Silicon Cones Research Articles

Formation of conical silicon tips with nanoscale sharpness by localized laser irradiation

The formation of conical silicon tips with nanoscale sharpness as a result of single-pulse localized laser irradiation is presented in this work. A Q-switched neodymium doped yttrium aluminum garnet nanosecond-pulse laser, emitting at its fourth harmonic of 266 nm, and a mask projection technique were used to generate circular laser spots, several microns in diameter. The irradiation of silicon-on-insulator films was performed in ambient, vacuum, or argon atmospheres, with the resulting structures and underlying substrate examined via atomic force microscopy and scanning electron microscopy. The laser fluence range within which tip formation occurs is strongly dependent on the irradiated spot size. Within this range, the height of the resulting tip increases with the fluence level, while nearly preserving the aspect ratio. The formation mechanism of these structures is briefly discussed in view of these results and other, related published work.

Spiral nucleation of silicon solidified on heterogeneous surface of carbon nanocones

Molecular dynamics studies are carried out to examine the spiral nucleation mode of silicon solidified on the surface of carbon nanocones (CNCs). The silicon atoms are concentrated to form magic multilayered nanocones composed of coaxial and equidistant conical shells. Heterogeneous nucleation that occurred on CNCs follows a spiral nucleating mechanism. The structures of so-formed silicon cones show strict structural matching and strong structural relevance with CNCs. The uniform internal potential field around the CNCs is responsible for the formation of the coaxial conical shells.

Fabrication of precisely controlled silicon wire and cone arrays by electrochemical etching

There is an exponentially growing need for well-oriented, vertical silicon nano/micro-structure arrays, particularly in high-density integrated electronic devices. Here, we demonstrate that precisely controlled vertical arrays of silicon wires and cones can be fabricated by a combined treatment strategy of electrochemical and chemical etchings. First, a periodically ordered array of silicon wires was readily fabricated at microscale by simple electrochemical etching in which the current density played a critical role in determining the wire diameter and interspacing. The microstructures fabricated by electrochemical etching were more precisely tuned by further chemical etching, thereby transforming into cone arrays with extremely sharp tips where the cone height was controlled by the etching time. This approach could have broad utility in many electronics requiring miniaturization and high-density integration such as field emitters, photovoltaic and thermoelectric devices.

Nanostructure Characterization of Conical Silicon Carbide Nanowires and Heterostructures Induced by Release Catalysis

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Morphology and Wettability Control of Silicon Cone Arrays Using Colloidal Lithography

In this paper we present a simple method to fabricate ordered silicon cone arrays with controllable morphologies on a silicon substrate using reactive ion etching with two-dimensional silica colloidal crystals as masks. The etching process and the morphologies of the obtained structure are quantified. Unlike works reported previously, we show that the surface roughness of the obtained silicon cone arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the behavior of water droplets when being used as antireflection coatings with superhydrophobicity. Moreover, this strategy is compatible with the methods we have established on controlling the arrangement of colloidal spheres, and thus silicon cone arrays with tunable periodicities, different lattice structures, and various patterns can be prepared. The obtained silicon cone arrays with strips can be used as hydrophobic substrates with anisotropic dewetting just like the leaves of rice. It is found that by adjusting the strip width with and without silicon cones, the water droplets can transform from isotropic dewetting to anisotropic dewetting.

Cone kinetics model for two-phase film silicon deposition

We study quantitatively the morphology of nanocrystalline silicon (nc-Si:H) cones that appear during amorphous silicon (a-Si:H) film growth by chemical vapor deposition from hydrogen-diluted silane. The shapes of the nc-Si:H inclusions are found to be spherical cones, consistent with our “cone kinetics” model for silicon film growth. This model predicts cone development when growth is isotropic and there is sparse nucleation of a second material phase with a higher growth rate. Application of the cone kinetics model provides insights into the deposition of technologically important thin film materials, including protocrystalline silicon, highly P-doped nc-Si:H, silicon heterojunctions, and silicon epitaxy.

Laser Fabrication of Sharp Conical Microstructures on Si Thin Films by Nd:YAG Laser Single Pulse Irradiation

ABSTRACTConical microstructures with nanoscale sharpness form on silicon films as a result of single-pulse, localized UV irradiation using a solid-state, Q-switched Nd:YAG laser. Projection imaging of pinhole apertures was employed to obtain micron-sized irradiation spots on the surface of silicon-on-insulator samples. The formation of these structures requires melting of the silicon film and was followed at different laser fluence levels and irradiation spot sizes. Atomic force microscopy (AFM) was used to characterize the structures. After fabricating small arrays of such micro-cones, the silicon top layer was selectively etched away in order to understand the role of the underlying silicon oxide. AFM images of such etched samples revealed that the topography of the oxide material below the cones had been significantly modified: bumps with heights that represent a significant fraction of the original Si cone height have formed. This suggests that substrate melting plays an important role in the mechanism of formation of the silicon cones.

