The solubility of As in Si as an active, noncomplexed dopant is determined by completely activating with laser annealing a metastable As concentration of about 1×1021/cm−3, and subjecting it to thermal-equilibrium annealing. The solubility is 9.3×1019 cm−3 at 700 °C and 3.2×1020 cm−3 at 1000 °C. MeV ion channeling measurements show that when As deactivation from the metastable concentration takes place at 900 °C, the As atoms stay almost completely substitutional. The deactivation process has a thermal activation energy of 2.0 eV at 350–410 °C.
Temperature profiles induced by a cw laser beam in a semiconductor are calculated. The calculation is done for an elliptical scanning beam and covers a wide range of experimental conditions. The limiting case of a circular beam is also studied. This calculation is developed in the particular cases of silicon and gallium arsenide, where the temperature dependence of the thermal conductivity has been taken into consideration. Using a cylindrical lens to produce an elliptical beam with an aspect ratio of 20, a 1-mm-wide area of an ion-implanted silicon wafer was annealed in a single scan. The experimental data are consistent with the extrapolation of solid-phase epitaxial regrowth rates to the calculated laser-induced temperatures.
A set of simultaneous equations for lattice temperature, carrier concentration, and carrier temperature is numerically solved for typical nanosecond laser pulses. The temperature dependences of the thermal conductivity and lattice absorption are included, as well as the free carrier absorption and reflection. Carrier diffusion and electronic heat conduction are taken into account, and Auger recombination is assumed to be the dominant recombination mechanism. The calculations show that while free carrier absorption plays a major role in annealing with 1.06-μm radiation, only lattice absorption is important at wavelengths corresponding to photon energies well above the band gap. The Auger recombination coefficient is not a sensitive parameter, and the energy relaxation time does not affect the annealing results unless it is comparable to the pulse length. The results of the calculations are consistent with the hypothesis that the observed increase in silicon reflectivity is due to surface melting of the material. When literature values are used for all relevant parameters, the model predicts melt threshold energies which are in agreement with published experimental values.
The rate of solid-phase epitaxial regrowth of implantation amorphized 〈100〉 Si was studied in intrinsic, phosphorus-doped and compensated (boron- and phosphorus-doped) materials. The anneals were performed in flowing Ar gas in the temperature range from 477 to 576 °C, and the regrowth was analyzed by 2.2-MeV 4He+ channeling techniques. The intrinsic and compensated samples exhibited nearly equal growth rates with thermal-activation energies of 2.85 eV (intrinsic) and 2.8 eV (compensated). The growth rate in the 31P-doped (constant concentration of 1.7×1020 cm−3) was enhanced by a factor of 6 to 8, while little change in the activation energy was observed.
Complete electrical activity was obtained by cw laser annealing of 7×1015 As/cm2 implanted into (100) Si at 100 keV. The peak concentration for these implantation conditions is 1.4×1021/cm3, both theoretically and experimentally. However, this peak concentration was found to be thermally unstable, relaxing to a value of 3×1020/cm3 in a period of less than 2 min at 900 °C. If the peak implanted concentration is below 3×1020/cm3, the electrical activation and crystal structure are unaffected by similar thermal processing. We conclude from these data that the solid solubility of As in Si at 900 °C is approximately 3×1020/cm3, which is almost an order of magnitude below the published value.
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