Growth regime reaction controlled

Initially, when the ApBq layer is very thin, the reactivity of the A surface is realised to the full extent because the supply of the B atoms is almost instantaneous due to the negligibly short diffusion path. In such a case, the condition kom kW]/x is satisfied. Therefore, if the surface area of contact of reacting phases A and ApBq remains constant, chemical reaction (1.1) takes place at an almost constant rate. In practice, this regime of layer growth is usually referred to as reaction controlled. The terms interface controlled regime and kinetic regime are also used, though less suited. 

Like the case of component B, it is possible to theoretically define the concept of the regime of growth of the ApBq layer with regard to component A as well. The growth regime of the ApBq layer is reaction controlled with regard to component A at... 

Diffusion controlled growth of the ApBq and ArBs layers Increasing the thickness of the ArBs layer will inevitably result in a change of its growth regime from reaction to diffusion controlled with regard to... 

An unambiguous criterion to distinguish between the growth regimes of any compound layer is the availability or lack of diffusing atoms of a given kind for other layers of a multiphase binary system. Under conditions of reaction (chemical) control these atoms are still available, while under conditions of diffusion control already not, and this is all that is necessary to explain the absence of some part of compound layers from the A-B reaction couple. 

If the growth regimes of all the layers are reaction controlled (in the theoretical definition given in Chapter 1) at least with regard to one component, then they can in principle grow simultaneously whatever their number. Note that in this case the layer-growth kinetics can hardly be expected to obey a parabolic law. This is characteristic of very thin compound layers, at most a few hundreds of nanometres thick, if not less. 

It might seem that the ArBs layer could grow in the A-ApBq-ArBs-AiBn-B system by the same mechanism as in the ApBq-ArBs-B and ApBq-ArBs-A Bn systems, i.e. at the expense of the phase transformation of ApBq into ArBs under the influence of reaction diffusion of the A atoms. However, this is not the case. If the growth regime of the ApBq layer in the A -ApBq-ArBs-AiBn-B system is reaction controlled with regard to component A (x < ), then there is an excess of A atoms in comparison with the... 

Assume that with time the growth regime of the ApBq layer with regard to component A became diffusion controlled (x> x fl), while the ApBq phase was partly transformed into the ArBs one by reaction (4.6). The A atoms released as a result of this transformation cannot, however, cross the ApBq ArBs interface in the A ApBq ArB -A]Bu- B system in the same manner as in the ApBq -ArBs-B or ApBq ArBs-AiB system. Those A atoms will immediately be combined into the ApBq compound at this interface (onto the surface of the ArBs phase from the side of ApBq) by reaction (3.12) which is opposite to reaction (4.6). It is clear that the total result of these reactions is zero. 

Growth kinetics of nanocrystals in the presence of capping agents is determined by several complex factors, and signatures of either the diffusion or the reaction-controlled regimes are... 

In the A sector (lower right), the deposition is controlled by surface-reaction kinetics as the rate-limiting step. In the B sector (upper left), the deposition is controlled by the mass-transport process and the growth rate is related linearly to the partial pressure of the silicon reactant in the carrier gas. Transition from one rate-control regime to the other is not sharp, but involves a transition zone where both are significant. The presence of a maximum in the curves in Area B would indicate the onset of gas-phase precipitation, where the substrate has become starved and the deposition rate decreased. 

Heat transfer is an extremely important factor in CVD reactor operation, particularly for LPCVD reactors. These reactors are operated in a regime in which the deposition is primarily controlled by surface reaction processes. Because of the exponential dependence of reaction rates on temperature, even a few degrees of variation in surface temperature can produce unacceptable variations in deposition rates. On the other hand, with atmospheric CVD processes, which are often limited by mass transfer, small susceptor temperature variations have little effect on the growth rate because of the slow variation of the diffusion with temperature. Heat transfer is also a factor in controlling the gas-phase temperature to avoid homogeneous nucleation through premature reactions. At the high temperatures (700-1400 K) of most... 

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