Gate stack technology for advanced high-mobility Ge-channel metal-oxide-semiconductor devices - Fundamental aspects of germanium oxides and application of plasma nitridation technique for fabrication of scalable oxynitride dielectrics

Germanium (Ge)-based high-mobility metal-oxide-semiconductor field-effect transistors (MOSFETs) have gained considerable attention because they perform better than common Si-based devices. Although degraded electrical property of germanium oxide (GeO2) gate insulators is considered the most serious obstacle for implementing Ge-channel for future MOSFETs, remarkable progress has been made recently. This article overviews both fundamental and technological aspects of thermally grown GeO2, and discusses strategies for achieving ultrathin gate insulators for high-performance Ge-based MOSFETs. Our experimental and theoretical studies revealed that, despite excellent electrical property of GeO2/Ge interface, its poor stability is a big concern, especially for ultrathin dielectrics. To overcome this problem, we investigated the impact of plasma nitridation of Ge and GeO2 surfaces, in terms of surface cleaning, stability, and electrical properties of the nitrides. On the basis of the experimental findings, we have proposed high-quality Ge oxynitride (GeON) gate dielectrics, which consist of stable nitrogen-rich capping layers on ultrathin oxides. We implemented the GeON gate dielectrics into Ge-channel pMOSFETs and successfully demonstrated hole mobility that was 2.4 times higher than Si universal mobility.