Successful growth of low carrier density $\alpha$-In$_2$Se$_3$ single crystals using Se-flux in a modified Bridgman furnace

TL;DR

This paper presents a novel Se-flux assisted modified Bridgman technique for growing high-quality, low-carrier-density Se crystals, crucial for ferroelectric applications, demonstrating significantly reduced defects and controlled stoichiometry.

Contribution

The study introduces a unique high-pressure Se-flux Bridgman method that effectively minimizes Se-vacancies in Se crystals, advancing synthesis techniques for volatile chalcogenides.

Findings

Achieved the lowest reported carrier density of 1.5-3.2 0^{16} cm^{-3} at 300K.

Confirmed pure Se with 3R stacking via structural analysis.

Demonstrated Se-flux control over Se-vacancies and defect suppression.

Abstract

Indium selenide (In$_2$Se$_3$) has garnered significant attention for its intriguing properties and applications in batteries, solar cells, photodetectors and ferroelectric devices. However, the controlled synthesis of single phase $\alpha$-In$_2$Se$_3$ remains challenging owing to its complex phase diagram, presence of multiple polymorphs and the high volatility of selenium that induces non-stoichiometry and unintentional carrier doping. For ferroelectric {\alpha}-In2Se3, minimizing the carrier density is essential because leakage current can obscure polarization switching. Here, we report the growth of $\alpha$-In$_2$Se$_3$ single crystals using a unique approach, the Se-flux assisted modified vertical Bridgman technique combined with liquid encapsulation under high pressure. This approach creates a high-pressure, Se-rich environment that effectively minimizes Se-vaporization. Structural and compositional analysis using X-ray diffraction, transmission electron microscopy and energy-dispersive X-ray spectroscopy confirm the formation of pure $\alpha$-In$_2$Se$_3$ single crystals with 3R stacking. Furthermore, the crystals exhibit remarkably low carrier density of 1.5-3.2 $\times$ 10$^{16}$ cm$^{-3}$ at 300K$-$the lowest reported to date, reflecting a significant suppression of Se-vacancies relative to the conventional Bridgman or melt-grown crystals. Through transport and ARPES measurements on different batches of crystals, we also demonstrate that the amount of Se-flux plays a crucial role in controlling Se-vacancies. Our results thus establish this modified Bridgman method as an effective strategy for synthesizing large $\alpha$-In$_2$Se$_3$ single crystals with reduced intrinsic defects. This technique can be broadly applied to grow other volatile chalcogenides with reduced defects and controlled stoichiometry.