Study on modulation of electrical properties in two-dimensional semiconductors

Yue, Dewu Sungkyunkwan university 2019 국내박사

In this dissertation, high-performance semiconductor devices based on two-dimensional materials are fabricated by various functional modulation process. Edge contact structure of graphene, air-stable ambipolar BP transistor, Ohmic contact in MoS2, WSe2, MoTe2, and BP (black phosphorous) FETs and plasma induced doping, layer-by-layer removal of WSe2 are studied in order to reveal the high-performance nature of 2D materials. For manufacturing high-performance 2D semiconductors, contact resistance impedes high-performance 2D semiconductors. The formation of “edge-contacted” graphene through the use of a controlled plasma processing technique generates a bond between the graphene edge and the contact metal, which controls the edge structure of the bond and significantly reduces the contact resistance. The contact resistance attained by using pre-plasma processing in “edge-contacted” graphene FETs was of 270 Ω·μm, which is a decrease of 77%. We also demonstrated the fabrication of solution-processed BV polymeric contacts for the preparation of high mobility MoS2, WSe2, MoTe2, and BP (black phosphorous) FETs with significantly lowered contact resistance. Ohmic contacts were achieved and produced 3-, 700-, 3000-, and 17-fold increases in electron mobilities. Carrier transport mechanism at metal−2D interface are investigated. For manufacturing high-performance 2D logical circuits, n- and p-type performance of 2D materials are necessary. Considering the n- and p-type nature of 2D materials, doping method were studied by chemicals and plasma. Surface charge transfer doping techniques, n-type BV doping, are introduced to 2D MoS2, WSe2, MoTe2, and BP (black phosphorous) FETs. By BV doping and BV interlayer pre-doping, ambipolar and p-type 2D materials based FETs could, therefore, be transformed into n-type FETs. Most importantly, our devices exhibit excellent stability in both ambient and vacuum. High performance p-type WSe2 field-effect transistors were achieved transferred from ambipolar behavior by O2 plasma treatment. High electronic performance of 2D transistors is strongly dependent on its thickness, down to monolayer FETs. We report layer-by-layer thinning of WSe2 via chemical KOH solution, by removing the surface top layer WOX formed by O2 plasma treatment. Monolayer WSe2 flakes were obtained after several consecutive etching, which indicates the layer-by-layer etching technology is reliable and stable. Finally, enhanced ambipolar performance of WSe2 FETs with 10- and 35-fold increases in electron and hole mobilities, respectively, was achieved from etched WSe2 field-effect transistors.

Exploring the surface of 2D semiconductors : observation on the electronic structure near the fermi level

