||Investigation of Polymer Resistive Memory with ZnO Nanoparticles and Graphene Quantum Dots
||Institute of Microelectronics
本論文利用旋轉塗佈的方式在氧化铟锡/玻璃基板上沉積絕緣層，並利用熱蒸鍍機在絕緣層表面鍍上Al當上電極來製作出結構為金屬/絕緣體/金屬(MIM)的電阻式記憶體。絕緣層的種類主要分成兩種：一種是使用聚二甲基矽氧烷（Polydimethylsiloxane，PDMS），另外一種是使用聚甲基丙烯酸甲酯（Poly(methyl methacrylate)，PMMA），並分別加入氧化鋅奈米粒子（Zinc Oxide Nanoparticles，ZnO NPs）及石墨烯量子點（Graphene Quantum Dots，GQDs）來觀察電阻式記憶體之切換特性與傳導機制。
在PDMS電阻式記憶體方面，首先將PDMS溶於甲苯來改善不同轉速下旋轉塗佈時的絕緣層厚度，之後再摻雜氧化鋅奈米粒子來提升元件寫入/讀取的次數(~ 300次)。而在PMMA電阻式記憶體方面，首先直接將GQD溶於PMMA來製作絕緣層，之後又在絕緣層與上電極之間沉積氟化鋰(LiF)來提升元件的穩定度與可靠度，有助於寫入/讀取的次數(~ 500次)及改善上電極鋁的氧化問題。
以上這兩種元件其導電機制在低阻態(low resistance state，LRS)時皆為歐姆傳導機制，高阻態(high resistance state，HRS)在低電壓時符合歐姆定律，而高電壓則為空間電荷限制電流機制，兩者都具有不錯的開關電流比(~ 104 )和耐久力(~ 104s)。因此可證實聚合物摻雜奈米粒子與量子點製作電阻式記憶體都具有優異的記憶體特性。
In this study, the insulator layer was deposited on the ITO/Glass substrate by spin coating method and Al was deposited on the insulator layer as the top electrode using thermal evaporation to form a resistive memory with the metal/insulator/metal (MIM) structure. The insulator layers used in this experiment were divided into two types: one was polydimethylsiloxane (PDMS), and the other was poly(methyl methacrylate (PMMA), and zinc oxide nanoparticles (ZnO NPs) and graphene quantum dots (GQDs) were doped respectively to observe the switching characteristics and the conduction mechanism of the resistive memory.
In PDMS resistive memory, PDMS was firstly dissolved in toluene to improve the thickness of the insulator layer under different spinning speed, and then doped with zinc oxide nanoparticles to increase the write/read times of the device (~300 times). In PMMA resistive memory, GQD was initially dissolved directly in PMMA to form the insulator layer, and then the lithium fluoride (LiF) was deposited between the insulator layer and the top electrode to improve the stability and reliability, the writes/reads times (~500 times) was enhanced and the oxidation of Al on the top electrode was suppressed of the device.
The conduction mechanism of the above two devices was ohmic conduction mechanism in low resistance state (LRS); In high resistance state (HRS) Ohm's law dominate at low voltage region and space charge limitation current mechanism at high voltage was observed and they all had good ON/OFF current ratio (~104) and retention time (~104s). Therefore, polymers doped with nanoparticles or quantum dots to fabricate resistive memory could help to obtain good memory characteristics.
Chinese Abstract I
English Abstract II
Figure Captions IX
Table Captions XII
Chapter 1 Introduction 1
1.1 History of Resistive Random access Memory 1
1.2 The Technology of Memory 1
1.3 Motivation 3
1.4 Article structure 4
Chapter 2 Theoretical Analysis of RRAM 7
2.1 Classification of Resistive Memory 8
2.1.1 Unipolar 8
2.1.2 Bipolar 9
2.2 Classification of RRAM by Switching Mechanism 10
2.2.1 Filamentary conducting path 10
2.2.2 Interface Effect 13
2.3 Classification of RRAM by Conduction Mechanism 15
2.3.1 Electrode-limited Conduction Mechanisms 16
(a.) Schottky Emission: 16
(b.) Fowler-Nordheim Tunneling / Direct Tunneling: 18
(c.) Thermionic-field Emission: 19
2.3.2 Bulk-limited Conduction Mechanisms 20
(a.) Poole-Frenkel Emission: 20
(b.) Hopping Conduction: 21
(c.) Ohmic Conduction: 22
(d.) Space-charge-limited Conduction: 23
(e.) Ionic Conduction: 25
(f.) Grain-boundary-limited Conduction: 27
Chapter 3 Experimental processes and measurements 29
3.1 Material Introduction 29
3.2 Fabrication process 32
3.2.1 ITO/Glass substrate cleaning steps 35
3.2.2 Device preparation 35
3.2.3 Electrical measurement 36
3.3 Experimental Instruments and Specifications 36
Alpha-Step (α-step): 36
Scanning Electron Microscope (SEM): 37
Atomic Force Microscope (AFM): 37
Chapter 4 Investigation of PDMS as insulator layer 38
4.1 To fabricate ITO/PDMS/Al RRAM 38
4.1.1 Fabrication process of PDMS thin film 38
4.1.2 Experimental Detail 39
4.1.3 Influence of electrical characteristics with different PDMS thickness 39
4.2 To fabricate ITO/PDMS:ZnO NPs /Al RRAM 45
4.2.1 Experimental Detail 45
4.2.2 Electrical Characteristics of ITO/PDMS:ZnO NPs/Al device 45
Thickness of insulator layer (SEM) 47
SET/RESET voltage 47
Cumulative Probability 48
Retention time and ON/OFF ratio 48
ZnO NPs film surface analysis (AFM) 48
4.3 Conduction Mechanism of ITO/PDMS:ZnO NPs/Al device 49
4.4 Summery 49
Chapter 5 Investigation of PMMA:GQD as insulator layer 61
5.1 To fabricate ITO/PMMA:GQD/Al RRAM 61
5.1.1 Experimental Detail 61
5.1.2 Electrical Characteristics of ITO/PMMA:GQD/Al device 62
5.2 To fabricate ITO/PMMA:GQD/Al RRAM with buffer layer of LiF 64
5.2.1 Experimental Detail 64
5.2.2 Electrical Characteristics of ITO/PMMA:GQD/LiF/Al device 64
Thickness of insulator layer (SEM) 65
SET/RESET voltage 65
Switching cycle 66
Retention time and ON/OFF ratio 66
Thin film surface analysis (AFM) 66
5.3 Conduction Mechanism of ITO/PMMA:GQD/LiF/Al device 67
5.4 Summery 67
Chapter 6 Conclusion and Future work 74
6.1 Conclusion 74
6.2 Future work 77
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