Porous 4H-SiC Research Articles

High-Temperature Reorganization Behavior of Single-Crystalline Porous 4H-SiC Thin Foils

This work reports on the high-temperature reorganization behavior of single-crystalline porous 4H-silicon carbide (4H-SiC) thin foils. Porous 4H-SiC thin foils are realized via state-of-the-art photoelectrochemical etching in hydrofluoric (HF) acid solution enabling for the first time a released foil with a diameter of 2 inches. Subsequent annealing under inert gas atmosphere and comparison between samples suggests that a temperature of 1500 °C allows for various degrees of compactification across the foil surface, whereas at 1600 °C single crystallinity can be preserved.

Improved passivation depth of porous fluorescent 6H-SiC with Si/C faces using atomic layer deposition

We investigated the effects of different growth facets of 6H-SiC and different voltage waveforms on the porous structure and luminescence properties. The structure formed on the surface after anodic etching significantly changed because of the difference in the growth plane, whereas dendritic and columnar pores were observed inside the Si- and C-face samples. These large porous structures were shown to promote the penetration depth of the atomic-layer-deposited Al2O3 films, and a recorded passivation depth of 30 μm layer was confirmed in C-face porous SiC. From the results using a fluorescence microscope and photoluminescence spectra measurement, it was concluded that the pulsed-voltage etching was preferable for fabricating uniform porous structures compared with the constant-voltage etching. However, the enhancement of the luminescence intensity needs to be further improved to realize high luminescent efficiency in porous fluorescent SiC.

Quasi-ballistic thermal conduction in 6H–SiC

The minimization of electronics makes heat dissipation of related devices an increasing challenge. When the size of materials is smaller than the phonon mean free paths, phonons transport without internal scattering and laws of diffusive thermal conduction need to be revisited. This work reports the temperature dependent thermal conductivity of doped epitaxial 6H–SiC and monocrystalline porous 6H–SiC below room temperature probed by time-domain thermoreflectance. Strong quasi-ballistic thermal transport was observed in these samples, especially at low temperatures. Doping and structural boundaries were applied to tune the quasi-ballistic thermal transport since dopants selectively scatter high-frequency phonons while boundaries scatter phonons with long mean free paths. Exceptionally strong phonon scattering by boron dopants are observed, compared to nitrogen dopants. Furthermore, orders of magnitude reduction in the measured thermal conductivity was observed at low temperatures for the porous 6H–SiC compared to the epitaxial 6H–SiC. Finally, first principles calculations and a simple Callaway model are built to understand the measured thermal conductivities. Our work sheds light on the fundamental understanding of thermal conduction in technologically-important wide bandgap semiconductors such as 6H–SiC and will impact applications such as thermal management of 6H–SiC-related electronics and devices.

Nanoporous 6H-SiC Photoanodes with a Conformal Coating of Ni-FeOOH Nanorods for Zero-Onset-Potential Water Splitting.

A surface-nanostructured semiconductor photoelectrode is highly desirable for photoelectrochemical (PEC) solar-to-fuel production due to its large active surface area, efficient light absorption, and significantly reduced distance for charge transport. Here, we demonstrate a facile approach to fabricate a nanoporous 6H-silicon carbide (6H-SiC) photoanode with a conformal coating of Ni–FeOOH nanorods as a water oxidation cocatalyst. Such a nanoporous photoanode shows significantly enhanced photocurrent density (jph) with a zero-onset potential. A dendritic porous 6H-SiC with densely arranged holes with a size of ∼40 nm on the surface is fabricated by an anodization method, followed by the hydrothermal deposition of FeOOH nanorods and electrodeposition of NiOOH. Under an illumination of AM1.5G 100 mW/cm2, the Ni–FeOOH-coated nanoporous 6H-SiC photoanode exhibits an onset potential of 0 V versus the reversible hydrogen electrode (VRHE) and a high jph of 0.684 mA/cm2 at 1 VRHE, which is 342 times higher than that of the Ni–FeOOH-coated planar 6H-SiC photoanode. Moreover, the nanoporous photoanode shows a maximum applied-bias-photon-to-current efficiency (ABPE) of 0.58% at a very low bias of 0.36 VRHE, distinctly outperforming the planar counterpart. The impedance measurements demonstrate that the nanoporous photoanode possesses a significantly reduced charge-transfer resistance, which explains the dramatically enhanced PEC water-splitting performance. The reported approach here can be widely used to fabricate other nanoporous semiconductors for solar energy conversion.

