Charge injection memory transistors and vertical schottky diodes using 2D transition metal dichalcogenides

Widiapradja Livia Janice Graduate School, Yonsei University 2023 국내박사

The successful isolation of graphene has shifted the research focus from bulk-layered materials to individual layers and created an explosion of two-dimensional transition metal dichalcogenides (2D TMD) research. A dangling bond-free surface, unique and excellent transport properties in nanoscale thickness, as well as a finite bandgap of 1~2 eV have brought 2D TMD materials into the spotlight of semiconductor field. A variety of electrical devices e.g. field-effect transistors and PN diodes have come in reports, but more advanced devices and broader applications are yet to be demonstrated. Among advanced devices with 2D semiconductors, charge injection memory field effect transistors (CIM FETs) might be one of the most important and practical ones. Reported CIM FETs utilize three layers (for tunneling, trapping, and bulk dielectric) in general, resulting in high switching voltages over 10 V. Here, nonvolatile CIM FET is fabricated with MoS2 channel and hetero-stack bilayer oxide dielectrics adopting 5 nm-SiO2/25 nm-HfO2, where the charge traps are expected at the SiO2/HfO2 oxide interface. It is nicely observed that low pulse gate voltage less than ±7 V is enough to make Program and Erase states, which originate from electron charges tunneling through the low-k SiO2. For comparison, another device with a reversed oxide sequence is also fabricated, only to show inferior memory performances even at higher pulse voltages of ±10 V due to the low E-field in high-k HfO2 tunnel oxide. Even though this could be re-utilized via PS-brush engineering on the top surface of HfO2, but it still requires high gate pulse voltages of ±8~±10 V for nonvolatile memory performances. The former with a thin-SiO2/thick-HfO2 hetero-stack bilayer oxide is now determined as an optimum one because of low operating voltages and less process complexity, and it is extended to a circuit application for a long-term memory switching of an organic light-emitting diode (OLED) pixel. Most of the 2D semiconductor-based devices are using in-plane direction current in their device architecture, to justify the novelty over the conventional 3D devices. Those ultrathin 2D devices, however, would unavoidably meet high contact resistance issue. In this study, thick 2D-layered crystals are rather chosen for a meaningful vertical device, which alleviates the contact resistance issue using a large contact area. Here, we have fabricated Pt/120 nm-thick MoSe2 Schottky diodes with different Ohmic metal contacts of Au, Ti/Au, and MoTi/Au. These diodes are then monolithically integrated with a capacitor of ~20 nm-thick h-BN or 50 nm-thick ALD Al2O3. This way, van der Waals crystal radios are successfully fabricated for wireless RF energy harvesting. In terms of crystal radio device performance, the best results come from Pt/n-MoSe2 Schottky diode with MoTi contact and h-BN capacitor combination. This superiority is attributed to the excellent Ohmic behavior of MoTi alloy contact. At last, when AM demodulation experiments are conducted with 1 MHz carrier frequency/audio frequency-mixed signals, the crystal radio with MoTi demonstrates the highest output DC voltage envelope, allowing loud audio sound. Suksesnya isolasi graphene telah mengalihkan fokus penelitian dari material berlapis ke lapisan individu dan menciptakan ledakan penelitian dalam bidang logam transisi dikalkogenida dua dimensi (TMD 