Mechanocapillary Forming of Filamentary Materials.

The hierarchical structure and organization of filaments within natural materials determine their collective chemical and physical functionalities. Synthetic nanoscale filaments such as carbon nanotubes (CNTs) are known for their outstanding properties including high stiffness and strength at low density, and high electrical conductivity and current carrying capacity. Ordered assemblies of densely packed CNTs are therefore expected to enable the synthesis of new materials having outstanding multifunctional performance. However, current methods of CNT synthesis have inadequate control of quality, density and order. In pursuit of these needs, a new technique called capillary forming is used to manipulate vertically aligned (VA-) CNTs, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation; followed by evaporation-induced shrinkage. Three-dimensional geometric transformations result from the heterogeneous strain distribution within the microstructures during the vapor-liquid-solid interface shrinkage. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using this technique. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 104 S/m respectively. These values are at least hundred-fold higher than as-grown CNT properties, and exceed the properties of typical microfabrication polymers. Responsive CNT-hydrogel composites are prototyped by combining isotropic moisture-induced swelling of the hydrogel with the anisotropic stiffness of CNTs to induce reversible self-directed shape changes of up to 30% stroke. Centimeter scale sheets are fabricated by mechanical rolling and capillary assisted joining of CNTs. The mechanical stiffness, strength and electrical conductivity of CNT sheets are comparable to those of continuous CNT microstructures; and can be tuned by engineering the morphology of the CNT joints. Finally, the applicability of mechanocapillary forming to other nanoscale filaments is demonstrated using silicon nanowires synthesized by metal assisted chemical etching. Further work using the methods developed in this dissertation could enable applications such as directional liquid transport, adhesives, and biosensors; toward an end goal of creating multifunctional surfaces having arbitrary structural, interfacial, and optical responsiveness.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/1/stawfick_1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/2/stawfick_2.pd

Developing germanium on nothing (GON) nanowire arrays

Advanced crystal growth techniques enable novel devices and circuit designs to further scale and integrate heterogeneous structures for CMOS, MEMS/NEMS, and optoelectronic applications. In particular, nanowires (NW) are among the promising structures derived from these developments. Research has demonstrated the utility of NWs as a channel material for gate-all-around transistors, high sensitivity biological/chemical sensors, photodetectors, as well as a whole spectrum of LEDs and lasers. However, NW based devices are not without their fabrication challenges. Relatively simple structures for CMOS or MEMS/NEMS processes are difficult to reproduce when many NW based devices rely on a dropcast process. This thesis demonstrates a method for producing Germanium on Nothing (GON) NW arrays on a Si substrate that forgoes dropcasting and, instead, creates NWs via selective material removal methods commonly utilized by industry. GON NW arrays are formed through the sequential use of E-beam lithography, selective wet chemical etching, and reactive ion etching. Global oxide thinning in BOE leaves a thin masking layer that protects the underlying Si, preventing etching in a TMAH solution. GON regions are defined by E-beam lithography and are subject to a RIE which creates release points in the remaining SiO2. Unmasked Si is then etched by a TMAH solution, undercutting the Ge lines, leaving an array of suspended Ge wires. NW dimensions are reached by thinning the Ge wire diameter with a H2O2 solution. NWs with ~50 nm diameters and ~ 200 nm lengths, as well as 10 [micron] by 10 [micron] membranes of Ge/SiO2, have been demonstrated in this thesis

