In Situ Study of Size and Temperature Dependent Brittle‐to‐Ductile Transition in Single Crystal Silicon

Silicon based micro‐ and nanometer scale devices operating at various temperatures are ubiquitous today. However, thermo‐mechanical properties of silicon at the small scale and their underlying mechanisms remain elusive. The brittle‐to‐ductile transition (BDT) is one such property relevant to these devises. Materials can be brittle or ductile depending on temperature. The BDT occurs over a small temperature range. For bulk silicon, the BDT is about 545 °C. It is speculated that the BDT temperature of silicon may decrease with size at the nanoscale. However, recent experimental and computational studies have provided inconclusive evidence, and are often contradictory. Potential reasons for the controversy might originate from the lack of an in situ methodology that allows variation of both temperature and sample size. This controversy is resolved in the present study by carrying out in situ thermo‐mechanical bending tests on single crystal silicon samples with concurrent control of these two key parameters. It is unambiguously shown that the BDT temperature reduces with sample size. For example, the BDT temperature decreases to 293 °C for a sample size 720 nm. A mechanism‐based model is proposed to interpret the experimental observations.     

Studies on Dielectric Properties of Silicon Nitride at High Temperature

In this paper, the dielectric properties of silicon nitride are studied using the dielectric polarization theories. According to the developed dielectric models, the temperature dependence of dielectric constant and loss of silicon nitride is mainly analyzed. In addition, the impact of Li^＋, K^＋, Ca^2＋, Al^3＋ and Mg^2＋ doping on the dielectric properties of silicon nitride are also estimated.     

Self‐Assembled Room Temperature Multiferroic BiFeO3‐LiFe5O8 Nanocomposites

Multiferroic materials have driven significant research interest due to their promising technological potential. Developing new room‐temperature multiferroics and understanding their fundamental properties are important to reveal unanticipated physical phenomena and potential applications. Here, a new room temperature multiferroic nanocomposite comprised of an ordered ferrimagnetic spinel α‐LiFe₅O₈ (LFO) and a ferroelectric perovskite BiFeO₃ (BFO) is presented. It is observed that lithium (Li)‐doping in BFO favors the formation of LFO spinel as a secondary phase during the synthesis of Li_xBi_1?xFeO₃ ceramics. Multimodal functional and chemical imaging methods are used to map the relationship between doping‐induced phase separation and local ferroic properties in both the BFO‐LFO composite ceramics and self‐assembled nanocomposite thin films. The energetics of phase separation in Li doped BFO and the formation of BFO‐LFO composites are supported by first principles calculations. These findings shed light on Li's role in the formation of a functionally important room temperature multiferroic and open a new approach in the synthesis of light element doped nanocomposites for future energy, sensing, and memory applications.     

Pipetting Nanowires: In Situ Visualization of Solid‐State Nanowire‐to‐Nanoparticle Transformation Driven by Surface Diffusion‐Mediated Capillarity

The most interesting applications of nanotubes include their use as storage media for atoms and small molecules, as nanoscale capsules for chemical reactions, and as nanopipettes for material delivery. The geometrical transformation of metallic copper nanowires, confined in graphitic coating, into crystalline nanoparticles of up to tenfold increased diameter is reported. In situ transmission electron microscopy images at 500 °C, recorded as movies, provide an exceptional real‐time visualization of Cu draining out of the carbon coating. The solid content of the carbon tube is effectively evacuated over micrometer distances towards the open end, transforming each nanowire into a single monocrystalline, facetted Cu particle. Kinetic Monte Carlo simulations propose that this dramatic morphological transformation is driven by surface diffusion of Cu atoms along the wire/tube interface, thus minimizing the total free energy of the system.     

