Single Crystalline Silicon Carbide Research Articles

Cohesive behavior of single crystalline silicon carbide scribing by nanosecond laser

Cohesive behavior of single crystalline silicon carbide scribing by nanosecond laser

Polytypic transformation behavior from 4H-SiC to 3C-SiC during high-speed scratching of single crystalline silicon carbide

The phase transformation behavior and associated mechanism of 4H-SiC induced by dynamic processing at room temperature conditions can be investigated with high-speed scratching tests. A dedicated high-speed scratching setup is developed to achieve the speed range over 10 m/s to 30 m/s while microscopic detection is performed to study the microstructural alteration of 4H–SiC. Raman spectra analyses indicate that the polytypic transformation from 4H to 3C occurs during high-speed scratching of single crystalline silicon carbide. Transmission electron microscopy demonstrates a shift in the stacking order of the atomic layers to face-centered cubic at the subsurface of the scratched groove, which reveals the formation mechanism of 3C-SiC. Furthermore, the 4H to 3C polytypic transformation exhibits a sensitive scratching speed correlation. The research can enhance the fundamental understanding of the mechanisms of damage formation during manufacturing of 4H-SiC components and provide g guidance for high-quality processing of 4H-SiC crystals.

Patterned Laser Ablation of Microgrooves with Controllable Cross‐Sections

AbstractFabricating precision microgrooves with controllable cross‐sections on difficult‐to‐machine materials is significantly valuable but still challenging. Herein, a patterned laser‐induced microjet‐assisted ablation method for cross‐sectional profiles controllable laser micromachining is proposed. During the liquid‐assisted laser ablation process, debris and bubbles that may disturb the laser energy deposition can be instantaneously expelled by a directional laser‐induced microjet. The Gaussian laser spot is spatially modulated into specific geometric shapes, such as triangles, to tune locally deposited laser energy to carve microgrooves with designed cross‐sections. To achieve customized microgrooves efficiently, a reliable geometrical model based on the ablation threshold theory is developed to guide the processing parameter selection, including the laser spot shape, polarization, pulse energy, and scanning strategies. The simulation and experimental results confirm that this method achieves the decoupled control of the groove depth and width in a single‐path laser ablation process. Using this method, the design and manufacturing of microgrooves with controllable cross‐sections on single crystalline silicon carbide are demonstrated. The patterned laser‐induced microjet‐assisted ablation method provides a new route for fabricating precision microgrooves with controllable cross‐sections on difficult‐to‐machine materials.

Combined Density Functional Theory and Microkinetics Study to Predict Optimum Operating Conditions of Si(100) Surface Carbonization by Acetylene for High Power Devices.

The Si(100) surface carbonization mechanisms by acetylene are explored using density functional theory calculations combined with microkinetic simulations. The most stable acetylene adsorption geometries and their subsequent decomposition mechanisms to form a carbon dimer on the Si surface are investigated. Microkinetics simulations are further used to examine the optimal reaction conditions for obtaining a single-crystalline silicon carbide (SiC). We find that the carbon dimer (C2*) as an end-bridge structure can be formed at 560 K, and the maximum of C2* can be obtained near 640 K. The acetylene adsorbed via the di-σ configuration starts to dehydrogenate when the heating rate is too fast and will form two possible carbon dimers (di-C2* and C2*), which will lead to a polycrystalline SiC buffer layer. We predict that 750 K and 10-6 bar will be the optimum temperature and pressure for obtaining a single-crystalline SiC buffer layer, respectively.

