Molecular-beam epitaxy growth of device-compatible GaAs on silicon substrates with thin (∼80 nm) Si1-x Gex step-graded buffer layers for high- κ III-V metal-oxide-semiconductor field effect transistor applications

The authors report the fabrication of TaN-Hf O2 -GaAs metal-oxide- semiconductor capacitors on silicon substrates. GaAs was grown by migration-enhanced epitaxy (MEE) on Si substrates using an ∼80-nm -thick Si1-x Gex step-graded buffer layer, which was grown by ultrahigh vacuum chemical vapor deposition. The MEE growth temperatures for GaAs were 375 and 400 °C, with GaAs layer thicknesses of 15 and 30 nm. We observed an optimal MEE growth condition at 400 °C using a 30 nm GaAs layer. Growth temperatures in excess of 400 °C resulted in semiconductor surfaces rougher than 1 nm rms, which were unsuitable for the subsequent deposition of a 6.5-nm -thick Hf O2 gate dielectric. A minimum GaAs thickness of 30 nm was necessary to obtain reasonable capacitance-voltage (C-V) characteristics from the GaAs layers grown on Si substrates. To improve the interface properties between Hf O2 and GaAs, a thin 1.5 nm Ge interfacial layer was grown by molecular-beam epitaxy in situ after the GaAs growth. The Ge-passivated GaAs samples were then transferred in air for the subsequent ex situ Hf O2 formation. This Ge interfacial layer in between Hf O2 and GaAs was necessary to avoid relatively flat C-V characteristics that are symptomatic of high interface state densities.