김민주 Graduate School, Yonsei University 2019 국내박사

2004년 그래핀의 발견 이후로, 2차원 물질에 관한 연구는 매우 급속도로 성장하고 있다. 2차원 물질이 각광받는 이유는 과학자들에게는 새로운 물리현상이 발현되는 시스템을 제공하기 때문이고 공학자들에게는 기존의 전자 및 광전소자영역의 한계를 극복할 수 있는 잠재성이 있기 때문이다. 표면 물리관점에서, 2차원 반도체의 경우 표면(z축)방향으로 dangling bond가 존재하지 않기 때문에 3차원 반도체의 표면보다는 비교적 쉽다고 생각할 수 있다. 하지만, 실제로 나타나는 전자소자/물리학적 현상을 보면 절대로 쉽다고 생각할 수 없다. 단층 2차원 반도체는 기본적으로 무극성을 갖고 층과 층 사이의 상호작용이 매우 약하다. 이것은 전계 또는 도펀트에 의한 분극이나 다른 물질과의 화학반응 (강한 상호작용)에 의해 2차원 반도체 본연의 물리적 성질이 사라질 수 있다는 것을 의미한다. 뿐만 아니라 물질 자체의 원자 결함이나 결정 대칭의 뒤틀림에 의해서도 물리적 성질이 크게 달라질 수 있다. 반대로, 전계 효과나 화학반응, 원자 결함, 결정 뒤틀림을 통해 우리가 원하는 물리적 성질을 강화시킬 수도 있다. 즉, 우리가 2차원 물질 고유의 표면과 위에서 언급한 물리적 현상들을 완전히 이해한다면, 2차원 반도체의 전자소자 응용과 새로운 물리현상을 발전시킬 수 있는 이론적 기반을 제공할 수 있다. 이러한 관점에서, 우리는 흑린 (black phosphorus, BP)의 표면의 degradation과 전이금속 디칼코제나이드 (transition metal dichalcogendies, TMDCs)에 표면산화에 의한 홀 도핑 메커니즘에 주목하였다. 먼저, BP는 매우 높은 전하이동도를 갖으며, 층 의존적 직접천이 밴드갭을 갖는다. 즉, 표면의 전자구조가 물질의 층수에 따라 크게 달라진다는 것을 의미한다. 우리는 BP의 층 의존적 전자구조가 BP의 표면 변화와 매우 밀접한 관계를 가질 것이라고 예상하였다. 우리는 원자력 간 현미경을 이용하여 48시간 동안 BP의 표면의 형상과 일함수 변화를 측정하여, 두 층의 BP보다 벌크 BP에서 산소 및 수분 결합이 빠르다는 것을 관측하였다. 제일원리계산과 마커스 이론을 결합한 이론 모델을 통해 층 의존적 산화 속도는 내재적으로 BP의 층 의존 전자구조에서 기인한다는 것을 규명한다. 우리의 이론 모델은 전도대의 전자의 밀도에 따라 표면산화속도가 매우 달라지는 것 또한 증명하였고 이는 홀 도핑을 통해 흑린의 안정성을 증가시킬 수 있다는 것을 제시한다. TMDC의 경우, 칼코겐 결함에 의한 의도하지 않는 전자도핑이 표면에 존재한다. 이러한 의도하지 않는 전자도핑은 p-type 특성을 저하시키고 메탈과의 페르미레벨 피닝을 유도하여 소자의 전기적 특성을 크게 저하시킨다고 알려져 있다. 그래서 많은 연구자들은 의도하지 않는 n-doping을 억제 시키기 위해 다양한 표면 처리방법을 사용하고 있다. 이 중에서, 우리는 TMDC의 표면산화를 이용한 홀 도핑에 주목하였다. 우리는 표면산화를 진행시키면서 TMDC의 표면의 화학적 상태와 페르미레벨 근처의 에너지레벨의 변화를 광전자/역광전자 분광을 통해 분석하였다. TMDC에서 표면산화는 칼코겐과 산소 결합과 칼코겐-산소 치환 2가지 형태로 나타나는데, 칼코겐-산소 치환의 경우 TMDC의 홀 도핑 효과가 크게 나타나는 것이 측정되었다. 이것의 원인은 칼코겐이 산소로 치환되면, transition metal의 d 오비탈의 전자를 산소가 직접 가져오기 때문에 TMDC의 홀의 농도가 크게 증가하기 때문이다. 즉 TMDC의 캐리어 농도를 효율적으로 조절하려면 전이금속의 d 오비탈의 전자량을 조절하는 것이 중요하다. Since the discovery of graphene in 2004, research on two-dimensional (2D) materials has tremendously grown. The reason why 2D materials are spotlighted is that they have the potential to overcome the limitation of conventional electronic devices while providing a new system that induced new physical phenomena, such as valleytronics, dark exciton and metal-semiconductor phase transition. From the point of view of surface physics, it can be considered that the two-dimensional semiconductor is relatively easier than the surface of the three-dimensional semiconductor because there is no dangling bond in the surface (z-axis) direction. However, 2D semiconductor-based electronic and physical phenomena are very sensitive to the surface of 2D semiconductor. A single layer of 2D semiconductors has fundamentally non-polar surface and weak interaction (vander Waals force) between layers. This means that the intrinsic properties of 2D semiconductors can be deteriorated by polarization by electric field or chemical reaction (strong interaction) other materials. Besides, their inherent properties can be greatly changed by atomic defect itself or the distortion of crystal symmetry. Conversely, it can be enhanced intrinsic properties by controlling electric field, chemical reaction, atomic defects, and crystal distortion. In other words, if we fully understand the physical phenomena raised on the surface of 2D material and on the surface abovementioned, we can provide a fundamental for developing the electronic device application of and their physical phenomena. In this regard, we have noted the degradation phenomena on the surface of black phosphorus (BP) and the hole doping mechanism by surface oxidation in transition metal dichalcogendes (TMDCs). Fast degradation remains one of the most significant challenges facing BP since the discovery of BP as a new two-dimensional material. To offer an ultimate solution for BP degradation, the complete understanding of the degradation mechanism is necessary. Despite this importance, the degradation mechanism is still lack due to the difficulties of correlating experimental measurements with the underlying physics. In this regards, we focused on the unveiling the degradation mechanism by using a scanning Kelvin probe microscopy and theoretical modeling using the Marcus-Gerischer theory and GW calculations. Our results demonstrate that there is an intrinsic correlation between the layer-dependent electronic structure and degradation. It provides not only a fundamental understanding of degradation but also the new strategy for improving the stability of BP. In the case of TMDC, the one of challenges is unintentional electron doping due to chalcogenide defects on the surface. To suppress this unintentional electron doping, various surface treatment methods are applied. Among them, we focused on surface oxidation of TMDC, which the unintentional electron doping is suppressed by hole doping by the surface oxidation. We analyzed the surface chemistry of the TMDC and the energy levels near the Fermi level by direct/inverse-photoelectron spectroscopy. Surface oxidation was observed in two forms: 1) chalcogen-oxygen bond. 2) chalcogen-oxygen substitution. In the case of chalcogen-oxygen substitution compared to chalcogen-oxygen bond, the hole doping was very effective. Its origin is that oxygen directly withdraw the electrons of the d orbitals of the transition metal when the chalcogen is replaced with oxygen.

