In the present paper investigation of electrical characteristics of p(-) - 3C-SiC /n(+) - 6H-SiC heterojunction grown by sublimation in vacuum was done. The electroluminescence spectrum (EL) of this diodes shows green band close in spectrum position to band of free-exciton annihilation in 3C-SiC. But the maximum position of this line was shifted to the shortwave region of the spectrum with about 0,05-0,07 eV in contrast with the typical position of exiton band in 3C-SiC. This shift can be explained by quantum-size effects at 3C-SiC/6H-SiC heteroboundary. Calculated position of the first level in quantum well near 3C-SiC/6H-SiC heteroboundary (E-0 = 0,05 eV) is in good agreement with experimental value of EL shift. Finally we conclude, that by sublimation epitaxy in vacuum (SEV) it is possible to grow p(-)-3C-SiC/n+- 6H-SiC heterostructure with doping level in p- and n - regions suitable for investigation of 2DEG at heterobondary.