Mikhail Anikin

State of the art in the modelling of SiC sublimation growth

Different computational tools have helped to provide additional information on the sublimation growth of SiC single crystals by the modified-Lely method. The modelling work was motivated by the need of a better control of the local temperature field inside the crucible. Because there is an environment of strong thermal radiation in which the SiC boule growth process occurs, heat transfer must therefore be coupled with gaseous species transport and reactivity. This highly coupled model must take into account all geometric modifications in crucibles which strongly influences the crystal growth process. Local thermochemical equilibrium linked to heat and mass transfer is the model proposed in this paper to give the magnitude of the growth rate and the shape of the crystal. This modelling field is still too young to propose a software package including all modelling aspects and a reliable material database. However, some parts of the modelling work have reached maturity like electromagnetic heating and thermal modelling coupled with simplified chemical models. We show in this paper selected examples in order to demonstrate the types of information which can be routinely available by simulation and how to approach defect formation from a macroscopic point of view. Minor geometric modifications of the holes for pyrometric measurements drastically change the magnitude of thermal gradients in the crucible. Geometric modifications of the crucible change the computed crystal shapes. The calculated results complete the experimental knowledge by a quantification of the local macroscopic fields.

Electrical and optical characterisation of vanadium in 4H and 6H–SiC

Abstract Vanadium deep acceptor level in bulk 6H–SiC and 4H–SiC has been studied by deep level transient spectroscopy (DLTS), optical absorption (OA), photoluminescence (PL) and deep level optical spectroscopy (DLOS). DLTS reveals two levels related to vanadium acceptor level (V3+/V4+) for 6H–SiC at Ec −0.68 eV (cubic sites) and Ec −0.74 eV (hexagonal site) and for 4H–SiC at Ec −0.82 eV. The good correlation obtained between DLOS and OA spectra allows us to identify OA lines and DLOS resonance band as V3+ internal transition between the 3A2 ground state towards excited states. The V3+ ion configuration in 4H–SiC is given.

Coherent X-ray imaging investigation of macrodefects and micropipes on SiC

We describe an X-ray phase imaging technique able to detect micro and macrodefects on SiC. This technique enables investigation of thick samples, not accessible to light transmission microscopy and provides additional information on the defects.

Defects formation in sublimation grown 6H-SiC single crystal boules

Abstract A perfect understanding of the origin of defects in connection with crystal growth conditions is of prime importance for the future of SiC based electronic. In the generally used modified Lely method (M-Lely), the growth takes place by incongruent sublimation from SiC powder on a monocrystalline SiC seed at low pressure and high temperature. We have recently proposed beginning the growth process at the inversed sign of temperature gradient and low argon pressure. In these conditions, nucleation at low temperature is suppressed and sublimation polishing etching takes place. Then the sign of the temperature gradient is slowly inversed to start the growth. The influence of this nucleation step on the nature and density of defects has been studied by structural analysis using X-ray diffraction, optical and electronic microscopy, atomic force microscopy and synchrotron white beam X-ray topography. The results are discussed in the light of our present understanding of the sublimation process.

Weak phonon modes observation using infrared reflectivity for 4H, 6H and 15R polytypes

Infrared reflectivity is investigated as a useful tool for SiC polytype identification. The oscillator parameters (energy, oscillator strength and damping) of the weak folded modes, arising from the folding of the axial optic branch, are determined in the cases of 4H, 6H and 15R polytypes. An example of polytype identification for a sample showing polytype conversion during SiC crystal growth observed in IR spectroscopy is given.

Influence of reactor cleanness and process conditions on the growth by PVT and the purity of 4H and 6H SiC crystals

We have investigated the influence of crucible purity on SiC crystals grown by physical vapor transport. A quadrupole gas analyser has been used to monitor the outgas stage of the growth system. Secondary ion mass spectroscopy (SIMS) and glow discharge mass spectroscopy (GDMS) analysis have been performed to study the purity of the graphite at different stages of the thermal treatment and to study the transfer of impurities into the grown crystals.

Modelling of SiC sublimation growth process: analyses of macrodefects formation

The influence of the temperature and its gradient on powder features and defect formation is discussed in the light of experimental results in the physical vapour transport process. The recrystallization of the source powder during crystal growth appears to be directly related to the temperature gradient field within the source material. Moreover, several calculations made on different experimental geometries of crucible show that the location of the macrodefects is strongly related to the temperature gradient in the crystal. Technical solutions in order to avoid or reduce the macrodefects formation are discussed.

Influence of growth conditions on the defect formation in SiC ingots

6H-SiC ingots with diameters of 25-35 mm with flat natural surfaces have been grown by the Modified Lely method. Influence of the growth conditions and crucible geometry on the growth front shape and on the enlargement of the seeds have been investigated. The developed growth process allows the growth of ingots under quasi-equilibrium conditions with and without enlargement at a growth rate between 1-1.5 mm h - 1 .

Optical assessment of purity improvement effects in bulk 6H and 4H-SiC wafers grown by physical vapor transport

We report on purity improvement effects which recently have been observed when comparing different series of wafers produced in two separate, but basically similar, home-made physical vapor transport (PVT) reactors. Looking in detail for the origin of this phenomenon, we have found a strong influence of the residual purity of the graphite material used to manufacture the crucibles. After proper optimization, a second effect has been found. It manifests when the residual level of impurities in the seed material is high and provides evidence for in situ auto-doping. Finally, quantitative analyses of C(V) characteristics and Raman spectra have been done. In this way we follow the trend in residual carrier concentration and mobility.

Macroscopic modelling of silicon carbide sublimation: toward a microscopic modelling of defect formation

The deposition of single SiC crystals has been processed inside a sealed enclosure at temperatures above 2300 K and pressures lower than 5 × 103 Pa by the modified Lely method. The purpose of this work is to present different optimized macroscopic models, thermodynamics, heat and mass transfers used in the simulation of the growth of such crystals. Thermodynamic modelling has been used to determine the most important reactive species involved in equilibrium conditions. Induction heating modelling has allowed the calculation of the actual temperatures inside the reactor which are not well known because of the difficulty associated with their measurement. Finally, mass transport modelling provided the calculated deposition rate. It was found that the calculated growth rates were close to the experimental ones which may indicate a good representation of the actual phenomena involved in the crucible. Moreover, defect formation, which is the primary obstacle to the production of large area devices, is explained by local sublimation of the growing ingot and related to the temperature gradients existing within the crystal and graphite holder.