Effects of laser scans on the diffusion depth and diffusivity of gallium in n-type 4H-SiC during laser doping

An n-type 4H-SiC substrate has been doped with gallium using a continuous wave Nd:YAG laser to heat the sample to high temperatures but below the peritectic temperature of SiC. Mathematical models have been presented for the temperature and Ga concentration distributions in the sample. The Ga atoms, which are produced due to the thermal decomposition of a metallorganic precursor, diffuse into the sample by the solid-phase diffusion process at high temperatures. This process is modeled by considering the temperature-dependent diffusion coefficient and the Ga concentration profile was measured by the secondary ion mass spectrometry (SIMS). The concentration of Ga (6.25 × 10²⁰ cm⁻³) at the substrate surface was found to exceed the solid solubility limit (1.8 × 10¹⁹ cm⁻³) of Ga in SiC. Comparing the SIMS data to the results of the diffusion model, the activation energy, pre-exponential factor and diffusion coefficient of Ga were determined for different doping conditions. Four doped samples were produced by scanning the samples with a laser beam for different number of passes. The sample prepared with four passes showed the highest diffusion coefficient of 5.53 × 10⁻⁷ cm²/s with activation energy 1.84 eV and pre-exponential factor 1.05 × 10⁻² cm²/s. The diffusion coefficient is five orders of magnitude higher than the typical diffusion coefficient of Ga in SiC. This indicates that the laser doping process enhances the diffusion coefficient of dopant significantly.