Multiscale Simulation Study of Interfacial Properties on Low Dimensional Materials: CNTs and SiNWs

Abstract

As the scale of the devices goes down and approaches to an atomic level, atomic scale phenomena especially at the interface have become extremely essential issues due to their significant effects on the performance of devices. Moreover, in the so-called ‘nanoscience and nanotechnology’ whose goal is to investigate and find the relationship between the structure of materials with the nanometer scale and their various properties, it is known that most properties of a system are determined by the interface rather than the intrinsic bulk properties. Carbon nanotubes (CNTs) and silicon nanowires (SiNWs), which have been considered as representative materials in nanoscience and nanotechnology, are no exception. Due to the size and dimensionality of CNTs and SiNWs exhibiting their unique mechanical, optical, electrical properties, and potential applications, most atoms constitute the interface when the devices or complex materials based on CNTs or SiNWs are fabricated. However, in actual experiment, it is difficult or even impossible to investigate the atomic structure and dynamics at the interface in nanoscale systems because of the difficulties of direct observation. Recently, computational science based on the well-organized theories, which is aimed to understand and predict various properties and phenomena of materials, has been regarded as a suitable method to overcome the limitations from the experiments. The limitations from the computational science, e.g. number of atoms to deal with and time scale have been also got over with the exponential growth in computing power and the development of algorithms. In this dissertation, the interfacial reactions on low dimensional materials, CNTs and SiNWs were investigated through various simulation techniques including the molecular dynamics (MD), density functional theory (DFT) and multiscale simulation method. This dissertation is composed of largely two parts,

1. Interface bonding strength effects in the CNT-reinforced composites 2. Interface roughness effects in the oxidized SiNWs

In the first part, the effects of interfacial bonding strength between CNTs and various matrixes on the mechanical properties were investigated. It is worth noting that CNT pull-out and load transfer on the strong CNT were identified as the main mechanism for the improved mechanical properties. Functionalization, which is a method to modify the surface of CNTs by introducing functional molecules for better reactivity, effects on the structural stability were also investigated. The calculation results showed that the optimal value of concentration of functional groups to have the effect load transfer from a matrix to CNT exists due to the competition between the enhancement of bond strength and the degradation of embedded CNTs. For CNT/Pt composites, the chemically active oxygen or nitrogen in the functionalized CNT surfaces can be a promising candidate for the fabricating agent of a CNT/Pt composite catalyst to promote better structural stability. The second part is to investigate the interface roughness effects in the oxidized silicon systems. First of all, the oxidation behavior of Si(100) substrate and SiNWs was studied. It was observed that the formation of nonstoichiometric suboxide layer at the interface between Si/SiO2 affects the band gap profiles, hence, the device characteristics. On the other hand, for the SiNWs, it is known that the oxidation rate on SiNWs strongly slows down due to its curvature effects resulting in the compressive stress at the interface between Si/SiO2. It is so-called ‘self-limiting oxidation’ and becomes more severe when the diameter of SiNWs is smaller. However, the MD simulation showed that the residual stress decreased with increasing curvature in the sub-10 nm regime of the diameter. In this regime, the unoxidized Si core of the thinner SiNWs deformed more to compensate for the volume expansion of the surface oxide layer, resulting in the smaller compressive stress, which is in contrast to the previous theories in which the silicon core is considered as a rigid matter. It successfully explained the experimental observation that thin SiNWs with a diameter in the sub-10 nm regime were fully oxidized without retardation. Finally, the interfacial roughness at the interface between Si/SiO2 in the oxidized SiNWs was characterized. In order to realize SiNWs as an alternative for new generation metal-oxide-semiconductor field-effect transistors, the understanding of the interface between Si and SiO2 at the atomic scale is required since the interface plays a crucial role to determine the device performance due to the interface roughness scattering. A unique multiscale simulation approach to investigate the interfacial roughness effects on the transport characteristics was proposed through the reactive MD, interfacial roughness characterization technique and NEGF calculations. Especially, this simulation technique provides quantitative information of the interfacial roughness in the oxidized SiNWs which is hardly observed or characterized in experimental methods. |소자의 크기가 점차 수 나노미터 단위로 작아짐에 따라, 소자의 특성에 큰 영향을 끼치는 원자 단위의 구조 및 현상, 특히 시스템 계면에서의 원자 구조 및 현상에 대한 정확한 이해는 매우 중요한 이슈가 되었다. 더욱이 나노 스케일 혹은 원자 단위의 크기로 구성된 재료를 다루는 나노 과학에서는 시스템의 여러 물성들이 벌크가 갖는 고유 특성 보다 계면에서의 특성에 의해 결정된다고 알려져 있기 때문에 계면의 중요성이 더욱 커진다 할 수 있다. 나노 과학의 대표 재료인 탄소 나노튜브와 실리콘 나노와이어 또한 예외가 아니다. 특히, 우수한 기계적, 광학적, 전기적 특성으로 인해 다양한 연구가 진행되고 있는 탄소 나노튜브와 실리콘 나노와이어는 나노미터 단위의 크기 및 1차원의 구조를 갖기 때문에 탄소 나노튜브와 실리콘 나노와이어를 이용한 복합 재료 및 소자를 구현할 경우, 구성하는 대부분의 원자들이 계면을 이루게 된다. 하지만 통상적인 실험 방법으로는 직접적인 관찰이 힘들기 때문에 나노 스케일의 계면에서의 원자 구조 및 움직임 등을 연구하기가 매우 비효율적일 뿐 아니라 어렵고 경우에 따라서는 불가능할 수 있다. 최근 들어 강력한 이론을 바탕에 둔 계산 과학은 이러한 실험적인 한계를 극복하는데 적절한 방법으로 재료의 다양한 물성 및 현상 등을 이해하는데 널리 쓰이고 있다. 특히 기존의 계산 과학 자체가 갖고 있던 여러 제약, 즉 다룰 수 있는 원자 개수, 시간 등이 급속히 발전하는 컴퓨터 성능과 우수한 알고리듬의 개발로 인해 빠르게 극복되어 가고 있다. 본 학위 논문에서는 저차원 재료, 탄소 나노튜브와 실리콘 나노와이어의 계면 구조 및 현상에 대해 분자동역학, 범밀도함수 계산 혹은 멀티스케일 시뮬레이션 등 다양한 시뮬레이션 방법을 통해 연구를 수행하였다. 본 연구는 크게 두 부분으로 이루어져 있다.

