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 accepter 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 post-deposition annealing and film thickness were studied. Oxidation of the NiO was observed at temperatures as low as 300 ^oC in 1 atmosphere of oxygen.  Films annealed at 400 ^oC 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.