Crystalline SiC Layer Research Articles

Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond.

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m2 K GW-1) compared to the sample without (107 ± 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

Development of SOI FETs Based on Core-Shell Si/SiC Nanowires for Sensing in Liquid Environments

Core–shell Si/SiC nanostructures appear as promising building blocks for sensing applications, thanks to the high chemical stability of SiC coupled with the semiconducting properties of Si. In order to optimize the fabrication process of such structures, Si nanowires were coated with a thin SiC layer, and integrated as back-gated field-effet transistors. Two approaches for the fabrication of the SiC shell were then investigated. The first approach involves chemical vapor deposition of amorphous SiC on Si nanowires, without the need for masking; the second approach involves carbonization of Si surfaces to produce a thin crystalline SiC layer, but requires a larger thermal budget. The resulting structures were analyzed using high-resolution transmission electron microscopy (HR-TEM), and the devices were characterized electrically. Electrical characterization shows that the carbonization approach induces a dramatic decrease in drain-to-source current associated with gate leakage, whereas the electrical performances were preserved in the case of chemical deposition.

Creation of leak-proof silicon carbide diffusion barriers by means of pulsed laser deposition

TRISO (tristructural isotropic) coated fuel particles are a crucial element of the HTR safety concept. While TRISO coated particles have been proven as a very efficient barrier for a large range of fission products in HTR experimental reactors, some particular fission products could still diffuse at a considerable rate. Most importantly, radioactive silver 110mAg was found to be released from coated particles. In future HTRs with active components like a gas turbine in the primary circuit, such silver contamination may severely limit maintainability of these parts with the result of reduced life-time performance.So far, experimental analyses on silver diffusion through silicon carbide have led to contradictory results. In this work, an alternative method was used to generate silicon carbide layers as a basis for analysis of silver diffusion. With pulsed laser deposition (PLD), it is possible to generate coatings of different materials and various kinds of compounds. In particular, this technology allows the generation of layers very well defined with respect to their composition, purity and density. The microstructure can precisely be manipulated through various parameters. Based on different silicon carbide coatings with well-defined properties, we are going to investigate the silver diffusion process. Our goal is to derive the properties of an ideal silicon carbide coating preventing silver diffusion entirely.In this paper we present the major aspects of our work creating crystalline SiC layers as well as silver and CsI layers both on plane and spherical substrates. Analyses with X-ray diffraction, X-ray spectrometry and secondary ion mass spectrometry show that complex multilayer systems comprising a graphite substrate, a crystalline SiC layer and an intermediate silver layer were successfully created. Major challenges to approach in the future are the handling of high-level intrinsic stresses forming in the layer structure as well as the high vapour pressure of silver and CsI, complicating subsequent crystalline SiC coating.

Formation and structural characterization of nanocrystalline Si/SiC multilayers grown by hot filament assisted chemical vapor deposition using CH 3SiH 3 gas jets

We report on the formation and the structural characterization of nanocrystalline Si/SiC ( nc-Si/SiC) multilayers on Si(100) by hot filament assisted chemical vapor deposition using CH 3SiH 3 gas pulse jets. Si rich amorphous SiC ( a-Si 1 − x C x , x ~ 0.33) was initially grown at the substrate temperature ( T s) of 600 °C with heating a hot filament at ~ 2000 °C. The following crystalline SiC layers were grown at T s = 850 °C without utilizing a hot filament. When the a-Si 1 − x C x layer was ultrathin (< 2 nm) on Si(100), this a-Si 1 − x C x layer was transformed to a single epitaxial SiC layer during the subsequent SiC growth process. The Si{111} faceted pits were formed at the SiC/Si(100) interface due to Si diffusion processes from the substrate. When the thickness of the initial a-Si 1 − x C x layer was increased to ~ 5 nm, a double layer structure was formed in which this amorphous layer was changed to nc-Si and nc-SiC was grown on the top resulting in the considerable reduction of the {111} faceted pits. It was found that nc-SiC was formed by consuming the Si atoms uniformly diffused from the a-Si 1 − x C x layer below and that Si nanocrystals were generated in the a-Si 1 − x C x layers due to the annealing effect during further multilayer growths.

