Development and implementation of new volatile Cd and Zn precursors for the growth of transparent conducting oxide thin films via MOCVD

For the growth of thin zinc group metal oxide films [i.e. cadmium oxide (CdO), cadmium stannate (Cd₂SnO₄), and zinc oxide (ZnO)] via metal-organic chemical vapor deposition (MOCVD), volatile Cd and Zn precursor families are needed. Starting with Cd, β-ketoiminates of varying substitution were prepared to elucidate structure-property relationships. The nature of the ligand substituents strongly influences the melting point (liquid precursors are desired). Unlike conventional Cd β-diketonates, these complexes are monomeric as determined by x-ray crystallography. Despite these advantageous characteristics, attempts to grow CdO thin films in a cold-wall MOCVD reactor using two such derivatives were not successful. This class of Cd complex appears to decompose thermally with time-- a likely cause of the poor performance. Therefore, a new series of more thermally stable Cd precursors was sought. Using the chelating diamine N,N,N′,N′-tetramethylethylenediamine (TMEDA), monomeric β-diketonates were prepared. The molecular structure of Cd(hfa)₂(TMEDA) (hfa = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) confirms the monomeric structural assignment. This series of Cd complexes is appreciably more volatile and sublime more cleanly than the aforementioned β-ketoiminates, as determined by vacuum thermogravimetric analysis (TGA). In addition to this advantageous characteristic, these complexes are easily prepared under ambient laboratory conditions from commercially available starting materials in a single step. Following the protocol established for Cd, a volatile series of Zn precursors was also prepared. For the Zn series, the melting point was effectively tuned through variation of both the β-diketonate and diamine ligands. The use of the Cd and Zn β-diketonate precursors in the successful growth of CdO and ZnO thin films, respectively, by MOCVD is also presented.