A new approach to the growth of ZnO by vapour transport

Abstract

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 O 2 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 (∆T) 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 (CO 2 ). The experiments with Ar have allowed the analysis of the effects of an inert gas on the transport processes. Meanwhile, the experiments with CO 2 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, CO 2 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.

Growth of ZnO crystals by vapour transport: Some ways to act on physical properties

“…Thus, an initial CO 2 pressure generates a mixture of CO 2 + CO inside the growth ampoule [30]. The partial disappearance of graphite covering after the reverse thermal profile period, in experiments with CO 2 as additional gas, confirms this assumption.…”

“…The colourless ZnO crystals obtained in experiment A indicates that near to stoichiometric conditions were attained. As said before, the initial pressure of CO 2 (∼ 1 atm) generates a mixture of CO 2 and CO, during the reverse thermal profile [30]. This gaseous mixture, together with a relatively fast mass transport rate (∼11.2x10 -3 g/h) established by the chosen experimental conditions, reduces the O 2 partial consumption (see eq.…”

“…As already shown [12,30], the ZnO growth in presence of graphite can be well understood as a process in which graphite reacts with the O 2 provided by the ZnO decomposition through the following reactions:…”

“…Recently, we have proposed a new frame to analyse the chemical role of carbon as an "additional species" in the ZnO growth process [12,30]. Carbon promotes a partial consumption of the O 2 provided by the ZnO decomposition and, consequently, produces a Zn excess.…”

“…Actually, the systematic study of the growth as a function of different experimental conditions (thermal difference between source material and crystallization zone, composition and pressure of additional gas) [30] provides information on the chemical reactions and hydrodynamic behaviour involved in the growth process. From an improved understanding of the global process, we present in this work some ways to act on the Zn excess during the growth process.…”