Effects of photogenerated-hole diffusion on 3C-SiC/Si heterostructure optoelectronic position-sensitive detector

Single-crystalline silicon carbide (3C-SiC) has been attracting significant attention in recent years due to its cost-effectiveness and high crystalline quality, mature fabrication techniques on Si-substrate and outstanding mechanical, chemical, and optoelectronic characteristics. Taking advantage of its large built-in potential, a promising application of 3C-SiC on Si (3C-SiC/Si) heterostructure is to develop position-sensitive detectors (PSDs) based on the lateral photovoltaic effect. The lateral photovoltage is generated under non-uniform illumination due to the asymmetry diffusion of photo-induced charge carriers. However, the full potential of 3C-SiC/Si heterojunction-based PSDs has not been elucidated yet. In this study, we investigate the influence of photogenerated hole and its diffusion path length on the sensing performance of the devices in attempts to obtain an optimal design and further pushing the limit of the PSD. Devices with different electrode spacings are fabricated on the 3C-SiC/Si heterostructure, and experiments are conducted under different illumination conditions to determine the position-sensitivity. Devices with short electrode spacings are found to have excellent position-sensitivity with the highest sensitivity of 470 mV mm−1 obtained in a device spacing of 300 µm under 980 nm (1000 µW) laser illumination. The physic mechanism underneath the experimentally observed behaviors are explained based on the generation and separation of electron–hole (e–h) pairs under the illumination, and charge carrier diffusion theory. The findings of this work will provide insights to design highly sensitive PSDs and explore its full potentials.

The Optical Properties, Energy Band Structure, and Interfacial Conductance of a 3C-SiC(111)/Si(111) Heterostructure Grown by the Method of Atomic Substitution

Epitaxial films of single-crystalline silicon carbide (3C-SiC cubic polytype) with 20- to 120-nm thickness grown on a silicon substrate by the method of atomic substitution have been studied by the spectroscopic ellipsometry technique at 0.5−9.3 eV photon energies. It is established that the dielectric permittivity of a thin intermediate layer formed at the 3C-SiC(111)/Si(111) interface is characteristic of a semimetal. This result was confirmed by quantum-chemical simulation of the properties of 3C-SiC(111)/Si(111) heterointerface, showing that the conductance of this intermediate layer is related to p-electrons of interfacial Si atoms that are most remote from Si atoms of the substrate.

Ultrafast pump-probe microscopic imaging of femtosecond laser-induced melting and ablation in single-crystalline silicon carbide

The temporal and spatial evolution of femtosecond laser-induced phase transitions and ablation on single-crystalline silicon carbide (SiC) were investigated via pump-probe microscopy in air on a time scale from approximately 100 fs to 1 ns. The largest reflectivity change was observed between 300 fs and 1 ps after excitation, which is due to free carrier generation, and the Drude model calculations indicated that the maximum free electron density was greater than 3.3 × 1021 cm−3. After a few picoseconds, there was direct evidence of the production of a rarefaction wave propagating towards the bulk, whose propagation velocity was estimated to be 3286 m/s. At delay times between a few hundreds of picoseconds and 1 ns, characteristic transient ring patterns were clearly observed and were related to the optical interference of the probe laser pulse reflected at the front surface of the ablating layer and at the interface of the non-ablating substrate. The estimated expansion velocity of the ablation front in 6H–SiC was found to be comparable to or slower than those reported for other semiconductors and dielectrics.

3C-SiC/Si Heterostructure: An Excellent Platform for Position-Sensitive Detectors Based on Photovoltaic Effect.

Single-crystalline silicon carbide (3C-SiC) on the Si substrate has drawn significant attention in recent years due to its low wafer cost and excellent mechanical, chemical, and optoelectronic properties. However, the applications of the structure have primarily been focused on piezoresistive and pressure sensors, bio-microelectromechanical system, and photonics. Herein, we report another promising application of the heterostructure as a laser spot position-sensitive detector (PSD) based on the lateral photovoltaic effect (LPE) under nonuniform optical illuminations at zero-bias conditions. The LPE shows a linear dependence on spot positions, and the sensitivity is found to be as high as 33 mV/mm under an illumination of 2.8 W/cm2 (635 nm). The structure also exhibits a linear dependence of the LPE over a large distance (7 mm) between two electrodes, which is crucial for PSDs as the region with a linear dependence of LPE is only usable for PSDs. The LPE at different spot positions and under different illumination conditions have been investigated and explained based on the energy-band analysis. The temperature dependence of the LPE and position sensitivity is also investigated. Furthermore, the two-dimensional mapping of the lateral photovoltages reveals the potential for utilizing the 3C-SiC/Si heterostructure to detect the laser spot position precisely on a plane.

