Probing the electronic properties of gap states near the surface of $n\text{−}{\mathrm{SrTiO}}_{3-\delta }/i\text{−}\mathrm{Si}\left(001\right)$ heterojunctions with high sensitivity

We have measured soft x-ray resonant photoemission for the $n\text{−}{\mathrm{SrTiO}}_{3-\delta }/i\text{−}\mathrm{Si}\left(001\right)$ $(\delta =\sim 0.0003)$ hybrid semiconductor heterojunction with the goal of probing occupied electronic states in the band gap near the surface. By utilizing both swept energy, angle-integrated and fixed-energy, and angle-resolved spectroscopies, we identify the atomic and orbital character of the in-gap state as well as the different electronic phases that contribute to it. Specifically, we isolate the state associated with trapped charge on the $n\text{−}{\mathrm{SrTiO}}_{3-\delta }$ surface that causes surface depletion from the localized gap state resulting from electron correlation effects in the complex oxide. Using an x-ray energy close to select dipole allowed $\mathrm{Ti}{L}_3\to 3d$ x-ray absorption resonances results in a 40-fold increase in sensitivity. This feature makes photoemission possible when probing electrons from dopants at concentrations as low as a few hundredths of a percent.