Akiko Okano

Theoretical studies of defect‐initiated particle emission from GaP(110) surfaces: Basis for a new technique of generating perfect surfaces

We have calculated the energy needed to eject P atoms from several types of defects on the GaP (110) surface as well as from the perfect surface. It is found that most types of defects have a Ga or P atom bonded less strongly than those on the perfect surface. In view of the result of calculation, and of recent experimental observation that less strongly bonded atoms are ejected by irradiation with laser pulses of lower fluences, we suggest a new method of producing perfect surfaces using laser irradiation to eliminate steps and adatoms and deposition to eliminate vacancies.

Defect-initiated emission of Ga atoms from the GaAs (110) surface induced by pulsed laser irradiation

The authors have measured the Ga0 emission yield from GaAs(110) surfaces for laser pulses of several photon energies near the band-gap energy 1.435 eV, ranging from 1.33 eV to 2.53 eV. Similarly to the case for emissions of Si atoms from Si(100) and of Ga atoms from GaP surfaces, they find that the Ga0 emission yield, under repeated irradiation with laser pulses at fluences smaller than that for ablating the surface, decreases from its initial value rapidly at first and then slowly, while repeated irradiation by laser pulses above the ablation threshold fluence increases the emission yield gradually. It is found that the Ga0 emission yield for laser pulses below the ablation threshold depends strongly on the photon energy h nu : the emission yield is relatively small for h nu 1.42 eV (region III). It is suggested that the emission for region I is induced by electronic excitation of defects on the surface, while that for region III is induced most dominantly by electronic transitions involving surface states. It is also found that the yield is a power function of the laser fluence with power indices 2-4 for the rapidly decaying component and 4-7 for the slowly decaying component, depending on the photon energy.

Nonthermal laser sputtering from solid surfaces

Abstract Current understanding of the phenomena of laser-induced sputtering of nonthermal origin from semiconductors and insulators is reviewed. Experimental observations on laser sputtering of typical insulators and semiconductors are compared, emphasizing laser sputtering by photons having subgap energies. It is pointed out that the sputtering is not induced at low laser intensities in semiconductors, while sputtering in proportion to the density of excitation is observed for alkali halides. The difference is attributed to the absence and presence of self-trapping of excitons in two different types of materials. In high-sensitivity measurements of laser-induced sputtering from GaP surfaces, the change in the sputtering yield as the pulsed-laser irradiation is repeated on the same spot is correlated with the change in the defect concentrations on the surface top layer. Hence, high-sensitivity measurements of nonthermal laser-induced sputtering can be a useful technique for defect and impurity analysis on the top layer of the surface. Theoretical aspects of the laser-induced sputtering, including the nature of excitons, energy localization, bond breaking and dynamics of localisation are discussed. It is suggested that two-hole localisation on surface defect sites is the origin of sputtering in materials in which no self-trapping of excitons is induced.

Characterization of surface defects by means of laser-induced Ga0 emission from GaP surfaces

The authors have carried out high-sensitivity measurements of the emission of Ga0 neutrals from a clean GaP (110) surface induced by laser pulses of subgap photon energies. They found that, below a sharp threshold laser fluence for evolution of surface damage, the emission yield of Ga0 atoms decreases, following two exponential functions, with repeating pulses of the same fluence. These two decays are ascribed to the ejection initiated by defects of two different types. These measurements are shown to be useful for characterizing defects of extremely small concentration.

Laser-induced fluorescence from collisionally excited Si atoms in laser ablation plume

Laser-induced fluorescence (LIF) from neutral Si atoms in a laser ablation plume is investigated using a probe laser beam at 251.6 nm. Fluorescence at 288 nm from the 4s(1P1) state is observed, aside from the deexcited fluorescence at 251.6 nm from the 4s(3P2) state. The coincidence of the 288 nm fluorescence and the 251 nm fluorescence strongly indicates that the Si atoms in the 4s(3P2) state are responsible for the 288 nm fluorescence. The 288 nm LIF signal is detectable only when the probe laser beam passes near the Si surface, and has maximum intensity for a time delay of 20 ns. The 288 nm LIF could be emitted when the Si atoms in the 4s(3P2) state, pumped by the probe laser, collide with other Si atoms in the gas phase, since a high-density gas phase of ejected particles exists near the surface. The LIF intensities from the ablated Si atoms decrease for large time delays of the probe laser (0.2 μs<td<100 μs), and the 288 nm fluorescence originating from the droplets (probably microparticles) is obser...

