We report synthesis of Au-free selective area epitaxy of InGaAs/InAs heterostructure nanopillars on patterned Si (111) substrate by metal-organic chemical vapor deposition. Effect of trimethylindium supply on InGaAs/InAs heterostructure nanopillars morphology and crystal structure is studied. Transmission electron microscopy images reveal subtle and consistent differences at the growth front of the InGaAs nanopillars for different indium-flow rate fraction. Notably, the nanopillar growth front exhibits zincblende-InAs cap for the low indium-flow rate fraction, whereas a sharp wurtzite-InAs segment is found at high indium-flow rate fraction. Nanopillar geometry profile fitting attributes that the wurtzite-InAs segment formation is ascribed to indium adatom diffusion on the sidewall surface along with direct impingement of indium growth species on the top facet of the nanopillar. Additionally, X-ray photoelectron spectroscopy confirms that indium segregation is more pronounced at the sidewall of the nanopillar under arsenic-limited region.