Bikowski, A.; Ellmer, K.: A comparative study of electronic and structural properties of polycrystalline and epitaxial magnetron-sputtered ZnO:Al and Zn1-xMgxO:Al Films - Origin of the grain barrier traps. Journal of Applied Physics 114 (2013), p. 063709/1-10
10.1063/1.4817376

Abstract:
Homoepitaxial and heteroepitaxial ZnO, ZnO:Al, and Zn1-xMgxO:Al films have been grown by magnetron sputtering from ceramic targets at substrate temperatures between 200°C and 500°C. We studied the relation between the electronic transport and structural properties for the epitaxially grown films and compared it to the properties of polycrystalline films by means of X-ray diffraction, transmission electron microscopy and optical reflectance and transmittance measurements. The results show that the epitaxial growth of ZnO:Al and Zn1-xMgxO:Al thin films, which has been observed for nearly all films prepared on single crystalline substrates, will not significantly improve the electronic transport properties in comparison to polycrystalline films unless the grain boundaries are eliminated completely. The grain boundary defect densities of about 3*10^13 cm^(-2) are nearly independent on the structural quality of the different polycrystalline, hetero- and homoepitaxial films. This clearly proves that the grain boundary defects are not caused by crystallographic defects, but, most probably, by the dopant aluminium.