Controlled growth of filamentary crystals and fabrication of single-crystal whisker probes

The growth of filamentary crystals (whiskers) on a single-crystal substrate through the vapour-liquid-solid mechanism is described. The possibility of fabricating oriented systems of whiskers on the basis of this mechanism of crystal growth is noted. A phenomenon that is important for nanotechnology is noted: the existence of a critical diameter of whiskers, below which they are not formed. The phenomenon of radial periodic instability, which is characteristic of nanowhiskers, is described and the ways of its elimination are shown. The possibility of transforming whiskers into single-crystal tips and the growth of crystalline diamond particles at their apices are noted as important for practice. Possible applications of systems of whiskers and tips are described briefly. Particular attention is paid to the latest direction in whisker technology—fabrication of single-crystal whisker probes for atomic force microscopy.     

Complex Formation of Homoionic Montmorillonites with Propylene Carbonate and Osmotic Swelling in Aqueous Electrolyte Solutions

Abstract

Homoionic montmorillonites saturated with several kinds of cations can form complexes with propylene carbonate (PC) by intercalation. The PC molecules interact with the interlayer cations by way of H₂O molecules between the silicate layers. The PC-montmorillonite complexes exhibit osmotic swelling even in aqueous electrolyte solutions. This is interpreted in terms of the formation of thick electric double-layers consisting of PC and H₂O between the silicate layers. Although the PC molecules are gradually hydrolyzed to propylene glycol between the layers, the hydrolyzed complex also shows a similar swelling power in aqueous electrolyte solutions.     

Fabrication and characterization of GaAs quantum well buried in AlGaAs/GaAs heterostructure nanowires

We developed a growth method for forming a GaAs quantum well contained in an AlGaAs/GaAs heterostructure nanowire using selective-area metal organic vapor phase epitaxy. To find the optimum growth condition of AlGaAs nanowires, we changed the growth temperature between 800 and 850 °C and found that best uniformity of the shape and the size was obtained near 800 °C but lateral growth of AlGaAs became larger, which resulted in a wide GaAs quantum well grown on the top (1 1 1)B facet of the AlGaAs nanowire. To form the GaAs quantum well with a reduced lateral size atop the AlGaAs nanowire, a GaAs core nanowire about 100 nm in diameter was grown before the AlGaAs growth, which reduced the lateral size of AlGaAs to roughly half compared with that without the GaAs core. Photoluminescence measurement at 4.2 K indicated spectral peaks of the GaAs quantum wells about 60 meV higher than the acceptor-related recombination emission peak of GaAs near 1.5 eV. The photoluminescence peak energy showed a blue shift of about 15 meV, from 1.546 to 1.560 eV, as the growth time of the GaAs quantum well was decreased from 8 to 3 s. Transmission electron microscopy and energy dispersive X-ray analysis of an AlGaAs/GaAs heterostructure nanowire indicated a GaAs quantum well with a thickness of 5−20 nm buried along the 〈1 1 1〉 direction between the AlGaAs shells, showing a successful fabrication of the GaAs quantum well.     

SiO2纳米线/纳米颗粒复合结构的制备及光致发光性能研究

采用热蒸发法制备了非晶SiO2纳米线/纳米颗粒复合结构,确定了非晶SiO2纳米线、微米颗粒及SiO2纳米线/纳米颗粒复合结构生长的工艺条件,并利用XRD、SEM、Raman、PL光谱等技术手段分析表征样品.实验结果表明,在不同的沉积温度范围内,生长样品的形貌和结构不同;SiO2纳米线/纳米颗粒复合结构的发光区与Si衬底明显不同,主要集中在黄绿光范围.     

A new technique for multilayer LPE

A novel technique is presented which allows LPE deposition of a large number of layers without the disadvantages arising from sliding container parts. Solutions of different compositions or containing different dopants are arranged in separate chambers. By rotation of the whole device the solutions move from one chamber to the next one without intermixing, and by controlled cooling, epitaxial layers are deposited onto the substrates fixed at the chamber walls. The sequence of the layers is determined by the sense and the angles of rotation, and the thickness of each layer is determined, among many other factors, by the cooling rate and dipping time. Up to 15 layers of III–V compounds have so far been produced in a double-screw device, with thicknesses between 0.1 to 10 μm and thickness reproductivity of about 10%. In addition, the fabrication of LED's, III–V lasers, and solar cells, and possibly superlattice devices by the MultiLPE technique can be considered.     

