Copper Bilayer Transparent Electrodes

Abstract

The shortcomings of ITO have led to the search for alternatives, such as single walled CNTs, graphene films, and UTMFs [1–4]. Copper in thin-film form is a very inexpensive material with excellent electrical and optical properties hence its wide use in microelectronics. However ultrathin (i.e. ≤10 nm) Cu films are prone to oxidation and corrosion, which alter significantly their electrical and optical properties. This chapter describes the development of bilayer transparent electrode (TE) structures consisting of an ultrathin and continuous Cu film covered by a protective ultrathin Ti or Ni film. The capping layer is chosen according to the functional requirements of specific devices. The top Ti or Ni capping layers are then typically in situ treated with O₂ plasma which notably increase their transmittance. Both the Ni and Ti enhance the stability of the Cu based TEs and, in addition, Ni also increases the work function. The measured optical performance of the fabricated films is in good agreement with theoretical estimation based on multiple reflection and refraction (MFMR) model [5, 6]. The obtained Cu-Ti/Ni bilayered structure meets the requirements of the majority of optoelectronics applications, such as organic LEDs and photovoltaic solar cells, liquid crystal displays, presenting large optical transparency, low electrical R_S, and stability against temperature and oxidation. Transparency as high as 86 % at 630 nm, R_S as low as 16 Ω/sq have been achieved in bilayer Cu based films, which also exhibit significantly higher stability than single layer Cu films against temperature in ambient atmosphere.