The formation of the positive, fixed charge at c-Si(111)/a-Si$_3$N$_{3.5}$:H interfaces

TL;DR

This study uses high throughput calculations to analyze c-Si/a-SiN:H interfaces, revealing how charge transfer causes positive fixed charges that influence device behavior.

Contribution

It provides a detailed atomic-level understanding of fixed charge formation at c-Si/a-SiN:H interfaces, combining computational modeling with interface physics.

Findings

Threefold coordinated Si atoms are present at the interface.

Charge transfer from a-SiN:H to c-Si causes positive fixed charge.

The fixed charge repels holes, affecting device performance.

Abstract

Modern electronic devices are unthinkable without the well-controlled formation of interfaces at heterostructures. These often involve at least one amorphous material. Modeling such interfaces poses a significant challenge, since a meaningful result can only be expected by using huge models or by drawing from many statistically independent samples. Here we report on the results of high throughput calculations for interfaces between crystalline silicon (c-Si) and amorphous silicon nitride (a-Si$_3$N$_{3.5}$:H), which are omnipresent in commercially available solar cells. The findings reconcile only partly understood key features. At the interface, threefold coordinated Si atoms are present. These are caused by the structural mismatch between the amorphous and crystalline part. The local Fermi level of undoped c-Si lies well below that of a-SiN:H. To align the Fermi levels in the device, charge is transferred from the a-SiN:H part to the c-Si part resulting in an abundance of positively charged, threefold coordinated Si atoms at the interface. This explains the existence of a positive, fixed charge at the interface that repels holes.