Fabrication of a Monolithic Implantable Neural Interface from Cubic Silicon Carbide

One of the main issues with micron-sized intracortical neural interfaces (INIs) is their long-term reliability, with one major factor stemming from the material failure caused by the heterogeneous integration of multiple materials used to realize the implant. Single crystalline cubic silicon carbide (3C-SiC) is a semiconductor material that has been long recognized for its mechanical robustness and chemical inertness. It has the benefit of demonstrated biocompatibility, which makes it a promising candidate for chronically-stable, implantable INIs. Here, we report on the fabrication and initial electrochemical characterization of a nearly monolithic, Michigan-style 3C-SiC microelectrode array (MEA) probe. The probe consists of a single 5 mm-long shank with 16 electrode sites. An ~8 µm-thick p-type 3C-SiC epilayer was grown on a silicon-on-insulator (SOI) wafer, which was followed by a ~2 µm-thick epilayer of heavily n-type (n+) 3C-SiC in order to form conductive traces and the electrode sites. Diodes formed between the p and n+ layers provided substrate isolation between the channels. A thin layer of amorphous silicon carbide (a-SiC) was deposited via plasma-enhanced chemical vapor deposition (PECVD) to insulate the surface of the probe from the external environment. Forming the probes on a SOI wafer supported the ease of probe removal from the handle wafer by simple immersion in HF, thus aiding in the manufacturability of the probes. Free-standing probes and planar single-ended test microelectrodes were fabricated from the same 3C-SiC epiwafers. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on test microelectrodes with an area of 491 µm2 in phosphate buffered saline (PBS) solution. The measurements showed an impedance magnitude of 165 kΩ ± 14.7 kΩ (mean ± standard deviation) at 1 kHz, anodic charge storage capacity (CSC) of 15.4 ± 1.46 mC/cm2, and a cathodic CSC of 15.2 ± 1.03 mC/cm2. Current-voltage tests were conducted to characterize the p-n diode, n-p-n junction isolation, and leakage currents. The turn-on voltage was determined to be on the order of ~1.4 V and the leakage current was less than 8 μArms. This all-SiC neural probe realizes nearly monolithic integration of device components to provide a likely neurocompatible INI that should mitigate long-term reliability issues associated with chronic implantation.

A facile method for direct bonding of single-crystalline SiC to Si, SiO2, and glass using VUV irradiation

Single-crystalline silicon carbide is an attractive material for power electronics. However, it is difficult to achieve the direct bonding of SiC to conventional Si-based materials (e.g., Si, SiO2, and glass) due to the large mismatch in coefficients of thermal expansion and lattice constants. To solve the bottleneck, we present a facile direct bonding method using vacuum ultraviolet (VUV) surface irradiation for a robust combination of SiC to Si, SiO2, and glass at low temperatures (≤200 °C). The mechanisms behind the VUV-irradiated bonding of SiC to Si-based materials were also investigated. According to surface characterizations, VUV irradiation can lead to smooth and hydrophilic surfaces, which are beneficial for direct bonding in humid air. The tight and defect-free SiC/Si, SiC/SiO2 and SiC/glass bonding interfaces were confirmed by transmission electron microscopy. In particular, the enriched carbon transition layers were formed on the side of silicon carbide because of the oxidation and sputtering of Si atoms during VUV irradiation. This will possibly improve the bonding interfaces and contribute to the enhanced bonding strengths. Moreover, the SiC/glass bonded pair exhibited relatively high optical transparency in the UV–Vis range. Therefore, the direct bonding of single-crystalline SiC and heterostructure Si-based materials offers great potentials for high-performance power electronics, as well as micro/nanofluidic devices.