Study on p-type graphene doping by using oxidized transition metal dichalcogenides

Huynh, Thi Thanh Tuyen Sungkyunkwan University 2023 국내석사

도핑은 2차원 (2D) 반도체 소재의 초얇은 구조 때문에 기술적인 도전입니다. 여기서는 비화학량론적 산화물 MOx (M은 Mo 또는 W)의 다른 유형의 영향이 그래핀 FET에서 TMDs (MoS2 또는 WSe2)의 산화로 인한 p-형 도핑과 이상을 유도하는 것과 관련하여 체계적으로 조사되었습니다. Raman 분광법에 의해 체계적으로 측정된 D 및 D' 피크의 시간 종속성을 기반으로, 최적의 자외선 오존 처리 시간은 결함을 생성하지 않고 MOx를 통해 그래핀 FET에서 높은 p-형 도핑 농도 (~2.5× 1013 cm-2)를 달성하는 데 도움이 될 수 있습니다. 또한, 두 종류의 장치의 시간에 따른 on-상태도 얻을 수 있었으며, WOx 기반 장치가 MoOx 기반 장치보다 더욱 안정성이 뛰어나다는 것을 나타냈습니다. 게다가 TEM 분석을 통해 그래핀 시트의 구조를 확인한 결과, WOx가 MoOx에 비해 기반층을 보호하는 데 더욱 우수함을 알 수 있었습니다. 이러한 결과는 CMOS 응용에서 표면 전하 이동 도핑을 더욱 효과적으로 활용하기 위한 지침을 제공합니다. Doping is a technological challenge of two-dimensional (2D) semiconductor materials due to their ultrathin body. Here, the impact of different types of non-stoichiometric oxide MOx (M are Molybdenum-Mo or Tungsten-W) are systematically investigated by oxidizing of Transition-metal dichalcogenides (TMDs) (Molybdenum Disulfide-MoS2 or Tungsten diselenide-WSe2) in graphene Field-effect transistors (FETs), related to p-type doping and inducing of disorders. Based on the time-dependency of the D and D’ peaks systematically measured by Raman spectroscopy, the optimal Ultraviolet -Ozone (UVO) treatment duration is able to identify to achieve high p-type doping concentration (~2.5× 1013 cm-2) in graphene FETs by MOx without generating defects. Furthermore, time-dependent on-states of both types of devices were also obtained, indicating better stability of WOx-based devices than MoOx-based devices. Moreover, through Transmission electron microscopy (TEM) analysis the structure of graphene sheets, WOx is much better at protecting the underlying layer than MoOx. These results offer guidelines for the further utilization of surface charge transfer doping in Complementary Metal-Oxide-Semiconductor (CMOS) applications.

