  Scanning electron microscopy (SEM) based electron channeling contrast imaging
(ECCI) is used to observe and quantify misfit dislocation (MD) networks formed
at the heteroepitaxial interface between ZnS and GaP grown by molecular beam
epitaxy (MBE). Below a critical thickness of 15-20 nm, no MDs are observed.
However, crystallographic features with strong dipole contrast, consistent with
unexpanded dislocation half-loops, are observed prior to the formation of
visible interfacial MD segments and any notable strain relaxation. At higher
film thicknesses (20 to 50 nm), interfacial MD lengths increase anisotropically
in the two orthogonal in-plane <110> line directions, threading dislocation
(TD) density increases, and a roughening transition is observed from atomically
smooth two-dimensional (2D) to a multi-stepped three-dimensional (3D)
morphology, providing evidence for step edge pinning via surface terminating
dislocations. The ZnS strain relaxation, calculated from the total MD content
observed via ECCI, matches the average strain relaxation measured by
high-resolution x-ray diffraction (HRXRD). The MD lengths are found to follow a
log-normal distribution, indicating that the combined MD nucleation and TD
glide processes must have a normal distribution of activation energies. The
estimated TD glide velocity ($v_{g}$) along [$\bar{1}$10] is almost twice that
along [110], but in both directions shows a maximum as a function of film
thickness, indicating an initial burst of plasticity followed by dislocation
pinning.
