Synthesis, characterization and optical properties of multipod ZnO whiskers

Multipod ZnO whiskers were synthesized successfully by two steps: pulsed laser deposition (PLD) and thermal evaporation process. First, a thin layer of Zn films were deposited on Si(1 1 1) substrates by PLD. Then the whiskers grew on Zn-coated Si(1 1 1) substrate by the simple thermal evaporation oxidation of the metallic zinc powder at 900 °C in the air without any catalysts or additives. The pre-deposited Zn films by PLD on the substrate can promote the growth of ZnO multipod whiskers effectively. The as-synthesized ZnO whiskers were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the whiskers are highly crystalline with the wurtzite hexagonal structure. Room temperature photoluminescence (PL) spectrum of the whiskers shows a UV emission peak at ∼393 nm and a broad green emission peak at ∼517 nm, which was assigned to the near band-edge emission and the deep-level emission, respectively.     

MgO和Si上NbN超薄薄膜的生长研究

我们在单晶MgO(100)、Si(100)和SiOx/Si基片上成功生长了纳米厚度的超薄NbN薄膜,利用现代分析手段:X射线衍射(XRD)、透射电子显微镜(TEM)、原子力显微镜(AFM)等技术分析研究了所制备的超薄NbN薄膜的微观结构、厚度、表面界面情况等物理特性。研究表明,在MgO(100)基片上获得了外延生长的单晶NbN超薄薄膜,在Si(100)和SiOx/Si基片上获得的是多晶NbN超薄薄膜。厚度均约6nm左右。这些超薄薄膜的超导转变温度分别为:MgO上薄膜是14.46K,Si和SiOx上薄膜分别是8.74K和9.01K.     

Photostimulated growth of whiskers in AgI-type superionic crystals

The growth of whiskers inside and on the surface of superionic crystals (AgI, CuI, RbAg₄I₅) is considered. The crystals are exposed to radiations with different spectral compositions at temperatures above and below the temperature of the superionic phase transition. The chemical composition, structure, and properties of whiskers are studied with optical microscopy, scanning electron microscopy, transmission electron microscopy, and photoluminescence. The mechanisms of photostimulated growth of whiskers in semiconductors with a high ionic conductivity are discussed, and the role of the “molten” cation sublattice during nucleation and growth of whiskers is considered.     

Morphological properties of AlN and GaN grown by MOVPE on porous Si(111) and Si(111) substrates

Metal Organic Vapour Phase Epitaxy (MOVPE) of AlN and GaN layers at a temperature of 1080 C were performed on porous Si(111) and Si(111) substrates. The thermal stability of porous silicon (PS) is studied versus growth time under AlN and GaN growth conditions. The surface morphology evolution of the annealed PS is revealed by scanning electron microscopy (SEM). Porous Si(111) with low porosity (40%) is more thermally stable than porous Si(100) with relatively high porosity (60%).AlN layers with various thicknesses were grown under the same conditions on the two substrates. Morphological properties of AlN were studied by atomic force microscopy (AFM) and compared taking into account the two different surfaces of the substrates. The two growth kinetics of AlN were found to be different due to the initial surface roughness of the PS substrate. The effect of AlN buffer morphology on the qualities of subsequent GaN layers is discussed. Morphological qualities of GaN layers grown on PS are improved compared to those obtained on porous Si(100) but are still less than those grown on Si substrate.     

GexSi1-x/Si应变层超晶格的分子束外延生长及其特性研究

在Si(100)衬底上用分子束外延在不同的温度下生长了不同组份的Ge_xSi_1-x/Si应变层超晶格。用反射式高能电子衍射、X射线双晶衍射、卢瑟福背散射、透射电子显微镜以及Raman。散射等测试方法研究了Ge_xSi_1-x/Si超晶格的生长及其结构特性。结果表明,对不同合金组份的超晶格,其最佳生长温度不同。x值小,生长温度高;反之,则要求生长温度低。对于x为0.1—0.6,在400—600℃的生长温度范围能够长成界面平整、 关键词：     

Epitaxial crystal growth of gadolinium on tungsten

Field electron microscopy is used to measure activation energies for multilayer diffusion of gadolinium over several different surfaces of tungsten and to prepare crystal layers of gadolinium by epitaxy on tungsten substrates. Nucleation, crystal growth and epitaxial relations are described.     

