Porous Silicon Carbide Layers Research Articles

A Novel Tool Layout and Process for Single Side Wet Electrochemical Processing of Porous Silicon Carbide Layers without Edge Exclusion

In this paper we present a novel tool layout for wet chemical processing of porous silicon carbide layers. The novel tool concept includes single side processing without edge exclusion. There is no need to contact the backside of the wafer. We show SEM cross sections of the porous layer made by different currents densities. With increased current density the porosity increases. After optimization of the process conditions, we achieve a layer thickness non-uniformity of 10%.

Impact of rapid thermal annealing on impurities removal efficiency from silicon carbide for optoelectronic applications

Impurities are of crucial interest in optoelectronic devices as they affect carrier lifetimes and electrical properties. In view of that, it is important to incorporate certain processing steps to reduce impurities effect on final devices. The aim of this work is to enhance silicon carbide SiC purity for silicon passivation. Low cost method of SiC purification is presented. This method combines three main steps consisting of formation of porous silicon carbide layers by acid vapor etching followed by a photo-thermal annealing at different temperatures. Finally, obtained SiC powder was subject to a chemical treatment to remove porous SiC layer. Effect of gettering temperature on purification efficiency was evaluated by inductively coupled plasma atomic emission spectrometry (ICP-AES). The gettering experiment was performed at 800–950 °C, and optimum results were obtained at 950 °C. SiC purity is improved from 3 N (99.977%) to 5 N (99.999%) with an impurity removal efficiency of 96.56%. Purified SiC was used as a target to synthesize SiC layers using pulsed laser deposition technique (PLD) for optoelectronic devices. The use of intrinsic amorphous silicon carbide (a-SiC) passivating layers was investigated especially by photoconductivity decay technique. Improved SiC target purity leads to a significant enhancement of a-SiC passivating properties attributed to the surface recombination velocity decrease. Electrical properties of a-SiC/c-Si(p) were also studied using I-V and C-V techniques.

Porous structures based on silicon carbide for photovoltaic cells

A method for producing porous silicon carbide by carbonization of porous silicon is considered. The surface morphology and phase composition of porous silicon carbide layers obtained by diffusion of carbon from the gas phase into the porous silicon layer are investigated. Diffusion and absolute reflection spectra of porous silicon carbide and porous silicon samples are obtained. A study of the specific surface resistance and lifetime of excess charge carriers in porous silicon carbide was carried out. Using of porous silicon carbide in photovoltaic cells to increase their efficiency is shown.

Dual layer SiC coating on matrix graphite sphere prepared by pack cementation and fluidized‐bed chemical vapor deposition

AbstractA dual layer silicon carbide (SiC) coating including inner porous SiC (p‐SiC) layer and outer dense SiC (d‐SiC) layer was fabricated on the matrix graphite (MG) spheres of high‐temperature gas‐cooled reactor fuel elements by pack cementation and fluidized‐bed chemical vapor deposition process to improve the oxidation‐resistant property. Microstructure of the coating demonstrates different density and structure of the two SiC layers with no obvious boundaries between them. Weight gain curves of oxidation tests at 1773 K for 200 hours show that the coating could effectively protected the MG sphere by isolating the air infiltration with p‐SiC layer as the main functional layer and d‐SiC layer as the transition layer to improve the bond strength. Due to the transition function of p‐SiC layer, the coated spheres could understand more than 50 times thermal shocking tests from 1773 K to room temperature with no stress cracking.

New process of silicon carbide purification intended for silicon passivation

In this work, we report on a new, efficient and low cost process of silicon carbide (SiC) powder purification intended to be used in photovoltaic applications. This process consists on the preparation of porous silicon carbide layers followed by a photo-thermal annealing under oxygen atmosphere and chemical treatment. The effect of etching time on impurities removal efficiency was studied. Inductively coupled plasma atomic emission spectrometry (ICP-AES) results showed that the best result was achieved for an etching time of 10 min followed by gettering at 900 °C during 1 h. SiC purity is improved from 3N (99.9771%) to 4N (99.9946%). Silicon carbide thin films were deposited onto silicon substrates by pulsed laser deposition technique (PLD) using purified SiC powder as target. Significant improvement of the minority carrier lifetime was obtained encouraging the use of SiC as a passivation layer for silicon.

Pool boiling CHF of reduced graphene oxide, graphene, and SiC-coated surfaces under highly wettable FC-72

This paper presents the results of a study of enhanced boiling heat transfer (BHT) and critical heat flux (CHF) for a bare indium tin oxide (ITO) surface, a nonporous few-layered graphene-deposited ITO surface, a nonporous SiC layer-deposited ITO surface, and porous graphene and silicon carbide (SiC) layer-deposited ITO surfaces. The experiments were conducted under atmospheric pressure using an FC-72 refrigerant under saturation conditions. Infrared thermometry was used to determine the temperature fields of the heater surfaces and the CHF conditions. The CHF values for the surfaces with highly thermally conductive nonporous graphene layers and nonporous SiC layers were found to be increased by 9% and 15.7%, respectively, compared to that of the bare ITO heating surface. For the heating surfaces with porous graphene layers and SiC layers, the CHF values were increased by 90% and 58%, respectively. All of the heating surfaces exhibited hydrophilic behavior with respect to the FC-72 fluid. The differences in CHF enhancement observed can be explained by differences in the thermal properties of graphene and SiC, their heat dissipation limits, differences in aspects of their surface morphologies, such as their porosities and permeabilities, and the effects of these on their hydrodynamic limits and capillary pumping limits.

