Investigation of Bipolar Resistive Switching Characteristics with Silicon Nitride device and bi-layer application by introducing the Al2O3 barrier layer

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부(하이브리드 재료), 2020. 8. 황철성.The resistance switching random access memory (ReRAM), which changes the resistance state of the device by the external electrical stimulation, is one of the promising candidates for a next-generation nonvolatile memory. Due to its simple metal-insulator-metal (MIM) structure, low power consumption, scalability, and complementary metal-oxide-semiconductor compatibility, ReRAM has been attracted enormous attention as a highly integrated memory to replace NAND flash memory. The transition metal oxides, such as NiO, TiO2, HfO2, and Ta2O5, were the main focus on the ReRAM device fabrication and mechanism analysis. On the other hand, the insulating nitride films, such as silicon nitride (Si3N4), have no reason for not being regarded as the feasible ReRAM material. In fact, the Si:N ratio of Si3N4 can be readily controlled to induce defects inside the film, which is already in massive use for charge-trap layer in NAND flash memory. The defect generation and possible percolation of the defects to form the so-called conducting filament (CF) is the main mechanism for the fluent ReRAM performance. Therefore, N-deficient Si3N4, i.e., Si3N4-x, can be a feasible resistance switching (RS) material. In the first part of this study, a bipolar resistive switching (BRS) property of a Si3N4-x thin film depending N-deficiency was investigated with Pt/Si3N4-x/TiN devices with various NH3 gas flow rate during plasma enhanced chemical vapor deposition process of Si3N4-x thin film. By X-ray photo-electron spectroscopy analysis, it was confirmed that the fraction of nitrogen element in Si3N4-x thin film decreased as NH3 gas flow rate decreased and the degree of N-deficiency could be controlled by changing NH3 gas flow rate. The change of N-deficiency affected the current-voltage (I-V) characteristics of Si3N4-x thin film, and the behavior of BRS and the optimized condition could be achieved. In addition, the series line resistance of Pt/TiN electrode was attributed to the self-compliance behavior, having a stable BRS behavior even without compliance current. The Si3N4-x devices didn't show cell area dependency of I-V characteristics, implying the formation and rupture of CF involved in the RS behavior. To further investigation of RS mechanism, the temperature dependent I-V behavior was examined. With a double logarithmic plot of the I-V curves, the slope of the best-linear-fitted data in the low and high voltage regions was ~1 and ~2, respectively, which coincide with the Childs law and space charge limited conduction mechanism dominated the conduction of Si3N4-x device. In addition, the activation energy and hopping distance for hopping conduction were calculated and both values decreased as NH3 gas flow rate decreased and after switching occured. From these results, CF is formed by the