Study On Resistive Switching Polarity Of ZnO Thin Films

Rapid advances in information technology rely on high-speed, large-capacity and lowpower consumption nonvolatile memories(NVMs). Flash memory, which is currently verypopular, has very high capacity, but is relatively slow and gradually approaching the physicallimits of scalability. For future information storage needs, emerging NVMs, such asphase-change random access memory (RAM), ferroelectric RAM, magnetic RAM, andresistive RAM (RRAM) have attracted a great deal of attention in recent years. Among thesepromising candidates, RRAM has the excellent merits of fast response, long retention time,multilevel storage, and low power consumption, as well as scaling better than currenttechnologies and compatibility of the CMOS process. Many transition metal oxides (TMOs),such as ZnO, TiO2, ZrO2, MnO2, and NiO exhibit reproducible resistance switching property.Among them, ZnO has been explored intensively because of its simple compositions, clearswitching properties, low switching voltages, higher the resistance ratio of high resistancestate (HRS) to low resistance state (LRS), and adjustable electronics property by doping ofdifferent dopants. RRAM based ZnO is investigated in this Master’s thesis, the doped ZnOfilm devices were fabricated on the Pt/Ti/SiO2/Si substrates by Sol-Gel method, and theresistive switching polarities are mainly explored by using the different device structure andtest conditions. Research works are summarized as follows:1. Metal/La-doped ZnO/Pt sandwich structures were constructed by depositing differenttop electrodes (Ag and Pt). Unipolar resistive switching (URS) and bipolar resistive switching(BRS) behaviors were investigated in Pt/La-doped ZnO/Pt and Ag/La-doped ZnO/Ptstructures, respectively. Compared with the undoped devices (Pt/ZnO/Pt and Ag/ZnO/Pt), theLa-doped devices exhibit superior resistive switching performances, such as narrowdistribution of the resistive switching properties, higher RHRS/RLRSratio, good retentioncharacteristics and sharp switching transition. Furthermore, we fabricated Pt/La-dopedZnO/ZnO or SrTiO3/Pt structures devices in which ZnO or SrTiO3was used as a buffer layer,and found that diffirent resistive switching behaviors depend on diffirent buffer layer,respectively. The stable unipolar and bipolar resistive switching behaviors were showed in thePt/La-doped ZnO/ZnO/Pt and Pt/La-doped ZnO/SrTiO3/Pt structures devices, respectively.Compared with the Pt/ZnLaO/Pt structure device, by embedding a thin buffer layer betweenthe ZnLaO and the Pt bottom electrode (BE), Pt/ZnLaO/buffer layer/Pt structure devicesexhibit much better RS performances. 2. The coexistence of bipolar and unipolar resistive switching modes is observed in thevanadium doped ZnO polycrystalline thin films fabricated by a Sol-Gel method. These twoswitching modes can be activated separately depending on the different compliance current(Icc) during the first voltage sweeping process: the fabricated device shows reproducible BRSbehavior with a low compliance current Icc=0.1mA, while with a high Icc=0.01A, URSbehavior was observed after electroforming. The conversion between BRS and URS isreversible, it is worth noting that three logical state can be carried out during the transitionthrough an appropriate measurement sequence. The conducting filament formation/rupturemodels with electrochemical redox reactions and thermal effects were suggested to explainthe BRS and URS behaviors respectively.3. The bipolar and unipolar resistive switching modes are observed to coexist inPt/Zn0.99Zr0.01O/Pt structure device. After the forming process, this device with URS behaviorexhibits either URS mode in the same direction or BRS mode in the opposite directionsduring the reset process. Controllable multi-state resistances in the low and high resistancestates for the BRS mode were obtained by imposing different compliance currents (Icc) andthe span of voltage sweeping in the reset process (Vstop). Similar results can be also carried outon sub-micron devices for tens of thousands of cycles with fast pulses. These results suggestthat our devices have high potential for the next generation of NVMs application.