4H-SiC Surface Research Articles

The Best Paper Award 2024 Congratulations!

The 15th Best Paper Award 2024 ceremony was held at Gakushikaikan, Tokyo, Japan, on September 20, 2024, attended by the winners and IJAT Editorial Committee members. The Best Paper was severely selected from among 58 papers published in Vol.17 (2023). The Best Paper Award winner received a certificate along with an honorarium of JPY100,000 (nearly US$1,000). We extend our heartfelt congratulations to the winners and sincerely wish them continued success in their future endeavors. The 15th Best Paper Award Thitipat Permpatdechakul, Panart Khajornrungruang, Keisuke Suzuki, and Shotaro Kutomi Study on a Novel Peeling of Nano-Particle (PNP) Process for Localized Material Removal on a 4H-SiC Surface by Controllable Magnetic Field Int. J. of Automation Technology, Vol.17 No.4 pp. 410-421, 2023

Fabrication of grooves on 4H–SiC using femtosecond laser vector beam

Femtosecond lasers are widely acknowledged for their capability to groove and generate laser-induced periodic surface structures (LIPSS) on various materials. LIPSS structures include LSFL (low spatial frequency LIPSS) and HSFL (high spatial frequency LIPSS). In this study, a segmented waveplate (SWP) was used to transform a linearly polarized femtosecond laser Gaussian beam with a wavelength of 515 nm into a vector beam with a donut-shaped energy peak distribution and polarization of azimuthal and radial. Grooving was conducted on three different SiC surface conditions: polished and pre-processed (LSFL and HSFL), using linear, azimuthal, and radial polarization. This study investigated the differences in microstructure morphology and compared the ablation depths of the grooves. It was found that grooving on pre-processed LSFL 4H–SiC surfaces resulted in grooves with higher ablation depths than those produced on polished surfaces. Additionally, using azimuthal polarization on the HSFL surface could create fine HSFL structures (with a period of approximately 100 nm) between the initial periodic structures (with a period of approximately 250 nm) formed during the pre-processing.

SNDM Study of the MOS Interface State Densities on the 3C-SiC / 4H-SiC Stacked Structure

The layer structure of 3C-SiC stacked on 4H-SiC is implemented by simultaneous lateral epitaxy (SLE). The SLE, involving spontaneous nucleation of 3C-SiC(111) on the 4H-SiC(0001) surface followed by step-controlled epitaxy, facilitates the creation of a single-domain 3C-SiC layer with an epitaxial relationship to the underlying 4H-SiC, establishing a coherent (111)//(0001) interface aligned in the basal plane. An extremely low state density at an interface between thermally grown SiO2 and SLE-grown 3C-SiC layer is revealed by local deep level transient spectroscopy (local-DLTS) based on scanning nonlinear dielectric microscopy (SNDM).

Simulation of chemical reactions with methanol and oxalic acid on 4H-SiC surfaces before and after nanoabrasion

Previous work by our team has shown that not only can oxalic acid added to a reactive methanol polishing solution significantly further improve SiC polishing removal rate but also surface quality. However, due to the lack of fundamental information on the nature of the interfacial reaction, the reaction mechanism involved in continuously improving SiC polishing performance remains unresolved. This study explores the chemical interactions of pristine and nano-scratched surfaces with the reaction medium using a molecular dynamics (MD) model combined with the ReaxFF reaction force field. The results show that –OH and –CHn, methanol can effectively break the Si-C bond on the surface of silicon carbide and the high amount of OH and C=O in oxalic acid favors the formation of surface silicon oxides and the softening of the silicon carbide surface. The synergistic effect of oxalic acid and methanol in the pre-scratch system indirectly supports the notion that nanoabrasion caused by nano-abrasives enhances the chemical reactivity of polishing fluid components. In conjunction with the experimental data, a proposed reaction mechanism for organic acid-containing alcohol fluids with SiC based on non-aqueous solvent systems is presented. This may stimulate ideas for better developing and applying novel non-aqueous-based polishing fluids.

Cristobalite formation on high-temperature oxidation of 4H-SiC surface based on silicon atom sublimation

Two innovative high-temperature oxidation methods for 4H-SiC were developed, with observations made via Optical Microscope, Scanning Electron Microscope and Raman spectrum. We successfully fabricated fully crystallized cristobalite films through oxidation and annealing processes, unveiling the lateral growth mechanisms of crystal planes during annealing. The study also delved into the unusual effects of Si sublimation on the oxidation rate of cristobalite during its formation and investigated the stress-induced cracking phenomena observed when the film thickness exceeds 1 μm. Additionally, a novel step oxidation technique was introduced to suppress vapor phase oxidation of Si at high temperatures by employing a barrier layer, facilitating the swift production of amorphous SiO2 thick films. The exceptional surface and interface flatness exhibited by these films underscore their significant potential for applications in the realm of functional films.