local repelling N to TiN BE and the percolation of the traps. The trap condition of the initial Si3N4-x played an important role in the formation of CF, otherwise the device underwent a hard breakdown. On the basis of the BRS property of a Si3N4-x thin film in the first part of this study, Al2O3 interfacial barrier layer (IBL) was inserted between the Si3N4-x thin film with optimized deposition condition (Si3N3.0) and Pt top electrode, forming the Pt/Al2O3/Si3N3.0/Ti devices with various Al2O3 film thickness (3-5 nm). The seperation between Al2O3 and Si3N3.0 layer could be identified with TEM image and EDS mapping image. While the Pt/Si3N3.0/Ti device showed filamentary BRS, the Pt/Al2O3/Si3N3.0/Ti devices showed electronic BRS with forming free property. In addition, the devices had self-rectifying and nonlinearity characteristics, which is necessary to prevent sneak current for big crossbar-array structure, due to the high band gap of Al2O3 IBL. The devices showed area dependency at HRS and LRS, indicating the interfacial electronic BRS dominated the RS mechanism. The temperature dependency analysis revealed the trap depth of trap sites in Si3N3.0 layer and schottky barreir height between Pt and Al2O3. Thus, the device switched its resistance state by trapping/detrapping of electrons at the trap sites in Si3N3.0 RS layer. To estimate the available maximum crossbar-array size (CBA), HSPICE simulation was performed and it was confirmed that ~106 density could be obtained. To form crossbar-array structure with ReRAM, sneak current is main issue for proper operation of selected cell. To suppress the sneak current, using transistor is a solution. However, relatively large size of transistor device hinders the scaling-down of ReRAM device. In this respect, selector device ,which has simple MIM sutucture, could replace a transistor while suppressing sneak current sufficiently. Therefore, it is needed to investigate the issue of integrated device with 1 selector and 1 RS material (1S1R). In the third part of this study, 1S1R device was fabricated with the Pt/Si3N4-x/TiN RS layer in the first part of this study and Pt/TiO2/TiN selector layer using the atomic-layer deposited TiO2 film. The device was fabricated via lift-off process with single cell, 2 by 2 and 9 by 9 crossbar-array pattern. By comparing each device, optimized deposition condition of selector and RS layer could be founded and additional issue to overcome was identified.외부 자극에 의해 소자의 저항상태를 변화시키는 저항 변화 메모리는 차세대 비휘발성 메모리의 유망한 후보 중 하나이다. 간단한 MIM 구조, 저전력 소모, 고 집적성 그리고 CMOS 적합성으로 인해 저항 변화 메모리는 NAND 플래시 메모리를 대체할 고집적 메모리로 많은 기대를 받고 있다. NiO, TiO2, HfO2 그리고Ta2O5와 같은 전이금속 산화물이 저항변화 메모리 소자 제작과 거동 분석의 주된 초점이였다. 반면에 Si3N4와 같은 질화막 또한 저항 변화 메모리로 쓰이지 않을 이유가 없을 것이다. 실제로 Si3N4의 Si와 N의 비율은 박막내의 결함을 유도하기위해 손쉽게 조절 가능하며, NAND 플래시 메모리에서의 charge trap layer로서도 이미 널리 쓰여지고 있다. 결함의 생성으로 인한 일명, 전도 필라멘트는 저항 변화 메모리의 주된 거동이다. 따라서 N 원소가 부족한 Si3N4 즉 Si3N4-x는 저항변화 물질로 사용 가능 할 것이다. 