Laser-induced phase transitions in a-Ge/Si heterostructures (amorphous Ge films on crystalline Si substrate) have been
studied by optical diagnostics and numerical simulation methods. The samples were irradiated by (i) a ruby laser with
pulse duration 80 ns (FWHM) and wavelength 694 nm and (ii) an ArF excimer laser (10 ns and 193 nm). Time resolved
reflectivity measurements showed the discrepancy in dynamics of reflectivity of probing beam for different regimes of
laser irradiation. This discrepancy can be explained by differing kinetics of solid-liquid phase transitions in Ge films: (i)
intermediate crystallization or (ii) simultaneous solidification of molten Ge layer from the surface and from the substrate.
The laser annealing of Ge/Si heterostructures with Ge quantum dots (QD's) embedded on the depth of 0.15 and 0.3 μm
has been studied. The samples were irradiated by 80-nanosecond ruby laser pulses. Irradiation energy density was close
to the melting threshold of Si surface. The nanocluster structure was analyzed by Raman spectroscopy. Changes in
composition of QD's were observed for both types of samples. The decrease in dispersion of nanocluster sizes after laser
irradiation was obtained for samples with QD's embedded on 0.3 μm depth. The numerical simulations on the basis of
Stefan problem showed that the maximum temperatures on the depth of QD's bedding differ by ~ 100 K. This difference
is likely to lead to different effects of laser annealing of heterostructures with QD's.
Crystal-liquid phase transitions induced in monocrystalline Si and GaAs surface layers by nanosecond ruby laser irradiation have been studied. The values of undercooling at crystallization stage were calculated on the basis of a nonequilibrium model of the phase transitions. The calculated values of undercooling are compared with experimental results obtained by a pyrometric method under irradiation of samples with (111) and (100) surface crystallography orientations. Calculated values of surface temperature at crystallization stage are in a reasonable agreement with experimental data. The revealed experimentally difference in melt undercooling at crystallization stage for (100) and (111) surface orientations is explained within the framework of the nonequilibrium model.
Phase transitions induced by laser irradiation in CdTe wafers have been studied in situ and experimental and computer simulation methods. The samples were irradiated by ruby laser with pulse duration 80 ns in energy density range from 0.02 to 0.5 J/cm2. Time-resolved reflectivity (TRR) measurements were carried out at the wavelengths of λ1=1.064 μm and λ2=0.532 μm. Dynamics of transmissivity was studied at λ1. Photoluminescence of CdTe, excited by the ruby laser single pulse, was also investigated. The character of TRR transients changes with the increase of irradiation energy density. The changes are more considerable at λ1 than at λ2. The time dependencies of reflectivity are explained by the changes of optical parameters of CdTe in course of laser-induced melting, solidification and evaporation. The experimental data obtained from transmissivity and photoluminescence measurements correlate with those from TRR transients. Laser-induced melting, crystallization and evaporation processes were studied on the basis of the computational solution of a two-phase moving boundary problem with two moving interfaces. The calculated dependency of melt duration on energy density is in a reasonable agreement with experimental data. From our investigation it follows that in the molten state CdTe is characterized by constant or weakly changing reflectivity in the temperature range from Tm to 3000 K.
Solid-liquid phase transitions induced in monocrystalline GaAs by nanopulsed radiation of a ruby laser were studied by means of time-resolved reflectivity and pyrometric measurements. GaAs samples were irradiated in transparent liquid media (alcohol’s and toluene) or in air ambient. Dependencies of the peak temperature and melt duration on laser energy density have been determined. Experimental data obtained for a case of irradiation of GaAs samples in air are compared with the results of computer simulation of the laser-induced phase transitions.
The phase transitions in A3B5 single crystals have been studied. GaAs, GaSb and InSb wafers oriented in (100) and (111) plains were irradiated by ruby (80 ns FWHM) and ArF excimer (10 ns, (lambda) equals193 nm) lasers. Dependence of melt duration (tau) on the irradiation energy density W was measured. The values of (tau) were determined from the time-resolved reflectivity measurements (TRR) at (lambda) equals1.06 (ruby laser) and 0.63 micrometers (ArF). The average discrepancies between (tau) 100 and (tau) 111 established for each semiconductor are related to somewhat slower velocity of crystal growth in <111> direction epitaxial process in this direction is characterized by lower crystallization temperature Tcr and by larger undercooling of the liquid phase. The values of Tcr(100) and Tcr(100) for InSb can differ by almost 10%. Increase in difference between (tau) 111 and (tau) 100 values going from GaAs to GaSb and to InSb correlates with the decrease in interface temperature and with the increase of average atomic mass, i.e., with the reduction of mobility and diffusion of atoms (ions) in binary melt.
Liquid-solid phase transitions induced in monocrystalline GaAs by two laser beam irradiation have been studied by a numerical modeling. The modeling was carried out on the basis of solving the Stefan problem in 1D approximation by a finite difference method. Two variants of combined irradiation by Q-switched ruby and CW Nd:YAG lasers were considered. In the first variant nanosecond radiation from a ruby laser induced the surface melting of a GaAs wafer and 'switches on' the absorption of additional CW intensive radiation directed from the back side of the wafer through its volume. Two laser beams are directed from one side in the second variant of combined irradiation. As it follows from the data obtained, the motion of the liquid-solid interface can be controlled by changing the intensity of CW radiation. Because of strong temperature dependence of optical absorption in solid GaAs at (lambda) equals 1064 nm, a heat wave moving toward Nd:YAG laser radiation can arise near the liquid-solid interface in opposite geometry and screen the melt from the CW laser beam. In the case of one- sided geometry the time dependence of melting depth has a nonmonotone character; the crystallization process can be terminated and the melting develops again.
Crystal 4—< liquid phase transitions induced in monocrystalline silicon surface layers by pulsed irradiation of a ruby laser have been studied using in situ methods and also by numerical modeling the laser - induced thermal processes. Hydrodynamic phenomena and convective heat transfer from the liquid surface absorbing laser radiation to the melt-crystal interface are possible at the melting stage. During epitaxial crystallization, the undercooling of liquid Si at <1 1 1< crystal growth direction is '-15 K more than the same for <100< and <1 10< directions. Two kinetics regimes characterize the epitaxial process to various directions. This regimes differ not only in undercooling, but also in morphology of the liquidsolid interface which can be atomically smooth or rough.
Solid-liquid phase transitions initiated by nanosecond heating of Si surface with monopulse radiation of a ruby laser have been studied. Changes in the aggregative state of Si were diagnosed by optical probing of irradiated zone at 0.53 or 1.06 micrometers and by means of thermal radiation pyrometry at the effective wavelength of 525 nm. Dynamics of the probing radiation reflection and scattering were studied by directing the probe beam at the central region of the irradiated zone both directly and from the backside through the specimen volume. Numerical modelling has been conducted of the phase transitions and optical absorption at (lambda) equals 1.06 micrometers under the internal reflection of probing radiation from moving liquid-solid interface. It has been shown that the surface temperature of the melt is nearly constant at the epitaxial crystallization stage. At the onset of melting and at the end of solidification stage, the Si surface is optically nonuniform due to coexistence of crystal and liquid microscale areas. The typical size of heterogeneous surface irregularity has been estimated. Local fluctuations at the moving liquid-solid interface result in irregularity of the liquid-solid interface that causes scattering of the probe radiation. Possibility has been demonstrated of using optical diagnostics of phase transitions occurring under laser irradiation of a semiconductor immersed in translucent liquid (water), where traditional optical probing is not effective.
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