Energy Levels, Electronic Properties, and Rectification in Ultrathin p-NiO Films Synthesized by Atomic Layer Deposition

NiO is an attractive p-type transparent semiconductor that is being explored for a variety of applications. We report a systematic study of the electronic properties, relevant to hole-transporting materials in solar energy conversion applications, of NiO synthesized by atomic layer deposition (ALD). The acceptor concentration, flat band potential, and valence band position were determined by electrochemical Mott-Schottky analysis of impedance data in aqueous electrolytes for films less than 100 nm in thickness on F:SnO2 (FTO)-coated glass substrates. The effects of postdeposition annealing and film thickness were studied. Oxidation of the NiO was observed at temperatures as low as 300 degrees C in 1 atm of oxygen. Films annealed at 400 degrees C and above in Ar exhibited signs of thermal decomposition. Thinner films were found to have a higher carrier concentration. F:SnO2 and thermally evaporated Ag were both observed to form ohmic contact to ALD-synthesized TiO2 and NiO. A p/n heterojunction diode was fabricated from the transparent ALD TiO2 and NiO layers with the structure FTO/NiO/TiO2/Ag that showed rectification.