||Influences of oxygen vacancy drifting on resistive switching behaviors in TiOx and TaOx based nonvolatile memories
||Department of Materials Science and Engineering
本實驗將針對氧化鈦及氧化鉭兩種材料為主的電阻式記憶體做討論，其試片結構分別為Pt/TiOx/Pt/TiOx/Pt (價態轉換型)和Ag/TaOx/Pt (電化學金屬型)。我們藉由觀察兩個元件的電流-電壓曲線，包括electroforming, 單極式電阻轉換，雙極式電阻轉換和抹除電流等特性來瞭解氧空缺的移動對這兩個元件造成的影響。
論文第一部份討論的是以二氧化鈦為基底的記體元件之材料特性及電性。在二氧化鈦為基底的記體元件中觀察到一個明確單極性且單一極性的電阻轉換行為。其中，此TiOx/Pt/TiOx 主動層是經由熱氧化Ti/Pt/Ti疊層所製備。藉由插入中間層Pt會在主動層中產生兩個額外的蕭基能障導致記憶體元件在高電阻態時有整流的特性。實驗結果顯示，氧空缺受電埸驅動移動會影響蕭基能障和導電燈絲 (conduction filament)的組成使元件具有單極性電阻轉換特性。此外，以AFM探針為上電極時，元件整流的特性依然存在，此結果顯示元件整流的特性在奈米維度下仍可使用。
論文第二部份的討論是以二氧化鉭為主的記憶體元件，討論在300 K和100 K溫度區間內electroforming和電阻轉換的行為。同時，也將會區別在此溫度區間銀離子和氧空缺所組成的導電燈絲之間的差異。我們發現不論是electroforming的電壓或是寫入的電壓，其電壓值在100 K時都明顯的高於300 K時的電壓值。藉由觀察脈衝轉換時間和溫度的關係發現銀離子在氧化鉭內移動有很高的活化能，表示銀離子移動速率會隨溫度降低而大幅下降。可知，electroforming的電壓或寫入的電壓在100 K和300 K時的差異是來自於銀離子移動速率的差別。此外，銀離子在高、低溫移動速率的差別也將導致電燈絲在300 K時是由銀所組成，而在100 K時由氧空缺和銀共同組成。氧空缺參與導電燈絲的組成會改變元件抹除電流的特性，從300 K時的緩降 (gradually descending )變成100 K 時的陡降(sharp drop)。此外，當溫度從100 K回到300 K，寫入電壓和抹除電流仍會維持100 K時的特性。此結果顯示導電燈絲的組成對Ag/oxide/Pt系統電阻轉換特性是關鍵的因素。
This dissertation is devoted to study the influence of oxygen vacancy on resistive switching behaviors in the valence change memories (VCM) and electrochemical metallization (ECM) memories.
The VCM and ECM memory devices were prepared with the device structures of Pt/TiOx/Pt/TiOx/Pt and Ag/TaOx/Pt, respectively. The current-voltage (I-V) characteristics including the electroforming, unipolar resistive switching (URS), bipolar resistive switching (BRS) and reset current characters were investigated and understood in terms of the migration of oxygen vacancies in VCM and ECM system.
In the first part this dissertation, the TiOx-based memory devices were fabricated for electrical and material characterization. A distinct unipolar but single-polarity resistive switching behavior is observed in a TiOx/Pt/TiOx active layer, formed by thermal oxidation of a Ti/Pt/Ti stack. Introduction of the Pt mid-layer creates two additional Schottky barriers, which mediate the band bending potential at each metal-oxide interface and attains a rectifying current conduction at the high resistance state (HRS). Experimental evidences proving the single-polarity switching behavior is a combination of bias-induced Schottky barrier modification and conduction filament construction, both associated with the bias-driven migration of oxygen vacancies. In addition, the rectifying conduction behavior is also observed with an AFM-tip as the top electrode (TE), which implies the rectifying property is still valid when miniaturizing the device to nano-meter scale.
In the second part of this dissertation, we explored the electroforming and resistive switching behaviors in the Ag/TaOx/Pt trilayer structure under a continual change of temperatures between 300 K and 100 K to distinguish the contributions of Ag ions and oxygen vacancies in developing of conducting filaments. We found that either electroforming or resistive switching, a significantly higher forming/set voltages is needed as the device is operated at 100 K, as compared to that observed when operating at 300 K. The temperature dependence of the pulsed switching time (tsw) measurement results indicates that larger activation energy (Ea) of Ag diffusion in TaOx (EaeV) will lead to a faster decay of Ag ions mobility in TaOx with decreasing temperature. Thus, the disparity in electroforming/set voltages of Ag/TaOx/Pt operating at 300 K and 100 K can attribute to the mobility difference of Ag ions at 300 K and 100 K. This mobility difference of Ag ions also leads a presence of metallic filament at 300 K while a filament composed of oxygen vacancy and Ag is formed at 100 K.
The presence of oxygen vacancy segment in the conducting filament also modifies the reset current from a gradually descending behavior (at 300 K) to a sharp drop (at 100 K). Furthermore, the characteristic set voltage (Vset) and reset current are irreversible as the operation temperature is brought from 100 K back to 300 K, indicating the critical role of filament constituents on the switching behaviors of Ag/oxide/Pt system.
List of Tables X
List of Figures XI
Chapter 1 Introduction 1
1-1 Back ground and motivation 1
1-2 Thesis organization 2
Chapter 2 Literature review 3
2-1 Volatile and Nonvolatile memories 3
2-2 Resistance random access memory (RRAM) and materials for RRAM 5
2-2-1 Inorganic materials and electrode materials 6
2-2-2 Organic materials 8
2-3 Resistive switching behaviors 10
2-3-1 Unipolar and bipolar resistive switching 10
2-3-2 Reset current characteristics 12
2-4 Resistive switching mechanisms 16
2-4-1 Valence change switching 18
2-4-2 Electrochemical Metallization 23
2-4-3 Interface type switching (Schottky barrier) 26
2-4-4 Simulation study on conducting filament formation/rupture 31
Single polarity resistive switching in TiOx-based memory device 36
3-1 Introduction 36
3-2 Experiment methods 38
3-3 Results and discussion 40
3-3.1 Material characterization 40
(TEM, GIAXRD and XPS analyses of TiOx/Pt/TiOx films) 40
3-3.2 Current-voltage characteristics of TiOx-based memory 42
3-3.3 Resistive switching mechanism of TiOx-based memory 49
3-4 Summary 50
Temperature dependence of electroforming and resistive switching behaviors in Ag/TaOx/Pt memory device 62
4-1 Introduction 62
4-2 Experiment methods 64
4-3 Results and discussion 66
4-3.1 TEM analysis of Ag/TaOx/Pt films 66
4-3.2 Current-voltage characteristics of TaOx-based memory 67
4-4 Summary 74
Chpater 5 Conclusions 83
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