Metal-insulator transition in epitaxial Ga-doped ZnO films via controlled thickness.

Understanding and tuning of metal-insulator transition (MIT) in oxide systems is one of the interesting and active research topics of condensed matter physics. We report thickness dependent MIT in Ga-doped ZnO (Ga:ZnO) thin films grown by pulsed laser deposition technique. From the electrical transport measurements, we find that while the thinnest film (6 nm) exhibits a resistivity of 0.05 Ω-cm, lying in the insulating regime, the thickest (51 nm) has resistivity of 6.6×10-4Ω-cm which shows metallic type of conduction. Our analysis reveals that the Mott's variable range hopping (VRH) model governs the insulating behavior in the 6 nm film whereas the 2D weak localization phenomena is appropriate to explain the electron transport in the thicker Ga:ZnO films. Magnetoresistance study further confirms the presence of strong localization in 6 nm film while weak localization is observed in 20 nm and above thicker films. From the density functional calculations, it is found that due to surface reconstruction and Ga doping, strong crystalline disorder sets in very thin films to introduce localized states and thereby, restricts the donor electron mobility.