Amorphous Silicon Carbide

Contrary to their crystalline counterparts, which boast an ordered arrangement of atoms, the lattice structure of a-SiC is randomly organized; yet this randomness gives this material remarkable strength.

Delft University of Technology researchers recently unlocked the mechanical properties of an innovative new material with unprecedented precision, opening up potential future uses for this incredibly strong yet flexible material.

Physical Properties

Silicon carbide is a wide band gap semiconductor material found both crystalline and amorphous forms. Amorphous forms have gained attention for their unique mechanical properties, chemical inertness, hardness, and chemical inertness which make them suitable for applications such as protective coatings in harsh working environments as well as nanomechanical sensors.

To achieve the desired physical and chemical properties of an a-SiC thin film, several optimization steps may need to be taken. This is particularly important when fabricating protective coatings out of this material as the film must remain stable after fabrication.

This can be a difficult challenge, and different approaches have been explored to enhance a-SiC properties. One key factor is its crystalline structure which is controlled by deposition conditions – for instance temperature and RF power during deposition have an impactful on its atomic configuration, which directly influences optical and electrical properties; as can hydrogen incorporation during the deposition process itself.

A-SiC atomic arrangement is often characterized by structural disorder, often due to angular distortion of band configuration or degradation by formation of Si-Si and C-Si bonds or even dangling bonds; these defects can be minimized through proper sputtering conditions – an essential step when working with applications requiring stable chemical structures.

At first glance, the atoms in a-SiC are fourfold coordinated; in crystalline SiC this pattern remains constant over long distances. But in amorphous SiC this structure changes, with some atoms not completely fourfold coordinated forming an irregular random network and some not bonding to their three other counterparts (known as “dangling bonds”). Hydrogen can help passivate these bonds and improve properties significantly for this material.

Chemical Properties

Silicon carbide, with its chemical formula SiC, exhibits a wide variety of atomic configurations and is an outstanding semiconductor with an energy gap between 3eV to 5eV.

Silicon carbide can be doped n-type with nitrogen or phosphorus and p-type with boron, aluminium or gallium to achieve different effects. Silicon carbide has also proven itself to be an excellent electrical conductor with metallic doping increasing conductivity even further. Due to its high melting point it makes an excellent candidate for ceramics and refractory materials production; naturally occurring moissanite occurs naturally within meteorite rocks as well as corundum deposits and kimberlite; however today most silicon carbide sold worldwide is synthetic.

As is true of its crystalline counterpart, amorphous silicon carbide boasts exceptional mechanical properties in terms of fracture toughness and compressive strength, with Young’s modulus that matches that of its counterpart.

These properties make amorphous silicon carbide an excellent material choice for neural interface applications that involve chronic implant of microelectrode arrays, where their transverse dimensions must remain stable during chronic use. Implant thickness must be thin enough to minimize inflammation responses while still remaining transverse dimension stable – with thinner implants coming closer to reaching their buckling threshold; larger Young’s moduli provide additional resilience in such cases, decreasing chances of the array becoming unstable after chronic use. Amorphous silicon carbide offers a great alternative to more rigid materials like titanium in these cases because its larger Young’s modulus provides less chance of array instability during chronic use compared with its stiffer materials counterparts like titanium in such applications – providing another strength advantage over stiffer materials like titanium by offering reduced chances of chronic instability during long term placement compared with stiffer materials like titanium’s stiffer Young’s modulus providing increased Young’s moduli reduce risk in such applications reducing instability risk during prolonged implantation periods of time thereby offering considerable savings over its counterpart.

University of Manchester researchers examined the tensile strength of amorphous silicon carbide to optimize it for neural interface applications, creating an innovative method of testing its strength. Instead of relying on traditional methods that might introduce inaccuracies due to anchoring methods for testing materials like this one onto test surfaces, they created an innovative microchip microscale solution by growing nanostrings of this material onto it and suspending them to determine maximum tensile strength.

