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

The in-depth distribution of hydrogen atoms in 100 nm thick amorphous-nanocrystalline silicon films (a-nc-Si:H) was estimated by Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) using previously described setup. The layer with nanocrystals were deposited on 50 nm amorphous layer by plasma enhanced chemical vapour deposition, using silane gas highly diluted with hydrogen. As seen by high resolution transmission electron microscopy (HRTEM), the films contained nanocrystals of silicon embedded in amorphous Si: H matrix. The size of crystals and crystal to amorphous fraction increased starting from substrate towards surface of the film. Amorphous matrix looked uniform except in the area close to a-Si:H/a-nc-Si:H interface where spots brighter than average appeared. These areas can be attributed to less density material, presumably voids. It is assumed that the surface of voids is “decorated” with hydrogen that saturates silicon “dangling bonds”. That is why distribution of hydrogen should indicate density fluctuation in material. Consistent with this assumption, TOF- ERDA showed non uniform distribution of hydrogen across the depth with maximum close to a-Si:H/a- nc-Si:H interface that coincidence with less density material seen by HRTEM. This supports the idea about important influence of voids in crystal formation, in particularly in nucleation phase. After heat treatment at 400oC, the distribution of hydrogen remained the same, while total hydrogen concentration decreased. This indicated that the type of hydrogen bonding was the same across the amorphous network and assumed that areas of less density materials consist of agglomerates of smaller voids.

Amorphous-nanocrystalline silicon; TOF-ERDA; Thin films; PECVD

NANOSMAT 2012 ; Jeff Th. M. De Hosson, Nasar Ali, Giuseppe Fierro, Mahmood Aliofkhazraei, Mircea Chipara (ur.).