Selective formation of silicon nanowires on pre-patterned substrates

In this paper, the selective growth of silicon nanowires (SiNWs) was studied. With the aid of photolithography, the vertically aligned silicon nanowires were selectively formed on the patterned substrates via an electroless metal deposition (EMD) method under normal conditions (room temperature, 1 atm). Low-pressure chemical vapor deposition (LPCVD) silicon nitride was used as the masking layer for SiNWs preparation. The scanning electron microscope was used to examine the etching results. Both the patterned and the unpatterned silicon substrates were used for study. The results indicated that the growth rates of the SiNWs upon the patterned and the unpatterned substrates are different. For the patterned substrates, the growth rate of SiNWs is dependent upon the pattern shape. The influence of length-to-width ratio for the rectangular-shaped patterns was studied. It is concluded that by designing the proper length-to-width ratio, the nanowires with different lengths can be fabricated simultaneously on the same substrate.     

Electric current-induced giant electroresistance in La_(0.36)Pr_(0.265)Ca_(0.375)MnO_3 thin films

The electroresistance(ER) of La_(0.36)Pr_(0.265)Ca_(0.375)MnO_3(LPCMO) epitaxial thin film was studied under various dc currents.The current effect was compared for the unpatterned film and patterned microbridge with a width of 50 μm.The value of ER in the unpatterned LPCMO film could reach 0.54 under a 1-mA current,which is much higher than ER under 1 mA for the patterned weak phase-separated La_(0.67)Ca_(0.33)MnO_3 and La_(0.85)Sr_(0.15)MnO_3 microbridges with 50-μm width.More interestingly,for the patterned LPCMO microbridge,the maximum of ER can reach 0.6 under a small current of 100 μ.A.The results were explained by considering the coexistence of ferromagnetic metallic phase with the charge-ordered phase,and the variation of the phase separation with electric current.     

Magnetization and polarized neutron reflectivity experiments on patterned exchange bias structures

Patterned Co/CoO thin film structures have been investigated by magnetization and polarized neutron reflectivity measurements in order to study the influence of finite size and shape anisotropy effects on the magnetization reversal. An anomaly was found in the upper branch of the hysteresis loops, probably caused by incomplete bias in the patterned structures. The asymmetry in magnetization reversal mechanism commonly found in the two branches of the hysteresis loops of unpatterned Co/CoO layers is altered in the patterned structures, consistent with the existence of interfacial domains.     

Frequency-selective near-field radiative heat transfer between photonic crystal slabs: a computational approach for arbitrary geometries and materials

We demonstrate the possibility of achieving enhanced frequency-selective near-field radiative heat transfer between patterned (photonic-crystal) slabs at designable frequencies and separations, exploiting a general numerical approach for computing heat transfer in arbitrary geometries and materials based on the finite-difference time-domain method. Our simulations reveal a tradeoff between selectivity and near-field enhancement as the slab-slab separation decreases, with the patterned heat transfer eventually reducing to the unpatterned result multiplied by a fill factor (described by a standard proximity approximation). We also find that heat transfer can be further enhanced at selective frequencies when the slabs are brought into a glide-symmetric configuration, a consequence of the degeneracies associated with the nonsymmorphic symmetry group.     

Femtosecond laser processing of nitride-based thin films to improve their tribological performance

The ability of femtosecond laser pulses to pattern coated tribological surfaces in order to improve their wear behavior was investigated. Experiments were performed with a Ti:sapphire laser (wavelength: 800 nm, energy density: 2 J/cm², pulse duration: 100 fs) on TiN- and on TiCN-coated surfaces. Morphological analyses of the laser-treated surfaces were carried out and did not reveal any film delamination or other coating damage after laser processing. Tribological tests simulating rapidly increasing contact pressures under boundary friction were performed on both unpatterned and laser-patterned coated surfaces using a steel counter body. The patterned surfaces showed significantly better tribological performance with respect to stability and the value of the friction coefficient during testing. EDX analyses of the tested unpatterned samples revealed complete coating removal and material transfer from the counter body to the sample surface. In the case of the laser-patterned surfaces, only slight coating damage and an accumulation of debris from the steel counter body in the laser-induced pores were observed. PACS 42.62.Cf; 81.40.Wx     

Magnetization reversal in exchange biased Co/CoO patterns

The influence of patterning on exchange bias has been investigated using arrays of micron-sized Co/CoO dots with different lateral confinement and length-to-width ratio. The patterned samples show higher coercive and exchange bias fields than a continuous Co/CoO bilayer. As in unpatterned film, magnetization reversal mechanisms on opposite sides of the hysteresis loops of the microstructured samples are different. However, with the increase of lateral confinement and shape anisotropy of the dots, the asymmetry in the magnetization reversal starts to differ from that observed in continuous Co/CoO films.     

