Surface chemistry on semiconductors studied by molecular-beam reactive scattering

This Report reviews the use of molecular-beam reactive scattering to study the surface reactions of gas molecules on semiconductors which have relevance to microelectronic technologies. Modern semiconductor fabrication techniques rely heavily on dry processes where gas-surface reactions are the basic premise. This article focuses on the use of supersonic molecular-beam-surface scattering to study the dynamics and kinetics of surface reactions connected with the growth and etching processes on semiconductor surfaces. The discussion on growth processes covers the oxidation of silicon and germanium, the tungsten-hexafluoride-based tungsten deposition, and the organometallic chemical vapor deposition of gallium arsenide. The discussion on etching processes covers the halogen-based etching of gallium arsenide and silicon. An overview of the experimental technique and the underlying principles in surface-reaction dynamics and kinetics is included for readers in the technology area. The potential use of the molecular beams for actual semiconductor materials processing is also discussed.     

Computing Raman and infrared wavenumbers of nanostructures: application to silicon nanowires

The characterization of nanostructures with spectroscopic methods is a fundamental tool in nanoscience. For novel nanostructures, the interpretation of spectral features is a challenging task. To address this issue, we present the “Symmetry‐Filtered Molecular Dynamics (SFMD)” method to calculate Raman and infrared wavenumbers from molecular dynamics (MD) simulations, employing only the symmetry of the atomic structure. Explicit and expensive calculations of the electric polarizability or the dipole moment are not required. Therefore, our method can be easily used with any standard MD software. On the basis of the density functional tight‐binding method for the MD simulations, we apply our method to bulk silicon and small‐diameter hydrogen‐passivated silicon nanowires. For bulk silicon, we study the wavenumber shift of the Raman peak with temperature and obtain results that are in good agreement with experiments. We further show that thermal lattice expansion is a minor effect (22%) and that temperature‐driven anharmonic effects (78%) are the main contributions to that wavenumber shift. By analyzing the bond lengths of different silicon nanowires, we found that surface stress manifests as a 0.37% shortening of bonds only in the outermost silicon layer. We further analyzed the diameter‐dependent wavenumber shift of a Raman peak in silicon nanowires. We found that the main contribution to the wavenumber shift comes from the phonon confinement effect and surface stress leads to an additional shift of 9–22%. Our results indicate that our method is able to produce quantitative results that can be compared with experiments. We propose our method to be used for the understanding of Raman and infrared spectra of novel bulk and nanostructures. Copyright © 2012 John Wiley & Sons, Ltd.     

Optical absorption spectroscopy of one-dimensional silicon nanostructures

During the last decade there has been a great development in nanoscience and nanotechnology. The technology of nanostructures synthesis and characterization has grown rapidly and optical spectroscopy has become a very useful characterization technique, since it provides information on the structural, electronic, optical and dynamical properties of materials. Nanostructures have unique physical properties that are different from bulk materials. A wealth of interesting and new phenomena are associated with nanometer-sized structures, such as size-dependent emission or excitation, metallic and semiconductor behavior, etc. Here we present an overview of the linear optical response of one-dimensional silicon nanostructures. In particular, we make a theoretical study of the effects of the size and shape of one-dimensional silicon structures on the absorption spectrum, focusing on the calculation of the linear optical response of clean and hydrogen-adsorbed armchair (6,6) silicon nanotubes. We discuss the changes of the absorption spectrum of silicon nanowires with different diameters and analyze the behavior of the band gap as we go from bulk silicon to one-dimensional silicon nanostructures with nanometer-size diameters.     

Confinement scaling laws for the conventional reversed-field pinch

Mechanical behavior of the Si(111)/Si(3)N4(0001) interface is studied using million atom molecular dynamics simulations. At a critical value of applied strain parallel to the interface, a crack forms on the silicon nitride surface and moves toward the interface. The crack does not propagate into the silicon substrate; instead, dislocations are emitted when the crack reaches the interface. The dislocation loop propagates in the (1; 1;1) plane of the silicon substrate with a speed of 500 (+/-100) m/s. Time evolution of the dislocation emission and nature of defects is studied.     

