憶阻記憶體元件之材料開發與探索

Material Exploration and Applications in Resistive Random Access Memory

黃建修(Jian-shiou Huang)

指導教授 ： 李紫原 ； 金重勳 ； 闕郁倫

國立清華大學/工學院/材料科學工程學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00522

摘要

本論文探討非揮發性記憶體，特別是憶阻式記憶體新穎材料與結構的開發，藉以改善記憶體表現，並藉由材料分析與理論基礎相互探討可能的傳導機制。
本論文的第一部分，非晶氧化矽憶阻材料，嘗試添加不同含量的銅(SCuO)，以提昇憶阻性質。首先嘗試調控反應性共濺鍍摻雜第三元素─銅的含量，以最佳化Cu/SCuO/Pt堆疊的憶阻特性。於最低瓦數(2 W)摻銅的元件中發現，其具有最佳無極性操作特性，即寫入/抹去過程與操作極性無關，也具有穩定的耐久度操作特性(>100次)，更有相對低的操作功率與操作電壓(SET在+0.75 V，RESET 在+0.45 V內可以完成)。再由X射線光電子能譜儀與溫度阻值係數的分析發現銅傳導路徑的存在呈上寬下窄的錐狀分布，依此我們嘗試提出一個熱輔助銅氧化還原於燈絲生成與斷裂的傳導機制。
第二部分則嘗試添加另一種第三元素─鋅─於非晶氧化矽中，嘗試調控鋅於非晶氧化矽中的摻雜量。發現隨著鋅摻雜量的不同，可以得到不同的臨界電壓轉換特性─即選擇器特性。當我們施加電壓抵一個臨界值，電流會驟升，但當電壓釋放後，電流回到初始態。而當施加較大保護電流時，可得到由選擇器特性轉變成憶阻特性的特殊電性轉換，並藉由不同的電壓電流源操作控制，可以得到穩定且差別明顯的憶阻窗。隨後利用理論模擬配合吸收光譜分析，可知在高阻態時，載子可於散佈在氧化矽中的奈米氧化鋅/鋅團簇間克服一個能障後躍遷。施加大電流更可以在奈米團簇中形成傳導燈絲，而形成低阻態，燈絲的長消造成阻態得以轉換。由於在同一物系中同時存在選擇器與憶阻器特性，在未來發展上，可嘗試調控匹配的電性，並進一步將兩層薄膜鍍製在一起，得到可抑制潛電流的元件特性。
在本文第三部分，嘗試發展一種成本取向的功能性薄膜。我們利用三極式電鍍槽電鍍氧化鋅薄膜，發現利用循環伏安法可以大幅提高氧化鋅薄膜的披覆性，相較於傳統的定電壓或是定電流電鍍，此法可應用在所有成核控制的電鍍物系上，以提高薄膜的披覆性。電鍍氧化鋅薄膜的憶阻特性與傳導機制，也可由空間限制電荷傳導解釋，並可萃取出載子濃度與載子遷移率。最後利用X射線光電子能譜儀來探測由表面到深層的氧鍵結情形，發現有大量的氧缺陷在氧化鋅薄膜表面，而氧化鋅薄膜內部則有較良好的鍵結。所以我們假設均質的氧缺陷受到操作極性的遷移，是造成憶阻性質轉換的主因。
在本文的四部分，我們設計並實現利用電場驅動透明導電薄膜中的氧移動，進而得到憶阻性質。此特性可在同一層透明導電薄膜中獲取，即同時具有憶阻特性與電傳導特性。如此以單層導電層與接觸電極(MM’)結構取代傳統的電極/絕緣體/電極(MIM)結構，可免去一層薄膜，此為憶阻器的一大創新，並可大幅減少半導體製程中的光罩數目。文中，我們使用掃描式電子探微儀分析平面的成分分布，發現變化主要來自於受到電驅動的氧載子移動，並造成憶阻特性。我們更進一步利用電子微影技術與導電式原子力顯微鏡來趨近最小線寬的操作可能。在未來發展方面，我們提出垂直結構的高密度堆疊結構，更可以大幅降低成本。