Liquid-phase epitaxy of AlGaAs heterostructures on profiled substrates

A method of fabrication of planar local structures using the selective epitaxial growth of GaAs and AIGaAs layers from liquid phase on profiled GaAs substrates was developed. The planar regrowth of the recesses formed in GaAs substrates by local etching was performed using the anisotropy of epitaxial growth rates and also by providing the uniformity of mass flow to the surface of local epilayer. The developed method of localized structures fabrication was used for improving the characteristics of discrete light emitting diodes — LED and for fabrication of DLE monolithic arrays.     

Coloured ANOF layers on aluminium

By addition of metal salts to a basic electrolyte as colouring components coloured ANOF layers can be produced. The colour shades of such layers are dependent on concentration and deposition parameters. Two application examples of coloured ANOF layers are demonstrated.     

Characterisation of structures grown by Movpe using x-ray diffraction

This paper describes the double and triple axis diffractometers available for characterising heteroepitaxial layer structures in the laboratory environment. The relative merits of the techniques are described.To demonstrate the materials information available from the diffractometer data (usually referred to as rocking curves) examples are given of the analysis of layer thickness and lattice parameter values for single heteroepitaxial layers, two layer structures and multiquantum well structures. For layers with a defect density of less than 10⁵ per cm² detailed analysis of rocking curve data is carried out by comparing experimental results with simulated data.X-ray diffraction also offers a non-destructive means of fully characterising the state of relaxation of strained layer systems.     

Synthesis of nanostructured zirconia by anodization at low potentials

This work reports the synthesis of nanostructured ZrO₂ by anodization of zirconium electrodes at potentials well below the range of 10–50 V used as a standard procedure. Zirconium was first anodized in a 1M (NH₄)₂SO₄ electrolyte and then further anodized in the presence of fluoride ions added to the electrolyte as NH₄F. The maximum potential applied to zirconium during the whole process was 1 V with respect to an Ag/AgCl (sat.) reference electrode. Amorphous films of ZrO₂ consisting of nanopores and nanowire‐like structures were produced with this low‐potential methodology.     

Formation of partially ordered organic planar systems based on the in situ control of their structural organization

The possibilities of significantly improving the quality of planar systems based on photoactive porphyrin–fullerene dyads, layers based on cytochrome c and cardiolipin, and lysozyme crystals and films using a complex of in situ X-ray methods and simulation are described. The potential of X-ray phase-sensitive and surface-sensitive methods developed by M.V. Koval’chuk and researchers from his school in monitoring all stages of synthesis of partially ordered organic structure is demonstrated. This approach shows its efficiency for in situ studies: starting from the formation of complexes in solutions up to the growth of protein films and crystals.     

LPE growth and characterization of InP/InGaAsP ridge-waveguide lasers at 1.3 μm-wavelength

This paper presents the fabrication and the lasing characteristics of 1.3 μm-wavelength ridge-waveguide laser. The epitaxial material used in this study was grown applying the step-cooling technique of liquid phase epitaxy (LPE). The growth conditions for InGaAsP layers lattice-matched to the (001) InP-substrate are reported for lattice compositions corresponding to photoluminescence peak wavelengths of 1.07, 1.14, and 1.31 μm. We have used a conventional multiple-bin sliding boat to grow the LPE layers and a second apparatus for achieving batches of melts of uniform compositions. In the LPE apparatus the various batch melts (In–Sn In–Zn; In–Ga–As of different composition) were saturated with phosphorus using the seed dissolution technique. The epitaxial layers were grown by a single phase technique at a constant temperature. This LPE growth technique is useful for the fabrication of double-heterostructure wafers with an uniform alloy composition and a well-defined layer thickness. Using these epitaxial materials, metal-clad ridge-waveguide (MCRW) lasers have been prepared with stripe widths of 3.5 μm. CW threshold currents of 18 mA at room temperature are achieved for 200 μm long cavities. These lasers have T₀ values ranging from 50 to 70 K and well linear L-I-characteristics.     

