We extensively investigated the physical origin of the threshold voltage VTH modulation by the Ge channel ion implantation (I/I) in HfO 2/TiN/polycrystalline Si gate stack systems. The possible V TH modulation factors were carefully distinguished from each other by changing the channel dopant concentration and the thickness of the gate dielectric films. It was found that the changes in energy-band offset and dopant concentration in the channel region are minor factors of the VTH modulation. However, it was also found that ΔVTH is sensitive to the oxidation condition of the interfacial oxide layer; a longer oxidation induces a larger ΔVTH. From the X-ray spectroscopy, we found that a Ge pile-up at the interfacial layer/channel interface occurs, and that the VTH modulation range correlates well with the amount of piled-up Ge. © 2011 The Japan Society of Applied Physics.