PNNL

Abstract

We discuss the effect of microstructure on the secondary phase segregation region and mobility of carbon impurities in case of poorly soluble Cr as a dopant in ZnO thin films. Thin films of Cr:ZnO ~50 nm in thickness were grown by metal organic chemical vapor deposition (MOCVD) of Zn(TMHD) and Cr(TMHD) precursors in reactive oxygen partial pressure environment. For an accurate comparison among the differences among the grain-boundary density and degree of orientation on the secondary phase segregation and impurity mobility, simultaneous thin film growths were carried out on single crystals of Si (100), c-plane oriented Al2O3 (c-ALO) and r-plane oriented Al2O3 (r-ALO) substrates. High-resolution transmission electron microscopy (HRTEM) measurements across the film substrate interface indicate that growths on Si(100) and c-ALO resulted in highly oriented Cr:ZnO films whereas a good epitaxial growth was observed on r-ALO. The trace carbon impurity detection, secondary phase formation and their mobility properties were studied by sensitive x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). We have observed that secondary phase segregation regions occur near the surface for Cr:ZnO films grown on ALO whereas the region moves near the interface for the growth on Si. Considering the presence of grain boundaries in Cr:ZnO grown on c-ALO and Si, it appears to be a weak relationship between grain boundary density and unfavorable dopant mobility as well as preferred segregation region. A near uniform stress distribution observed at r-ALO/ZnO interface indicates good epitaxial growth by domain matching epitaxy process. We also observe that low carbon impurity distribution in the studied thickness regime remains more or less uniform inside Cr:ZnO. This gives strong evidence that trace amount of carbon is soluble in the Cr:ZnO system as a direct result of oxygen vacancy defects.