Fast pore etching on high resistivity n-type silicon via photoelectrochemistry

In this paper, five factors, namely the HF (hydrofluoric acid) concentration, field strength, illumination intensity as well as the oxidizing-power and conductivity of electrolytes were found to strongly affect the fast pore etching. The oxidizing power of aqueous HF electrolyte of different concentrations was especially measured and analysed. A positive correlation between optimal bias and HF concentration was generally observed and the relationship was semi-quantitatively interpreted. Pore density notably increased with enhanced HF-concentration or bias even on patterned substrates where 2D (two-dimensional) nuclei were densely pre-textured. The etch rate can reach 400μm/h and the aspect ratio of pores can be readily driven up to 250.     

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator layer

Effect of patterning on thermal agglomeration of ultrathin silicon-on-insulator (SOI) layer has been studied. A square-shaped 12 nm thick SOI layer was patterned by lithography and by selective etching with a KOH solution. The structural change by ultrahigh vacuum annealing in a temperature range of 900–1100 °C was observed by atomic force microscopy. The agglomeration takes place preferentially from the pattern edges at a lower annealing temperature than that for the unpatterned layer, indicating enhanced diffusion of Si atoms at the edges. Additionally, the patterning causes formation of smaller islands than those for the unpatterned layer, reflecting that the patterning limits the amount of Si atoms supplied for the island formation.     

Pulsed electrodeposition and magnetism of two-dimensional assembly of controlled-size Co particles on Si substrates

We present an extremely simple and inexpensive way to obtain controlled-size and density Co metallic particles on Si(1 1 1) using electrodeposition. When unpatterned substrates are used, the particle density and size can be controlled by adjusting the pulse frequency and the total deposition time. Randomly arranged cobalt particles with diameters of few tens of nanometres are obtained for short deposition times. Continuing the deposition, the particle size and density can be increased until coalescence. Magnetic force microscopy images show magnetically coupled/uncoupled particles depending on the size and distance between them. For small decoupled particles, no in-plane uniaxial anisotropy is found, in agreement with transmission electron microscopy observations which show randomly oriented single crystal particles. As the particle coalescence increases, the in-plane anisotropy evaluated from magnetization loops increases as well. When deposited on focused ion beam patterned substrates, well organized nanoparticles with adjustable magnetic anisotropy are obtained. Ferromagnetic resonance measurements performed on these samples reveal that the magnetic anisotropy originates mainly from the particle shape.     

High repetition rate laser ablation for vapor–liquid–solid nanowire growth

A high repetition rate (500 kHz) solid state laser was used for the ablation process in a Pulsed Laser Deposition (PLD)/Vapor–Liquid–Solid (VLS) growing. A ZnO target was ablated with laser powers between 0.6 W and 1.2 W, and a variable number of pulses per train and trains frequencies. ZnO structures were grown on gold patterned and unpatterned substrates surfaces. Enhanced growth of the nanostructures could be noticed on the catalyst patterned surfaces. Better nanowire morphologies were also observed for bigger number of laser pulses per train. The enhancements are more evident for low laser powers. Based on plume expansion investigations, by using a high speed camera and a Particle-In-Cell (PIC) Monte-Carlo based simulations, the nanostructure morphology variations could be understood on the basis of the plume particles diffusion process and thin film versus VLS growing competition.     

Improved light extraction of GaN-based light-emitting diodes with surface-patterned ITO

The surface patterning of the indium tin oxide (ITO) transparent current layer has been investigated to improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs). LEDs with periodic micro-hexagon patterned ITO have been fabricated utilizing standard lithography techniques and inductively coupled plasma (ICP) technology. The luminance intensity of the LED chips with patterned ITO following 160 s ICP etching was enhanced by about 50% compared to the LED chips with unpatterned ITO. Detailed processing parameters are provided. scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the micro-structures. The results indicate that the surface-patterned ITO technique could have potential applications in high-power GaN-based LEDs.     

