Colour, polytypes

This Datareview outlines the data on optical absorption and refractive index in the various polytypes of SiC. The optical transitions give rise to the characteristic colour of each polytype. Values for both the ordinary and extraordinary refractive indices versus wavelength are given. 

The interband transitions in SiC polytypes give rise to the characteristic colours of green (6H), yellow (15R) and green-yellow (4H). Cubic SiC exhibits a pale yellow colour when undoped and greenish yellow in the doped state. The variations of refractive index with wavelength for the 3C, 2H, 4H, 6H and p-SiC polytypes are listed. 

Blue LEDs made in the 6H-SiC polytype, utilising a donor-acceptor pair recombination, are now commercially available. Boron-doped 6H-SiC produces a yellow colour while green... 

The crystal structure of SiC corresponds to either cubic ZnS, the same as Si and GaAs, or hexagonal ZnS wurtzite (the colour of 3C-SiC is yellow and a-SiC is colourless like CdS). The crystal lattices are usually given in the standard hexagonal or cubic forms. It is evident that substitutional impurities can occupy different sites depending on the polytype. Most of the polytypes, except 2H, are metastable. However, 3C does transform to 6H at temperatures above 2000 °C and other polytypes can transform at temperatures as low as 400 °C [11,12]. 

SiC- modification (colour) Addition to SiC (weight-%) Chemical analysis (weight-%) Al B Polytype analysis (weight-%) 6H 15R 4H 3C Micro- structure ... 

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