The Irradiance-Based Growth of Surface Structures Induced by Nanosecond Laser Pulses on Si and Ge and Their Correlation With Plasma Ion Kinetic Energies and Densities

Abstract

The laser irradiance-based surface structural growth on Si and Ge has been correlated first time with plasma parameters. The better control over plasma parameters makes manufacturing of various sized and shaped surface structures on the semiconducting materials. The effect of laser irradiances on surface morphology of Si and Ge has been explored. For this purpose, Nd: YAG laser (532 nm, 6 ns, 10 Hz) has been employed as an irradiation source at the various laser irradiances ranging from 4 to 7.1 GW/cm2 under the vacuum condition. Surface modifications of laser-ablated Si and Ge were analyzed by performing scanning electron microscope (SEM) analysis. It has been revealed that laser irradiance plays a significant role in the growth of the micro- and nanostructures on the laser-irradiated target surfaces. The surface morphology of laser-ablated Si and Ge exhibited the formation of various structures such as laser-induced periodic surface structures (LIPSS), cracks, spikes, ridges, and cones. Density and size of these structures have been found to be strongly dependent upon the laser irradiances. SEM analysis exhibits the cones formation at central ablated region of both Si and Ge. These cones become more distinct and pronounced with increasing the laser irradiance due to more energy deposition with Gaussian profile distribution at the central region. Microspikes were observed at boundaries of laser-ablated Si. Whereas, in case of Ge-ablated boundaries, wave-like ridges have been observed, which are then converted into globules at higher laser irradiances up to 7 GWcm−2. LIPSSs were seen at outer boundaries of laser-ablated Ge, whose periodicity varies with the laser irradiances. Faraday cup has been employed in order to probe the kinetic energy and density of laser-induced Si and Ge plasma ions at the similar values of laser irradiances. A correlation at similar values of laser irradiances has been established between the evaluated plasma ion parameters (kinetic energy and density of plasma ions) and observed structures for both materials. This correlation reveals the dependence of kinetic energy and density of plasma ions on the corresponding surface modification of both laser-ablated Si and Ge, as well as enables us for the better understanding of the laser-induced plasma to be used as ion source in various fields ion implantation, surface structuring, and material modification. The results of ion energies are explained by the generation of ambipolar field or self-generated electric field (SGEF) in the expanding plasma due to the charge separation and double-layer structure. The values of SGEF have also been evaluated at different laser irradiances.