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Germanium (Ge) has gained great attention not only for future nanoelectronics but for back-end of line (BEOL) compatible monolithic three-dimensional (M3D) integration recently. For high performance and low power devices, various high-k oxide/Ge gate stacks including ferroelectric oxides have been investigated. Here, we demonstrate atomic layer deposited (ALD) polycrystalline (p-) HfO2/GeOX/Ge stack with an amorphous (a-) HfO2 capping layer. The consecutively deposited a-HfO2 capping layer improves hysteretic behaviors ( ${\Delta } {V}$ ) and interface state density ( $\text{D}_{\mathrm{ it}}$ ) of the p-HfO2/GeOX/Ge stack. Furthermore, leakage current density ( ${J}$ ) is significantly reduced $(\times 100)$ by passivating leakage paths through grain boundaries of p-HfO2. The proposed HfO2 layer with the graded crystallinity suggests possible high-k/Ge stacks for further optimized Ge MOS structures.

We report on the effect of Al2O3 surface passivation on electrical properties of beta-gallium oxide (β-Ga2O3) nanomembrane field-effect transistor (FET). The fabricated bottom-gate β -Ga2O3(100) FET exhibits enhanced channel conductance and reduced hysteresis after the conformal atomic layer deposited Al2O3 passivation investigated by high-resolution transmission electron microscope (HR-TEM) analysis. Moreover, abnormal positive threshold voltage (VTH) shifts under negative bias stress are turned into negative VTH shifts, and off-state breakdown characteristics is improved as well. A modeling work using physics-based TCAD shows reduced surface depletion effect after the surface passivation. The results demonstrate that high-quality ALD-Al2O3 surface passivation is an effective method to improve electrical properties of the bottom-gate β-Ga2O3 FET and its device applications.

In this paper, we report on the electrical and thermal instability of β-Ga2O3 nanomembrane field-effect transistor with a bottom-gate configuration. The fabricated device exhibits high electrical performance of field-effect mobility of up to 60.9 cm2/V·s, on/off-current ratio of 109 and subthreshold slope of 210 mV/dec. However, we observe abnormal positive threshold voltage (VTH) shifts under negative bias-temperature stress at an elevated operating temperature of 80 °C as well as under temperature-dependent transfer characteristics up to 200 °C. This abnormal instability is significantly influenced by the surface depletion effect, and is discussed using energy band diagram. The opposite VTH shift was achieved by applying atomic-layer deposited Al2O3 passivation layer.