Positron annihilation studies on the behaviour of vacancies in LaAlO3/SrTiO3 heterostructures

The formation and diffusion of vacancies are studied in LaAlO3/SrTiO3 heterostructures. Oxygen vacancies (V OS) appear easily in the SrTiO3 substrate during LaAlO3 film growth at 700 °C and 10−4 Pa oxygen pressure rather than at 10−3–10−1 Pa, thus the latter two-dimensional electron gas should come from the polarity discontinuity at the (LaO)+/(TiO2)0 interface. For SrTiO3−δ /LaAlO3/SrTiO3, high-density V OS of the SrTiO3−δ film can pass through the LaAlO3 film and then diffuse to 1.7 µm depth in the SrTiO3 substrate, suggesting that LaAlO3 has V OS at its middle-deep energy levels within the band gap. Moreover, high-density V OS may combine with a strontium/titanium vacancy (V Sr/Ti) to form V Sr/Ti–O complexes in the SrTiO3 substrate at 700 °C.