40Å Platinum–porous SiC gas sensor: Investigation sensing properties of H2 gas

Abstract The present paper reports on a new structure for H2 gas sensing based on thin porous silicon carbide (PSiC) films. The PSiC layer has been formed by electrochemical etching of SiC films previously deposited onto p-type silicon substrate by pulsed laser deposition (PLD) using 6H-SiC target. Current–voltage (I–V) and current–time (I–t) characteristics have been measured. A thin platinum (Pt) film (40 A thickness) deposited onto PSiC layer has been used as a catalytic metal. The Schottky diode parameters such as ideality factor (n), barrier height (ϕBp) and series resistance (RS) have been evaluated under different concentrations of H2 gas. The experimental results show that upon exposure to H2 gas the barrier height, the ideality factor and the series resistance change significantly. The different changes in the electrical parameters of the structure (increase and decrease as a function of the H2 concentration) have been explained by the formation of two inversion layers. The first one forms as soon as the gas is in contact with the sensor and the second when the concentration reaches 90 ppm. Subsequently, the effect of gas concentration on the maximum sensitivity value of the sensor has been investigated. A high sensitivity (ΔI/I) value around 86% is found at about 1 V bias voltage. In addition, the response and recovery times were determined to be around 55 s and 160 s, respectively. Finally, the structure shows a reversible response for low gas concentration at room temperature.

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.

Influence of thickness and porous structure of SiC layers on the electrical properties of Pt/SiC-pSi and Pd/SiC-pSi Schottky diodes for gas sensing purposes

Abstract The aim of this work was to study the influence of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky diodes for gas sensing purposes by using a platinum (Pt) or a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous SiC (PSC) layers. The Schottky diodes were used for the first time for H2 and hydrocarbon (C2H6) gas sensing. The non-porous and porous SiC layers were realized on a p-type silicon (Si(1 0 0)) substrate by pulsed laser deposition using a KrF laser (248 nm) and thermal deposition of a thin Pt and Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The properties of the porous SiC layers formed by this method were investigated by scanning electron microscopy (SEM). The electrical measurements were made at room temperature (295 K) in an air ambience using a cryostat chamber. The effect of the porous surface structure and the thickness of the SiC layer was investigated by evaluating electrical parameters such as the ideality factor (n), barrier height (ϕBp) and series resistance (Rs). The thickness of the porous layer significantly affects the electrical properties of the Schottky diodes. Analysis of current–voltage (I–V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I–V characteristics was found to be dependent on the SiC thickness. For a thin SiC layer (0.16 μm) n was around 1.293 with a barrier height 0.847 eV, while for a thick layer (1.6 μm), n and ϕBp were 1.110 and 0.812 eV, respectively, for both Pt/SiC-pSi and Pd/SiC-pSi. The low value of series resistance obtained using Cheung's method clearly indicated the high performance of the Schottky diode for large SiC layer thickness. This effect showed the uniformity of the SiC layer. C–V analysis revealed a decrease in the capacitance with increasing voltage. This decrease was less for a thin SiC layer (0.16 μm) than a thick SiC layer (1.6 μm), clearly indicating the role of the SiC thickness in the determining sensor capacitance values. Finally, high sensitivity (ΔI/I = 90%) and selectivity of the sensors were reached at low voltages below 1 V, by using the PSC layer with catalytic metals of Pt and Pd.

Morphology and optical properties of titanium-doped porous silicon carbide layers

The effect of rapid thermal annealing (RTA) in oxygen on the properties of titanium-doped porous silicon carbide (por-SiC) layers has been studied. The data on the surface morphology and the photoluminescence (PL) spectra show that the high-temperature diffusion annealing is accompanied by the formation of titanium silicides, by an increase in the grain size of the material, and by the emergence of pores on the surface of the por-SiC-Ti sample. The results of PL measurements are consistent with data on the phase composition and the surface morphology of the samples. It is established that RTA leads to modification of the titanium-doped por-SiC structure.

