Ion-implanted, shallow-energy, donor centres in diamond: the effect of negative electron affinity

Abstract

A summarised overview of selected results is presented, and their modelling, which have been generated by the research program pursued by myself in South Africa. It is shown that shallow donor states can be inserted into diamond by ion implantation of either oxygen- or nitrogen ions followed by low temperature (<600°C) annealing. It is argued that such low-temperature annealing is required to “quench” these donor flaws into the diamond lattice, because their energy levels are higher than the vacuum level. An analysis of the interface between such an n-type diamond and the vacuum, based on the accepted principles of band theory, leads to the additional conclusion that a dipole layer has to form at the surface. As in the case of a Schottky diode, this dipole generates a barrier to electron migration, from the n-type diamond. However, in contrast to a Schottky diode, a forward potential does not generate a forward current, but rather increases the barrier to electron egression. In order to ensure that electrons can overcome this barrier, the surface of the semiconductor has to be doped to a high density, such that electrons can tunnel out of the diamond's surface. Electron-current flow into the anode initiates when these extracted electrons fill the whole gap between the diamond surface and the anode. All experiments, to date, show consistently that, after current flow has initiated, the electrons between the diamond surface and the anode are able to form a stable, highly conducting phase. By applying accepted, and proven, concepts from thermodynamics and quantum mechanics, it is concluded that the formation of such a phase is inevitable, and that it has to be superconducting.