Shallow melting of thin heavily doped silicon layers by pulsed CO2 laser irradiation

We show that an extremely shallow (≲800 Å) melt depth can be easily obtained by irradiating a thin (∼200 Å) heavily doped silicon layer with a CO2 laser pulse. Since the absorption of the CO2 laser pulse is dominated by free-carrier transitions, the beam heating occurs primarily in the thin degenerately doped film at the sample surface, and there is little energy deposited in the underlying lightly doped substrate. For CO2 pulse-energy densities exceeding a threshold value of about 5 J/cm2, surface melting occurs and the reflectivity of the incident laser pulse increases abruptly to about 90%. This large increase in the reflectivity acts like a switch to reflect almost all of the energy in the remainder of the CO2 laser pulse, thereby greatly reducing the amount of energy available to drive the melt front to deeper depths in the material. This is in contrast to the energy deposition of a laser pulse that has a photon energy exceeding the band gap, in which case the penetration depth of the incident radiation is only weakly affected by the free-carrier density. Transmission electron microscopy shows no extended defects in the near-surface region after CO2 laser irradiation, and van der Pauw electrical measurements verify that 100% of the implanted arsenic dopant is electrically active. Calculated values for the melt depth versus incident pulse-energy density (EL) indicate that there exists a window where the maximum melt-front penetration increases slowly with increasing EL and has a value of less than a few hundred angstroms. For silicon specimens having a thin degenerately doped film at the surface and a lightly doped substrate, the two primary reasons for using a CO2 laser pulse to achieve very shallow melt depths are (1) the pulse energy is deposited only in the thin surface layer and (2) the melting of this layer causes the reflectivity to jump abruptly to a value of almost unity.