利用鈷摻雜優化Pt/ZnO/Pt電阻式記憶體之開關特性

Abstract

In recent years, resistive random access memory (RRAM) has emerged as a potential candidate for replacing conventional nonvolatile memory based on flash memory technology. RRAM shows many advantages, such as simple structure, good retention and endurance properties and high density storage. The operation of RRAM devices originates from electrical resistance switching phenomenon of memory cells, which is the change in the resistance as a result of appropriate applied biases (ON state with its low resistance, and OFF state with its high resistance). Despite the simple working principle, RRAM devices usually have an unstable distribution of resistance switch parameters, such as the set-voltage (VSET) and reset-voltage (VRESET), a huge disadvantage. The value of set-voltage (VSET), reset voltage (VRESET), reset current (IRESET), low resistance state (RON) and high resistance state (ROFF) usually distribute in a wide range. Therefore, it is important to find solutions that may improve the uniformity of operation parameters. In this thesis, the resistive switching in the ZnO thin film sandwiched by the Pt bottom and top electrodes was studied. The ZnO film was deposited by sol-gel technique, and Co was doped in the ZnO layer during the spin-coating process to improve the resistive switching performance of oxide layer. The method given shows improvements in the electrical parameter distributions. The significant improvement occurs at high doping concentrations (10% and 15% cobalt doping). The distribution of VSET in these doped devices is from 0.9 to 1.6 V, whereas VSET of pure ZnO device is from 0.9 V to 3.2 V. Material characterizations were used to study the physical mechanisms behind the improvement in the RRAM performance of the Pt/Co-doped ZnO/Pt capacitor structure. The SEM and AFM images show similar morphology microstructure of pure and cobalt doped ZnO devices. Photoluminescence study shows that the amount of pre-existing oxygen vacancies of ZnO decreases with doping Co. Therefore, the filaments in Co-ZnO RRAM devices are created by only one kind of oxygen vacancy, resulting in better resistive switching properties. Raman spectra exhibit the segregation of cobalt oxide phase in high doping concentration, causing the degradation of resistance ratio.