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The transition between relaxed and unrelaxed amorphous silicon can be obtained by thermal treatment of the unrelaxed amorphous or by low dose ion irradiation of the relaxed material. In both cases a variation in the short range order has been invoked to explain the behavior of the structural changes probed by various techniques. In this work we study the influence of such changes on the optical properties of a-Si in the region of the transition between the relaxed and the unrelaxed states. We show that a progressive variation of the optical constant in the visible-near infrared region upon derelaxation occurs. Therefore, significant modifications of the electron density of state in the region above the optical gap are associated with the changes in the short range order probed by Raman spectroscopy.
Direct picosecond laser measurements of the critical fluence for melting have been performed for the first time, giving unambiguously consistent differences in the energy required for surface melting of relaxed and unrelaxed amorphous silicon. The different optical coupling cannot account for this variation which can only be explained in term of different melting temperatures. Heating of unrelaxed amorphous silicon samples at temperatures close to the melting point may result in relaxation of the material even when the treatment occurs in the nanosecond time scale. However nanosecond UV irradiation of relaxed and unrelaxed amorphous silicon samples have provided informations on the specific heat of the two amorphous states. The melting temperature of unrelaxed amorphous silicon has been derived independently via both picosecond data and via free energy calculations.
The difference in the melting temperature of ion implanted and relaxed amorphous silicon has been measured. Pulsed laser irradiation (λ=347 nm, τ=30 ns) has been used to induce surface melting in the amorphous layer and time resolved reflectivity to detect the melting onset. The threshold energy density for surface melting in the relaxed amorphous was found 15.9±.3% higher than that in the unrelaxed one. The estimate of the variation of the thermal parameters in amorphous silicon upon relaxation allowed a determination of ΔTM=45±10 K between relaxed and unrelaxed amorphous silicon.
NiSi and Ni2Si layers on silicon substrates as well as high fluence Si(As) ion implanted layers,have been rapidly melted by 30 ns Nd laser pulse irradiation.The energy density ranged between 0.4 and 1.2 J/cm2. Bilayer structures have been observed when the energy density has been chosen properly.
Buried epitaxial layers together with an amorphous or a policrystalline layer on top,have been detected by RBS and TEM measurements.
Thermally grown NiSi layers on <111> Si substrates were irradiated by 35 nsec Nd glass laser pulses in the energy density range 0.3−2.0 J/cm2. Time resolved reflectivity measurements were performed during the irradiation to detect surface melting. The samples were analyzed by 2.0 MeV He+ Rutherford Backscattering Spectrometry in combination with channeling effect. The measured threshold for surface melting was 0.5 J/cm2. Irradiation at energy densities higher than 1.3 J/cm2 changed the silicides layer composition because of the mixing with the underlying silicon. In the intermediate energy density range (0.7−1.1 J/cm ) slight changes in composition were observed, a strong alignement of NiSi molecules along the <111> substrate direction was however observed. The measured Xmin was about 30%. It seems then that an epitaxial NiSi phase can be grown by pulsed laser irradiation with a suitable choice of the incident energy density. Work is in progress to identify this new NiSi phase by TEM. However this ordered phase is a metastable one since after anunealing at 250°C, 30min the channeling yield reduction disappeared without any appreciable change in composition.
Thermally grown Ni2 Si and NiSi2 layers on <111> Si substrates were irradiated by 40 ns Nd laser pulses in the energy density range 0.3–2.0 J/cm2. The samples were analyzed by time-resolved reflectivity, 2.0 MeV He+ Rutherford backscattering in combination with channeling and by transmission electron microscopy. In the NiSi2/Si system the melt starts at the free surface (1280 K) and propagates towards the inside. Dissolution of substrate silicon atoms occurs when the silicon temperature reaches the liquidus temperature (1400 K). In the Ni2Si/Si samples the melt starts instead at the interface when it reaches the eutectic temperature (1250 K). The subsequent propagation towards the surface is limited by the mass transport of silicon atoms to maintain a composition near that of the eutectic. In some cases the surface may melt also at the congruent melting temperature (1570 K), giving rise after solidification to a quite complex structure. The different behaviour of the two silicides/silicon systems is explained in terms of phase diagram.
Impurity redistribution in Bi-implanted Si and in As-implanted Si has been investigated after irradiation with 25 ps Nd(λ=l.06 μm) laser pulse in the energy range 0.1–1.5 J/cm2 . Channeling effect in combination with 2.0 MeV He+ backscattering in glancing detection has been used to characterize the epitaxial crystallization, the impurity location and its depth distribution. The amorphous to single crystal transition occurs at an energy density of about 0.4 J/cm 2 . Bi atoms are located after crystallization in substitutional lattice sites for the in depth part of the distribution. Part of the Bi atoms accumulated at the sample surface and the amount of segregation increases with the pulse energy density and depends on the substrate orientation. A computer model has been also developed to calculate several parameters of interest, as the melt threshold,the melt duration, the carrier temperature etc including a detailed description of the absorption and of the energy relaxation processes. The calculations indicate that the simple thermal description accounts quantitatively for the experimental data on melt duration and impurity segregation.
The crystallization onset and the annealing thresholds have been nmeasured as a function of the absorbed energy density in ion implanted amorphous silicon irradiated with nanosecond Nd pulse. Thin amorphous layers (∼500 Å) require higher thresholds ccapared with thick (∼4000 Å) amorphous layers. This result can be explained in terms of balance between absorbed energy and heat flow. For a given thickness of the amorphous layer the thresholds depend on the absorption coefficient of the amorphous material. This last parameter has been varied frcm 104 to 102 CM−1 by low temperature (T<400°C) pre-treatment of the ion implanted sample. The observed drastic variations of both crystallizazion and annealing thresholds agree well with nunerical evaluation of heat flow.
Heat flow calculations, based on the main assumption that the energy of the incident pulse is instantaneously and locally converted into heat, are used to review the results of laser pulse annealing and impurities behaviour in ion implanted semiconductors. The crystallization behaviour of amorphous layers, the velocity of the solid-liquid interface and impurity redistribution are detailed with the main emphasis on the relevance of the parameters (laser wavelen gth, pulse duration and energy density, substrate temperature, etc.) that can be experimentally controlled. Comparison with the latest experimental results is also given.
Amorphous, implanted, Si layers have been melted by pulsed electron irradiation. Implanted As has been used as a marker for determining melt duration. Systematic differences between As diffusion in initially amorphous or crystalline Si are interpreted in terms of different enthalpies of melting between amorphous (1220 J/g) and crystalline (1790 J/g) Si. The amorphous Si layers melt and crystallize at significantly lower electron energies than those required to melt crystalline Si, indicating that amorphous Si melts at 1170K compared to 1685 K for crystalline Si. We have used these thermodynamic parameters to successfully predict some of the phenomena associated with the laser induced melting and crystallization of amorphous Si.
The segregation phenomena of In, Ga and Bi in Si have been investigated as a function of the liquid-solid interface velocity following laser irradiation. The crystallization velocity has been changed within the range 0.8–5 m/s by varying either the substrate temperature during irradiation or the laser pulse duration. The measured interfacial segregation coefficients depend critically on the velocity and on the crystal orientation of the solidifying plane.
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