The causes for variations in quantum efficiency (QE) for boron‐implanted midwave infrared diodes formed in mercury cadmium telluride on CdTe buffered GaAs/Si substrates have been investigated. Smooth layers have QE≳30% with many of the diodes in the 50%–60% range. Layers having a faceted morphology produce diodes with QE<30%. The faceted layers have been shown by infrared absorption measurements and secondary ion mass spectrometry depth profiles to possess microinhomogeneities in composition. Carrying out comparative growths on GaAs substrates have shown that these microinhomogeneities are not due to the growth process or fundamentally due to the interdiffused multilayer process used to grow the alloy layers. A study of the origins of the faceting behavior on GaAs/Si substrates, usinginsitulaser reflectance, has shown that they form from three‐dimensional island growth, nucleating on the substrate and growing outwards to dominate the growth mode after 2 000 Å of buffer growth.