A high quality interfacial layer (IL) is important to realize advanced high-k/Ge gate stacks for Ge metal oxide semiconductor (MOS) devices. Here, GeO2 can be regarded as one of the most promising interfacial layers (ILs). However, significant degradation of ultra thin GeO2 IL after atomic layer deposition (ALD) of high-k films has been reported, making it difficult to integrate GeO2 with high-k gate dielectrics. In this study, an in-situ plasma nitridation method is employed for plasma oxidized GeO2 layers in order to realize GeO2-based ILs robust against ALD processes, resulting in a GeON layer with tunable nitrogen concentration. The impact of the nitrogen content on the properties of GeON IL is examined. It is found that the GeON IL with low nitrogen content exhibits good MOS interface properties, inherent to the GeO2/Ge interface, as well as its tolerance for ALD-induced damages. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3599065] All rights reserved.