In this article, we demonstrate plasma-enhanced atomic layer deposition (PEALD) as an effective passivation technique to establish high interfacial quality of high-k/InGaAs structures. Performing PEALD-AlN as an interfacial passivation layer (IPL), excellent capacitance-voltage (C-V) characteristics have been achieved for the HfO2/n, p-In0.53Ga0.47As MOSCAPs. The effects of AlN-IPL on the effective work function (EWF) of metal and band alignment of the HfO2/AlN/In0.53Ga0.47As structure have also been investigated. In addition, the multilayer TiNi alloys were applied as gate dielectric for a suitable metal EWF with a small work function variation (WFV). A sub-nm equivalent oxide thickness (EOT) HfO2/In0.53Ga0.47As MOS device with low density of interface states and low leakage current density was obtained utilizing AlN-IPL and post remote-plasma (PRP) gas as pre-and post-gate treatments in an in-situ ALD process. The device performance and reliability of HfO2/In0.53Ga0.47As nMOSFETs with in-situ PEALD treatments have also been characterized.