Novel isotopically enriched and aluminum (Al) delta-doped silicon crystals with exceptional material properties are proposed and developed, in an effort to bridge superconducting quantum information with silicon-based quantum devices for a new generation of solid-state quantum computing. Quantum computing platforms based on semiconducting and superconducting qubits are two powerful candidates. For semiconductor spin qubits, long coherence times can be achieved when using isotopically enriched 28Si as a host material. However, qubit coupling in semiconductor qubits is difficult to achieve due to the nanometer scale of the devices. For superconductor qubit devices, millimeter scale with long spatial coherence length makes them relatively easy to couple multiple qubits, but losses at the material interface are limiting the device performance. Our ultimate objective is to create a hybrid system where both Si spin qubits and superconductor qubits are coupled in a nuclear spin-free and interface-free material. To achieve this, a superconducting semiconductor with monocrystalline structure is proposed. One possible candidate is through Al delta-doped 28Si, as highly Al doped Si is expected to have a 10× higher critical temperature (Tc) than highly boron (B) doped Si (Tc ≈ 0.6 K). The focus of this thesis is to attack two materials limitations for realizing a monocrystalline, super-semiconducting hybrid architecture: 1) substantially reducing 29Si that limits coherence of semiconducting spin qubits and 2) exploring supersaturated, Al-doped silicon as a system for localized superconductivity within silicon with a potential Tc > 1K.In the first part of this thesis, we demonstrate the advancements in isotopically enriched 28Si in terms of chemical purity, enrichment, and electrical properties. In this work, a new, compact UHV version of the hyperthermal Penning ion source was designed and built. An improvement in the chemical purity from 98.47 % to 99.97 %, while maintaining a 28Si enrichment of > 99.999987 % (0.832 ppm 29Si) has been achieved. We broaden the material variety of 28Si at different levels in the quantum information community by demonstrating the ability to grow isotopically enriched 28Si epitaxial films with precisely controlled (≈ 90 % accuracy) enrichment levels, ranging from natural abundance to < 1 ppm 29Si. In addition, to better assess the quality of our 28Si material, we have successfully fabricated and measured 28Si MOSFET devices, and compared to those from natural abundance Si on the same substrate. The charge carrier mobility on isotopically enriched 28Si is found to be approximately a factor of 3 lower compared to the natural abundance Si, a result of the short-range scattering (impurity scattering). In the second part of this thesis, we report on the material growth and characterization of super-saturated Al delta layers in Si and explore the possibility for superconductivity. To reach a critical density needed for superconducting transition, the first step is to study the saturation density of this dopant and a way to confine it in 3D. Using a combination of different characterization tools, the maximum 2D atom density of one atomic layer of Al on Si(100) surface before cluster formation is found to be 3.4 × 1014 cm-2. We also studied the effects of different material growth methods on electrical conduction and the possibility of reaching higher 3D density of Al in this Si-Al-Si heterostructure. We found that Al delta doping in Si behaves differently compared to other dopants: the incorporation anneal does not change the dopant activation efficiency. Standard molecular beam epitaxy (MBE) and locking layer (LL) growth on Al layer is not successful and Al dopant activation is found to be < 50 %, most likely due to the tendency of Al atoms to move toward the surface and the cluster states been developed from the thermal anneals. The electrical conduction of this delta layer at low temperature is also studied and modeled using a temperature dependent two-carrier type model, which is the first reported conducting Al delta layer in Si. We believe that reaching the superconducting transition using an Al delta layer as a dopant in Si is possible, but this requires further studies both experimentally and theoretically to minimize the Al segregation in order to achieve a high enough 3D dopant density.
