The observation of metallic interface between band insulators LaAlO$_3$ and SrTiO$_3$ has led to massive efforts to understand the origin of the phenomenon as well as to search for other systems hosting such two dimensional electron gases (2-DEG). However, the understanding of the origin of the 2-DEG is very often hindered as several possible mechanisms such as polar catastrophe, cationic intermixing and oxygen vacancy (OV) etc. can be operative simultaneously. The presence of a heavy element makes KTaO$_3$ (KTO) based 2-DEG a potential platform to investigate spin orbit coupling driven novel electronic and magnetic phenomena. In this work, we investigate the sole effect of the OV, which makes KTO metallic. Our detailed textit{ab initio} calculations not only find partially filled conduction bands in the presence of an OV but also predict a highly localized mid-gap state due to the linear clustering of OVs around Ta. Photoluminescence measurements indeed reveal the existence of such mid-gap state and O $K$-edge X-ray absorption spectroscopy finds electron doping in Ta $t_{2g}^*$ antibonding states. This present work suggests that one should be cautious about the possible presence of OVs within KTO substrate in interpreting metallic behavior of KTO based 2-DEG.
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