본 연구의 첫번째 파트에서는 N원소의 부족한 정도에 따른 Si3N4-x의 양극성 저항변화 특성 (BRS) 을 조사하기위해 Si3N4-x박막의 plasma enhanced chemical vapor deposition 과정 중에 사용되는 NH3가스 유량을 다양하게 하여 Pt/Si3N4-x/TiN 소자를 제작하였다. X-ray photo-electron spectroscopy 분석을 통해, NH3 가스 유량이 감소함에 따라 Si3N4-x 박막내의 질소 원소의 분율이 감소함을 확인할 수 있었고 NH3가스 유량을 조절함으로서 질소 원소의 부족한 정도를 조절할 수 있음을 알아내었다. 질소 원소의 부족한 정도는 Si3N4-x의 전류-전압 특성과 BRS 거동에 영향을 미치며 최적화된 조건을 찾을 수 있었다. 또한 Pt/TiN 전극의 직렬 선저항으로 인해 compliance 전류 없이 안정적인 BRS 거동을 보이는self-compliance 거동이 나타남을 확인하였다. Si3N4-x 소자는 전류-전압 특성에서 면적 의존성을 보이지 않았으며 이는 전도 필라멘트의 형성과 끊어짐에 의해 저항 변화 거동이 보임을 암시하였다. 저항 변화 거동의 추가적인 분석을 위해 전류-전압 특성의 온도의존성 측정을 진행하였다. 전류-전압 그래프의 더블 로그 플롯을 통해, 저전압과 고전압 영역대에서 피팅을 한 결과 1과 2가 각각 나옴을 확인하였으며 이는 childs law를 따르며 Si3N4-x의 전도가 space charge limited conduction에 의해 이루어짐을 알 수 있었다. 또한 hopping conduction을 위한 활성화 에너지와 hopping거리도 계산하였으며 두개의 값 모두 NH3가스 유량이 감소함에따라, 그리고 스위칭이 나타난 후에 감소함을 알 수 있었다. 이러한 결과로부터 질소 원소가 TiN 하부전극으로 밀려나가며 trap들이 뭉침으로 인해 전도 필라멘트가 형성됨을 확인할 수 있었다. 또한 초기 Si3N4-x의 trap 상태가 전도 필라멘트 형성에 중요한 역할을 하며 그렇지 않으면 소자는 망가지는 것을 확인하였다. 본 연구의 첫 번째 파트에서의 Si3N4-x 박막의 BRS 특성에 기초하여, Al2O3 계면 장벽 층이 최적화 된 증착 조건으로 만들어진 Si3N3.0층과 Pt 상부 전극 사이에 삽입되으며 그 결과 다양한 Al2O3 막 두께 (3-5 nm)를 갖는 Pt/Al2O3/Si3N3.0/Ti 소자를 형성 하였다. TEM 이미지와 EDS 매핑 이미지로 Al2O3와 Si3N3.0 층 사이의 분리를 확인 가능하였다. Pt/Si3N3.0/Ti 소자는 필라멘트의 BRS 특성을 나타내지만, Pt/Al2O3/Si3N3.0/Ti 소자는 forming-free특성을 갖는 e-BRS특성을 나타냈다. 또한 이 소자는 자체 정류 특성 및 비선형성 특성을 가지는데, 이러한 특성은 큰 사이즈의 크로스바 어레이 (CBA) 구조에서 누설전류를 방지하는데 도움이 되며, Al2O3층의 높은 밴드 갭으로 인해 이러한 특성이 나타나게 된다. 장치는 HRS 및 LRS에서 면적 의존성을 보여 주었으며 이는 계면의 e-BRS 저항 변화 메커니즘을 지배 한다는 것을 나타낸다. 또한 온도 의존성 분석을 통해 Si3N3.0 층에 존재하는 trap site의 trap 깊이 및 Pt와 Al2O3 사이의 쇼트키 배리어 높이를 알아낼 수 있었다. 따라서, 해당 소자는 Si3N3.0 저항 변화층의 trap site에서 전자를 트래핑 / 디트랩핑하여 저항 상태를 바꾼다는 걸 확인 가능하였다. 또한 가능한 최대 CBA 크기를 추정하기 위해 HSPICE 시뮬레이션을 수행하였으며 약 106 의 값을 얻을 수 있음을 확인하였다. 저항 변화 메모리로 CBA 구조를 형성할 때, 선택된 셀의 적절한 구동을 위해선 누설 전류가 가장 큰 문제이다. 누설 전류를 억제하기 위해, 트랜지스터의 사용은 하나의 해결책이 될 수 있다. 하지만, 상대적으로 큰 사이즈의 트랜지스터 소자는 저항 변화 메모리의 고집적을 방해하게 된다. 이러한 점에서 간단한 MIM구조를 갖는 선택소자는 누설전류를 충분히 억제하면서 트랜지스터를 대체할 수 있을것이다. 따라서 1 선택소자 1 저항변화 물질의 적층된 소자 (1S1R) 에서 생길 수 있는 문제점에 대해 조사할 필요가 있다. 본 연구의 세번째 파트에서는, 첫번째 파트에서의 Pt/Si3N4-x/TiN 저항 변화 소자와 ALD TiO2 박막을 사용한 Pt/TiO2/TiN 선택소자를 이용하여 1S1R소자를 제작하였다. 해당 소자는 lift-off 공정을 통해 단일소자, 2 by 2 그리고 9 by 9 CBA 패턴으로 제작되었다. 각각의 소자를 비교하면서 선택소자와 저항변화 층의 최적화된 증착 조건을 찾아내었고 극복해야할 추가적인 문제또한 확인 하였다.1. Introduction 16 1.1. Resistive switching Random Access Memory 16 1.2. Research scope and objective 20 2. Bipolar resistive switching property of Si3N4-x thin film depending on N-deficiency 22 2.1. Introduction 22 2.2. Experimental 25 2.3. Results and Discussions 26 2.4. Conclusion 45 3. Area-type electronic bipolar resistive switching of Pt/Al2O3/Si3N3.0/Ti with forming free, self-rectification, and nonlinearity characteristics 46 3.1. Introduction 46 3.2. Experimental 49 3.3. Results and Discussions 51 3.4. Conclusion 69 4. 1S1R property with Pt/Si3N4-x/TiN resistive switching device and Pt/TiO2/TiN selector device 71 4.1. Introduction 71 4.2. Experimental 73 4.3. Results and Discussions 74 4.4. Conclusion 81 5. Bibliography 82 6. Conclusion 88 List of publications 92 Abstract (in Korean) 104Docto

