(Invited) Significant Enhancement of High-Ns Electron Mobility in Ge n-MOSFETs with Atomically Flat Ge/GeO2 Interface

The demonstration of high mobility Ge n-MOSFETs has been a great challenge for high-performance Ge CMOS, because electron mobility was limited by a large amount of interface states density (D_it) near the conduction band edge, which acts as Coulomb scattering centers. We significantly reduced D_it (<10¹¹ eV^-1cm^-2) at the Ge/GeO₂ interface with high-pressure oxidation (HPO), and very high electron mobility in Ge n-MOSFETs was demonstrated thanks to the dramatic reduction of Coulomb scattering. However, the rapid degradation of high-N_s electron mobility in Ge n-MOSFETs is still one of the greatest concerns in Ge CMOS technology. Although there are many possible origins so far considered, the degradation mechanism is still unclear. In this work, we present (i) the atomically flat Ge(111) surface, followed by (ii) the layer-by-layer oxidation for achieving the atomically flat Ge/GeO₂ interface, and examine the effect of surface roughness (SR) scattering on high-N_s electron mobility in Ge n-MOSFETs. The layer-by-layer oxidation on atomically flat Ge(111) surface was investigated. It was found that surface roughness was formed by conventional oxidation process, while low-temperature high-pressure oxidation provided the atomically flat Ge/GeO₂ interface. On the basis of this understanding, we have examined the effect of surface roughness scattering on high-N_s electron mobility in Ge n-MOSFETs. The highest electron mobility of 582 cm²/Vs at N_s=1x10¹³ cm^-2 was achieved by the formation of atomically flat Ge/GeO₂ interface thanks to the layer-by-layer oxidation.