2D). Sifat permukaannya yang bebas dari ikatan menjuntai, juga kemampuan transportasi yang unik dan luar biasa dalam ketebalan skala nano, serta celah pita yang terbatas (sebesar 1~2 eV) telah membuat material TMD 2D menjadi pusat perhatian dalam bidang semikonduktor. Meskipun berbagai perangkat elektronik seperti transistor efek medan dan dioda PN telah banyak diteliti, namun perangkat yang lebih canggih dan aplikasi yang lebih luas masih memerlukan eksplorasi lebih lanjut. Di antara perangkat-perangkat canggih berbasis semikonduktor 2D, transistor memori injeksi muatan merupakan salah satu devais paling penting dan praktis. Secara umum, penelitian mengenai perangkat ini menggunakan tiga lapisan (untuk tunneling, trapping, dan dielektrik bulk), yang menghasilkan tegangan pengalihan yang tinggi, yaitu di atas 10 V. Dalam penelitian ini, transistor memori injeksi muatan dibuat dengan menggunakan kanal MoS2 dan dielektrik oksida lapisan ganda yang terdiri dari 5 nm-SiO2/25 nm-HfO2, di mana perangkap muatan diharapkan terdapat pada antarmuka oksida SiO2/HfO2. Melalui metode ini, ditemukan bahwa tegangan pulsa rendah (kurang dari ± 7 V) sudah cukup untuk menghasilkan keadaan Program dan Erase, yang berasal dari muatan elektron yang menembus lapisan tipis SiO2 dengan konstanta dielektrik rendah. Sebagai perbandingan, perangkat lain dengan urutan oksida yang sebaliknya juga dibuat, namun menunjukkan kinerja memori yang lebih rendah. Hal ini disebabkan karena konstanta dielektrik yang tinggi pada HfO2 membuat medan listrik pada lapisan tersebut sangatlah rendah, sehingga dibutuhkan tegangan pulsa yang lebih tinggi, yaitu sekitar ±10 V. Meskipun devais ini dapat dimodifikasi dengan menggunakan teknik PS-brush pada permukaan HfO2, namun pulsa tegangan yang dibutuhkan masih berada pada kisaran ±8~±10 V untuk mencapai kinerja memori non-volatile. Oleh karena itu, devais pertama dengan lapisan ganda SiO2 (tipis)/HfO2 (tebal) ditentukan sebagai perangkat yang paling optimal karena tegangan operasi yang rendah dan kompleksitas proses fabrikasi yang lebih sederhana. Devais ini juga dapat diaplikasikan ke dalam sirkuit untuk melakukan peralihan memori jangka panjang pada piksel dioda cahaya organik (OLED). Dalam dunia devais semikonduktor 2D, penggunaan arus sebidang dalam desain perangkat merupakan sebuah pembeda dan kebaruan dari perangkat-perangkat konvensional 3D. Namun, devais 2D yang sangat tipis tersebut tidak dapat terhindar dari masalah resistansi kontak yang tinggi. Dalam studi ini, kristal 2D yang relatif tebal digunakan untuk mengatasi masalah resistansi kontak dengan memperluas area kontak dan memanfaatkan arah arus keluar bidang. Melalui strategi ini, dioda Schottky dengan menggunakan Pt/MoSe2 (120 nm) berhasil dipadukan dengan berbagai kontak logam Ohmik, yaitu Au, Ti/Au, dan MoTi/Au. Dioda ini kemudian diintegrasikan secara monolitik dengan kapasitor h-BN setebal ~20 nm atau ALD Al2O3 dengan tebal 50 nm untuk merealisasikan radio kristal van der Waals yang dapat digunakan untuk memanen energi RF nirkabel. Dalam hal kinerja perangkat radio kristal, hasil terbaik diperoleh dari kombinasi dioda Schottky Pt/n-MoSe2 dengan kontak MoTi dan kapasitor h-BN. Keunggulan ini dapat dikaitkan dengan perilaku Ohmik yang sangat baik dari kontak alloy MoTi. Sebagai langkah terakhir, ketika dilakukan percobaan demodulasi AM dengan frekuensi pembawa 1 MHz dan sinyal campuran frekuensi audio, radio kristal dengan kontak MoTi menunjukkan tegangan keluaran DC envelope tertinggi, sehingga dapat menghasilkan suara audio yang jelas dan nyaring.

