Ma mb

for each component, i, in the system.

When a system is not at equilibrium, the thermodynamic driving force to restore equilibrium is:

This also is the driving force for epitaxial growth. In MOCVD, a nonequilibrium situation is intentionally created, in which the chemical potential of the solid phase differs from that of the vapor. The system restores equilibrium by producing the desired solid phase. The maximum quantity of solid produced is that which is needed to establish equilibrium. It is thus limited by the thermodynamics and size of the system.

The growth rate observed in MOCVD is almost always less than that calculated from thermodynamic principles. Kinetic factors, that is, either surface reaction rates or the diffusion of gas species through the vapor phase, are what limits the growth rate because they hinder the establishment of equilibrium. Driving forces for epitaxial growth are created in the vapor phase and the diffusion boundary layer, and at the interface between the solid and vapor, as depicted in Fig. 3(a). This diagram shows the driving forces necessary to sustain the flux for mass-transport processes (ApD) and surface reactions (Aps).

Input gas | Boundary phase 1 layer

Interface

Solid

t

N

i

\

0 0

Post a comment