Quantum Defects from First Principles
Point defects in semiconductors or insulators are a promising platform to realize quantum information science, composed of quantum computing, quantum communication, and quantum metrology. These so-called quantum defects are particularly appealing because they are fixed in a controlled solid-state environment, hold the promise of room temperature operation, and will benefit from mature semiconductor fabrication techniques for integration and scaling. First-principles calculations based on density functional theory have been indispensable for the study of point defects: such calculations provide crucial microscopic insight that may be inaccessible in experiments. In this dissertation, we develop and apply first-principles methodologies to treat quantum defects.
Nonradiative transitions are integral to the control and operation of quantum defects. Indeed, nonradiative transitions dissipate energy through vibrations and thus can impact the quantum efficiency of a given quantum defect. We developed the Nonrad code, which implements a quantum-mechanical formalism to evaluate the nonradiative transition rate from first principles. We also put into effect several important modifications that are essential for attaining accurate rates.
Identifying novel quantum defects is of vital importance for their widespread utilization in quantum information science. Boron nitride is an ultra-wide-band-gap material with excellent thermal and chemical stability, making it a promising host for quantum defects and for applications in electronic devices. Control over conductivity is essential to utilize boron nitride in the proposed applications. In cubic boron nitride, we assess potential dopants and their ability to produce n-type conductivity.
In hexagonal boron nitride, bright single-photon emitters have been observed in the visible spectrum; however the microscopic origin of the emission has eluded researchers. Here we propose boron dangling bonds as the origin of the emission and provide a thorough characterization of their properties. We find that boron dangling bonds possess an optical transition with minimal coupling to phonons; we also calculate the magnetic-field dependence and show it to be in agreement with experiments. In a monolayer, we find that the boron dangling bond will behave similarly to when it is embedded in bulk material. Furthermore, we demonstrate the importance of out-of-plane distortions on the dangling bond, a result that has implications for other quantum defects in two-dimensional materials. Finally, in a fruitful collaboration with the experimental group of Prof. Lee Bassett at the University of Pennsylvania, we elucidated the optical dynamics of boron dangling bonds.
In total, this work advances the study of quantum defects through the development and application of first-principles techniques.