Strain-relaxed, compositionally graded InGaP layers grown by atmospheric-pressure metalorganic vapor phase epitaxy (APMOVPE) have previously been found to exhibit unusual contrast in transmission electron microscopy (TEM). The features that generate this contrast were termed "branch defects." Branch defects have been shown to pin threading dislocations and are thus undesirable features for the realization of low dislocation density semiconductors'. In this study, we compare the properties of branch defects formed during optimized, relaxed, graded InGaP buffer deposition in two different reactor configurations: a commercial, multiwafer, low-pressure reactor and a custom-built, atmospheric-pressure research reactor. Branch defect formation is further characterized via the introduction of in situ annealing interruptions during graded buffer deposition in the atmospheric-pressure system. Branch defects are observed in material from both reactor systems, suggesting that they are a phenomenon intrinsic to InGaP graded buffer growth. Careful TEM studies of the resulting samples reveal that the phase space for the formation of branch defects is similar in both reactor configurations. During standard optimized graded buffer growth, higher growth temperatures delay the onset of branch defect formation to higher indium fractions in the graded buffer. Low growth temperatures produce branch defects at lower indium fractions, and these defects tend to be more closely spaced. In addition, the formation of branch defects is favored by low V/III ratios and in situ growth interruption and annealing. Annealing is found to create anisotropic strain relaxation in the graded buffer, which we attribute to the blocking of gliding threading dislocations by preferentially oriented branch defects. Based on the observed properties of branch defects and the factors that affect their formation, it appears that these defects are a manifestation of local variations in indium concentration that develop on the sample surface during MOVPE and are buried in the bulk due to kinetic limitations. (C) 2004 American Vacuum Society.