SiC Nuclei Research Articles

MBE growth and properties of SiC multi-quantum well structures

Multi-quantum well structures with 3C–SiC wells between α-SiC barriers were grown in a two-step procedure by solid-source molecular beam epitaxy. First, one-dimensional wire-like 3C–SiC was nucleated selectively on terraces of the well-prepared off-axis α-SiC(0 0 0 1) substrates at low temperature (T<1500 K) . Next, 3C–SiC lamellae were incorporated into the hexagonal layer material via simultaneous step-flow growth mode of both the 3C–SiC nuclei and the hexagonal substrate material at higher T and Si-rich conditions. In comparison to homopolytypic SiC layers, photoluminescence investigations revealed additional signals, which can be explained by optical transitions within the thin cubic well layers.

Ion beam synthesis of buried SiC layers in silicon: Basic physical processes

For the ion beam synthesis (IBS) of buried stoichiometric epitaxial layers of 3C–SiC in silicon, a complicated structure in the as-implanted state has been identified as a favourable starting structure prior to annealing. It consists of a nearly rectangular depth distribution of equally sized, oriented 3C–SiC nanocrystals sandwiched between two amorphous zones. The basic physical processes leading to such a distribution of amorphous and crystalline phases during high-dose, high-temperature carbon implantation into silicon are reviewed in this article. In particular, the precipitation of carbon in a self-organized lattice of amorphous inclusions, the formation of stable 3C–SiC nuclei via precursors, beam-induced nucleation of SiC in amorphous Si:C, the different growth of crystalline SiC precipitates in crystalline and amorphous silicon and the ballistic destruction of SiC nanoclusters by the continued bombardment with energetic ions – resulting in a carbon-induced amorphization at elevated temperatures – are described.

Effects of additives on densification, microstructure and properties of liquid-phase sintered silicon carbide

Dense SiC ceramics were obtained by hot pressing of β-SiC powders using Al2O3-Y2O3 and La2O3-Y2O3 additive systems. The effect of the addition of an amount of ultrafine SiC to commercial silicon carbide powder was evaluated. Sintering behaviour and microstructure depended on type and amount of liquid phase, as densification proceeded via a classical solution-reprecipitation mechanism. A core/rim structure of SiC grains indicated that reprecipitation of a solid solution of SiC containing Al and O occurred on pure SiC nuclei. Grain boundary phase was constituted of crystalline YAG and amorphous silicates. Values of flexural strength up to ∼750 MPa at RT and up to ∼550 MPa at 1000 °C were measured. At 1300 °C a strong degradation of strength was attributed to softening of the amorphous portion of grain boundary phase. In highly dense materials toughness ranged from 2.95 to 3.17 MPa·m1/2 and hardness from 21 to 23 GPa.

Synthesis of β-SiC nanowires with SiO 2 wrappers

β-SiC nanowires with uniform amorphous SiO 2 wrappers have been synthesized by carbothermal reduction of the sol-gel derived silica containing carbon nanoparticles. The product was characterized by IR, TEM, HREM, and EDX. The nanostructures are more than 20 μm in length. The diameters of β-SiC nanowires are in the range 10–25 nm, while the outer diameters of the SiO 2 wrappers are between 20 and 70 nm. The formation of β-SiC nanowires is ascribed to the large quantities of SiC nuclei and the nanometer-sized SiC nucleus sites on carbon nanoparticles. The SiO 2 wrappers come from the combination reaction of the SiO vapor and O 2 decomposed from the unreacted SiO 2.

