Fabrication and Optimization of Type II Silicon Clathrate Films

Summary

The two-thermal annealing method enables the fabrication of thin films of type II semiconductor silicon clathrate. Post-synthesis treatments, including thermal pressing and reactive ion etching, were performed to improve the material properties. This approach provides an accessible pathway for fabricating tunable clathrate films suitable for next-generation electronic and photonic devices.

Abstract

Silicon clathrates (SiCL) are a class of cage-structured materials with tuneable optoelectronic properties, offering significant potential for photovoltaic applications. Among them, type II SiCL (Na_xSi₁₃₆) are particularly attractive due to their unique ability to reversibly accommodate guest atoms without compromising the crystal structure. Traditional synthesis methods, such as synthesis under extreme pressure or fabrication under glove boxes, although essential for probing the intrinsic properties of clathrates under well-controlled conditions, are not well-suited for scaling up and are often energy-intensive and require specialized equipment. To overcome these limitations, a scalable, glovebox-free synthesis method based on two thermal decomposition steps was developed to produce silicon clathrate films. Post-synthesis treatment, including thermal pressing and reactive ion etching, was employed to enhance the electrical properties of the material. Phase formation and crystallinity are confirmed by X-ray diffraction and Raman spectroscopy; morphology is assessed by scanning electron microscopy (SEM); and optoelectronic properties are evaluated by photoluminescence. This approach provides a reproducible and scalable route to fabricate type II silicon clathrate films, with promising potential for integration into optoelectronic devices.

Introduction

Since the 1950s, silicon-based materials have been at the heart of modern optoelectronic and energy technologies¹, particularly in photovoltaic and semiconductor industries². Silicon fulfils the abundance, thermal stability, aging resistance³, and non-toxicity criteria^4,5. However, despite its commercial success, diamond silicon (d-Si) suffers from intrinsic limitations, such as an indirect band gap and weak light absorption⁶, requiring thick active layers and energy-intensive processing to achieve extremely high elemental purity⁷. In recent years, alternative allotropes and exotic phases of silicon, particularly clathrate phases⁸, have garnered increasing attention as promising candidates for overcoming these limitations⁹. Applications are promising due to their open-framework structure and tunable electronic properties¹⁰. Among these, type II SiCL (Na_xSi₁₃₆) has emerged as a metastable yet robust material with a quasi-direct band gap in the range of 1.6-2 eV^9,11.

The synthesis of SiCL relies on high-pressure methods¹², such as flow growth¹³ or redox chemistry in ionic liquids¹⁴, which are often complex, not always compatible with scale-up and thin-film processing. Nevertheless, these traditional approaches were essential to probe the intrinsic properties of clathrates by suppressing extrinsic effects such as oxidation or secondary phase^15,16,17. In contrast, this glove box-free two-step thermal decomposition protocol enables the synthesis of silicon clathrate thin films involving the reactive Na₄Si₄, intermediate compound, using moderate temperatures and vacuum conditions. This protocol enables precise control of the sodium content throughout the duration of the thermal treatment, thereby altering the behaviour of Na_xSi₁₃₆ from metal to semiconductor depending on the occupation of sodium in the silicon cages. In this process, a Na₄Si₄ precursor film is first formed by the thermal reaction of sodium with crystalline silicon under an inert atmosphere (Figure 1). Subsequent thermal annealing under dynamic vacuum promotes the extraction of sodium from the film, allowing the rearrangement of the Zintl phase into the desired clathrate framework, with minimal contamination^18,10 (Figure 2).

While the structural formation of clathrates through this method has been demonstrated, further insight into their tuneability and semiconducting behaviour is achieved after two successive post-synthesis treatments, thermal pressing, and reactive ion etching. Thermal pressing is performed to reduce the clathrate film thickness by approximately half, resulting in a dense, smooth layer that was better suited for further processing. Etching has been shown to further improve the homogeneity of the surface state while preserving the cage framework, making it a powerful tool for tuning electrical and optical properties¹⁹.

While the number of laboratories having successfully reported^20,21,22 SiCL film fabrication can be counted on the fingers of one hand, we believe that this is due to the fact that the processing parameters must be carefully tuned for successfully obtaining the desired phase. With this current protocol, while ensuring safety using appropriate measures, SiCL film fabrication requires only one tubular furnace in argon and one dynamic vacuum furnace, pure sodium, and a c-Si wafer¹⁰. This approach demonstrates the feasibility of fabricating and engineering SiCL films through a simple and scalable synthesis process, with minimal resources and equipment, paving the way for future device integration.

Protocol

The reagents and the equipment used in this study are listed in the Table of Materials.

