The epitaxial SiP film grows on top of the Si template. The SiP has a smaller lattice constant than Si (since P atoms are smaller than Si atoms). Therefore, the SiP must undergo a tensile strain to atomically fit on top of the Si structure . This tensile strain is connected to the dynamics of the epi growth and decides how the epi will grow . There are three parts of epi growth dynamics 1) Epi height 2) Dopant concentration 3) Crystallinity. The fundamental physics of epi growth tells us that we cannot win on all three factors. We must lose on at least one of the three (Height, doping, crystallinity). This epi trilemma is at the center of epi process optimization.