Porous SiC Layers Research Articles

In English

We report the synthesis of β-SiC/por-Si/mono-Si heterostructure by a hybrid method, consisting of the electrochemical etching of the single-crystal silicon surface with a subsequent carbidization by a thermal annealing in a methane atmosphere. This method has a number of advantages over the known ones, because it is cheap enough and allows one to form the silicon carbide layers of high- quality. The formed structure was studied by means of SEM, EDX and XRD methods. As a result, the dense β-SiC layer, consisting of an array of the spherical islands with diameters of 2–6 μm, coated with the small pores, was formed on the por-Si/mono-Si surface. The geometric dimensions of the islands were studied by calibrating the sample image in the ImageJ software package. The maximum value of the linear size (diameter) of the island dmax = 5.95 μm and the minimum value dmin = 2.11 μm were found in the studied area. In general, the average diameter of the islands is d = 3.72 μm. The distribution has the left-sided asymmetry, that is, the smaller islets predominate. Roundness (the ratio of the area to the square of the larger axis) R = 0.86. According to the results of EDX analysis, it was found that the synthesized structure surface consists exclusively of the carbon and silicon atoms, indicating the high quality of the formed structures. It was found that the SiC film crystallizes in the cubic phase. The formation of the islands is explained by means of the layer-plus-island growth model according to Stranski-Krastanow mechanism, which is characterized by the formation of the dense wetting layer with the massive island complex on the surface. It should be also noted that the porous SiC layers of island type can, in turn, show the perspective as the buffers with the heteroepitaxy of the silicon substrate materials.

Exciton-light coupling and polaritons in SiC nanocrystals

The paper presents the results of SiC:N nanocrystal characterization using the photoluminescence (PL) and X-ray diffraction (XRD) techniques. Photoluminescence study of porous SiC:N (PSiC) layers with different PSiC thicknesses and SiC nanocrystal (NC) sizes reveals the intensity stimulation mainly for the high energy PL bands. The early investigation of temperature dependences of the high energy PL bands has shown that these PL bands related to the free exciton emission in the different SiC polytypes. The SiC polytypes in the original n-type SiC:N wafers and in porous SiC layers were confirmed by XRD study. The intensity enhancement of exciton-related PL bands in big size (50–250nm) SiC NCs is attributed to the realization of the exciton week confinement and exciton-light coupling with the formation of polaritons in the SiC NCs. The numerical simulation of exciton radiative recombination rates and radiative lifetimes for the different exciton emissions has been done using a model of exciton–light coupling in SiC NCs. The experimental and numerically calculated results have been compared and discussed.

Effect of metal diffusion into polycrystalline 6H–SiC prior to its anodization on luminescence response

p-Type porous SiC layers were fabricated by anodization of resistive p-type 6H–SiC samples using HF/ethylene glycol solution. Thin films of lithium (Li) and aluminum (Al) as donor and acceptor elements were vacuum deposited and diffused onto SiC substrates prior to anodization. The aim of this work is to investigate the properties of the nanoporous SiC layer formed by this method and to deduce the effect of diffused lithium as donor and Al as acceptor atoms on their photoluminescence response (PL). The profile distribution of lithium and aluminum diffused atoms was carried out using secondary ion mass spectrometry (SIMS). The photoluminescence spectra of the anodized Al-diffused samples exhibit a broad emission band centered at about 475nm, while the Li-diffused samples exhibit luminescence with one broad peak located at 655nm, attributed to Li-related defect centers. In addition, the PL intensity of lithium diffused samples varies with varying the etching time. Finally, the results are expected to have important applications in modern optoelectronic devices and electrode materials of lithium batteries.

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)

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.

Electrochemical etching of n-type 15R-SiC and 6H-SiC without UV illumination

We report the formation of porous n-type 15R- and 6H-SiC and analyze the porous layers using scanning electron microscopy, Raman scattering and X-ray diffraction (XRD). The crystal structures and the preparation conditions of porous SiC are shown to have an effect on the structural and electrical properties of the material obtained. Raman spectra of porous SiC layers have shown some specific features compared with those of bulk SiC. For the porous 15R-SiC, the semi-cylindrical structure of the porous network has been observed and the porosity is about 66%.

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.

Effect of thickness and porous structure of SiC layers on the spectral response of the Pd/SiC–pSi Schottky photodiodes

In this work, we study the effect of the thickness and porous structure of silicon carbide (PSC) layers on the electrical properties of Schottky photodiodes by using a palladium (Pd) layer deposited on non-porous silicon carbide (SiC) and porous-SiC (PSC) layers. The non-porous and porous-SiC layers were realized on a p-type silicon (Si(100)) substrate by pulsed laser deposition using a KrF laser (248nm) and thermal deposition of a thin Pd layer. The porous structure of the SiC layer deposited was developed by an electrochemical (anodization) method. The electrical measurements were made at room temperature (295K) in an air ambience. The effect of the porous surface structure and the thickness of the SiC layer were investigated by evaluating electrical parameters such as the ideality factor (n) and barrier height (ϕBp). The thickness of the porous layer significantly affects the electrical properties of the Schottky photodiodes. 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 a value For a thin SiC layer (0.16μm) n was around 1.325 with a barrier height 0.798eV, while for a thick layer (1.6μm), n and ϕBp were 1.026 and 0.890eV, respectively for Pd/SiC–pSi. These results indicate Schottky photodiodes with high performance are obtained for thicker SiC layer and for thin layer of PSC. This effect showed the uniformity of the SiC layer. In the same case the ideality factor (n) decreases for Pd/PSC–pSi(100) for low SiC thickness by report of Pd/PSC–pSi(100) Schottky photodiodes, but for Pd/PSC–pSi(100) n increase for large SiC thickness layer. We notice that the barrier height (ϕBp) was reversely depend by report of ideality factor. A spectral response value of (SR) of 34mA/W at λ=400nm was measured for Pd/0.16μm SiC–pSi Schottky photodiode with low SiC thickness. On the other hand, a value of SR=0.14mA/W at λ=900nm was obtained when we used PSC layer (Pd/PSC–pSi(100)). A reverse behaviour occurs for thicker SiC layer. Finally, it was found that the thickness and surface porous structure have strong effect on sensitivity.