Anomalous giant piezoresistance in AlAs 2D electron systems with antidot lattices

An AlAs two-dimensional electron system patterned with an antidot lattice exhibits a giant piezoresistance effect at low temperatures, with a sign opposite to the piezoresistance observed in the unpatterned region. We suggest that the origin of this anomalous giant piezoresistance is the nonuniform strain in the antidot lattice and the exclusion of electrons occupying the two conduction-band valleys from different regions of the sample. This is analogous to the well-known giant magnetoresistance effect, with valley playing the role of spin and strain the role of magnetic field.     

Direct photoetching of single crystal SiC by VUV-266 nm multiwavelength laser ablation

2 O₂:H₂O and HF:H₂O. The analysis of the etched samples by scanning electron microscopy (SEM) and scanning probe microscopy (SPM) indicates that an array of square holes having well-defined patterned structures and clean substrate surfaces were obtained. The X-ray photoelectron spectroscopy (XPS) analysis indicates that the SiC samples etched by VUV-266 nm multiwavelength laser have a similar stoichiometry after chemical post-treatment as the virgin SiC. The mechanism of high-quality ablation using VUV-266 nm multiwavelength laser is discussed in comparison with ablation using 266 nm single wavelength. The chemical post-treatment contributes to removing the residues from the laser photolysis of SiC. Received: 26 August 1996/Accepted: 17 October 1996     

Antireflection microstructures fabricated upon fluorine-doped SiO(2) films

Two-dimensional periodic structures were fabricated upon a fluorine-doped SiO(2) film in which the fluorine content changed gradually in the direction of film thickness. The films were deposited by plasma-enhanced chemical-vapor deposition. The film was periodically patterned into a 1-mum period and an ~1-mum -groove depth by inductive coupled plasma reactive-ion etching followed by chemical etching in a diluted HF solution. A surface reflectance of 0.7% was attained at 1.85-mum wavelength, a value that is one fifth as large as the 3.5% Fresnel reflection of a SiO(2) substrate with a flat surface.     

Altering the catalytic activity of thin metal catalyst films for controlled growth of chemical vapor deposited vertically aligned carbon nanotube arrays

The growth rate and terminal length of vertically aligned carbon nanotube arrays (VANTAs) grown by chemical vapor deposition have been dramatically improved through pulsed KrF excimer laser pretreatments of multilayer metal catalyst films. Silicon wafers coated with Al, Mo, and Fe layers were laser processed in air with single laser shots of varying fluence through circular apertures, then heated to ∼750°C and exposed to acetylene and ferrocene containing gas mixtures typically used to grow vertically aligned nanotube arrays. In situ videography was used to record the growth kinetics of the nanotube arrays in both patterned and unpatterned regions to understand the changes in catalytic activity, growth rates, and termination of growth. The height of the patterned regions varied with fluence, with the most successful treatment resulting in 1.4 cm tall posts of nanotubes embedded in a 0.4 cm tall nanotube carpet. High-resolution transmission electron microscopy images from the nanotubes in the posts revealed fewer walls, smaller diameters, and a much narrower distribution of diameters compared to the nanotubes grown in the carpet. This information, along with data obtained from weighing the material from each region, suggests that pulsed laser processing can also significantly increase the areal density of VANTAs. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy.     

SERS study of Ag nanoparticles electrodeposited on patterned TiO2 nanotube films

In this work, Ag nanoparticles (NPs) were deposited on patterned TiO₂ nanotube films through pulse‐current (PC) electrodeposition, and as a result patterned Ag NPs films were achieved. Scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X‐ray diffraction (XRD) were used, respectively, to study the morphology, uniformity, and phase structure of the patterned Ag NP films. The size and density of the as‐deposited Ag NPs could be controlled by changing the deposition charge density, and it was found that the patterned Ag NP films produced under a charge density of 2.0 C cm⁻² gave intense UV–vis and Raman peaks. Two‐dimensional surface‐enhanced Raman scattering (SERS) mapping of rhodamine 6G (R6G) on the patterned Ag NP films demonstrated a high‐throughput, localized molecular adsorption and micropatterned SERS effect. Copyright © 2010 John Wiley & Sons, Ltd.     