STRUCTURAL AND OPTICAL PROPERTIES OF POLYCRYSTALLINE6H-SiCAND CRYSTALLINESiCFILM GROWN ONTO SILICON SUBSTRATE BY PLD

In this paper, we present a comparative study of structural and optical properties of polycrystalline p-type 6H-SiC and thin SiC layer growth onto Si . The thin SiC layer was grown on a p-type Si(100) substrate by pulsed laser deposition (PLD) using KrF excimer laser from a 6H-SiC hot pressed target. The properties of polycrystalline 6H-SiC and thin SiC layer were investigated by scanning electronic microscopy (SEM), high resolution X-Ray Diffraction (XRD), secondary ion mass spectrometry (SIMS), FT-IR spectroscopy and photoluminescence spectrometry. XRD analysis showed that the two materials have the same hexagonal structure (6H-SiC) as identified by ASTM 72-0018. In addition, a SIMS analysis gives a ratio Si/C of the thin SiC layer around 1.15 but the ratio Si/C of the target was found equal to 1.06, whereas one should have 1.0. This is due to the higher ionization efficiency of Si by the report of C atoms and in photoluminescence, the two materials exhibit the same emission bands (blue and green). Finally, a crystalline thin SiC layer of 1.6 μm was elaborated using the PLD method at low-temperature indicating that the technique reproduces the same macroscopic property (optical, structural, mechanical, etc.) of the target.

Determination of stoichiometry in silicon carbide materials using elastic backscattering spectrometry

Elastic backscattering spectrometry (EBS) was performed on SiC materials, using 4He particles at energies ranging from 2 to 4 MeV, in order to establish the energy values that lead to an accurate measurement of the Si/C ratio. Analysis of the random yield of “bulk” SiC single crystals indicates that energy values of 3.25 and 3.75 MeV are the most suitable for chemical composition determination; backscattering yield of carbon is enhanced compared to the yield measured at 2 MeV, while the excitation of strong resonances above 3.75 MeV are suppressed. Random backscattering yield measurements were then carried out at an energy of 3.25 MeV on unhydrogenated SiC thin films grown on Si(1 0 0), by pulsed laser deposition, at different substrate temperatures. The Si and C atomic concentrations in the films were determined with an uncertainty of 1% and little interference from the underlying substrate. The films were found to be stoichiometric with a Si/C ratio of 1.03 ± 0.05, independent of deposition temperature, which indicates that the films were grown under congruent ablation conditions. The analysis proved to be applicable to both amorphous and crystalline SiC layers, as confirmed by the results obtained for films deposited at 400 and 950 °C, respectively.

Crystallization of amorphous SiC and superhardness effect in TiN/SiC nanomultilayers

TiN/SiC nanomultilayers with various constituent layer thicknesses were prepared by magnetron sputtering using TiN and SiC ceramic targets. X-ray diffractometer, scanning electron microscope, energy dispersive spectrometer, high-resolution transmission electron microscope, atomic force microscope and nanoindenter were employed to study the growth, microstructure and mechanical properties of these films. Experimental results revealed that amorphous SiC, which is more favorable under normal sputtering conditions, was forced to crystallize and grew epitaxially with TiN layers at thicknesses of less than 0.8nm. The resultant films were found to form strong columnar structures, accompanied with a remarkable hardness increment. Maximal nanoindentation hardness as high as 60.6GPa was achieved when SiC thickness was ∼0.6nm. A further increase of SiC thickness caused the formation of amorphous SiC, which blocked the epitaxial growth of the multilayers, resulting in the decline of film's hardness. Additionally, investigations on multilayers different in TiN layer thicknesses showed that they are insensitive in both microstructure and hardness to the fluctuation of TiN layer thickness. The formation of epitaxially grown structure between crystalline SiC and TiN layers was found to be responsible for the obtained superhardness in multilayers.

