Xuejie Liu

The adsorption and migration of sulphur (S) atoms on the diamond (001) surface were investigated through first principles calculations to discover the inherent law in S-doped diamond film growth. Results indicated that deposited S atoms could abstract the hydrogen atom on the surface. The adsorption energies were in a range of 2.47 to 5.5 eV when S atoms were deposited on the hydrogen terminated surface or the surface with open radical sites (ORSs). The S atom could migrate on the surface of the 3ORS slabs and the energy barrier was approximately 1.35 eV. The calculations of the projected density of states and the analysis of the magnetic moments presented an interesting result, which demonstrated the evolving phenomena in S-doped diamond film growth and discovered the inherent laws. On the 2ORS slabs, the magnetic moment of the S atom became 0.000 μ_B after bonding with the two carbon atoms. In such case, a new doped C atom combined with the S atom with a triple bond, and then the C–S molecule was desorbed from the surface. The abstraction of the adsorbed S atom results from the fact that S atoms have six electrons in their outermost electron shell. This finding revealed the reason behind the low S incorporation and the growth rate decrease in S-doped diamond film deposition. This discovery also indicated that atoms with six electrons in their outermost electron shell might hardly be doped into the diamond films during the deposition process.

In this study, N-doped diamond films were prepared through microwave plasma chemical vapor deposition with NH₃/CH₄/H₂ gas mixtures. The effects of the ammonia addition to the process gas mixture on the morphology and structure of diamond films were systematically investigated through characterization by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). This work focuses on the ammonia addition to the process gas mixtures in the narrow range of N/C ratios from 0.4% to 1.0%. The results reveal that different N/C ratios can affect the morphology, the preferred crystal orientation, and the sp³/sp² ratio in the films. When the N/C ratio of the process gas mixture ranges from 0.6% to 1.0%, the XRD and SEM results show that ammonia addition is beneficial for the growth of the (110) faceted grains. When the N/C ratio of the process gas mixture ranges from 0.8% to 1.0%, the XPS and Raman results indicate that the diamond films exhibit a considerable enhancement in the sp³ fraction.

The elastic properties and electronic structure of interfaces in Ti–Si–N nanocomposite films were calculated using first principles based on density functional theory (DFT). The results showed that the mechanical moduli of the single-substitution interface (1Si–6N) were higher than those of the double-substitution interface and interstitial interface (1Si–4Ti4N). The single-substitution interface (1Si–6N) was revealed to be characterized as the more elastically isotropic structure in different directions, whereas the Young’s moduli significantly varied in different directions in the interstitial interface (1Si–4Ti4N). The electronic structures of interfaces indicated that the structures were conductors with intersecting bands. Strongly delocalized d states of titanium and silicon ions were spread over a wide region of about 10–12 eV and were strongly hybridized with the nitrogen 2p states. The overall appearance of the calculated cross-sections of the electron density difference changed drastically.

The adsorption and migration activation energies of boron atoms on a hydrogen-terminated diamond (001) surface were calculated using first principles methods based on density functional theory. The values were then used to investigate the behavior of boron atoms in the deposition process of B-doped diamond film. On the fully hydrogen-terminated surface, the adsorption energy of a boron atom is relatively low and the maximum value is 1.387 eV. However, on the hydrogen-terminated surface with one open radical site or two open radical sites, the adsorption energy of a boron atom increases to 4.37 eV, and even up to 5.94 eV, thereby forming a stable configuration. When a boron atom deposits nearby a radical site, it can abstract a hydrogen atom from a surface carbon atom, and then form a BH radical and create a new radical site. This study showed that the number and distribution of open radical sites, namely, the adsorption of hydrogen atoms and the abstraction of surface hydrogen atoms, can influence the adsorption and migration of boron atoms on hydrogen-terminated diamond surfaces.