非揮發性記憶體為現今許多消費性電子產品中不可或缺之重要元件，電阻式隨機存取記憶體(resistance RAM, RRAM)因具備高密度、低成本、低功耗及非揮發特性等優點，故已廣泛的被視為研究重點之一。目前對於電阻轉換機制的討論仍尚未有明確的定論，因而影響電阻式記憶體商品化的可靠度與時機。
本論文針對高分子電阻式非揮發性記憶體進行系統性的研究，以旋轉塗佈的方式製作高分子主動層(active layer)薄膜，其中內含包覆硫醇分子之金奈米顆粒(Au nanoparticles，Au NPs)，高分子薄膜的角色為主動層基質(matrix)和電子捐贈者(electron donor)，而Au NPs則作為電子接受者(electron acceptor)。將記憶元件製作成金屬(Al)/主動層(Au NPs+polymer)/金屬(Al)的結構，觀察與探討此「有機+無機」複合材料組成的記憶體元件，其電性量測結果(I-V characteristics，Retention test，C-f curves)與電阻轉換行為的相關性，進一步推測其電阻轉換機制，並改善記憶體之功能特性。
本論文研究主要分為四個階段，第一階段為探討記憶體效能與金奈米顆粒外所包覆的硫醇分子碳鏈長度之關連性。研究中以不同碳鏈長度硫醇分子(1-octanethiol(C8)，1-dodecanethiol(C12)，和1-octadecanethiol(C18))修飾的金奈米顆粒，再添加於polystyrene(PS)和8-hydroxyquinoline(8HQ)混合高分子薄膜內，作為元件主動層。結果發現不同碳鏈長度的硫醇分子對於電子穿遂的阻抗，並未表現在臨界電壓的大小上，而於金奈米顆粒外包覆C18硫醇分子的記憶元件，須設定較高的設限電流始能穩定轉換至低電阻態，但在低電阻態時卻觀察到電流擾動與衰退現象，此特徵與被金奈米顆粒捕捉的電子，其相互間的庫倫排斥(Coulomb repulsion)作用有關。
因poly(N-vinylcarbazole)(PVK)高分子在化學結構上，沒有-OH或是-COOH等親水基團(hydrophilic groups)，相對於8HQ有較佳之抗水氧的能力。因此第二階段的主體材料改以摻雜金奈米顆粒之PVK高分子薄膜，作為記憶元件之主動層。而於改變“PVK-Au NPs”系統中金奈米顆粒添加量(重量比Au NPs/PVK=0.000，0.083 和0.200)的實驗發現，不論金奈米顆粒添加量為何，此三種元件皆具記憶元件高-低電阻雙穩態的表現；然而於低電阻態記憶保持(retention)時間的量測中，只有Au NPs/PVK=0.083的記憶元件於量測時間內保持穩定電流值。從電容-頻率(capacitance-frequency，C-f)特性曲線量測可發現，此三種元件於高-低電阻態皆呈現相同的負電容特徵，此表示載子的傳輸行為是由PVK高分子基質所主導。
第三階段則藉由變溫電流-電壓曲線的量測發現，Au NPs/PVK=0.083(重量比)的記憶元件可運作於154 oC之大氣環境下，表現絕佳的熱穩定性。而且元件於高電阻態的電流傳導主要為蕭特基發射(Schottky emission)，而於低電阻態為歐姆(Ohmic)傳導機制。從電流傳導與溫度的相依性可發現，高電阻態之蕭特基發射傳導行為與金屬(Al or Au)-PVK接面energy-band offsets相關。而電阻轉換行為與PVK在電場施加下，因電洞濃度改變(亦即Fermi level位置改變)而造成空乏區寬度的變化有關。
第四階段則是於元件插入poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS)高分子薄膜，製作成Al/ PVK:Au NPs/PEDOT-PSS/Al電阻式記憶體，可於1 V電壓下得到約5×109超高的電流比，且此元件相對於只含PVK:Au NPs主動層的記憶元件，有相同的臨界電壓(Vth)與相對較小的臨界電壓偏移。而於改變PVK:Au NPs主動層厚度(120和360 nm)的實驗結果亦證實高分子記憶元件之高低電阻態電流比的調變，不能單純只靠增加高分子主動層的厚度。

Nonvolatile memory devices are essential to almost all consumer electronic products. Among the emerging memory devices, resistance random access memories (RRAMs) have several advantages, such as high density, low cost, low power dissipation, and nonvolatile property, which result in a great amount of attention. Nevertheless, RRAMs have not been fully commercialized yet because the resistive switching mechanisms are still controversial up to now.
