A new approach to the growth of ZnO by vapour transport



The crystal growth of ZnO by vapour transport is classically made in presence of additional species which enhance the growth process. Usually, additional species have been considered as chemical transport agents that promote a typical CVT (Chemical Vapour Transport) process. Recently, we have proposed a new interpretation of the chemical role of some of these species. This new interpretation considers that, in some cases, the additional species promote a partial consumption of the O2 provided by the ZnO decomposition and, consequently, a Zn excess is generated. This excess of Zn pressure activates the ZnO decomposition and the growth rate is enhanced. Among those species, carbon shows an additional beneficial role in the ZnO growth process. The deposition of a graphite layer on the inner walls of the generally used growth silica ampoules avoids the reaction between the ZnO crystals and the silica. In order to gain a further insight into the growth process, the time dependence of the transported mass in presence of graphite has been studied using an in-situ dynamic technique. A systematic study of the mass transport rate dependence on the thermal difference between the source material and the crystallisation zone (ΔT) has been made. On the other hand the influence of a residual gas on the transport rate has been also systematically analysed. Two different gases have been used, argon (Ar) and carbon dioxide (CO2). The experiments with Ar have allowed the analysis of the effects of an inert gas on the transport processes. Meanwhile, the experiments with CO2 have shown its chemical role on the generation of a non-reacted Zn which is necessary to activate the ZnO decomposition. This role paves the way to control the Zn excess during the growth experiments. Thus, CO2 can be used to act on the off-stoichiometry degree of the ZnO crystals and consequently to control, at least partially, some of the physical properties of this material. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)