Electronic bipolar resistance switching (eBRS) in an Al/TiOx/Al structure, where the TiOx layer was reactively sputter-deposited, was examined in conjunction with a structural analysis using transmission electron microscopy. A thin (3-5 nm) insulating Al(Ti)Ox layer was formed at the bottom Al electrode interface, which provided the necessary asymmetric potential barrier for the eBRS to emerge, whereas the top Al electrode interface appeared to have provided the fluent carrier (electron) injection. The set and reset switching were related to the trapping and detrapping of the carriers at the trap centers, the characteristic energy of which was ∼0.86 eV, across the entire electrode area. The general features of this material system as the feasible RS memory were insufficient: endurance cycle, <∼8000, and retention time at 85 °C, 10(6) s. However, the detailed analysis of the switching behavior based on the space-charge limited current conduction mechanism, and its variation with the switching cycles, provided useful information on the general features of the eBRS, which could also be applicable to other binary (or even ternary) metal-oxide RS systems based on the electronic switching mechanism.

Two-step reset behaviors in the resistance switching properties of the top Al/TiOx/bottom Cu structure were studied. During the electroforming and set steps, two types of conducting filaments composed of Cu and oxygen vacancies (Cu-CF and V(O)-CF) were simultaneously (or sequentially) formed when Al was negatively biased. In the subsequent reset step with the opposite bias polarity, the Cu-CFs ruptured first at ~0.5 V, and formed an intermediate state. The trap-filled V(O)-CFs were transformed into a trap-empty state, resulting in a high-resistance state at ~1 V. Matrix phase in the electrochemical metallization cell can play an active role in resistance switching.

Reliability and uniformity in resistance switching behaviours in top electrode Cu-sputtered TiO2-bottom electrode Pt memory structure were greatly improved by inserting an interface layer of 5 nm-thick HfO2 between Cu and 50 nm-thick TiO2. The thin HfO2 layer, with much smaller cluster size than TiO2, limited the Cu migration appropriately and induced more uniform Cu conducting filament distribution. The repeated rejuvenation and rupture of Cu filament was limited within the HfO2 layer, thereby improving the switching reliability and uniformity. This also greatly decreased operation power compared to a memory cell without the thin HfO2 layer.

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...

Bipolar resistive switching property was studied in Al/TiOx/Al structure. This structure exhibits uniform high resistance and low resistance among cell-to-cell test. The switching mechanism ascribes to space-charge-limited conduction. On the other hand, Al/TiOx/Cu sample was deposited as a reference. This structure shows non-uniform variation. The reasons for uniformity of Al/TiOx/Al sample and non-uniformity of Al/TiOx/Cu sample were demonstrated in this work.