Porous SiC Layer Research Articles

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.

An improved design of TRISO particle with porous SiC inner layer by fluidized bed-chemical vapor deposition

Tristructural-isotropic (TRISO) particle has been successful in high temperature gas cooled reactor (HTGR), but an improved design is required for future development. In this paper, the coating layers are reconsidered, and an improved design of TRISO particle with porous SiC inner layer is proposed. Three methods of preparing the porous SiC layer, called high methyltrichlorosilane (MTS) concentration method, high Ar concentration method and hexamethyldisilane (HMDS) method, are experimentally studied. It is indicated that porous SiC layer can be successfully prepared and the density of SiC layer can be adjusted by tuning the preparation parameters. Microstructure and characterization of the improved TRISO coated particle are given based on scanning electron microscope (SEM), X-ray diffraction (XRD), Raman scattering and energy dispersive X-ray (EDX) analysis. It can be found that the improved TRISO coated particle with porous SiC layer can be mass produced successfully. The formation mechanisms of porous SiC layer are also discussed based on the fluidized bed-chemical vapor deposition principle.

Effect of por-SiC buffer layer on the parameters of thin Er2O3 layers on silicon carbide substrates

Using optical absorption and Auger spectrometry techniques, we studied the effect of rapid thermal annealing (RTA) on the properties of erbium oxide films deposited onto a porous silicon carbide buffer layer formed on 4H-SiC substrates. An analysis of atomic composition of the films under investigation as a function of RTA duration was performed. It is shown that phase composition of erbium oxide films on silicon carbide substrates with a porous SiC layer can be changed by varying RTA duration.

Influence of the anodic etching current density on the morphology of the porous SiC layer

In this report, we fabricated a porous layer in amorphous SiC thin films by using constant-current anodic etching in an electrolyte of aqueous diluted hydrofluoric acid. The morphology of the porous amorphous SiC layer changed as the anodic current density changed: At low current density, the porous layer had a low pore density and consisted of small pores that branched downward. At moderate current density, the pore size and depth increased, and the pores grew perpendicular to the surface, creating a columnar pore structure. At high current density, the porous structure remained perpendicular, the pore size increased, and the pore depth decreased. We explained the changes in pore size and depth at high current density by the growth of a silicon oxide layer during etching at the tips of the pores.

Structural and optical properties of thin films porous amorphous silicon carbide formed by Ag-assisted photochemical etching

In this work, we present the formation of porous layers on hydrogenated amorphous SiC (a-SiC: H) by Ag-assisted photochemical etching using HF/K 2S 2O 8 solution under UV illumination at 254 nm wavelength. The amorphous films a-SiC: H were elaborated by d.c. magnetron sputtering using a hot pressed polycrystalline 6H-SiC target. Because of the high resistivity of the SiC layer, around 1.6 MΩ cm and in order to facilitate the chemical etching, a thin metallic film of high purity silver (Ag) has been deposited under vacuum onto the thin a-SiC: H layer. The etched surface was characterized by scanning electron microscopy, secondary ion mass spectroscopy, infrared spectroscopy and photoluminescence. The results show that the morphology of etched a-SiC: H surface evolves with etching time. For an etching time of 20 min the surface presents a hemispherical crater, indicating that the porous SiC layer is perforated. Photoluminescence characterization of etched a-SiC: H samples for 20 min shows a high and an intense blue PL, whereas it has been shown that the PL decreases for higher etching time. Finally, a dissolution mechanism of the silicon carbide in 1HF/1K 2S 2O 8 solution has been proposed.

Effect of electrochemical etching solution composition on properties of porous SiC film

Porous amorphous SiC (a-SiC) layer with pore size in the nanometer region was fabricated on the a-SiC/Si substrates by the electrochemical etching method using HF/H2O/surfactant solution. Systematic study showed that the HF concentration in the etching solution (in the 1–73% region) strongly affects the structure (both the pore size and the pore density) of the porous a-SiC layer. It was also observed the changing of the photoluminescence properties of the porous a-SiC layer when its structure has been changed.

