Nanoscale concentration and strain distribution in pseudomorphic films Si_(1-x)Ge_x/Si processed by pulsed laser induced epitaxy

摘要

We report on the structural analysis of Si_(1-x)Ge_x pseudomorphic layers synthesized by pulsed laser induced epitaxy (PLIE) using a nanosecond excimer laser. We focus here on the local determination of strain and related Ge concentration. First, a Ge amorphous layer is predeposited on a Si substrate. Successive laser pulses induce the incorporation of Ge atoms in the molten substrate layer and lead to the synthesis of a graded Si_(1-x)Ge_x alloy over a depth which depends on the laser fluence. The Si_(1-x)Ge_x layers are coherently strained and free of defects. The in-depth Ge concentration distribution is investigated by RBS and HAADF STEM. The strain fields are specifically explored using the new dark-field electron holography (Holodark) technique, offering mapping of the full strain tensor in two dimensions with a high precision. Independently determined strain and Ge concentration distributions over a distance of 150 nm from the surface are found to be well consistent. An unexpected but reproducible depletion of Ge is evidenced inside the SiGe layer. This feature is shown to be related to the shape of the temporal characteristics of laser pulses. In particular, the second contribution, which occurs 32 ns later, is involved in a two-stage solidification process.