Quasi-coarse-grained dynamics: modelling of metallic materials at mesoscales

A computationally efficient modelling method called quasi-coarse-grained dynamics (QCGD) is developed to expand the capabilities of molecular dynamics (MD) simulations to model behaviour of metallic materials at the mesoscales. This mesoscale method is based on solving the equations of motion for a chosen set of representative atoms from an atomistic microstructure and using scaling relationships for the atomic-scale interatomic potentials in MD simulations to define the interactions between representative atoms. The scaling relationships retain the atomic-scale degrees of freedom and therefore energetics of the representative atoms as would be predicted in MD simulations. The total energetics of the system is retained by scaling the energetics and the atomic-scale degrees of freedom of these representative atoms to account for the missing atoms in the microstructure. This scaling of the energetics renders improved time steps for the QCGD simulations. The success of the QCGD method is demonstrated by the prediction of the structural energetics, high-temperature thermodynamics, deformation behaviour of interfaces, phase transformation behaviour, plastic deformation behaviour, heat generation during plastic deformation, as well as the wave propagation behaviour, as would be predicted using MD simulations for a reduced number of representative atoms. The reduced number of atoms and the improved time steps enables the modelling of metallic materials at the mesoscale in extreme environments.     

Pulsed laser deposition of uniform semiconductor nanodot arrays

Uniform arrays of silicon (Si), gallium arsenide (GaAs) and zinc oxide (ZnO) nanodots have been deposited using Pulsed Laser Deposition (PLD) technique combined with a contact mask consisting of nano-holes fabricated by E-beam lithography (EBL). These nanocrystalline semiconductor nanodots have been deposited by PLD on Si and GaAs substrates at room temperature. Characterization of the nanodots has been carried out using different techniques including X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), Auger Electron Spectroscopy (AES), and Raman spectroscopy. This work demonstrates a novel technique for deposition of uniform array of semiconductor nanostructures using a contact mask at room temperature for photonic applications.     

Electron spectroscopy of objects for nanoelectronics

An electron-spectroscopic analysis is made of layered nanostructures and clusters at the surface and in the bulk of a solid. A new method of forming metal/insulator/semiconductor (superconductor) nanostructures is proposed based on ion-stimulated metal segregation effects at the surface of low-temperature gallium arsenide and a 123 high-temperature superconductor. The geometric parameters and electronic structure of these nano-objects are studied. It is shown that their electronic properties can be controllably varied in situ by acting on the surface. The dimensional transformation of the electronic properties of metal clusters is studied for clusters in the insulator SiO₂, in the superconductor LTMBE-GaAs, and on silicon and graphite surfaces. The nature of this transformation is clarified. A diagnostics for cluster ensembles is developed by which one can determine the parameters needed to describe singleelectron transport: the average number of atoms per cluster, the average distance between clusters and isolated atoms, and the chemical state of the atoms. Ensembles of silver clusters with specified parameters are obtained on a silicon surface. It is shown that these ensembles are potentially useful for developing single-electron devices. Zh. Tekh. Fiz. 69, 85–89 (September 1999)     

Plasma treatment studies of MIS devices

Silicon nitride films have emerged as the possible future dielectrics for ultra large scale integration (ULSI). Because the interface state density of silicon nitride/silicon interface in metal insulator semiconductor (MIS) configuration is more than an order of magnitude larger than that of silicon dioxide/silicon interface, plasma treatment studies on silicon nitride films have been undertaken for the possible improvement. Accordingly, silicon nitride films of various composition have been prepared by plasma enhanced chemical vapor deposition (PECVD) system using silane(SiH₄) and ammonia(NH₃) with nitrogen(N₂) as the diluent and MIS devices have been fabricated with as well as without plasma treated silicon nitride as the insulator. A considerable improvement in the silicon nitride/silicon interface is observed on ammonia plasma treatment while nitrous oxide(N₂O) plasma treatment studies have resulted in the establishment of a novel plasma oxidation process.     

Light-emitting nanocrystalline silicon by low-pressure chemical-vapor deposition of disilane

Summary Porous silicon is an attractive material for silicon optoelectronics. The great advantage of porous silicon lies on the simple way of production which makes silicon nanostructures easily available. After several papers have been published on this topic, we are able to identify some disadvantages connected to the porous nature of the material and to the method of fabrication. Other dry processes can be used to produce Si nanostructures. In this paper we present a method fully compatible with the standard semiconductor technology. The optical and structural properties of the nanocrystalline films so far obtained are presented, together with some promising results indicating good electrical properties. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce) Torino, 12–13 October 1995.     

