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

This paper reports the successful growth of high-quality, single-phase $\alpha$ -In $_2$ Se $_3$ single crystals with record-low carrier densities by employing a Se-flux assisted modified vertical Bridgman technique under high pressure, which effectively suppresses selenium evaporation and minimizes intrinsic defects.

Original authors: Soumi Mondal, Sreekant Anil, Saurav Islam, Yingdong Guan, Sai Venkata Gayathri Ayyagari, Aaron Pearre, Sandra Santhosh, Nasim Alem, Nitin Samarth, Zhiqiang Mao

Published 2026-09-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Soumi Mondal, Sreekant Anil, Saurav Islam, Yingdong Guan, Sai Venkata Gayathri Ayyagari, Aaron Pearre, Sandra Santhosh, Nasim Alem, Nitin Samarth, Zhiqiang Mao

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the world of materials science, some substances are prized not just for what they are, but for what they can do when they are perfect. One such material is indium selenide, a compound made from two elements that can act as a switch for electricity, a sensor for light, or a storage medium for data. Among its many forms, a specific version known as alpha-indium selenide has captured the imagination of researchers because it can hold an electric charge in a way that flips back and forth, a property called ferroelectricity. This behavior is the key to building faster, more efficient electronic devices. However, growing this material into a single, flawless crystal is notoriously difficult. The elements involved are temperamental; one of them, selenium, is highly volatile, meaning it wants to escape as a gas when heated. When it escapes, it leaves behind empty spaces in the crystal structure, known as vacancies. These missing atoms act like unwanted guests, flooding the material with extra electrical carriers that drown out the delicate switching behavior scientists are trying to study. For years, the best crystals grown by standard methods were still too full of these defects to be truly useful for advanced applications.

A team of researchers at Pennsylvania State University has now found a way to tame this volatility and grow crystals with far fewer defects than ever before. They developed a new approach to crystal growth that combines a specialized furnace design with a protective liquid layer and high pressure. Instead of trying to grow the crystal from a simple melt, they added a large amount of extra selenium to the mix, creating a selenium-rich environment that acts as a flux, or a solvent, to help the crystal form correctly. To keep the volatile selenium from escaping into the air, they covered the molten mixture with a layer of liquid boron oxide, which acts like a shield, and then cranked up the pressure inside the furnace to ten times the normal atmospheric pressure. This setup forced the selenium to stay put, allowing it to fill the crystal structure completely rather than leaving gaps behind.

The results of this experiment were striking. The researchers successfully grew large, single crystals of alpha-indium selenide that were chemically pure and structurally perfect. When they examined the crystals under powerful microscopes and analyzed their atomic arrangement, they confirmed that the material had formed in the correct three-layer stacking pattern, known as the 3R phase, which is essential for its unique properties. More importantly, when they measured how easily electricity moved through the material, they found that the number of unwanted electrical carriers had dropped to a record low. At room temperature, the carrier density was between 1.5 and 3.2 times ten to the power of sixteen per cubic centimeter, a value that is one to two orders of magnitude lower than what has been achieved with traditional methods. At very cold temperatures, this number dropped even further, to just four times ten to the power of fifteen. This level of purity suggests that the selenium vacancies, which are the primary source of electrical noise in these crystals, have been suppressed more effectively than in any previous attempt.

The team also discovered that the amount of extra selenium used during the growth process was the critical factor in determining the quality of the final product. They grew two separate batches of crystals: one with nearly thirty percent excess selenium and another with twenty percent excess. The batch with the higher amount of selenium produced crystals that were almost insulating, meaning they resisted the flow of electricity, which is exactly what is needed to observe the material's ferroelectric switching without interference. In contrast, the batch with less excess selenium still showed a significant number of defects and behaved more like a metal, conducting electricity too freely. This comparison proved that the selenium-rich environment was not just a minor adjustment but the key to controlling the crystal's internal chemistry. By using a higher concentration of the flux, the researchers were able to push the material closer to its ideal, defect-free state.

Despite these significant improvements, the journey is not entirely complete. While the new crystals are the cleanest ever made, they still contain enough residual defects to create a small leakage of electrical current. This leakage was strong enough to prevent the researchers from measuring the material's ability to switch its electric polarization, a crucial test for its use in future devices. The authors suggest that the solution lies in pushing the selenium concentration even higher in future experiments. They believe that by further increasing the amount of selenium flux, they can eliminate the remaining vacancies and finally achieve the perfect, insulating crystals required for practical applications. This work establishes a new, reliable method for growing not just indium selenide, but potentially other difficult materials that contain volatile elements, opening the door to a new generation of high-performance electronic components.