Study of damage mechanism on single crystal 4H-SiC surface layer by picosecond laser modification (PLM)

The stable structure, high hardness, and brittleness of single-crystal 4H-SiC present challenges in achieving efficient and damage-free polishing processing. Ultra-short pulsed laser-induced surface modification provides a new solution to enhance the manufacturability of 4H-SiC. This paper aims to investigate the underlying mechanism of the picosecond laser-induced surface structural changes in 4H-SiC and the process of material removal from solid to vapour, elucidating the interaction mechanism between picosecond laser and 4H-SiC. A temperature gradient distribution model for picosecond laser irradiation on 4H-SiC was built to reveal the formation mechanism of subsurface crack damage. The results show that a combination of phase explosion and thermal effects controlled picosecond laser-modified 4H-SiC surfaces deposited spherical SiO2 particles and the process. The study demonstrates that the cracking behaviour in the subsurface of picosecond laser-modified SiC predominantly occurs in the recast region. The fundamental cause of cracking is attributed to the tensile stresses generated by thermal effects and the volumetric changes in the molten material resulting from the overlapping of neighbouring spots in alternating hot–cold cycles. The nanoindentation demonstrated that laser modification can effectively enhance the machinability of SiC. The findings of this study can provide a theoretical basis for efficient non-destructive machining of hard and brittle materials.

Growth behavior of cristobalite SiO2 coating on 4H–SiC surface via high-temperature oxidation

When SiO2 films undergo oxidation on 4H–SiC, a distinctive crystalline phase, cristobalite, emerges at elevated temperatures. We deeply explored the growth mechanism of the crystallite, focusing on its dependence on the silicon sublimation process (SSO). Activation energy calculations confirmed that the oxidation after SSO above 1350 °C exhibits a lower activation energy. The reduced activation energy suggests the elimination of the solid-phase diffusion process during oxidation. We devised a controllable growth process for the quartz phase to achieve a seamless transition from 0 to 100 % in the area proportion of cristobalite in SiO2. This strategic approach facilitates the precise preparation of oxide layers and holds potential applications in semiconductor device manufacturing.

Imaging ultra-weak UV light below 100 pW cm-2 using a 4H-SiC photodetector with an Al2O3 interfacial layer.

Weak light detection is crucial in various practical applications such as night vision systems, flame monitoring, and underwater operations. Decreasing the dark current of a photodetector can effectively mitigate noises, consequently enhancing the signal-to-noise ratio and overall weak light detection performance. Herein, we demonstrate a 4H-SiC UV photodetector capable of detecting extremely weak UV light. This device comprises a photosensitive layer of 4H-SiC, two TiN electrodes and an atomically thin Al2O3 interfacial layer between TiN and the C surface of 4H-SiC. Under 360 nm UV light illumination, the proposed Al2O3 device demonstrates an ultra-low dark current of 18 fA, possibly benefiting from the effective passivation of interfacial carriers, and a boosted photo-to-dark current ratio of 6.7 × 107. Consequently, it achieves a weak-light detection limit as low as 31.8 pW cm-2, significantly outperforming the control device lacking Al2O3. When compared to previously reported SiC photodetectors, our Al2O3 device boasts an exceptional large linear dynamic range of 172 dB. Leveraging this, we construct a photodetector array capable of clearly imaging an object under ultra-weak light illumination below the 100 pW cm-2 level. The proposed photodetector represents a significant advancement in the development of highly sensitive image sensors for weak light detection.

Bismuth as a buffer layer for metal contact with silicon carbide studied by In situ photoelectron spectroscopy

Silicon carbide (SiC) is a promising third-generation semiconductor due to its wide bandgap. However, the high Schottky barrier and metal-induced gap states (MIGS) at the metal/SiC interface present significant challenges for device fabrication, leading to high contact resistance and poor current delivery. This study proposes the use of bismuth (Bi), with its semimetallic properties and gap-state saturation effect, as a contact buffer layer to address these issues. We conducted a systematic investigation of the chemical and electronic characteristics of the Pt/Bi/4H-SiC(0001) system, fabricated via molecular beam epitaxy (MBE), using in situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Our findings reveal weak bonding between the Bi buffer layer and the 4H-SiC(0001) surface, resulting in a slight downward band bending effect and the formation of a substantial dipole across the Bi/4H-SiC(0001) interface. Moreover, UPS spectra indicate a reduction in the work function of Pt/Bi/4H-SiC(0001), suggesting the potential for achieving low contact resistance. Notably, the Pt/Bi/4H-SiC(0001) system remains stable when exposed to 1.6×109 Langmuir of oxygen at room temperature, while a bare Bi buffer layer undergoes partial oxidation. These results provide a comprehensive understanding of the Pt/Bi/4H-SiC(0001) interfaces and strategies for improving metal/SiC contacts.