Field Emission Characteristics of AlN Coated Silicon Nanocone Arrays

Nanoscaled silicon cone arrays were formed on mirror-polished silicon wafers by plasma etching using hot filament chemical vapor deposition (HFCVD) system. A mixture of CH4 and H2 was introduced during silicon cone formation. AlN films were coated on Si cone arrays using radio frequency (RF) magnetron sputtering system. Scanning electrons microscopy (SEM) was employed to characterize the morphology of silicon cone arrays before and after AlN coating. The field emission characteristics of AlN coated silicon cone arrays，uncoated silicon cone arrays and AlN films were studied and compared, and the silicon cone arrays with AlN coating showed the best enhanced electron emission properties due to the negative electron affinity of AlN coating layer and the high aspect ratio of silicon cone. For AlN coated silicon cone arrays, a slight hysteresis between the upward and downward voltage sweeps was also observed and the field emission currents from AlN coated Si nanocone arrays decreased with the increase of the thickness of AlN films, which could be mainly attributed to the space charge buildup in AlN film with wide band gap.

Roles of microcrystalline silicon p layer as seed, window, and doping layers for microcrystalline silicon p-i-n solar cells

The roles of the hydrogenated microcrystalline silicon (μc-Si:H) p layer in the μc-Si:H p-i-n solar cell fabricated by plasma-enhanced vapor deposition are determined through evaluation of the photovoltaic characteristics of solar cells fabricated by varying the deposition time of p layer. Mechanisms of p-layer growth are analyzed with in situ Auger electron spectroscopy and ex situ Raman scattering spectroscopy. Each successive regime of film growth including an amorphous silicon layer, an incubation layer containing crystalline silicon nuclei, and a layer filled with conical crystalline silicon grains that evolves in the p-layer process leads to diverse changes in the crystalline development of the subsequent μc-Si:H i layer and in the characteristics of the solar cell.

Simulation of optical near and far fields of dielectric apertureless scanning probes

We study apertureless field enhancing optical probes beyond thespherical approximation in a smooth transition towards up to3 µm long conical silicon tips. Such tips are used in apertureless scanning near fieldoptical microscopy, which holds the promise of sub 10 nm lateral resolution. A fullythree-dimensional numerical solution to the Maxwell equations is obtained with themultiple multipole method giving simultaneously both near fields and scattered far fields.The significance of focused beam excitation for work with long tips is illustrated and therelative influence of relevant length scales such as tip length, excitation wavelength, andbeam waist radius is discussed. In the limit of vanishing tip apex radius, the near fieldgrows without bounds, whereas the far field remains finite. We verify that forsmall apex radii the near field confinement at the tip apex, which is related tothe achievable lateral resolution, scales with the inverse of the radius. We find,however, that long tips exhibit a markedly lower confinement than spherical or veryshort tips. Relevant for experimental studies, we demonstrate how scanning theexcitation field with long conical tips can be a useful technique for mapping the focalvolume. We show that the normalized near field at the tip apex is robustly tolerantagainst small misalignments or misorientations of illumination focus and tip apex.

Fabrication of ordered arrays of silicon cones by optical diffraction in ultrafast laser etching with SF6

We report the fabrication of ordered rows of conical spikes on a Si(111) substrate by etching with a femtosecond pulsed laser in the presence of SF6 gas. This is achieved by the creation of an optical near-field diffraction pattern when a copper wire (diameter ∼140 μm) is placed on the surface. Measurements of the height, base width and average separation of the silicon cones at two irradiation wavelengths (780 nm and 390 nm) confirm the important role of the optical parameters in the photochemical etching process. Moreover, the dependence of average cone separation on the laser repetition rate indicates that long-timescale processes such as post-pulse chemical interactions are important in cone formation.

Localization of light energy on the nanometer scale in a silicon cone

The spatial structure of light fields in a metallized cone filled with a medium with complex dielectric function was studied on the basis of the exact solution of the eigenwave problem. It is suggested that silicon can be used as a core of optical probe in the visible spectral region. It is shown that the density of light energy at the output of optical probe can be drastically increased if silicon is used instead of glass fiber.

Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses

We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF6 produces an absorptance of 0.9 for radiation in the 1.2–2.5 μm wavelength range, higher than any of the other gases. Use of Cl2, N2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat crystalline silicon, for which the absorptance is roughly 0.05–0.2 for a 260 μm thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration.

