Fabrication and Optimization of Type II Silicon Clathrate Films

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.