Photo-excitation and carrier relaxation in semiconductor tip under high dc electric field during laser-assisted atom probe tomography analysis

Abstract : Laser-assisted atom probe tomography (LA-APT) technique allows the 3D reconstruction of a small volume (a tip with ~100 nm in diameter) of different materials with near-atomic resolution. High voltage applied to the specimen generates strong dc field at the tip apex that ionizes its surface atoms. Ultrashort laser pulse illuminates the tip apex and triggers the field extraction of ions one by one. Then ions are collected by a position sensitive detector and chemically identified by time-of-flight mass-spectrometry. Recently, it was shown that for metals laser illumination plays the role of a thermal pulse when the tip is heated due to the absorption of laser energy or enhances the electric field at the specimen apex due to optical rectification effect. However, the mechanism ruling the laser-assisted field evaporation of semiconducting and insulating materials is not yet understood. Here, we present the results of numerical simulation of the interaction of the laser pulse with indirect (Si,Ge) and direct (GaAs) band gap semiconductors in the presence of high static electric field. The role of Franz-Keldysh effect, photo-generated charge separation and hole accumulation at the surface in the field evaporation process is considered. The tip temperature at the surface after the hot-carrier relaxation and recombination is obtained for IR, visible and UV laser light. The comparison of numerical and experimental results is presented.