Blue luminescent silicon nanocrystals prepared by short pulsed laser ablation in liquid media

The pulsed laser processing in liquid media is an attractive alternative to produce room temperature luminescent silicon nanocrystals (Si-ncs). We report on a blue luminescent Si-ncs preparation by using nanosecond pulsed laser (Nd:YAG, KrF excimer) processing in transparent polymer and water. The Si-ncs fabrication is assured by ablation of crystalline silicon target immersed in liquids. During the processing and following aging in liquids, oxide based liquid media, induce shell formation around fresh nanocrystals that provides a natural and stable form of surface passivation. The stable room temperature blue-photoluminescent Si-ncs are prepared with maxima located around ∼440 nm with corresponding optical band gap around ∼2.8 eV (∼430 nm). Due to the reduction of surface defects, the Si-ncs preparation in water, leads to a narrowing of full-width-half-maxima of the photoluminescence spectra.     

Self-organized silicon microcolumn arrays generated by pulsed laser irradiation

Cumulative nanosecond pulsed excimer laser irradiation of silicon produces an array of high-aspect-ratio microcolumns that protrude well above the initial surface. The growth of these microcolumns is strongly affected by the gas environment, being enhanced in air or in other oxygen-containing atmosphere. An array of very large and complex conical structures that also protrude above the surface is formed if the irradiation is performed in sulfur hexafluoride (SF₆). Kinetics studies of microcolumn growth show that: (i) A certain number of pulses is required to initiate growth of microcolumns; (ii) column nucleation is inhomogeneous, taking place always at the edges of deep grooves or pits; (iii) growth is fast with the earlier pulses but slows down to a halt when the columns reach a certain length. These studies show that columns nucleate and grow by continuous influx of silicon with each laser pulse. It is proposed that the axial growth of microcolumns and cones is due to the deposition of atoms or clusters at their tips. The column/cone tips are melted during irradiation and act as preferred sites for deposition, resulting in a very high axial growth rate. The contribution of etching and ablation to the flux of silicon-rich vapor produced during irradiation is discussed. The mechanism of columnar growth is compared with the vapor-liquid-solid method to grow silicon whiskers.     

Tip-based nanoscale selective growth of discrete silicon nanowires by near-field laser illumination

Growth of discrete silicon nanowires is reported with nanoscale location selectivity by employing near-field laser illumination. An uncoated dielectric atomic force microscope (AFM) tip provides a nanometer-scale light source that is sufficiently localized to induce nucleation and subsequent growth of a single nanowire under its optical near-field. Far-field laser-induced heating is additionally supplied to the substrate to both relieve the required near-field light budget and also assist stable epitaxial growth. Specific catalysts are selected for the nanowire growth by non-contact mode AFM imaging. Through real-time monitoring of the deflection of the AFM cantilever during the growth process, the gap between the tip and the sample and hence truly near-field illumination are maintained throughout the growth process. The study shows that tip-based near-field laser illumination could be an effective tool for the direct integration of semiconductor nanowires.     

Photovoltaic properties of black silicon microstructured by femtosecond pulsed laser

Prototype devices based on black silicon have been fabricated by microstructuring 250 μm thick multicrystalline n doped silicon wafers using femtosecond pulsed laser in ambient gas of SF₆ to measure its photovoltaic properties. The enhanced optical absorption of black silicon extends across the visible region and all the black silicons prepared in this work exhibit enhanced optical absorption close to 90% from 300 nm to 800 nm. The highest open-circuit voltage (V_oc) and short-circuit current (I_sc) under the illumination of He–Ne continuous laser at 632.8 nm were measured to be 53.3 mV and 0.11 mA, respectively at a maximum power conversion efficiency of 1.44%. Upon excitation with He–Ne continuous laser at 632.8 nm, external quantum efficiency (EQE) of black silicon as high as 112.9% has also been observed. Development of black silicon for photovoltaic purposes could open up a new perspective in achieving high efficient silicon-based solar cell by means of the enhanced optical absorption in the visible region. The current–voltage characteristic and photo responsivity of these prototype devices fabricated with microstructured silicon were also investigated.     