Focused ion beam machining for ultra-precision fabrication of surface plasmon resonance devices using optical fiber

조항은 Graduate School, Yonsei University 2016 국내박사

표면 플라즈몬 공명 장치는 나노 렌즈, 태양광발전 장치, 굴절률 센서 등의 다양한 분야에서 연구되고 있다. 집속이온빔 가공법은 나노패턴을 미세한 영역에 직접 가공할 수 있어 나노 광학 장치의 프로토타입 제작 기술로 널리 사용된다. 최근 표면 플라즈몬 공명 장치의 형상 및 구조가 복잡해지면서 집속이온빔 가공법이 박막 코팅, 식각 공정 등의 일련의 공정과 함께 수행되고 있다. 이러한 일련의 공정 중에는 금속-유전체 다층박막이 만들어질 수 밖에 없고, 이에 따라, 이 다층박막 재료에 대한 집속이온빔 응용 초정밀 가공이 요구된다. 이 논문에서는 금속-유전체 다층박막 구조에 대한 집속이온빔의 가공 정밀도를 향상시키기 위한 여러 방법들을 제안하고, 이를 활용하여 광섬유 응용 표면 플라즈몬 공명 광필터 및 센서를 제작하였다. 먼저, 집속이온빔을 이용한 유전체 가공시 필수적으로 적층하는 전도성 코팅 재료의 효과적인 활용법에 대해 연구하였다. 전도성 코팅의 물질이 금속-유전체의 집속이온빔 가공시 횡방향 가공 정밀도에 미치는 영향을 연구하였다. 금속-유전체 가공에 대한 가공시 발생하는 이온 스퍼터링 현상을 이온-물질 상호작용으로 분석하여 코팅재료에 따른 이온침투범위와 패턴 벽면의 기울기의 관계를 분석하였다. 또한, 깊이방향의 가공정밀도 향상시키기 위해 집속이온빔의 가공 조건을 정밀하게 설계하기 위한 방법으로 3차원 박막두께측정법 및 이온빔 가공 시뮬레이션을 제안하였다. 이차전자이미지 모니터링 기반 실시간 경계면 검출법 (material interface detection, MID)을 제안하여 두께를 모르는 다층박막재료에 대해서도 집속이온빔 가공법의 깊이 방향 가공정밀도를 향상시킬 수 있었다. 제안된 집속이온빔 응용 초정밀 가공 방법들을 이용하여 표면 플라즈몬 장치인 플라즈모닉 리소그래피 헤드, 광섬유 응용 광필터, 광섬유 응용 굴절률 센서를 제작하였다. 금속-유전체의 경계면에 요철을 가진 나노개구를 고체침지렌즈 내부에 제작하였고, 리소그래피 실험을 통해 그 성능을 평가하였다. 마지막으로, 나노 홀 어레이를 이용한 광섬유 응용 표면 플라즈몬 광필터 및 굴절률 센서를 제안하였다. 플라즈모닉 광필터는 싱글모드 및 멀티모드 광섬유 끝단면에 각각 제작되었고, 가시광선 영역에서 투과 성능을 평가하였다. 또한, 유한차분 시간영역 방법을 이용하여 통신파장 영역에서 특이 광투과 현상을 발생시키는 나비넥타이 모양의 나노 홀 어레이를 설계하였다. 나노 홀 어레이를 광섬유의 끝단면 뿐만 아니라 옆면에도 제작하여 표면 플라즈몬 효과 발생 영역을 증가시켰고, 이렇게 제작된 소자는 광투과 효율이 극대화된 광섬유 응용 광필터 및 굴절률 센서로 활용될 수 있다. 이 연구에서 제안된 집속이온빔 응용 초정밀 가공 기술은 평판형 가공 대상물에 제한되었던 기존의 마이크로-나노 공정 기술의 적용 범위를 확장함으로써 다양한 분야에서 주목받고 있는 자유형상 및 곡면형 초미세 광학 소자 제작에 활용될 수 있을 것으로 기대한다. Surface plasmon resonance (SPR) devices have been investigated for various applications such as plasmonic lens, photon sorters, photovoltaic devices, and refractive index sensors. A focused ion beam (FIB) technique is widely used as a prototyping tool for fabrication of SPR devices because it is a direct-writing technique for used on a microscopic region. As the SPR devices have become more complicated, a nanofabrication process includes not only FIB machining but also thin film coating and etching in series. Since a metal-dielectric multilayer is a typical configuration produced during the series of nanofabrication processes, ultra-precision fabrication for metal-dielectric multilayers should be studied. In this dissertation, accuracy in FIB machining of metal-dielectric multilayers were improved by several methods and fiber-optic SPR devices, such as a color-filter and a sensor, was fabricated by the proposed methods. First, an effective usage of a conductive coating was investigated for FIB machining of dielectrics. In order to study effects of conductive coating materials on the lateral accuracy, ion-solid interaction for FIB machining of metal-dielectric material was hypothetically studied. It was experimentally verified by analyzing a correlation between lateral