Impact of Substrate Types on Structure and Emission of ZnO Nanocrystalline Films

Zinc oxide (ZnO) films were simultaneously synthesized by an ultrasonic spray pyrolysis (USP) method on p-type Si (100), silicon carbide polytype [6H-SiC (0001)], porous 6H-SiC and amorphous glass substrates with the aim of studying the impact of substrate types on the structure and emission of ZnO nanocrystalline films. Porous silicon carbide (P-SiC) was prepared by the electrochemical anodization method at a constant potential of 20 V and etching time of 12 min. ZnO films grown on the SiC and P-SiC substrates are characterized by a wurtzite crystal structure with preferential growth along the (002) direction and with grain sizes of 90–180 and 70–160 nm, respectively. ZnO films grown on the Si substrate have just some small irregular hexagonal islands. The amorphous glass substrate did not promote the formation of any regular crystal forms. The obtained x-ray diffraction and photoluminescence (PL) results have shown that the better ZnO film crystallinity and high PL intensity of near-band edge emissions were achieved in the films grown on the porous SiC and SiC substrates. The preferential growth and crystalline nature of ZnO films on the SiC substrate have been discussed from the point of view of the lattice parameter compatibility between ZnO and SiC crystals.

Effective optimization of surface passivation on porous silicon carbide using atomic layer deposited Al2O3

Significant enhancement of photoluminescence in ALD Al2O3 passivated porous 6H-SiC.

Stacked Layers of Different Porosity in 4H SiC Substrates Applying a Photoelectrochemical Approach

Porous 4H-SiC layers were prepared from monocrystalline samples applying photo-electrochemical etching in hydrofluoric acid. The influence of both current and voltage controlled mode during photo-electrochemical porosification was investigated. It was found that the resulting degree of porosity, the homogeneity in porosity as well as the pore morphology mainly depend on the applied voltage, whereas the current level has an almost negligible impact on these important parameters. Based on these results, it is proposed that the formation of porous SiC during photo-electrochemical etching can be described by fractal growth. Finally the gathered knowledge allowed to detach the porous 4H-SiC layers, which comprised several sub-layers of alternating degree of porosity, from the 4H-SiC substrate. Such layers of tailored porosity are key components for several advanced device concepts such as optical filters or membranes for biological applications.

The role of aluminium as an additive element in the synthesis of porous 4H-silicon carbide

Addition of 5 at.% aluminium to silicon and carbon in the process of forming SiC monoliths for use as diesel particle filters and catalyst supports, creates mechanically stable, well connected, and highly porous 4H-SiC structures. These monoliths have 65% accessible porosity with pore diameters of 17−20μm. Mixtures of silicon, graphite, aluminium and water, are extruded into honeycomb structures and heated under nitrogen and argon. The 2H-AlN crystalline assembly formed during initial heating under nitrogen at 850°C acts as a template for the subsequent reaction between silicon and graphite under argon to yield 3C-SiC. During a final high temperature step under argon at 1950°C, Al-vapour/liquid is crucial for the transformation of 3C-SiC to 4H-SiC. This final step also alters the SiC crystal morphology significantly and produces large by-product crystals of Al4C3·mSiC·nAlN. The polytypic conversion and recrystallization mechanism were found, in this case, to be independent phenomena.

Fabrication and surface passivation of porous 6H-SiC by atomic layer deposited films

Porous 6H-SiC samples with different thicknesses were fabricated through anodic etching in diluted hydrofluoric acid. Scanning electron microscope images show that the dendritic pore formation in 6H-SiC is anisotropic, which has different lateral and vertical formation rates. Strong photoluminescence was observed and the etching process was optimized in terms of etching time and thickness. Enormous enhancement as well as redshift and broadening of photoluminescence spectra were observed after the passivation by atomic layer deposited Al2O3 and TiO2 films. No obvious luminescence was observed above the 6H-SiC crystal band gap, which suggests that the strong photoluminescence is ascribed to surface state produced during the anodic etching.

