Electrical Transport and Structural Properties of Oxygen Deficient Strontium Titanate Thin Films Fabricated by Pulsed Laser Deposition

Bulk SrTiO₃ (STO) is a band insulator with a 3.2 eV band gap, but its electronic properties can be changed to yield metallicity through charge doping and introduction of defects by exciting photocarriers. In thin films of STO, charge doping can be achieved easily by introducing oxygen deficiency, thus yielding insulator-metal transitions which are driven by both carrier concentration and disorder. We have deposited epitaxial STO thin films on LaAlO₃ (LAO) and STO substrates using pulsed laser deposition, with the goal of investigating the effects of deposition parameters and lattice-mismatch strain on the metal-insulator transition. We observe that STO thin films on LAO substrates grown in high vacuum show metal-insulator transitions, while STO films grown on stoichiometric STO substrates show metallic behavior down to 50 K with very low resistivity. Interestingly, we also find that growth of STO films on STO substrates in high vacuum causes the substrate itself to become highly conducting, as has also been observed by previous researchers. When the substrate becomes highly conducting, it becomes necessary to eliminate the substrate contribution from measured electrical resistivity. This is an aspect that has been overlooked in previous reports of electrical properties of STO films. We will present our electrical resistivity results of the STO films on LAO and STO substrates, and we will discuss methods to separate substrate and film contribution from resistivity calculations. We will also present our results of the thickness dependence of electrical resistivity and lattice parameters on both the substrates, indicating the role of lattice-mismatch strain, and we will show that these electronic changes reflect a transformation in the optical band gap of the STO films and substrates. We will also discuss the variation of surface morphology as a function of deposition parameters using atomic force microscopy images.