Synthesis and characterization of TiO2 nanotube Schottky diodes

Abstract

Metal-semiconductor interfaces on one-dimensional (1D) nanostructures represent crucial building blocks for next-generation nanoelectronics. Over the past two decades, chemically synthesized titanium dioxide nanotubes (TiO2-NTs) have gained a considerable amount of interest due to their high specific surface areas, tunable geometries, and favorable electrical properties. Explored for a variety of applications such as solar cells, hydrogen production, memristors, and batteries, TiO2 is a transition metal oxide material that contains unique wide-bandgap semiconductor properties. This research seeks to fabricate a conformal, rectifying metal-semiconductor interface, or Schottky junction, throughout an ordered array of semiconducting nanotubes. The atomic layer deposition (ALD) technique offers a precise, conformal growth mechanism and was used to deposit a continuous platinum (Pt) Schottky contact throughout the inner walls of the TiO2-NTs. Any defects found in the nanotubes, such as cracks, led to Pt atoms reaching all areas, forming an electrical short between the anode and cathode. To address this issue, initial experiments were centered on synthesizing defect-free, ordered TiO2-NT arrays using electrochemical anodization as it offers precise geometric control over the NT growth. Fabrication parameters, such as anodization time, electrolyte concentrations, and annealing environments, were investigated until the resulting NTs were free from structural defects. Demonstrated with low vacuum annealing, oxygen vacant, nonstoichiometric TiO2 was synthesized and investigated as a diode material. By comparing the current-voltage characteristics between stoichiometric and nonstoichiometric TiO2 diodes, we find that the oxygen vacant, nonstoichiometric TiO2 diodes displayed an improved ideality factor from 3.7 to 2.4. A 350 [degrees] C post-fabrication thermal treatment, however, led to both stoichiometric and nonstoichiometric TiO2 diodes having similar ideality factors of 2.0 and similar shifts in trap concentration and depth. Nonstoichiometric TiO2 devices exhibited a unique shunt conduction regime after thermal treatment where the shunt resistance was found to be on the order of 105 [omega]. These results present valuable experimental observations into understanding the effects of oxygen vacancies in TiO2 and effectively modifying the electronic properties of a conformal Pt/TiO2 nanostructured junction using a facile post-fabrication thermal treatment.