  The high-mobility conducting interface (CI) between LaAlO_{3}(LAO) and
SrTiO_{3}(STO) has revealed many fascinating phenomena, including exotic
magnetism and superconductivity. But, the formation mechanism of the CI has not
been conclusively explained. Here, using in situ angle-resolved photoemission
spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown
samples, we observed a built-in potential (V_{bi}) proportional to the polar
LAO thickness starting from the first unit cell (UC) with CI formation
appearing above 3 UCs. However, we found that the V bi is removed by
synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered
by oxygen gas (O_{2}(g))-exposure. Furthermore, after UV-irradiation, the CI
appears even below 3UC of LAO. Our results demonstrate not only the
V_{bi}-driven CI formation in asgrown LAO/STO, but also a new route to control
of the interface state by UV lithographic patterning or other surface
modification.