Suspended 1D metal oxide nanostructure-based gas sensor

Department of Materials Science and EngineeringWe developed a novel batch fabrication technology for the ultralow-power-consumption metal oxide gas sensing platform consisting of a suspended glassy carbon heating nanostructure and hierarchical metal oxide nanostructures forests fabricated by the carbon-micro electromechanical systems (carbon-MEMS) and selective nanowire growth process. We have developed a new manufacturing process for suspended glass carbon nanostructures such as single nanowire, nano-mesh and nano-membranes fabricated using carbon-MEMS consisting of the UV-lithography and the polymer pyrolysis processes. We designed a gas sensing platform consisting of suspended glassy carbon heating nanostructures and suspended hierarchical metal oxide nanostructure forests for the sensing part. Glassy carbon structure produced by the carbon-MEMS has many advantages such as high thermal & chemical stabilities, good hardness, and good thermal & electrical characteristics. The electrical conductivity of glassy carbon nanostructures has been increased more than three times by using rapid thermal annealing (RTA) process owing to the inferior heating property of glassy carbon nano-heater in the electrical conductivity. In order to divide the suspended glassy carbon nano-heater and the suspended hierarchical metal oxide nanostructures forests, the insulating layer of HfO2 materials is a high dielectric constant and is deposited uniformly using a atomic layer deposition (ALD) process on a suspended glassy carbon nano-heater. Suspended hierarchical metal oxide nanostructures forests were grown circumferentially on the suspended HfO2/glassy carbon nano-heater using a hydrothermal method consisting of the seed deposition and the growth processes. For selective metal oxide seed layer deposition process, a short-time exposed polymer patterning process was performed using the positive photoresist. After the polymer patterning process, a metal oxide seed layer is deposited using the rf-sputtering system, followed by a metal oxide nanostructure growth process. The distinguishing architecture of a suspended hierarchical metal oxide nanostructures forests/HfO2/glassy carbon nanostructure ensures efficient mass transport to the metal oxide nanostructure detection point of the gas analyte, resulting in highly sensitive gas detection. In the absence of an external heating system, the ultralow-power-consumption gas sensing platform of a suspended hierarchical metal oxide nanostructures forests/HfO2/glassy carbon nanostructure has excellent the gas sensing characteristics.ope

