Nano- and microdot array formation by laser-induced dot transfer

Fabrication of FeSi₂ nano- and microdot array was performed by utilizing droplet ejection through laser-induced forward transfer, which we named laser-induced dot transfer (LIDT). An amorphous FeSi₂ alloy source film on a transparent support was illuminated from the support by a nanosecond excimer laser pulse patterned into migcrogrid form, resulting in size- and site-controlled dot deposition. Micro-Raman spectroscopy confirmed β-FeSi₂ semiconducting crystalline phase even on unheated substrates. Moreover, the microdots exhibited near-infrared photoluminescence at the peak wavelength of 1.57 μm, which comes from the β-FeSi₂ crystalline phase precipitated during the LIDT process. The dot size was successfully reduced to approximately 500 and 300 nm in diameter and height, respectively. This technique is useful for integrating functional nano- and microdots under atmospheric room-temperature conditions.     

Low temperature thermal annealing-induced α-FeSi2 derived phase in an amorphous Si matrix

Thermal annealing-induced recrystallisation in Fe ion-implanted Si was investigated by transmission electron microscopy. Single crystals of Si(111) were implanted with 120 keV Fe ions to a fluence of 1.0×10¹⁷ cm^-2 at cryogenic temperature. A buried amorphous Fe-Si layer in an amorphous Si matrix was formed in the as-implanted sample. Nanobeam electron diffraction revealed that metastable α-FeSi₂ precipitates embedded in the amorphous Si matrix were formed after annealing at 350 °C for 8 h. The formation of this α-FeSi₂-derived phase was unusual, because it has been observed only in epitaxially grown thin films. Based on the Fe_1-xSi (0<x<0.5) phase with the CsCl structure, which is another metastable phase in the Fe-Si binary system, we discuss the formation process of the metastable α-FeSi₂ in the amorphous matrix. PACS 61.43.Dq; 61.14.Lj; 61.80.Jh     

Optical waveguides in laser crystals

This article reviews the recent research on different types of planar and channel crystalline optical waveguides, fabrication methods such as liquid phase epitaxy, pulsed laser deposition, thermal bonding, reactive ion or ion beam etching, wet chemical etching, ion in-diffusion, proton exchange, ion beam implantation, and femtosecond laser writing, as well as waveguide laser operation of rare-earth and transition-metal ions in oxide crystalline materials such as Al₂O₃, Y₃Al₅O₁₂, YAlO₃, KY(WO₄)₂, and LiNbO₃. To cite this article: M. Pollnau, Y.E. Romanyuk, C. R. Physique 8 (2007).     

Laser and electrical current induced phase transformation of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> semiconductor thin film on Si(111)

Phase transformation of thin film (∼30 nm)In₂Se₃/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In₂Se₃ is of interest for non-volatile phase-change memory. Amorphous In _x Se _y was deposited at room temperature on Si(111) after pre-deposition of a crystalline In₂Se₃ buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In₂Se₃ single crystal with its {0001} planes parallel to the Si(111) substrate and parallel to Si , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm²) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous In _x Se _y film.     

Irradiation of amorphous Ta<Subscript>42</Subscript>Si<Subscript>13</Subscript>N<Subscript>45</Subscript> film with a femtosecond laser pulse

Films of 260 nm thickness, with atomic composition Ta₄₂Si₁₃N₄₅, on 4″ silicon wafers, have been irradiated in air with single laser pulses of 200 femtoseconds duration and 800 nm wave length. As sputter-deposited, the films are structurally amorphous. A laterally truncated Gaussian beam with a near-uniform fluence of ∼0.6 J/cm² incident normally on such a film ablates 23 nm of the film. Cross-sectional transmission electron micrographs show that the surface of the remaining film is smooth and flat on a long-range scale, but contains densely distributed sharp nanoprotrusions that sometimes surpass the height of the original surface. Dark field micrographs of the remaining material show no nanograins. Neither does glancing angle X-ray diffraction with a beam illuminating many diffraction spots. By all evidence, the remaining film remains amorphous after the pulsed femtosecond irradiation.     

Crystallization of amorphous Ge2Sb2Te5 films induced by a single femtosecond laser pulse

Crystallization is achieved in amorphous Ge₂Sb₂Te₅ films upon irradiation with a single femtosecond laser pulse. Transmission electron microscopy images evidence the morphology of the crystallized spot which depends on the fluence of the femtosecond laser pulse. Fine crystalline grains are induced at low fluence, and the coarse crystalline grains are obtained at high fluence. At the damage fluence, ablation of the films occurs.     

