Influence of the growth conditions on the formation of macro-steps on the growth interface of SiC-Crystals

• Gas phase composition affects step morphology on the surface of SiC crystals. • Step bunching due to doping influences the step flow kinetics on the main facet. • Step bunching due to low C/Si ratio seems energetically driven. • Large surface steps are hindered in their lateral movement at defects. • Fine surface steps in C-rich conditions contribute to a better polytype stability. Several SiC crystals were grown in the same temperature field with a variation of the gas phase and the resulting step morphologies on the surface are analyzed using Laser Scanning Microscopy (LSM). It is shown that the step morphology changes due to doping, but also due to different C/Si ratios in the gas phase. By the addition of graphite into the source powder a reduction of Si-excess was achieved and an ordered step structure with an average height of 0.014 µm and width of 5.0 µm was formed. In the n -type doped case (n = 5.9∙10 18 cm −3 ) the morphology on the facet differs strongly. Bunched steps with heights of 0.17 µm and widths of 152 µm are found. Between the diverse large macro-steps, a very ordered structure of unit cell height steps with a step spacing of 13.3 µm is found by Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). A different change in the step morphology occurs over the whole crystal surface when elemental silicon is added into the source material. In contrast to the doping induced case, the bunched steps are more closely spaced and exhibit average heights of 0.20 µm and widths of 22 µm. The steps are noticeably retained at defects like micropipes or threading dislocations. The N 2 induced step bunching is related to step kinetics on the main facet but the formation of macro-steps in the Si-rich case is assigned to the formation of stable surface steps. The variation of the inert gas pressure in the growth cell also changes the C/Si ratio in the gas phase as the diffusion from source to seed of the reactive gas species is affected differently. With a low inert gas pressure of 0.2 mbar a larger C/Si ratio in the gas phase is formed leading to steps with heights too small for analysis in the LSM, while the growth in 40 mbar promotes a smaller C/Si ratio leading to the formation of bunched steps with heights of 0.10–0.20 µm and with an average distance of 4.3 µm between them. The crystals grown in a gas phase comprising a large C/Si ratio show a better polytype stability than the crystals grown in Si-rich conditions. The instabilities can partly be attributed to the pile up of the macro-steps at defects exposing large 000 1 ¯ terraces, where 2D-nucleation of foreign polytypes can occur.