Mr Kevin Rietwyk
Department of Physics
La Trobe University
Bundoora, Victoria 3086
Research Group: Atom-scale Research Laboratory, La Trobe University
Phone: 9479 1430
Current Research ActivitiesThe focus of my research has been to investigate various surface terminations of semiconductor materials and the possibility for these surfaces to participate in surface charge transfer doping by adsorbed molecules. Surface charge transfer doping provides an elegant method of inducing a 2 dimensional conductivity in semiconductors.
Previously, I have studied the p-type doping of hydrogen terminated diamond by C60F48 molecules. The hydrogen termination creates a surface dipole with a slight negative charge on the surface carbon atom that acts to lower the internal energies of the diamond relative to a vacuum level, lowering the ionisation energy. Electrons from a hydrogen-terminated surface are able to transfer to adsorbed molecules with high electron affinities (in this case C60F48), creating a hole accumulation layer in the surface. The holes are mobile but are restricted to a plane parallel to the surface by electrostatic forces with the charged molecules. This creates a high 2 dimensional p-type conductivity which is comparable in magnitude to bulk doped silicon.
The surface dipole can be reversed by terminating the surface with a more electronegative element than carbon (typically oxygen), causing an increase in the ionisation energy of the diamond and removing the possibility for p-type doping. In a recent publication, Iíve shown how dissociation of fluorine atoms from C60F48 molecules can react and fluorine terminate an otherwise hydrogen terminated surface. The dissociation in this case, was induced by soft X-ray photons and can be controlled by adjusting the flux and position of the X-ray beam to produce distinct patterns limited by the spot size of the beam. This provides a relatively simple method to create areas of conduction and non-conduction on the same surface, without the need of additional contacts or masking.
My current efforts have been to better understand the fluorine terminated diamond surface, examining the electronic energy levels to understand how the surface may behave when interfaced with other materials. Photo-emission measurements will be utilised to simultaneous determine the work function and electron affinity. The results of these measurements allow energy levels to be determined relative to the universal vacuum level. A comparison with corresponding DFT and many body perturbation theory calculations on these qualities will be carried out. Aside from providing insight into the electronic properties, the calculations will also provide details about the surface reconstruction valuable for determining the surface dipole. The surface dipole model, when coupled with the parallel plate capacitor approximation, has been shown to accurately predict the energy alignment of adosrbed molecules with the surface. Altogether, the results of the study will present a thorough understanding of the surface chemistry and electronic properties that govern interactions of the fluorine terminated diamond surface with interfacial materials.