Enhancement of the efficiency of optoelectronic device based on 2D semiconductor

Bang, Seungho Sungkyunkwan university 2019 국내박사

Beyond graphene, two-dimensional (2D) semiconductor (transition metal dichalcogenides; TMDCs) has been attracted in next-optoelectronic fields due to their outstanding physical properties as well as the direct bandgap in the monolayer (1L). However, when using TMDC as a commercial electronic and optoelectronic device application, there are some problems (Low optical quantum yield, absence of air-stable and easy doping process, and encapsulation process for air stability). To solve these issues, I describe the researches have been done during my integrated Ms.-Ph.D. course. (i) Low optical quantum yield of TMDCs Recently, various researches to increase the low optical efficiency of 1L-TMDCs have been proceeded by the surface plasmon effect. In Chapter 2, we introduce the surface plasmon coupling for enhancing the low optical quantum yield of 1L-TMDCs. We have observed that the strong surface plasmons in high-density Ag NWs networks affect greatly enhanced optical QY of 1L-TMDC. The highest PL enhancement factor was observed as ~560, and the relative QY of 1L-molybdenum disulfide (MoS2) was enhanced by a factor of 200 in the Ag NWs/1L-MoS2 structure. Furthermore, Raman scattering and absorption efficiency were significantly improved by strongly integrated gap plasmon. Finally, I fabricated the Ag NWs/1L-MoS2 photodetectors on silicon dioxide (SiO2) substrate, the photocurrents were significantly enhanced (250-fold enhancement). The photoresponsivity of the hybrid photodetectors was ~ 59.60 A/W, which is greater than the pristine 1L-MoS2 photodetectors (0.05 A/W). (ii) Absence of air-stable and easy doping process The carrier type conversion in 2D semiconductors is basically importance for realizing complementary logic inverter. In order to fabricate 2D-based complementary metal oxide semiconductor (CMOS) device, the n-type and p-type TMDCs are required. Therefore, an efficient and practical doping method for TMDCs should be studied. In the chapter 3, we describe a chemical doping method by using polyvinylpyrrolidone (PVP), which excellent usability, reproducibility and air stability. The n-type characteristic of 1L-MoS2 is converted to semi-metallic properties, and the ambipolar characteristic of 1L-WSe2 is converted to the n-type properties. Finally, the 2D WSe2 CMOS FETs have been successfully fabricated through the n-type characteristics of PVP/1L-WSe2. Furthermore, to confirmed optoelectronic properties of the PVP/1L-MoS2 structure, an excitation laser source having 2.7 eV was irradiated to the 1L-MoS2 and the PVP/1L-MoS2 phototransistor. Persistent photocurrent at the PVP/1L-MoS2 phototransistor was observed by time-dependent photocurrent measurement. (iii) Encapsulation process for air stability The optoelectronic devices based on TMDCs should be environmentally stable. However, recent studies have shown that TMDCs are vulnerable by O2 and H2O. Therefore, the encapsulation studies are essential. A typical encapsulation method allows the carrier type of TMDC to be converted to the n-type property. In the case of p-type 2D semiconductors, specific techniques such as encapsulation and improvement of p-type characteristics are required. To overcome these issues, We describe a hybrid structure by growing zinc oxide nanorods (ZnO NRs) on a 1L-tungsten diselenide (WSe2) using the hydrothermal method in Chapter 4. Consequently, we confirmed not only enhanced air stability of 1L-WSe2 but also improved the p-type properties by fabricating the hybrid phototransistor.