關鍵字

憶阻記憶體 ； 氧化矽 ； 銅摻雜 ； 鋅摻雜 ； 氧化鋅 ； 循環伏安電鍍 ； 錫摻雜氧化銦

並列摘要

Fast-emerging next-generation nonvolatile memories include resistive random access memory (ReRAM), phase-change random access memory (PRAM), magnetoresistive random access memory (MRAM), ferroelectric random access memory (FeRAM). In this dissertation, our purpose were the development of new functional material and novel structure for ReRAM application, aiming to improve the memristive characteristics.
The first topic of this dissertation focuses on the electric induced resistive switching phenomena based on Cu-doped amorphous SiOx functional films (SCuO). Electrical switching of resistive memory is highly interface-dependent. We studied such a switching of Cu-doped amorphous SiOx thin-films in sandwich stacks Cu/SCuO/Pt. The stacks were prepared using radio frequency sputtering except Cu co-doping which utilized direct current (DC) power from 2 W to 15 W. We characterized electrical switching behavior by a Keithley 4200 semiconductor analyzer. Cu/SCuO/Pt devices with Cu-doping at DC-sputter 2 W exhibit the best switching performance showing reproducible forming-free and non-polar switching. The endurance is more than 102 cycles, electrical resistance ratio more than 10, and operating voltages as low as: ±0.75 V for SET and ±0.45 V for RESET. The switching mechanism of Cu/SCuO/Pt stacks is explained based on both filamentary conduction and diffusion of Cu ions/atoms in SiOx. Both ‘temperature coefficient of electrical resistance’ and ‘bonding status’ at different depth-profiles as analyzed by using X-ray photoelectron spectroscopy provide robust evidences of the mechanisms. Cu-doped amorphous SiOx thin-films are thus potential for resistive memory.
The second topic of this dissertation focuses on the electric induced resistive switching phenomena based on Zn-doped amorphous SiOx films, (SZO). We demonstrated dual resistive switching capability of SZO films. Both mono-stable selector-switching and bi-stable memristive switching are tuning via Zn-doping content and appropriate operation conditions. Voltages of selector-switching in Pt/SZO/ITO stacks can be noticeably modulated by varying Zn-doping. The selector-switching is stable for more than 100 cycles with a resistance ratio of 104 at voltages within +3 V. Stable memristive switching is obtainable by current-controlled RESET and voltage-controlled SET. We found that selector-switching arises from generalized trap-assisted tunneling of electrons provided by zinc addition. The dual-switching-mode of SZO is proposed to facilitate implementation of cross-bar RRAM.
The third topic of this dissertation, aims at developing a cost-effective method for ReRAM application. Cyclic voltammetry deposition (CV-D) was applied to deposit ZnO films on indium-tin-oxide (ITO) glass. The result is much superior coverage of the CV-D thin films as compared to those obtained by conventional electrochemical deposition. The Pt/CV-D ZnO/ITO devices in which ZnO prepared by CV-sweeping within ± 0.9 V for 6 cycles then fix-potential-deposited at - 0.75 V for 300 s show reproducible forming-free bipolar switching operation at voltages ≤ ±1 V. The cycle-life is at least 200 cycles. The electrical conduction belongs to space-charge-limited-current conduction mechanism, which is fitted to extract carrier mobility 0.97 cm2/Vs and carrier concentration 8.6x1018 cm-3. Gradient oxygen bonding status in ZnO film, typical of cyclic voltammetry deposition, was found to facilitate the electrical switching at low voltages.
In the fourth topic of this dissertation, is to work out a simplest ReRAM configuration based on single layer transparent conducting oxides (TCO). Bias-polarity-induced transformation of point contact resistive switching memory is demonstrated on three kinds of TCO layers, including tin-doped indium oxides (ITO), fluorine-doped tin oxides (FTO), and aluminum-doped zinc oxides (AZO) as conducting electrode as well as memristive material by the controllably electrical field simultaneously. Voltage-controlled SET and current-controlled RESET were utilized to obtain much more stable endurance results. The special RS behavior based on the TCO single layer provides a new material selection and the simplest geometry to realize the highest stacking density at development of three dimensional (3D) point-contact ReRAM application.

並列關鍵字

Resistive random-access memory ； Silicon oxide ； Cu-dopped ； Zn-doped ； Zinc oxide ； Cyclic-voltammetric deposition ； Tin-dopped indium oxide

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