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In this letter, we examine the role of the Ti capping layer in HfO _x -based resistive random access memory (RRAM) devices on the memory performance. It is found that with a thicker Ti capping layer, the fresh device initial leakage current increases and as a result, the forming voltage decreases. In addition, with a thin Ti layer of <;3 nm (on top of 8-nm HfO _x ), there is no resistive switching, while by inserting a thicker Ti layer of 10 nm, the memory window enlarges to about two orders. Very good uniformity has also been observed in thick Ti capping devices, demonstrating the effectiveness in RRAM device engineering. It is believed that the Ti layer serves as an oxygen reservoir, by extracting oxygen during device formation and electrical forming process and facilitates resistive switching thereafter.