Electrical and Optical Properties of Index-Matched Transparent Conducting Oxide Layers for Liquid Crystal on Si Projection Displays

Abstract

In this study, the characteristics of several kinds of index-matched transparent conducting oxide layers (TCO) were investigated for applications to liquid crystals on silicon (LCoS) projection displays. The purpose of exploring the characteristics of these materials is to find alternatives to currently used index-matched indium-doped tin oxide (ITO) layers, since index-matched ITO suffers from high production costs and materials shortages. Based on zinc oxide (ZnO) TCO layers, metal dopants such as Al, In, or Ga were added to improve the optical and electrical properties; 15 nm thick ZnO, aluminum-doped ZnO (AZO), indium–gallium-doped ZnO (IGZO), indium-doped ZnO (IZO), and gallium-doped ZnO (GZO) layers were sputtered on glass substrates using radio frequency (RF) and direct current (DC) magnetron sputtering. The measured transparency in the visible radiation range was above 94% for all prepared index-matched TCO layers. Among them, the transparency of AZO layers was the highest, reaching 97.5%. The sheet resistance of the TCO layers was around 100 Ω cm-2, with 82.6 Ω cm-2 being the lowest measured value obtained from a 15 nm thick AZO layer. Furthermore, the sheet resistance uniformity measured by samples with an area of 200×200 mm2 was below 5%. Atomic force microscopy measurement results show that the root-mean-square surface roughness values were lower than 0.01 nm in ZnO and AZO, and 0.128 and 0.261 in IGZO and GZO, respectively. The contact angle, which is another key factor in index-matched TCO-coated substrates, was around 25°, which meets the requirements for LCoS projection display panels. Among the tested TCO layers, AZO exhibited superior characteristics in terms of optical and electrical properties. Therefore, AZO represents an alternative to currently used index-matched ITO layers in LCoS projection displays.