Study on Carbon Particle Inclusions during 4H-SiC Growth by Using Physical Vapor Transport System

A study on carbon particle inclusions during 4H-SiC bulk growth is presented. Special attentions were paid to design of graphite growth compartment, size of SiC source materials, and process of seed crystal handling. It was found that common carbon inclusions with size of 30μm or less were attributed to carbon particles from graphitized SiC source. Less common carbon inclusions with size of over 100μm were also found and were attributed to poor seed crystal mounting process. In order to reduce carbon inclusions, several experiments were designed by using a NAURA Advanced Physical Vapor Transport (PVT) System APS130G. A graphite plate separator was inserted into the growth compartment to prevent the carbon particles from transporting to the growth surface. SiC powder materials with larger diameters were selected to reduce source graphitization. Additional clean process was performed to remove carbon particle residuals on graphite parts during seed mounting. The results showed significant improvement of carbon inclusion problems in SiC ingots and thus high-quality SiC wafers were made successfully.

Metastable Eutectoid Transformation in Spheroidal Graphite Cast Iron: Modeling and Validation

This paper presents a new microstructural model of the metastable eutectoid transformation in spheroidal graphite cast irons. The model takes into account the nucleation and growth of pearlite nodules. The nucleation is assumed to be continuous and dependent on the metastable undercooling associated with the upper limit of the three-phase field, while the growth rate is considered to be ruled by the silicon partitioning between ferrite and cementite at the pearlite/austenite front. The initial conditions for the metastable transformation are obtained from a microstructural simulation of solidification, graphite growth, and stable eutectoid transformation. These microstructural models are coupled with the thermal balance solved at a macroscopic level via the finite element method. The experimental validation of the metastable eutectoid model achieved by comparison with measured values of ferrite, graphite, and pearlite fractions at the end of the cooling process demonstrates the sound predictive capabilities of the proposed model.

Revisiting Models for Spheroidal Graphite Growth

Recent experiments resolved nucleation and growth of graphite during solidification of ductile cast iron in 4D using synchrotron X-ray tomography. A numerical model for microstructure formation during solidification is compared with the experiments. Despite very good overall agreement between observations of spheroidal graphite growth and model results, significant deviations exist towards the end of solidification. We use the experimental observations to analyse the relation between graphite growth rate and the state of the particle neighbourhood to pinpoint possible links between growth rate of individual graphite spheres and the overall solidification state. With this insight we revisit existing models for growth of spheroidal graphite and discuss possible modifications in order to correctly describe the critical final stage of solidification.

Crystallography of Growth Blocks in Spheroidal Graphite

A better understanding of spheroidal graphite growth is expected in a near future thanks to widespread use of transmission electron microscopy. However, common transmission electron microscopy is quite time consuming and new indexing techniques are being developed, among them is transmission Kikuchi diffraction in a scanning electron microscope, a recent technique derived from electron backscatter diffraction. In the present work, on-axis transmission Kikuchi diffraction in scanning electron microscope, completed by transmission electron microscopy, was used with the objective of producing new observations on the microstructure of spheroidal graphite. This study shows that disorientations between blocks and sectors in spheroidal graphite are quite large in the early growth stage, which may be indicative of a competition process selecting the best orientations for achieving radial growth along thecdirection of graphite.