Research team was able to achieve results which came very close to the theoretical value of this material’s tensile strength by carefully inspecting its structure and geometry of nanostrings. They confirmed their method of suspension was compatible with its chemical inertness; dry etching undercuts had minimal perturbations to suspension; thus measuring force required to break one nanostring was also possible.

Mechanical Properties

Silicon carbide’s amorphous state provides for an outstandingly high ultimate tensile strength and elastic properties, and resistance to fatigue, cracking, and deformation – qualities which make it an excellent material choice for applications that demand structural integrity and durability, such as MEMS devices or aerospace components.

Mechanical properties of amorphous silicon carbide have been evaluated using nanoindentation and bulge techniques, along with measuring Young’s modulus and Poisson’s ratio. Hydrogenation during deposition has proven to have an immense effect on these properties; in particular for alpha polymorph (a-SiC) with Wurtzite crystal structure; by comparison beta polymorph (a-SiC:H) featuring zinc blende crystal structure has proven more stable, though not suitable for industrial use due to low hardness.

Silicon carbide was first employed in semiconductor electronics as light emitting diodes and detectors for early radios in 1907. Since then, its resistance to high temperatures and voltages has made it a key component in numerous electronic products and its use ranges from spacecraft heat shields to jewels and insulators.

Over the past several years, amorphous Silicon Carbide (a-SiC) has increasingly gained in popularity as an enabler technology for advanced microelectronics. The material’s unique amorphous nature allows designers to experiment with novel design approaches, leading to higher performance at reduced costs than with crystalline materials.

Even with this surge of interest, a-SiC still faces considerable resistance within industry. But its scalability offers hope as production can take place efficiently in large volumes at minimal costs.

a-SiC’s unique properties have provided an opportunity for innovation across various applications, from protective coatings to high-temperature catalysts and fuel cells. Furthermore, its amorphous nature has allowed researchers to devise new techniques for fabricating patterned resonators – ideal for sensing applications including strain gauge monitoring – with their fabrication also offering strain gauge capabilities for monitoring purposes.

Electrical Properties

Amorphous silicon carbide (ASC) is an attractive semiconductor with a wide band gap and high thermal conductivity, making it suitable for optoelectronics and microelectronics applications due to its low energy loss. ASC can be doped with nitrogen, phosphorus and aluminium dopants for doping n- or p-type operation with metallic conductivity achieved via heavy doping of boron or gallium dopants. Furthermore, its high breakdown electric field strength makes ASC ideal for applications including super capacitors and MOSFETs.

Silicon carbide thin films have found applications across engineering applications, from wide-bandgap electronic devices and sensors, MEMS sensors, photovoltaic solar cells and light emitting diodes to light emitting diodes and hardmask films. Silicon carbide films have drawn particular interest due to their ability to be produced via different fabrication techniques to meet specialized properties needed for particular uses. Nanoscale amorphous silicon carbide materials remain poorly understood. Therefore, the authors of this study developed an innovative technique for characterizing nanoscale materials.

Researchers used a femtosecond pump-probe technique with lasers that can be tuned to different wavelengths to analyze amorphous silicon carbide thin films at the nanoscale using femtosecond pump-probe measurements with lasers tuned to specific wavelengths, with lasers capable of tuning to multiple wavelengths for accurate chemical and physical characterization of their constituent atoms and molecular arrangements, measuring surface compositions, examining constituent arrangements and further refine this analysis using Raman and XPS measurements; the results revealed two distinct phases; one phase containing carbon while another contained silicon.

Researchers also measured the electrical properties of a-SiC films. They discovered that their electrical properties were generally ohmic under low electric fields but showed non-ohmic behavior at higher electric fields. Based on this observation, it can be surmised that when exposed to higher electric fields the films experienced a phase transition leading to an increase in Si-Si and Si-C bonds as well as decreased overall atomic density density.

Understanding this phenomenon is important, because it could enable the development of amorphous silicon carbide materials with improved conductivity and reliability for neural interfaces. Neural interfaces require electrode arrays with low resistance and buckling thresholds to withstand chronic implant, something difficult to achieve with thinner, amorphous materials that are susceptible to tissue damage and biomechanical failures.