Electronic Structures Of Color Centers In Wide Band-gap Semiconductors And Their Application In Quantum Bit

Developing robust quantum systems that can be easily manipulated is quite crucial for future use in advanced information and communication. Point defects in semiconductors are currently the most possible candidates for reaching this goal. It is not only because their electronic states can be used as an individually addressable, solid-state quantum bit (qubit) even at room temperature, but also because they are atomic-scale quantum systems, which meets the development of information and communication in future. So far, the most successful defect is the so-called nitrogen-vancancy (NV) center in diamond, which composes of a substational nitrogen atom and an adjacent vacancy defect in the charge state of-1. The electronic spin of the negatively charged NV center can be efficiently manipulated and measured using microwave and optical excitation, which meets the requirement of qubit operation. These exceptional quantum properties have motivated efforts to identify similar defects in other semiconductors, as they may offer an expanded range of functionality not available to the diamond NV center.In recent years, progress has been achieved. For example, some defects in silicon carbide have been found possess potential application in spin qubit. However, there are many problems to be solved. The kind of point defects in semiconductors is abundant, which can broaden our choice. Simultaneously, it comes up with the question of how to make choice among such lots of defects. Secondly, a very important criterion for a qubit candidate is that its abundance can be manipulated. One way is the formation via local particle irradiation, which requires subsequent annealing. Theoretical investigation of their formation and annealing behavior seem necessary. Additionally, we should reveal the excited properties of the individual quantum systems and transitions between their electronic states, which may help in understanding their optical spectra. In fact, it is theoretical analysis and calculation can be employed to reveal these problems and also compare with experimental conclusions. Group theory and the first-principles calculations on the basis of density-functional-theory (DFT) are now useful theoretical methods in studying microscopic structures and properties of materials. It can calculate geometric and electronic structures, annealing behaviors and dynamics of chemical reactions, spin-polarization and magnetism, optical absorption and excitation, etc. Depending on the current development of solid-state qubit, in this thesis we focus on:(1) the formation and annealing behaviors of the proposed potential spin qubit defects in silicon carbide;(2) whether there are other potential qubit defects in silicon carbide or even other semiconducting materials such as gallium nitride. Below are the main results and conclusions.(1) What kind of defective centers are qubit candidates? We employed group theory analysis to investigate defect levels induced by vacancy-related defects, including their degeneracy, energetic alignments, etc. On the basis of this, we proposed that type of defects (anion or cation vacancy defect) and their local symmetry should meet some requirements. Thus, we further performed first-principles calculations to characterize the geometry, formation energy, and electronic structures of an AlsiVc center in cubic silicon carbide crystals, a complex defect consisting of a carbon vacancy and an adjacent substitutional aluminum impurity on a silicon site. It is cation vacancy related defect, different from the three anion vacancy related qubit defects. We found that the AlsiVc center has a stable C3V symmetric geometry with lower formation energy. Its defect levels in3C-SiC align in a way different from those of diamond NV center, i.e., the twofold degenerated exy level is lower than the singlet levels in energy. These defect levels reside in the conduction bands and thus are unsuitable for qubit operations. These results agreed with those of group theory analysis.(2) As theoretically proposed currently, negatively charged silicon vacancy Vsi, complex defect NcVsf similar to diamond NV center, neutral divacancy VcVsi, etc. can be employed as solid-state qubit candidates. Their experimental preparation becomes important. However, the formation mechanism and annealing behaviors are still unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.(3) Next we consider the possibility of defects in other semiconductors instead of diamond and silicon carbide (group IV), such as Ⅲ-Ⅴ gallium nitride. The unique properties of gallium nitride crystal, such as a wide band-gap and high thermal conductivity, make it ideal material for electronic and optoelectronic devices. To apply in spin qubit, there are some criterions for the defects as proposed by Weber et al., such as room temperature ferromagnetism. In previous works, gallium vacancy (VGa) was expected to act ferromagnetically at room temperature, corresponding to a Curie temperature of about1400K. However, it has not been proved experimentally. Using an accurate hybrid exchange-correlation functional, we show that the Curie temperature is only150K at the VGa density of1.28x1021cm-3. This room temperature paramagnetism of cation vacancy related defects in GaN seems meet the requirement of spin qubit from this point of view. So, further investigation is needed to reveal their properties.(4) According to the case of diamond and silicon carbide, we think that there are two sorts of cation vacancy related defects in GaN that may meet the requirement of qubit. One is a gallium vacancy with one of its nearest N atoms replaced by an O atom forming a complex VoaON, the other is divacancy VoaVo. In chapter6, we took VGaON for example to study qubit centers in GaN. Based on first-principles calculations, we predicted that this VGaON center has much in common with the famous NV center in diamond, but the excitation energy is very low. The electron spin-polarization of the centers can be tuned by changing the charge states. The neutral VGasON center has the v↓and e↓xy defect states being well isolated from the bulk bands with appropriate spacing which are suitable for achieving spin qubit operation with low excitation energy. Inspired by our work, similar defect VAION in AIN is proposed as a qubit candidate by others. All these efforts emphasize the existence of qubit candidates in III-V semiconductors. Some of their properties are different from those of diamond NV center, and maybe even better. For example, lower excitation energy, stronger zero-phonon-lines (ZPL) of qubit centers in SiC.