硅纳米线的电学特性

总结了硅纳米线在电学特性方面的研究进展,重点分析了本征及掺杂硅纳米线的载流子浓度与迁移率、场发射及电子输运特性。研究表明通过对硅纳米线进行掺杂可提高载流子浓度及迁移率、场发射和电子输运性能,随硅纳米线直径的减小其电学性能增强。因此,硅纳米线在场效应晶体管及存储元件等纳米器件方面具有极大的应用前景。     

3D Out‐of‐Plane Rotational Etching with Pinned Catalysts in Metal‐Assisted Chemical Etching of Silicon

Pinned structures in conjunction with shaped catalysts are used in metal‐assisted chemical etching (MACE) of silicon to induce out‐of‐plane rotational etching. Sub‐micro‐ and nanostructures are fabricated in silicon, which include scooped‐out channels and curved subsurface horns, along with vertically oriented thin metal structures. Five different etching modes induced by catalyst and pinning geometry are identified: 1) fully pinned–no etching, 2) rotation via twist, 3) rotation via delamination, 4) in‐plane bending, and 5) swinging. The rotation angle is roughly controlled through catalyst geometry. The force and pressure experienced by the catalyst are calculated from the deformation of the catalyst and range between 0.5–3.5 μN and 0.5–3.9 MPa, respectively. This is a new, simple method to fabricate 3D, heterogeneous sub‐micro‐ and nanostructures in silicon with high feature fidelity on the order of tens of nanometers while providing a method to measure the forces responsible for catalyst motion during MACE.     

Wear‐Resistant Nanoscale Silicon Carbide Tips for Scanning Probe Applications

The search for hard materials to extend the working life of sharp tools is an age‐old problem. In recent history, sharp tools must also often withstand high temperatures and harsh chemical environments. Nanotechnology extends this quest to tools such as scanning probe tips that must be sharp on the nanoscale, but still very physically robust. Unfortunately, this combination is inherently contradictory, as mechanically strong, chemically inert materials tend to be difficult to fabricate with nanoscale fidelity. Here a novel process is described, whereby the surfaces of pre‐existing, nanoscale Si tips are exposed to carbon ions and then annealed, to form a strong silicon carbide (SiC) layer. The nanoscale sharpness is largely preserved and the tips exhibit a wear resistance that is orders of magnitude greater than that of conventional silicon tips and at least 100‐fold higher than that of monolithic, SiO‐doped diamond‐like‐carbon (DLC) tips. The wear is well‐described by an atom‐by‐atom wear model, from which kinetic parameters are extracted that enable the prediction of the long‐time scale reliability of the tips.     

超短脉冲激光对无机硅材料的损伤

通过控制作用于材料表面的激光能量和脉冲数量,实验研究了800nm,50fs,1kHz激光作用下融石英玻璃和硅片的破坏机制和损伤规律,计算了材料的损伤阈值与脉冲能量以及脉冲数量的依赖关系,并采用简化的理论模型计算了熔石英玻璃材料的损伤阈值与激光脉宽以及光子能量之间的依赖关系。对这两种无机硅材料在飞秒脉冲作用后的微区结构改变进行了扫描电子显微镜(SEM)测试,研究了其形貌特征。结果表明,硅片是由缺陷中的导带电子作为种子电子引发雪崩电离导致材料损伤,而熔石英玻璃是由多光子电离激发出导带电子引发雪崩电离导致材料损伤。     

氮气氛中高温热处理硅片表面的直接氮化

研究了直拉硅单晶片在氮气氛下热处理时的表面氮化,利用了XPS(X射线光电子谱)、SEM(扫描电子显微镜)、金相显微镜、XRD(X射线衍射仪)等手段研究了在高纯氮和非高纯氮保护条件下不同温度热处理后的样品表面,结果发现只有用高纯氮保护和温度高于110 0℃的条件下,氮气才能与硅表面发生反应,生成氮化硅(Si3 N4 )薄膜,否则氮保护中微量的氧气会和硅表面发生反应,生成二氧化硅(SiO2 )薄膜.     

Regulated Breathing Effect of Silicon Negative Electrode for Dramatically Enhanced Performance of Li‐Ion Battery

Si is an attractive negative electrode material for lithium ion batteries due to its high specific capacity (≈3600 mAh g^–1). However, the huge volume swelling and shrinking during cycling, which mimics a breathing effect at the material/electrode/cell level, leads to several coupled issues including fracture of Si particles, unstable solid electrolyte interphase, and low Coulombic efficiency. In this work, the regulation of the breathing effect is reported by using Si–C yolk–shell nanocomposite which has been well‐developed by other researchers. The focus is on understanding how the nanoscaled materials design impacts the mechanical and electrochemical response at electrode level. For the first time, it is possible to observe one order of magnitude of reduction on breathing effect at the electrode level during cycling: the electrode thickness variation reduced down to 10%, comparing with 100% in the electrode with Si nanoparticles as active materials. The Si–C yolk–shell nanocomposite electrode exhibits excellent capacity retention and high cycle efficiency. In situ transmission electron microscopy and finite element simulations consistently reveals that the dramatically enhanced performance is associated with the regulated breathing of the Si in the new composite, therefore the suppression of the overall electrode expansion.     

