Name: Fabrication and Optimization of Type II Silicon Clathrate Films (Video) | JoVE Journal
Uploaded: 2025-10-14T13:50:00.000+00:00
Duration: 6 min 53 s
Description: Learn to synthesize type II silicon clathrate films using a simple two-thermal annealing method without a glove box. This cost-effective protocol enables fabrication of advanced semiconductor materials for electronic and photonic applications.

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5 minutes

This research was supported by the French National Research Agency (ANR) under the Exosil-Exotic Silicon Clathrate Films (ANR-22-CE50-0025). The authors thank the XRD platform of IPCMS and the staff of the C3Fab platform of ICube.

The reagents and the equipment used in this study are listed in the Table of Materials.
1. Preparation of materials
<ol>
	<li>Preparation of silicon (c-Si (001))
	<ol>
		<li>Precisely cleave with a diamond-tipped pen a 525 &#181;m &#177; 25 &#181;m thick, single-side polished, n-doped silicon (001) wafer into a rectangle with dimensions of 56 &#215; 38 mm2.</li>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Sodium pretreatment
	<ol>
		<li>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.</li>
		<li>Immediately place the sodium slice in an airtight glass (sealed with a ground-glass joint) container filled with anhydrous cyclohexane (&#8805;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).</li>
	</ol>
	</li>
	<li>Assembly in the crucible
	<ol>
		<li>Wash with pure ethanol and dry at 300 &#176;C for 4 h in an Inconel alloy boat (54 &#215; 18 &#215; 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.</li>
		<li>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.</li>
		<li>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.</li>
		<li>Seal the tube at both ends with fittings that allow only argon (purity &#8805; 99.9999%) to circulate inside the tube.</li>
		<li>Purge the tube with argon at a constant pressure of 1.6 bar for 15 min to ensure an inert atmosphere throughout the reaction.</li>
	</ol>
	</li>
</ol>
2. Synthesis of SiCL
<ol>
	<li>Thermal decomposition for the formation of Na4Si4
	<ol>
		<li>Start the programmable horizontal tube furnace with the following annealing cycle.
		<ol>
			<li>Raise the temperature until reaching the 600 &#176;C plateau with ramp rate of 5 &#176;C/min.&#160;Maintain this temperature stable at 600 &#176;C for 19 h.Allow the system to cool down naturally for over 7 h.</li>
		</ol>
		</li>
		<li>When the furnace and tube are back to room temperature, flush the tube with a continuous flow of argon (purity &#8805; 99.9999%).</li>
	</ol>
	</li>
	<li>High vacuum dynamic post-treatment
	<ol>
		<li>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.
		<ol>
			<li>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 &#215; 10-7 mbar is reached.</li>
		</ol>
		</li>
		<li>Ramp to 400 &#176;C over 30 min; hold at 400 &#176;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.</li>
	</ol>
	</li>
</ol>
3. Post-treatment
<ol>
	<li>Press annealing
	<ol>
		<li>Use a manual table-top hydraulic press equipped with two square stainless-steel plates (15 &#215; 15 cm2) and an integrated heating system capable of reaching temperatures up to 300 &#176;C for the press annealing process.</li>
		<li>Fix the upper plate and apply force by raising the lower plate. Operate the imprinting process in an ambient atmosphere.</li>
		<li>Place the silicon clathrate sample between the two plates inside the thermal press.</li>
		<li>Lift the lower plate to gradually increase the force until the pressure reaches a value of around 2 kN.</li>
		<li>When the force reaches 2 kN, increase the temperature of the two metal plates in contact with the sample to 250 &#176;C and maintain it for 30 min.</li>
		<li>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.</li>
		<li>After 30 min, stop heating. Apply rapid cooling using a cold-water circulation system (20 &#176;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 &#181;m to 31 &#181;m. The grain size of the pressed material reaches approximately 200 nm, which is four times higher than that of the as-synthesized one.</li>
	</ol>
	</li>
	<li>Dry etching
	<ol>
		<li>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.</li>
		<li>Place the cleaned sample onto the lower electrode inside the RIE etch system.</li>
		<li>Pump down the chamber to reach a base pressure of 5 &#215; 10-7 mbar.</li>
		<li>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.</li>
		<li>Set the substrate temperature to 5&#8239;&#176;C using the integrated cooling system.</li>
		<li>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.</li>
		<li>Perform etching tests with durations of 15 s. Under these conditions, the etching rate was measured to be 1 &#181;m&#183;min-1 on the pressed sample.</li>
		<li>After etching, turn off both the ICP and RF power supplies to stop the plasma.</li>
		<li>Evacuate remaining process gases, then slowly vent the chamber to atmospheric pressure.</li>
	</ol>
	</li>
	<li>Remove the etched sample from the chamber.</li>
</ol>