Brittle and ductile fracture of semiconductor nanowires – molecular dynamics simulations

Fracture of silicon and germanium nanowires in tension at room temperature is studied by molecular dynamics simulations using several interatomic potential models. While some potentials predict brittle fracture initiated by crack nucleation from the surface, most potentials predict ductile fracture initiated by dislocation nucleation and slip. A simple parameter based on the ratio between the ideal tensile strength and the ideal shear strength is found to correlate very well with the observed brittle versus ductile behaviours for all the potentials used in this study. This parameter is then computed by ab initio methods, which predict brittle fracture at room temperature. A brittle-to-ductile transition (BDT) is observed in MD simulations at higher temperature. The BDT mechanism in semiconductor nanowires is different from that in the bulk, due to the lack of a pre-existing macrocrack that is always assumed in bulk BDT models.     

Thermal stability of silicon nanowires: atomistic simulation study

Using the Stillinger--Weber (SW) potential model, we investigate the thermal stability of pristine silicon nanowires based on classical molecular dynamics (MD) simulations. We explore the structural evolutions and the Lindemann indices of silicon nanowires at different temperatures in order to unveil atomic-level melting behaviour of silicon nanowires. The simulation results show that silicon nanowires with surface reconstructions have higher thermal stability than those without surface reconstructions, and that silicon nanowires with perpendicular dimmer rows on the two (100) surfaces have somewhat higher thermal stability than nanowires with parallel dimmer rows on the two (100) surfaces. Furthermore, the melting temperature of silicon nanowires increases as their diameter increases and reaches a saturation value close to the melting temperature of bulk silicon. The value of the Lindemann index for melting silicon nanowires is 0.037.     

Modified Fowler–Nordheim tunnelling for modelling charge injection into Si3N4 in an Al/Si3N4/Si structure

Charge injection behaviours in silicon nitride of an Al/Si₃N₄/n-Si metal?Cinsulator?Csemiconductor (MIS) device are systematically studied before and after applying different high constant DC bias conditions with the aim of controlling charge accumulation in the dielectric when a high actuation voltage is applied. We found that both polarity and magnitude of charge accumulation in silicon nitride depend on the biasing direction. Charge injection from the semiconductor to the silicon nitride always dominates over charge injection from the Al electrode to the silicon nitride. Negative charge accumulation happens in silicon nitride when the Al electrode is positively biased, and positive charge accumulation occurs in silicon nitride when the Al electrode is negatively biased. The positive charge accumulation is much bigger than the negative charge accumulation under the same magnitude of stress voltage. Furthermore, the experimental results also show that the charge injection level exponentially increases with the applied voltage across the silicon nitride. These observed experimental results can be well explained by a modified Fowler?CNordheim tunnelling charge injection model, which takes into account the roles of both electrons and holes in the process of charge injection.     

Porous silicon templates for electrodeposition of nanostructures

We report the fabrication and characterization of porous silicon templates for electrodeposition of high aspect ratio one-dimensional metallic nanostructures (nanowires/nanoparticles) in them. Even though nanostructures/nanowires in the past have been fabricated in alumina, polymer or silica templates, the advantages of this approach are the possibility for seamless integration of nanostructures with other silicon components, and silicon based sensors because of better physical and electrical interconnection between the nanostructure and the silicon substrate. In this work, fabrication and characterization of nanowires/nanostructures such as single-segment Ni–Fe and Au and two-segment Ni–Fe/Au electrodeposited in the porous silicon template are presented. The templates with ordered and controlled nanometer-sized pores, 40 nm and 290 nm in diameter, were created through porous Si etching. The morphology, composition and structural characteristics of the template and of the single-segment Ni–Fe and Au and two-segment Ni–Fe/Au nanostructures of diameter 275±25 nm, length up to 100 μm and pitch of 1 μm were analyzed using scanning electron microscopy and X-ray diffraction techniques. The micrographs confirm that the plating parameters have a strong influence on morphology and composition of the structures. Further, the Ni–Fe images show the formation of both vertical and branched nanowires along with nanoparticles, from breakage/discontinuous growth of nanowires. Ni–Fe nanostructures were further analyzed for temperature-dependent magnetization and magnetization vs. magnetic field measurements using a commercial physical property measurement system. They reveal no magnetic anisotropy of the nanostructures probably due to a balance between ‘reduced’ shape anisotropy from branched and rough pore surfaces and magnetocrystalline anisotropy. PACS 61.46.+w; 75.75.+a; 81.07.-b; 81.16.Be     

