Flat band voltage (V-FB) roll-off in long channel devices at thin equivalent oxide thickness (EOT) is studied on SiO2/nitrided-HfSiO stacks. V-FB increases when SiO2 interfacial layer thickness decreases, and charges pumping (CP) frequency sweep analysis shows higher trap density near Si/SiO2 interface. Based on this observation, an atomic diffusion model is introduced. Higher concentration of nitrogen atom in the HfSiO(N) layer diffuses to the Si/SiO2 interface through the SiO2 layer in thinner SiO2 device, and accumulates near Si/SiO2 interface which can introduce higher density of interfacial traps. Lifetime extracted from negative bias temperature instability (NBTI), and mobility are also degraded in thinner SiO2 devices due to the higher interfacial trap density. The V-FB roll-off can be improved by lowering nitrogen concentration in the HfSiO(N) layer from optimizing plasma nitridation pressure, decreasing post deposition anneal temperature, or using defect absorbing layer on the high-k oxide. (C) 2011 Elsevier Ltd. All rights reserved.