Abstract: An integrated circuit (IC) substrate (32) comprising a germanium layer (26), an aluminium oxide layer (22), and an interfacial layer (28) provided on the germanium layer between the germanium layer and the aluminium oxide layer, which interfacial layer provides control of electrical properties at an interface between the germanium layer and the interfacial layer. The electrical properties may comprise charge carrier trap density, and the interfacial layer may provide control of the charge carrier trap density to minimise the trap density. The interfacial layer is used to ensure an intimate, high-quality germanium layer—interfacial layer interface. A method manufacturing an IC substrate is also provided, along with a gallium arsenide circuit integrated in a system-on-chip (SOC) comprising an IC substrate, and a germanium electronic circuit in combination with a gallium arsenide circuit, integrated in a system-on-chip—(SOC), comprising an IC substrate.