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<li>Kume, T., Ohashi, F., Nonomura, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Group+IV+clathrates+for+photovoltaic+applications%5BTitle%5D)+AND+%22Jpn+J+Appl+Phys%22%5BJournal%5D)">Group IV clathrates for photovoltaic applications.</a> Jpn J Appl Phys. 56 (5S1), 05DA05(2017).</li>
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<li>Vollondat, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Tunability+of+silicon+clathrate+film+properties+by+controlled+guest-occupation+of+their+cages%5BTitle%5D)+AND+%22J+Chem+Phys%22%5BJournal%5D)">Tunability of silicon clathrate film properties by controlled guest-occupation of their cages.</a> J Chem Phys. 158 (16), 164709(2023).</li>
<li>Kishimoto, K., Koda, S., Akai, K., Koyanagi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Thermoelectric+properties+of+sintered+type-II+clathrates+%28K%2C+Ba%2924%28Ga%2C+Sn%29136+with+various+carrier+concentrations%5BTitle%5D)+AND+%22J+Appl+Phys%22%5BJournal%5D)">Thermoelectric properties of sintered type-II clathrates (K, Ba)24(Ga, Sn)136 with various carrier concentrations.</a> J Appl Phys. 118 (12), 125103(2015).</li>
<li>Liu, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Formation+of+type+II+silicon+clathrate+with+lithium+guests+through+thermal+diffusion%5BTitle%5D)+AND+%22Inorg+Chem%22%5BJournal%5D)">Formation of type II silicon clathrate with lithium guests through thermal diffusion.</a> Inorg Chem. 62 (18), 6882-6892 (2023).</li>
</ol>

The authors confirm that they have no financial conflicts of interest.

The synthesis of type II silicon clathrate films (NaxSi136) 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...

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 criteria4,5. 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 (NaxSi136) has emerged as a metastable yet robust material with a quasi-direct band gap in the range of 1.6-2 eV9,11.
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 phase15,16,17. 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 NaxSi136 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 contamination18,10 (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 reported20,21,22 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&#160;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.

<table><tbody><tr><td>Acetone</td><td>Sigma-Aldrich</td><td>32201-2.5L-M</td><td></td></tr><tr><td>Airtight glass</td><td></td><td>custom made</td><td></td></tr><tr><td>Analytical balance</td><td>Sartorius</td><td>1409 001</td><td></td></tr><tr><td>Anhydrous cyclohexane</td><td>Thermo Scientific</td><td>11403087</td><td>&amp;ge;99% purity</td></tr><tr><td>Argon gas</td><td>LINDE GAS</td><td>2890122 2010</td><td>Bottle size S11Purity 99,9999%</td></tr><tr><td>Diamond tipped pen</td><td>Oxford Instruments Sarl</td><td>T5357</td><td></td></tr><tr><td>Dry Scroll Pump</td><td>Edwards</td><td>XDS10&amp;nbsp;</td><td></td></tr><tr><td>Ethanol</td><td>VWR Chemicals</td><td>20821.365</td><td></td></tr><tr><td>Horizontal tube furnace</td><td>CARBOLITE GERO</td><td>EST12150B-230SN</td><td>Programmable furnace</td></tr><tr><td>Hydraulic press</td><td>Rondol</td><td></td><td>Manual, table-top, with heating plates</td></tr><tr><td>Hydrofluoric acid (HF)</td><td>VWR Chemicals</td><td>20319.291</td><td>48% HF used to make 10% vol. solution</td></tr><tr><td>Inconel alloy boat</td><td>ANALAB</td><td>AR9001</td><td>54 x 18 x 13.3 mm&lt;sup&gt;3&lt;/sup&gt;</td></tr><tr><td>Isopropanol</td><td>VWR Chemicals</td><td>20842.367</td><td></td></tr><tr><td>Metallic sodium</td><td>Thermo Scientific Chemicals</td><td>L13285.24</td><td>99% purity</td></tr><tr><td>Quartz tube</td><td>ALC Quartz</td><td>custom made</td><td>external diameter: 65.5 &amp;plusmn; 0.3 mm, internal diameter: between 57 and 60 mm, length: 550 mm</td></tr><tr><td>RIE etch system</td><td>CORIAL</td><td>210IL</td><td></td></tr><tr><td>segmented blade cutter</td><td>Facom</td><td>844.S9PB</td><td>9 mm blade width</td></tr><tr><td>Silicon (c-Si (001)) wafer</td><td>BT Electronics</td><td>from stock</td><td>525 &amp;plusmn; 25 &amp;micro;m thick, single-side polished, n-doped (P) silicon (001)</td></tr><tr><td>Stainless steel tube</td><td>Hositrad</td><td>custom made</td><td>Used for sealed assembly, material SST304</td></tr><tr><td>Sulfur hexafluoride (SF6)</td><td>Air Liquide</td><td>P0964S05R0A001</td><td>Bottle size S05 purity 99,9990%</td></tr><tr><td>Tantalum (Ta) wire</td><td>Thermo Scientific Chemicals</td><td>10349.G9</td><td>0.5 mm diameter 99.95% purity, 25 m total</td></tr><tr><td>Turbopump</td><td>Edwards</td><td>EXT255H</td><td></td></tr></tbody></table>

fabrication and optimization of type ii silicon clathrate films

Fabrication and Optimization of Type II Silicon Clathrate Films

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 (NaxSi136) 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.