The Ups and Ups of Silicon

APPLIED PHYSICS The preparation of large areas of a substrate with periodic structures usually requires several steps of processing and patterning and can be a time-consuming part of the fabrication process. Removing the need for a patterning process altogether, Wysocki et al . show that a self-assembled layer of silica microspheres can be used to form a periodic array of silicon cones in a single step. The microspheres focus light from a single shot of a high-power laser onto the silicon surface and melt it. As the molten silicon cools, it begins to solidify, starting farthest away from the focal point directly under the microspheres and then toward the center. Because of the thermal properties of silicon, the volume of silicon expands as it cools, and so the center region is pushed up into a well-defined cone shape. The technique should prove particularly useful for the fabrication of field emission displays. — ISO Appl. Phys. Lett. 82 , 692 (2003).

Single-step fabrication of silicon-cone arrays

A regular lattice of SiO2 microspheres on a quartz support is used as a microlens system for laser-induced single-step fabrication of arrays of silicon cones on a (100) Si surface. The experiments were performed with single-pulse 248 nm KrF laser radiation.

Fabrication and Field Emission of High-Density Silicon Cone Arrays

Fabrication and Field Emission of High-Density Silicon Cone Arrays

Control of shape of silicon needles fabricated by highly selective anisotropic dry etching

The process to fabricate a needle-shaped silicon crystal (silicon needle) has been developed. These silicon needles are fabricated by highly selective anisotropic dry etching, where the etching mask is oxygen precipitation in the silicon substrate. In order to apply these silicon needles to field emitters, the shape of the silicon needle should be controlled. This is because a high aspect ratio can lead to electric field enhancement around tips and a flared base of the silicon needle can lead to mechanical and thermal stability. In this article we studied how to control the shape of the silicon needles. We found that the silicon needles with a high aspect ratio could be obtained by either lowering the deposition rate of sidewall passivation film or increasing etching depth, unlike conventional silicon cones fabricated by isotropic dry etching. Furthermore, a high aspect ratio could induce a flared base of the silicon needle, since incident ions were reduced in the vicinity of the base due to a shadowing effect by the silicon needle with a high aspect ratio.

Mechanisms of visible photoluminescence from nanoscale silicon cones

We explore the origin of visible photoluminescence in nanoscale silicon cones fabricated by reactive ion etching in silicon-on-insulator substrates utilizing rough silver films as masks. Photoluminescence (PL) visible to the naked eye was observed after oxidation and annealing. Samples oxidized at 900 °C exhibit intense yellow/green photoluminescence centered at about 530 nm. Samples oxidized at 1000 °C luminesce in the red-to-infrared region with peak positions between 650 and 730 nm. Transmission electron microscopy characterization is employed to show that PL at 530 nm can be understood in terms of defect states, while the PL at 650–730 nm can be explained by a combination of defect state and quantum confinement effects.

Field-emission properties of multihead silicon cone arrays coated with cesium

Field emission from multihead silicon (Si) cones was substantially improved by cesium (Cs) coating. Increasing the Cs coating lowered the emission turn-on field (for 10 μA/cm2) from 25 V/μm to a saturated value of 13 V/μm, while the threshold field (for 10 mA/cm2) decreased by 30%, dropping from 27 V/μm for Si cones coated with 1.8 monolayers (ML) of Cs to a saturated value of 19 V/μm with 4.1 ML of Cs. The Cs-treated Si cones could give an emission current density that was three to ten times that delivered by bare Si cones. The work function reduced by a factor of 1.43 for Si cones coated with 4.9 ML of Cs with reference to the untreated Si cones. From the slope of Fowler–Nordheim plot, the field enhancement factor β was found to increase by a factor of 2.02 for Si cones coated with 2.5 ML of Cs and then reduce to 1.57 after the 4.9 ML of Cs deposition. Reduction of the factor β might occur because of a thick Cs layer, which could flatten the sharp cone features. Stability test showed that no current decay was observed at a current density of 0.8 mA/cm2 under a constant applied field of 16 V/μm during the 10 h investigation.

Multi-tip cones induced by ion-bombardment

Arrays of multi-tip silicon cones covering large areas of Si(1 0 0) substrates were fabricated using argon ion beam bombardment and nickel as a seeding material. Each cone head split into a number of needle tips with a lateral size of tens of nanometers. The morphology of the cones was studied with scanning electron microscopy (SEM). The transmission electron microscopy (TEM) analysis revealed nickel constituents on the needle tips. In terms of experimental observation the formation mechanism of the multi-tip cones was discussed. Studying the field electron emission properties of fabricated silicon multi-tips showed a turn-on field to be about 25 V/μm.