Response of fusion plasma-facing materials to nanosecond pulses of extreme ultraviolet radiation

AbstractThe experimental study of damage to tungsten (W), molybdenum (Mo), and silicon carbide (SiC) surfaces induced by focused extreme ultraviolet laser radiation (λ ~ 47 nm/~1.5 ns/21–40 µJ) is presented. It was found that W and Mo behaved similarly: during the first shot, the damaged area is covered by melted and re-solidified material, in which circular holes appear – residua of just opened pores/bubbles, from which pressurized gas/vapors escaped. Next cracks and ruptures appear and the W has a tendency to delaminate its surface layer. Contrary, single-crystalline SiC has negligible porosity and sublimates; therefore, no escape of “pressurized” gas and no accompanying effects take place. Moreover, SiC at sublimating temperature decomposes to elements; therefore, the smooth crater morphology can be related to local laser energy density above ablation threshold. When more shots are accumulated, in all three investigated materials, the crater depth increases non-linearly with number of these shots. The surface morphology was investigated by an atomic force microscope, the surface structure was imaged by a scanning electron microscope (SEM), and the structure below the surface was visualized by SEM directed into a trench that is milled by focused ion beam. Additionally, structural changes in SiC were revealed by Raman spectroscopy.

Damage effects of Au&He dual ion irradiated silicon carbide

Synergistic effects of Au and He dual ion beam irradiations were investigated by comparing the damage accumulated in single crystalline silicon carbide (SiC). By implanting Au ions at 600 K and He ions at room temperature into 6H-SiC, the resulting damage was characterized using several advanced techniques, including Raman spectroscopy, Rutherford backscattering spectrometry in channeling mode (RBS/C), nano-indentation and high-resolution x-ray diffraction (HRXRD). Here, we report the complex effects induced by dual ion beam irradiations and we also investigate the isothermal annealing effects on dual ion irradiation damage. Synergy effects of Au and He dual irradiations have been observed. Irradiation damage produced by Au&He ions is larger than single ion irradiation. The displacement damage induced by heavy recoil atoms and helium atoms interact with each other, causing more serious damage. Interestingly, Au + He successive irradiation induced a larger damage level than He + Au successive irradiation, probably due to thermal annealing effect during Au irradiation. Finally, the hardness and Young’s modulus of irradiated SiC were found to be highly related with single or dual ion irradiations, implantation sequences and the corresponding fluences.

Surface modification of single-crystalline silicon carbide by laser irradiation for microtribological applications

Carbide-derived carbon layers were locally formed on the surface of 4H-SiC single crystalline wafers by irradiation with an infrared laser (λ = 9.3 μm) in a low vacuum. Longer laser irradiation time produced thicker carbon layers and improved the bonding continuity. For an irradiation duration of 4.5 s, the modified layer was formed without surface damage and identified as amorphous carbon containing gradient amount of Si with a layer thickness of several nm. The results of friction tests conducted on the modified layer under micro-loads using a friction force microscope revealed the improvement of lubricity of the modified layer with a friction coefficient of 0.01 that is lower than that of the original SiC surface. No wear tracks were detected after scanning the Si probe over the layer surface 10,000 times with maintaining lubricity. The layer also preserved its lubricity after heating up to 550 °C in an air atmosphere; however, the lubricity disappeared at 600 °C owing to decomposition of the lubricating layer caused by oxidation.

Unintentionally Doped Epitaxial 3C-SiC(111) Nanothin Film as Material for Highly Sensitive Thermal Sensors at High Temperatures

There is a growing interest and demand to develop sensors that operate at high temperatures. In this work, we investigate the temperature sensing properties of unintentionally doped n-type single crystalline cubic silicon carbide (SiC) for high temperatures up to 800 K. A highly sensitive temperature sensor was demonstrated with a temperature coefficient of conductivity (TCC) ranging from ${1.96}\times {10}^{4}$ to ${5.18}\times {10}^{4}$ ppm/K. The application of this material was successfully demonstrated as a hot film flow sensor with its high signal-to-noise response to air flow at elevated temperatures. The high TCC of the single crystalline SiC film at and above 800 K strongly revealed its potential for highly sensitive thermal sensors working at high temperatures.

