This paper examines the conduction band offset, ∆Ec, and interface charge density, σ, of disordered InGaP and GaAs heterointerfaces by controlling the AsH 3 cover time and flow rate at the growth interval from GaAs to InGaP. Short AsH, cover time (0.05 min) creates high ΔEc of 0.2 eV and low σ of 6.3×l0 10cm −2. Extending the AsH 3 cover time by 50 times cuts ΔEc to almost 0 eV and increases σ by one order. Interface morphology for long-AsH3 -cover-time samples observed by atomic force microscopy shows a terrace structure on GaAs surface, which means the surface is As rich. These results suggest that an As-poor GaAs surface is essential to achieving high-quality InGaP/GaAs heterointerfaces.