| Abstract [eng] |
In recent years, a lot of attention started shifting towards the investigation of point defects in view of their potential applications in quantum information processing and quantum emitters. Nowadays, one of the most interesting defects is the NV center in diamond, but the diamond has its technological shortcomings in real world applications, such as high costs and fabrication difficulties. Therefore, it is important to search for defects analogous to the diamond NV center in other wide-bandgap semiconductors. h-BN has one of the largest band gaps among III-V compounds. Its value was determined to be 5.955 eV by means of optical spectroscopy, so it is expected to host optically active defects that have ground and excited states within the gap. In 2016 scientists were able to measure quantum emission from localized defects in h-BN monolayers and bulk crystal even at room temperature. However, it is still not clear what defects are probable source of the measured emission. Ab-initio calculations which are based on density functional theory (DFT), could bring a lot of valuable information about defects: they can predict whether a desired defect is likely to form in a given material and would it be stable once formed, etc. In this study we investigate boron and nitrogen vacancies, which are likely to form in h-BN during the growth process. Spin-polarized calculations were performed to examine neutral and charged states of these defects and their alternative forms. Formation energies of boron and nitrogen vacancies were calculated. In both cases (boron-rich and nitrogen-rich conditions) formation energies are very high (8–10 eV for boron vacancy, 6–8 eV for nitrogen vacancy), therefore concentration of these type of defects in h-BN is not expected to be high. Spin state of the neutral boron vacancy was determined to be S = 3/2. According to the selection rules, optical transitions could appear at about 880 nm, polarized in the z direction. Spin of the neutral nitrogen vacancy was determined to be S = 1/2. It was shown that there is no intra-defect transition in any of the charge states. The spin state of the neutral boron vacancy complex with oxygen ([V_B-O_N]) is S = 0. No intra-defect transitions are expected in the neutral state. An alternative form of the boron vacancy [N_B -V_N] has spin state of S = 1/2 in its neutral state. It can have three optical transitions in the 512-832 nm region. Such transitions would be polarized in either y (along the symmetry axis) or z (perpendicular to the boron nitride planes) directions. Measurements of the excitation and emission polarization from a single defect in the z direction need to be carried out. |