Atomic layer deposition and properties of mixed Ta2O5 and ZrO2 films

Thin solid films consisting of ZrO2 and Ta2O5 were grown by atomic layer deposition at 300 degrees C. Ta2O5 films doped with ZrO2, TaZr2.75O8 ternary phase, or ZrO2 doped with Ta2O5 were grown to thickness and composition depending on the number and ratio of alternating ZrO2 and Ta2O5 deposition cycles. All the films grown exhibited resistive switching characteristics between TiN and Pt electrodes, expressed by repetitive current-voltage loops. The most reliable windows between high and low resistive states were observed in Ta2O5 films mixed with relatively low amounts of ZrO2, providing Zr to Ta cation ratio of 0.2. (C) 2017 Author(s).Peer reviewe

Impact of oxygen exchange reaction at the ohmic interface in Ta2O5-based ReRAM devices.

Interface reactions constitute essential aspects of the switching mechanism in redox-based resistive random access memory (ReRAM). For example, the modulation of the electronic barrier height at the Schottky interface is considered to be responsible for the toggling of the resistance states. On the other hand, the role of the ohmic interface in the resistive switching behavior is still ambigious. In this paper, the impact of different ohmic metal-electrode (M) materials, namely W, Ta, Ti, and Hf on the characteristics of Ta2O5 ReRAM is investigated. These materials are chosen with respect to their free energy for metal oxide formation and, associated, their impact on the formation energy of oxygen vacancy defects at the M/Ta2O5 interface. The resistive switching devices with Ti and Hf electrodes that have a negative defect formation energy, show an early RESET failure during the switching cycles. This failure process with Ti and Hf electrode is attributed to the accumulation of oxygen vacancies in the Ta2O5 layer, which leads to permanent breakdown of the metal-oxide to a low resistive state. In contrast, the defect formation energy in the Ta2O5 with respect to Ta and W electrodes is positive and for those highly stable resistive switching behavior is observed. During the quasi-static and transient-pulse characterization, the ReRAM devices with the W electrode consistently show an increased high resistance state (HRS) than with the Ta electrode for all RESET stop voltages. This effect is attributed to the faster oxygen exchange reaction at the W-electrode interface during the RESET process in accordance to lower stability of WO3 than Ta2O5. Based on these findings, an advanced resistive switching model, wherein also the oxygen exchange reaction at the ohmic M-electrode interface plays a vital role in determining of the resistance states, is presented

Evolution of Resistive Switching Characteristics in WO3-x-based MIM Devices by Tailoring Oxygen Deficiency

