Cryogenic Hybrid Superconducting Electronics In article 2416957, Alessandro Paghi and co-workers propose the InAs-on-Insulator (InAsOI), a new platform for developing superconducting electronics. An epilayer of semiconducting InAs with different electron densities is grown onto a cryogenic insulating InAlAs metamorphic buffer, used to decouple adjacent devices electrically. Josephson junctions with a high supercurrent density are fabricated employing Al as a superconductor and InAsOI with different electron densities.
Cryogenic Hybrid Superconducting Electronics In article 2416957, Alessandro Paghi and co-workers propose the InAs-on-Insulator (InAsOI), a new platform for developing superconducting electronics. An epilayer of semiconducting InAs with different electron densities is grown onto a cryogenic insulating InAlAs metamorphic buffer, used to decouple adjacent devices electrically. Josephson junctions with a high supercurrent density are fabricated employing Al as a superconductor and InAsOI with different electron densities.
Abstract Superconducting circuits based on hybrid InAs Josephson Junctions (JJs) play a starring role in the design of fast and ultra‐low power consumption solid‐state quantum electronics and exploring novel physical phenomena. Conventionally, 3D substrates, 2D quantum wells (QWs), and 1D nanowires (NWs) made of InAs are employed to create superconducting circuits with hybrid JJs. Each platform has its advantages and disadvantages. Here, the InAs‐on‐insulator (InAsOI) is proposed as a groundbreaking platform for developing superconducting electronics. An epilayer of semiconducting InAs with different electron densities is grown onto an InAlAs metamorphic buffer, efficiently used as a cryogenic insulator to decouple adjacent devices electrically. JJs with various lengths and widths are fabricated employing Al as a superconductor and InAs with different electron densities. A switching current density of 7.3 µA µm−1, a critical voltage of 50‐to‐80 µV, and a critical temperature equal to that of the superconductor used are achieved. For all the JJs, the switching current follows a Fraunhofer‐like pattern with the out‐of‐plane magnetic field. These achievements enable the use of InAsOI to design and fabricate surface‐exposed Josephson Field Effect Transistors with high critical current densities and superior gating properties.
The performance and coherence of superconducting quantum systems are fundamentally tied to the quality of the materials from which they are built. In superconducting transmon qubits, many loss mechanisms and sources of decoherence originate from material imperfections or unwanted oxide formation. Likewise, semiconductor–superconductor hybrid systems depend on high-quality interfaces to ensure strong coupling between electronic states in the two materials. In such hybrid devices, the accessible operating regime is often dictated by the intrinsic material properties of either the semiconductor or the superconductor. Advancing quantum device performance therefore requires exploring new material systems and optimizing their physical characteristics. In this thesis, we present the development of several superconducting material platforms for quantum technologies, including superconducting qubits, single-photon detectors, and semiconductor superconductor hybrid devices. We focus on the growth and characterization of superconducting films and heterostructures, and we establish clear connections between their material properties and the resulting device performance. First, we will present a novel semiconductor-superconductor hybrid material based on in-situ deposited epitaxial layers of Nb and Al on a shallow InAs quantum well. In this structure, the Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs two dimensional electron gas. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms. Then, we discuss a different approach in the next chapter where we develop a hybrid material based on a molecular beam epitaxy grown InAsSb surface quantum well with an in-situ deposited Nb layer directly on top of the quantum well. Relative to conventional Al-InAs based systems, the InAsSb surface quantum well offers a lower effective mass and stronger spin-orbit interaction, while the Nb layer has a higher critical temperature and a larger critical magnetic field. The in-situ deposition of the Nb layer directly on top of the quantum well results in a high-quality interface that enables strong coupling to the InAsSb quantum well. Transport measurements on Josephson junctions reveal an induced superconducting gap of 1.3 meV and a critical current density of 3.75 × 10³A/cm². Furthermore, a planar asymmetric SQUID is realized, exhibiting gate-tunable superimposed oscillations originating from both the individual Josephson junction and the full SQUID loop. The large induced superconducting gap combined with strong spin-orbit interaction position this material as an attractive platform for experiments exploring gate-tunable superconductivity and topological superconducting devices. In the next chapter, we will focus on establishing a link between Nb film crystal properties and their performance in superconducting circuits. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10⁶ at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasi-particle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Then, we explore yet a different material platform by attempting to realize onchip NbTiN single photon detectors on LiNbO₃, an emerging photonic platform with high potential for scalable quantum information processing. We use DC reactive magnetron sputtering to grow high-quality NbTiN thin films and investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (𝑇𝑐) of 12.3 K grown at a substrate temperature of 825 K. We demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LiNbO3 platform and investigate the obtained interface quality. Finally, we fabricate superconducting nanowire single photon detectors from the NbTiN film on LiNbO₃ and characterize the detector performance. Lastly, we report a gate-tunable Josephson diode effect in hybrid nanowire junctions consisting of a spin-orbit-coupled semiconductor core coated with epitaxial ferromagnetic insulator and superconductor shells. The wires display a hysteretic superconducting window as a function of axial magnetic field. In the superconducting regime, the devices exhibit nonreciprocal current transport, with the diode efficiency showing a strong dependence on back-gate voltage. The effect persists in a remanent magnetization state following a controlled demagnetization procedure, establishing zero-field operation. The combination of results presented in this thesis stress the importance of a material-focused approach in advancing the performance of quantum hardware. The novel hybrid material platforms developed in this thesis expand the operating regime of devices, which could enable future experiments involving gate-tunable superconductivity or topological properties. Similarly, we demonstrate that optimizing superconductor crystal structure benefits quantum devices, such as single-photon detectors and superconducting resonators.
