  Understanding the physics of doping a charge transfer insulator is the most
important problem in high-temperature superconductivity. In this work, we show
that an in-gap bound state emerges from the localized hole of the doped charge
transfer insulator. We propose an approximate ground state wavefunction based
on one localized Zhang-Rice singlet and the Neel state. By calculating the
excitation states with one hole added and removed from this ground state, we
successfully identify the existence of bound states inside the charge transfer
gap. This feature is further proved by the MPS-based Lanczos study of a system
of $4\times4$ CuO$_2$ unit cells. How these bound states evolve into metallic
states is further discussed. Our findings identify the key component of recent
STM results on lightly doped Ca$_2$CuO$_2$Cl$_2$ and provide a new
understanding of hole-doped charge transfer insulators.
