  In order to study the origin of metallization of VO$_2$ induced by electron
injection, we deposit K atoms onto the surface of VO$_2$ films grown on TiO$_2$
(001) substrates, and we investigate the change in the electronic and crystal
structures using ${in~situ}$ photoemission spectroscopy and x-ray absorption
spectroscopy (XAS). The deposition of K atoms onto a surface of insulating
monoclinic VO$_2$ leads to a phase transition from insulator to metal. In this
metallization state, the V-V dimerization characteristic to the monoclinic
phase of VO$_2$ still exists, as revealed by the polarization dependence of the
XAS spectra. Furthermore, the monoclinic metal undergoes a transition to a
monoclinic insulator with decrease in temperature, and to a rutile metal with
increase in temperature. These results indicate the existence of a metallic
monoclinic phase around the boundary between the insulating monoclinic and
metallic rutile phases in the case of electron-doped VO$_2$.
