Influence of passivating interlayer on Ge/HfO2 and Ge/Al2O3 interface band diagrams

The energy band alignment between Ge, HfO2 and Al2O3 was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO2, Ge3N4, Si/SiOx). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO2 interfaces, the latter being universally characterized by conduction and valence band offsets of 2.1 and 3.0 eV, respectively. However, in the case of HfO2 growth using H2O-based atomic layer deposition, the Ge oxide IL appears to have a narrower bandgap, 4.3 eV, than the 5.4–5.9 eV gap of bulk germania. Accordingly, formation of this IL yields significantly reduced barriers for hole and, particularly, electron injection from Ge into the insulator. Changing to a H-free process for HfO2 and Al2O3 deposition suppresses the formation of the narrow-gap Ge oxide.