Simplified Theory of Reactive Close‐Spaced Vapor Transport

Abstract

A simplified model is proposed to determine the thermodynamic constants of the transport reaction in a close-spaced configuration, even when the reacting species are unknown. For example, the enthalpy variation, the activation energies of condensation, and sublimation in the water vapor transport of CdTe are determined (AHeq. = 22, Qcond. ----9.2, and QsubL = 31.2 in kcal/ mole-D. The model is useful for interpreting the experimental results in epitaxial processes. In the low pressure iodine transport of silicon, it is shown that the growth rate is not a diffusion controlled process; from the energy AHeq : --16.5 kcal/mole -1 for the reaction Si + 2I ~--SiI2, we found Qcond. "----8 and Qsubl. : --24.5 (kcal/mole-D. An important result of the analysis is that determination of enthalpy variation cannot be obtained only from the slope of the growth rate-temperature dependence.The use of a close spacing in chemical transport systems for growing epitaxial layers of pure or compound semiconductors has been emphasized by Sirtl (1), Robinson (2), and Nicoll (3). The most important features of the technique are the use of a close spacing between source and substrate (1 mm) between which is maintained a temperature gradient, and the fact that a gas is used to react with the source to form a volatile compound which is decomposed at the surface of the substrate. Generally, the transport reaction occurring in the closed spaced system is difficult to establish, and it is the aim of this paper to propose a simple theoretical model for the mass transfer even if the transporting species are unknown. Although the deposition rate is strongly sensitive to the surface preparation and the crystallographic orientation of the substrate (4, 5) as well as to the pressure, or the flow rate, of the reactive gas, the substrate and source temperatures are of major importance in the process only in so far as the single crystallinity of the layer is not concerned. The proposed model is shown to be useful for endothermic as well as exothermic process, i.e., for positive or negative heat of reaction at constant pressure.

Vapor transport of ZnO in closed ampoules

“…However, the experiments in Ref. [23] used $50 mg of ZnO in 15 mm ID ampoule; thus higher fluxes may be expected when larger amounts of the source are used. At higher temperatures the kinetics would be faster and corresponding limitations to the sublimation rate would be reduced.…”

“…Growth rates of a few mm/min were observed. The work and subsequent conclusions about kinetic limitations to sublimation of ZnO were based on the paper by Bailly et al [23]. That paper describes determination of kinetic limitations by experiments in close-spaced configuration (CSVT).…”

Appareillage d'épitaxie de composés semiconducteurs par transport réactif à courte distance

“…4) et se trouve être en accord avec les valeurs publiées par d'autres auteurs [10,11,15]. Un modèle simplifié, de portée générale, du processus de croissance a été proposé, qui permet d'atteindre les constantes thermodynamiques de la réaction de transport, même dans le cas où les espèces obtenues par la réaction ne sont pas déterminées [19,20]. Un autre paramètre important de la méthode, la pression et le débit du gaz réactif, a été étudié ; on a pu noter que d'une manière générale la vitesse de croissance présentait un maximum pour une certaine valeur de la pression partielle de vapeur d'eau (Fig.…”