A rapid and cost-effective metallization technique for 3C-SiC MEMS using direct wire bonding.

This paper presents a simple, rapid and cost-effective wire bonding technique for single crystalline silicon carbide (3C–SiC) MEMS devices. Utilizing direct ultrasonic wedge–wedge bonding, we have demonstrated for the first time the direct bonding of aluminum wires onto SiC films for the characterization of electronic devices without the requirement for any metal deposition and etching process. The bonded joints between the Al wires and the SiC surfaces showed a relatively strong adhesion force up to approximately 12.6–14.5 mN and excellent ohmic contact. The bonded wire can withstand high temperatures above 420 K, while maintaining a notable ohmic contact. As a proof of concept, a 3C–SiC strain sensor was demonstrated, where the sensing element was developed based on the piezoresistive effect in SiC and the electrical contact was formed by the proposed direct-bonding technique. The SiC strain sensor possesses high sensitivity to the applied mechanical strains, as well as exceptional repeatability. The work reported here indicates the potential of an extremely simple direct wire bonding method for SiC for MEMS and microelectronic applications.

Superior Robust Ultrathin Single-Crystalline Silicon Carbide Membrane as a Versatile Platform for Biological Applications.

Micromachined membranes are promising platforms for cell culture thanks to their miniaturization and integration capabilities. Possessing chemical inertness, biocompatibility, and integration, silicon carbide (SiC) membranes have attracted great interest toward biological applications. In this paper, we present the batch fabrication, mechanical characterizations, and cell culture demonstration of robust ultrathin epitaxial deposited SiC membranes. The as-fabricated ultrathin SiC membranes, with an ultrahigh aspect ratio (length/thickness) of up to 20 000, possess high a fracture strength up to 2.95 GPa and deformation up to 50 μm. A high optical transmittance of above 80% at visible wavelengths was obtained for 50 nm membranes. The as-fabricated membranes were experimentally demonstrated as an excellent substrate platform for bio-MEMS/NEMS cell culture with the cell viability rate of more than 92% after 72 h. The ultrathin SiC membrane is promising for in vitro observations/imaging of bio-objects with an extremely short optical access.

Fabrication of Deep 4H-SiC Microstructures Via Inductively Coupled SF6/O2 Plasma

High aspect ratio deep reactive ion etching (DRIE) of single-crystalline silicon carbide (SiC) holds promise for fabricating next-generation harsh-environment (e.g. high temperature, high power, reactive environments) micro- and nanoelectromechanical systems (MEMS/NEMS). However, unlike Si, SiC microfabrication via DRIE remains in its infancy. In particular, high aspect ratio SiC DRIE poses a significant challenge in SiC MEMS development. To tackle this issue, SiC DRIE process parameters (e.g. cycle time, power, backside substrate cooling) were investigated to achieve high aspect ratio SiC microstructures with close to vertical sidewalls. Overall, the SiC processing work described here helps to bridge the knowledge gap between Si and SiC microfabrication and support the adaption of single-crystalline SiC-based micro- and nanodevices.

Stress and thermal characterization of 4H-SiC microelectromechanical structures

In this letter, the potential of single crystalline 4H-polytype silicon carbide (4H-SiC) based microelectromechanical structures as resistance thermometer for high temperature sensing were explored. A dopant-selective photoelectrochemical etching process was applied to release the sensing element – suspended microstructures on 4H-SiC substrate. Residual stress and stress gradient in the microstructure before and after release was examined by micro-Raman spectroscopy. Electrical resistance of the suspended microstructures at different temperatures were characterized and analyzed by a temperature-dependent electron mobility model.

