MoO x modified ZnGaO based transparent conducting oxides

Abstract

We report here the growth of high work function bilayered structures of thin MoOx (2.0<x<2.75) layer (few nanometers) on Zn0.95Ga0.05O films by pulsed laser deposition (PLD) on glass and sapphire substrates for transparent electrode applications. It was found that the films were highly textured along [0001] direction. The crystalline quality of the films deposited at different substrate temperatures was investigated by x-ray diffraction, transmission electron microscopy (TEM) imaging, and selected area diffraction pattern (SAED). In the MoOx layer, molybdenum exists in Mo4+, Mo5+, and Mo6+ oxidation states, and the ratio of (Mo4++Mo5+) to Mo6+ was determined to be ∼2:1. The bilayer films showed good optical transparency (≥80%) and low resistivity of ∼10−4 Ω cm. Different transport behavior of the MoOx/ZnGa0.05O films grown at different Ts (substrate temperature) was observed in temperature-dependent resistivity measurements. The bilayer film at higher Ts showed metallic conductivity behavior down to 113 K. Moreover, a blueshift of the absorption edge in the transmission spectrum was observed with the increase in Ts, indicating an increase in the carrier concentration. It was observed that the ZnGa0.05O films with ultrathin MoOx (∼1–2 nanometers) overlayer showed a higher work function (varying from 4.7 to 5.1 eV) as compared to the single layer ZnGa0.05O film work function (∼4.4 eV). A correlation between the surface work function and MoOx layer thickness is observed. The higher work function of the MoOx overlayer is envisaged to improve the transport of the carriers across the heterojunction in a solid state device, thus resulting an increase in device efficiency.