Peculiarities of filamentation of sharply focused ultrashort laser pulses in air

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of R _pl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.     

Synthesis of monoclinic KGd(WO4)2 nanocrystals by two preparation methods

The monoclinic crystalline phase of KGd(WO₄)₂ is a well-known solid-state laser host. The present article describes how monoclinic nanocrystals of KGd(WO₄)₂ are synthesized by two different methodologies: the sol–gel modified Pechini method and femtosecond pulsed laser ablation of bulk material in deionized water. Transmission Electron Microscopy images show KGdW nanocrystals between 15 and 100 nm for both methods. The nanocrystals are studied by X-ray powder diffraction (XRD) in order to identify the crystalline phase and to refine the unit cell parameters. For the sol–gel modified Pechini method, differential thermal analyses and XRD techniques were also used to optimize the method of preparation.     

Width of the ferromagnetic resonance line in highly dispersed powders of crystalline and amorphous Co-P alloys

The resonance characteristics (inhomogeneous FMR linewidth ΔH) in highly dispersed (d=0.1–3 μm) powders of crystalline and amorphous Co-P alloys are investigated as a function of the composition, particle size, and atomic structure. It is established that ΔH for powders of amorphous Co-P alloys is two to three times larger than ΔH for crystalline Co-P powders. According to the investigations performed, this is caused by thermodynamically stimulated segregation of nonmagnetic Co₂P inclusions, apparently an effective relaxation channel, in the amorphous state of Co-P powders. Fiz. Tverd. Tela (St. Petersburg) 41, 464–467 (March 1999)     

Porous silicon based β-FeSi2 and photoluminescence

Highly-pure iron powder was covered on porous silicon for fabricating semiconducting β-FeSi₂ structures. X-ray diffraction and Raman scattering results confirm the formation of pure-phase β-FeSi₂ after high-temperature annealing at 1100°C and then long-time persistence at 900°C. Scanning electron microscope observations reveal that large-size (>μm) β-FeSi₂ grains mainly form in the pores of porous silicon and some nanocrystals grow on local surfaces. The temperature-dependent photoluminescence spectra disclose that the observed ∼1.54 μm emission arises from free exciton recombination, which is confirmed via the activation energy (0.25 eV) measurement. Our method provides a way to synthesize single-phase β-FeSi₂ materials.     

Pulsed laser ablation and deposition of silicon

A KrF laser was used to ablate a polycrystalline Si target for deposition of Si on MgO and GaAs substrates at room temperature. The deposition was performed in 10⁻⁸ mbar, with two types of laser beams: a homogeneous beam being imaged onto the target (2.9 J/cm²), and a non-homogeneous which is nearly focused (2 J/cm², 6.5 J/cm²). In both cases, the beam was scanned over an area of 1 cm². For the homogenous beam, we observed only a limited number of droplets (<0.1 μm). A high number of micron-sized (<5 μm) droplets were observed on the film by the higher fluence nonhomogeneous laser beam. Raman spectroscopy showed that the micron-sized droplets are crystalline while the film is amorphous. The generation of the large droplets is most likely related to the cone structures formed on the ablated target. We also compared cone formation on a polycrystalline Si target and a single crystalline Si wafer, using multiple laser pulses onto a single spot.     

Energy efficiency of femtosecond laser ablation of polymer materials

We present the results of an experimental study of the ablation spectral energy thresholds for a number of polymer materials ((C₂F₄) _n , (CH₂O) _n ) exposed to femtosecond (τ_0.5 ~ 45–70 fs) laser pulses (λ ~ 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10–2 Pa). We have analyzed the energy thresholds and the efficiency of optical, thermophysical, and gasdynamic processes in laser ablation vs. the laser pulse duration and photon energy.     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

Time-resolved,stress-induced,and anisotropic phase transformation of a piezoelectric polymer

We investigate the time-dependent and anisotropic phase transformation of poly (vinylidene difluoride) (PVDF) under bending. Using combined techniques of an atomic force microscope and a Fourier transform infrared spectroscope, observation of surface morphology and phase transformation in time was made. Results showed that bending stress induces the transformation of amorphous, α,β, and γ crystalline phases. Specifically, the amorphous phase was transformed into the β phase when the bending force was applied. In addition, the transformation observed was time and direction dependent. The anisotropic behavior observed brings insights into the origin of the piezoelectricity of PVDF.     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t∼10⁻¹³ s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms (“cold liquid”) or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. Fiz. Tverd. Tela (St. Petersburg) 41, 1462–1466 (August 1999)     

