Type conversion of intentionally undoped ZnO layers grown by pulsed laser deposition

We have investigated intentionally undoped p-type ZnO films grown on Si(111) substrates by pulsed-laser deposition as a function of the oxygen pressure. It is shown that the undoped ZnO films experience type conversion from n-type to p-type when the oxygen pressure changes from 7.98×10−3 to 3.99×10−2 Pa. Ti/Au contacts give ohmic behaviour to n-type ZnO (∼1017 cm−3), but leaky Schottky behaviour to p-type ZnO (∼1018 cm−3). It is argued based on PL results that native defects, such as oxygen and zinc vacancies, could play an important role in determining the conductivity of these nominally undoped ZnO films.