Atomic Insights of Self-Healing in Silicon Nanowires

The self-healing capability is highly desirable in semiconductors to develop advanced devices with improved stability and longevity. In this study, the automatic self-healing in silicon nanowires is reported, which are one of the most important building blocks for high-performance semiconductor nanodevices. A recovery of fracture strength (10.1%) on fractured silicon nanowires is achieved, which is demonstrated by in situ transmission electron microscopy tensile tests. The self-healing mechanism and factors governing the self-healing efficiency are revealed by a combination of atomic-resolution characterizations and atomistic simulations. Spontaneous rebonding, atomic rearrangement, and van der Waals attraction are responsible for the self-healing in silicon nanowires. Additionally, the self-healing efficiency is affected by the fracture surface roughness, the nanowire size, the nanowire orientation, and the passivation of dangling bonds on fracture surfaces. These new findings shed light on the self-healing mechanism of silicon nanowires and provide new insights into developing high-lifetime and high-security semiconductor devices.     

Direct Observation at Room Temperature of the Orthorhombic Weyl Semimetal Phase in Thin Epitaxial MoTe2

The direct observation at room temperature (RT) of the noncentrosymmetric orthorhombic topological Weyl semimetal phase in epitaxial thin films of MoTe₂ grown on InAs(111)/Si(111) substrates by molecular beam epitaxy (MBE) is reported. The orthorhombic phase is typically found at lower temperatures but its observation at RT in this work is attributed to the enlarged lattice parameters, influenced by the substrate, which stabilize an interlayer antibonding state compatible with the orthorhombic stacking. First‐principles calculations predict eight type II Weyl nodes which are located below (but near) the Fermi energy making them accessible to charge transport and creating the prospect for practical applications exploiting the nontrivial topological properties. The orthorhombic phase coexists with an unconventional triclinic layer stacking which is different than the monoclinic or orthorhombic structures but it is centrosymmetric and topologically trivial.     

Room‐Temperature Crosslinkable Natural Polymer Binder for High‐Rate and Stable Silicon Anodes

Natural polymers with abundant side functionalities are emerging as a promising binder for high‐capacity yet large‐volume‐change silicon anodes with a strong and reversible supramolecular interaction that originates from secondary bonding. However, the supramolecular network solely based on hydrogen bonding is relatively vulnerable to repeated deformation and has an insufficient diffusivity of lithium ions. Herein, reported is a facile but efficient way of incorporating the natural polymers with an ionically conductive crosslinker, which can construct a robust network for silicon anodes. The boronic acid in the crosslinker spontaneously reacts with natural polymers to generate boronic esters at room temperature without any kind of triggers, which gives a strong and dynamic covalent bonding to the supramolecular network. The other component in the crosslinker, polyethylene oxide, contributes to the enhanced ionic conductivity of polymers, leading to outstanding rate performances even at a high mass loading of silicon nanoparticles (>2 mg cm^?2). The small portion of the proposed crosslinker can modulate the strength of the entire network by balancing the covalent crosslinking and self‐healing secondary interaction along with the fast lithium‐ion diffusion, thus enabling the extended operation of silicon electrodes.     

Ultralow Thermal Conductivity of Single‐Crystalline Porous Silicon Nanowires

Porous materials provide a large surface‐to‐volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Herein, a structure–property relationship is established by measuring the porosity and thermal conductivity of individual electrolessly etched single‐crystalline silicon nanowires using a novel electron‐beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 W m^?1 K^?1 at 300 K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young's modulus, as well as the smallest structural size ever reported in crystalline silicon (<5 nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials toward electronics and nanoelectromechanical systems.     

