Materials Science Forum Vols. 483-485

Abstract: Two-dimensional model of conjugate heat and mass transport in Chemical Vapour Infiltration (CVI) process was developed. The model was used for simulation of Isothermal CVI production of SiC matrix composite. Flow pattern, thermal field and distributions of species concentrations in the gas region of the reactor and in the porous preform were analysed. Temperature and species concentration distributions were found to be noticeably non-uniform. The non-uniformity of parameter distributions leads to non-uniform densification of the preform both along the preform thickness and height.

Abstract: A brief historical development of porous SiC and GaN is given. SEM images of nine porous morphologies in 4H, 6H and 3C SiC are shown along with anodization details. Similarly, two porous GaN morphologies are presented. Applications and future prospects are discussed.

Abstract: 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.

Abstract: We have studied effects of thermal treatment in vacuum and wet oxidation on the optical transmission of SiC samples with porous layer on the Si face in the visible and near infrared spectral range. An analysis of changes in the absorption coefficient shows that the process of graphitization in vacuum begins at a temperature below 700°C and can be observed at wet oxidation at 1000°C.

Abstract: The methods of X-ray topography and diffractometry have been applied to characterize the structure of epilayers grown on porous layers. Two geometrical configurations of defects determined to be stacking faults (SF) were revealed: i) with the images of triangular shape with the edge size 560 µm along the <10-10> directions; ii) linear shape along the [11-20] direction. The sources of SFs are located within the epilayer and start from the epilayer / porous layer interface. We propose that the source of SFs is connected with graphitization of porous layer at the temperature of epitaxy.

Abstract: Thermal stability of porous SiC (PSC) with nano-, micro- and double-layer porous structure is assessed through annealing the material at T=900–1700 0C in vacuum and Ar. Changes in composition of PSC under thermal treatment are correlated with porous structure modification. Limitations in PSC technology and applications due to compositional and structure evolution at high temperatures are discussed.

Abstract: The defects at the 3C-SiC/SiO2 interface have been studied by X-band EPR spectroscopy in oxidized porous 3C-SiC. One interface defect is detected; its spin Hamiltonian parameters, spin S=1/2, C3V symmetry, g//=2.00238 and g⊥=2.00317, central hyperfine interaction (CHF) with one carbon atom and AB//[001]=48G and superhyperfine (SHF) interaction with three equivalent Si neighbour atoms and TB//[001]=12.4G, allow us to attribute the center to a sp3 coordinated carbon dangling bond center, PbC.

Abstract: The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO2 interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000°C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: PbC centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.

Abstract: Etching temperature and time are important parameters in the etching of SiC single crystals in molten KOH for defect studies. However, comparison of results of different research groups is difficult because of the way temperature measurements are being carried out. Until now the temperature of the melt has been measured indirectly with a temperature sensor placed outside the melt on the outer walls of the crucible of the etching furnace, resulting in varying etching conditions for varying setup designs. In this paper we developed an etching furnace with the capability of measuring the absolute temperature in-situ directly in the KOH melt. A new thermoelement, resistant to hot molten KOH was developed. Temperature profile measurements of the molten KOH were carried out and a calibration curve of the furnace was obtained. Based on our temperature measurements, we found that etching at 530 °C for 5 minutes was optimal for defect characterisation, both for defect statistics and for distinguishing between the etch pit morphologies. At 550 °C the etch pits become too large, overlap each other and the etching is no longer defect selective.

Abstract: Co-sputtering of silicon and carbon in a hydrogenated plasma (20%Ar-80%H2) at temperatures, Ts, varying from 200°C to 600°C has been used to grow SiC thin films. We report on the influence of Ts on the crystallization, the ratio Si/C and the hydrogen content of the grown films. Film composition is determined by ion beam analysis via Rutherford backscattering spectrometry, nuclear reaction analysis via the 12C(d,p0)13C nuclear reaction and elastic recoil detection analysi(ERDA) for hydrogen content. Infrared absorption (IR) has been used to determine the crystalline fraction of the films and the concentration of the hydrogen bonded to Si or to C. Complementary to IR, bonding configuration has been also characterized by Raman spectroscopy. As Ts is increased, the crystalline fraction increases and the hydrogen content decreases, as observed by both ERDA and IR. It also appears that some films contain a few Si excess, probably located at the nanograin boundaries.