Nanosecond pulsed laser ablation of silicon in liquids

Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.     

Room temperature fabrication of silicon nanocrystals by pulsed laser deposition

Reaction of 10 nm gold nanoparticles (AuNPs) with a thiol-functionalized bipyridine copper(II) complex, Cu[(N-(6-mercaptohexyl)-2,2′-bipyridinyl-5-carboxamide)]Cl₂ (3), and (1-mercaptohex-6-yl)tri(ethylene glycol) (5) in different ratios resulted in mixed monolayer modified NPs with varying surface coverage of capping agent. The copper complex modified NPs were used for surface plasmon resonance (SPR) promoted homogeneous catalysis applied to the hydrolysis of the nerve agent methyl parathion (MeP) at pH 8.0. Low power green laser (532 nm) irradiation of solutions of modified AuNPs with MeP resulted in significant increase in the rate of phosphate ester hydrolysis which could not be attributed to a thermal process. Ratios of initial rates (laser/dark) at high substrate concentrations of MeP as a function of copper catalyst coverage were determined. A possible mechanism for catalytic enhancement involving dissociation of catalytically inactive hydroxy-bridged Cu(II) dimer is discussed.     

Plasma induced by pulsed laser and fabrication of silicon nanostructures

It is interesting that in preparing process of nanosilicon by pulsed laser, the periodic diffraction pattern from plasmonic lattice structure in the Purcell cavity due to interaction between plasmons and photons is observed. This kind of plasmonic lattice structure confined in the cavity may be similar to the Wigner crystal structure. Emission manipulation on Si nanostructures fabricated by the plasmonic wave induced from pulsed laser is studied by using photoluminescence spectroscopy.The electronic localized states and surface bonding are characterized by several emission bands peaked near 600nm and 700nm on samples prepared in oxygen or nitrogen environment. The electroluminescence wavelength is measured in the telecom window on silicon film coated by ytterbium. The enhanced emission originates from surface localized states in band gap due to broken symmetry from some bonds on surface bulges produced by plasmonic wave in the cavity.     

Optical properties of silicon microcolumn grown by nanosecond pulsed laser irradiation

We have investigated the optical properties of silicon pillars formed by cumulative nanosecond pulsed excimer laser irradiation of single-crystal silicon in vacuum created under different repetition rates. The changes in optical characteristics of silicon pillar were systematically determined and compared as the number of KrF laser shots was increased from 1 to 15,000.The results show that silicon pillar PL curves exhibit a blue band around 430 nm and an ultraviolet band peaking at 370 nm with the vanishing of the green emission at 530 nm. A correlation between the intensity of the blue PL band and the intensity of the Si-O absorption bands has been exploited to explain such emission, whereas, the origin of the ultraviolet band may be attributed to different types of defects in silicon oxide.     

Hyperdoping of silicon with deep-level impurities by pulsed YAG laser melting

Hyperdoping with deep-level impurity is a promising method to prepare intermediate band semiconductors. We prepared silicon hyperdoped with deep-level impurities, sulfur and titanium, by ion implantation followed by pulsed YAG laser melting. The processes of sulfur and titanium hyperdoping are comparatively studied. The amorphous sulfur and titanium ion-implanted layers changed to monocrystal by following pulsed laser melting. The depth profile of sulfur impurity after pulsed laser melting is similar to that of ion-implanted sample, while large segregation is observed for titanium hyperdoping. The crystallinity and degree of segregation depend on the laser shot number and initially implanted titanium dose. There is a trade-off between crystallinity and depth profile of impurity for titanium hyperdoping. From a viewpoint material processing, formation of high-quality silicon monocrystal hyperdoped with sulfur is easier than that with titanium. Correlation between the mid-infrared optical absorption and photoconductivity is also discussed for sulfur-hyperdoped sample.     

