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-
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-
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.
Abstract Fabrication of coplanar waveguide resonators with internal quality factors near 10 6 remains challenging. Here, high-purity superconductors are implemented through metamorphic epitaxial aluminum that is grown via molecular beam epitaxy on silicon and sapphire substrates. X-ray diffraction and scanning transmission electron microscopy indicate an abrupt highly ordered interface that results in crystal relaxation within a few monolayers of the substrate interface and no measurable interfacial contamination. Quarter-wave coplanar waveguide resonators are fabricated using optical lithography and measured at temperatures below 100 mK. Post measurement characterization with charge contrast imaging in a scanning electron microscope identifies processing artifacts at the waveguide sidewalls, on the exposed substrate area and on the exposed aluminum surface. Of primary importance are processing induced corrosion defects on aluminum sidewalls, nanoparticle contamination, and photoresist residue that is difficult to remove without affecting the superconductor material. Likely correlations between these artifacts and the measured quality factor are discussed in context of device to device variations in resonator performance.
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.
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.
Abstract Quantum bits (qubits) with long coherence times are an important element for the implementation of medium- and large-scale quantum computers. In the case of superconducting planar qubits, understanding and improving qubits’ quality can be achieved by studying superconducting planar resonators. In this paper, we fabricate and characterize coplanar waveguide resonators made from aluminum thin films deposited on silicon substrates. We perform three different substrate surface treatments prior to aluminum deposition: one chemical treatment based on a hydrofluoric acid clean; one physical treatment consisting of a thermal annealing at 880 °C in high vacuum; and one combined treatment comprising both the chemical and the physical treatments. The aim of these treatments is to remove the two-level state (TLS) defects hosted by the native oxides residing at the various samples’ interfaces. We first characterize the fabricated samples through cross-sectional tunneling electron microscopy, acquiring electron energy loss spectroscopy maps of the samples’ cross sections. These measurements show that both the chemical and the physical treatments almost entirely remove native silicon oxide from the substrate surface and that their combination results in the cleanest interface. Additionally, we analyze the effects of the various substrate treatments on the roughness of the silicon surface by means of atomic force microscopy surface morphology mapping. We then study the quality of the resonators by means of microwave measurements in the ‘quantum regime’, i.e., at a temperature T ∼ 10 mK and at a mean microwave photon number 〈 n ph 〉 ∼ 1 . In this regime, we find that both surface treatments independently improve the resonator’s intrinsic quality factor by ≈172%. The highest quality factor is obtained for the combined treatment, Q i ≈ 0.82 million, corresponding to an improvement by ≈256%. Finally, we find that the TLS quality factor averaged over a time period of 3 h is ∼3 million at 〈 n ph 〉 ∼ 10 , indicating that substrate surface engineering can potentially reduce the TLS loss below other losses such as quasiparticle loss and flux noise.
We explore the molecular beam epitaxy synthesis of superconducting aluminum thin films grown on c-plane sapphire substrates at cryogenic temperatures of 6 K and compare their behavior with films synthesized at room temperature. We demonstrate that cryogenic growth increases structural disorder, producing crystalline grains that modify the optical, electrical, and superconducting properties of aluminum. We observe that cryogenic deposition changes the color of aluminum from fully reflective to yellow and correlate the pseudodielectric function and reflectance with structural changes in the film. We find that smaller grain sizes enhance the superconductivity of aluminum, increasing its critical temperature and critical field. We then estimate the superconducting gap and coherence length of Cooper pairs in aluminum in the presence of disorder. Finally, we fabricate superconducting microwave resonators on these films and find that, independently of the growth temperature, the system is dominated by two-level system loss with similar quality factors in the high- and low-power regimes. We further measure a higher kinetic inductance in the cryogenically grown films.