Materials Science Forum Vols. 556-557

Abstract: For several years the major focus of material issues in SiC substrates was laid on the reduction of macroscopic defects like polytype inclusions, low angle grain boundaries and micropipes. Since then significant improvements have been achieved and micropipe densities could be reduced to values below 1 cm-2. Nevertheless the fabrication of high quality substrates at high volume and low cost is still challenging. Therefore preconditions for reproducible process and quality control will be discussed. Since it is obvious that dislocations are the main reason for degradation in power devices the prevailing attention has also been shifted to that field of material research. Intense studies were utilized on dislocation and stacking fault formation during sublimation growth. For this reason we systematically varied crucial parameters of the crystal growth process and applied several specific characterization methods, e.g. KOH-defect-etching, electron microscopy and optical microscopy, to evaluate resulting material properties. The investigations were accompanied by failure analysis on devices of the Schottky-type. We found out that for the improvement of substrate quality emphasis has to be laid on the reduction of thermoelastic stress in the growing crystal. The results of numerical calculations enabled us to derive moderate growth conditions with reduced temperature gradients and correspondingly low defect concentration.

Abstract: SiC single crystal ingots were prepared onto different seed material using sublimation PVT techniques and then their crystal quality was systematically compared. In this study, the conventional SiC seed material and the new SiC seed material with an inserted SiC epitaxial layer on a seed surface were used as a seed for SiC bulk growth. The inserted epitaxial layer was grown by a sublimation epitaxy method called the CST with a low growth rate of 2μm/h. N-type 2”-SiC single crystals exhibiting the polytype of 6H-SiC were successfully fabricated and carrier concentration levels of below 1017/cm3 were determined from the absorption spectrum and Hall measurements. The slightly higher growth rate and carrier concentration were obtained in SiC single crystal ingot grown on new SiC seed materials with the inserted epitaxial layer on the seed surface, maintaining the high quality.

Abstract: We have carried out the growth and basic characterization of isotopically enriched 4HSi 13C crystals. In recent years the growth of 13C enriched 6H-SiC has been performed in order to carry out fundamental materials studies (e.g. determination of phonon energies, fundamental bandgap shift, carbon interstitial defect study, analysis of the physical vapor transport (PVT) growth process). For electronic device applications, however, the 4H-SiC polytype is the favored material, because it offers greater electron mobility. In this paper we present the growth of 4H-Si13C single crystals with up to 60% of 13C concentration. From a physical point of view we present first results on phonons as well as the fundamental bandgap energy shift due to 13C incorporation into the SiC lattice.

Abstract: New results on bulk growth of 6H-SiC crystals along the [01-15] direction are presented. The aim of our work is to improve the quality of the crystal grown by classical PVT method by employing alternative growth directions, other than conventional [0001]. Using a specially designed graphite crucible, crystals with an expansion angle of 30 degrees and diameters up to 40 mm have been grown. No polycrystalline rim develops at the contact with the graphite wall. Concerning specific defect content in the [01-15]-oriented crystals, they appear completely free of micropipes and screw or threading edge ([0001]-oriented) dislocations. The [01-15] crystal relaxes adopting a network of in-plane (0001) dislocations. They are not uniformly distributed reaching the maximum density of about 106 cm-2.

Abstract: C-plane substrates with off-orientation to <1120 > may stabilize the grown polytype, but the stacking fault density (SFD) increases from zero in the on-axis sample to 4500 cm-1 (7.7° off). The SF form preferentially at the seed-crystal-interface by a kinetically induced rearrangement of surface ad-atoms on m-facets. Most SF start in bundles with an average distance of 100 .m, which are subdivided in smaller bundles with 8 .m distance. They start preferentially from the upper corner of the vertical non-polar plane of bunched steps, which may be composed of small pyramids with m-facet surfaces. The dislocation density could decrease with increasing SFD by a pinning mechanism.

Abstract: We investigated the effects of hydrogen addition to the growth process of SiC single crystal using sublimation physical vapor transport (PVT) techniques. Hydrogen was periodically added to an inert gas for the growth ambient during the SiC bulk growth. Grown 2”-SiC single crystals were proven to be the polytype of 6H-SiC and carrier concentration levels of about 1017/cm3 was determined from Hall measurements. As compared to the characteristics of SiC crystal grown without using hydrogen addition, the SiC crystal grown with periodically modulated hydrogen addition definitely exhibited lower carrier concentration and lower micropipe density as well as reduced growth rate.

Abstract: We report the effect of changing the growth conditions in the case of bulk 3C-SiC crystals grown by the Travelling Zone Method when the seed is placed on the top graphite rod. First, we investigated the effect of changing the temperature gradient and the cooling ramp. Next we studied the effect of changing the seed polytype and misorientation. Every time, working in the 1700 °C temperature range, the grown polytype was 3C. From X-ray analysis we evidenced a better hetero-epitaxial relationship between the seed and layer when a low misorientation angle was used. Better quality and homogeneity were obtained on the first 500 .m of the layer and, beyond this thickness, micro-Raman measurements show that the effect of solvent (Si) incorporation is not yet fully under control.

Abstract: Homoepitaxial growth of 4H-SiC and characterization of deep levels obtained mainly in the authors’ group have been reviewed. The growth rate has been increased to 24 om/h with keeping very good surface morphology and low trap concentration on 8o off-axis 4H-SiC(0001) by hot-wall chemical vapor deposition at 1650oC. The increased growth rate has resulted in the enhanced conversion of basal-plane dislocations into threading edge dislocations in epilayers. The Z1/2 and EH6/7 concentrations can be decreased to about 1·1012 cm-3 by increasing the C/Si ratio during CVD. Extensive investigation on as-grown and electron-irradiated epilayers indicates that both the Z1/2 and EH6/7 centers may be attributed to the same origin related to carbon displacement, probably a carbon vacancy. Deep levels observed in as-grown and irradiated p-type 4H-SiC are also presented.

Abstract: Growing good quality SiC epitaxial layers at temperature lower than 1400°C is a challenging problem which could help reducing the costs, increasing the safety of the process or even give new perspectives. Toward this aim, liquid based growth techniques have been used. The Si-based melts should be carefully chosen considering several criteria. Furthermore, the implementation of a liquid phase for growing SiC epilayer can be performed in various manners (dipping or VLS mechanism) so that one has to choose the more appropriate technique. The discussion is illustrated with several results showing that the growth of SiC from a liquid phase at low temperature can address various important technological points such as experimental safety, ptype doping, on-axis or selective epitaxy. The recent demonstration of single-domain 3C-SiC heteroepitaxial layers on hexagonal SiC substrates confirms that liquid based growth has still unexpected qualities.

Abstract: Growth of thick epitaxial SiC layers needed for high power devices is presented for horizontal hot-wall CVD (HWCVD) reactors. We demonstrate thickness of epilayer of 100 μm and more with good morphology, low-doping with no doping variation through the whole thick layer and reasonable carrier lifetime which mainly depends on the substrate quality. Typical epidefects are described and their density can dramatically be reduced when choosing correctly the growth conditions as well as the polishing of the surface prior to the growth. The control of the doping and thickness uniformities as well as the run-to-run reproducibility is also presented. Various characterization techniques such as optical microscopy, AFM, reflectance, CV, PL and minority carrier lifetime have been used. Results of high-voltage SiC Schottky power devices are presented.