EPITAXIAL GROWTH AND CHARACTERIZATION OF 28Si AND ALUMINUM DELTA LAYERS FOR HYBRID QUANTUM COMPUTING
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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.