Al incorporation during metal organic chemical vapour deposition of aluminium zinc oxide

Abstract

Aluminium zinc oxide (AZO) is a promising transparent conductive oxide this is difficult to fabricate by metal-organic chemical vapour deposition (MOCVD) because of prereactions between the precursors for Al and oxygen. One way of reducing prereactions is to use separate injectors. In this work, we investigated the effect of separate injectors and various flow configurations on the MOCVD fabrication of AZO using diethyl zinc (DEZn), tert-butanol (tBuOH), and trimethyl aluminium (TMAl) precursors.

X-ray diffraction revealed that the resulting films maintained a purely wurtzite ZnO structure, and that the Al incorporation changed the c lattice parameters from 5.207 to 5.181 Å in agreement with theoretical models of mixing on the Zn sub-lattice. This indicates that Al was mainly incorporated onto the Zn sub-lattice. The maximum Al concentration was 6 × 1021 cm

.

The Al concentration was directly influenced by the TMAl flow rates, but the incorporation efficiency of Al also depended on the O/Zn balance as determined by the tBuOH supply and the total flow rate through the TMAl+DEZn injector. The tBuOH supply decreased the incorporation efficiency, while it increased for higher total flow in the TMAl+DEZn injector although both changes nominally increases the O/Zn balance. This dual, but contradicting, dependency on the O/Zn suggested that the incorporation efficiency depended not only on precursor supply, but also on the chemical stability of the Al species present at the growth interface. The concept of a reaction depth of TMAl was introduced to discuss the observed trends, and a qualitative model was suggested to explain how the Al-species present at the growth interface are determined by the relative depths of the reaction depth and the boundary layer. These findings illustrate the limits of the O/Zn parameter when using separate injectors because the properties of the boundary layer can affect Al incorporation during MOCVD growth. This should be relevant for MOCVD growth of other compound semiconductor systems that are grown using separate injectors.