A pulsed synthesis of β-FeSi2 layers on silicon implanted with Fe ions

Continuous layers and fine-grained films of β-FeSi₂ were synthesized using the implantation of Fe⁺ ions into Si (1 0 0) with subsequent pulsed nanosecond ion-beam treatment of the implanted layers. The X-ray diffraction studies showed that the pulsed ion-beam treatment brings about the formation of a mixture of two phases: FeSi and β-FeSi₂ with strained crystal lattices. Subsequent rapid thermal annealing led to the complete transformation of the FeSi phase into the β-FeSi₂ phase with the formation of a textured layer. The data obtained using Raman spectroscopy corroborate the formation of the β-FeSi₂ phase with a high degree of silicon crystallinity.The results of measuring the optical absorption point to the formation of β-FeSi₂ layers and precipitates with a direct-gap structure, an optical gap of E_g≈0.83 eV. The photoluminescence band peaked at λ≈1.56 μm and caused by direct band-to-band transitions in β-FeSi₂ was observed at temperatures lower than 210 K.     

Transmission electron microscopy investigation of crystalline silicon surface irradiated by femtosecond laser pulses in different background atmospheres

This article aims to obtain structural and compositional characteristics of a crystalline silicon surface irradiated by femtosecond laser pulses in SF₆, N₂, air, and vacuum background atmospheres by performing transmission electron microscopy observation of ??110?? cross-sectional specimens. Conical microstructures covered with defective outer layers were formed in SF₆ gas. The elemental sulfur dopants in the surface microstructure, which located in close proximity to defects, were mainly concentrated at the tip region of the microcones, and about several hundred nanometers thick. In N₂ atmosphere, the defects produced regularly on the silicon surface were of the same types with those formed in SF₆ gas and confirmed to be stacking faults and overlapped twins. Furthermore, silicon crystalline grains with different orientations were observed on the silicon surface irradiated in N₂, air, and vacuum atmospheres. Especially, ??-Si₃N₄ crystalline grains were found to be formed in N₂ and air as chemical products when elemental nitrogen exists, and the SiO₂ amorphous phase was formed in air by the oxidation effect. Based on these experimental results, the relevant interaction mechanisms between pulsed laser and crystalline silicon were suggested to be mainly attributed to laser-assisted chemical etching and laser ablation, i.e., if volatile silicon compounds can be produced in a reactive gas atmosphere (e.g., SF₆), the strong laser-assisted chemical etching dominates over the laser irradiation process. Otherwise, laser ablation is the dominant mechanism such as in N₂, air, and vacuum.     

Study of opto-mechanical characteristics of interaction of ultrashort laser pulses with polymer materials

The opto-mechanical characteristics, such as the specific mechanical recoil momentum, the specific impulse, and the energy efficiency, of the laser ablation of flat polymer targets ((C₂F₄) _n , (CH₂O) _n ) have been determined experimentally for the first time for the case of excitation with femtosecond pulses (τ ∼ 45–70 fs) of UV-IR (λ ∼ 266, 400, 800 nm) laser radiation (I ₀ up to 10¹⁵ W/cm²) under normal atmospheric and vacuum (p ∼ 10⁻⁴ mbar) conditions. The efficiency of mechanical recoil momentum generation is analyzed for various regimes of the laser irradiation.     

Excimer laser deposited CuO and Cu<Subscript>2</Subscript>O films with third-order optical nonlinearities by femtosecond <Emphasis Type="Italic">z</Emphasis>-scan measurement

By controlling the oxygen pressure, single-phase CuO and Cu₂O thin films have been obtained on quartz substrates using a pulsed laser deposition technique. The structure properties and linear optical absorption of the films were characterized by X-ray diffraction and UV–VIS spectroscopy. By performing z-scan measurements using a femtosecond laser (800 nm, 50 fs), the real and imaginary parts of the third-order nonlinear susceptibility, Re χ ⁽³⁾ and Im χ ⁽³⁾, of the films were determined. Both CuO and Cu₂O films exhibited large optical nonlinearities, which is comparable to those in some representative semiconductor films such as ZnO and GaN films using femtosecond laser excitation. Compared with Cu₂O films, the CuO films showed larger third-order nonlinear optical effects under off-resonance excitation. Furthermore, the mechanisms of the optical nonlinearities in CuO and Cu₂O films are explained in the main text. It was suggested that the reasons of the difference in their nonlinear refractive effects may be related to the different electronic structure in CuO and Cu₂O materials.