1. Preparation of materials

1. Preparation of silicon (c-Si (001))
   1. Precisely cleave with a diamond-tipped pen a 525 µm ± 25 µm thick, single-side polished, n-doped silicon (001) wafer into a rectangle with dimensions of 56 × 38 mm².
   2. Immerse the silicon substrate in a hydrofluoric acid solution with a concentration of 10% vol., using a 48% HF bottle, for exactly 2 min to remove the native silicon dioxide that could be present on the substrate surface.
      NOTE: All HF-containing liquids and any contaminated rinsed/solvents were collected in dedicated, compatible HDPE (High-Density Polyethylene) containers labelled with the producer unit and mixture composition, stored on retention trays in the designated waste area, and kept segregated from other acids, with HF specifically isolated for safety. Final disposal was arranged through SPSE-UNISTRA/Suez in accordance with the University of Strasbourg disposal rules, which follow the French environmental code and dangerous-goods transport rules.
      CAUTION: Hydrogen fluoride (HF) is highly toxic and corrosive. Handle only under a fume hood, wearing gloves, a full-face mask, and appropriate protective clothing.
   3. After exposure to HF, thoroughly rinse the substrate with deionized water to remove any acid. Dry the wafers using a nitrogen gun to blow away any residual water droplets and prevent residue formation on the surface.
2. Sodium pretreatment
   1. Carefully cut a piece of metallic sodium (99% purity) using a cutter to obtain a small rectangular slice weighing approximately 0.22 g, measured with an analytical balance.
   2. Immediately place the sodium slice in an airtight glass (sealed with a ground-glass joint) container filled with anhydrous cyclohexane (≥99% purity). Ensure that the sodium is fully immersed to prevent oxidation.
      CAUTION: Metallic sodium reacts violently with water. Always handle under a fume hood, wearing gloves, safety goggles, an appropriate lab coat, and take any other precautions necessary to ensure safety (class D fire extinguisher).
3. Assembly in the crucible
   1. Wash with pure ethanol and dry at 300 °C for 4 h in an Inconel alloy boat (54 × 18 × 13.3 mm³). Then, place the previously prepared sodium slice into a cleaned Inconel alloy boat and position the silicon wafer directly above it, with the polished surface pointing down the boat.
   2. Carefully insert the prepared assembly (Boat-Sodium-Silicon) into the centre of a sealed stainless-steel tube (ISO-KF with O-ring seal), in a programmable horizontal tube furnace.
      NOTE: The tube must be thoroughly cleaned with acetone and ethanol, and then dried for 1.5 h.
   3. Insert a high-purity tantalum (Ta) wire into the tube to trap any traces of oxygen. The wire used has a diameter of 0.5 mm, a purity of 99.95%, and a total length of 3.4 cm.
   4. Seal the tube at both ends with fittings that allow only argon (purity ≥ 99.9999%) to circulate inside the tube.
   5. Purge the tube with argon at a constant pressure of 1.6 bar for 15 min to ensure an inert atmosphere throughout the reaction.

2. Synthesis of SiCL

1. Thermal decomposition for the formation of Na₄Si₄
   1. Start the programmable horizontal tube furnace with the following annealing cycle.
      1. Raise the temperature until reaching the 600 °C plateau with ramp rate of 5 °C/min. Maintain this temperature stable at 600 °C for 19 h.Allow the system to cool down naturally for over 7 h.
   2. When the furnace and tube are back to room temperature, flush the tube with a continuous flow of argon (purity ≥ 99.9999%).
2. High vacuum dynamic post-treatment
   1. Transfer as fast as possible the obtained samples into a quartz tube and connect it to the dynamic vacuum furnace. The tube was pre-cleaned by sequential rinsing with acetone and deionized water, followed by drying with nitrogen gun. The exposure time to ambient air was kept below 40 s in a laboratory environment with relative humidity maintained below 40% which was found to yield reproducible and uncontaminated films.
      1. After connection of the tube to the pumping system, evacuate first with the primary pump, then engage the turbomolecular pump until a high vacuum of 5 × 10^-7 mbar is reached.
   2. Ramp to 400 °C over 30 min; hold at 400 °C for 4 h; then switch off the tubular furnace and allow the sample to cool naturally to room temperature. After that, unload the sample from the furnace.

