Characterization of undoped and Cu-doped ZnO films for surface acoustic wave applications

Abstract

Cu-doped ZnO (denoted by ZnO:Cu) films have been prepared by RF magnetron co-sputtering of a ZnO target with some Cu-chips attached. X-Ray diffraction (XRD) spectra of deposited ZnO:Cu films were measured and texture coefficient (TC) values for (002)-orientation were estimated. Optimal ranges of RF powers and substrate temperatures for obtaining high TC values were determined. Effects of Cu-doping conditions (such as Cu-chip sputtering areas and O2/(Ar+O2) mixing ratios) on TC values, electrical resistivities, and relative Cu-compositions of deposited films have been systematically investigated. X-Ray photoelectron spectroscopy (XPS) study suggests that the relative densities of metallic copper (Cu0) atoms and CuO (Cu2+)-phases within deposited films may play an important role in determining their electrical resistivities. Highly resistive (>1010 Ωcm) ZnO films with high TC values (>80%) can be achieved by Cu-doping. Surface acoustic wave (SAW) devices with ZnO:Cu (or ZnO)/interdigital transducer (IDT)/SiO2/Si configuration were also fabricated to estimate the effective electro-mechanical coupling coefficient (keff2) and insertion loss. The devices using Cu-doped ZnO films have higher keff2 and lower insertion loss, compared with those using undoped films.