Quantification of nanoscale density fluctuations in hydrogenated amorphous silicon

TL;DR

This study uses advanced scattering techniques to quantify nanoscale voids and dense domains in hydrogenated amorphous silicon, linking nanostructure to electronic properties and defect formation.

Contribution

It provides the first detailed quantification of voids and dense domains in a-Si:H, validating a structural model and connecting nanostructure to material properties.

Findings

Identification of 1.2 nm voids forming a superlattice

Detection of 1 nm dense ordered domains

Quantification of void concentrations up to 6×10^19 ccm

Abstract

The nanostructure of hydrogenated amorphous silicon (a Si:H) is studied by a combination of small-angle X-ray (SAXS) and neutron scattering (SANS) with a spatial resolution of 0.8 nm. The a-Si:H materials were deposited using a range of widely varied conditions and are representative for this class of materials. We identify two different phases which are embedded in the a-Si:H matrix and quantified both according to their scattering cross-sections. First, 1.2 nm sized voids (multivacancies with more than 10 missing atoms) which form a superlattice with 1.6 nm void-to-void distance are detected. The voids are found in concentrations as high as 6*10^19 ccm in a-Si:H material that is deposited at a high rate. Second, dense ordered domains (DOD) that are depleted of hydrogen with 1 nm average diameter are found. The DOD tend to form 10-15 nm sized aggregates and are largely found in all a-Si:H materials considered here. These quantitative findings make it possible to understand the complex correlation between structure and electronic properties of a-Si:H and directly link them to the light-induced formation of defects. Finally, a structural model is derived, which verifies theoretical predictions about the nanostructure of a-Si:H.