Orbital gating driven by giant stark effect in tunneling phototransistor

Hwang, Geunwoo Sungkyunkwan University 2023 국내박사

터널링 광 트랜지스터에서 거대한 슈타르크 효과에 의해 구동되는 오비탈 게이팅 황근우 에너지과학과 성균관대학교 2차원 금속 및 반도체 물질은 2004년 단층 그래핀이 발견된 이후 광범위하게 연구되어 왔다. 원자적으로 얇은 구조에도 불구하고, 2D 반도체 물질은 빛과 외부에 갇힌 전하에 강력한 상호작용할 수 있다. 이는 2D 반도체를 기반으로 한 새로운 전자 및 광전자 장치에 대한 독창적인 연구를 가능하게 하였다. 특히 넓은 대역폭과 게이트 조정성이라는 독특한 특징은 나노미터 규모의 디바이스의 유망성을 보여주고 있다. 실리콘 반도체 디바이스와 함께, 트랜지스터 기술은 무어의 법칙에 따라 빠르게 발전해 왔다. 실리콘 기술과 폰 노이만 아키텍처는 트랜지스터가 3 nm 이하로 축소되면서 트랜지스터가 양자역학과 전하역학의 문제로 제작 한계에 직면해 있다. 이러한 기술적 문제를 해결하기위해 많은 연구자들이 2D 재료를 사용한 메커니즘 또는 새로운 개념의 구조를 제안하고 있다. 이 학위논문에서는 2D 물질을 기반으로 한 수직적 복합구조를 이용하여 광-전하 상호작용을 조절한 디바이스에 대한 연구 내용이다. I. 거대 슈타르크 효과에 의해 구동되는 오비탈 게이팅 먼저 외부 전기 게이트가 없이 새로운 광 게이팅 현상을 이용하는 h-BN과 MoTe2의 복합구조를 기반으로 하는 새로운 개념의 디바이스를 구상했다. 이 디바이스는 느린 전하 트래핑 역학과 관련된 이전의 문제들을 극복할 것으로 예상한다. 오비탈 게이팅은 특정한 오비탈 특성을 갖는 전자가 게이팅 동작을 위해 디바이스 채널에 축적될 수 있음을 의미한다. 기존의 토마스-페르미 스크리닝으로는 설명할 수 없는 층별 전위 변화는 빛에 의해 켜고 끌 수 있다. MoTe2에서 몰리브덴 원자의 dz2 궤도에서 나온 전자는 h-BN 층의 고정 전하에 의해 유도된 거대한 슈타르크 효과를 통해 원자적은 얇은 반도체에서 수직으로 효과적인 빌트인 포텐셜을 실현시킨다. 그러나 MoTe2에서 텔루륨 원자의 p-궤도로부터의 전자는 인접한 원자층의 p-궤도 사이의 강한 결합으로 인해 게이팅에 거의 기여하지 않는다. 따라서, 반도체 MoTe2의 상단 원자층의 전자 대역은 빛과 선택적 오비탈 게이팅에 의해 최대 100 meV까지 변화할 수 있었다. 이 새로운 오비탈 게이팅 디바이스는 낮은 암전류(1 nA 수준), 높은 광응답성(3357 A/W), 0.5 ms의 빠른 동작 속도를 달성했는데, 이는 이차원 소재를 기반은 한 디바이스 중 최고의 성능이다. 이런 최적화된 성능은 응답성이 높고 느린 트랩 역학을 가진 광학 게이팅 장치에서의 중요한 문제를 해결하였다. 이를 통해 2차원 물질을 기반으로 하는 새로운 개념의 광전자 디바이스를 개발할 수 있을 것으로 기대한다. II. 전기 및 광학 신호를 이용한 2차원 시냅스 디바이스 스파이킹 신경망(SNN)의 하드웨어는 효율적인 에너지 방식으로 통합될 수 있는 새로운 재료가 필요했다. 따라서 연구자들은 폰 노이만 아키텍처의 병목 현상을 극복하고 차세대 컴퓨팅 및 메모리 장치를 개발하기 위해 노력했다. 이와 관련하여, 카버 미드는 인간 뇌의 성능을 모방하여 컴퓨팅과 메모리(즉, 메모리 내 컴퓨팅 또는 에지 컴퓨팅)를 동시에 수행할 수 있는 뉴로모픽 시스템을 제안하였다. 이 학위논문을 통해, h-BN에서 2H-MoTe2 기반의 새로운 시냅스 디바이스를 개발하였다. 이 디바이스는 다양한 전기 및 광학 입력으로 작동할 수 있다. 이를 이용해, 단기 가소성(STP)과 장기 가소성(LTP), 그리고 자외선 조사에 의한 단기에서 장기 가소성으로의 전이를 입증하였다. 시냅스 소자 작동 외에도, 새로운 도핑 방법으로 기본 전계 효과 트랜지스터 성능을 달성할 수 있다. 이 디바이스는 인간 뇌의 가소성을 모방함으로써 인간 뇌의 "학습 경험" 행동을 성공적으로 모방한다. 본 연구 결과는 광전자 장치와 관련된 더 많은 뇌과학적 성능에 대한 연구가 진행되고, 하드웨어 기반 AI에 대한 지식이 확장에 기여할 것으로 기대한다. Orbital Gating Driven by Giant Stark Effect in Tunneling Phototransistor Two-dimensional (2D) metallic and semiconducting materials have been extensively studied since the discovery of exfoliated monolayer graphene in 2004. Despite their atomically thin geometry, 2D semiconducting materials can interact strongly with light and external trapped charges, which leads to original research to develop novel electronic and optoelectronic devices based on 2D semiconductors. In particular, their wide bandwidth and gate tunability have been considered unique and promising characteristics to achieve nanometer-scale devices. With the use of silicon, transistor technology has been rapidly developed following Moore’s law. However, silicon technology and the von Neumann architecture are facing their fabrication limits as transistors have been scaled down to below 3 nm, where quantum mechanical charge dynamics are involved. To solve these technical issues with transistors and the von Neumann architecture, many researchers have proposed novel concepts involving mechanisms or structures with 2D materials. In this study, I investigated various devices fabricated with 2D materials, particularly by controlling the vertical heterostructures and manipulating the light-charge interaction. 1. Orbital Gating Driven by Giant Stark Effect First, I conceived a new conceptual device, “Orbital gating driven by giant Stark effect in a tunneling phototransistor.” based on a stack of h-BN and MoTe2 layers, which uses a new optical gating phenomenon without an external electric gate. It is expected to overcome the former issues related to slow charge trapping dynamics. The orbital gating indicates that electrons with a certain orbital nature can be accumulated in the device channel for gating operation. The layer-by-layer potential gradient, which cannot be described by conventional Thomas-Fermi screening, could be turned on and off by light illumination. Electrons from the dz2 orbitals of molybdenum atoms in the MoTe2 realize an effective built-in potential vertically in the atomically thin semiconductor via the giant Stark effect induced by fixed charges in the h-BN layer. However, the electrons from the p-orbitals of tellurium atoms in the MoTe2 contribute little to the gating because of the strong coupling between the p-orbitals of adjacent atomic layers. Thus, the electronic bands of the top atomic layer of the semiconducting MoTe2 could be modulated by up to 100 meV through light illumination and selective orbital gating. This novel gating device achieved a low dark current (1 nA level), high photo-responsivity (3357 A/W), and fast-rising/fall time of 0.5 ms, which were better than the performances of other devices based on 2D materials. Please note that the optimized performances resolved critical issues in optical gating such as slow dynamics with high responsivity. Accordingly, optoelectronic devices based on this new concept could be developed with 2D materials. 2. 2D Materials-based Synaptic Device with Electric and Optical Signals. The hardware demonstration of spiking neural networks (SNNs) has required new materials that can be integrated in an energy-efficient way. Thus, researchers have tried to develop next-generation computing and memory devices to overcome the bottleneck imposed by the von-Neumann architecture. In this regard, Carver Mead proposed a neuromorphic system that can simultaneously perform computing and memory functions (i.e., in-memory computing or edge computing), mimicking the performance of the human brain. In the current study, a new synaptic device was developed based on 2H-MoTe2 on h-BN, which can operate with various electrical and optical inputs. We demonstrated short-term plasticity (STP) as well as long-term plasticity, and their transition in the device by UV light irradiation. Beyond the synaptic device operation, a basic field effect transistor performance could be also achieved with a new doping method. Our device successfully mimicked the “learning-experience” behavior of the human brain, by mimicking its plasticity. It is expected that additional characteristics related to our optoelectrical devices will be investigated and the knowledge of hardware-based AI will be expanded in future work.

