Analysis of amorphous-nanocrystalline silicon thin films by time-of-flight elastic recoil detection analysis and high-resolution electron microscopy

Abstract

The in-depth distribution of hydrogen atoms in 100nm-thick, amorphous-nanocrystalline, silicon films (a-nc-Si:H) was estimated by time-of-flight elastic recoil detection analysis (TOF-ERDA) using a previously described set-up. The layer with nanocrystals was deposited on a 50nm amorphous layer by plasma-enhanced chemical vapor deposition (PECVD), using silane gas that was diluted with hydrogen. High-resolution transmission electron microscopy (HRTEM) showed that the films contained nanocrystals of silicon embedded in an amorphous Si:H matrix. The size of the nanocrystals and the crystal-to-amorphous ratio increased in the direction from the substrate toward the surface of the film. The amorphous matrix appeared uniform, except for the area close to the a-Si:H/a-nc-Si:H interface, where spots that were brighter than average appeared. These areas can be attributed to the presence of less-dense material, presumably voids. It is assumed that the surface of the voids is “decorated” with hydrogen that saturates the silicon “dangling bonds”. This is why the distribution of hydrogen should indicate a density fluctuation in the material. Consistent with this assumption, the TOF-ERDA showed a non-uniform distribution of hydrogen across the depth, with a maximum value close to the a-Si:H/a-nc-Si:H interface that coincides with the less-dense material seen by the HRTEM. This supports the idea about the important influence of voids on crystal formation, particularly in the nucleation phase. After a heat treatment at 400°C, the distribution of hydrogen remained the same, while the total hydrogen concentration decreased. This indicated that the type of hydrogen bonding was the same across the amorphous network and assumed that the areas of less-dense material are agglomerates of smaller voids.