Influence of the expansion dynamics of laser-produced gold plasmas on thin film structure grown in various atmospheres

Abstract

The expansion dynamics of plasma produced by excimer laser ablation of a gold target was measured by means of spatially resolved real time emission spectroscopy. The emission line of the Au(I) neutral gold species at 267.65 nm was used to monitor the expansion dynamics of the plasma in several background gases (He, N2, O2, and Ar). The measurements were performed as a function of the gas pressure (from 4×10−5 to 4 Torr) and target-to-substrate distance (from 1.5 to 11 cm). Gold thin films were prepared in the same conditions and their structure was analyzed by x-ray diffraction. All films prepared fall into one of three categories: highly (111) oriented, mixed, or polycrystalline. All the films prepared herein show a transition from highly (111) oriented to mixed and then to polycrystalline as the velocity of the Au(I) neutral gold species decreases. In the case of inert background gases (He, N2, and Ar), the velocity at which the transition between the various types of structure occurs is remarkably constant. Highly (111) oriented films are obtained for Au(I) neutral gold species exceeding v1=2.4 km s−1 (5.8 eV), while a polycrystalline (nanocrystalline) film is formed when the velocity falls below v2=0.8 km s−1 (0.6 eV). The conditions of distance and pressure at which these velocities are attained differ greatly from one atmosphere to the other, reflecting the fact that the interaction between the expanding plasma and the background gases varies with the molar mass of the gas. In the case of O2, the transition velocities between the different structures are higher than those observed in He, N2, and Ar [v1=8.3 km s−1 (70.4 eV) and v2=3.4 km s−1 (11.8 eV)]. This reflects a significant difference in the growth mechanism of these films compared to those prepared in an inert atmosphere.