Aluminium oxide tunnelling junctions: preparation and inelastic electron tunnelling spectroscopy

Abstract

The preparation and characterisation of aluminium oxide tunnelling junctions suitable for inelastic electron tunnelling spectroscopy are discussed. Three processes which may be used to produce thin oxide films on aluminium were investigated; thermal oxidation, immersion of an unbiased aluminium sample in a glow discharge of pure oxygen, and plasma anodisation in pure oxygen. It was found that of these only plasma anodisation was capable of growing oxide films of a controllable thickness on a reproducible basis. A model for the plasma anodisation process based on a balance of the ionic and electronic currents flowing to the plasma-oxide interface has been developed. The model shows that plasma anodisation will only proceed to completion within a reasonable time if the electron density is ≥ 10 10/cm2, which accounts for anodisation being successful only in the negative glow of the discharge. The model does not account for the low current efficiency of the process, indicating that a non-tunnelling contribution to the electronic current becomes significant for thick barriers. A comparative IETS study of the barriers grown thermally and in the plasma anodisation process, shows that in the thermal technique the vibrational characteristics of the oxide barrier are markedly affected by the presence of a high concentration of A1H (≈ 0.2 molecules/Å2) resulting from a reaction between the aluminium electrode and adsorbed water vapour. Fitting the I-V characteristics of plasma anodised aluminium-oxide-aluminium samples to the tunnelling model of McBride et al [1974] yields a barrier height of 1.8±2eV. Whilst this parameter is supported by photoemission measurements, neither the McBride model nor an earlier model due to Stratton [1963] can adequately predict the oxide thickness determined from capacitance measurements.