Electroforming-Free and Multilevel Resistance Switching Properties in Al/TiOx/Cu Structure

Abstract

Multilevel resistance switching with electroforming behavior was studied in Al/TiO x /Cu structure. Electroforming-free was obtained through reducing the thickness of TiO x thin film, resulting in a relatively low reset current. Three stable resistance states were achieved from two step resets, indicating the multilevel resistance switching. Coexistence of conductive filaments of Cu and oxygen vacancies was proposed to explain multilevel resistance switching. The relationship among electroforming-free, low reset current and the concentration of Cu in TiO x was confirmed by Auger electron spectroscopy. A switching model for explaining electroforming-free and multilevel resistance switching properties was proposed.Resistance random access memory (RRAM) in various oxide materials, especially binary oxide materials such as TiO 2 , 1 ZrO 2 , 2 HfO 2 , 3 and ZnO, 4 has attracted considerable attention as one of the promising applications in next-generation nonvolatile semiconductor memory devices. Although the resistance switching (RS) materials can be classified into several groups according to switching mechanisms, this study focuses on combined different mechanisms effects on the RS performance in Al/TiO x /Cu. As Cu is an active metal in the electrochemical metallization memory (ECM), 5 adopting Cu as electrode may induce Cu conductive filaments (CFs). Meanwhile, because of the higher oxidation potential of Al compared with Ti, 6 adopting Al on TiO x may induce the oxygen vacancy (V o ) related RS mechanisms, such as electronic switching related with the trapping and detrapping of carriers along the V o channels. 7 Particularly, when RRAM devices is programmed, a filament with trap-controlled space charge limited current (SCLC) could be formed, while a metallic CFs could also be formed. Therefore, multilevel resistance states may be obtained by forming different filaments inside the device. However, research on combining these two mechanisms remains to be clarified. During the resistance switching process, the "electroforming" process is usually needed to obtain the resistance switching. However, this process often results in the random fluctuations of switching parameters and needs a much higher bias, which are generally unfavorable for the device fabrication and operation. 8 In this paper, the characteristics of multilevel resistance switching, electroforming-free and low I reset (the maximum current level changes from LRS to HRS) were investigated through Al/TiO x /Cu structure. The relationship between electroforming-free and low I reset will be elucidated in this paper, as well as the multilevel resistance switching mechanism.Experimental TiO x thin films with the different thickness of 50 nm and 200 nm were deposited on Cu via DC magnetron sputtering at room temperature, using a 99.999% Ti target and O 2 /Ar sputtering gas (1 Pa total pressure, 5% O 2 ), and subsequently Al was deposited on TiO x /Cu through a metal shadow mask (400 μm hole diameter). Both electrodes were deposited by electron beam evaporator...

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