Morphological, structural, optical, and electrical study of nanostructured thin films: Charge transport mechanism of p-type Co3O4

Abstract

The normal cubic spinel Co3O4 () is the most stable crystalline cobalt oxide and is a promising material for many applications due to its high abundance, low cost, and low toxicity. Co3O4 is successfully used as magnetic material [1], as supercapacitor [2,3], as electrochromic windows [4], as well as corrosion protective coatings [5]. It is noteworthy that Co3O4 is a direct p-type semiconductor and is extensively studied in photovoltaics cells [6], in gas sensors [7], in anode materials in lithium-ion batteries [8] and in photocatalysis [9]. Low band gap, large surface exchange and high conductivity are essential properties for the achievement of efficient photocatalytic materials. A large variety of physical and chemical deposition methods are available to tailor the films microstructures and their inherent physical and electrochemical properties. In particular, various Co3O4 deposition methods have been reported as epitaxy [10,11], magnetron sputtering [12,13], pulsed laser deposition [6], thermal oxidative decomposition method [4], chemical spray pyrolysis [3,14], sol–gel processes like dip coating [15], metal organic chemical vapor deposition (MOCVD) [16], plasma enhanced MOCVD [9], pulsed liquid injection MOCVD [5], and atomic layer deposition (ALD) [17,18]. Moreover, the electrical properties are strongly related to the deposition method. MOCVD [16] and ALD [18,19] dense thin films show the lowest resistivity ranging from 0.5 to 100 Ω cm, while sol-gel [20], chemical precipitation [21], and spray pyrolysis [3,[22], [23], [24]] deposition methods show lower conductive films with resistivity of about two orders of magnitude higher. The main hypothesis to explain this significant difference is the morphology and the grain size. The conduction paths in the highest conductive Co3O4 thin films is discussed as grain boundaries [18] or through the grains by variable range hopping (VRH) conduction of holes as proposed by Cheng et al. [16]. However, to the best of our knowledge, the latter hypothesis is not demonstrated experimentally using nano-scale characterization. In this context, the main objective of this paper is to investigate the influence of the Co3O4 morphology on the electrical properties at macro and nanoscale.In the following, we study the effect of two substrate temperatures (Ts) of 400 °C and 500 °C on the morphology, on the optical and the electrical properties of ~140-nm-thick Co3O4 films deposited by thermally activated pulsed liquid-gas injection MOCVD.