Thermal and kinetic surface roughening studied by scanning tunneling microscopy and atomic force microscopy

Recent studies of thermal roughening on Si surfaces and kinetic roughening of some growing films, copper and tungsten, by using scanning tunneling microscopy and atomic force microscopy are reviewed. A logarithmic divergence of the surface height fluctuations of Si(111) vicinal surfaces is confirmed, in agreement with the theoretical prediction of rough surface in thermal equilibrium. For the kinetically formed rough surfaces, power law dependences of the interface width on the system size are clearly observed. Furthermore, the tungsten films show a short-range scaling regime and a long-range “smooth” regime. The roughness exponents α are compared with theoretical predictions: for the typical Cu electrode position condition (α=1/2), the exponent appears to be close to that found for local growth models, and for tungsten films (0.7～0.8), it is consistent with recent predictions for growth where surface diffusion is predominant.     

Cones with a dual structure formed on Ar+-bombarded Si surfaces

Transmission electron microscopy disclosed that cones formed on crystalline Si surfaces bombarded with a few keV Ar⁺ ions were of a dual structure inexplicable by the existing models of cone formation. The outer region of the cones was composed of unoriented crystallites of Si, whereas the inner region was a single crystal oriented in the 〈111〉 direction. The polycrystalline region was too thick to explain its formation in terms of ion-induced disordering of the monocrystalline phase, suggesting that the redeposition of sputtered Si atoms was deeply concerned in evolving the cones. It is supposed that the present cone evolution involved particle supply processes underlying the growth of Si whiskers from the vapor phase.     

Growth peculiarities during vapor–liquid–solid growth of silicon nanowhiskers by electron-beam evaporation

One-dimensional (1D) silicon (Si) nanostructures were grown by electron-beam evaporation catalyzed by gold nanoparticles on silicon substrates following the vapor–liquid–solid growth mechanism. We report three strikingly different growth morphologies of the 1D Si nanostructures and discuss their formation. The morphology of the silicon nanostructures strongly depends on gold layer thickness, annealing temperature before deposition and growth temperature during the deposition. The formation of nanoscale silicon features such as nanobelts, nanowires and nanowhiskers was observed. The nanoscale silicon features were characterized by transmission and scanning electron microscopy using imaging, diffraction and energy-dispersive X-ray spectroscopy, atomic force microscopy and UV micro-Raman spectroscopy. PACS 68.37.Lp; 68.70.+w; 78.30.-j; 81.15.Jj     

Strain-driven morphology of Si1-xGex islands grown on Si(100)

A study has been carried out on the morphology and structure of three-dimensional (3D) SiGe islands grown by molecular beam epitaxy (MBE) on Si(100) substrates. Samples of Si1-xGex alloys have been prepared to investigate the effects either of the alloy composition or of the growth temperature. Atomic force microscopy (AFM) evidenced the growth of 3D islands and transmission electron microscopy (TEM) demonstrated wetting layer growth on Si(100), independently on the deposition conditions. Energy dispersive spectroscopy (EDS) micro-analyses carried out on cross-sections of large Si1-xGex islands with defects allowed a measurement of the Ge distribution in the islands. To the best of our knowledge, these have been the first experimental evidences of a composition change inside SiGe islands. The interpretation of the experimental results has been done in terms of strain-enhanced diffusion mechanisms both of the growing species (Si and Ge) and of small islands.     