III-V족 화합물 반도체 터널 전계 효과 트랜지스터 개발

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2022. 8. 최우영.리소그래피 기술의 놀라운 발전은 10 nm 이하의 논리 트랜지스터를 상용화했다. 게이트 길이 스케일링은 모스펫 (MOSFET)의 전력 소비를 줄이기 위한 노력의 큰 부분을 차지한다. 그러나 이러한 접근 방식은 리소그래피의 물리적 한계와 누설 전류 제어와 같은 몇 가지 문제에 직면했다. 모스펫의 근본적인 문제는 현재 전송 메커니즘의 한계로 인해 60 mV/dec 미만의 임계값 기울기 (SS)에 도달할 수 없다는 것이다. Si 터널링 전계효과 트랜지스터 (TFET)의 여러 연구자들이 60 mV/dec 미만의 결과를 보고했지만, Si 동종 접합 터널링 전계효과 트랜지스터는 간접 대역 갭 물질의 터널링 확률이 낮아 전류상으로 불충분하다. P-I 접합부에서의 터널링 확률은 터널링 전계효과 트랜지스터의 동작전류에 영향을 미치기 때문에 작은 직접 밴드갭을 가지고 유효질량이 낮은 III-V 화합물 반도체는 임계값 기울기가 60 mV/dec 미만인 높은 터널링 전류를 달성할 수 있는 가장 유망한 재료이다. 또한 밴드 오프셋이 다른 재료를 선택함으로써, 스태거드 또는 브로큰 갭을 형성함으로써 터널링 전류를 현저하게 증가시킬 수 있다. P-I 접합부의 터널링이 터널 전계효과 트랜지스터 소자의 전류 공급원이기 때문에 많은 연구자들이 분자빔 에피택시 (MBE) 방식으로 성장한 p형 도핑 농도가 높은 III-V 웨이퍼로 제조된 터널 전계효과 트랜지스터의 성능을 보고해왔다. 그러나 높은 도핑 농도와 가파른 도펀트 프로파일을 갖는 p형 InGaAs를 성장하기가 까다롭기 때문에 금속-유기 화학 기상 증착 (MOCVD) 성장 에피택셜 층에서 제조된 InGaAs TFET 소자는 거의 보고되지 않았다. 이에 따라 본 연구는 TFET 소자 제작을 위한 고품질 에피택셜 층을 성장시키기 위한 MOCVD 성장 기술을 선보인다. 종래의 TFET 소자에 대해서는 동종 접합 p-i-n InGaAs 에피택셜층을 성장시키고, p++-Ge/i-InGaAs/n+-InAs 나노선을 성장시켜 TFET 소자 성능 향상 가능성을 확인하였다. MOCVD에 의해 성장한 에피택시 층에서 제조된 TFET 소자의 잠재성을 확인하기 위해 평판과 나노선 에피택셜 층에서 제작된 TFET 소자의 성능이 확인되었다. MOCVD 방법을 이용하여 고품질의 에피택셜 층이 성장되었다. MBE에 비해 가성비, 높은 처리량, 우수한 결정 품질이 MOCVD의 가장 큰 장점이다. 이에 여러 성장 조건을 변화시키면서 InP (001) 기판 위로 InGaAs 필름층의 성장이 연구되었다. 소스 유량, 온도 및 V/III 비율이 성장된 InGaAs 필름층의 품질에 끼치는 영향이 연구되었다. 또한 MOCVD InGaAs 성장 기술에서 n형 및 p형 도펀트의 농도를 높이는 것과 도펀트 프로파일을 가파르게 하는 것이 도전적이므로 탄소 및 텔루륨 도핑을 통해 가파른 도펀트 프로파일을 보이는 고농도의 p형 및 n형 InGaAs층을 성장하였다. 성장된 에피택셜 필름층은 TFET 소자를 제작하여 평가하였다. TFET 소자 제작 전에 우선 TFET 소자의 채널 길이가 전기적 시뮬레이션 결과에 의해 선택되었다. MOCVD를 이용하여 도핑 프로파일이 가파른 고품질의 수직 p-i-n 에피텍셜 구조가 한번에 성장되었다. 에피택셜 성장 후에 TFET 소자는 수직 방향의 습식 식각을 통해 제작되었다. 옴 (Ohmic) 공정과 에어브릿지 공정도 소자 제작을 위해 최적화되었다. P형 도핑 농도에 대한 영향과 MOCVD 성장 중에 생긴 전위에 대한 영향이 TFET 성능을 통하여 확인되었다. 제조된 TFET 소자는 60 mV/dec에 가까운 SS와 괜찮은 온/오프 전류 비율을 보여주었는데, 이는 최초로 보고되는 MBE에서 성장된 웨이퍼에서 만들어진 TFET 소자와 비교할 수 있는 소자이다. 이 결과는 고품질의 MOCVD로 성장한 III-V TFET 소자의 양산 가능성을 보여준다. 이 연구의 다음 부분은 나노선 TFET 제작이다. 전자소자 제작을 위한 III-V 나노선 성장에는 몇 가지 장점이 있다. 다양한 종류의 웨이퍼에 다양한 특성을 가지는 헤테로 구조를 형성할 수 있다는 것이 큰 장점이다. 충분히 작은 직경으로 성장된 나노선은 웨이퍼와 다른 격자 상수를 가지더라도 전위 없는 계면을 가진다. 다양한 유형의 밴드 정렬이 만들어질 수 있으며, 이는 TFET의 터널링 정류를 증가시키는 데에 있어 중요한 요소이다. 또한 직경이 작은 나노선은 칩으로 제작되었을 때 더 나은 소자 밀도, 향상된 게이트 제어성, 성장 시간 단축을 통한 처리량 향상이 가능하다. InGaAs 나노선은 선택적 영역 성장법 (SAG) 성장되었다. 하드마스크 층으로서 InP (111)B 및 Ge (111) 웨이퍼에 SiO2 층이 증착 되었다. 성장 모드가 다르기 때문에 InGaAs 평판 필름층 성장과는 크게 다른 성장 조건을 테스트하였다. 나노선의 선택적 성장은 온도, V/III 비율 및 소스 유량을 최적화하여 확인하였다. 그 결과 InP (111)B와 Ge (111) 웨이퍼에서 InAs와 InGaAs 나노선을 성공적으로 성장시켰다. P형 물질로는 p++도핑된 Ge (111) 웨이퍼를 사용하였다. 인트린식 InGaAs와 InAs 나노선이 그 위에 선택적으로 성장되었다. 마지막으로 실리콘 도펀트를 가진 n형 InAs 나노선이 후속적으로 성장되었다. 성장된 나노선은 수직 나노선 TFET을 제작하여 평가되었다. 높은 단계 커버리지와 양호한 인터페이스 상태 밀도를 위하여 ALD HfO2 및 ALD TiN 공정과정이 최적화되었다. 개발된 ALD 공정을 적용함으로써 수직형 나노선 Ge/InGaAs 헤테로 접합 TFET의 동작이 성공적으로 확인되었다.The remarkable development of lithography technology commercialized the sub-10 nm logic transistors. Gate length scaling is a large portion of the effort to reduce the power consumption of metal-oxide-semiconductor field-effect transistors (MOSFETs). However, this approach faces several problems, such as the physical limitation of lithography and leakage current control. The fundamental problem of MOSFETs is that they cannot reach subthreshold-slope (SS) below 60 mV/dec due to their current transport mechanism. Several researchers of Si tunneling field-effect transistors (TFETs) reported sub-60 mV/dec, but Si homo-junction TFETs show insufficient on-current due to the poor tunneling probability of indirect-band gap materials. As tunneling probability at the p-i junction influences the on-current of TFETs, III-V compound semiconductors, which have a direct small band gap and low effective masses, are the most promising materials to achieve high tunneling current with SS below 60 mV/dec. Also, the tunneling current can be remarkably increased by forming a staggered or broken gap by choosing materials with different band offsets. Since the tunneling at a p-i junction is the current source of TFET devices, many researchers have reported the performance of TFETs fabricated from III-V wafers with high p-type doping concentration grown by the molecular beam epitaxy (MBE) method. However, very few InGaAs TFET devices fabricated on MOCVD-grown epitaxial layers have been reported due to the challenging techniques for achieving p-type InGaAs with high doping concentration and steep dopant profile. Accordingly, this work demonstrates the metal-organic chemical vapor deposition (MOCVD) growth techniques to grow a high-quality epitaxial layer for TFET device fabrication. Homo-junction p-i-n InGaAs epitaxial layers were grown for conventional TFET devices, and hetero-junction p++-Ge/i-InGaAs/n+-InAs nanowires were grown to confirm the possibility of boosting the TFET device performance. The TFET device performance at both epitaxial layers was characterized to confirm the potential of TFET devices fabricated on the epitaxy layers grown by the MOCVD method. The high-quality epitaxial layers were grown using the MOCVD method. Compared