Comparative investigation of optical and structural properties of porous SiC

Paper presents results of the non-destructive characterization of porous SiC (PSiC) layers using atomic force microscope, Raman scattering, scanning electronic and X-ray diffraction spectroscopes. A comparative study of the Raman spectroscopy on the PSiC layers prepared at the different technological routine with the variation of the nanocrystallite sizes and the thickness of PSiC layers has shown a number of new features specific for nanocrystallite materials. The latter stimulates the modification of Raman scattering spectra, which have been discussed.

Porous SiC layers on Si nanowire surface

The preparation method and characterization data are described for porous SiC layers grown on the surface of porous silicon (P-Si) wires. Raman scattering spectroscopy, energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM) were employed for the characterization. It is shown that in the first approximation, the wire structure of the porous SiC repeats the surface morphology of the porous Si. Some peculiarities of P-SiC surface structure were also observed whose connection with the P-Si surface is not yet clarified. The study of the Raman spectra has permitted to determine the crystalline structure (polytypes) for the SiC layers and shown a feature specific for nano-crystallite materials, which has been analyzed and discussed.

Optical and structural evaluation of SiC nanocrystallites

This paper presents results of the non-contact and non-destructive characterization of porous SiC layers using Raman scattering spectroscopy, scanning electron microscopy as well as atomic force microscopy methods. The comparative study of the Raman spectroscopy on the bulk SiC and porous SiC layers has shown a number of new features specific for nanocrystallite materials, which have been analyzed and discussed.

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.

Preparation of Porous 4H-SiC by Surface Anodization

The preparation of porous 4H-SiC by electrochemical etching of SiC crystals was investigated. The porous layer was created at the porous SiC (PSC)/SiC interface but not from the SiC/electrolyte interface. The nanopores at the adjacent region of PSC/SiC interface were bigger than those at the top region. In the visible light region, the optical reflectance from PSC exhibits interference fringes. In the Reststrahlen region, the fourier transform infrared (FTIR) reflectance of porous 4H-SiC shows a splitting into more bands: a broad band with high reflectivity at low frequency and several sharp peaks near the LO frequency. The width and shape of FTIR spectra depended on the anodization current density. The anodization current density is a crucial parameter which determined the porosity, porosity depth profile, and the thickness of PSC layers. A pore transformation of porous structure was observed after chemical vapor deposition process.

Raman scattering investigation on porous SiC layers

This paper presents the results of the non-contact and non-destructive characterization of micro-porous SiC layers subjected to reactive ion treatment using Raman scattering, Scanning Electron Microscopy, and Atomic Force Microscopy. A comparative study of the Raman spectroscopy on the SiC wafer and the porous SiC layers has shown a number of new features specific for nano-crystallite materials, such as crystal disordering, changes of Raman polarizability tensors, appearance of the surface polariton modes and enhancement of the LO phonon-plasmon coupling. The influence of all these effects on modification of Raman scattering spectra have been discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence and Raman spectroscopy in porous SiC

This paper presents results of porous SiC characterization using photoluminescence, Raman scattering and Atomic Force Microscopy. A comparative optical spectroscopy study on bulk SiC and porous SiC layers has shown a number of new features specific to nano-crystallite materials. The role of these effects on optical spectroscopy data in porous SiC accessed by photoluminescence and Raman scattering is discussed.

Raman-scattering and structure investigations on porous SiC layers

Raman scattering spectroscopy, scanning electron microscopy, and scanning acoustic microscopy were studied on porous SiC layers prepared by different technological routes and subjected to reactive ion treatment. The Raman spectra revealed a number of features specific for nanocrystallite materials, which can be used for characterization and diagnostics of porous SiC layers for technological applications.

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.

Semi-insulating silicon carbide layers obtained by diffusion of vanadium into porous 4H-SiC

Semi-insulating silicon carbide layers have been obtained by diffusion of vanadium into porous 4H-SiC. The diffusion was performed from a film deposited by cosputtering of silicon and vanadium, with the content of the latter equal to 20%. The diffusion profile of vanadium in porous silicon carbide has a complex structure with a fast diffusion coefficient of 7×10−15 cm2/s. The activation energy of the resistivity of vanadium-diffusion-doped porous SiC layers is 1.45 eV. The resistivity of vanadium-doped semi-insulating layers is 5×1011 Ω cm at 500 K, which exceeds the resistivity of undoped porous SiC by two orders of magnitude. The results obtained indicate that porous SiC is a promising material for semi-insulating substrates in device structures based on wide-bandgap semiconductors.

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.