Influence of electrolytic plasma spatial distribution on nanoporous structure etching on 4H-SiC surface

Single-crystal SiC nanoporous structure has important applications in the field of micro electromechanical systems, chemical sensors and biomedical devices. However, SiC exhibits a strong chemical inertness due to the short interatomic distance and high binding energy, making it significantly challenging to fabricate nanoporous structure. Electrolytic plasma-assisted chemical etching (EPACE) based on electrochemical anodic oxidation and high-activity electrolytic plasma etching is an ideal method for SiC nanoporous structure etching. In which, the spatial distribution of electrolytic plasma has an important impact on EPACE performance, such as etching efficiency and stability. In order to analyze the impact mechanism, this paper studies EPACE in tool electrode mounting posture of vertical and horizontal types by visual observation, real-time recording of voltage-current waveforms during processing, and the morphology analysis of SiC nanoporous structure after etching. It can be found that in the vertical etching type, EPACE can proceed stably because the workpiece immersion depth is greater than the bubble accumulation thickness. However, in the horizontal etching type, bubbles accumulate seriously between the workpiece and the liquid surface, which may cause bubble cavitation and abnormal discharge, leading to damage to the etching surface. The etching current in the vertical etching type is about 1.3 times that of the horizontal etching type, indicating that the bubble mass transfer in the vertical etching type is faster and the etching impedance is smaller, which is beneficial to improve the etching efficiency and obtain a uniform nanoporous structure. Finally, a SiC nanoporous layer with a thickness of 10.6 μm was prepared at etching time t = 50 min in vertical etching type, and the etching efficiency was 212 nm/min. In addition, the prepared nanoporous structure (nanofiber) was connected to the SiC substrate, indicating that it has a good bonding strength.

Plasma assisted remediation of SiC surfaces

This paper describes a three-step process to remediate surface and sub-surface defects on chemo-mechanically polished SiC surfaces. In this process, a CF4-based inductively coupled plasma with reactive ion etch was used to remove material to a depth, which is unaffected by surface and subsurface polishing damage. This produced a planarized but carbon-rich fluorinated surface. This surface was then exposed to a 2 min rapid thermal oxidation in air at 1000 °C to oxidize and volatilize the excess carbon and fluorinated species, respectively. The resulting surface oxide was then stripped using a dilute hydrofluoric acid in water solution. This process, referred to as plasma assisted remediation, reproducibly yielded planarized, stoichiometric surfaces with low levels of carbon and oxygen contamination suitable for subsequent device fabrication. In the supporting studies described here, 4H- and 6H-SiC(0001) surfaces were remediated and characterized by x-ray photoelectron spectroscopy and atomic force microscopy at each stage of the process. Experimental studies under ion-rich and radical-dominant conditions are also reported which provide greater insight into the underlying chemistry and physics of the process.

Generation of deep levels near the 4H-SiC surface by thermal oxidation

Abstract Deep levels near the surface of 4H-SiC after dry oxidation were investigated. A large and broad peak appeared in the low-temperature range of deep level transient spectroscopy (DLTS) spectra after oxidation of SiC at 1300 °C, indicating multiple deep levels energetically located near the conduction band edge are generated inside SiC by thermal oxidation. Analyses of the DLTS spectra acquired with changing the bias voltage revealed that the majority of deep levels is located very near the SiC surface, within about 6 nm deep region from the surface. The area density of the observed deep levels is higher than 3 × 1012 cm−2.