Novel isotopically enriched and aluminum (Al) delta-doped silicon crystals with exceptional material properties are proposed and developed, in an effort to bridge superconducting quantum information with silicon-based quantum devices for a new generation of solid-state quantum computing. Quantum computing platforms based on semiconducting and superconducting qubits are two powerful candidates. For semiconductor spin qubits, long coherence times can be achieved when using isotopically enriched 28Si as a host material. However, qubit coupling in semiconductor qubits is difficult to achieve due to the nanometer scale of the devices. For superconductor qubit devices, millimeter scale with long spatial coherence length makes them relatively easy to couple multiple qubits, but losses at the material interface are limiting the device performance. Our ultimate objective is to create a hybrid system where both Si spin qubits and superconductor qubits are coupled in a nuclear spin-free and interface-free material. To achieve this, a superconducting semiconductor with monocrystalline structure is proposed. One possible candidate is through Al delta-doped 28Si, as highly Al doped Si is expected to have a 10× higher critical temperature (Tc) than highly boron (B) doped Si (Tc ≈ 0.6 K). The focus of this thesis is to attack two materials limitations for realizing a monocrystalline, super-semiconducting hybrid architecture: 1) substantially reducing 29Si that limits coherence of semiconducting spin qubits and 2) exploring supersaturated, Al-doped silicon as a system for localized superconductivity within silicon with a potential Tc > 1K.In the first part of this thesis, we demonstrate the advancements in isotopically enriched 28Si in terms of chemical purity, enrichment, and electrical properties. In this work, a new, compact UHV version of the hyperthermal Penning ion source was designed and built. An improvement in the chemical purity from 98.47 % to 99.97 %, while maintaining a 28Si enrichment of > 99.999987 % (0.832 ppm 29Si) has been achieved. We broaden the material variety of 28Si at different levels in the quantum information community by demonstrating the ability to grow isotopically enriched 28Si epitaxial films with precisely controlled (≈ 90 % accuracy) enrichment levels, ranging from natural abundance to < 1 ppm 29Si. In addition, to better assess the quality of our 28Si material, we have successfully fabricated and measured 28Si MOSFET devices, and compared to those from natural abundance Si on the same substrate. The charge carrier mobility on isotopically enriched 28Si is found to be approximately a factor of 3 lower compared to the natural abundance Si, a result of the short-range scattering (impurity scattering). In the second part of this thesis, we report on the material growth and characterization of super-saturated Al delta layers in Si and explore the possibility for superconductivity. To reach a critical density needed for superconducting transition, the first step is to study the saturation density of this dopant and a way to confine it in 3D. Using a combination of different characterization tools, the maximum 2D atom density of one atomic layer of Al on Si(100) surface before cluster formation is found to be 3.4 × 1014 cm-2. We also studied the effects of different material growth methods on electrical conduction and the possibility of reaching higher 3D density of Al in this Si-Al-Si heterostructure. We found that Al delta doping in Si behaves differently compared to other dopants: the incorporation anneal does not change the dopant activation efficiency. Standard molecular beam epitaxy (MBE) and locking layer (LL) growth on Al layer is not successful and Al dopant activation is found to be < 50 %, most likely due to the tendency of Al atoms to move toward the surface and the cluster states been developed from the thermal anneals. The electrical conduction of this delta layer at low temperature is also studied and modeled using a temperature dependent two-carrier type model, which is the first reported conducting Al delta layer in Si. We believe that reaching the superconducting transition using an Al delta layer as a dopant in Si is possible, but this requires further studies both experimentally and theoretically to minimize the Al segregation in order to achieve a high enough 3D dopant density.
I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. ii The growth of Aluminum (Al) on semiconductors and dielectrics is a cornerstone in the quest for scalable quantum computing systems. Indeed the electrical properties of Al make it an exceptional candidate for the realization of superconducting resonators, pivotal tools for understanding and operating superconducting qubits. Such resonators have been fabricated recently on Sapphire substrates, using Molecular Beam Epitaxy (MBE), and displayed quality factors above a million. Complementary studies of these resonators have demonstrated that the metal-substrate interface was the primary source of decoherence and losses, highlighting the importance of pristine interfaces (free of contaminants), and high quality epitaxial growth in order to minimize the native defects level. In this work we investigate different substrate materials in order to yield equivalent or higher qual-
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.
