“Morphological and electronic properties of strongly boron-doped diamond films prepared by chemical vapor deposition.”
Very thin (< 200 nm) polycrystalline diamond layers with grains smaller than 100 nm, also called nanocrystalline diamond (NCD) films, are systematically grown and investigated. Because these layers are very thin, it only takes minutes to a couple of hours to grow them. They can easily be deposited on large surfaces which makes them interesting for industrial applications. N- and p-type dopants can be introduced into the NCD films to make them electrically conductive and for boron doping even superconductive below 12 K. Even biomolecules can also be attached to the surface of these layers to create highly sensitive biosensors. Still, there is a lack of fundamental knowledge which is necessary to optimize the properties of thin NCD layers for applications. The work in this thesis unravels some properties which makes it possible to tune thin NCD layers for a variety of applications.
First, the surface energy of hydrogen terminated NCD (NCD:H) is determined and a process to oxygen terminate NCD is developed.
Tuning the average grain size of equally thick NCD layers resulted in a set of samples with for each sample a different mean grain size. This was done by changing the growth conditions and after characterization it was shown that the quality of the NCD layers increased as a function of grain size.
When NCD is doped with a large amount of boron (B:NCD), it becomes conductive in a broad temperature range. Due to its relative high resistivity, compared to metals, and low mobility, due to grain boundaries and doping, only low Hall voltages can be generated with a strong magnetic field. A way of measuring low Hall voltages and the influence of “good” and “bad” contacts on Hall voltage measurements is described.
A set of strongly boron-doped NCD layers, each with a different mean grain size was grown. Magnetotransport measurements revealed a decrease of resistivity and a large increase of mobility, approaching the values obtained for single crystal diamond as the average grain size of the layers increases. In all layers, the temperature dependence of the resistivity decreases with larger grains and the mobility is thermally activated. It is possible to separate the intra- and intergrain contributions for the resistivity and the mobility, which indicates that in these complex systems Matthiessen's rule is followed. The concentration of active charge carriers is reduced when the B:NCD is grown with a lower C/H-ratio. This is due to a lower boron-incorporation, which is confirmed by neutron depth profiling.
Superconducting B:NCD layers with critical onset temperatures below 12 K are grown. The amount of grain boundaries in the different types of diamond films influences the temperature difference between the critical onset and the critical offset temperature. This can be attributed to Josephson coupling between grains.
B:NCD layers were also optimized and terminated with DNA to fabricate a biosensor which detects point mutations by impedance spectroscopy.