In this study, the characterization and operation of nonvolatile polymer-based resistive memory devices using a spin-coated polymer film containing gold nanoparticles (Au NPs) capped with alkanethiols as the active layer are systematically investigated. The polymer thin film basically serves as the matrix and electron donor, while Au NPs act as the electron acceptors. The “organic+inorganic” hybrid memory devices are fabricated as the metal (Al)/active layer/metal (Al) structure to investigate the correlation between electrical characters (I-V characteristics，Retention test，C-f curves) and resistive switching behaviors to acquire further insight into the resistive switching mechanism and improve the functionality of the memory devices.
The thesis is divided into four phases; the first phase is to investigate how the length of the molecules which encapsulates Au NPs affects the device performance in terms of ON/OFF current ratio, turn-on voltage, retention time, etc. Electrical bistability is demonstrated in a polymer memory device using polystyrene (PS) containing organic conjugated compound 8-hydroxyquinoline (8HQ) and Au NPs capped with different alkanethiol of carbon chain lengths (1-octanethiol (C8), 1-dodecanethiol (C12), and 1-octadecanethiol (C18)) as the active layer. The threshold voltages (Vth) of thiol-derivatized Au NPs based organic memory devices are almost the same. Au NPs capped with C18 based memory device with a higher current compliance setting (1 mA) shows current fluctuation and degradation at the low-resistance state. This feature is related to charging and discharging of Au NPs because of the Coulomb repulsion between electrons confined in nanocrystals.
Poly(N-vinylcarbazole) (PVK) is more resistant to moisture and oxygen than other organic materials (such as 8HQ blended into polystyrene matrix) for nonvolatile memory application owing to the absence of hydrophilic groups. Therefore, the host material in the second phase are changed to PVK incorporated with C12 capped Au NPs with various weight ratios between PVK and Au NPs (zero, 0.083, and 0.200). Electrical bistability is demonstrated for all three PVK-based devices, regardless of Au-NP ratios. However, good current stability is only obtained for the Au NPs:PVK=0.083:1 device. Capacitance-frequency (C-f) curves present the comparable negative capacitance feature for all three devices in both low-resistance state (LRS, “ON”) and high-resistance state (HRS, “OFF”), indicating that the carrier transport is dominated by the PVK matrix.
In the third phase, the electrical conduction measurement at various temperatures reveals that Au NPs/PVK=0.083 memory device can be programmed and exhibits excellent thermal stability up to 154 oC in ambient atmosphere. The current conduction is dominated by Schottky emission at HRS and exhibits Ohmic behavior at LRS. The dependence of the current conduction on temperature reveals the connection between the conduction character and the energy band offsets at the metal (Al or Au)-PVK junctions. In addition, the resistive switching is correlated with the width of depletion region in PVK, which varies with the change of hole carrier concentration upon applying electrical field.
In the fourth phase, an ultrahigh ON/OFF current ratio of 5×109 at 1 V is demonstrated for the Al/PVK:Au NPs/PEDOT-PSS/Al resistive memory device without magnifying the threshold voltage and improves the consistency of threshold voltage. The investigation into different thickness (120 and 360 nm) of PVK:Au NPs active layer also suggests that the modulation of ON/OFF current ratio for the memory device can not be achieved by simply varying the thickness of the polymer active layer itself.

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