accuracy and ion-range. Also, 3D film thickness measurement and FIB machining simulation were proposed for improving FIB machining accuracy in milling depths by designing the machining parameters. A real-time material interface detection (MID) for FIB machining was proposed to improve the accuracy in depth direction for multilayers with indeterminable thicknesses. Ultra-precision fabrication of the SPR devices was achieved by the proposed methods. A metallic nano-aperture inserted inside of a solid immersion lens was fabricated and its performance was evaluated by maskless nanolithography. Finally, a fiber-optic color-filter and a refractive index sensor using nanoholes array was precisely fabricated in order to evaluate the advanced FIB machining. The plasmonic color-filter was fabricated on singlemode and multimode fibers, respectively, and their performance in transmission was evaluated in the wavelengths of visible ranges. The bowtie-shaped nanoholes array operating in telecommunication wavelengths was designed by a finite-difference time-domain method. The nanoholes array was fabricated not only on the facet of fibers but also on the curved surface of the periphery of fibers. The nanoholes array on a periphery of fibers was proposed as a one configuration of fiber-optic SPR devices for improving the effective transmitted power for the SPR signals by enlarging the active area. This dissertation can extend the scope of FIB technique that was limited to a flat substrate for used in optical micro/nano-devices. An advanced FIB process with an ultra-precision methods that proposed in this dissertation have potentials to manufacture the complex-shaped components for photonic devices such as free-form lens and curved display substrate.

Enhancement and polarization manipulation of photonic spin hall effect with waveguided-SPR method

Monu Nath Baitha Graduate School, Yonsei University 2023 국내박사

광자 스핀 홀 효과(Photonic spin Hall effect, PSHE)는 plane-polarized wave가 광 표면에서 반사와 굴절 이후 right and left-handed circularly-polarized wave로 분리되는 현상이다. 광자 스핀 홀 효과는 전자의 홀 효과와 매우 유사하며, 다양한 모델과 구조내에서 연구되어 왔지만 아직까지 그 수치가 작고 편광에 의존적이어서 현실적으로 적용하기 어렵다. 플라즈모닉 시스템에서 스핀 홀 효과의 증폭은 가능했으나, 수평 편광이라는 조건에 한정되어 있었다. 광자 스핀 홀 효과를 더 증폭시키고 편광 의존성을 제어하기 위해 새로운 도파로-표면 플라즈몬 공명 (Waveguided surface plasmon resonance, WG-SPR) 방법을 채택하였다. 본 논문에서의 사용한 구조는 금속층 위의 얇은 유리층으로 구성되어 있으며, 수직 및 수평 편광 모두 적용이 가능하다. 입사광의 편광 상태는 광자 스핀 홀 효과에 상당한 영향을 미친다. 본 논문은 도파로-표면 플라즈몬 공명을 이용한 거대 광자 스핀 홀 효과를 이론적으로 분석하였으며, 그 수치를 밀리미터 수준까지 향상시켰다. 이 연구는 플라즈몬 공명 기반 모델을 사용하여 빛의 수직 및 수평 편광 모두에 대해 거대 광자 스핀 홀 효과를 관찰한 첫 번째 연구이다. 또한, 도파관의 층 두께를 조정함으로써 편광을 능동적으로 조작할 수 있으며, 이러한 효과는 광 스핀을 주로 이용하는 수직 및 수평 편광 기반의 양자 광학 장치 및 센서에 적용할 수 있다. 더 나아가, 편광의 전환이 가능한 스핀 광자 홀 효과를 얻기 위해 새로운 방법을 제안하였다. 결과적으로 구조의 물리적인 변화없이 광자 스핀 홀 효과를 관찰 할 수 있다. 이 연구는 광자 스핀 홀 효과를 향상시키는 입사 공명 각도만 변경하여 편광 모드를 능동적으로 제어하는 데 매우 적합하며, 이러한 효과는 스위치, 빔 스플리터, 필터 등의 광 편광 기반 장치에 적용할 수 있다. 