Abstract Aluminum (Al) is the most critical material in the burgeoning field of superconducting quantum computing due to its self-limiting native oxide and reliable Josephson effect. However, material-induced decoherence remains a major bottleneck currently limits the scalability of quantum processors. As highlighted in seminal roadmaps, dielectric loss from amorphous interfaces and structural defects in standard evaporated Al films are the primary sources of noise. While current international efforts focus on engineering-level mitigation (e.g., circuit design), this review proposes a materials-level solution through the lens of research conducted in Taiwan. We will focus on the paradigm shift enabled by molecular beam epitaxy (MBE) growth of ultrahigh-quality Al nanofilms. Unlike traditional “dirty-limit” superconductors where transition temperature (Tc) increases are linked to disorder, researchers in Taiwan have demonstrated a doubling of Tc (reaching ≈ 2.4 K) in the ultraclean limit. We will discuss the underlying physics, such as surface phonon softening and quantum size effects, and why these clean-limit films are superior for practical applications. Notably, in situ Al2O3 passivation ensures long-term environmental stability, enabling MBE-grown-Al-based resonators to maintain internal quality factors exceeding 106 even after extended air exposure. Current literature frequently reviews atomic layer deposition (ALD) or disordered “granular aluminum” for high kinetic inductance. However, there is a significant gap in surveying epitaxial, wafer-scale Al for applied devices. Our review aims to provide a balanced global survey, showcasing how the synergy between MBE expertise and low-temperature physics in Taiwan provides a viable technological pathway to address the materials challenges of the quantum era.
Several solid-state quantum information processor proposals are based on silicon [16-18, 20, 52], including the original architecture conceived by Kane in 1998 [22], in which the quantum information was stored in the spin states of 31P donors. The interest raised by silicon in the field of quantum information is twofold. On the one hand, it represents the main semiconductor used for large- scale fabrication. On the other hand, it owns interesting physical properties suitable for preserving andmanipulating quantum states, such as long coherence times and relatively low disturbance of nuclear spins, which are further enhanced by employing purified 28Si. The employment of individual donors in silicon covers all the aspects of quantum information encoding and processing, like quantumbit (qubit) storage, implementation of quantum logic gates, coherent transfer of quantum states, and readout schemes. This chapter is devoted to the physics of few dopants in nanometric silicon devices and their applications to quantum information processing purposes. In the first part, an overview of the physics of themost common donors and acceptors in silicon is presented. Next, the quantum information key concepts and their implementation in donor-based architectures are discussed. Finally, decoherence effects and the concept of measurement of quantum states are presented for the case of electron states bound to donors in silicon. Between the sixties and the early eighties of the twentieth century, all the theoretical and experimental aspects related to the doping of bulk silicon have been investigated and explained. A silicon crystal consists of a diamond lattice constituted by two interleaved cubic lattices (face-centered cubic, FCC), the second of which has the origin in the center of the tetrahedron given by the origin (0,0,0) and the centers of the faces 100. Each Si atom has four valence electrons, which create a covalent bond with the shared electrons of four other Si atoms. The substitution of silicon atoms with atoms of a group V element (indicated as donors, typically P, As, and Sb in silicon) generates an n-doped silicon crystal (n-Si). Each donor introduces an eccess electron in the crystal, and it provides new energy levels in the band gap, close to the conduction band edge. Similarly, substituting silicon atoms with atoms of a group III element (an acceptor, typically B in silicon) generates a p-doped silicon crystal (p-Si). Each acceptor introduces an electron hole in the crystal, and it provides new energy levels in the band gap, close to the valence band. The diffusion of impurities in semiconductors alters the conduction properties from an insulating to a metallic regime. Depending on the different doping concentration, the wavefunction of the electron (hole) states introduced by each donor (acceptor) may overlap negligibly or substantially with those of neighboring sites. Such a transition from low doping to high doping as a function of the average distance between neighboring sites is described in terms of an Anderson-Mott transition and produces additional impurity bands (Hubbard bands) below the conduction band edge at sufficiently high concentration [1, 34]. The transport is governed by mechanisms based on localized states at low density, while it is based on delocalized states at high density. Four conventional regimes of impurity concentration are defined [50]. The dilute concentration of impurities holds for n < 1 · 1016 cm−3. There, the problem of the donor is that of a hydrogen atom with a scaled Rydberg and radius. Between the densities n of 1 ·1016 cm−3 and 2 · 1017 cm−3 (rNN = 11.9 nm), respectively, the regime is called semidilute, and it is characterized by formation of pairs. Above 2 · 1017 cm−3 and below the metallic behavior, which occurs at 3.7 · 1018 cm−3, the regime is called intermediate, and the formation of random clusters leads to effects generally accounted by the Hubbard band formation. Above n > 3.7 · 1018 cm−3, silicon is treated as a metal. In this section the physics of the first three regimes of donor concentration and the Anderson-Mott transition are described. The Anderson-Mott transition has been observed down to microscopic scale by employing arrays of few deterministically implanted As ions (see chapter 5) in Si transistors [39]. Analogous arguments and treatment can be given to acceptor concentration regimes.
