Cobalt and beryllium in diamond

: experimental and first-principles calculations of magnetic and electronic properties

Abstract

Diamond is a special material with extraordinary mechanical and electrical properties. To extend its applications to more areas, in this thesis, we implemented the preparation and characterisation of magnetic diamond film composites and the theoretical study of n-type doping of  diamond. Diamond with modified magnetic and electronic properties may find applications in electromagnetic devices and be used in extreme environments.

In the experimental section, we studied the preparation and properties of magnetic diamond composites, in which patterned magnetic material(Co nanoparticles or thin films) were used as part of the substrate to grow diamond film by chemical vapour deposition (CVD), aiming at imbuing ferromagnetic properties to the diamond film. Co was patterned by laser cutting or lift-off fabrication. After CVD diamond growth, the patterned Co was coated with a diamond film. The magnetic signal from the underlying Co was detected by magnetic force microscopy (MFM) through the 1 – 2 mm thick diamond film. Due to the robustness of diamond, the diamond film is expected to work as a protective layer, which will provide a comprehensive protection to magnetic devices.

The basic components required for making semi conductor devices are n- and p-type semiconductor materials. While p-type diamond can be made easily by boron doping, the preparation of n-type diamond remains problematic. The maturity of density-functional theory and the development of high-performance computing make it possible to simulate the electronic or magnetic properties of a given system. In the theoretical sections, we used the CRYSTAL 17 package to study the probability of using Be or Be-N clusters as dopants for making n-type diamond. Various substitutional and interstitial defect positions were investigated in terms of the thermodynamic, magnetic and electrical properties. Our calculation results suggest that it is hard to bring useable n-type semi conductivity to diamond at room temperature by single-element doping of Be. The ground-state of Be in diamond is a single substitutional defect, which imparts p-type semi conductivity to the diamond, but this has a high activation energy of ~ 1.2 eV. Although Be situated at an alternative tetrahedral interstitial site makes diamond n-type semi conductive and works as a shallow donor with the defect level 0.47 eV below the conduction band, this defect has a formation energy ~ 8.7 eV higher than that of the single substitutional defect, and so is unlikely to form.

For the theoretical study of Be-N clusters in diamond, Be and N were found to enhance the incorporation of each other, and therefore they are situated at adjacent single substitutional sites to reach the minimum energy state. The Be_sN₃ and Be_sN₄ clusters(a single substitutional Be in the centre, surrounded by 3 or 4 substitutional N atoms) behave as shallow donors in diamond, with donor levels of 0.51 and 0.46eV below the conduction band minimum. The formation of such clusters requires a coincidence of Be and its surrounding N atoms. A two-step preparation scheme was proposed for making such complex clusters, i.e., VN₃ and VN₄complexes can be made first, followed by the ion implantation of Be into the vacancies. The theoretical study in this thesis may provide some new ideas to the preparation of n-type diamond.