The electronic structure and memory device applications of tetrahedral amorphous carbon

The introduction of nitrogen dopant sites into tetrahedral amorphous carbon produces changes in the structure and the electronic density of states tha t can be modelled using molecular dynamics. In this work we use both a tigh t-binding approach and a Car-Parrinello density functional theory approach. In a comparison of these, we found that the former tends to overestimate t he strain energy of 3 membered carbon rings relative to the latter and to e xperiment, explaining the reduced occurrence of 3 membered rings in network s simulated using tight-binding. Experiment shows that at approximately 3% of nitrogen, the network begins to change rapidly with nitrogen content. In this form, an additional electronic conduction mode is found experimentall y, of the Poole-Frenkel type. which can be turned on and off at will. The c onduction is turned on by negative voltage excursion and quenched by a posi tive one. This conduction bistability can be exploited to produce a simple new type of memory device in which the high conductivity state ("on") is a digital "1" and the low conductivity state ("off") is a digital "0". The op erating characteristics of the device are excellent, with more than one mil lion read cycles having been demonstrated without deterioration of the disc rimination between the "on" and "off" states. Molecular dynamics is used to study the configuration of the nitrogen atoms, yielding a possible candida te for the site responsible for the Poole-Frenkel conduction.