This paper reports on a GaAs high- k MIS structure having an MBE-grown Si interface control layer (ICL) which has recently shown a promising result. It has a HfO 2/SiN x /Si ICL/GaAs structure where an ultrathin SiN x buffer layer is produced by direct nitridation of Si ICL. In this study, a particular attention is paid to optimize the initial thickness of Si ICL by correlating the interface structure studied by in situ X-ray photoelectron spectroscopy with the electronic interface quality studied by capacitance–voltage measurements. It was found that the presence of ML-level Si ICL at the interface after the formation of the SiN x is vitally important to obtain low values of interface trap density ( D it). Excess initial thickness of Si ICL also resulted in increase of D it. Initial Si ICL thicknesses of 5–6 MLs were found to be optimum, and gave U-shaped D it distributions with minimum D it values around 1 × 10 11 cm −2 eV −1 or below.