Silicon carbide (SiC) is a promising semiconductor material for a variety of new device applications. These include high-power devices, high-voltage switching applications, hightemperature electronics, and high-power microwave applications. Due to its superior material properties SiC provides many advantages over Si. The properties of SiC have been known for decades, but SiC is still under development as a semiconductor material. The first growth technique for semiconductor quality material was the Lely technique developed by J.A. Lely in 1955. These crystals still show the best crystal quality, but they are small in size. Today the most common technique and the basis for commercial growth systems is the modified Lely method, developed by Tairov and Tsvetkov in 1978.

The main limitation of the technology is in the area of the crystal growth. The problems lie in the structural quality of the material, defects and the control of polytypes. Different kinds of defects like micropipes, dislocations, stacking faults, and domain boundaries exist in the material. The dislocation densities, and especially micropipe densities are to be reduced. 35 mm diameter wafers with micropipe densities as low as 0.8 cm^-2 have been reported. 75 mm diameter wafers have also been demonstrated, but still larger wafer sizes are desired for industrial applications.

The micropipes are apparently the largest problem for the commercialization of SiC technology. There are different explanations for mechanisms causing the micropipes. Most views are based on Frank's theory considering the micropipe being a hollow core of a screw dislocation with a large Burgers vector. The diameter of the micropipe is related to the magnitude of the Burgers vector. However, there are several mechanisms for formation of hollow cores: kinetic - high growth rates, thermodynamic - variations in constituent overpressures, dislocation centers, and defect formation after the growth. In all cases the process control, stability and cleanliness must be taken into account.

The modified Lely method is a seeded sublimation process where the growth rate is proportional to the supersaturation of a vapor phase. This method is used in the present work. In paper 1 the effect of the growth parameters on the growth rate and its components has been investigated. The rate-determining step under different growth conditions has been defined. In papers 2 and 3 the structural defects in the grown material have been investigated. In paper 2 structural defects in wafers of 4H and 6H polytype have been studied by means of synchrotron X-ray topography and optical microscopy. The effect of seed crystal attachment, orientation, growth front shape, reloading and continued growth are considered. The results are discussed in connection with and related to the applied growth conditions. In paper 3 the temperature distribution is modified, and the influence on secondary evaporation, domain and micropipe formation is studied. As a comparison the commercially available 4H polytype wafers are studied as to their structural quality in paper 4. The domain structure and the misorientation is investigated. In papers 5 and 6 defect analysis of Lely platelets is performed by synchrotron X-raytopography. The type of dislocations along with the Burgers vectors are determined and relationships between the observed defects and their formation mechanisms in the growth processes are discussed. These kind of platelets are used as a seed crystal material in the growth experiments. Finally, in paper 7, a high-resolution X-ray diffraction study of the domain occurrence in SiC epitaxial layers grown by sublimation is performed. It has been shown that the size and the misorientation of domains in the grown material can be controlled by selecting proper growth conditions.