Tailoring the Mechanical Properties of High‐Aspect‐Ratio Carbon Nanotube Arrays using Amorphous Silicon Carbide Coatings

The porous nature of carbon nanotube (CNT) arrays allows for the unique opportunity to tailor their mechanical response by the infiltration and deposition of nanoscale conformal coatings. Here, we fabricate novel photo‐lithographically defined CNT pillars that are conformally coated with amorphous silicon carbide (a‐SiC) to strengthen the interlocking of individual CNTs at junctions using low pressure chemical vapor deposition (LPCVD). We further quantify the mechanical response by performing flat‐punch nanoindentation measurements on coated CNT pillars with various high‐aspect‐ratios. We discovered new mechanical failure modes of coated CNT pillars, such as “bamboo” and brittle‐like composite rupture as coating thickness increases. Furthermore, a significant increase in strength and modulus is achieved. For CNT pillars with high aspect ratio (1:10) and coating thickness of 21.4 nm, the compressive strength increases by an order of magnitude of 3, towards 1.8 GPa (from below 1 MPa for uncoated CNT pillars) and the elastic modulus increases towards 125 GPa. These results show that our coated CNT pillars, which can serve as vertical interconnects and 3D super‐capacitors, can be transformed into robust high‐aspect‐ratio 3D‐micro architectures with semiconductor device compatible processes.     

直拉单晶硅中氧沉淀的高温消融和再生长

重点研究了直拉(CZ)硅中氧沉淀在快速热处理(RTP)和常规炉退火过程中的高温消融以及再生长行为.实验发现,RTP是一种快速消融氧沉淀的有效方式,比常规炉退火消融氧沉淀更加显著.硅片经RTP消融处理后,在氧沉淀再生长退火过程中,硅中体微缺陷(BMD)的密度显著增加,BMD的平均尺寸略有增加;而经过常规炉退火消融处理后,在后续退火过程中,BMD的密度变化不大,但BMD的尺寸明显增大.氧沉淀消融处理后,后续退火的温度越高,氧沉淀的再生长越快.     

直拉单晶硅中氧沉淀的高温消融和再生长

重点研究了直拉(CZ)硅中氧沉淀在快速热处理(RTP)和常规炉退火过程中的高温消融以及再生长行为.实验发现,RTP是一种快速消融氧沉淀的有效方式,比常规炉退火消融氧沉淀更加显著.硅片经RTP消融处理后,在氧沉淀再生长退火过程中,硅中体微缺陷(BMD)的密度显著增加,BMD的平均尺寸略有增加;而经过常规炉退火消融处理后,在后续退火过程中,BMD的密度变化不大,但BMD的尺寸明显增大.氧沉淀消融处理后,后续退火的温度越高,氧沉淀的再生长越快.     

退火温度对室温沉积的ITO薄膜与p-Si接触性能的影响

室温下用射频磁控溅射法在玻璃和p型单晶硅衬底上沉积ITO薄膜,并对其进行不同温度的退火处理.采用XRD衍射仪测试薄膜结晶性,用紫外-可见分光光度计和霍尔效应测试试样光电性能,用吉时利2400表测试ITO/p-Si接触的I-V曲线,用线性传输线模型测试比接触电阻.研究结果表明:室温下沉积的ITO薄膜与p-Si形成欧姆接触,但比接触电阻较大.退火处理可以进一步优化接触性能,200℃退火后试样保持欧姆接触且比接触电阻下降为8.8×10-3 Ω·cm2.随着退火温度进一步升高到300℃,比接触电阻达到最低值2.8×10-3 Ω·cm2,但接触性能变为非线性.     

Orientation‐Controlled Selective‐Area Epitaxy of III–V Nanowires on (001) Silicon for Silicon Photonics

Monolithic integration of III–V nanowires on silicon platforms has been regarded as a promising building block for many on‐chip optoelectronic, nanophotonic, and electronic applications. Although great advances have been made from fundamental material engineering to realizing functional devices, one of the remaining challenges for on‐chip applications is that the growth direction of nanowires on Si(001) substrates is difficult to control. Here, catalyst‐free selective‐area epitaxy of nanowires on (001)‐oriented silicon‐on‐insulator (SOI) substrates with the nanowires aligned to desired directions is proposed and demonstrated. This is enabled by exposing {111} planes on (001) substrates using wet chemical etching, followed by growing nanowires on the exposed planes. The formation of nanowire array‐based bottom‐up photonic crystal cavities on SOI(001) and their coupling to silicon waveguides and grating couplers, which support the feasibility for on‐chip photonic applications are demonstrated. The proposed method of integrating position‐ and orientation‐controllable nanowires on Si(001) provides a new degree of freedom in combining functional and ultracompact III–V devices with mature silicon platforms.