Effect of interference pattern on femtosecond laser-induced ripple structure

Combined structures composed of a micron-sized periodic structure and a nano-sized quasi-periodic ripple structure were generated by a single process of multiple shots of an interfering femtosecond laser. The former structure was generated by an interference pattern and controlled by a number of beams. The latter structure was generated by a multiple shot method. As a result, multiple periodic structures were generated. The periodicity of the structures was analyzed by two-dimensional (2D) fast Fourier transform (FFT). The most probable period of ripple structure was downsized to 177 nm by restricting irradiated region by a TM wave interference pattern. The smallest period was 105 nm. The dispersion angle of a ripple decreased to about 50%. The period and direction of the ripples were for a first time controlled by using interference.     

Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC

Raman spectroscopy was performed to investigate microscopic structural changes associated with a ripple structure formation initiated by femtosecond laser irradiation on the surface of single-crystal silicon carbide. The amorphous phases of silicon carbide, silicon, and carbon were observed. The intensity ratio between amorphous silicon carbide and silicon changed discretely at the boundary between fine and coarse ripples. The physical processes responsible for the formation of the ripple structure are discussed.     

Modeling for laser-induced surface thermal lens in semiconductors

Received: 14 December 1997/Revised version: 19 May 1998     

Polarization-dependent laser-induced giant mass transport in glassy semiconductors

Direct experimental evidence of the validity of the two-phase model of chalcogenide glass has been obtained. It has been shown that the polarization-light irradiation of glassy semiconductor films induces giant mass transport in the direction perpendicular to the polarization of the incident light. It has been revealed that a surface relief appears in the irradiated films. The shape of the relief depends on the laser polarization state. A macroscopic model qualitatively describing the observed phenomenon is presented.     

Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N _DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ～150 fs, central wavelength ～800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from ?40 ps to +40 ps with a resolution of ～0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ～790 nm towards ～550 nm is demonstrated for delay changes of less than ～2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (～760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.     

A simple theory of the laser-induced damage in semiconductors

A simple theory is presented for the laser-induced damage in semiconductors in the range of low irradiation intensities. This theory avoids the solution of coupled differential equations, and takes into account characteristic physical properties of semiconductors—i.e. energy gap, carrier lifetime, and surface recombination velocity. The deduced equations permit us to estimate the damage threshold, or the minimum irradiation time required for damage to occur. Comparison is made with some experimental results reported in the literature and a reasonable agreement is found.     

Femtosecond laser-induced formation of spikes on silicon

We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF₆. The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes. Received: 10 September 1999 / Accepted: 7 January 2000 / Published online: 8 March 2000     

Mechanism of the formation and evolution of periodic surface relief nanostructures under the scanning laser-induced inelastic photodeformation of semiconductors

A defect-deformational (DD) mechanism is proposed for the self-organization of laser-induced point defects (vacancies and interstitials) under low-threshold (far from the melting point) local (10–100 μm) light-induced heating with the scanning periodic pulsed laser irradiation of a semiconductor resulting in an inelastic deformation of micron-sized regions of Ge. A linear theory of DD instability is developed within the model of a biaxially stressed defective film. This model describes the main experimental data on the formation of two-and one-dimensional periodic nanostructures on a semiconductor surface relief.     

Laser-induced defect formation in semiconductors

The electron-deformation-thermal theory of pulsed laser-induced point defect generation in strongly absorbing semiconductors is developed. The theoretical results obtained are in a good agreement with the results of experiments carried out in Ge, GaAs, and GaP.     

Comparison of laser-induced etching behavior of III–V compound semiconductors

Laser-induced maskless etching of III–V compound semiconductors (InSb, GaAs, and InP) in a KOH aqueous solution by irradiation with a focused argon-ion laser has been investigated to obtain high etching rates and aspect ratios of etched grooves. The etching rate at low laser power was found to depend on the carrier density of the sample and its type. With the increase of the laser power, the etching reaction becomes primarily a thermochemical reaction. High etching rates and aspect ratios have been achieved with a single scan of the laser beam. The damage induced by laser wet etching is less than that by laser dry etching, and the damage at the etched side wall is less than that at the etched bottom. Grooves with locally controlled depth and slab structures have been fabricated for application.     