Morphological, structural and optical properties of thin SiC layer growth onto silicon by pulsed laser deposition

Crystalline silicon carbide thin layers were grown on a p-type Si(1 0 0) substrate by pulsed laser deposition (PLD) using KrF excimer laser at λ=248 nm from a 6H–SiC hot-pressed target. The target “SiC” used to elaborate our SiC films is realized from a mixture of 1SiO 2 with 3C (carbon) “1SiO 2+3C” heated in an oven at 2500 °C (the target was a hot-pressed material and supplied by Goodfellow). The morphological, structural and optical properties of SiC layers were investigated by scanning electronic microscopy (SEM), high-resolution X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and UV–visible spectrophotometer. XRD analysis of the target showed that this latter is a hexagonal structure (6H–SiC). The XRD pattern shows that a 1.6 μm crystalline SiC layer was formed. In addition, a SIMS analysis gives a ratio Si/C of the thin SiC layer around 1.15 but the ratio Si/C of the target was found equal to 1.06, whereas one should have 1.0. This is due to the degree of the sensitivity of the SIMS technique and due to the higher ionization efficiency of Si compared to C atoms, all these which give different ratios. It is known that the PLD technique reproduces the same macroscopic property (optical, mechanical, structural, etc.) of the target. An optical gap ( E Gap) of the SiC layer of about 2.51 eV was obtained by reflectance measurement. Finally, a crystalline thin SiC layer of 1.6 μm was elaborated using PLD method at low-temperature deposition.

A comparative study of the electrical properties of heavily Al implanted, single crystalline and nanocrystalline SiC

The electrical properties of heavily Al doped single and nanocrystalline 4H–SiC layers on semi-insulating 4H–SiC substrate, prepared by multienergy, high-fluence Al implantation and subsequent furnace annealing, are investigated by sheet resistance and Hall effect measurements. Ion beam induced crystallization is used to prepare the nanocrystalline SiC layers. The doping levels are chosen around the solid solubility limit of 2×1020cm−3 in the range from 5×1019to1.5×1021cm−3. The comparison of the results shows that heavily Al doped single crystalline SiC layers have superior conduction properties. The lowest resistivities measured at room temperature are 0.08 and 0.8Ωcm for the single crystalline and nanocrystalline samples, respectively. Recent results on enhanced Al acceptor activation in nanocrystalline SiC cannot be confirmed. There is an upper limit for the hole concentration in the nanocrystalline samples independent of the Al supersaturation level in the as-implanted state due to outdiffusion of Al in excess to the solid solubility limit during annealing. In contrast to the nanocrystalline SiC layers the as-implanted Al profile in single crystalline material remains stable after annealing even for concentrations above the solid solubility limit. Therefore, in single crystalline material efficient impurity band conduction due to strongly interacting acceptors can be achieved in the range of supersaturation. For lower doping levels impurity band conduction is more effective in nanocrystalline SiC.

Preparation of silicon carbide nanotubes by hydrothermal method

One-dimensional SiC nanotubes with small diameter without metallic catalysts were prepared under supercritically hydrothermal conditions. The SiC nanotubes were characterized by x-ray diffraction, energy dispersive x-ray spectrum, selected area electron diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and Raman and infrared spectra. The results show that the samples are SiC nanotubes composed with crystalline β-SiC structure and some residual carbon nanoscale grains. The SiC nanotubes with small inner pores have an outer diameter of less than 10nm and length of micrometers. The structures of SiC nanotubes are hollow inner pores, multiwall crystalline SiC layers, and thin amorphous silicon oxide outer layers. The substitution reaction model is proposed to explain the formation of SiC nanotubes. C atoms diffuse into Si nanotubes and Si atoms in Si nanotubes are partially substituted for C atoms by substitution chemical reaction, resulting in the formation of SiC nanotubes.

Mechanical and field emission properties of CGed Si(C,N) films synthesized by PECVD from HMDS precursor

Compositionally graded (CGed) Si(C,N) films were prepared by Ar/H 2/N 2 plasma enhanced chemical vapor deposition from liquid injected hexamethyldisiloxane precursor. The films were characterized by scanning/transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Monolithic crystalline SiC and amorphous SiN x films were produced from Ar/H 2 and Ar/H 2/N 2 thermal plasma, respectively. The CGed SiC–SiN x film was obtained by changing N 2 flow rate from 2 L/min to zero in Ar/H 2/N 2 during the deposition process, and it was composed of an uppermost crystalline SiC layer, a thin intermediate layer containing nanocomposite c-SiC/a-SiN x and an innermost layer of amorphous SiN x . The CGed SiN x –SiC film, in which SiN x acts as a top layer with a SiC layer underneath, was fabricated by an inverse change of the plasma gas supply from initial Ar/H 2 to Ar/H 2/N 2. Microhardness increase and promising field emission properties were obtained from these CGed films in comparison with monolithic SiC and SiN x films.

