Theoretical Studies Of Impurities In Cubic BC₂N And The Optical Properties Of Be₃N₂

Diamond is well known for the hardest material which has a notable mechanical property in the world. It has wide applications in the industry for its outstanding optical, thermal, acoustical and electrical properties. For the defects are ineluctably exist in all the crystals, and it can bring new defect electron levels in the forbidden band which makes a decisive effect on the character of the crystal. People have make great effort on the defect and impurity which would bring new level in the forbidden band, and it is consummate day by day, but the theory calculation result could not match the experiment result as well.In this work, we do a deep research on oxygen substitute in the cubic structures of BC₂N, and have completely theoretical analysis of the structural and electronic properties and the optical property using first-principles method. And the Co atom considered as the accelerant in composing BC₂N has also exist in the BC₂N crystal with a high concentration, so in our work we also do some research on the Co atom as a intrinsic defect; The burgeoning superhard semiconductor material Be₃N₂ with a wide band gap have a wide application foreground, but the Be is a toxin element which lead a small application in experiment, in our calculation we also do a deep research on the band structure, DOS, optical property, and the dynamics property. The main contents are as follows:â… . Our first-principles calculations show that it has lower formation energy after Oxygen replace the nitrogen in the c-BC₂N. From the electronic DOS we can see that the band structure has come together and the Fermi level get up a little higher and the DOS move to the lower energy after Nitrogen substituted. With pressure increase, the defect level near the conductive band cut through the Fermi level from F to Z which belongs to the high symmetry point in the BZ, and at the other side from F to Z point, the defect level go far away from the Fermi level which make it easy for the electrons in the defect level activated. It can be seen as the transformation of the semiconductor from P type to N type. The different charge in the O-N structure has also brought different defect level. From the optical calculation on the O-N structure shows that the real part of the dielectric function increase with the energy become larger which has a largest number when the energy come to 6.67eVo The main peak of the refractive index appears at the energy region 2.5-15eV, the largest peak corresponding to the 6.89eV, the extinction number get smaller along with the energy get bigger when the energy bigger than 12.46ev. The intrinsic defect of Co in C-BC₂N has formation energy of9.5eV, the formation volume is 0.9A , which prove that Co is hard to exist in BC₂N as an intrinsic defect and it will make the crystal expand.â…¡. We investigate three different structures of Be₃N₂, the calculated result shows that theα-Be₃N₂ is a direct band semiconductor with a band width of 4.36eV;β-Be₃N₂ is an indirect semiconductor with a band gap of 7.40eV. What's more theγ-Be₃N₂ is a perfect direct semiconductor with a band width of 2.50eV.We also found that the Bulk Modulus ofα-Be₃N₂ andβ-Be₃N₂ increase distinctively compared with theγ-Be₃N₂, and theα-Be₃N₂,β-Be₃N₂ have nearly the same changing trend.For theα-Be₃N₂, the largest number of the real part of the complex dielectric function appears at 7.0eV, and the imaginary part of complex dielectric function has the largest number of 11.OeV. We also do some optical calculations on this three structures, these results prove that the absorption function of theγ-Be₃N₂ appears three obvious peaks, the first peak has a energy of 10.12eV which comes from the electron transition from the 2s orbit of Be to the 1s orbit of N; the second peak mainly contributed from the 1s obit of Be to the 1s orbit of N with the energy of 15.28eV; the third peak comes from the 1s orbit of Be to 2p orbit of N with the highest energy of 16.88eV.