The effect of trace amounts of copper on the microstructure, stability and oxidation of macroporous silicon carbide

Trace amounts of copper have advantageous effects when creating macroporous silicon carbide monoliths to be used for combined diesel particle filter and catalyst support systems. These structures were produced from a slurry of silicon, graphite, aluminium and copper. Mixed with water, extruded and dried, the resulting bodies were pyrolyzed, sintered, and then partially oxidized, to yield a mechanically stable porous 4H-SiC microstructure with an average pore diameter of 20μm and average accessible porosity of 57%. The Cu alloys with the Si and Al to create the sintered body via a liquid phase, and prevents the build-up of undesirable Al-containing ternary carbide crystals in the microstructure. The Cu promotes oxidation of SiC to form a 80nm SiO2 layer that serves as a good catalyst support. The accelerated oxidation can be intercepted by dissolving the Cu component from the monoliths with an acid solution.

A Combination of Metal Assisted Photochemical and Photoelectrochemical Etching for Tailored Porosification of 4H SiC Substrates

Porous 4H-SiC layers were prepared with a tailored sequence of metal assisted photochemical etching (MAPCE) and photoelectrochemical etching (PECE) steps. Porous layers resulting from MAPCE provided initiation sites for a subsequent PECE process. Finally, the porosity of the porous layers was increased to the desired degree with a second MAPCE step. By applying this sequence, the problem of skin and cap layer formation occurring during pure PECE is avoided. In addition, layers with a homogeneous porosity depth profile are obtained, which is not possible when only pure PECE is applied.

Communication—The Role of the Metal-Semiconductor Junction in Pt-Assisted Photochemical Etching of Silicon Carbide

Porous 4H-SiC layers were fabricated by photochemical etching of n-type 4H-SiC samples with varying resistivity. An etching solution of Na2S2O8 and HF was used while Pt deposited at the 4H-SiC surface served as catalyst for the reduction of Na2S2O8. The contact resistance at the Pt/4H-SiC junction was decreased by annealing and surface near phosphorous doping. This enabled the porosification of 4H-SiC with photochemical etching.

Effects of B4C on the microstructure and phase transformation of porous SiC ceramics

Porous SiC ceramics sintered with B4C additive exhibit excellent corrosion resistance in hot acidic and basic solutions compared with traditional porous ceramic membrane supports. The effects of B4C on the density, phase composition, and microstructure of porous 6H–SiC ceramics were investigated in this study. The phase composition and microstructure of the as-prepared samples were characterized through X-ray diffraction, Raman spectrum, scanning electron microscopy, and electron probe microanalyzer equipped with wavelength dispersive spectrometry. B4C promoted the sintering of porous SiC ceramics within the temperature range of 2100–2200°C. The B4C content higher than 0.5wt% induced the phase transformation of 6H–SiC to 4H–SiC at 2200°C, resulting in the formation of large plate-like 4H–SiC grains, which significantly improved the flexural strength of the porous ceramics. The mechanisms of phase transformation and grain formation were also elucidated.

Development of elongated 6H-SiC grains in reaction-bonded porous SiC ceramics

Porous 6H-SiC ceramics were fabricated at a low temperature by an in situ reaction bonding process. After subsequent heat treatment, porous SiC ceramics with elongated 6H-SiC grains and improved strength were obtained. The results indicated that in situ growth of elongated 6H-SiC grains was caused by rare earth elements together with the liquid Ba–Al–Si–O system. The type and content of rare earth oxides had a large influence on the growth of elongated 6H-SiC grains.

Emission related to exciton‐polariton coupling in porous SiC

AbstractThe paper presents the results of SiC nanocrystal (NC) characterization using the photoluminescence (PL), its temperature dependence and X‐ray diffraction (XRD) techniques. It is revealed that original n‐type 6H‐SiC wafers and porous 6H‐SiC layers consisted inclusions of 2H‐SiC, 4H‐SiC, and 8H‐SiC polytypes. The photoluminescence study has shown that in porous SiC layers the emission intensity of high energy PL bands enlarges. Temperature dependences of high energy PL bands testify that these PL bands related to excitons bounded at nitrogen donors in different SiC polytypes. The PL intensity enhancement of donor‐bound exciton PL bands in the big size (50‐250nm) SiC nanocrystals has been attributed to the realization of exciton weak confinement and exciton‐polariton coupling in SiC nanocrystals. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Elastic Waves in Nano-Columnar Porous 4H-SiC Measured by Brillouin Scattering

We report measured and calculated frequencies of elastic waves propagating in columnar porous 4H-SiC, an analytically tractable system. The sample was prepared using photo-electrochemical etching followed by mechanical polishing. The frequencies were measured using Brillouin scattering spectroscopy in backscattering geometry. The effective elastic constants, ordinary and extraordinary indices of refraction, and mass density, all obtained using effective medium models, are used to calculate the frequencies. Although the quasistatic limit is not strongly satisfied, the agreement between the measured and calculated values is good.