Study of exciton-polaritons in 2D semiconductor via strong light-matter coupling

Jin-Woo Jung DGIST 2024 국내박사

Coupling Two-Dimensional (2D) Nanoelectromechanical Systems (Nems) with Electronic and Optical Properties of Atomic Layer Molybdenum Disulfide (MoS2)

Yang, Rui Case Western Reserve University ProQuest Dissertat 2016 해외박사(DDOD)

The discovery of two-dimensional (2D) materials has attracted tremendous interest and led to a great deal of investment due to their unique properties that are not present in three-dimensional (3D) or one-dimensional (1D) materials. Though graphene as the flagship 2D material has been extensively studied, it is a semimetal without a natural bandgap, and the difficulties in creating a useful bandgap has limited its applications in logic circuits, photonic devices and tunable devices. 2D semiconductors such as molybdenum disulfide (MoS2) compensate for graphene because they have a natural sizable bandgap, and thus can largely extend the applications of 2D materials. In order to fully exploit the distinct properties of these 2D semiconductors toward advantageous performance as applicable devices, it would be ideal to synthetically consider the electronic, mechanical, and optical properties of these materials. While MoS2 field-effect transistors (FETs), nanoelectromechanical systems (NEMS), and optoelectronic devices have been demonstrated, there are still numerous problems that need to be solved before applying the devices for sensing, computing, and communication applications that require high performance (sensitivity, reliability, responsivity, etc.).In this dissertation, state-of-the-art studies of MoS2 electronics are first introduced and surveyed. The electrical breakdown limit of MoS2 FETs is investigated because it determines the current carrying capability and failure modes, which are critical for integrated circuit applications. A completely-dry transfer method combined with vacuum thermal annealing is developed to fully harness the intrinsic properties of MoS2 without inducing residue on the surface. Then the mechanical properties and devices of MoS2 are presented. The first MoS2 nanomechanical resonator on a flexible PDMS substrate that is tolerant to a large amount of bending and straining is demonstrated, showing promise for flexible and foldable electronics. The temperature dependence of MoS2 resonators is also studied. Finally, the coupling of electrical and mechanical properties of MoS2 are explored using the first all-electrical readout of 1-, 2-, 3-layer MoS2 NEMS resonators, with the thickness confirmed with both Raman and photoluminescence (PL) characterization. The devices take the form of vibrating-channel transistors, with multimode resonances highly tunable by the gate voltage, which holds promises and intriguing potential for real-time sensing and signal processing applications.