Ti-assisted β-SiC nanowhiskers by pyrolysis of PTFE: synthesis and mechanical properties

Large quantities of Ti-catalyzed β-SiC nanowhiskers were efficiently prepared via a SHS process by pyrolysis of poly (tetrafluoroethylene) (PTFE). The as-synthesized β-SiC whiskers appear to be structurally uniform with width from 50?nm to 2?μm and length of up to several hundred microns, and they are single crystalline in nature. Results demonstrated that Ti particles could significantly accelerate the absorption and dissolving of Si- and C-related fragments. A Ti–Si–C alloy droplet was detected at the end of an individual β-SiC whisker, which indicated that β-SiC whiskers were grown via the vapor–liquid–solid (VLS) growth mechanism. The elastic bending modulus of individual whiskers was measured by an in-situ transmission electron microscopy (TEM) process; the average value of the elastic bending modulus of individual as-synthesized whiskers was 554?GPa.     

AuPd CATALYTIC NANOPARTICLE SIZE EFFECT ON THE FORMATION OF AMORPHOUS SILICON NANOWIRES

Amorphous silicon (a-Si) nanowires have been prepared on SiO₂/Si substrates by AuPd nanoparticles / silane reaction method. Field-emission scanning electron microscopy and transmission electron microscopy were used to characterize the samples. The typical a-Si nanowires we obtained are of a uniform diameter about 20 nm and length up to several micrometers. The growth mechanism of the nanowires seems to be the vapor-liquid-solid mechanism. The catalytic particle size effect on the formation of the nanowires and the cause of forming amorphous state Si nanowires are discussed.     

Epitaxial growth of high quality β-FeSi2 layers on Si(1 1 1) under the presence of an Sb flux

β-FeSi₂ layers were grown on Si(1 1 1) substrates by Fe deposition and simultaneous reaction with Si under the presence of an Sb flux. High quality epitaxial layers were obtained at the substrate temperature of 650°C with smooth interface between the reactive resultant β-FeSi₂ layers and Si substrates, in comparison to the layers grown by conventional reactive deposition epitaxy (RDE). Sb/Fe flux ratio dependence of the structural property was examined using X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Photoluminescence spectra were also measured at low temperature.     

Formation of CuO nanowires by thermal annealing copper film deposited on Ti/Si substrate

Nanowires of various inorganic materials have been fabricated due to the realization of their applications in different fields. Large-area and uniform cupric oxide (CuO) nanowires were successfully synthesized by a very simple thermal oxidation of copper thin films. The copper films were deposited by electron beam evaporation onto Ti/Si substrates, in which Ti film was first deposited on silicon substrate to serve as adhesion layer. The structure characterization revealed that these nanowires are monoclinic structured single crystallites. The effects of different growth parameters, namely, annealing time, annealing temperature, and film thickness on the fabrication of the CuO nanowires were investigated by scanning electron microscopy. A typical procedure simply involved the thermal oxidation of these substrates in air and within the temperature range from 300 to 700 °C. It is found that nanowires can only be formed at thermal temperature of 400 °C. It is observed that the growth time has an important effect on the length and density of the CuO nanowires, whereas the average diameter is almost the same, i.e.50 nm. Different from the vapor-liquid-solid (VLS) and vapor-solid (VS) mechanism, the growth of nanowires is found to be based on the accumulation and relaxation of the stress.     

Si/SiO<Subscript>2</Subscript> core shell clusters probed by Raman spectroscopy

Using a pulsed microplasma source, clusters were produced through the ablation of a Si cathode and successive supersonic expansion. The Si cluster beam was deposited onto different substrates and the partial oxidation of the cluster surface avoided the growth of large agglomerates, preserving their nanocrystalline morphology. Micro-Raman spectroscopy was used for an accurate size diagnosis of the deposited nanoparticles. The size of the Si dots ranges between 2 and about 15 nm. The Si dots appear to have a Si oxide shell, as confirmed also by structural and compositional analysis through transmission electron microscopy and atomic force microscopy. Double Raman peaks were attributed to small Si agglomerates having a thin substoichiometric Si-O interface.     