多孔硅制备的二维光子晶体生长过程监控

采用单槽电化学腐蚀法在预置有倒金字塔结构的n型单晶硅上制备用于二维光子晶体的多孔硅。利用基于LabVIEW的虚拟仪器技术对实验仪器编程,搭建实时测控系统,实现对反应过程中所需电源的控制,并且实时显示采集到的随时间变化的电压和电流信号,将采集的数据存储在计算机里。实验表明,恒电流供电模式下致使电压剧烈变动,导致多孔硅侧向...     

Transient-enhanced Si diffusion on natural-oxide-covered Si(0 0 1) nano-structures during vacuum annealing

The morphology of annealed patterned Si(0 0 1) wire templates was studied by several techniques. We found an enormous Si-mass transport on the Si surface at usual oxide desorption temperatures around 900°C under UHV conditions. Heat treatment of 5 min transforms the initially rectangular wire profiles with a height of 300 nm to flat (<100 nm) and faceted triangular ridges exhibiting thermodynamically preferred {1 1 1}- and {3 1 1}-facets.It was found that the natural SiO₂ on the predefined wire pattern must be responsible for the degradation of the wire structure. Removing the SiO₂ layer from the Si wires ex situ with an HF dip preserves the rectangular structures during high-temperature annealing. The Si–SiO₂ interface was investigated with high-resolution transmission electron microscopy to image the Si wire surface and the natural oxide layer in detail.     

Wettability and osteoblast cell response modulation through UV laser processing of nylon 6,6

With an ageing population the demand for cheap, efficient implants is ever increasing. Laser surface treatment offers a unique means of varying biomimetic properties to determine generic parameters to predict cell responses. This paper details how a KrF excimer laser can be employed for both laser-induced patterning and whole area irradiative processing to modulate the wettability characteristics and osteoblast cell response following 24 h and 4 day incubation. Through white light interferometry (WLI) it was found that the surface roughness had considerably increased by up to 1.5 μm for the laser-induced patterned samples and remained somewhat constant at around 0.1 μm for the whole area irradiative processed samples. A sessile drop device determined that the wettability characteristics differed between the surface treatments. For the patterned samples the contact angle, θ, increased by up to 25° which can be attributed to a mixed-state wetting regime. For the whole area irradiative processed samples θ decreased owed to an increase in polar component, γ^P. For all samples θ was a decreasing function of the surface energy. The laser whole area irradiative processed samples gave rise to a distinct correlative trend between the cell response, θ and γ^P. However, no strong relationship was determined for the laser-induced patterned samples due to the mixed-state wetting regime. As a result, owed to the relationships and evidence of cell differentiation one can deduce that laser whole area irradiative processing is an attractive technology for employment within regenerative medicine to meet the demands of an ageing population.     

Controlling the defect density to improve the output power of InGaN/GaN-based vertical light-emitting diodes by using substrates patterned with SiO2 lenses

In this study, we investigated the effect of SiO₂ lenses on the output power of InGaN/GaN-based vertical light-emitting diodes (VLEDs; wavelength = 445 nm) and compared the results to those of reference VLEDs without the SiO₂ lenses (planar samples). Arrays of SiO₂ lenses (pitch = 3 μm, width = 2.5 μm, height = 1.0 μm) were formed on c-plane sapphire substrates. The external quantum efficiency (EQE) of the packaged VLEDs with planar and patterned substrates was characterised. At 5 mA, the EQE of the patterned samples was 150% higher than that of the planar samples. A patterned, N-polar, n-GaN sample contained far fewer nanopipes (approximately 2.2 × 10⁵ cm^?2) than a planar n-GaN sample (approximately 2.4 × 10⁶ cm^?2). Furthermore, the patterned samples contained far fewer threading dislocations (approximately 1.0 × 10⁸ cm^?2) than the planar samples (approximately 5.0 × 10⁸ cm^?2). Scanning electron microscopy (SEM) images showed that the photoelectrochemical (PEC)-etched patterned samples contained cones that were 150% larger than that of the PEC-etched planar samples. In addition, SEM images, cathode luminescence measurements and finite-difference time-domain simulations were used to characterise the improved light output of the patterned samples.