1. 탄소 나노튜브를 이용한 복합 재료에서의 계면 결합 강도의 효과 2. 산화된 실리콘 나노와이어에서의 계면 roughness의 효과

첫 번째 부분에서는 탄소 나노튜브와 다양한 매트릭스 사이의 계면 결합 강도가 복합 재료의 기계적 특성에 미치는 효과에 대한 연구를 수행하였다. 복합 재료의 기계적 특성을 증가시키는 주된 요인은 탄소 나노튜브의 pull-out 거동과 매트릭스에 가해진 외부 응력이 우수한 기계적 성질을 갖는 탄소 나노튜브로 이동하는 것으로 밝혀졌다. 또한, 탄소 나노튜브 표면의 반응성을 좋게 하여 탄소 나노튜브와 매트릭스 사이의 결합 강도를 증진시키기 위한 방법인 functionalization의 효과에 대해서 연구를 수행하였다. Functional group들의 최적의 농도가 존재함을 확인하였는데, 이것은 functional group으로 인한 탄소 나노튜브와 매트릭스 사이의 결합력 증진과 탄소 나노튜브 자체의 기계적 특성 저하, 두 요인의 경쟁으로 인한 것임을 알 수 있었다. 또한 탄소 나노튜브와 플래티늄 복합 재료에서는 화학적으로 활발한 산소 원자 혹은 질소 원자가 functional group으로 사용될 경우 탄소 나노튜브와 플래티늄 복합 재료의 구조 안정성이 좋아지는 것을 확인하였다. 두 번째 부분은 산화된 실리콘 시스템에서의 계면 roughness 효과에 대한 연구 결과이다. 우선, Si(100) 기판과 실리콘 나노와이어에서의 산화 거동에 대한 연구를 수행하였다. 실리콘과 실리콘 산화층 사이의 계면에서 형성된 비화학양론 산화물, 즉 산소가 결핍된 실리콘 산화층(suboxide)이 에너지 밴드갭 및 소자 특성에 큰 영향을 끼치는 것을 확인하였다. 실리콘 나노와이어는 부피 팽창을 수반하는 산화 과정이 실리콘 나노와이어의 곡률로 인해 실리콘과 실리콘 산화층 사이의 계면에 응력을 발생시켜 산화 과정이 느려지거나 완전히 멈추는 현상을 보인다. Self-limiting oxidation으로 알려져 있는 이 현상은 실리콘 나노와이어의 지름이 작아질수록 그 효과가 더욱 심해진다고 다양한 이론 및 실험 결과들에 의해 알려져 있다. 하지만 분자동역학을 이용하여 10 nm 이하의 직경을 갖는 실리콘 나노와이어의 산화 시뮬레이션을 수행한 결과, 실리콘과 실리콘 산화층 사이의 계면에 생기는 응력이 곡률이 커짐에 따라, 즉 지름이 작아짐에 따라 작아지는 것을 확인하였다. 내부의 실리콘을 변형이 없는 강체로 가정한 기존의 이론과는 반대의 경향을 보이는 이 현상은 표면의 산화층이 부피 팽창을 함에 따라 내부의 실리콘 결정이 함께 변형을 하면서 계면에서의 응력을 낮추는 것으로 확인되었다. 따라서 최근 10 nm 이하의 실리콘 나노와이어에서 self-limiting oxidation이 아닌 full oxidation이 보이는 이유를 성공적으로 설명하였다. 마지막으로 실리콘 나노와이어에서의 실리콘과 산화층 사이의 계면 roughness에 대한 연구를 수행하였다. 실리콘 나노와이어를 이용한 반도체 소자에서 계면 roughness로 인한 scattering 이 소자 특성을 결정하는데 중요한 역할을 하는 것으로 널리 알려져 있다. 따라서 실리콘 나노와이어를 기존의 반도체 소자를 대체할 재료로 사용하기 위해서는 계면 roughness에 대해 정량적인 이해가 필요하다. 이에 따라 본 연구에서는 분자동역학, 계면 roughness 측정 시뮬레이션 그리고 NEGF 계산을 통한 멀티스케일 시뮬레이션 방법을 제안하였다. 특히, 이 멀티스케일 시뮬레이션 방법을 통해 실험적으로 불가능에 가까운 산화된 실리콘 나노와이어 계면에서의 roughness의 정량적 측정 및 분석을 수행하였다.; As the scale of the devices goes down and approaches to an atomic level, atomic scale phenomena especially at the interface have become extremely essential issues due to their significant effects on the performance of devices. Moreover, in the so-called ‘nanoscience and nanotechnology’ whose goal is to investigate and find the relationship between the structure of materials with the nanometer scale and their various properties, it is known that most properties of a system are determined by the interface rather than the intrinsic bulk properties. Carbon nanotubes (CNTs) and silicon nanowires (SiNWs), which have been considered as representative materials in nanoscience and nanotechnology, are no exception. Due to the size and dimensionality of CNTs and SiNWs exhibiting their unique mechanical, optical, electrical properties, and potential applications, most atoms constitute the interface when the devices or complex materials based on CNTs or SiNWs are fabricated. However, in actual experiment, it is difficult or even impossible to investigate the atomic structure and dynamics at the interface in nanoscale systems because of the difficulties of direct observation. Recently, computational science based on the well-organized theories, which is aimed to understand and predict various properties and phenomena of materials, has been regarded as a suitable method to overcome the limitations from the experiments. The limitations from the computational science, e.g. number of atoms to deal with and time scale have been also got over with the exponential growth in computing power and the development of algorithms. In this dissertation, the interfacial reactions on low dimensional materials, CNTs and SiNWs were investigated through various simulation techniques including the molecular dynamics (MD), density functional theory (DFT) and multiscale simulation method. This dissertation is composed of largely two parts,