Nanostructuring of silicon surface by femtosecond laser pulse mediated with enhanced near-field of gold nanoparticles

High-energy synchrotron X-ray diffraction and imaging experiments were performed at the Advanced Photon Source on two ancient Chinese bronzes from the Art Institute of Chicago with the goal to nondestructively study their microstructure. The first object, a bronze fragment from an early Western Zhou dynasty vessel (Hu, 11th/10th century B.C.), was investigated with spatially-resolved diffraction to reveal the depth and composition of the surface corrosion layer as well as the composition and grain size of the underlying bronze core. The second object, a bronze dagger-axe (Ge, 3rd/2nd century B.C.) with a silver-inlaid sheath, was studied under both diffraction and imaging conditions. It was found to have been cast as a single object, answering longstanding scholars’ questions on whether the ceremonial object concealed an interior blade. PACS 81.00; 01.75.+m     

Experimental evidence of the vacancy-mediated silicon self-diffusion in single-crystalline silicon

We have determined silicon self-diffusivity at temperatures 735-875 degrees C based on the Raman shift of longitudinal optical phonon frequencies of diffusion annealed 28Si/30Si isotope superlattices. The activation enthalpy of 3.6 eV is obtained in such low temperature diffusion annealing. This value is significantly smaller than the previously reported 4.95 eV of the self-interstitial mechanism dominating the high temperature region T>855 degrees C and is in good agreement with the theoretical prediction for the vacancy-mediated diffusion. We present a model, containing both the self-interstitial and the vacancy terms, that quantitatively describes the experimentally obtained self-diffusivity between 735 and 1388 degrees C, with the clear crossover of the two diffusion mechanisms occurring around 900 degrees C.     

Optical characterization of thin silicon films deposited by CVD and annealed by pulsed laser

Polycrystalline Si layers 100 to 800 nm thick have been deposited on Si single crystal substrates by CVD and annealed with a Q-switched ruby laser at energies up to 1.5 J/cm². The optical characteristics of these layers have been measured by SEM and ellipsometry. The results can be attributed to a change in surface roughness with film thickness and laser energy.     

Defects in pulsed laser and thermal processed ion implanted silicon

The evolution of the nature and concentration of the defects produced by 100 or 300 keV As ions at fluences 1 to 4×10^–12 cm^–2 inn-type, Fz Silicon doped with 10¹⁵ to 10¹⁶ cm^–3 has been studied as function of thermal treatments (in the range 500°–900 °C) and of the energy density (in the range 0.3–0.6 J cm^–2) of a light pulse from a ruby laser (15 ns, 0.69 m). Deep-level transient spectroscopy (DLTS) combined with capacitance — voltage (C-V) measurements were used to get the characteristics (energy level, crosssection for the capture of majority carriers) of the defects and theirs profiles. The difficulties encountered in the analysis of the results, due to the large compensation of free carriers in the implanted region and to the abrupt defect and free carrier profiles, are discussed in detail and the corrections to apply on the C-V characteristics and the DLTS spectra are described. The defects resulting from the two types of treatments are found to be essentially the same. Only, for laser energies higher than 0.5 J cm^–2, the laser treatment appears to introduced new defects (atE0.32 eV) which should result from a quenching process. The fact that a laser energy smaller than the threshold energy for melting and recrystallization is able to anneal, at least partially, the defects produced by the implantation, demonstrates that the annealing process induced by the laser pulse is not a purely thermal process but is enhanced by a mechanism involving ionization.     

Generation of multiply charged silicon and germanium ions by excimer laser pulses

Plasma bursts were produced by focusing excimer-laser (XeCl, 308 nm) pulses on Ge and Si targets. At moderate laser fluences (30 MW/cm²) high-intensity Ge³⁺ and Si³⁺ ion pulses were extracted from the laser-produced plasma. A peculiar electrical circuit allows a self-bunching of the beam. By time-of-flight method, the currents produced by the ions of different charge number were measured. Peak currents of 620 mA and 800 mA were recorded for Ge³⁺ and Si³⁺ ions, respectively, with an extraction voltage of only 400 V.     