MOVPE growth of GaInP/GaAs hetero-bipolar-transistors using CBr4 as carbon dopant source

Carbon doping of GaAs with carbon tetrabromide (CBr₄) in low pressure MOVPE has been investigated and applied to the fabrication of GaInP/GaAs HBTs. Especially the hydrogen incorporation and the associated acceptor passivation has been studied. The hydrogen found in single GaAs:C layers is predominantly incorporated during cooling the sample under AsH₃ after growth, n-Type capping layers can block this H indiffusion and GaAs:C base layers in HBTs show much lower H concentrations than GaAs:C single layers without a cap. A further reduction of acceptor passivation is possible by optimization of the growth procedure. First HBTs processed from layers with a base that was doped using CBr₄ show promising DC and HF performance (β = 45, for 2 × 20 μm² devices).     

Silicon nanowire growth on poly‐silicon‐on‐quartz substrates formed by aluminum‐induced crystallization

The vertical growth of Si nanowires on non‐monocrystalline substrates is of significant interest for photovoltaics and other energy harvesting applications. In this paper, we present results on using poly‐Si layers formed by aluminum‐induced crystallization (AIC) on fused quartz wafers as an alternative substrate for the vapor‐liquid‐solid (VLS) growth of vertical Si nanowires. Oxidation of the Al surface to Al₂O₃ before the a‐Si deposition was shown to be a key requirement in the formation of the poly‐Si template since it promotes the crystallization of the a‐Si into Si(111) which is required for vertical silicon nanowire growth. The effect of Al deposition technique (DC sputtering versus thermal evaporation) on a‐Si crystallization and Si nanowire growth was investigated. The use of Al thermal evaporation yielded AIC poly‐Si layers with the highest fraction of 〈111〉 grains as measured by orientation imaging microscopy (OIM) which enabled the growth of vertical Si nanowires. Cross‐sectional transmission electron microscopy analysis confirmed that the 〈111〉 Si nanowires grew epitaxially off of {111}poly‐Si grains in the AIC layer. This study demonstrates the potential of using AIC poly‐Si as a template layer for the vertical growth of silicon nanowires on amorphous substrates.     

Electrical properties of photoanodically generated thin oxide films on n-GaN

The characteristics of photoelectrochemically (PEC) generated gallium oxide films on n-GaN using an 0.002 M KOH electrolyte are described. The chemical composition of the resistive layers was analyzed by Auger electron spectroscopy. The DC and HF characteristics of Al/Ti/PEC-Ga₂O₃ (gallium sesquioxide)/GaN structures were studied with current–voltage and capacitance–voltage measurements, respectively. Under reverse bias we found extremely low leakage currents (<10⁻⁸ Acm⁻² at −15 V) and a very low interface state density; high-temperature operation (up to 166°C tested) motivates the integration of the described dielectric layer forming technique into GaN based device process schemes. Our method may also be employed as gate recess technology.     

Preparation and Characterization of MBE-grown Si/CaF2 Heterostructures

Growth and fabrication of silicon-on-insulator structures, based on heteroepitaxial growth of insulating films on Si, is an area of research that has rapidly developed in recent years. Thin CaF₂ films and Si/CaF₂ multilayers were prepared on {111}-oriented Si substrates by molecular beam epitaxy (MBE) and were investigated by RHEED and RBS. For epitaxial growth the Si substrates were cleaned using the ultraviolet/ozone surface cleaning method which is an effective tool to remove contaminants from the surface by low-temperature in-vacuo preheating. The growth of CaF₂ on such Si{111} substrates provides epitaxial layers with a high structural perfection. When this layer system, however, is used as a substrate for epitaxial Si growth the Si layers show always twin formation. Si layers without twins could be obtained only after deposition of thin amorphous Si buffer layers at room temperature, immediately followed by Si growth at 700 °C.     

Ion transport through LiCoO2/(Li2O)x · (B2O3) thin-film bi-layers

e-Gun deposited bi-layer structures of LiCoO₂/(Li₂O)_x · (B₂O₃) cathode material and solid electrolyte have been investigated by SEM and impedance spectroscopy. The crystallinity of the substrates, on which the structures are deposited, influences the extent of crystallization, and consequently the ionic conductivity, of the electrolyte layer. Comparison of impedance spectra between bi-layers and electrolyte single layers indicates that the total ionic conductivity of the electrolyte in the bi-layer is decreased with respect to the single layer, due to higher crystallization.     

