Using a photon emission setup and observing through the substrate the light associated with the avalanche, we have investigated the breakdown characteristics of silicon carbide p(+)-n-n(+) 4H-SiC diodes. After characterizing the devices in forward operation, we show that whereas the light emission is uniform throughout the surface in the forward regime, it occurs preferentially along parallel stripes at breakdown. These stripes are independent of diode geometry, they do not correspond to microplasmas and do not lead to catastrophic failure of the devices. A hypothesis involving doping non-uniformity is proposed.