氧化鋯電阻式記憶體搭配氧化鎢之雙極性與自我限流特性研究

Abstract

本論文主要探討以氧化鋯電阻式記憶體結構搭配不同氧化鎢厚度所產生的雙極性電阻轉換與自我限流之特性。 第一部分討論加入薄氧化鎢後之元件特性。此元件需經兩段形成過程後才會顯示有效的雙極性電阻轉態現象。氧化鎢薄膜改變導通路徑斷裂的位置，使控制轉態的區域由氧化鈦變為氧化鎢。在極薄的2奈米氧化鎢元件時，因氧化鎢與氧化鈦之自由能相近，故鎢離子與鈦離子皆與可移動氧離子結合，造成導通路徑斷裂位置不固定，操作電壓變凌亂。若增加氧化鎢厚度到5奈米，操作電壓變回穩定狀態，這是因氧化鎢厚度足以阻擋鈦離子與氧離子結合，使轉態區域控制在氧化鎢層。未加氧化鎢之元件可操作超過兩萬次；2奈米氧化鎢元件可操作超過一萬兩千次。兩者在攝氏150度下皆可儲存約105秒，且連續以0.3V讀取超過104秒。 第二部分著重於厚氧化鎢對元件之影響。在未加氧化鎢或加薄氧化鎢時，元件無法在低電流區域進行電阻轉態。而加入厚氧化鎢層，元件無須較大電壓的形成過程且在低電流區即可操作，並具有自我限流現象。此由於外加電壓使氧離子遷移，改變介面處能帶，進而使電子在加負偏壓時能輕易流動，造成電阻值之變化。加入30奈米氧化鎢的結構能直流操作超過一萬次。

In this thesis, the bipolar and self-compliance resistive switching characteristics are investigated in the structure of ZrO2-based resistive random access memory (RRAM) with different thickness of WO3 layer. The first section is the Pt/Ti/WO3/ZrO2/W structure with no, 2nm and 5nm WO3 film. Under low current measurement, thin WO3 devices exhibit no switching behavior. Nevertheless, the devices show bipolar switching characteristics with double forming process. The location of filaments formation and rupture is altered after inserting a WO3 film. In order to recombine with mobile oxygen ions, there is a competition between tungsten ions and titanium ions due to the close Gibbs free energy of WO3 and TiO2. As a result, the reset voltages of 2nm WO3 device become disordered compared with no WO3 device. However, increasing the WO3 layer to 5nm, the operating parameters including Von and Voff return to stable conditions. The reason is that the 5nm WO3 film is thick enough to avoid the recombination of titanium ions and oxygen ions. Therefore, the switching region of filaments formation and rupture is confined in the WO3 film. The endurance of no WO3 and 2nm WO3 device are over 20000 cycles and 12000 cycles. At 150oC, both of them have good characteristics on the retention test for 105s and on the stress test for 104s. The second section focuses on the Pt/Ti/WO3/ZrO2/W structure with thick WO3 layer, such as 20nm and 30nm. Without forming process, the thick WO3 devices have resistive switch phenomena on low current region while the thin or no WO3 device does not show any resistance change. Furthermore, the thick WO3 devices has self-compliance characteristic which means no compliance current is needed. External field induce the migration of mobile ions. The accumulation of oxygen vacancies cause a band bending in the ZrO2 layer. For this reason, electrons can flow through the structure and these devices reveal a resistance ratio on a negative bias. The 30nm WO3 device has the endurance over 10000 cycles.