Improved Spatial Resolution in Electron Probe Microanalysis of Multilayer Structures

A method is described to measure composition profiles of layer systems on substrates by electron probe microanalysis with an improved effective spatial resolution down to 0.1 μm. The resolution problem in microanalysis of angle lapped layers and the related experimental problems are discussed and examples of composition profile measurements at GaAs-Al_xGa_1–xAs double-heterostructures are presented.     

Epitaxial growth of ferroelectric oxide films

This review paper begins with a brief overview of the most common ferroelectric materials, the perovskites and the Aurivillius families. The epitaxial growth of ferroelectric epitaxial films can be a viable approach to improve the ferroelectric properties, in particular for the layered perovskites. Defects related to a polycrystalline structure, which lead to a degradation of ferroelectric properties like remanent polarisation, piezoelectric coefficient, charge retention, and may cause time-dependent fatigue problems, can be prevented. However, it is also to be considered that effects connected with thin films like substrate clamping, strain or finite thickness may limit the film properties. Substitution of elements allows the adjustment of the film characteristics to the device function. Additionally, the orientation of the films can be controlled by the appropriate choice of the substrate, which is important due to the anisotropy of the ferroelectrics.The deposition methods commonly used for ferroelectric oxide layers are reviewed, particularly with regard to epitaxial growth. The conditions under which stoichiometric, crystalline growth can be obtained are described. The paper primarily focuses on the MO-CVD technique.Furthermore whether epitaxial growth of ferroelectric films occurs or not depends on several conditions like lattice mismatch between film and substrate, surface orientation and crystal symmetry of the substrate, thermal expansion of film and substrate. The influence of these parameters on epitaxial growth is discussed. Local epitaxial growth of ferroelectric layers on metallic electrodes is also mentioned due to its importance in device fabrication. The site-engineering concept is shortly reviewed as the substitution of elements constitutes a simple way to modify film properties in thin film technology.     

Plasma‐enhanced chemical vapor deposition of 99.95% 28Si in form of nano‐ and polycrystals using silicon tetrafluoride precursor

The process of plasma chemical deposition of silicon was investigated from its tetrafluoride containing 99.99% of ²⁸Si isotope in the form of thin layer of nano‐crystalline silicon on silicon substrate and of thick layer of polycrystalline silicon on the inner surface of quartz reactor. The layers are characterized by the methods of X‐ray diffraction and Raman spectroscopy. Using the SIMS method the mechanism of isotopic dilution was investigated in the PECVD process (the content of ²⁸Si isotope in layers was 99.95‐99.98%). A necessity is indicated in thorough special preparation of the reactor for minimization of isotopic dilution in case of fabrication of silicon containing ≥99.9% of ²⁸Si. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A review of nanowire growth promoted by alloys and non-alloying elements with emphasis on Au-assisted III–V nanowires

Seed particles of elements or compounds which may or may not form alloys are now used extensively in promoting well-controlled nanowire growth. The technology has evolved following the well-known Vapour–Liquid–Solid (VLS) model which was developed over 40 years ago. This model indicates that a liquid alloy is formed from the seed particle and the growth precursor(s), resulting in crystal growth by precipitation from a supersaturated solution. The enhanced growth rate compared to the bulk growth from the vapour is typically attributed to preferential decomposition of precursor materials at or near the particle surface. Recently, however, there has been much interest in further developing this model, which was developed for Au-assisted Si whiskers (with diameter on the micrometre scale), in order to generally describe particle-assisted growth on the nanoscale using a variety of materials and growth systems. This review discusses the current understanding of particle-assisted nanowire growth. The aim is first to give an overview of the historical development of the model, with a discussion of potential growth mechanisms. In particular, the enhancement of growth rate in one dimension due to preferential deposition at the particle–wire interface will be discussed. Then, the particular example of III–V nanowires grown by metal–organic vapour phase epitaxy using Au particles will be revised, with details of the various growth processes involved in this system. The aim of this review is not to provide a conclusive answer to the question of why nanowires grow from seed particle alloys, but to describe the progress made towards this goal of a unified theory of growth, and to clarify the current standing of the question.