to the MBE method, cost-effectiveness, high throughput, and excellent crystal quality are the significant advantages of the MOCVD method. The growth of InGaAs film layers on InP (001) substrate with several growth conditions was studied. The effects of source flow rate, temperature, and V/III ratio on the quality of grown InGaAs film layers were studied. As the high-concentration and steep dopant profile of n-type and p-type dopants are challenging in MOCVD InGaAs growth technique, carbon and tellurium doping techniques were introduced to achieve highly-doped p-type and n-type InGaAs layer with steep dopant profile. The grown epitaxial film layers were evaluated by fabricating the TFET device. Before the TFET device fabrication, the dimensions of the TFET device were selected by electrical simulation results of TFET devices with different structures. For TFET device fabrication, a high-quality vertical p-i-n epitaxial structure with a steep doping profile was successively formed by MOCVD. After epitaxial growth, the TFET devices were fabricated by the vertical top-down wet etching method. The ohmic process and air-bridge process were also optimized for device fabrication. The effect of p-type doping concentration and the dislocations formed during MOCVD growth was confirmed by TFET performance. The fabricated TFET devices showed SS of near-60 mV/dec and sound on/off current ratio, which was by far the first reported device comparable to TFET devices fabricated on the MBE-grown wafers. This result represents the possible mass-production of high-quality MOCVD-grown III-V TFET devices. The next part of this study is nanowire TFET fabrication. The growth of III-V nanowires for electronic device fabrication has several advantages. The significant advantage is that hetero-structures with various characteristics can be formed on various wafers. The nanowires grown by a sufficiently small diameter show a dislocation-free interface even if nanowires have a different lattice constant compared to the wafer. Various types of band-alignment can be formed, and this is a crucial factor in boosting the tunneling current of TFETs. Also, nanowires with a small diameter show better device density in a chip, improved gate controllability, and enhanced throughput by reducing growth time. The InGaAs nanowires were grown by the selective area growth (SAG) method. As a hard-mask layer, a SiO2 layer was deposited on InP (111)B and Ge (111) wafers. Growth conditions far different from InGaAs film layer growth were tested due to the different growth modes. Selective growth of nanowires was identified by optimizing temperature, V/III ratio, and source flow rate. As a result, InAs and InGaAs nanowires were successfully grown on InP (111)B and Ge (111) wafers. For p-type material, the p++-doped Ge (111) wafer was used. The intrinsic InGaAs and InAs nanowires were selectively grown on the patterned substrate. Finally, n-type InAs nanowires with silicon dopant were grown subsequently. The grown nanowires were evaluated by fabricating the vertical nanowire TFETs. ALD HfO2 and ALD TiN processes were optimized for high step coverage and good interface state density. By applying the developed ALD processes, a successful demonstration of vertical nanowire Ge/InGaAs hetero-junction TFET was observed.Contents List of Tables List of Figures Chapter 1. Introduction 1 1.1. Backgrounds 1 1.2. III-V TFETs for Low Power Device 5 1.3. Epitaxy of III-V Materials 12 1.4. Research Aims 17 1.5. References 20 Chapter 2. Epitaxial Growth of InGaAs on InP (001) Substrate 24 2.1. Introduction 24 2.2. Temperature Dependent Properties of Intrinsic-InGaAs on InP (001) Substrate 33 2.3. In-situ Doping Properties of InGaAs on InP (001) Substrate 37 2.4. Conclusion 51 2.5. References 52 Chapter 3. Demonstration of TFET Device Fabricated on InGaAs-on-InP (001) Substrate 56 3.1. Introduction 56 3.2. Simulation of Basic Operations of TFET Device 60 3.3. Process Optimization of TFET Fabrication 68 3.4. Process Flow 74 3.5. Characterization of TFETs Fabricated on MBE-grown and MOCVD-grown Wafers 78 3.6. Conclusion 96 3.7. References 97 Chapter 4. Selective Area Growth of In(Ga)As Nanowires 101 4.1. Introduction 101 4.2. Process Flow of Nanowire Growth 108 4.3. Impact of Different Growth Variables on the Growth of InAs Nanowires 113 4.4. Impact of Different Growth Variables on the Growth of InGaAs Nanowires 127 4.5. Conclusion 144 4.6. References 146 Chapter 5. Demonstration of Vertical Nanowire TFET 149 5.1. Introduction 149 5.2. Optimization of ALD HfO2 High-k Stack 152 5.3. Optimization of ALD TiN Gate Metal 169 5.4. Detailed Demonstration of Vertical Nanowire TFET Fabrication Processes 182 5.5. Characterization of Fabricated Vertical Nanowire TFETs 196 5.6. Conclusion 203 5.7. References 205 Chapter 6. Conclusions and Outlook 209 6.1. Conclusions 209 6.2. Outlook 211 Appendix. 213 A. n+-InAs Nanowire Doping Concentration Evaluation by TLM Method 213 B. n+-InAs Nanowire Doping Concentration Evaluation by C-V Method 219 C. References 205 Abstract in Korean 226 Research Achievements 230박