We report on resistive switching (RS) characteristics of W/WO3-x/Pt-based thin film memristors modulated by precisely controlled oxygen non-stoichiometry. RS properties of the devices with varied oxygen vacancy (VO) concentration have been studied by measuring their DC current voltage properties. Switchability of the resistance states in the memristors have been found to depend strongly on the VOs concentration in the WO3-x layer. Depending on x, the memristors exhibited forming-free bipolar, forming-required bipolar and non-formable characteristics. Devices with high VOs concentration (~1*1021 cm-3) exhibited lower initial resistance and memory window of only 15, which has been increased to ~6500 with reducing VOs concentration to ~5.8*1020 cm-3. Forming-free, stable RS with memory window of ~2000 have been realized for a memristor possessing VOs concentration of ~6.2*1020 cm-3. Investigation of the conduction mechanism suggests that tailoring VOs concentration modifies the formation and dimension of the conducting filaments as well as the Schottky barrier height at WO3-x/Pt interface which deterministically modulates RS characteristics of the WO3-x based memristors

Resistive Switching Mechanisms on TaOx and SrRuO3 Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

The local electronic properties of tantalum oxide (TaO[subscript x], 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO[subscript 3]) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO[subscript x] films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO[subscript 3], we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO[subscript x] and SrRuO[subscript 3] differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-1419807

Memristors using solution-based IGZO nanoparticles

Solution-based indium-gallium-zinc oldde (IGZO) nanoparticles deposited by spin coating have been investigated as a resistive switching layer in metal-insulator-metal structures for nonvolatile memory applications. Optimized devices show a bipolar resistive switching behavior, low programming voltages of +/- 1 V, on/off ratios higher than 10, high endurance, and a retention time of up to 104 s. The better performing devices were achieved with annealing temperatures of 200 degrees C and using asymmetric electrode materials of titanium and silver. The physics behind the improved switching properties of the devices is discussed in terms of the oxygen deficiency of IGZO. Temperature analysis of the conductance states revealed a nonmetallic filamentary conduction. The presented devices are potential candidates for the integration of memory functionality into low-cost System-on-Panel technology.National Funds through FCT - Portuguese Foundation for Science and Technology [UID/CTM/50025/2013, SFRH/BDP/99136/2013]; FEDER [POCI-01-0145-FEDER-007688]info:eu-repo/semantics/publishedVersio

Memristors: a short review on fundamentals, structures, materials and applications

The paper contains a short literature review on the subject of special type of thin film structures with resistive-switching memory effect. In the literature, such structures are commonly labeled as "memristors". The word "memristor" originates from two words: "memory" and "resistor". For the first time, the memristor was theoretically described in 1971 by Leon Chua as the 4th fundamental passive electronics element with a non-linear current-voltage behavior. The reported area of potential usage of memristor is enormous. It is predicted that the memristor could find application, for example in the domain of nonvolatile random access memory, flash memory, neuromorphic systems and so forth. However, in spite of the fact that plenty of papers have been published in the subject literature to date, the memristor still behaves as a "mysterious" electronic element. It seems that, one of the important reasons that such structures are not yet in practical use, is unsufficient knowledge of physical phenomena determining occurrence of the switching effect. The present paper contains a literature review of available descriptions of theoretical basis of the memristor structures, used materials, structure configurations and discussion about future prospects and limitations

Low-temperature amorphous oxide semiconductors for thin-film transistors and memristors: physical insights and applications

