We present in-depth profiling of chemical bonding features and defect state density in ultrathin HfSiOxNy (Hf/(Hf+Si)=~43%) films with average nitrogen contents up to ~18at.% by using x-ray photoelectron spectroscopy (XPS) and total photoelectron yield spectroscopy (PYS) in combination with oxide thinning in a dilute HF solution. The films were prepared on pre-cleaned Si(100) by an atomic layer chemical vapor deposition (ALCVD) method and followed by plasma nitridation. By annealing at 1050°C in N2 ambience, Si-N bonding units in the films are increased as a result of thermal decomposition of Hf-Nx(x=2 and 3) units and the interfacial oxidation accompanied with nitrogen incorporation is caused. For the annealed samples, Hf ions coordinated with two N atoms are distributed with a profile peaked around 1nm from the top surface. Also, from the depth profiles of chemical compositions, which were determined from the change in the intensity at each thinning step, we found that the oxygen content becomes its minimum around ~1.2nm from the surface while the nitrogen content becomes its maximum within ~1.5nm from the surface. The result suggests that the surface re-oxidation is promoted coincidentally with the diffusion of N atoms generated by thermal decomposition of the Hf-Nx units during the N2- annealing. The photoelectron yield from filled defect states in the dielectric stacks was increased in the early stages of oxide thinning and then decreased with further progressive thinning. The depth profile of the defect states, which was derived from the change in the yield, shows that the defect state density becomes its maximum in the near-surface region where oxygen deficiency becomes significant. It is likely that the imbalance in chemical coordination between anions and cations is responsible for the defect generation.