Investigation of porous silicon carbide as a new material for environmental and optoelectronic applications

In this paper, we present an experimental study on the chemical and electrochemical etching of silicon carbide (SiC) in different HF-based solutions and its application in different fields, such as optoelectronics (photodiode) and environment (gas sensors). The thin SiC films have been grown by pulsed laser deposition method. Different oxidant reagents have been explored. It has been shown that the morphology of the surface evolves with the etching conditions (oxidant, concentration, temperature, etc.). A new chemical polishing solution of polycrystalline 6H-SiC based on HF:Na 2O 2 solution has been developed. Moreover, an electrochemical etching method has been carried out to form a porous SiC layer on both polycrystalline and thin SiC films. The PL results show that the porous polycrystalline 6H-SiC and porous thin SiC films exhibited an intense blue luminescence and a green-blue luminescence centred at 2.82 eV (430 nm) and 2.20 eV (560 nm), respectively. Different device structures based on both prepared samples have been investigated as photodiode and gas sensors.

Surface phonons and exciton–polariton coupling in SiC nanocrystals

The paper presents the results of SiC nanocrystal characterization using scanning electron microscopy (SEM), Raman scattering and photoluminescence spectroscopy techniques, as well as X-ray diffraction (XRD). The Raman scattering investigation of porous SiC with different NC sizes have shown new features specific for nano-crystallite materials: appearing of surface phonon modes and the enlargement of diffusing background scattering. Photoluminescence study of PSiC layers with different thicknesses and SiC NC sizes reveals the intensity stimulation for exciton related PL bands. The intensity enhancement for exciton-related PL bands is attributed to exciton recombination rate increasing due to the realization of exciton confinement and exciton –polariton effects in big size SiC NCs of different polytypes (6H-PSiC with inclusions of 15R- and 4H-PSiC). XRD study has confirmed that the investigated porous 6H-SiC layers contain inclusions of 4H-SiC and 15R-SiC polytypes.

X-ray diffraction and electron paramagnetic resonance study of porous 6H-SiC

The article presents the results of porous SiC (PSiC) characterizations using x-ray diffraction (XRD) and electron paramagnetic resonance (EPR) techniques. Two types of PSiC samples, electrochemically as-etched and electrochemically etched and reactive ion treated, have been investigated. The XRD study shows that original SiC wafers and porous SiC layers have mainly the 6H-SiC crystal type with inclusions of 4H-SiC and 15R-SiC polytypes. With increasing porosity and porous layer thickness a new XRD band appears and grows that is assigned to the phase of amorphous graphite at the PSiC surface. The EPR spectra of PSiC layers measured at room temperature reveal low intensity signal in the g-value region of g=2.0027 and a peak-to-peak width of about 0.24–0.42 mT. The intensity of this signal increases simultaneously with the rise of PSiC thickness. The EPR study shows that the carbon dangling bond centers at and near the 6H-SiC/SiO2 interface are the dominant defects for electrochemically etched porous 6H-SiC samples.

Exciton related photoluminescence stimulation in SiC nanocrystals

The paper presents the results of porous SiC characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence spectroscopy techniques. XRD study shows the investigated porous 6H-SiC layers contain inclusions of 4H-SiC and 15R-SiC polytypes as well as the amorphous graphite phase. Photoluminescence study of PSiC layers with different thicknesses and SiC NC sizes reveals the intensity stimulation for exciton and defect-related PL bands. The intensity stimulation of defect-related PL bands is due to the increase, at the etching process, of the concentration of surface defects, apparently to deal with carbon. The intensity enhancement for exciton-related PL bands is attributed to the exciton recombination rate increasing due to the realization of exciton confinement and exciton–polariton effects in big size SiC NCs of different polytypes (6H-PSiC with inclusions of 15R- and 4H-PSiC).