All-nitride superconducting qubits based on atomic layer deposition

TL;DR

This work introduces all-nitride superconducting qubits based on NbN/AlN/NbN trilayers deposited entirely by atomic layer deposition (ALD). By controlling the AlN barrier thickness through ALD cycles, the authors achieve a seven-order range in Josephson critical current density and demonstrate transmon qubits with microsecond relaxation times at temperatures exceeding 300 mK, enabled by the high Tc NbN. The devices employ a flip-chip architecture that eliminates dielectric spacers and enables independent optimization of junctions and readout circuitry, with structural and chemical analyses confirming sharp interfaces and uniform barriers. Collectively, these results establish ALD-grown nitride trilayers as a scalable, CMOS-compatible platform for elevated-temperature superconducting quantum circuits and point toward broader material combinations and industrially integrable fabrication routes.

Abstract

The development of large-scale quantum processors benefits from superconducting qubits that can operate at elevated temperatures and be fabricated with scalable, foundry-compatible processes. Atomic layer deposition (ALD) is increasingly being adopted as an industrial standard for thin-film growth, particularly in applications requiring precise control over layer thickness and composition. Here, we report superconducting qubits based on NbN/AlN/NbN trilayers deposited entirely by ALD. By varying the number of ALD cycles used to form the AlN barrier, we achieve Josephson tunneling through barriers of different thicknesses, with critical current density spanning seven orders of magnitude, demonstrating the uniformity and versatility of the process. Owing to the high critical temperature of NbN, transmon qubits based on these all-nitride trilayers exhibit microsecond-scale relaxation times, even at temperatures above 300 mK. These results establish ALD as a viable low-temperature deposition technique for superconducting quantum circuits and position all-nitride ALD qubits as a promising platform for operation at elevated temperatures.

Figures (8)

• Figure 1: ALD growth sequence and atomic-scale characterization of NbN/AlN/NbN trilayer.a, ALD process flow for trilayer deposition, following the arrows. NbN is first deposited on a c-plane sapphire substrate using the precursor TBTDEN. The Al-containing precursor (TMA) is then introduced and purged, followed by plasma-enhanced nitridation that converts surface-adsorbed Al into AlN. Residual ions and byproducts are subsequently removed in a second purge. Repeated cycles form the AlN barrier and NbN top layer, completing the NbN/AlN/NbN trilayer stack. b, 1D atomic concentration profile extracted from APT of a 15 nm-diameter cylindrical volume. The y-axis origin (0 nm) corresponds to the center of the AlN barrier; negative and positive distances denote the bottom and top NbN layers, respectively. c, Overview cross-sectional STEM image. d, Zoom-in STEM image of NbN/AlN/NbN interfaces. e, Magnified STEM image of NbN/sapphire interface.
• Figure 2: Device schematics and IV characterization at 4 K.a, A flip-chip qubit model, where the C-chip, consisting of a readout resonator and a transmission line, is at the bottom, while the Q-chip, hosting a junction, is on top. Blue, green, and yellow represent NbN, AlN, and gold, respectively. The lower-left panel shows a zoom-in model of the junction area, where two shunting pads ensure strong coupling of a qubit to the readout resonator. The lower-right panel shows a close-up view of a device used for DC characterization, in which the unetched bottom NbN film serves as the ground plane and the junction top electrode is flip-chip bonded to NbN-gold wires for readout. b, Dependence of $J_\text{c}$ on the number of AlN deposition cycles. Blue dots represent experimental data from wafers grown over a year, while the orange curve is an exponential fit. The arrow indicates that 21 ALD cycles deposit $\sim$1.6 nm-thick AlN, as estimated by STEM. The inset microscopic image shows nine junctions with two alignment crosses for flip-chip bonding. The inset IV curves correspond to a junction with 7 $\mathrm{\upmu m}$ diameter.
• Figure 3: Qubit performance at the base temperature.a, Power-dependent qubit spectroscopy. The power refers to the output level of a signal generator. The color bar represents the voltage amplitude of the transmitted probe tone. b, Chevron plot of Rabi oscillations. c, Ramsey oscillations measured near $f_\text{q}$ with varying $\Delta_\text{d}$. d, Microscopic image of a qubit and part of its readout resonator. e, $T_1$ characterization. The inset histogram summarizes the $T_1$ values from 100 repeated measurements taken over a two-hour period. The blue dots in the main graph represent averaged data from these repeated measurements. The orange curve is an exponential fit, yielding an average $T_1$ of 3.0 $\mathrm{\upmu s}$. f, Extracted Ramsey oscillation from c, at $\Delta_\text{d}$ = –5.2 MHz. Orange curve is from a fitting model: $A_0+Ae^{-\tau/T_2^*}\mathrm{cos}(2\pi\Delta_\text{d}\tau+\phi_0)$.
• Figure 4: Qubit performance at elevated temperatures.a, Rabi oscillation of qubit A3 as a function of pulse duration at 310 mK. b, orange squares and blue dots represent the $T_1$ values of qubits A2 and A3, respectively, across a temperature sweep. The green and gray curves show the theoretical temperature dependence of $T_1$, as anticipated by the spin-boson model for NbN-based qubits and the quasiparticle relaxation model for Al-based qubits, respectively, both assuming $T_1$ = 3 $\mathrm{\upmu s}$ at 10 mK.
• Figure 5: STEM image of NbN/AlN/NbN trilayer and corresponding FFTs.a, Cross-sectional annular dark-field (ADF) STEM image of the trilayer. Regions outlined in red and yellow are selected for local FFT analyses. b, Global FFT of the entire field of view in a, indicating twin domains along the [111] direction. c, FFT of red-marked regions in a, showing grains oriented along the [0$\overline{1}$1] direction. d, FFT of yellow-marked regions in a, showing grains oriented along the [01$\overline{1}$] direction. The shaded regions in c,d highlight the inner diffraction patterns that reflect mirror symmetry along the [111] direction.