Study on OH radical oxidation of 4H-SiC in plasma based on ReaxFF molecular dynamics simulation

The application of hydroxyl (OH) radicals to promote chemistry during the polishing of the SiC substrates has received increasing attention from researchers. Compared to catalyzing OH radicals in solution, catalyzing OH radicals in the gaseous state have faster speed and more environmentally friendly generation. In this study, a stable OH radical plasma jet was generated for the oxidation modification of 4H-SiC surface, by excitation and ionizing water molecules in plasma. The deposition of OH radicals on the 4H-SiC surface was simulated with ReaxFF molecular dynamics to study the nanoscale oxidation mechanism. The experimental results demonstrate that the OH radical can achieve surface oxidation of 4H-SiC, thus effectively reducing the hardness. The simulation results revealed that the deposition of OH radicals on the 4H-SiC surface generated varieties of different reaction processes, such as adsorption, dissociation, decomposition, rebound, collision, and injection. The OH radical deposition rate and substrate temperature determine the reaction process. The results of this study provide theoretical support for further revealing the mechanism of action of OH radicals on 4H-SiC and for realizing efficient plasma-assisted polishing.

The investigation of initial decomposition paths of methyltrichlorosilane on (0001) and (000[formula omitted]) surfaces of 4H-SiC: A DFT study

Methyltrichlorosilane (CH3SiCl3, MTS), a favored precursor for chemical vapor deposition (CVD) process of silicon carbide (SiC) epitaxial layer fabrication in a wide variety of temperatures, has been investigated prevalently. In this work, the investigation focused on the initial decomposition paths of MTS with active surface sites on 4H-SiC surfaces. The surfaces employed in this study include both (0001) face and (0001¯) face, which represent Si face and C face, respectively. The decomposition pathways of MTS decomposed with the active surface sites of 4H-SiC surfaces were researched theoretically with employment of transition state theory (TST). The barriers of Gibbs free energies and the estimated reaction rate constants were acquired to discuss the decomposition reactivity of MTS with the active sites on 4H-SiC surfaces. This work shows that MTS has tendency to decompose on both (0001) face and (0001¯) face of 4H-SiC, the H and Cl atoms can be abstracted by active surface sites on (0001) face and (0001¯) face, and impingement of MTS to active surface sites on (0001) and (0001¯) surfaces can yield C-containing and Si-containing surface species.

Schottky contacts on sulfurized silicon carbide (4H-SiC) surface

In this Letter, the effect of a sulfurization treatment carried out at 800 °C on silicon carbide (4H-SiC) surface was studied by detailed chemical, morphological, and electrical analyses. In particular, x-ray photoelectron spectroscopy confirmed sulfur (S) incorporation in the 4H-SiC surface at 800 °C, while atomic force microscopy showed that 4H-SiC surface topography is not affected by this process. Notably, an increase in the 4H-SiC electron affinity was revealed by Kelvin Probe Force Microscopy in the sulfurized sample with respect to the untreated surface. The electrical characterization of Ni/4H-SiC Schottky contacts fabricated on sulfurized 4H-SiC surfaces revealed a significant reduction (∼0.3 eV) and a narrower distribution of the average Schottky barrier height with respect to the reference untreated sample. This effect was explained in terms of a Fermi level pinning effect induced by surface S incorporation.

Study on schottky barrier of Cu/Graphene/4H-SiC interface based on first principles

High stability 4H-SiC ohmic contact is currently a key technical challenge that silicon carbide devices urgently need to overcome. In the paper, the interfacial structures, atomic interactions and Schottky barrier height (SBH) of Cu/Graphene/4H-SiC were studied using the first-principles method. According to research, the SBH for Cu/G/4H-SiC is lower than the SBH for Cu/4H-SiC. The reasons for this phenomenon mainly include the following: 1. The graphene C atoms saturate the dangling bonds on the 4H-SiC surface and the influence of the metal-induced-gap-states (MIGS) at the interface is decreased. 2. A new phase is formed by inserting graphene between the Cu and 4H-SiC have low work functions. 3. An interfacial-electric-dipole layer (IEDL) formed at the interface of 4H-SiC and graphene may also reduce the SBH. These results make them to be promising candidates for future radiation resistant electronics.

First Principle Study on Schottky Barrier at Ni/Graphene/4H-SiC Interface

High stability 4H-SiC ohmic contact is currently a key technical challenge that silicon carbide devices urgently need to overcome. It is important to reduce the Schottky barrier height (SBH) at the Ni/4H-SiC interface to optimize ohmic contact. In this paper, the mechanisms of graphene layer changing Ni/4H-SiC interface Schottky barrier height (SBH) are studied based on the first-principles method within the local density approximation. Theoretical studies have shown that graphene intercalation can reduce the SBH of Ni and 4H-SiC interfaces. The reason of SBH reduction may be that the graphene C atoms saturate the dangling bonds on the 4H-SiC surface and the influence of the metal-induced energy gap state at the interface is reduced. In addition, the new phase formed at the interface of graphene and silicon carbide has a lower work function. Furthermore, an interfacial electric dipole layer may be formed at the 4H-SiC/graphene interface which may also reduce the SBH. These results make them to be promising candidates for future radiation resistant electronics.

