Influence of embedding Cu nano-particles into a Cu/SiO2/Pt structure on its resistive switching
© Liu et al.; licensee Springer. 2013
Received: 18 February 2013
Accepted: 22 March 2013
Published: 8 April 2013
Cu nano-particles (Cu-NPs) were embedded into the SiO2 layer of a Cu/SiO2/Pt structure to examine their influence on resistive switching characteristics. The device showed a reversible resistive switching behavior, which was due to the formation and rupture of a Cu-conducting filament with an electrochemical reaction. The Cu-NPs enhanced the local electric field within the SiO2 layer, which caused a decrease in the forming voltage. During successive switching processes, the Cu-NP was partially dissolved, which changed its shape. Therefore, the switching voltages were not reduced. Moreover, the Cu-NPs caused a non-uniform Cu concentration within the SiO2 layer; thus, the Cu-conducting filament should be formed in a high Cu concentration region, which improves switching dispersion. The Cu-NPs within the SiO2 layer stabilize the resistive switching, resulting in a larger switching window and better endurance characteristics.
KeywordsCu nano-particle Resistive switching SiO2 73.50.-h 73.40.Rw 73.61.Ng
Portable electronic products are common in daily life. A requirement of portable electronic products is low power consumption. Non-volatile memory (NVM) can retain information without a power supply, which is suitable for portable products. Flash memory is currently the mainstream product in NVM devices. However, it will eventually reach its physics limitations with continuous scaling, which causes retention degradation and serious reliability issues. Therefore, numerous novel devices for replacing flash memory have been proposed. Among these devices, the resistive random access memory (RRAM) with a simple metal/insulator/metal structure shows a reversible resistive switching behavior . The device resistance can switch between a high-resistance state (HRS) and a low-resistance state (LRS) using dc voltages or pulses. Numerous materials with various resistive switching behaviors, such as NiO , HfO2, SrZrO3, and SiO2 have been proposed. Several switching mechanisms such as electrochemical , thermochemical , and valance change effect  have been proposed to explain the various switching behaviors. However, resistive switching is unstable, which may cause operating issues [9, 10]. Several methods such as doping , process optimization , interface control , and embedding nano-particles [14–16] have been adopted to improve the switching dispersion in various switching behaviors. All studies used inactive materials for their embedded nano-particles when examining their effect on switching behavior [14, 17]. The inactive nano-particles enhanced the local electric field within the resistive layer, which decreased the operating voltages and improved the switching dispersion .
Pt nano-particles were embedded into the resistive layer in our previous study  to examine their influence on the resistive switching of an electrochemical-based RRAM device. The improvement of the switching dispersion resulted from the enhancement of the local electric field within the resistive layer. An electrochemical-based RRAM device generally has an active electrode and a counter inert electrode. The active metal is partially dissolved and acts as a cation supplier. The cations migrate in an electric field through the resistive layer and are reduced at the inert cathode. Thereafter, a metallic filament grows toward the anode and connects the two electrodes. The growth of the conducting filament is through the preferred ionic drift path within the resistive layer. Thermadam et al. proposed that the Cu concentration of the resistive layer influenced the resistive switching behavior . The influence of the embedded nano-particles of an active metal on electrochemical-based RRAM has not been examined. The nano-particles of active metals within the resistive layer may change the distribution of the local electric field and cation supply. In this study, Cu nano-particles (Cu-NPs) were embedded into a Cu/SiO2/Pt structure to examine the role of Cu-NPs on resistive switching. The forming voltage was reduced in the Cu-NP sample; this was due to the enhancement of the local electric field. The improvement of switching dispersion may be caused by the non-uniform Cu concentration in the SiO2 layer.
Four-inch p-type silicon wafers were used as substrates. After a standard Radio Corporation of America cleaning, a 200-nm-thick SiO2 layer was thermally grown in a furnace to isolate the Si substrate. Thereafter, a 5-nm Ti layer and a 100-nm Pt layer were deposited by an electron-beam evaporator to form a Pt/Ti/SiO2/Si structure. The Pt layer was adopted as the bottom electrode. A 20-nm SiO2 layer was deposited using radio frequency (rf) sputtering at room temperature on the Pt electrode. A 10-nm Cu layer was deposited with a thermal evaporator at room temperature on the 20-nm SiO2 layer to examine the influence of Cu-NPs. Thereafter, a rapid thermal annealing was performed at 600°C for 5 s in a nitrogen ambient to form the Cu-NPs. A 20-nm SiO2 layer was subsequently deposited on the Cu-NPs. Furthermore, the 150-nm Cu top electrodes patterned by a metal mask were deposited using a thermal evaporator coater to fabricate a Cu/Cu-NP embedded SiO2/Pt device (Cu-NP sample). The area of the device was approximately 5×10−5 cm2. A Cu/SiO2/Pt device (control sample) was additionally fabricated without the Cu-NPs formation procedures for comparison purposes. The cross section of the Cu-NP sample was observed with a high-resolution transmission electron microscopy (HRTEM, TEM-3010, JEOL, Ltd., Tokyo, Japan). The distribution of the Cu concentration within the structure was analyzed using energy-dispersive X-ray spectroscopy (EDX). Electrical measurements were performed using an HP 4155B semiconductor parameter analyzer (Hewlett-Packard Company, Palo Alto, CA, USA) at room temperature. The bias voltage was applied on the Cu top electrode while the bottom electrode was grounded. The applied voltage was swept with a step of 20 mV, and the compliance current was 1 mA.
Results and discussion
Cu-NPs were embedded into the SiO2 layer of the Cu/SiO2/Pt structure to examine their influence on resistive switching behavior. The Cu-NPs enhanced the local electrical field during the forming process, which decreased the magnitude of the forming voltage and improved the switching dispersion. However, during the subsequent switching processes, the Cu-NPs were partially dissolved and their particle shape was altered; thus, the local electrical field was not enhanced by the Cu-NPs and did not decrease the magnitude of the operating voltages. The Cu-NP fabrication process and partial dissolution of the Cu-NPs in the switching process caused non-uniform Cu concentration within the SiO2 layer. Non-uniform Cu distribution caused the Cu-conducting filament to form in a high Cu concentration region, which improved the switching dispersion. The Cu-NPs stabilized the resistive switching, and subsequently improved endurance characteristics.
CYL is an associate professor at the Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Taiwan. JJH is a master student at the Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Taiwan. CHL (Lai) is an associate professor at Department of Electronic Engineering, National United University, Taiwan. CHL (Lin) is a master student at the Department of Electronic Engineering, National Kaohsiung University of Applied Sciences, Taiwan.
Energy-dispersive X-ray spectroscopy
High-resolution transmission electron microscopy
Resistive random access memory
The authors thank the National Science Council of R.O.C. for their financial supports under project no. NSC 101-2221-E-151-044 and the facility support from National Nano Device Laboratories.
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