Analysis of Semiconductor Silicon-Based Heterostructure Quantum Dots for Qubit Systems

Quantum computing is a breakthrough computation paradigm capable of overcoming the constraints of classical computers. Several semiconductor-based solutions are currently under research to implement qubits, with one of the most fascinating techniques being the utilization of electron spin qubits inside quantum dots, thanks to their flexibility, small footprint, and compatibility with the CMOS integration technique. This work explores three silicon-based quantum dot heterostructures: two double quantum dot heterostructures (one realized with SiGe and the other one SiMOS) and a four quantum dot SiGe heterostructure. The above structures have been analyzed using the low-level FEM-based simulator QTCAD to derive essential physical parameters critical for creating electron spin qubits. This analysis can be useful for testing different device configurations without the need for experimental fabrication. For example, using QTCAD, it is possible to easily modify the gate distance, gate dimensions, layers materials and dimensions, and perform low-temperature simulations, tasks that are typically complex to perform in a physical laboratory. Although QTCAD may not be entirely accurate compared to real-world experiments, it provides useful insights of device behaviour. The goal is to leverage these simulations to evaluate the device's feasibility for real-world applications and efficiently assess how it responds to structural parameter modifications. As a result, changes to the structure can be made by simply modifying the simulation code, avoiding the need for repetitive and expensive lithographic processes. The study begins with a transport analysis on the two double quantum dot heterostructures, which will yield Coulomb peaks, Coulomb diamonds, and charge stability diagrams. Before calculating these quantities, both the nonlinear Poisson equation and the single-particle Schrödinger equation must be solved. However, when dealing with multiple quantum dots, solving the single-particle Schrödinger equation alone is not enough. Hence, a many-body solver is adopted to carry out transport analysis. The results indicate a single-electron regime and reveal a strong interdependence between the dots, even if they are well-defined. This highlights the complexity of quantum dot systems and the importance of understanding their coupling mechanisms for precise control and manipulation. Indeed, the voltage applied to one dot’s gate directly impacts the behaviour of the other dot through this capacitive interaction. After analyzing the previously mentioned structures, a four-quantum-dot SiGe heterostructure was examined. The aim is to verify the possibility of investigating a four-dot quantum structure using QTCAD and to study the formation and behavior of the four quantum dots compared to the previous cases. The results show that, despite having a smaller gate separation and dimensions than the double quantum dots scenario, the four quantum dots remain well-defined within the quantum layer. However, as expected, the dots are more interdependent than in earlier examples. Finally, the results of this study can be used to create four or two logic gates using these heterostructures. The four quantum dots device, in particular, can be utilized to create complex logic circuits that include more than one quantum logic gate. Furthermore, it would be intriguing to explore the outcomes of simulating hole spin qubits with Germanium quantum dots in QTCAD.