>Structural characterization of 6H- and 4H-SiC polytypes by means of cathodoluminescence and x-ray topography
The cathodoluminescence (CL) technique is used to analyse the radiative recombination properties of four distinct silicon carbide (SiC) samples: a 6H-SiC n+-type Lely wafer, two off-axis 4H-SiC epitaxial layers of n type and p type, and a (11\bar 20 )-oriented 4H-SiC n+-type substrate. The CL spectra, recorded at various temperatures and at various excitation conditions, show strong differences between the polytypes, indicating a better homogeneous distribution of radiative centres inside the 6H polytype than in the 4H one, and also between the different orientations. For the (11\bar 20 )-oriented 4H sample, luminescence features decrease when the excitation intensity increases, probably due to a more significant indirect transition band. The CL spectra also vary for the same sample, due to the impurity and the microscopic defect density variations. Comparisons between two local spectra taken in two distinct areas of the (11\bar 20 )-oriented 4H sample, and with images obtained by x-ray topography in the same areas, allow us to establish that some structural defects are involved in luminescence centres. A deep centre involved in green luminescence (at 1.80 eV) is found to be associated with basal plane dislocations with the Burgers vector b=(1/3)\langle 11\bar 20\rangle .
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