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Atomic layer deposition (ALD) provides a unique opportunity to integrate high-quality gate dielectrics on III-V compound semiconductors. The physics and chemistry of a III-V compound semiconductor surface or interface are problems so complex that even after three decades research understanding is still limited. We report a simplified surface preparation process using ammonium hydroxide (NH4OH) to remove the native oxide and make the hydroxylated GaAs surface ready for ALD Al2O3 surface chemistry. The effectiveness of GaAs passivation with ALD is demonstrated with small hysteresis, 1%-2% frequency dispersion per decade at accumulation capacitance, and a mid-bandgap D it of 8 times1011 to 1times 1012 cm-2 ldr eV-1determined by the Terman method. The results from ammonium sulfide [(NH4)2S-,and hydrofluoric acid (HF)-, and hydrochloric acid (HCl)-treated surfaces and a surface with native oxide are also presented to compare with the results from the ammonium-hydroxide-treated surface. Fermi-level unpinning is also easily demonstrated on the ALD HfO2 and p-type GaAs interface.