A model of laser ablation in nonmetallic inorganic solids

A model is presented of laser ablation, primarily applicable to nonmetallic solids. The model does not include any critical phenomena, but assumes evolution of vacancy clusters due to the emission of atoms bonded weakly around vacancies and vacancy clusters on surfaces. The result of calculation shows a high power dependence of the yield on the laser fluence, agreeing with the experimental results.

Electronic processes in laser-induced Ga0 emission and laser ablation of the GaP(110) and GaAs(110) surfaces

We have carried out sub-monolayer sensitivity measurements of laser-induced Ga0 emission from the GaP(110) and GaAs(110) surfaces using a resonant ionization technique. The laser ablation threshold fluence is defined as the critical fluence above which an increase of the Ga0 emission yield is observed by repeated irradiation with laser pulses on the same spot but below which a decrease of the yield is observed. The photon-energy dependences of the laser ablation threshold fluence and the sub-ablation emission yield have been investigated for photons below, near and above the bulk band-gap energies in both GaP and GaAs. For photons below the band-gap energies, the sub-ablation emission and laser ablation are observed and ascribed to photon absorption by defects on the surfaces. The sub-ablation emission is found to be reduced substantially at a photon energy slightly below the band-gap energies for both GaP and GaAs; the reduction is of resonance type for GaAs and stepwise for GaP. Only a small amount of emission is observed for GaP for photons above the band-gap energy. The ablation threshold fluence for GaAs shows a resonant-type increase, corresponding to me resonance-type reduction of the sub-ablation emission yield, while the ablation threshold fluence for GaP does not change on crossing the band-gap energy. For photons above the band-gap energies, the ablation threshold fluence decreases with increasing photon energy in a similar manner for both GaP and GaAs. The decrease in the ablation threshold fluence for the GaAs(110) surface appears to be correlated to the increase in the sub-ablation emission yield as observed previously. These results of the photon energy dependence of the sub-ablation emission yield and ablation threshold fluence are explained in terms of the electronic excitation of defects on surfaces and of surface occupied states.

Neutral silicon clusters produced by laser ablation in vacuum

Abstract Small neutral silicon clusters Sin0 (n=1–11) produced by pulsed 1.2-eV laser irradiation in vacuum are investigated. Clusters are photoionized by a 6.4-eV excimer laser and detected by a time-of-flight mass spectrometer. A low fluence of 30–350 mJ/cm2 is used for inducing ablation, which is about one third lower than the fluence at visible white plume creation. In this moderate fluence range, the size distribution of photoionized clusters does not change much except for a relative decrease of larger clusters, while the total yield of clusters changes by five orders of magnitude. Velocity distributions for Si1–Si4 and Si9 at 300 mJ/cm2 are well fitted by Maxwell–Boltzmann (MB) shapes. Evaluated temperatures from the MB distributions depend on cluster size and are too high to reconcile with surface temperature; 4870 K for Si2, 12 420 K for Si4 and 47 350 K for Si9. These experimental results suggest that collisional effects at the near surface are small and that the thermal equilibrium theorem cannot be applied to cluster emission. Neutral clusters are considered to be emitted directly from the surface by mechanisms which differ from classical thermal evaporation.

The DIET from semiconductor surfaces by excitation of valence electrons

Abstract We discuss the desorption induced by electronic transitions (DIET) of constituent atoms from several types of non-metallic solids, particularly the DIET from semiconductors by valence electron excitations. We first classify the non-metallic solids into type A, in which no self-trapping of excitons occurs, and type B, in which self-trapping occurs. We argue that in type B solids the localization of electron-hole pairs or excitons through the self-trapping on the surfaces induces the Menzel-Gomer-Redhead-type anti-bonding state resulting in DIET. For the DIET from type A non-metals, typically semiconductors, in which the self-trapping is not induced, we derive two important characteristics: (1) the emission is related to defects on the surfaces and (2) single electronic excitation cannot induce the emissions. The recent experimental observations of laser-induced emissions satisfying these characteristics for the DIET from semiconductors are surveyed. Furthermore we present experimental evidence demonstrating that the observed emissions are of the electronic origin: the emission occurs dominantly when the excitation is localized on the surface. Finally, surface phenomena, such as laser ablation and dry etching, related to the DIET from semiconductors, and the applications of the DIET from semiconductors are discussed.

Defect initiated particle emission from semiconductor surfaces by laser irradiation

Abstract We review current studies of high sensitivity measurements of laser-induced emission of Ga atoms from GaP and GaAs surfaces, emphasizing its application to characterization of defects on surfaces and to studies of the reactions of defects with adsorbates of small coverage.