Optical and Structural Properties of Single Phase Epitaxial p‐Type Transparent Oxide Thin Films

Abstract

Transparent conductive oxides (TCOs) are widely used in optoelectronic devices. For example, Sn-doped In2O3 (indium tin oxide, ITO), Al-doped ZnO, and F- or Sb-doped SnO2 have been practically adopted as transparent electrodes in flat panel displays, solar cells, and organic light-emitting diodes. [1] As these materials are all electron doped, the electrical conductivity of these materials is controlled by the conduction of electrons. In other words, they are n-type TCOs. In order to fully explore the potential applications of TCOs in transparent electronics, the development of p-type TCOs is critical as well. [2] For instance, the formation of active devices such as p–n junctions based on TCOs can lead to functional windows, where the junction not only transmits visible light but also generates electricity by the absorption of the ultraviolet (UV) radiation. [2] Since the discovery of p-type electrical conduction in the transparent CuAlO2 thin film, a class of delafossite AMO2 materials (A= Cu or Ag, M = Al, Ga, Fe, or In) has been extensively investigated in the last several years. [2–5] The crystal structure of delafossite CuAlO2 consists of an alternating stack of Cu ion layers and AlO2 octahedron layers. Each Cu atom is linearly coordinated with two oxygen atoms to form an O-Cu-O dumbbell unit placed parallel to the c-axis. O-atoms of O-Cu-O dumbbell link all Cu layers with the AlO2 layers. [2] Synthesizing thin film of this material is complicated due to the variety of possible Cu-Al-O phases. [6] A few attempts have been made to prepare CuAlO2 films using techniques such as pulsed laser deposition (PLD), DC- and rfsputtering, chemical vapor deposition (CVD), ion-exchange reactions, spray, and sol-gel methods. [2,5–18] The chemical solu