또한, 모든 입사각에 대해 편광 독립적인 광자 스핀 홀 효과를 관찰 할 수 있는 연구를 진행하였다. 모든 입사각, 모든 평면 편광 입사파에 대해 수직 편광 광자 스핀 홀 효과와 수평 편광 광자 스핀 홀 효과가 같음을 보였다 (δ_±^H=δ_±^V). 이 연구는 모든 입사각에 대해 광자 스핀 홀 효과의 편광 의존성을 없애기 위해 빛의 스핀-궤도 결합을 조작한 첫 번째 연구이다. 이 연구의 결과는 편광 독립적인 양자 장치 및 센서에 적용할 수 있다. The photonic spin Hall effect (PSHE) explains the separation of right- and left-handed circularly-polarized waves after reflection and refraction of a plane-polarized wave from an optical interface. It is a direct analogy to the electronic Hall effect. To date, PSHE has been explored for many different models and structures, but still, its small value and polarization dependency limits the practical application. In plasmonic systems, the enhanced spin Hall effect (SHE) was previously possible but only for horizontal polarization. To further enhance the PSHE and control the polarization dependency, the novel waveguided surface plasmon resonance (WG-SPR) method has been adopted. The proposed structure comprises a thin glass layer over a metal layer that produces a hybrid mode of transverse magnetic and a regular waveguiding transverse electric mode. The polarization state of the incident light has a substantial impact on the photonic spin Hall effect (PSHE). This thesis reports a theoretical analysis of the giant photonic spin Hall effect (G-PSHE) using WG-SPR. An enhancement of millimeter-scale (more than 2 mm to submillimeter) is achieved. To the best of our knowledge, this is the first study to achieve G-PSHE for both vertical and horizontal polarization modes of light with the SPR-based model. Other findings also indicate the manipulation of active polarization mode by only tuning the wave-guiding layer thickness. This study enables the scope for potential applications of both H- and V-polarized based quantum optical devices and sensors, where light spin plays a pivotal role. Further, a novel method has been proposed to get the polarization-switchable PSHE. Outcomes offer the opportunity of eliminating any physical alteration of the given structure to govern the desired results. This research is highly suited to controlling the active polarization mode of enhanced PSHE by only altering the incident resonance angle. It will be a simple alternative for modern light polarization (spin) based devices such as switches, beam splitters, filters, etc. Furthermore, another study revealed an opportunity to polarization-independent photonic spin Hall effect (PSHE), for all incident angles. This work demonstrates that the horizontal (H) and vertical (V) polarized PSHE will remain the same; (�±ு = �±௏) for any plane-polarized incident wave at any incident angle. This is the first study where the Spin-Orbit coupling of light has been manipulated to vanish the polarization dependency of PSHE for all incident angles. The results of this study pave the way for feasible future applications of new polarization-independent quantum devices and sensors.