1. Interface bonding strength effects in the CNT-reinforced composites 2. Interface roughness effects in the oxidized SiNWs

In the first part, the effects of interfacial bonding strength between CNTs and various matrixes on the mechanical properties were investigated. It is worth noting that CNT pull-out and load transfer on the strong CNT were identified as the main mechanism for the improved mechanical properties. Functionalization, which is a method to modify the surface of CNTs by introducing functional molecules for better reactivity, effects on the structural stability were also investigated. The calculation results showed that the optimal value of concentration of functional groups to have the effect load transfer from a matrix to CNT exists due to the competition between the enhancement of bond strength and the degradation of embedded CNTs. For CNT/Pt composites, the chemically active oxygen or nitrogen in the functionalized CNT surfaces can be a promising candidate for the fabricating agent of a CNT/Pt composite catalyst to promote better structural stability. The second part is to investigate the interface roughness effects in the oxidized silicon systems. First of all, the oxidation behavior of Si(100) substrate and SiNWs was studied. It was observed that the formation of nonstoichiometric suboxide layer at the interface between Si/SiO2 affects the band gap profiles, hence, the device characteristics. On the other hand, for the SiNWs, it is known that the oxidation rate on SiNWs strongly slows down due to its curvature effects resulting in the compressive stress at the interface between Si/SiO2. It is so-called ‘self-limiting oxidation’ and becomes more severe when the diameter of SiNWs is smaller. However, the MD simulation showed that the residual stress decreased with increasing curvature in the sub-10 nm regime of the diameter. In this regime, the unoxidized Si core of the thinner SiNWs deformed more to compensate for the volume expansion of the surface oxide layer, resulting in the smaller compressive stress, which is in contrast to the previous theories in which the silicon core is considered as a rigid matter. It successfully explained the experimental observation that thin SiNWs with a diameter in the sub-10 nm regime were fully oxidized without retardation. Finally, the interfacial roughness at the interface between Si/SiO2 in the oxidized SiNWs was characterized. In order to realize SiNWs as an alternative for new generation metal-oxide-semiconductor field-effect transistors, the understanding of the interface between Si and SiO2 at the atomic scale is required since the interface plays a crucial role to determine the device performance due to the interface roughness scattering. A unique multiscale simulation approach to investigate the interfacial roughness effects on the transport characteristics was proposed through the reactive MD, interfacial roughness characterization technique and NEGF calculations. Especially, this simulation technique provides quantitative information of the interfacial roughness in the oxidized SiNWs which is hardly observed or characterized in experimental methods.