While amorphous oxides semiconductors (AOS), namely InGaZnO (IGZO), have found market application in the display industry, their disruptive properties permit to envisage for more advanced concepts such as System-on-Panel (SoP) in which AOS devices could be used for addressing (and readout) of sensors and displays, for communication, and even for memory as oxide memristors are candidates for the next-generation memories. This work concerns the application of AOS for these applications considering the low thermal budgets (< 180 °C) required for flexible, low cost and alternative substrates. For maintaining low driving voltages, a sputtered multicomponent/multi-layered high-κ dielectric (Ta2O5+SiO2) was developed for low temperature IGZO TFTs which permitted high performance without sacrificing reliability and stability. Devices’ performance under temperature was investigated and the bias and temperature dependent mobility was modelled and included in TCAD simulation. Even for IGZO compositions yielding very high thermal activation, circuit topologies for counteracting both this and the bias stress effect were suggested. Channel length scaling of the devices was investigated, showing that operation for radio frequency identification (RFID) can be achieved without significant performance deterioration from short channel effects, which are attenuated by the high-κ dielectric, as is shown in TCAD simulation. The applicability of these devices in SoP is then exemplified by suggesting a large area flexible radiation sensing system with on-chip clock-generation, sensor matrix addressing and signal read-out, performed by the IGZO TFTs. Application for paper electronics was also shown, in which TCAD simulation was used to investigate on the unconventional floating gate structure. AOS memristors are also presented, with two distinct operation modes that could be envisaged for data storage or for synaptic applications. Employing typical TFT methodologies and materials, these are ease to integrate in oxide SoP architectures

Niobium and tantalum oxides as model materials for resistive switching effect

Celem współczesnej nauki jest znalezienie nowych rozwiązań dla wciąż zmieniającego się świata. Jednym z wyzwań jakie stawiają sobie naukowcy jest znalezienie nowego materiału dla pamięci nieulotnych o dużej gęstości zapisu, który w coraz to mniejszej scali, nanoskali, pozwoli na zapisanie coraz większej ilości danych. Takimi materiałami mogą być tlenki metali przejściowych, które bazując na reakcji redoks, wykazują zdolność do zmiany oporu pod wpływem przyłożonego pola elektrycznego. Jednakże wiedza o fizycznych podstawach tego zjawiska jest wciąż ograniczona. Do tej pory nie zostało jasno i klarownie przedstawione wyjaśnienie natury zjawiska przełączania rezystywnego, a co za tym idzie jego aplikacja w urządzeniach elektronicznych, może być nadal problematyczna. Niniejsza praca doktorska została poświęcona tlenkom metali przejściowych jakim są tlenki niobu i tantalu. Chociaż jak się często podkreśla są to materiały wciąż badane od wielu lat i wydaje się, że posiadamy już duży zasób wiedzy na ich temat, to nadal są miejsca, gdzie materiały te potrafią nas zaskoczyć. Praca ta została podzielona na dwie części. Pierwsza została poświęcona monokryształowi Nb₂O₅ natomiast w drugiej badania były skoncentrowane na cienkich warstwach Nb-O i Ta-O. W pracy przedstawiono wyniki badań podstawowych właściwości fizykochemicznych materiału przed oraz po redukcji termicznej. Temperatury od 800°C -1000°C znacząco redukują monokryształ Nb₂O₅. Natomiast w cienkich warstwach amorficznych Nb-O czy Ta-O o złożonej strukturze wewnętrznej, w której skład wchodzą warstwy pięciotlenków, zaobserwowano ten efekt w znacznie niższych temperaturach. Nawet niewielka zmiana temperatury (300°C dla Nb-O i 600°C dla Ta-O) wpływa na stopień redukcji warstwy. Wpływ temperatury ma również silenie znaczenia na przewodnictwo. W cienkich warstwach zaobserwowano przełączanie oporności typu bipolarnego. Natomiast w krysztale ten sam efekt również był zauważalny, lecz znacznie słabszy. Można było, podobnie jak w cienkich warstwach, zmodyfikować jego powierzchnie w celu zapisania informacji przy pomocy igły z mikroskopu sił atomowych. Reasumując przedstawione wyniki badań w tej pracy pozwalają w szerszy sposób spojrzeć na problem chemicznej i strukturalnej niestabilności tlenków metali przejściowych (Nb, Ta) w krysztale jak i cienkich warstwach. Pokazują wpływ tych zmian na ich przewodnictwo, które może być lokalnie kontrolowane, pozwalając na łatwiejszą aplikacje tych materiałów