...we introduced an integrated circuit consisting of arrays of planar Josephson junctions based on two-dimensional electron gas (2DEG) in In0.75Ga0.25As heterostructure. The methods that are used for ballistic 2D Josephson junction and hybrid quantum integrated circuit fabrication will be demonstrated. Furthermore, the temperature and magnetic field dependencies of the phase coherent quantum transport in 2D Josephson junctions, the induced superconducting properties into 2DEG, and the subharmonic energy gap structures (SGS) due to Andreev reflections at the S-Sm interfaces will be discussed. Reference: [1] K. Delfanazari, et al, Adv. Mater. 29, 1701836 (2017). [2] K. Delfanazari, et al, J. Magn. Magn. Mater. 459, 282-284 (2018). [3] K. Delfanazari, et al, IEEE Trans. Appl. Supercond. 28, 1100304 (2018).
Time-division multiplexing of cryogenic signals is a promising approach to reduce space requirements, shorten cooldown times, and increase the number of quantum devices measured per cooldown. We demonstrate time-division multiplexing of non-dissipative supercurrents using voltage-controlled hybrid superconducting demultiplexers. These chips integrate superconducting Josephson Field Effect Transistors including Al superconducting electrodes, proximitized semiconducting InAs channels, and hafnium oxide gate insulators. Each transistor fully suppresses the switching current and increases the resistance 20 times under a gate voltage of −4.5 V. A demultiplexer with one input and eight outputs showed a non-dissipative input range of ±2 μA, operating up to 100 MHz in signal frequency, and 100 kHz in switching frequency at 50 mK. It achieved near-zero insertion loss in the superconducting state and an OFFON ratio of 17.5 dB. By optimizing the signal layout, the operation was extended up to 4 GHz using a demultiplexer with two outputs. The authors report a 1-input-8-ouput superconducting demultiplexer consisting of 14 Al-InAs-Al Josephson Field Effect Transistors, enabling time-division multiplexing of non-dissipative current up to several GHz. This strategy can reduce the number of I/O lines to drive cryogenic electrical devices in quantum applications.
Further to the development of scalable hybrid quantum circuits introduced by us [1-5], here we report on the discovery of an unconventional Josephson effect in hybrid In0.75Ga0.25As two-dimensional electron gas-Nb junctions that are designed and fabricated for scalable hybrid quantum circuit integration. Reference: [1] K. Delfanazari, et al, Adv. Mater. 29, 1701836 (2017). [2] K. Delfanazari, et al, J. Magn. Magn. Mater. 459, 282-284 (2018). [3] K. Delfanazari, et al, IEEE Trans. Appl. Supercond. 28, 1100304 (2018).
In this paper, the author describes studies on electronic devices and circuits utilizing both of superconductors and semniconductors. First, a design of an interface circuit composed of Josephson junctions and MOSFETs is described. The Josephson-MOS interface circuit will be used for realizing high-speed data communication between a Josephson digital circuit and a semiconductor one. Second, fabrication and characterization of MOSFETs with superconductive NbN gate electrodes are described. The use of superconductive gates makes it easy to hybridize MOSFETs with Josephson junctions. Finally, studies of Josephson-CMOS hybrid memory circuits are introduced.