Takeyoshi Nakayama

Mechanism of neutral particle emission from electron-hole plasma near solid surface

Abstract A new model is presented for neutral particle emission from the surface of laser-irradiated non-metallic solids. The model accounts for the experimental result that the energy distribution of the particles emitted from the laser-irradiated surface is not maxwellian and deviates to higher energy than expected from the lattice temperature derived from a Raman experiment.

Laser-induced sputtering of ZnO, TiO2, CdSe and GaP near threshold laser fluence

Abstract Sputtering of ZnO, TiO 2 , CdSe and GaP induced by 10 ns laser pulses from a tunable dye laser has been investigated. It is shown that all of these materials exhibit the following characteristics: (1) non-linear dependence of sputtering yield on the excitation laser fluence, (2) existence of the threshold laser fluence, (3) dependence of the threshold laser fluence on laser wavelength, and (4) non-Maxwellian velocity distribution of sputtered particles. These results indicate that the laser-induced sputtering near threshold fluences is not a thermal effect but an effect of dense electron-hole plasma.

Electronic excitation mechanism of sputtering and track formation by energetic ions in the electronic stopping regime

Abstract Atomic processes, such as sputtering and track formation, induced by electronic encounters of energetic heavy ions with nonmetallic compounds are discussed critically on the basis of electronic excitation mechanism. First, the behaviors of excitons in terms of electron-phonon coupling are surveyed and nonmetallic solids are divided into three categories. Possible atomic processes in each of these solids are discussed semi-quantitatively. It is pointed out that the electronic excitation mechanism can account for the observed yield of sputtering and of track formation, even though the mechanism differs from material to material depending on the type of nonmetallic compound according to the categorization given in the present paper. Some of the laser-induced processes that are not due to melting are also discussed.

Non-linear photo-induced desorption of GaP

Abstract Laser-induced desorption of P from GaP at various photon energies near the absorption edge has been measured. The desorption yield is found to start increasing above a certain threshold laser fluence, of which the dependence on the photon energy exhibits a sharp dip near the indirect band gap besides a gradually decreasing component from the indirect band gap to the direct band gap energy. The sharp dip is ascribed to desorption induced by dense excitation of the surface states.

Laser-induced sputtering of oxides and compound semiconductors

Abstract The damage and sputtering of oxides and compound semiconductors induced by laser irradiation are discussed. Emphasis is placed on phenomena that occur under a non-melting condition. These include the resultant decomposition of GaAs and GaP induced by a μs laser pulse and damage induced by irradiation near the indirect band gap of GaP. This non-thermal laser-induced damage is ascribed to dense electronic excitation at states localized near the surface. Several features of the non-thermal aspect of laser-induced sputtering are also described, e.g. non-Maxwellian velocity distribution and sputtering by photons near the indirect band gap. The threshold laser faiences for damage and sputtering are compared; The mechanisms of non-thermal laser-induced damage and sputtering are discussed and it is pointed out that the mechanism previously suggested by the present authors explains the results most satisfactorily.

Formation of ion damage tracks

The formation of damage tracks in insulators from the passage of energetic (MeV/amu) ions indicates that the energy lost by an ion to electronic excitation is partially transferred to atomic motion. It is known that a track consists of localized regions of extended defects that are separated by lengths that exhibit only point defects. The utility of tracks for selective detection of various types of ions arises because of preferential chemical etching along the track as compared to etching the bulk material. In this letter we propose a new model to explain both the localized damage regions and the preferential etching of damage tracks. The formation of each region of extended defects is initiated by the Auger decay of a vacancy produced in an inner electronic shell of an atom of the insulator by the incident ion. This decay produces an intense source of ionization within a small volume around the decaying atom, which causes decomposition of the material in a manner similar to that observed in pulsed laser irradiation. The resulting chemical or crystalline modification of the material is the latent track, which because of its changed structure can be preferentially etched.

Studies of Higher Triplet Exciton Bands through T-T Absorption in Naphthalene Single Crystal

Two T - T transitions in naphthalene crystals have been identified: one with a series f vibronic progressive absorption bands between 3500 A and 4500 A is assigned as the 3 B 1 g - ← 3 B 2 u + transition, and the other with a series of absorption bands between 4500 A and 6000 A is assigned as the 3 A 1 g + ← 3 B 2 u + transition. The shape of the absorption band for the former transition has been studied between 20 K and room temperature, with the result that the shape has no temperature dependence above 80 K while the shape and the polarization change at lower temperatures. The absorption is concluded to be due to a band-to-band transition and the width of the 3 B 1 g - exciton band is determined to be 1050 cm 1 .

Electronic-excitation mechanism in sputtering induced by high density electronic excitation☆

On the basis of the assumption that two holes are attractive in a dense electron-hole plasma near the surface because of the Anderson negative-U interaction, the yield of laser- and ion-induced sputtering in the electronic excitation regime was calculated as a function of electron-hole concentration in a plasma. It is found that the model gives an order-of-magnitude account of the yields of these two sputtering processes.

A new type of laser-induced surface damage of GaAs

Abstract The damage of surface layers of GaAs induced by a laser pulse with 15 ns and 600 ns duration has been studied by means of channeling. It is found that damage induced by 600 ns pulses is much more pronounced than that by shorter pulses and that 600 ns pulses produce As-rich layers at the surface. Results are discussed on the basis of calculated temperature rise during and after a laser pulse.

Dependence of laser‐induced damage of surface layers of GaP on pulse width and wavelength

Disorder induced by irradiation with a laser pulse on surface layers of GaP has been studied by means of the Rutherford backscattering channeling technique. It is found that a 0.6‐μs laser pulse near the direct band gap and a 15‐ns laser pulse near the indirect band gap induce damages more pronounced than a 15‐ns laser pulse near the direct band gap of the same fluence. Moreover, decomposition of the surface layers into Ga‐rich and P layers is found to be induced by irradiation with a 0.6‐μs laser pulse. It is suggested that such a decomposition occurs when heating is not substantial.