Femtosecond pulsed laser ablation of diamond-like carbon films on silicon

Femtosecond pulsed laser ablation (τ = 120 fs, λ = 800 nm, repetition rate = 1 kHz) of thin diamond-like carbon (DLC) films on silicon was conducted in air using a direct focusing technique for estimating ablation threshold and investigating the influence of ablation parameter on the morphological features of ablated regions. The single-pulse ablation threshold estimated by two different methods were ?_th(1) = 2.43 and 2.51 J/cm². The morphological changes were evaluated by means of scanning electron microscopy. A comparison with picosecond pulsed laser ablation shows lower threshold and reduced collateral thermal damage.     

Theoretical considerations regarding pulsed CO2 laser annealing of silicon

We present a calculation of the surface temperature and investigate the “thermal runaway” phenomenon during pulsed CO₂ laser (λ = 10.6 μm) annealing of silicon. In calculating the temperature variation of free carrier absorption in n-Si, we have taken into account acoustic deformation potential scattering, optical deformation potential scattering, and ionized impurity scattering. The deformation potentials are adjusted to fit the experimentally observed values at 300°K. Also, we discuss the contribution of free carrier absorption during annealing with a Nd:glass laser (λ = 1.06 μm).     

Barium ferrite films with in-plane orientation grown on silicon by pulsed laser deposition

For the first time, barium ferrite films with in-plane orientation were prepared at 700°C by pulsed laser deposition technique (PLD) on Si(1 1 1) without any post-annealing. An amorphous Ba–Fe–O film is used as underlayer to facilitate the crystallization and improve the orientation of films. Sharp (1 1 0) and (2 2 0) peaks appeared in the XRD pattern. The surface morphologies observed by SEM are similar to the typical computer generated grain arrangements obtained by Suzuki et al. in their micromagnetics study. Furthermore, a particular kind of structure of film cross-section was identified in PLD for barium ferrite films. The grain size is about 3 μm, and the coercivity is around 1500 Oe.     

Amorphous to crystalline phase transition in pulsed laser deposited silicon carbide

SiC thin films were grown on Si (1 0 0) substrates by excimer laser ablation of a SiC target in vacuum. The effect of deposition temperature (up to 950 °C), post-deposition annealing and laser energy on the nanostructure, bonding and crystalline properties of the films was studied, in order to elucidate their transition from an amorphous to a crystalline phase. Infra-red spectroscopy shows that growth at temperatures greater than 600 °C produces layers with increasingly uniform environment of the Si-C bonds, while the appearance of large crystallites is detected, by X-ray diffraction, at 800 °C. Electron paramagnetic resonance confirms the presence of clustered paramagnetic centers within the sp² carbon domains. Increasing deposition temperature leads to a decrease of the spin density and to a temperature-dependent component of the EPR linewidth induced by spin hopping. For films grown below 650 °C, post-deposition annealing at 1100 °C reduces the spin density as a result of a more uniform Si-C nanostructure, though large scale crystallization is not observed. For greater deposition temperatures, annealing leads to little changes in the bonding properties, but suppresses the temperature dependent component of the EPR linewidth. These findings are explained by a relaxation of the stress in the layers, through the annealing of the bond angle disorder that inhibits spin hopping processes.     

Generation and characterization of Nd-Fe-B-C nanoparticles by pulsed Nd:YAG laser ablation in liquid

We have generated Nd-Fe-B-C nanoparticles by Nd:YAG (1064 nm) laser irradiation in distilled water. Exposure times were 1, 5, and 10 min. Characterization of such nanoparticles in terms of their size distribution, shape, and chemical composition was carried out by transmission electron microscopy, energy-dispersive X-rays, and Fourier transform infrared spectroscopy. To investigate the nanoparticle stability, the size distribution of nanoparticles was measured two weeks after the nanoparticle generation, using dynamic light scattering. Investigations with the help of the atomic force microscope and magnetic force microscope showed other aspects of the generated nanoparticles.