A STUDY ON THE PHOTOLUMINESCENCE PROPERTIES OF POROUS 6H-SiC(p) AND SiC FILM ON p-Si SUBSTRATE

P-type porous SiC layers were fabricated by anodization of a hot-pressed p-type 6H-SiC (30 kΩcm) and a 1.6 μm SiC film was deposited onto p-type Si (100) substrate by Pulsed Laser Deposition (PLD) in HF. In order to facilitate the electrochemical etching of the substrate, ethylene glycol electrolyte has been added to the solution and a thin metallic film of aluminium ( Al ) has been deposited onto a SiC prior to anodization. The structure and properties of the porous SiC layer formed by this method were investigated by Scanning Electron Microscopy (SEM) and Photoluminescence (PL). It shows that photoluminescence spectra exhibit two emission bands centered at 2.20 eV (blue band) and an extended broad band from 2.20 to 3.22 eV (green band) from porous 6H-SiC (PSC). On the contrary, porous thin 1.6 μm SiC layer exhibits only a blue band centered at 2.95 eV. Finally, the results indicate clearly that porous SiC gives off green and blue light. Also, there is a shift to blue luminescence with the same order of magnitude when using a thin SiC layer deposited onto silicon (p- Si (100)).

Morphological and photoluminescence study of porous thin SiC layer grown onto silicon

AbstractA porous SiC layer was fabricated by anodization of a 1.6 µm‐thin SiC layer deposited onto p‐type Si(100) substrate by pulsed laser deposition (PLD), using a hot‐pressed 6H‐SiC(p) as sputtered target. P‐type porous SiC layers were realized by anodization in HF/ethylene glycol (ETG) electrolyte (1:1 by vol.) at different etching times. The properties of the porous SiC layer formed by this method were investigated by SIMS, SEM, Fourier transform Infra‐Red spectroscopy (FT‐IR) and photoluminescence (PL). The results show that the growth layer was crystalline, with the photoluminescent spectra showing a blue band emission. In addition, the results clearly indicate an increase in PL intensity by 10 times of magnitude compared to that exhibited by the unetched sample. Copyright © 2008 John Wiley & Sons, Ltd.

Morphology and photoluminescence of anodized aluminium‐coated 6H‐SiC samples

AbstractIn this work, we report the effect of surface modification by aluminum metallization, prior to anodization in a solution of HF/Ethylene glycol, on the morphology and photoluminescence of p‐type porous SiC. For this purpose, a thin metallic film of aluminum has been deposited on a highly resistive 6H‐SiC. The morphology of the porous SiC layer has been investigated by Scanning Electron Microscopy (SEM). The results show that aluminum play a crucial role on the dissolution of SiC; it initiates and facilitates the electrochemical etching. The photoluminescence spectra of the anodized Al‐coated samples exhibit two emission bands centered at about 430 nm and 560 nm. While for the uncoated samples, a strong luminescence with a peak located at 560 nm is observed. Finally, the role of Al on the photoluminescence mechanism is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of anodization time on photoluminescence of porous thin SiC layer grown onto silicon

A porous SiC (PSC) layer was fabricated by anodization of a 1.6 μm thin SiC layer deposited onto p-type Si(1 0 0) substrate by pulsed laser deposition (PLD), using a hot-pressed 6H-SiC( p) as sputtered target. p-Type PSC layers were fabricated by anodization in HF/ethylene glycol electrolyte (1:1 by vol.) at different etching times. The properties of the PSC layer formed by this method were investigated by X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), scanning electron microscopy (SEM) and photoluminescence (PL). The results show, that the growth layer was crystalline and PL spectra exhibit blue band emission centered at 2.95 eV. In addition, the results indicate clearly an increase in PL intensity by ten times of magnitude compared to that exhibited by the unetched sample.

Oxidation Behavior of Silicon-Infiltrated Carbon/Carbon Composites in High-Enthalpy Convective Environment

Thermal response and oxidation behavior of commercial metal-silicon-infiltrated carbon/carbon composites (MICMATTM; Si-CC) were evaluated in a high-enthalpy convective environment using an arc jet facility (an arc wind tunnel). Composite specimens were put into a supersonic plasma air stream having a gas enthalpy of 12.7–18.8 MJ/kg for 50–600 s. Cold-wall heat fluxes measured by a Gardon-type calorimeter ranged from 1.0 to 1.8 MW/m2, and the maximum surface temperature reached 1300°–1660°C. After the arc jet testing, no surface recession was observed in the samples, and the mass loss rate of the composites was far less than that of graphite. The excellent oxidation resistance was caused by formation of a porous SiC layer at the surface of the composite. Oxidation behavior of the composites is discussed based on a simplified airflow blocking model of the porous SiC layer. The composites exhibited excellent oxidation resistance for short-term exposure in high-enthalpy airflow.