Design And Fabrication Of Chemiresistor Typemicro/nano Hydrogen Gas Sensors Usinginterdigitated Electrodes

Hydrogen sensors have obtained increased interest with the widened application of hydrogen energy in recent years. Among them, various chemiresistor based hydrogen sensors have been studied due to their relatively simple structure and well-established detection mechanism. The recent progress in micro/nanotechnology has accelerated the development of small-scale chemical sensors. In this work, MEMS (Micro-Electro-Mechanical Systems) sensor platforms with interdigitated electrodes have been designed and fabricated. Integrating indium doped tin dioxide nanoparticles, these hydrogen sensors showed improved sensor characteristics such as sensitivity, response and selectivity at room temperature. Design parameters of interdigitated electrodes have been studied in association with sensor characteristics. It was observed that these parameters (gap between the electrodes, width and length of the fingers, and the number of the fingers) imposed different impacts on the sensor performance. In order to achieve small, robust, low cost and fast hydrogen micro/nano sensors with high sensitivity and selectivity, the modeling and process optimization was performed. The effect of humidity and the influence of the applied voltage were also studied. The sensor could be tuned to have high sensitivity (105), fast response time (10 seconds) and low energy consumption (19 nW). Finally, a portable hydrogen instrument integrated with a micro sensor, display, sound warning system, and measurement circuitry was fabricated based on the calibration data of the sensor

Platinum Nanoparticle Decorated SiO2 Microfibers as Catalysts for Micro Unmanned Underwater Vehicle Propulsion

Micro unmanned underwater vehicles (UUVs) need to house propulsion mechanisms that are small in size but sufficiently powerful to deliver on-demand acceleration for tight radius turns, burst-driven docking maneuvers, and low-speed course corrections. Recently, small-scale hydrogen peroxide (H2O2) propulsion mechanisms have shown great promise in delivering pulsatile thrust for such acceleration needs. However, the need for robust, high surface area nanocatalysts that can be manufactured on a large scale for integration into micro UUV reaction chambers is still needed. In this report, a thermal/electrical insulator, silicon oxide (SiO2) microfibers, is used as a support for platinum nanoparticle (PtNP) catalysts. The mercapto-silanization of the SiO2 microfibers enables strong covalent attachment with PtNPs, and the resultant PtNP–SiO2 fibers act as a robust, high surface area catalyst for H2O2 decomposition. The PtNP–SiO2 catalysts are fitted inside a micro UUV reaction chamber for vehicular propulsion; the catalysts can propel a micro UUV for 5.9 m at a velocity of 1.18 m/s with 50 mL of 50% (w/w) H2O2. The concomitance of facile fabrication, economic and scalable processing, and high performance—including a reduction in H2O2 decomposition activation energy of 40–50% over conventional material catalysts—paves the way for using these nanostructured microfibers in modern, small-scale underwater vehicle propulsion systems