Band structure engineering of 2D semiconductor heterostructure for quantum-optoelectronics

Yoon Seok Kim 고려대학교 KU-KIST융합대학원 2023 국내박사

Band structure engineering represents manipulating the electronic states of the lowest states of the conduction bands and the highest states of the valence bands. It acts as a key parameter that determines the properties and functionality of semiconductor devices. Through the heterostructure, controlling the valence and conduction bands at interfaces by introducing the other materials, a few quantum optoelectronic devices such as photodetectors, solar cells, LASERs, and light emitting devices are widely reported. Furthermore, a strategy has been proposed to increase the device performance by controlling the conduction and valence band using the local strain on semiconductor materials. From this point of view, band structure engineering has become an indispensable strategy to achieve novel device concepts. However, in the case of bulk semiconductors (3D), there is difficulty in realizing the extensive range of band structure engineering. Because the bulk semiconductor based heterostructure has to fulfill the lattice matching. Furthermore, the bulk semiconductor has dangling bonds, which act as a factor in performance degradation when forming the heterostructures. Due to these reasons, the demand for novel materials which can overcome these limitations is increasing. The van der Waals materials have attracted much attention among the many materials that can overcome the inherent limitations mentioned above. Since van der Waals materials are two-dimensional (2D), there are no dangling bonds. This characteristic makes a big difference from bulk semiconductors. Therefore, van der Waals based heterostructures can provide a viable platform for forming a wide range of heterostructures and maximizing the interaction between the layers. In addition, since the van der Waals material can realize the atomic layer thickness with single crystal crystallinity, it is possible to realize external stress, Moiré pattern, defects, and heterostructure. Van der Waals materials also have excellent optical properties due to high exciton binding energy and light-matter interaction, enabling the realization of various optoelectronic devices with atomic layer thickness. In order to realize various optoelectronic devices, a type-I quantum well heterostructure must be preceded. However, it is not easy to consist of a type-I heterostructure because most of van der Waals material based heterostructures form type-II heterostructure. Even in terms of heterostructures, since other materials must be introduced, the possibility of intrusion of artifacts and defects increases. In this thesis, the realization of the quantum well structure of the van der Waals materials was presented, and the multiple-quantum-well structure based on a monolithic process was introduced by overcoming the limitations. Furthermore, by realizing the lateral quantum well based single photon light source, we proposed a novel concept that can achieve van der Waals material based quantum optoelectronic devices. In order to consist of a quantum well structure with an ideal interface, layer-by-layer oxidation was introduced to realize the WOX/WSe2 multiple-quantum-well structure through monolithic based band engineering and sequential van der Waals stacking process. We also verified that WOX and WSe2 build a heterostructure through ultraviolet photoelectron spectroscopy (UPS) and UV/Vis absorption. The PL intensities in the triple quantum well were increased about 5~6 times compared to the single quantum well. We confirmed that the WOX layer acts as a decoupling between the WSe2 layer and suppresses the bandgap transition. Moreover, to evaluate the other mechanism of improving the performance, we evaluated the exciton dynamics in quantum wells. Through these exciton dynamics measurements, we could confirm that excitons were confined in the quantum well, and the multiple-quantum-wells were realized. In the next step, using the monolithic based lateral quantum well, we realized the deterministic and stable single photon light source. In the previous reports, most of van der Waals materials based single photon light sources have a limitation on realizing the quantum well band structure and site-control simultaneously. To overcome these limitations, we designed a novel platform to realize the lateral quantum well structure and local strain for maximizing the exciton confinement in atomic defects. It was confirmed that the lateral WOX/WSe2 heterostructure consists of a quantum well structure by measuring the Fermi level difference between the WOX and WSe2. Single photon emitting in the lateral quantum well was verified with the zero-phonon-line with 264 eV linewidth. In addition, single photon emitting behavior was confirmed with g(2)(0)≈0.25. In addition, we observed that the compressive strain of -0.8% was induced in the WSe2 pattern using transmission electron microscopy and near-field scanning microscopy-based Raman spectroscopy. These results confirmed the possibility of implementing a lateral quantum well based single photon light source capable of position control and integration into an photonic circuit.