Abstract The quest for a universal quantum computer has renewed interest in the growth of superconducting materials on semiconductor substrates. High-quality superconducting thin films will make it possible to improve the coherence time of superconducting quantum bits (qubits), i.e., to extend the time a qubit can store the amplitude and phase of a quantum state. The electrical losses in superconducting qubits highly depend on the quality of the metal layers the qubits are made from. Here, we report on the epitaxy of single-crystal Al (011) layers on GaAs (001) substrates. Layers with 110 nm thickness were deposited by means of molecular beam epitaxy at low temperature and monitored by in situ reflection high-energy electron diffraction performed simultaneously at four azimuths. The single-crystal nature of the layers was confirmed by ex situ high-resolution x-ray diffraction. Differential interference contrast and atomic force microscopy analysis of the sample’s surface revealed a featureless surface with root mean square roughness of 0.55 nm. A detailed in situ study allowed us to gain insight into the nucleation mechanisms of Al layers on GaAs, highlighting the importance of GaAs surface reconstruction in determining the final Al layer crystallographic orientation and quality. A highly uniform and stable GaAs (001)- ( 2 × 4 ) reconstruction reproducibly led to a pure Al (011) phase, while an arsenic-rich GaAs (001)- ( 4 × 4 ) reconstruction yielded polycrystalline films with an Al (111) dominant orientation. The near-atomic smoothness and single-crystal character of Al films on GaAs, in combination with the ability to trench GaAs substrates, could set a new standard for the fabrication of superconducting qubits.
The understanding of quantum mechanics enabled the development of technology such as transistors and has been the foundation of today’s information age. Actively using quantum mechanics to build quantum technology may cause a second revolution in handling information. However, to execute meaningful algorithms, largescale quantum computers have to be built. Such systems are constructed from many qubits, the quantum version of the classical bit. While exciting progress is being made across a range of different qubit platforms, achieving the radical scalability that is necessary to build a largescale processor could be a roadblock. Huge challenges are put on reproducibility, inand output connectivity and material quality. Qubits based on the spins of electrons and holes confined in semiconductor quantum dots may have an important advantage in constructing quantum processors. This platform can profit from the advanced semiconductor industry that was responsible for the first computing revolution. Group IV semiconductors such as silicon and germanium have a high compatibility with industrial semiconductor manufacturing and contain stable isotopes with zero nuclear spin. The materials can be isotopically purified and serve as excellent hosts for spins with long quantum coherence. In Chapter 3 we present quantum dot arrays in silicon metaloxidesemiconductor (SiMOS), strained silicon (Si/SiGe) and strained germanium (Ge/SiGe). A nearly identical integration scheme based on an overlapping gate structure can be used to define quantum dots in each platform. Each platform has its own opportunities, which are carefully assessed. By employing charge sensing we confirm that all quantum dots can be depleted to the singleelectron regime. We compare capacitive crosstalk and find it to be the smallest in SiMOS, relevant for the tuning of quantum dot arrays. Using this crossplatform integration, we can study qubits in each platform with minimal overhead. Long coherence times, excellent singlequbit gate fidelities and twoqubit logic have been demonstrated with SiMOS spin qubits, making it one of the leading platforms for quantum information processing. However, due to the high disorder at the Si/SiO2 interface compared to Ge/SiGe and Si/SiGe interface, quantum dots defined in SiMOS are small and achieving sufficient control over single electrons has been a long standing challenge. In Chapter 5 we show experiments on a double quantum dot that can be isolated from its reservoir. We demonstrate a tunable tunnel coupling between single electrons up to 13 GHz and tunable tunnel rates down to below 1 Hz. These results mark an important step towards the required degree of control over the location of and coupling between quantum dots, necessary for the operation of a large array.