Role of native point defects and Hg impurities in the electronic properties of Bi$_4$I$_4$

TL;DR

This study investigates how native point defects and Hg impurities influence the electronic properties of Bi$_4$I$_4$, revealing defect-induced doping effects and pathways for tuning its electronic behavior.

Contribution

The paper combines experimental transport measurements with density functional theory calculations to identify dominant native defects and impurity effects in Bi$_4$I$_4$, providing new insights into defect-controlled electronic properties.

Findings

Native defects and Hg impurities significantly alter resistivity.

Bi antisites create resonant states at band edges.

Iodine antisites and Bi vacancies induce n- and p-type doping.

Abstract

We studied the effects of point defects and Hg impurities in the electronic properties of bismuth iodide (Bi$_4$I$_4$). Our transport measurements after annealing at different temperatures show that the resistivity of Bi$_4$I$_4$ depends on its thermal history, suggesting that the formation of native defects and impurities can shape the temperature dependence of electrical resistivity. Our density functional theory calculations indicate that the bismuth and iodine antisites, and bismuth vacancies are the dominant native point defects. We find that bismuth antisites introduce resonant states in the band-edges, while iodine antisites and bismuth vacancies lead to a $n$-type and $p$-type doping of Bi$_4$I$_4$, respectively. The Hg impurities are likely to be found at Bi substitutional sites, giving rise to the $p$-type doping of Bi$_4$I$_4$. Overall, our findings indicate that the presence of native point defects and impurities can significantly modify the electronic properties, and, thus, impact the resistivity profile of Bi$_4$I$_4$ due to modifications in the amount and type of carriers, and the associated defect(impurity) scattering. Our results suggest possible routes for pursuing fine-tuning of the electronic properties of quasi-one-dimensional quantum materials.