Porous SiC ceramics fabricated by reactive infiltration of gaseous silicon into charcoal

Basswood has been converted to a porous biocarbon template by carbonizing under vacuum at 1200 °C. Subsequent reactive infiltration of gaseous silicon at 1500–1700 °C for 0.5–8 h in inert atmosphere resulted in the formation of a porous, cellular β-SiC ceramic. X-ray diffraction (XRD) studies show the resulting SiC, obtained at 1600 °C for 6 h in static argon atmosphere, is β-SiC. Scanning electron microscopy (SEM) reveals the resulting SiC ceramic reproduces the biomorphic cellular morphology of basswood tissue with high precision. Gaseous Si diffuses into charcoal through its continuous tracheidal cell channels, and reacts with carbon in the cell wall to form SiC. Growth of SiC layer is dependent of the diffusion of Si vapor through the primary porous SiC layer and further gas–solid reaction of Si(g) and C(s) at C/SiC interface. The densification of SiC strut material may occur at moderate temperatures and holding time.

Semi-insulating porous SiC substrates

Semi-insulating SiC substrates were fabricated on a base of porous silicon carbide made from 4H and 6H commercial SiC wafers. Porous SiC (PSC) layers, 3 µm in thickness, were made by surface anodization of SiC wafers and then impregnated with excessive Si from a sputtered SiO2 film during thermal processing at 1200 °C. The processed structures were studied by Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS). The Si/C ratio in the processed porous layers showed a measurable change from 1:1 to 2.3:1 due to the diffusion of Si from the SiO2 film into the PSC layer during thermal processing. The value of specific resistivity achieved was found to be 5 × 1011 Ω cm at 470 K and 2 × 106 Ω cm at 700 K, respectively, with an activation energy of resistivity of 1.53 eV. Preliminary evaluations of AlN films grown by HVPE on the prepared substrates are also presented.

Structure and properties of silicon carbide grown on porous substrate by vacuum sublimation epitaxy

A SiC layer was grown by vacuum sublimation epitaxy on porous silicon carbide. A porous SiC layer about 10 µm thick was fabricated by electrochemical etching of an off-axis 6H-SiC substrate. The epitaxial layer was ∼ 10 µm thick. Structural and optical properties of the initial substrate and the porous and epitaxial layers were investigated by X-ray, IR-reflection and photoluminescence methods. An epitaxial SiC layer grown on porous SiC exhibits improved characteristics when compared to a SiC layer on a conventional substrate.

Chemical vapor deposition of 4H–SiC epitaxial layers on porous SiC substrates

Epitaxial 4H–SiC layers were grown by chemical vapor deposition (CVD) on porous silicon carbide. Porous SiC substrates were fabricated by the formation of a 2 to 15 μm thick porous SiC layer on commercial off-axis 4H–SiC substrates. The thickness of CVD grown layers was about 2.5 μm. The concentration Nd–Na in the layers was about 7×1015 cm−3. The layers were investigated for their surface roughness, crystal structure, deep level concentration, and minority carrier diffusion length. It was found that the characteristics of SiC epitaxial layers grown on porous SiC substrates were significantly improved compared to those of SiC layers grown on standard SiC substrates.

Polarization memory in an oxidized porous SiC layer

Polarization memory in an oxidized porous SiC layer

Oxidation Mechanism of C-SiC Composite Fiber Prepared by Precursor Method

The oxidation mechanism of C-SiC composite fiber, which was prepared from carbon fiber (Kynol) and ethyl silicate, has been investigated. The oxidation rate was determined by means of a thermobalance in an atmosphere of O2 at temperatures from 673 to 1473K. The oxidation curves were characterized by S-shape. In the second stage, the oxidation obeyed a linear rate law. The oxidation rate of the composite fiber was lower than that of the carbon fiber. Below 973K, the activation energy for the oxidation was estimated to be 115kJ·mol-1, and the gas diffusion through the porous SiC layer was a rate-determining step. Above 973K, the activation energy was estimated to be 24kJ·mol-1. The oxidation was considered to be controlled jointly by the gas diffusion to the surface of the fiber and the gas diffusion through the porous SiC layer.