InGaAs epitaxial growth on InP substrates at low temperatures (below 560 degrees C) was examined by low-pressure metalorganic vapor-phase epitaxy using TMIn, TEGa, and TBAs as growth precursors in a horizontal reactor. It was not possible to grow InGaAs layer below 480 degrees C under usual growth conditions, and the limiting factor to lower the growth temperature in this growth system is found to be the decomposition of TEGa in the presence of TMIn. A new method in which InP dummy wafer is introduced upstream from the substrate in the gas flow to decompose the TEGa is successfully applied to obtain InGaAs epitaxial layers at lower growth temperatures. InGaAs layers of good optical quality were grown even at 420 degrees C. No observable difference in the amount of carbon than in the bulk substrate was found in the InGaAs layer grown at 420 degrees C by second ion mass spectroscopy measurement.