利用鈷摻雜優化Pt/ZnO/Pt電阻式記憶體之開關特性

Abstract

近年來研究結果顯示，電阻式隨機存取記憶體（Resistive random access memory, RRAM）具有取代傳統非揮發性快閃記憶體之潛力。電阻式記憶體具有許多優點，如結構簡單化、記憶時間長、耐久性好與儲存密度高。電阻式記憶體工作機制源自於記憶單元的電阻轉換現象，記憶單元的電阻會隨著施加適當偏壓而改變(低電阻為ON狀態，高電阻為OFF狀態)。雖然電阻式記憶體工作原理簡單，但其 set-voltage (VSET), reset-voltage (VRESET), reset-current (IRESET), low resistance state (RON) 及 high resistance state (ROFF) 分佈範圍廣，使得電阻轉換參數的不穩定為此元件之巨大缺點。因此，解決此缺點而提高操作參數之一致性十分重要。 本研究以鉑作為電極，與氧化鋅薄膜組成鉑/氧化鋅/鉑之三明治結構，分析此元件電阻轉化之現象。氧化鋅前驅物以溶膠凝膠法製備，接著利用旋轉塗佈法沉積氧化鋅薄膜，並於薄膜沉積過程中摻雜鈷金屬來提升元件表現，改善參數分佈穩定性，摻雜濃度提高(10%、15%)對於元件之表現有顯著提升。摻雜鈷之氧化鋅記憶元件VSET分佈為0.9至1.6伏特，而純氧化鋅之記憶元件VSET之分佈則為0.9至3.2伏特。利用一系列材料分析來研究此含鈷之氧化鋅記憶元件表現提升之物理機制。掃描式電子顯微鏡與原子力顯微鏡分析結果顯示，純氧化鋅與摻雜鈷之氧化鋅具有相近之形貌與微結構。螢光光譜儀分析結果顯示，藉由摻雜鈷金屬可減少氧化鋅薄膜之氧缺陷，因而只有一種氧缺陷會於記憶元件中產生導通絲狀路徑(filaments)，使元件得到較佳之電阻轉換特性。拉曼光譜儀分析結果顯示於高摻雜濃度下，氧化鈷有相分離之現象，造成電阻率之下降。

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.