al2O3 and TiN (Fig. 1d). In other words, the dielectric layer is fully strained to match the underlying metal layer; such high strain can distort local bonding environments and thus create new TLSs or shift existing TLSs into resonance. Beyond imperfections within the bulk of the dielectric layer, there are noteworthy interfacial defects: roughening of both TiN/γ-Al2O3 interfaces deviate from atomically flat boundaries (Fig. 1c), while anion interdiffusion across both interfaces, as evident from core-loss electron energy-loss spectral (EELS) maps (Fig. 1e), leave behind vacancies and undercoordinated metal atoms, thereby creating 1.5 – 2 nm nonstoichiometric diffuse interfacial layers, both of which can cause TLS loss. In the superconducting TiN layers, we again use NBED to identify 180°-reflected specular twinned domains, indicating distinct heteroepitaxial relationships of [111]TiN//[0006]Al2O3 and [002¯]TiN//[22¯02¯]Al2O3 (Fig. 2a) [14]. Crystal twinning of the TiN epilayer is not only spatially distributed at the nanoscale, as shown using virtual dark-field (VDF) images (Fig. 2b), but is also widespread at the microscale, as shown using SEM electron backscatter diffraction (EBSD) (Fig. 2c). While microwave performance is not limited by a single factor, it is clear that defective quantum microstructure across length scales collectively contribute to dielectric loss, and in turn, TLS-related decoherence. By correlating nanoscale crystallographic and spectroscopic information with microscale diffraction measurements, our work can guide interface engineering during material growth, paving the way towards further improvements in quantum device performance. Cross-sectional samples were prepared by subsequent 30 kV, 8 kV and 5 kV Xe+ plasma focused ion beam (FIB) using the Thermo Scientific Helios 5 Hydra CX DualBeam Plasma FIB/SEM. Samples were then plasma cleaned using South Bay Technology PC-2000 Plasma Cleaner with a 3:1 mixture of Ar:O2 gas at 20∼40 W RF power for 15 s at 150 mTorr. High-angle annular dark-field (HAADF-) and annular bright-field (ABF-) STEM images were collected on a probe-corrected JEOL JEM-ARM200CF S/TEM operating at 200 kV using JEOL detectors. The STEM convergence angle, HAADF collection angle, and ABF collection angle were 27 mrad, 90 – 370 mrad, and 23 mrad, respectively. EELS data were collected on a JEOL JEM-ARM300F GrandARM S/TEM operating at 300 kV using the Gatan GIF Continuum K3 IS system. The data were collected using a 2.5 mm aperture and a 750 eV wide energy slit at a dispersion of 0.18 eV/Ch and a camera length of 2 cm. 4D-STEM data were collected on the same microscope and detector system using a 0.5 – 1.5 mrad convergence angle and a 20 eV wide energy slit at a camera length of 4 cm. All STEM data processing was performed using the Gatan Microscopy Suite software. Top-down EBSD measurements were performed in the Thermo Scientific Helios 5 CX DualBeam FIB/SEM at 25 kV using the Oxford Symmetry S3 detector. EBSD data processing was performed using the Oxford AZtecCrystal software [15]. Defects in γ-Al2O3. a) HAADF-STEM image of a TiN/γ-Al2O3/TiN tri-layer stack on a α-Al2O3 (0001) substrate. The thickness of γ-Al2O3 is approximately 13.5 nm. b) 4D-STEM NBED pattern of the γ-Al2O3 layer, confirming the {111} out-of-plane orientation. c) Fourier-filtered atomic-resolution ABF-STEM image of the γ-Al2O3 dielectric layer, taken from the cyan-colored square region in a), emphasizing the planar defects along both TiN/γ-Al2O3 interfaces and within the γ-Al2O3 layer itself–interfacial roughness (yellow lines) and antiphase boundaries (colored parallelograms), respectively. The scale bar is 2 nm. d) In-plane and out-of-plane strain maps of the heteroepitaxial tri-layer stack, with approximately 6.5∼6.8% strain in both directions. The units of the intensity bar are such that 100% strain would be equivalent to 1.0. e) STEM-EELS maps of Ti-L2,3, N-K, and O-K edges, along with a superimposed image of the three, taken from the same region as c). The scale bars are all 5 nm. Defects in TiN. a) 4D-STEM NBED patterns and b) VDF images of nanoscale TiN {111} and {002} twins. c) SEM EBSD pole figures in the three face-centered cubic crystallographic directions, revealing that the 60° rotation twinning behavior (with the [111] twin axis) is spread across the microscale.
Microscopy and Microanalysis•Jul 1, 2026•0 citations