Ohmic contact formation mechanism of Ge-doped 6H-SiC

•Only two native-defect-induced shallow energy levels are found in Ge-doped-6H-SiC.•Raman and RBS/c reveal high quality of Ge-doped 6H-SiC.•Ge is a substitutional impurity.•The GeC bond is easy to break to generate carriers through the induced levels. We present a comprehensive investigation of mechanism of ohmic contact characteristic in Ge-doped 6H-SiC single crystals. Raman mapping and Rutherford backscattering spectrometry reveal high crystalline quality of Ge-doped 6H-SiC and the Ge as a substitutional impurity. Deep-level transient spectroscopy (DLTS) spectrum was measured. Its Arrhenius fitting shows two shallow energy levels. They are below the conductive band bottom 0.298 eV and 0.323 eV respectively. The first-principles calculations indicate that the no new levels are in (Ge)Si configuration, but (Ge)C can induce deep levels in 6H-SiC. Thus, the shallow energy levels are caused by native defects which arise in a process of crystal growth. The two characteristics shallow energy levels, the small trap cross sections and positions near to conductive band bottom, can emit electron under effect of an applied field. In addition, the GeSi bonds are relatively easily to break for generating electrons to jump into conductive band bottom though the shallow energy levels.