Kinetical study of the nonstoichiometric vapour growth process in the system ZnSe:I₂

Abstract

We have studied the growth of ZnSe crystals by chemical transport in a closed system in nonstoichiometric conditions, and we have deduced that the interface kinetics is the phenomenon limiting the growth process. The effect on the growth process of the deviation from stoichiometry of the II-VI compound was investigated using a mathematical model that involves indirect data computed from directly obtained experimental values. The experimental crystallization rate was compared with the maximum value of the transport flux calculated using the Arizumi-Nishinaga model. The influence of the stoichiometry of the source material and of the variations in the growth parameters (supercooling, geometrical dimension, specific loading of the ampoule and iodine concentration) on the ZnSe crystal growth process has also been studied. IntroductionThe growth of the bulk wide gap II-VI semiconductor compounds, such as zinc selenide (ZnSe) for optical and opto-electronic applications, rises problems because they have high melting points and they decompose in volatile components below this temperature. Alternative methods for the crystals growth are the growth from the vapour phase by physical transport [1][2][3] or chemical transport in a closed system using different transport agents (iodine [4][5][6][7][8][9][10][11][12][13][14], NH 4 Cl [15], Zn(NH 4 ) 3 Cl 5 [16][17][18]). In the last system the growth rate is controlled either by the interface kinetical processes or by the mass transport in the vapour phase [19]. The first thermodynamic analysis of the chemical transport reaction in closed tube system has been done by Schäfer [20][21][22] who explained the transport of the solid material considering only the diffusion and laminar flows and neglecting all the other influences on the transport in the vapour phase. Lever [23] has developed a model for the transport of the solid between two regions situated at different temperatures and he has deduced a transport function for an one-dimensional, steady-state diffusion limited process, in a single heterogeneous equilibrium system. Arizumi-Nishinaga [24][25][26] has introduced another transport function taking into account equilibrium considerations, Fick's diffusion law and Schäfer expression for the transport rate.The method to grow single crystals from the vapour phase by chemical transport was firstly presented by Nitsche [27] and further developed by Kaldis [28]. Lately the interest for this growth method has increased and computational studies of different systems such as ZnSe+I 2 have been developed [12][13][14].This paper presents some results about the influence of the stoichiometry of the starting material on the crystallization process. To investigate the effect of the deviation from stoichiometry and thermal regime on the growth process of ZnSe crystals by iodine transport, we have considered two different successive phenomena: the transport of the species present in the gaseous phase (transport kinetics) and the incorporation of the gaseous species at the crystal-vapou...