First stages of low temperature and low pressure carbonization of Si (0 0 1) in acetylene

The thermal growth of 3CSiC films at 820°C in acetylene (chosen among other hydrocarbons because of its high reactivity with Si) was performed and the resulting films were analyzed by nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Films were grown on clean single domain 2 × 1 reconstructed Si (0 0 1) vicinal surfaces either in natural (C 2H 2) or in 99% deuterium enriched (C 2D 2) acetylene under 2 × 10 −6Torr. In order to characterize the electronic structure and the short range order of the films, XPS and its by-product X-ray photodiffraction (XPD) were performed in situ in an analysis chamber connected to the preparation chamber. The growth kinetics was followed by measuring, ex situ, the amount of incorporated carbon using the 12C(d,p) 13C reaction at 970 keV, while for determining the hydrogen incorporation in the films the D( 3He,p) 4He reaction at 700 keV was used. Also ex situ, the film morphology was followed by SEM as the thickness of the layer increased. By using these complementary techniques, phenomena like H incorporation, C in-diffusion and 3CSiC nucleation have been evidenced at the very beginning of the growth (amount of 12C incorporated in the films smaller than 10 16 atoms/cm 2). Besides, it was observed that the imperfect coalescence of 3CSiC nuclei determine the morphology of films grown during longer times.

Micropipe defects and voids at β-SiC/Si(100) interfaces

Abstract The application of an uncommon silicon substrate pretreatment and a carbonization procedure without temperature ramp in a CVD reactor was found to create a new kind of defect besides the usual voids at β-SiC/Si(100) interfaces. SiC-covered micropipes of minute size and of high area densities evolve into the substrate by Si outdiffusion and simultaneous ingrowth of SiC. Results of our systematic investigations of micropipes and voids, obtained mainly by transmission electron microscopy (TEM), are summarized. The micropipe formation is obviously determined by very specific conditions such as, e.g., a high density of SiC nuclei at high carbonization temperatures. Recent new results demonstrate that micropipes together with voids can also develop in carbonization experiments during a relatively slow temperature rise at very low hydrocarbon concentrations.

Micropipes and voids at β¨SiC/Si(100) interfaces: an electron microscopy study

The microstructure of β-SiC/Si(100) interfaces generated by carbonization and subsequent growth in a chemical vapor deposition (CVD) reactor was investigated by transmission electron microscopy (TEM). Differently prepared cross section and planar specimens allowed a detailed characterization of interface defects. Besides pyramidal voids, which were frequently reported to appear at SiC/Si interfaces within the substrate, recently discovered micropipes are of special interest. Both kinds of defects form by outdiffusion of silicon during the carbonization process. In contrast to voids. which initially remain empty, micropipes develop by simultaneous ingrowth of SiC. The area densities of micropipes were found to be orders of magnitude higher than those of voids. Micropipe formation may be due to a high density of SiC nuclei preexisting on the substrate surfaces after pretreatments. The simultaneous development of voids and micropipes is discussed on the basis of results obtained from a short-time carbonization experiment.

Formation of buried epitaxial silicon carbide layers in silicon by ion beam synthesis

RBS/channeling, X-ray diffraction and transmission electron microscopy (TEM) as well as cross-sectional transmission electron microscopy (XTEM) are used to study the formation of well-defined, epitaxial 3C-SiC layers in Si(100) and Si(111) by high dose implantation of 180 keV C ions and subsequent thermal annealing at 1250 °C. The dose dependence of the carbon redistribution during the post-implantation anneal is studied in detail, revealing the possibility to grow well-defined substoichiometric and stoichiometric silicon carbide layers as well as 3C-SiC layers with a large concentration of excess carbon atoms. The presence of crystalline SiC nuclei in the as-implanted state and their depth distribution are shown to be important for the carbon redistribution into a discrete layer during annealing. XRD monitoring of the epitaxial 3C-SiC component formed during implantation indicates the build-up of lattice distortions close to the stoichiometry dose. After annealing, the approximately 170 nm thick continuous SiC layers are covered by 300 nm thick crystalline silicon top layers, containing individual SiC precipitates. Cross-sectional TEM investigations reveal sharp interfaces between Si and SiC layers and almost unstrained Si on top and underneath the layers. First results are reported indicating that SiC can be formed also in homogenous deep buried layers using MeV ion beam synthesis.

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T p < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si02 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T p < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T p > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.