Carrier concentration modulation and contact resistance reduction of 2D TMDCs-based transistors

Qu, Deshun Sungkyunkwan university 2018 국내박사

In this dissertation, field effect transistors (FETs) based on two-dimensional (2D) semiconducting materials are fabricated. A systematic modulation of the carrier concentration in molybdenum ditelluride (MoTe2) FETs is described, through rapid thermal annealing (RTA) under a controlled O2 environment (hole concentration modulation) and benzyl viologen (BV) doping (electron concentration modulation). Al2O3 capping is then introduced to improve the carrier mobilities and device stability because of reduced trap densities and isolation from ambient air. Degenerate p-type doping is also realized through a mild O2 plasma doping technique. The O2 plasma doping is found to be effective for tungsten ditelluride (WSe2) too. It is then used to selectively dope the contact regions of MoTe2 and WSe2 transistors. With high doping concentration in the contact area, WSe2 transistor is modulated to unipolar p-type from pristine n-type while MoTe2 transistor shows ambipolar transfer characteristics. Different polar transition mechanisms for MoTe2 and WSe2 transistors are revealed. The accessibility to both n- and p-type carrier conduction with controllable carrier concentration makes it possible to build 2D materials-based integrated circuits with diverse functions. Except for doping, carrier concentration can also be modulated by gate bias. The charge carrier distribution under the back-gate modulation and its affection to the multi-layer MoS2 transistor performance with respect to different contact configurations are also studied in this thesis.

Engineering Hybrid Interfaces of Organic-Inorganic 2D Semiconductors

Cheng, Che-Hsuan ProQuest Dissertations & Theses University of Mich 2022 해외박사(DDOD)

The unique properties of two-dimensional (2D) materials have inspired widespread interests in integrating distinct 2D materials into van der Waals (vdW) heterojunctions for innovative device configurations. The organic-inorganic heterojunctions, combining atomically thin inorganic semiconductors with a wide variety of organic molecules, provide versatile platforms not only for the exploration of novel physical phenomena at nanoscale but also for the development of emerging device applications with promising functionalities. In this thesis, we explore the science and applications of two hybrid interfacial systems that consist of organic molecules and inorganic transition metal dichalcogenides (TMD) monolayers.In the first part, we investigate the energy transfer mechanisms across the hybrid interface of j-aggregates of organic dye and monolayer molybdenum disulphide (MoS2) by using phototransistor’s photoresponsivity. The hybrid interface combines high absorption of organics with high charge mobility of inorganics. Besides, the spectral alignment between the emission of j-aggregates and the absorption of MoS2 B-exciton in the material system enables the study of Forster resonance energy transfer (FRET) across the hybrid interface. The hybrid phototransistors show nearly 93 ± 5 % enhancement of photoresponsivity in the excitonic spectral overlap regime due to efficient energy transfer from j-aggregate to MoS2. We also report a short Forster radius of 1.88 nm for the hybrid system. Based on the study, we then investigate the energy transport dynamics of hybrid charge transfer exciton (HCTE), a quasi-particle formed by another energy transfer mechanism Dexter energy transfer (DET) in this hybrid system. Following photoexcitation, highly diffusive hot HCTEs are formed in about 36 ps via scattering with optical phonons at the hybrid interface. Once the energy drops below the optical phonon energy, the excess kinetic energy is relaxed slowly via acoustic phonon scattering. As a result, the energy transport that is initially dominated by highly diffusive hot HCTEs transition into slower cold HCTEs in about 110 ps. By using Frohlich and deformation potential theory, we model the exciton-phonon interactions and attribute the prolonged transport of hot HCTEs to phonon bottleneck. We also find that the diffusivity of HCTEs in both hot and cold transport regions is higher than the diffusivity of MoS2 A exciton.In the second part, we explore another organic-2D TMDs hybrid system and utilize nanoscale strain engineering to create a self-erasable and rewritable optoexcitonic platform. We employ the reversible structural change of azobenzene based (A3) molecules to strain the overlying monolayer tungsten diselenide (WSe2), and consequently, tune its optical bandgap. By using such a hybrid material combination, we are able to generate large (>1%) local strain that results in dramatic photoluminescence (PL) wavelength shift (> 11 nm). The strain in layered A3 molecules can be relaxed under visible light exposure or can be retained up to seven days under dark condition. Based on the study, we use the same hybrid materials system and apply the principles to develop high performance hybrid transistor devices. The strain is created in an ultrathin monolayer WSe2 conducting channel by the photoisomerization of A3 molecules. The carrier mobility of the hybrid transistor can be tuned from 50 to as high as 125 cm2/Vs using the induced strain. Besides, the hybrid transistor exhibits a superior UV responsivity of 155 A/W, more than four times higher than in bare WSe2.