High-quality GeON gate dielectrics formed by plasma nitridation of ultrathin thermal oxides on Ge(100)

High-quality germanium oxynitride (GeON) gate dielectrics for Ge-based metal-oxide-semiconductor (MOS) devices were fabricated by plasma nitridation of ultrathin thermal oxides on Ge(100) substrates. Although ultrathin oxides with abrupt GeO₂/Ge interfaces can be formed by conventional dry oxidation, air exposure results in serious electrical degradation. It was found that plasma nitridation forms a nitrogen-rich capping layer on the ultrathin oxide and significantly improves thermal stability of the GeON layer. The nitrogen-rich layer effectively suppresses electrical degradation during air exposure and provides excellent insulating properties. Consequently, we were able to achieve Ge-MOS capacitors with GeON dielectrics of an equivalent oxide thickness (EOT) as small as 1.7 nm. Minimum interface state density (D_it) values of GeON/Ge structures, i.e., as low as 3 × 10¹¹ cm^-2eV^-1, were successfully obtained for both the lower and upper halves of the bandgap.