Studies on bombardment-enhanced conductivity. IV: IV. On the chemical state of oxygen-bombarded Nb 2O 5

Abstract

It is now a well-established result that an increased oxygen content in bombarded metal targets influences the yields of secondary ions or photons. A similar influence appears to have been identified also for a decreased oxygen content in the case of bombarded oxide targets. The chemical state of targets which have gained or lost oxygen is less often specified and the present work was therefore undertaken to determine this state in a particular system, oxygen-bombarded Nb 2O 5. The results relate to conductivity, annealing, and electron diffraction. Specifically, intermediate doses (⩽ 4 × 10 ions cm 2 ) of 35 keV oxygen ions led, as with many other substances, to amorphization with unchanged conductivity. Doses in excess of 2 × 10 17 ions cm 2 , however, caused an increase in the bulk conductivity of a factor of about 4 × 10 8 as well as a corresponding stoichiometry change such that Oxygen was lost and a thin layer of crystalline NbO was formed. These changes persisted when bombarded specimens were heated in vacuum at 150–800 °C. The results are discussed from the standpoint of three mechanisms, namely thermal-spike induced vaporization, internal precipitation, and preferential sputtering. Vaporization could be excluded on the basis of the oxygen partial pressures for the process Nb 2 O 5 (1) = 2 NbO (1) + 3 2 O 2(g) being too low, while precipitation could be tentatively excluded on the basis of the vacuum annealings. There were no obvious objections, however, to a model for oxygen loss based on preferential oxygen sputtering even if the details of the model were unclear.