Investigations on surface morphology and bandgap engineering of single crystal boron-doped silicon irradiated by a nanosecond laser.

We irradiate the single crystal boron-doped silicon (Si) at various laser fluences with 100 laser shots in ambient air at room temperature using an Nd:YAG laser and investigate its surface morphology and optical properties. The optical microscopy gives evidence of the formation of a crater and reveals that the heat-affected zone and melted area are increased with increase in laser fluence from 1.1 to 15.4  J/cm. The micrographs obtained by scanning electron microscopy (SEM) show that the micro- and nano-structures such as microcracks, bubbles, nucleation sites, clusters, redeposited layered material, nanoparticles, and alike water droplet structures are formed on a laser-exposed Si surface. The optical profilometry of the irradiated Si further confirms the ablation and redeposition of the material and shows that the depth of the crater is increased from 12.1 to 15.2 μm with increase in fluence from 1.1 to 15.4  J/cm. Raman spectroscopy of the samples shows that the irradiation generates anneal effects due to higher temperature, which increases the crystallinity of the Si. The ellipsometric analysis shows that the irradiation of Si with increasing laser fluence changes its optical constants (refractive index and extinction coefficient), which further influence its optical properties, e.g., reflectivity, absorptivity, and energy bandgap. The absorptivity of laser irradiated Si tends to increase with increasing laser fluence, and the energy bandgap is decreased accordingly due to increase in structural disorders. Our study shows that the controlled laser irradiation can tune the energy bandgap of exposed Si, and it makes the Si materials useful for the fabrication of optoelectronic devices such as solar cells, photovoltaic cells, and LEDs.