HfO₂/Si interface is among the most studied heterostructure materials due to the use of HfO2 in the mainstream Si microelectronic technology. Following the discovery of new functionalities in HfO₂ such as ferroelectric and reversible resistance-switching properties, we study ultrathin HfO₂ films grown on highly doped (p⁺ and n⁺) Si by means of synchrotron-based soft-x-ray spectroscopy techniques, such as x-ray photoelectron spectrosopy (XPS) and angle resolved photoelectron spectroscopy (ARPES). With angular resolution, we directly obtain the electronic dispersions E(k) of the single-crystalline Si substrate in contact with the HfO₂ overlayer, depending on the Si doping and heat treatment, and determine the k-resolved band offset at the interface. Analysis of the Hf and Si core-level energies and line shapes as a function of photon energy yields band bending in HfO₂ and Si. The evolution of the Hf 4f linewidth upon annealing points to development of a potential distribution across HfO₂ due to charged defects at the surface and interface with Si. The effect of intense x-ray beam on the HfO₂/Si interfaces, distorting their pristine electronic structure, is evaluated from the time evolution of line shape and position under irradiation. We propose a model explaining the effects of both heat treatment and x-ray irradiation on the HfO₂/Si electronic structure in terms of oxygen vacancies generated at the surface of HfO₂ and its interface to Si, where the released O atoms react with Si to form SiO_x at the interface. The knowledge of the irradiation-dependent band bending is essential for precise determination of the k-dependent band offset locally at the HfO₂/Si interface.