Optimizing thermal oxidation temperature for enhanced structural and electrical properties of Ho2O3 ultrathin-film gate dielectric on Ge semiconductor substrate

Abstract

This study systematically characterized the structural, morphological and electrical properties of sputtered Ho/Ge interface after subsequent thermal oxidation in O₂ ambient varying deposition temperature from 450 °C–550 °C for metal oxide semiconductor capacitor (MOSCAP) device. The XRD analysis confirmed presence of cubic c-Ho₂O₃ and tetragonal and cubic GeO₂ while the effect of crystallite size and micro-strain on electrical performance has been reported. As observed in XPS, 500 °C thermal oxidation condition has been conducive for exhibiting more stoichiometric phase of Ho₂O₃. This condition helped to limit irregular O diffusion toward substrate facilitating more controlled Ge activation and minimized uneven intermixing. Thereby, inhibited unstable interfacial (IL) layer sub-oxides, GeO_x resulting more stable and compact interface Ho₂O₃/ IL (GeO₂ + GeO_x). However, thermal budget of 550 °C led thicker interfacial layer due to over-oxidation corresponding severely defected Ho₂O₃/(GeO₂ + GeO_x) interface degrading device operation. HRTEM further confirmed double stacked amorphous Ho₂O₃/IL structure ranging physical thickness from 7.04–10 nm. The reduction of structural defect at optimum oxidation condition 500 °C imposed a conduction band offset of 1.77 eV which impeded uneven electron transportation from Ge CB edge into the interface of Ho₂O₃/IL leading enhanced electrical breakdown at $J_g$ 10^-6 Acm⁻² withstanding electrical breakdown field, $E_{\mathrm{BD}}$ 7.93 MVcm⁻¹. Therefore, controlling the temperature set up for oxidized Ho/Ge could offer thinner GeO_x layer improving the $k$ 12.54 corresponding EOT 2.65 nm while reducing $Q_{\mathrm{eff}}$ and $Q_{\mathrm{it}}$ and $D_{\mathrm{it}}$ 10¹² eV⁻¹ cm⁻². In outline, Ho₂O₃ is projected to be a potential candidate as dielectric insulator for Ge-based MOSCAP devices.