Roles of V/III ratio and mixture degree in GaN growth: CFD and MD simulation study

To understand the mechanism of Gallium nitride (GaN) film growth is of great importance for their potential applica- tions. In this paper, we investigate the growth behavior of the GaN film by combining computational fluid dynamics (CFD) and molecular dynamics (MD) simulations. Both of the simulations show that V/III mixture degree can have important impacts on the deposition behavior, and it is found that the more uniform the mixture is, the better the growth is. Besides, by using MD simulations, we illustrate the whole process of the GaN growth. Furthermore, we also find that the V/III ratio can affect the final roughness of the GaN film. When the V/III ratio is high, the surface of final GaN film is smooth. The present study provides insights into GaN growth from the macroscopic and microscopic views, which may provide some suggestions on better experimental GaN preparation.     

Activation of a semiconductor surface by a pulsed magnetic field

The possibility of semiconductor surface activation, which shows up as a long-term increase in the adsorption capacity in response to a short exposure to a pulsed magnetic field, is demonstrated for the first time. Magnetic-field-induced surface activation is studied on silicon, germanium, and gallium arsenide crystals. The effect revealed extends the capabilities of thin-film growth on the semiconductor surface.     

Simulation of screw dislocation motion in iron by molecular dynamics simulations

Molecular dynamics (MD) simulations are used to investigate the response of a/2<111> screw dislocation in iron submitted to pure shear strain. The dislocation glides and remains in a (110) plane; the motion occurs exclusively through the nucleation and propagation of double kinks. The critical stress is calculated as a function of the temperature. A new method is developed and used to determine the activation energy of the double kink mechanism from MD simulations. It is shown that the differences between experimental and simulation conditions lead to a significant difference in activation energy. These differences are explained, and the method developed provides the link between MD and mesoscopic simulations.     

Thermal transport in semiconductor nanostructures,graphene,and related two-dimensional materials

We review experimental and theoretical results on thermal transport in semiconductor nanostructures(multilayer thin films, core/shell and segmented nanowires), single-and few-layer graphene, hexagonal boron nitride, molybdenum disulfide, and black phosphorus. Different possibilities of phonon engineering for optimization of electrical and heat conductions are discussed. The role of the phonon energy spectra modification on the thermal conductivity in semiconductor nanostructures is revealed. The dependence of thermal conductivity in graphene and related two-dimensional(2 D) materials on temperature, flake size, defect concentration, edge roughness, and strain is analyzed.     

Nearly golden-ratio order in Ta metallic glass

The formation of mono-atomic tantalum(Ta)metallic glass(MG)through ultrafast liquid cooling is investigated by ab-initio molecular dynamics(MD)simulations.It is found that there exists nearly golden ratio order(NGRO)between the nearest and second nearest atoms in Ta MG,which has been indirectly confirmed by Khmich et al.and Liang et al..The NGRO is another universal structural feature in metallic glass besides the local five-fold symmetry(LFFS).Further analyzing of electronic structure shows that the obvious orientation of covalent bond could be attributed to the NGRO in amorphous Ta at 300 K.     

镶嵌在氢化氮化硅中纳米非晶硅粒子光吸收的模拟

采用量子限制效应模型对镶嵌有纳米非晶硅粒子的氢化氮化硅薄膜的光吸收进行了理论模拟,探讨了由吸收谱分析给出该结构薄膜光学参数的方法,并通过对不同氮含量样品的讨论给出了量子限制效应和纳米硅粒子表面的结构无序对薄膜光吸收特性的影响规律。分析结果表明,随氮含量的增加,薄膜有效光学带隙增大,该结果与薄膜中纳米硅粒子平均尺寸的减小引起的量子限制效应的增强相关,而小粒度纳米硅粒子比例增加所引入的较高微观结构无序度和较多缺陷将会导致薄膜低能吸收区吸收系数增加。