3. Post-treatment

1. Press annealing
   1. Use a manual table-top hydraulic press equipped with two square stainless-steel plates (15 × 15 cm²) and an integrated heating system capable of reaching temperatures up to 300 °C for the press annealing process.
   2. Fix the upper plate and apply force by raising the lower plate. Operate the imprinting process in an ambient atmosphere.
   3. Place the silicon clathrate sample between the two plates inside the thermal press.
   4. Lift the lower plate to gradually increase the force until the pressure reaches a value of around 2 kN.
   5. When the force reaches 2 kN, increase the temperature of the two metal plates in contact with the sample to 250 °C and maintain it for 30 min.
   6. During the thermal pressing, allow the force to increase gradually from the initial 2 kN to around 8.5 kN by the end of the process.
   7. After 30 min, stop heating. Apply rapid cooling using a cold-water circulation system (20 °C/min) until the temperature returns to ambient. Release the pressure.
      NOTE: At the end of the process, the thickness, determined from cross-sectional SEM images, is reduced from 82 µm to 31 µm. The grain size of the pressed material reaches approximately 200 nm, which is four times higher than that of the as-synthesized one.
2. Dry etching
   1. Clean the sample sequentially using acetone and isopropanol (IPA), then dry it with a nitrogen gun, which blows off residual solvent droplets and prevents residue formation.
   2. Place the cleaned sample onto the lower electrode inside the RIE etch system.
   3. Pump down the chamber to reach a base pressure of 5 × 10^-7 mbar.
   4. Introduce the etching gas (SF₆) into the chamber at a flow rate of 50 sccm.
      NOTE: All SF₆ exhaust from etching processes must pass through an abatement system. Direct venting to the atmosphere is prohibited. The scrubber must be activated before gas flow begins and kept running until purging is complete.
   5. Set the substrate temperature to 5 °C using the integrated cooling system.
   6. Ignite the plasma by applying 800 W ICP power to generate a high-density plasma, and simultaneously apply 400 W RF power (13.56 MHz) to the lower electrode to control ion bombardment energy.
   7. Perform etching tests with durations of 15 s. Under these conditions, the etching rate was measured to be 1 µm·min^-1 on the pressed sample.
   8. After etching, turn off both the ICP and RF power supplies to stop the plasma.
   9. Evacuate remaining process gases, then slowly vent the chamber to atmospheric pressure.
3. Remove the etched sample from the chamber.

Results

This study explored the structural and optical evolution of type II clathrate Na_xSi₁₃₆ films before and after pressing and etching SiCL films, using a combination of X-ray diffraction, Raman spectroscopy, and photoluminescence measurements. The first step is to ensure that the right phase is obtained after fabrication and after post-processing steps. Figure 3 shows the X-ray diffraction (XRD) θ-2θ pattern (Cu K_α source) which confirms the formation of the typ...

Discussion

The synthesis of type II silicon clathrate films (Na_xSi₁₃₆) via the two-step thermal decomposition method involves two critical steps that determine the success and quality of the final material. The initial formation of the Na₄Si₄ Zintl precursor is pivotal (Figure 1), as it serves as the reactive intermediate phase²⁷. This step requires the exposure of a silicon wafer to sodium vapor under an inert argon atmospher...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetone	Sigma-Aldrich	32201-2.5L-M
Airtight glass		custom made
Analytical balance	Sartorius	1409 001
Anhydrous cyclohexane	Thermo Scientific	11403087	≥99% purity
Argon gas	LINDE GAS	2890122 2010	Bottle size S11Purity 99,9999%
Diamond tipped pen	Oxford Instruments Sarl	T5357
Dry Scroll Pump	Edwards	XDS10
Ethanol	VWR Chemicals	20821.365
Horizontal tube furnace	CARBOLITE GERO	EST12150B-230SN	Programmable furnace
Hydraulic press	Rondol		Manual, table-top, with heating plates
Hydrofluoric acid (HF)	VWR Chemicals	20319.291	48% HF used to make 10% vol. solution
Inconel alloy boat	ANALAB	AR9001	54 x 18 x 13.3 mm3
Isopropanol	VWR Chemicals	20842.367
Metallic sodium	Thermo Scientific Chemicals	L13285.24	99% purity
Quartz tube	ALC Quartz	custom made	external diameter: 65.5 ± 0.3 mm, internal diameter: between 57 and 60 mm, length: 550 mm
RIE etch system	CORIAL	210IL
segmented blade cutter	Facom	844.S9PB	9 mm blade width
Silicon (c-Si (001)) wafer	BT Electronics	from stock	525 ± 25 µm thick, single-side polished, n-doped (P) silicon (001)
Stainless steel tube	Hositrad	custom made	Used for sealed assembly, material SST304
Sulfur hexafluoride (SF6)	Air Liquide	P0964S05R0A001	Bottle size S05 purity 99,9990%
Tantalum (Ta) wire	Thermo Scientific Chemicals	10349.G9	0.5 mm diameter 99.95% purity, 25 m total
Turbopump	Edwards	EXT255H