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In pursuit of understanding the mechanism of two-dimensional electron gas (2DEG), artificial heterostructures based on SrTiO3 have sparked a wealth of exciting experimental and theoretical results. However, to date, the physical origin of this phenomenon still remains controversial. In our endeavor to unravel the mystery, we have synthesized a series of LaTiO3+δ/SrTiO3 films by controlling growth temperature and oxygen pressure, respectively. X-ray absorption results identify the transition of Ti ions from Ti3+ to Ti4+. It is proved that electrical transport properties in the LaTiO3/SrTiO3 film are dominated by oxygen vacancies in SrTiO3. In addition, the metallic behavior observed in the amorphous LaTiO3/SrTiO3 film suggests contributions from growth-induced donor defects to 2DEG. Our work presents direct evidence of competing conduction mechanisms for the observed 2DEG at the LaTiO3/SrTiO3 interface and highlights the important role of oxygen diffusion from the substrate SrTiO3. Meanwhile, we emphasize the significance of LaTiO3 as an oxygen scale meter to investigate interfacial properties of oxide heterostructures. [ABSTRACT FROM AUTHOR]