Crystallographically-dependent ripple formation on Sn surface irradiated with focused ion beam

The metallographically polished polycrystalline Sn surface was sputtered by 30 kV focused Ga⁺ ions at room temperature. The experiment was carried out using various FIB incidence angles (0°, 15°, 30°, and 45°) over a wide range of doses (10¹⁶–10¹⁸ ions/cm²). The surface morphology was carefully characterized under the optical microscope, scanning electron microscope (SEM) and atomic force microscope (AFM). Ripples were observed on the irradiated areas even at the normal FIB incidence angle, which is not consistent with the Bradley–Harper (BH) rippling model. The orientation of ripples relies on crystallographic orientation rather than projected ion beam direction as predicted by BH model. The ripple wavelength is independent of ion dose, while ripple amplitude increases with ion dose. It is found that the ripples are formed by self-organization due to anisotropic surface diffusion in the low melting point metal.     

Chemical effects during ripple formation with isobaric ion beams

The formation of nanostructures on SiGe surfaces by erosion using mixed beams of isobaric species (Cs/Xe) is shown to depend on the Cs/Xe ratio. The nanostructures exhibit different wavelengths (longer wavelengths for higher Cs concentrations) contrary to the present theoretical understanding. Moreover, experiments with pure Cs and Xe beams also demonstrate that such differences are enhanced at lower bombarding energies. Such effects are primarily due to the fact that the retentivity and mobility of cesium at the sample surface gets enhanced at lower bombarding energies. The phenomenon could be explained theoretically by including an additional diffusion term in the growth equation describing the mobility of the primary ions on the irradiated surface. Semi-empirical calculations done in this direction also confirm this phenomenon.     

Subwavelength ripple formation on planar and nonplanar surfaces by femtosecond laser scanning

The self-formation of periodic subwavelength ripples by linear polarized femtosecond laser scanning planar and non-planar tungsten targets on the employed laser wavelength, scanning speed, and energy fluence are examined systematically. The results show that, for a certain laser wavelength, the scanning conditions have no obvious effect to the morphological features of grating structures in the threshold range of laser fluence. The spatial structured period of gratings can be self-consistently interpreted by recently presented physical model of surface two-plasmon resonance. The subwavelength structures on cylindrical surface would be a good method to realize unique surface functions on complex surface of micro-devices.     

Search for laser-induced formation of antihydrogen atoms

Antihydrogen can be synthesized by mixing antiprotons and positrons in a Penning trap environment. Here an experiment to stimulate the formation of antihydrogen in the n = 11 quantum state by the introduction of light from a CO2 continuous wave laser is described. An overall upper limit of 0.8% with 90% C.L. on the laser-induced enhancement of the recombination has been found. This result strongly suggests that radiative recombination contributes negligibly to the antihydrogen formed in the experimental conditions used by the ATHENA Collaboration.     

Photoinduced defect formation in chalcogenide vitreous semiconductors

Journal of Applied Spectroscopy -     

In-situ formation of local inhomogeneities in semiconductors

The transport properties of ionic conductors such as α-AgI and Li_0.23La_0.69Fe_0.3Ti_0.7O₃ and of electronically conducting semiconductors such as α-Ag₂S and Ag_xWO₃ as a result of the application of voltages smaller than the decomposition voltage by two ionically blocking electrodes were investigated. The mobile ionic as well as the mobile electronic species are shifted from one electrode side to the other one. The change in the stoichiometric composition within the sample as a result of this transport process causes the formation of local inhomogeneities leading to changes in the electrical and optical properties. The experimental results also show that Nernst's law is not only valid for ionic conductors but also for semiconductors as well when a steady state is reached for ionically blocking electrodes. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Heat of formation of ternary chalcopyrite semiconductors

Heat of formation of I-III-VI₂ and II-IV-V₂ groups of ternary chalcopyrite semiconductors has been calculated using plasma oscillations theory of solids. Two simple relations between plasmon energy and heat of formation have been proposed. One is based on spectroscopic model of Phillips and Van Vechten and other is based on the best-fit data of heat of formation. The calculated values of heat of formation from both the equations are compared with the experimental values and the values reported by earlier workers. A fairly good agreement has been obtained between them.