Technical Summary: Successful Growth of Low Carrier Density $\alpha$ -In $_2$ Se $_3$ Single Crystals

Problem Statement
Indium selenide (In $_2$ Se $_3$ ), particularly the $\alpha$ -phase, is a material of significant interest for applications in batteries, solar cells, photodetectors, and ferroelectric devices due to its room-temperature ferroelectricity and bulk photovoltaic effect. However, the controlled synthesis of single-phase $\alpha$ -In $_2$ Se $_3$ single crystals faces substantial challenges. The material exhibits a complex phase diagram with multiple polymorphs ( $\alpha, \beta, \gamma, \delta$ ) and stacking variants (2H and 3R), often leading to mixed-phase crystals. Furthermore, the high volatility of selenium induces non-stoichiometry and unintentional carrier doping via selenium vacancies ( $V_{Se}$ ). These vacancies act as electron donors, resulting in high carrier densities (typically $5\times10^{17}$ – $2\times10^{18}$ cm $^{-3}$ ) that generate leakage currents, obscuring polarization switching and limiting ferroelectric performance. Previous attempts to reduce carrier density through post-growth annealing were often performed on polycrystalline or "almost single crystals" lacking rigorous structural characterization, leaving the intrinsic defect levels of high-quality single crystals unverified.

Methodology
To address these synthesis challenges, the authors employed a modified vertical Bridgman technique utilizing a double-crucible geometry (DCVB) combined with liquid encapsulation under high pressure. The furnace design allows for a high-pressure, selenium-rich environment intended to suppress Se vaporization.

Growth Strategy: While the double-crucible setup was designed for continuous feeding, the high volatility of In $_2$ Se $_3$ prevented effective material transfer from the upper to the lower crucible. Consequently, the authors adopted a Se-flux assisted approach without continuous feeding.
Flux and Encapsulation: Two batches were grown with varying excess selenium concentrations relative to stoichiometric In $_2$ Se $_3$ : Batch-1 with 29.3% excess Se and Batch-2 with 20% excess Se. To prevent Se loss, boron oxide (B $_2$ O $_3$ ) was used as a liquid encapsulant due to its chemical inertness, low density, and melting point (~450°C).
Process Conditions: Growth occurred in an alumina crucible within a high-pressure chamber (10 atm Ar). The melt was homogenized at 980°C (above the In $_2$ Se $_3$ melting point of 890°C) and then translated downward at 0.9–1.0 mm/hr through a temperature gradient of ~25°C/cm to 770°C.
Characterization: The resulting crystals were analyzed using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDXS), four-probe electrical transport measurements, and angle-resolved photoemission spectroscopy (ARPES).

Key Results

Phase Purity and Structure: Structural analysis confirmed the growth of pure single-phase $\alpha$ -In $_2$ Se $_3$ with 3R stacking (rhombohedral, $R3m$ space group). TEM and selected-area electron diffraction (SAED) verified the ABC stacking sequence characteristic of the 3R polytype.
Carrier Density Reduction:
- Batch-1 (29.3% excess Se): Exhibited remarkably low carrier densities of $1.5\times10^{16}$ cm $^{-3}$ at 300 K, decreasing to $4\times10^{15}$ cm $^{-3}$ at 10 K. This represents the lowest carrier density reported to date for $\alpha$ -In $_2$ Se $_3$ single crystals, being 1–2 orders of magnitude lower than conventional Bridgman-grown crystals. The resistivity at 300 K was 20 $\Omega$ cm, indicating insulating behavior.
- Batch-2 (20% excess Se): Showed a higher carrier density of $2.32\times10^{17}$ cm $^{-3}$ at 300 K and metallic-like behavior, suggesting that reducing the Se flux concentration led to a higher concentration of Se vacancies.
Role of Se-Flux: The comparison between batches demonstrates that the amount of Se-flux is critical in controlling Se vacancies. The higher Se excess in Batch-1 effectively suppressed vacancy formation, whereas the lower excess in Batch-2 resulted in higher doping.
Electronic Structure: ARPES measurements revealed that Batch-1 samples were nearly undoped with no bulk conduction near the Fermi level in some pieces, while Batch-2 samples showed clear bulk conduction. Band gap estimates from ARPES were ~1.7 eV for Batch-1 and ~1.0 for Batch-2, consistent with the transport data.

Significance and Claims
The paper establishes the Se-flux assisted modified Bridgman method, combined with liquid encapsulation under high pressure, as an effective strategy for synthesizing large, single-phase $\alpha$ -In $_2$ Se $_3$ single crystals with significantly reduced intrinsic defects. The authors claim that the achieved carrier density ( $1.5-3.2\times10^{16}$ cm $^{-3}$ ) is the lowest reported for this material, reflecting a substantial suppression of Se vacancies compared to conventional growth methods.

However, the authors maintain a modest tone regarding the immediate applicability to ferroelectric measurements. Despite the record-low carrier density, reliable polarization-electric field (P-E) hysteresis loops could not be obtained due to leakage currents, indicating that the crystals are not yet sufficiently insulating for definitive ferroelectric characterization. The paper suggests that further increasing the Se-flux concentration may be necessary to achieve the near-stoichiometric composition required to eliminate residual bulk conduction. Ultimately, the work highlights the sensitivity of In $_2$ Se $_3$ growth to Se-flux ratios and encapsulation conditions, offering a promising pathway for growing other volatile chalcogenides with controlled stoichiometry and reduced defects.

Technical Summary: Successful Growth of Low Carrier Density α\alphaα-In2_22​Se3_33​ Single Crystals

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Technical Summary: Successful Growth of Low Carrier Density $\alpha$ -In $_2$ Se $_3$ Single Crystals