Dynamics Simulation Of Nanosecond Pulsed Laser Ablation Of Ge Crystal

Pulsed laser ablation (PLA) technology is widely used in high technology fields due to its large numbers of outstanding advantages. Especially have playing very important role in micro-electronics/optoelectronic elements manufacturing, nanomaterials preparation and new components forming fields. The deep research of pulsed laser ablation process and its basic mechanisms will further promote the development of PLA technology.In the paper, one-dimensional dynamic semiconductor model was presented based on the conductor metal laser ablation model. The UV laser ablation of germanium crystal and the expansion of ablated plume in the background helium gas were researched. The target heating, the plume expansion in a background gas, plasma formation and laser absorption, the electron thermal conduction, the binary diffusion and the back flux onto the target were considered simultaneously in this model. In addition, the system of equation was solved by Newton-Raphson method, finite difference method and Riemann solver.The model has been used to investigate the influence of different laser irradiance and ambient gas pressure on the laser ablation and the expansion of plume. In addition, the laser ablation of Ge crystal and the characters of plume in a background gas at 1 torr and the conditions under consideration (laser pulse with 1×10~8w/cm~2 and 4.0×10~8w/cm~2 peak irradiance, 248nm wavelength, and 7ns FWHM) were studied. The simulated temporal evolution of surface temperature of the target, depth of target evaporation, evaporation rate and spatial distribution temperature, velocity and number density of ablated plume were investigated. The conclusions are as follows:(1) The laser ablation is greatly influenced by laser irradiance. The more intensive the laser irradiance is, the higher surface temperature it would have. In this case, the depth of evaporation, the expansion velocity of ablated plume and plume length will also increase with the rising of laser irradiance. Meanwhile the more intensive the laser irradiance is, the earlier the plasma shielding would appear. For the given conditions, the threshold of plasma shielding is between 1×10~8w/cm~2 and 1.5×10~8w/cm~2.(2) In the background of inert gas existing ambient, the target surface temperature and depth of evaporation depends lightly on the change of ambient pressure. However, the ablated plume expansion velocity and plume length decrease with the increase of gas pressure. (3) The plume expansion is greatly influenced by the presence of ionization and laser absorption in the plume. A part of the absorbed energy from the laser can be transferred into the plume kinetic energy.(4) The spatial distribution of ionization degrees at 17ns under the fixed laser ablation condition is studied. The results show that the ablated plume has been fully ionized. The first-order ionization degree of Ge always dominates in the region close to the target surface. In the centre of plume, the second-order ionization degree of Ge is a little bigger than the first-order ionization degree of Ge. In the shock wave region, the ionization degree of Ge depends critically on the high temperature caused by the shock front heating.