Fabrication and characterization of germanium n-MOS and n-MOSFET with thermally oxidized yttrium gate insulator: Formation of underlying germanium oxide and its electrical characteristics

Abstract

A high-quality insulating film on germanium is essential for germanium applications. In this study, we oxidized metal yttrium at 500–550 °C to generate an yttrium oxide (Y-oxide) layer on germanium. According to transmission electron microscopy images, Y-oxide comprised three layers, which were Y2O3, YGeO3, and GeOx from the top side. n-Type metal-oxide-semiconductor (n-MOS) capacitors and n-type MOS field-effect transistors (n-MOSFETs) with the Y-oxide gate insulator showed typical electrical characteristics. The n-MOS capacitor with the Y-oxide gate insulator had a lower interface state density (Dit) and border trap density (Nbt) than the n-MOS capacitor with a thermally oxidized GeOx insulator, suggesting that defects were terminated by Y atoms in the GeOx layer and GeOx/Ge interface. In contrast, the Dit–energy distribution and Nbt temperature dependence of the Y-oxide gate insulator were similar to those of the GeOx gate insulator, indicating that the defect signals originated in GeOx underlying YGeO3. The structural analysis showed that the temperature of metal yttrium oxidation affected only the GeOx thickness. Therefore, adjusting the conditions of metal yttrium oxidation may produce Y-oxidized gate stacks with a thinner GeOx layer or even direct contact of YGeO3 and Ge, which may result in different Dit values, Nbt values, and Nbt–temperature trends.