电场中不同能量密度激光烧蚀制备纳米硅晶粒分布特性

在室温和10 Pa氩气环境中,引入平行于靶面方向的直流电场,通过改变脉冲激光能量密度烧蚀单晶硅靶,在与羽辉轴线呈不同角度的衬底上沉积纳米硅晶薄膜。利用扫描电子显微镜和拉曼散射谱对沉积样品进行分析,结果表明：随着激光能量密度的增加,位于相同角度衬底上的晶粒尺寸和面密度逐渐变大;在同一激光能量密度下,零度角处衬底上的晶粒尺寸和面密度最大,且靠近接地极板处的值比与之对称角度处略大。通过朗缪尔探针对不同能量密度下烧蚀羽辉中硅离子密度变化的诊断、结合成核区内晶粒成核生长动力学过程,对晶粒分布特性进行了分析。     

电场中不同能量密度激光烧蚀制备纳米硅晶粒分布特性

在室温和10 Pa氩气环境中,引入平行于靶面方向的直流电场,通过改变脉冲激光能量密度烧蚀单晶硅靶,在与羽辉轴线呈不同角度的衬底上沉积纳米硅晶薄膜。利用扫描电子显微镜和拉曼散射谱对沉积样品进行分析,结果表明:随着激光能量密度的增加,位于相同角度衬底上的晶粒尺寸和面密度逐渐变大;在同一激光能量密度下,零度角处衬底上的晶粒尺寸和面密度最大,且靠近接地极板处的值比与之对称角度处略大。通过朗缪尔探针对不同能量密度下烧蚀羽辉中硅离子密度变化的诊断、结合成核区内晶粒成核生长动力学过程,对晶粒分布特性进行了分析。     

Fabrication of aligned carbon nanotubes on Cu catalyst by dc plasma-enhanced catalytic decomposition

Aligned multi-walled carbon nanotubes (ACNTs) are deposited using copper (Cu) catalyst on Chromium (Cr)-coated substrate by plasma-enhanced chemical vapor deposition at temperature of 700 °C. Acetylene gas has been used as the carbon source while ammonia is used for diluting and etching. The thicknesses of Cu films on Cr-coated Si (100) substrates are controlled by deposition time of magnetron sputtering. The growth behaviors and quality of ACNTs are investigated by scanning electron microscopy (SEM) and transmission electron microscopy. The different performance of ACNTs on various Cu films is explained by referring to the graphitic order as detected by Raman spectroscopy. The results indicate that the ACNTs are formed in tip-growth model where Cu is used as a novel catalyst, and the thickness of Cu films is responsible to the diameter and quality of synthesized CNTs.     

Annealing effects on the microstructure of sputtered gold layers on oxidized silicon investigated by scanning electron microscopy and scanning probe microscopy

The structure of Au layers deposited by sputtering on oxidized p-type Si(100) substrates is investigated by a combination of scanning electron microscopy and scanning probe microscopy. The effect of the temperature on the grain structure of the layers has been determined, revealing that an annealing temperature of 300° C results in a larger grain size and smoother surfaces but generates some cracks in the film surface. At an annealing temperature of 500° C, further grain growth is observed, but a high density of cracks and voids also results while there is little enhancement regarding the smoothness of the grain surfaces.     

Synthesis of GaN nanowires by Tb catalysis

Rare earth metal seed Tb was employed as catalyst for the growth of GaN wires. GaN nanowires were synthesized successfully through ammoniating Ga₂O₃/Tb films sputtered on Si(1 1 1) substrates. The samples characterization by X-ray diffraction and Fourier transform infrared indicated that the nanowires are constituted of hexagonal wurtzite GaN. Scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy showed that the samples are single-crystal GaN nanowire structures. The growth mechanism of the GaN nanowires is discussed.