Current Flow in Very Thin Films of Al2O3 and BeO

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Abstract
This paper describes a study of current flow through very thin insulating films. Several possible conduction mechanisms are presented and their properties are discussed. The dominant conduction process in the layers studied is quantum‐mechanical tunneling. Previous work on this mechanism is extended by an exact calculation which shows the current‐voltage characteristic to be divided into three regions of different logarithmic curvature. The dielectric constant of the insulator is found to have a large effect, via the image force, on the tunneling current. The theoretical results are fitted to measurements made on insulating layers less than 100 Å thick, sandwiched between metal electrodes. For Al2O3 layers the fit is excellent, resulting in barrier heights of 2.0–2.5 eV and thicknesses approximately 25 Å less than the values determined from capacitance, as expected. In contrast the conduction characteristics of BeO layers can be explained by tunneling only in the low‐voltage region; barrier height and thickness are similar to the Al2O3 case. At intermediate voltages the currents are lower than predicted and at high voltages reproducible hysteresis effects are observed with time constants of the order of minutes. These effects are only weakly temperature dependent. They appear to be due to tunneling from one of the metals into trap states in the forbidden gap of the insulator, creating space charge which distorts the barrier for tunneling.