First-principles calculations of iron-hydrogen reactions in silicon

TL;DR

This study uses first-principles calculations to investigate how hydrogen interacts with iron impurities in silicon, revealing insights into defect structures, electronic levels, and implications for silicon contamination control.

Contribution

It provides detailed computational analysis of FeH complexes in silicon, clarifying their electronic properties and stability, which was previously not well understood.

Findings

Fe_iH pair has a migration barrier similar to Fe_i

Fe_iH_2 acts as a hole trap at E_v+0.32 eV

Fe_sH complexes are deep acceptors with little impact on minority carriers

Abstract

Controlling the contamination of silicon materials by iron, especially dissolved interstitial iron (Fe$_{\mathrm{i}}$), is a longstanding problem with recent developments and several open issues. Among these we have the question whether hydrogen can assist iron diffusion, or if significant amounts of substitutional iron (Fe$_{\mathrm{s}}$) can be created. Using density functional calculations we explore the structure, formation energies, binding energies, migration, and electronic levels of several FeH complexes in Si. We find that a weakly bound Fe$_{\mathrm{i}}$H pair has a migration barrier close to that of isolated Fe$_{\mathrm{i}}$ and a donor level at $E_{\mathrm{v}}+0.5$~eV. Conversely, Fe$_{\mathrm{i}}$H$_{2}(0/+)$ is estimated at $E_{\mathrm{v}}+0.33$~eV. These findings suggest that the hole trap at $E_{\mathrm{v}}+0.32$~eV measured by capacitance measurements should be assigned to Fe$_{\mathrm{i}}$H$_{2}$ . Fe$_{\mathrm{s}}$H-related complexes show only deep acceptor activity and are expected to have little effect on minority carrier life-time in $p$-type Si. The opposite conclusion can be drawn for $n$-type Si. We find that while in H-free material Fe$_{\mathrm{i}}$ defects have lower formation energy than Fe$_{\mathrm{s}}$, in hydrogenated samples Fe$_{\mathrm{s}}$-related defects become considerably more stable. This would explain the observation of an EPR signal attributed to a Fe$_{\mathrm{s}}$H-related complex in hydrogenated Si, which was quenched from above 1000$^{\circ}$C to iced-water temperature.