Surface Metallization of Silicon Carbide By Electroless Deposition

Silicon carbide (SiC) is one of the most promising semiconductor materials for a next generation of power devices. Its physical properties such as breakdown electric field and thermal conductivity are higher and more suitable for efficient power devices than those of silicon, which is common semiconductor material for the present devices. The metallization of SiC surfaces is a key technology to use SiC for power devices. The autocatalytic electroless deposition, which is a conventional method to metallize non-conductive substrates, has several advantages, however, cannot produce adhesive metal films directly on SiC. We recently developed a new surface-activation process for the direct electroless deposition of adhesive metal films on silicon substrates (1, 2). This process uses gold (Au) nanoparticles epitacially grown on silicon by an electroless displacement deposition (3, 4). In this study, this process is applied for formation of adhesive metal films on SiC. The single crystalline n-type 4H-SiC (0001) wafers were used as the substrates. Au nanoparticles were deposited on SiC substrates using a HAuCl4 solution containing HF or KOH under photoillumination using a Hg-Xe short arc lamp. Then, nickel-phosphorus alloy (Ni-P) films were electrolessly formed on the SiC substrates by using NiSO4 solution containing NaPH2O2as a reducing agent. The adhesion of the electrolessly deposited Ni-P films on SiC substrates was examined by a tape test based on Japanese Industrial Standard JIS H8504 corresponding to ISO 2819. Figure 1 shows SEM images of Au-nanoparticle-deposited SiC substrates. The size and particle density of Au deposited using the solution containing KOH (Fig. 1 (b)) were larger than that using HF (Fig.1 (a)). The surface of SiC substrates was roughened by the electroless deposition of Au nanoparticles. No photoillumination significantly attenuated the Au deposition. These results indicate that this Au nanoparticle deposition is a photoelectrochemical displacement reaction. The autocatalytic electroless deposition of Ni-P films was initiated on the Au-nanoparticle-deposited SiC substrate. No deposition occurred on a bare SiC substrate. The adhesion of metal films formed on the SiC substrates depended on the deposition time of Au nanoparticles. Immersion of SiC substrates in the Au-deposition solution containing KOH for 15 min obtained high adhesion. No peeling occurred up to 1.3 μm in thickness of Ni-P films by the tape test. This result is similar to that in the case of single crystalline silicon substrates (1 - 4). In conclusion, we successfully formed adhesive metal films directly on single crystalline SiC substrates by using wet processes that are electroless displacement deposition of Au nanoparticles and common autocatalytic electroless deposition of Ni-P alloy films. Acknowledgements The present work was partly supported by KAKENHI (26289276) from JSPS and A-STEP from JST. References 1) S. Yae, M. Enomoto, H. Atsushiba, A. Hasegawa,N. Fukumuro, S. Sakamoto, and H. Matsuda., ECS Trans., 53 (6), 99 (2013). 2) H. Atsushiba, Y. Orita, S. Sakamoto, N. Fukumuro, and S. Yae., ECS Trans., 61(10), 9 (2014). 3) N. Yamada, H. Atsushiba, S. Sakamoto, N. Fukumuro, and S. Yae., ECS Trans., 69(39), 59 (2015).4) N. Yamada, S. Sakamoto, N. Fukumuro, and S. Yae., Symposium Z01 in this meeting (2016). Figure 1

Amorphization resistance of nano-engineered SiC under heavy ion irradiation

Silicon carbide (SiC) with a high-density of planar defects (hereafter, ‘nano-engineered SiC’) and epitaxially-grown single-crystalline 3C-SiC were simultaneously irradiated with Au ions at room temperature, in order to compare their relative resistance to radiation-induced amorphization. It was found that the local threshold dose for amorphization is comparable for both samples under 2 MeV Au ion irradiation; whereas, nano-engineered SiC exhibits slightly greater radiation tolerance than single crystalline SiC under 10 MeV Au irradiation. Under 10 MeV Au ion irradiation, the dose for amorphization increased by about a factor of two in both nano-engineered and single crystal SiC due to the local increase in electronic energy loss that enhanced dynamic recovery.