Fabrication and characterization of laminated SiC ceramics with self-sealed ring structure

One of the most creative achievements of researchers in the structural ceramics field is ceramic-matrix composite materials. Like so many materials science accomplishments, uni-directionally aligned continuous fibre reinforced ceramic composites [1–3] or laminated plate-form ceramic composites [4–12] actually partially imitate the structure of a natural material, in this case wood, either in one or two dimensions. The advantage of layered structures is that they perform the function of fibre-reinforced ceramic composites, but are much easier to fabricate. Plate-form laminated ceramic composites with a weak interface such as the SiC/graphite system, usually show a high work of fracture and fracture toughness as high as 14 MPam1/2 [4, 5]. However, the major problem associated with the plate-form laminate is that it possesses two delamination directions, which prevents the laminates from enjoying widespread applications as structural components. Therefore other structures are needed, either at the microor meso-/macro-level, to address this problem. From the meso-/macro-structure point of view, one possible solution to the intrinsic delamination problem in plate-form ceramic laminates is to once again look to a natural material and imitate the ring structure of wood in three-dimensions. This strategy reduces the potential delamination directions in a laminated ceramic material from two to zero when the structure of a laminate changes from plate form to a self-sealed ring, i.e., a highly anisotropic structure. The objective of the present work is to create a ceramic laminate by imitating the concentric cylinder tree trunk structure and to examine the delamination resistance and fracture behaviour of the structure. A simple shaping technique, a modified slip casting method, has been used to achieve a self-sealed ring structure for a variety of ceramic laminates. In our experiment, the silicon carbide/carbon system was chosen as an example to demonstrate this structure. This system has two characteristics: (1) the graphite not only happens to be a successful sintering aid for SiC but also provides a weak interface [4, 5] and (2) our previous research [4] showed that the carbon layers can be converted into porous silicon carbide layers. Hence a single-phase SiC ceramic with a better oxidation resistance can be obtained while at the same time retaining a suitably weak interface. SiC slurry was prepared by mixing 88 wt% of SiC, 8 wt% of Al2O3 and 4 wt% of Y2O3 with water with solid to liquid ratio of 30/70 by volume. The concentration of carbon in the water based graphite slurry was between 2.5 and 5% by volume. SiC/graphite laminates were slip-cast with alternate SiC and graphite layers in a plaster of Paris mould with a casting chamber 10 mm in diameter and 50 mm in length. All the laminates were fabricated in such a way that the outermost layer and core were SiC. The thickness of the SiC layer was varied from 50 to 600 μm and that of the graphite layer from 5–20 μm by adjusting the viscosity of the slurry and casting time. The number of SiC layers in the structure was varied from 20 to 25. After slip casting, the green bodies were slowly dried in air for 48 h and sintered at temperatures ranging from 1,800 to 1,850◦C for 1 h in an Ar atmosphere at 1 atm pressure. Bulk densities were measured by the Archimedes method. The theoretical density of the laminates was calculated on the basis of the rule of mixtures. Three-point bending tests on specimens with a span of 25.4 mm were conducted using an Instron machine (8502) at room temperature. The crosshead speed was 0.06 mm/min. Since ceramic rods with circular crosssections were used for the mechanical property tests, the work of fracture/failure work was used to characterize the fracture resistance of the silicon carbide/carbon laminates. The total work per unit volume during a bending test can be written as:

Features of the structure of a porous silicon carbide layer obtained by electrochemical etching of a 6H-SiC substrate

Transverse sections of the (11–20) cuts of a 6H-SiC substrate-porous SiC layer-epitaxial 6H-SiC layer structure were studied using electron microscopy. An intermediate layer is revealed between pores and unetched SiC which consists of a damaged region containing two-dimensional defects and a completely amorphous region. Energy-dispersive X-ray spectra measured within local (∼3 nm) areas in various regions of the transverse sections of the structure studied showed that the intermediate layer is enriched with carbon in comparison to the stoichiometric substrate composition. The excess carbon content is retained in the layer of epitaxial SiC contacting the porous layer.

Nondestructive determination of the characteristics of porous silicon carbide layers

It shown that the porosity and thickness of a porous silicon carbide layer can be nondestructively monitored provided that the sample weight loss upon electrochemical etching is known and the reflectance spectrum exhibits interference.

Effects of substrate doping on the properties of porous silicon carbide layers

Effects of the (n)6H-SiC substrate doping in the interval from 4×1017 to 4.2×1018 cm−3 on the properties of porous silicon carbide layers (pore diameter, relative porosity, pore concentration) were studied for samples prepared by the Lely method. It is established that porous SiC layers obtained by this method in strongly doped substrates are characterized by smaller pore diameter and volume and several times greater pore concentration as compared to analogous values for slightly doped substrates.

The effect of high-temperature epitaxial SiC layer growth on the structure of porous silicon carbide

Porous silicon carbide layers obtained by electrochemical etching of 6H-SiC at three anode current densities were investigated. X-ray double-and triple-crystal diffractometry and scanning electron microscopy were used to study the structure of porous SiC layers before and after high-temperature sublimation growth of 6H-SiC epilayers. The density of pores in the structure is found to be independent of the current density in electrochemical etching. The effective diameter of pores increases with increasing current density, resulting in higher porosity of the structure. High-temperature annealing modifies the structure without changing the sample porosity. The structural rearrangement is accompanied by coalescence of single pores and an increase in their diameter.