pn diodes have recently been fabricated from 3C-SiC material heteroepitaxially grown atop on-axis 4H-SiC mesa substrate arrays [1,2]. Using an optical emission microscope (OEM), we have investigated these diodes under forward bias, particularly including defective 3C-SiC films with in-grown stacking faults (SFs) nucleated on 4H-SiC mesas with steps from screw dislocations. Bright linear features are observed along <110> directions in electroluminescence (EL) images. These features have been further investigated using electron channeling contrast imaging (ECCI) . The general characteristics of the ECCI images—together with the bright to dark contrast reversal with variations of the excitation error—strongly suggest that the bright linear features are partial dislocations bounding triangular SFs in the 3C-SiC films. However, unlike partial dislocations in 4H-SiC diodes whose recombination-enhanced dislocation motion serves to expand SF regions, all the partial dislocations we observed during the electrical stressing were immobile across a wide range of current injection levels (1 to 1000 A/cm2).