Resistive memory switching behavior depending on voltage sweep direction is studied by intentionally creating oxygen vacancies within titanium dioxide (TiO(2)). By inserting a reactive Ti layer on the TiO(2). oxygen deficient TiO(2-x) layer is created, which then causes TiO(2-x)/TiO(2) which has an oxygen vacancy gradient. This gradient of oxygen vacancy makes it possible to create an insulating TiO(2) layer on the bottom electrode during the first reset with a negative bias at the top electrode. This insulating layer makes counterclockwise directional bipolar switching more stable. On the other hand, under the clockwise directional voltage sweeping, the first set switching is prevented by the insulating TiO(2) layers created during the first and second reset, which leads to a short circuit due to local heating eventually. (C) 2011 Elsevier Ltd. All rights reserved.