LPE of Silicon Carbide Using Diluted Si-Ge Flux

Epitaxial growth of 4H-SiC has been carried out at temperatures up to 1650 oC on 4HSiC substrates dipped in strongly diluted Si-based solutions. Liquid Phase Epitaxy (LPE) in conditions of low supersaturation was shown to be an effective technique to overgrow micropipe defects (MPs) in SiC wafers prepared by the Physical Vapour Transport (PVT) technique. The aim of this work was to investigate the SiC growth morphology and the dependence of MP elimination efficiency on Si-Ge flux composition. Macroscopically flat, single crystalline SiC layers of a thickness up to 10 µm were grown with a growth rate of about 0.5 µm/h. Stepped growth morphology was observed independent of the melt composition. Micropipes with the diameter below 5 µm were closed with an efficiency of about 80%. SEM investigations as well as inspection under reflected/transmitted light did not show any specific distortion of the growth morphology at the micropipe healing place.

Formation of a surface SiC layer by carbon-ion implantation into silicon

Formation and characterisation of a thin surface layer of silicon carbide on a silicon substrate, created by ion-beam synthesis, are discussed. Low-energy (30 keV) carbon ions were implanted into a (1 0 0) silicon substrate in a single step to a dose of 10 18 cm −2. Due to a combination of surface sputtering, a dynamically changing target density and diffusion during annealing, a fairly uniform and approximately stoichiometric mixture of silicon and carbon can be obtained. Subsequent high-temperature thermal annealing produced a crystalline SiC layer of 50–80 nm thick on the substrate surface. Optical techniques (infra-red reflectance, Raman spectroscopy and ellipsometry) and Auger spectroscopy were used to characterise the SiC film.

Growth of GaN layers on SiC/Si(111) substrate by molecular beam epitaxy

High quality GaN layers were grown by molecular beam epitaxy (MBE), with and without AlN buffer, on SiC/Si(111) substrates obtained by C implantation into Si(111) at 500 °C to avoid amorphization. Thermal annealing at 1150 °C for 6 h produced crystalline SiC layers embedded into the Si(111). The SiC top stoichiometry is controlled by reactive ion etching (RIE), after what, all SiC layers show a fairly flat (4 nm) and specular surface. Photoluminscence spectra reveal that all GaN layers are under tensile biaxial strain of thermal origin. GaN layers grown on stoichiometric SiC have lower mosaicity, but also less photoluminescence efficiency and tensile strain, as compared to GaN on bare non-stoichiometric SiC. This, most likely, relates to changes in microcrystals size and grain boundaries density, that depend strongly on the GaN nucleation process. Despite the partial polycrystalline nature of the SiC/Si(111), the grown GaN quality is as high or even better than that from GaN grown on Si(111).

Formation of crystalline SiC films by CH 4 plasma immersion ion implantation into Si

Hydrocarbon ions are implanted into silicon by pulse biasing Si to a high voltage of –40 kV in a methane plasma. As implantation of Si at beam heating conditions ( T < 500 K) results in the formation of amorphous silicon carbide compounds, an infrared heating system is used to obtain implantation temperatures up to 1100 K. RBS/C measurements show, that it is possible to obtain the stoichiometric 1:1 ratio of Si:C for all implantation temperatures. The amorphous layer thickness considerably decreases with increasing temperature. For implantation temperatures of 1000 K crystalline SiC precipitates in an amorphous SiC matrix are observed by cross-section transmission electron microscopy (XTEM). Further increasing the implantation temperature to 1100 K results in the formation of a crystalline SiC layer during implantation itself. The phase formation of the cubic silicon carbide phase is shown and an epitaxial relationship between Si and SiC is given.

Formation of silicon carbide and amorphous carbon films by pulse biasing silicon to a high voltage in a methane electron cyclotron resonance microwave plasma

Silicon was pulse biased to −45 kV in a methane plasma generated by microwave excitation in the electron cyclotron resonance (ECR) mode. Hydrocarbon ions were accelerated in the electrical field and implanted into the silicon. Rutherford backscattering (RBS) measurements showed that it is possible to incorporate a concentration of up to 95 at.% C into the Si. Cross-section transmission microscopy (XTEM) showed that the resulting surface layer was amorphous. Annealing at 1250 °C resulted in the formation of 10 to 60 nm thick, crystalline SiC layers.