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Electron traps in HfO2 have been investigated by measuring thermally stimulated current (TSC). Two different interface regions have been identified where captured electrons interact with energy band states. The two domains are separated by and close to the ldquointerlayerrdquo of SiOx commonly present in high-k/silicon stacks. On the inner side, between SiOx and silicon, we find an irregular silicon crystal which we interpret as a source for interface states. At the SiOx/HfO2 interface, we find a high concentration of unstable traps with a strong influence on the electric field distribution in the gate stack.