Although a large amount of interface states density (Dit) at dielectric/Ge interface has been reported, the success of extensive research efforts for GeO2/Ge interface passivation encourages us to realize high-mobility Ge complementary metal oxide semiconductor (CMOS) beyond Si CMOS. However, there are still two great concerns in the GeO2/Ge stack, namely, thermal instability and water solubility, which inevitably incur difficulty in Ge-based device process as well as the reliability issue.
In the present work, we demonstrate a significant improvement of both thermal stability and water resistance in GeO2/Ge stack by replacing GeO2 with yttrium-doped GeO2 (Y-GeO2). The excellent electrical properties of Y-GeO2/Ge stack with low Dit (~1011eV-1cm-2) are presented as well as enhancement of dielectric constant in Y-GeO2 layer, which is beneficial for further equivalent oxide thickness (EOT) scaling of Ge gate stack.
On the basis of this understanding of Y-GeO2, we have achieved 0.47 nm-thick EOT in HfO2/Y-GeO2/Ge gate stack with 3 A/cm2 of gate leakage current at VG=-1V. The superior interface properties of this gate stack are maintained without any degradation up to 550oC thanks to the thermally robust Y-GeO2 interfacial layer. The influence of Y doping on the material properties of GeO2 is discussed from a thermodynamic viewpoint that Y can lower the oxygen potential of GeO2. In addition, a systematic comparison is also carried out between different M2O3 (Sc2O3, Y2O3 and La2O3) as doping materials into GeO2 and their similarity and difference are discussed.