CO₂ laser processing of diffusion induced lattice imperfections in silicon: Experiment and theory

The high-temperature diffusion of phosphorus into crystalline silicon causes the formation of electrically inactive phosphorus-rich precipitates near the surface. These precipitates decrease the carrier lifetime and mobility in the diffused layer, and thus lead to less than optimal diode characteristics of electrical junctions formed by diffusion of phosphorus into a p-type substrate. We show that the free-carrier absorption of a CO₂ laser pulse can be used to completely dissolve the precipitates and remove dislocations in the diffused layer. Furthermore, we find that there are distinct advantages in depositing the pulse energy by way of free-carrier transitions, since the energy can be preferentially deposited in either confined doped layers or diffusion wells that are surrounded by lightly doped material. Our transmission electron microscopy results show that the annealing of the extended lattice defects is caused by melting of the near-surface region and subsequent liquid-phase epitaxial regrowth. Van der Pauw measurements are used to study the carrier concentration, mobility, and sheet resistivity of the samples before and after laser irradiation. The results of the electrical measurements show that there is a large increase in the carrier concentrations and a corresponding drop in the sheet resistivities of the laser irradiated samples. Using a Fourier transform infrared spectrometer, we find that significant changes occur in the transmittance and reflectance spectra after CO₂ laser annealing. Secondary ion mass spectrometry measurements are used to determine the redistribution of the phosphorus as a function of the pulse energy density. A time resolved pump-and-probe technique is utilized to measure the threshold for the onset of surface melting and the melt duration. We find that for energy densities greater than about 3 J/cm², the reflectivity of the probe laser (at 633-nm wavelength) jumps rapidly to 70%, which is consistent with the reflectivity of liquid silicon. The interpretation of the laser induced changes in the electrical, optical, and structural properties is based on a thermal model, in which surface melting occurs for incident pulse energy densities exceeding a threshold value. Comparative calculations are reported for the melt depths and duration of surface melting, and good agreement is found. Other calculated results for the transient heating and cooling of the near-surface region are also reported.