This study explored the structural and optical evolution of type II clathrate NaxSi136 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 Kα source) which confirms the formation of the typ...

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.

Silicon clathrates (SiCL) are a class of cage-structured materials with tuneable optoelectronic properties, offering significant potential for ...

We present a protocol for synthesizing type II silicon clathrate films that involve two thermal and alien films that does not require a glove box. This approach is changed forward and cost-effective. Currently, a few laboratories produced silicon clathrate films.Our goal is to share the simple fabrication method with the scientific community emphasizing adherences to all specified experimental safety measures. Begin by immersing the silicon substrate in a 10%hydrofluoric acid solution for exactly two minutes to remove the native silicon dioxide present on the substrate surface. After exposure to hydrofluoric acid, rinse the silicon substrate thoroughly with deionized water to remove any residual acid.Using a nitrogen gun, blow dry the substrate to eliminate remaining water droplets and prevent surface residue. Using a cutter, carefully cut a piece of metallic sodium to obtain a small rectangular slice weighing approximately 0.22 grams. Immediately place the sodium slice in an airtight glass container filled with anhydrous cyclohexane.Ensure that the sodium is fully immersed in the cyclohexane to prevent oxidation. Now, place the previously prepared sodium slice into a cleaned in canal alloy boat. Then position the silicon wafer directly above the sodium slice with its polished surface facing downward toward the boat.Carefully insert the prepared assembly into the center of a sealed stainless steel tube and an o-ring seal situated inside a programmable horizontal tube furnace. Insert a high purity tantalum wire with a diameter of 0.5 millimeters, a length of 3.4 centimeters and 99.95%purity into the tube to capture trace oxygen. Seal both ends of the stainless steel tube using fittings that allow circulation of only argon.Purge the sealed tube with argon at a constant pressure of 1.6 bar for 15 minutes to establish an inert atmosphere throughout the reaction. Then raise the temperature of the furnace at a ramp rate of five degrees Celsius per minute until it reaches 600 degrees Celsius and maintain it for 19 hours. When the furnace and tube have cooled back to room temperature, flush the system with a continuous flow of argon.Transfer the obtained samples as quickly as possible into a quartz tube, and connect the tube to a dynamic vacuum furnace. After connecting the quartz tube to the pumping system, first, evacuate the system using the primary pump. Then activate the turbomolecular pump and continue pumping until a high vacuum five times 10 to the power of negative seven millibars achieved.Ramp the furnace temperature to 400 degrees Celsius over 30 minutes and hold for four hours. Then switch off the tubular furnace and allow the sample to cool naturally to room temperature. After cooling, unload the sample from the furnace.Lift the lower plate to gradually increase the force until the applied pressure reaches approximately two kilo Newtons. Then release the applied pressure. Place the clean sample onto the lower electrode inside the reactive ion etching system.Pump down the chamber until a base pressure of five times 10 to the power of negative seven millibar is reached. Once the etching process is complete, turn off both the inductively coupled plasma and radio frequency power supplies to stop the plasma. Evacuate any remaining process gases from the chamber.Then slowly vent the chamber back to atmospheric pressure. Finally, remove the etched sample from the chamber. The x-ray diffraction pattern confirmed the formation of the type two silicon clathrate phase in both the pressed and pressed etch samples with sharp peaks matching the ICDD 010895534, and weak reflections indicating the presence of a minor type one silicon cloth rate secondary phase.After etching, the peak positions remained unchanged, but showed a slight reduction in intensity. Raman spectroscopy revealed that both pressed and pressed edge samples exhibited characteristic peaks near 185, 290, and 460 inverse centimeters corresponding to the SI 20 and SI 28 cages in the type two clathrate structure. Photo luminescence measurements showed a broad emission band centered around 1.75 electron volts in both pressed and pressed etch samples consistent with the quasi direct band gap of semiconducting silicon clathrates, SEM top view images of the pressed sample revealed a smoother surface with significantly reduced grain boundaries while cross-sectional views showed improved film density and structural connectivity.After SF6 dry etching, SEM top view images showed a transformation to a textured surface morphology and cross-sectional images confirmed changes in surface structure compared to the unetched film. We successfully fabricated type two silicon clathrate films and improved the property through two post synthesized regimens, thermal pressing and reactive ion etching process. We employed multiple characterization techniques including XRD, and jam spectroscopy to confirm the successful formation of the clathrates and conducting films.We believe that this work is a separate stone for further application of silicon clathrates into semiconductor technology.

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Research

JoVE Journal

Chemistry

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Learn to synthesize type II silicon clathrate films using a simple two-thermal annealing method without a glove box. This cost-effective protocol enables fabrication of advanced semiconductor materials for electronic and photonic applications.

Fabrication and Optimization of Type II Silicon Clathrate Films

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