Professor Jackman holds UCL's chair in Electronic Devices
Outside of UCL I am the immediate past-chairman of the British Vacuum Council, and sit on the IOP’s Semiconductor Physics Group committee. I am on the organising and/or programme committees of most of the international diamond conferences/meetings, and am co-chairing symposia on ‘Diamond Electronics’ at the MRS Fall 2006 and 2007 meetings in the USA.
Diamond is a truly remarkable material. It has very high carrier mobilities, saturated carrier velocities and electric field breakdown strength. It has the highest thermal conductivity of any material. It has a very low dielectric constant. It can display ‘negative electron affinity’. It can be considered to be a wide band gap semiconductor (5.5eV) that can be doped p-type or n-type. It is chemically and physically robust, and radiation ‘hard’ – electronics formed from diamond should not only perform at the highest levels, but should also be capable of operation in extreme environments. It has unusual optical properties. In short, using diamond as a gemstone is a waste of its true potential! It can also be considered to be biocompatible, in that it is simply carbon, and is also not prone to unwanted cell adhesion or particulate generation when inside a living body. The Diamond Electronics Group within the LCN, which I head, is actively engaged in the growth and doping of diamond using chemical vapour deposition methods, and its use within a wide range of nano-electronic devices.
Fully connected network of neurons (from Mice) – these have been grown on diamond and communication can be achieved through field-effect transistors fabricated on the surface of the diamond and the nodes of the neuronal material. An exciting step towards the realisation of implantable devices for repair of the nervous system. The red colour is as a result of staining for F-actin, and indicates the position of the neural material. The diamond is black in this image.
A visually attractive picture of polycrystalline diamond grown by microwave plasma enhanced CVD here at UCL. Different crystallographic planes can be seen, along with multiple twins within crystals
An important development in the field of diamond electronics has been the production of n-type electrical characteristics following homoepitaxial diamond growth on 111 diamond in the presence of phosphorus-containing gases. Several studies have reported that a phosphorus donor level forms with an activation energy in the range of 0.43 – 0.6 eV; the ground state for the donor level is considered to be at 0.6 eV. Little is currently known about other electrically active defects that may be produced alongside the donor state when phosphorus is introduced. In this paper we report upon the use of impedance spectroscopy, which can isolate the differing components that contribute to the overall conductivity of the film. In Cole-Cole plots, two semicircular responses are observed for all temperatures above 75 ° C; a single semicircle being seen at temperatures below this. The results suggest the presence of two conduction paths with activation energies of 0.53 and 0.197 eV. The former can be attributed to the phosphorus donor level, being lower than 0.6 eV due to reduced mobility within the film at elevated temperatures. The latter is discussed in terms of defects in the P+-doped region under the Ohmic contacts being used.
We report on the operation of ungated surface conductive diamond devices in electrolytic solutions. The effect of electrolyte pH on the channel conductivity is studied in detail. It is shown that fully hydrogen terminated diamond surfaces are not pH sensitive. However, a pronounced pH sensitivity arises after a mild surface oxidation by ozone. We propose that charged ions from the electrolyte adsorbed on the oxidized surface regions induce a lateral electrostatic modulation of the conductive hole accumulation layer on the surface. In contrast, charged ions are not expected to be adsorbed on the hydrogen terminated surface, either due to the screening induced by a dense layer of strongly adsorbed counter-ions or by the absence of the proper reactive surface groups. Therefore, the modulation of the surface conductivity is generated by the oxidized regions, which are described as microscopic chemical in-plane gates. The pH sensitivity mechanism proposed here differs qualitatively from the one used to explain the behavior of conventional ion sensitive field effect transistors, resulting in a pH sensitivity higher than the Nernstian limit.
Diamond has a number of unique properties that make it an attractive electronic and bio-electronic material. Here we show the ordered growth of mammalian neurons, the principal electrogenic cells of the nervous system, on diamond. Proteins were specifically patterned on diamond surfaces by micro-contact printing. Mouse cortical neurons were then cultured on these substrates. Neuron adhesion and outgrowth was specific for those areas of the diamond that had been stamped with laminin, resulting in ordered growth of high resolution. Neurons survived in culture for the duration of the experiment, and laminin patterns were stable for at least 1 week in culture. The relative biocompatibility of diamond and the suitability of neuron interfacing with the hydrogen surface conductivity layer make this an interesting model for the formation of defined neuronal networks and for implants.
Within UCL I act as the Undergraduate Admissions Tutor for the Department of Electronic and Electrical Engineering, where I also teach Device Physics and Technology to BEng and MEng students. At MSc level, I teach ‘Nanoscale Processing and Characterisation’ and ‘Nano-electronic devices’ to students on UCL’s programme in ‘Nanotechnology’.