Nature of the stable oxide layer formed on an aluminum surface by work function measurements

Abstract

The interaction of oxygen with clean aluminum results in formation of a stable oxide layer on the surface of the metal. This stable layer has a lower work function than that of clean aluminum. The nature of this stable layer is studied by measurement of work function changes. Heating of the stable layer, formed at room temperature and low oxygen pressures' (~10 −8 torr), in ultra-high vacuum resulted in a further decrease in the work function. The extent of this additional decrease was a function of temperature and the changes in work function caused by heating were irreversible in nature. At high oxygen pressures (p O 2 > 10 −2 torr) the effect of growth of incorporated oxide on the net limiting work function change appears to be small and the reversible changes in the work function are mainly a result of the variation in the amount of surface oxide. At 250°C, the contribution of incorporated oxide to net limiting work function change was dependent on oxygen pressure when it was formed at low oxygen pressures ( P < 10 −7 torr). However, when the incorporated oxide was formed at oxygen pressures above 10 −7 torr, its contribution to the limiting work function change and, perhaps, the structure of the incorporated oxide itself were nearly independent of oxygen pressure. The difference in limiting work function change at various oxygen pressures was mainly because of the difference in the limiting amounts of surface oxide. Similar behavior is expected at room temperature.