On the opto-electrical properties of ion-implanted single-crystal diamond in the visible and near-visible regime (2012)

<br>Diamond is potentially the ultimate material for a vast range of optical and quantum-computing applications. The fabrication of diamond-based optical and photonic devices by ion implantation requires knowledge of the diamond’s modified optical properties. The purpose of this thesis is to determine how the optical properties of ion-implanted diamond depend on the ion implantation fluence. Ion-implanted diamond has been studied structurally, electrically and optically in the range 200 – 1700 nm. Optical micro-waveguides are of fundamental importance to integrated optics for the transport of light into, out from and around diamond-based quantum devices and other photonic devices. A number of micro-waveguides have been modelled; the dimensions are designed to maintain single-mode propagation in the unimplanted core of the waveguide. Disagreement in previous studies suggests qualitatively different mechanisms for ion-beam modification, at least at low fluences, between light-ion and heavy-ion implantation. This thesis supports the observation of the lowering of refractive index by low-fluence heavy-ion bombardment. There exists a region of implantation fluences in which the refractive index is lower than that of pristine diamond, while the absorption coefficient is still low enough to enable fabrication of efficient waveguides. The achieved reduction in the refractive index, <i>n</i>, at 1.95 eV (637 nm vacuum wavelength) was ∆<i>n </i> ≈ -0.06; typically waveguides have ∆<i>n </i> ≈ -0.003. The measured extinction coefficient, <i>k</i>, was 0.037 (α ≈ 7x10<sup>3</sup> cm<sup>-1</sup>). The physical size of photonic components is largely influenced by the refractive index contrast between the two materials; a large contrast allows for smaller structures. However, diamond/air structures need to be very small to maintain single-mode propagation; they are inherently fragile. The smaller refractive index contrast achieved in this work permits the structures to be larger, and hence mechanically sound. Furthermore, the attenuation in the cladding region is inconsequential, due to the short operational lengths of the waveguides. Refractive index determinations have been performed by spectral ellipsometry, white light reflectance and spectral transmittance, and compared with measurements of the electrical conductivity and the ion-induced surface swelling. The optical measurements all show quantitative agreement with each other. Furthermore, a consistent qualitative relation is shown between the optical measurements and the electrical conductivity measurements, which are comparable with previous measurements in diamond implanted with heavy ions. It is a further claim of this thesis that the influence of the implanted atoms is negligible compared to the structural modifications that occur upon ion implantation. Furthermore, it is proposed that the lattice-induced pressure is the responsible mechanism that inhibits the decrease of the refractive index under high-energy light-ion implantation. The conclusion reached is that the behaviour of the refractive index can be completely understood in terms of physical properties; namely material density, dangling-bond density, electric polarisability and electrical conductivity. The magnitude of these effects is highly dependent on the ion fluence; their individual contributions vary depending on the amount of lattice damage. The information contained within this thesis provides a feasible foundation for the production of waveguides and cavities; critical components in the realisation of a room-temperature scalable quantum computer.