Femtosecond Laser-Induced Phase Transformation on Single-Crystal 6H-SiC.

Silicon carbide (SiC) is widely used in many research fields because of its excellent properties. The femtosecond laser has been proven to be an effective method for achieving high-quality and high-efficiency SiC micromachining. In this article, the ablation mechanism irradiated on different surfaces of 6H-SiC by a single pulse under different energies was investigated. The changes in material elements and the geometric spatial distribution of the ablation pit were analyzed using micro-Raman spectroscopy, Energy Dispersive Spectrum (EDS), and an optical microscope, respectively. Moreover, the thresholds for structural transformation and modification zones of 6H-SiC on different surfaces were calculated based on the diameter of the ablation pits created by a femtosecond laser at different single-pulse energies. Experimental results show that the transformation thresholds of the Si surface and the C surface are 5.60 J/cm2 and 6.40 J/cm2, corresponding to the modification thresholds of 2.26 J/cm2 and 2.42 J/cm2, respectively. The Raman and EDS results reveal that there are no phase transformations or material changes on different surfaces of 6H-SiC at low energy, however, decomposition and oxidation occur and then accumulate into dense new phase material under high-energy laser irradiation. We found that the distribution of structural phase transformation is uneven from the center of the spot to the edge. The content of this research reveals the internal evolution mechanism of high-quality laser processing of hard material 6H-SiC. We expect that this research will contribute to the further development of SiC-based MEMS devices.

Removal mechanism on 4H–SiC single crystal by picosecond laser ablation-assisted chemical mechanical polishing (CMP)

The properties of the single-crystal 4H–SiC are complex and difficult to understand. Consequently, determining the effect of the surface and subsurface structures and composition changes of picosecond laser ablation (PLA) on the material removal mechanism during the CMP stage is challenging. In this study, we used PLA of 4H–SiC surfaces to investigate the mechanism underlying the effect of PLA-assisted CMP, which is considerably larger than the laser ablation threshold, on the removal of 4H–SiC to improve the material removal rate and surface quality of CMP. The TEM and contact angle detection results indicated that the reduction in the fracture strength and increase in the surface energy of high-power PLA contributed to the removal form and chemical action of CMP. Subsequent experimental results showed that the contact angle and hardness of the PLA-treated surfaces in the hydrophilic range were reduced by 45% and 48.6%, respectively. The material removal rate (MRR) of PLA-CMP (alumina slurry) at 45 min was 3.67 μm/h that was a 70% improved CMP removal relative to no picosecond laser ablation (NPLA). The mechanism of PLA-assisted CMP removal was revealed to be related to the plastic cutting of the abrasive and high pressure and temperature, leading to the softening-removal-softening cycle of the material. This study provides a new perspective on the ultraprecise and high-efficiency fabrication of high-quality single-crystal 4H–SiC.

Piezocatalysis for Chemical-Mechanical Polishing of SiC: Dual Roles of t-BaTiO3 as a Piezocatalyst and an Abrasive.

Chemical mechanical polishing (CMP) offers a promising pathway to smooth third-generation semiconductors. However, it is still a challenge to reduce the use of additional oxidants or/and energy in current CMP processes. Here, a new and green atomically smoothing method: Piezocatalytic-CMP (Piezo-CMP) is reported. Investigation shows that the Piezo-CMP based on tetragonal BaTiO3 (t-BT) can polish the rough surface of a reaction sintering SiC (RS-SiC) to the ultra-smooth surface with an average surface roughness (Ra) of 0.45nm and the rough surface of a single-crystal 4H-SiC to the atomic planarization Si and C surfaces with Ra of 0.120 and 0.157nm, respectively. In these processes, t-BT plays a dual role of piezocatalyst and abrasive. That is, it piezo-catalytically generates in-situ active oxygen species to selectively oxidize protruding sites of SiC surface, yielding soft SiO2, and subsequently, it acts as a usual abrasive to mechanically remove these SiO2. This mechanism is further confirmed by density functional theory (DFT) calculation and molecular simulation. In this process, piezocatalytic oxidation is driven only by the original pressure and friction force of a conventional polishing process, thus, the piezo-CMP process do not require any additional oxidant and energy, being a green and effective polishing method.